POSTERS BOOK
Posters - Alphabetical Order
Konstantinos
Patrik
Markus
Institution
Country
Title
Authors
Agalou
CNR
Italy
Ahlberg
Department of Engineering Sciences
Ahlskog
University of Jyväskylä
Studies of Isocyanate oligomer mixed with Graphene Oxide
Konstantinos Kouroupis-Agalou
Growth, synthesis techniques and integration methods
Sweden
CVD grown graphene evaluated with Raman and optical microscope
Patrik Ahlberg, Si Chen, Shi-Li Zhang, Ulf Jansson and Zhibin Zhang
Growth, synthesis techniques and integration methods
Finland
AFM-studies of humidity M. Ahlskog, M. dependence of friction Hokkanen, J. Lievonen, in graphene and other A. Helle, K. Holmberg 2D materials
Other 2 dimensional materials
Russia
Computer Simulation of Epitaxial Graphene Assembly on Silicon Carbide Surface with Using Semi-Empirical Quantum Chemistry Methods
N.I.Alekseyev, V.V.Luchinin, A.S.Ivanov
Growth, synthesis techniques and integration methods
United Kingdom
Two-phonon scattering in graphene in the quantum Hall regime
Arseny Alexeev and Mikhail Portnoi
Quantum transport
St.Petersburg
Nick
Alekseyev Electrotechnical University
Arseny
Miguel
Patricia
Irina
Irina
Chloé
Alejandro
Alexeev
University of Exeter
Alonso Pruneda
Centro de Investigacion en Nanociencia y Nanotecnologia (CIN2)
Alvarez
National Institute of Carbon, Spanish Council for Research
Antonova
A.V. Rzhanov Institute of Semiconductor Physics
Antonova
A.V. Rzhanov Institute of Semiconductor Physics
École Archambault Polytechnique de Montréal
Arzac
POLYMAT
Topic
Spain
Magnetoelectric and Flexomagnetic effects in J. M. Pruneda, R. Martinez-Gordillo hybrid C/BN nanostructures
Spain
Different chemical approaches to produce graphene derivatives
Russia
Advantage of few-layer graphene in comparison I.V. Antonova, I.A. Kotin, V.Ya Prinz with graphene for applications
Russia
Fluorographene with N.A. Nebogatikova, I.V. nanoswell surface relief Antonova, V.A. Chemistry of Graphene obtained by hydrofluoric Volodin, V.Ya Prinz acid treatment
Canada
Charge transfer engineering in graphene Chloé Archambault, nanoribbons using Alain Rochefort metallic contacts and organic adsorbed layers
Chemistry of Graphene
Spain
Emulsion mixing technique for preparation of poly(buthylacrylate/methyl Alejandro Arzac, methacrylate)/graphene Radmila Tomovska electrically conductive composite films
Applications (gaz sensors, composites, nanoelectronic devices...)
P. Álvarez, C. Botas, R. Santamaría, C. Blanco, M. Granda, R. Menendez
Other 2 dimensional materials
Chemistry of Graphene
Applications (gaz sensors, composites, nanoelectronic devices...)
POSTERS
Surname
1
Name
Name
Andres
Ayuela
DIPC and CFM CSICUPV/EHU
Country
Spain
Bacsa
LCC/CNRS
France
Jenny
Baker
Swansea University
United Kingdom
University of York
United Kingdom
Jurgis
Baldwin
Barkauskas
Vilnius University
Title Edge states and flat bands in graphene nanoribbons with arbitrary geometries Few layer graphene decorated with Pd nanoparticles: synthesis, characterisation and catalytic applications in the electrochemical oxidation of alcohols Development of a technique based on methylene blue for characterizing specific surface area of graphenes and other carbon nanostructures
Authors A. Ayuela, W. Jaskólski, M. Pelc, H. Santos, and L. Chico
Topic Magnetism and Spintronics
Bruno F. Machado, Revathi R. Bacsa, Julien Beausoleil, F. Vizza, C. Chemistry of Graphene Bianchini, Philippe Serp
J. Baker, D. Gethin
A generalised, tightbinding transport model Jack Baldwin and Y. description for random Hancock edge-defected ZGNRs
Chemistry of Graphene
Quantum transport
Graphene research in Lithuania with subsequent application Lithuania in bioanalysis, energy storage and optical materials
Jurgis Barkauskas, Aldona Beganskiene, Julija Razmiene, Gediminas Raciukaitis, Rasa Pauliukaite, ZĂŝŵŽŶĚĂ ĞůŝĞƓŝƵƚĞ
Growth, synthesis techniques and integration methods
Bernhard
Bayer
University of Cambridge
United Kingdom
In-situ Characterization of Graphene Growth
B. C. Bayer, R. S. Weatherup, P. R. Kidambi, S. Hofmann
Growth, synthesis techniques and integration methods
Simon
Bending
University of Bath
United Kingdom
Superconductivity in Two-dimensional Crystals
S. J. Bending, M. S. El Bana, David Hudson, Saverio Russo
Quantum transport
Austria
Large area Micro- and Nanostructuring of Graphene
Iris Bergmair, Wolfgang Hackl, Maria Losurdo, Maria Giangregorio, Giovanni Bruno, Christian Helgert, Thomas Pertsch, ErnstBernhard Kley, Thomas Mueller, Thomas Fromherz and Michael Muehlberger
Growth, synthesis techniques and integration methods
P. Bergonzo, J.A. Garrido, K.P. Loh
Applications (gaz sensors, composites, nanoelectronic devices...)
Sagar Bhandari, Wei L. Wang, Robert Westervelt, Efthimios Kaxiras
Iris
Philippe
POSTERS
Institution
Revathi
Jack
2
Surname
Sagar
Bergmair
PROFACTOR GmbH
Bergonzo
CEA LIST
France
Assembling graphene with diamond as novel platforms for biointerfacing and photovoltaics
Bhandari
Harvard University
United States
Direct Imaging of atomic scale ripples in few-layer graphene
Spectrocopies and microscopies
Abdul Manaf
Jan
Francesco
Alberto
Mohamed
Danil
Jan
Izaskun
Andrea
Marcel
Bianco
Bin Hashim
Institution
IMIP CNR
Universiti Teknologi Malaysia
Country
Italy
Malaysia
Title
Authors
Plasmonic gold nanoparticle deposition on pristine and functionalized graphene
Giuseppe V. Bianco, Maria M. Giangregorio, Maria Losurdo, Pio Capezzuto, Giovanni Bruno
Gate Control of Nonlinear Shaharin Fadzli Abd Characteristics in Rahman, Seiya Kasai, Chemically Doped Abdul Manaf Hashim Graphene Three-Branch Junction Device
Topic
Chemistry of Graphene
Applications (gaz sensors, composites, nanoelectronic devices...)
Poland
Interaction of epitaxial graphene with SiC substrate studied by Raman spectroscopy
K.Grodecki, J.A.Blaszczyk, A.Drabinska, W.Strupinski, A.Wysmolek, Z͘^ƚħƉŶŝĞǁƐŬŝ͕ and J. M. Baranowski
Spectrocopies and microscopies
United Kingdom
Exfoliation and Sorting of Graphite flakes and inorganic twodimensional materials
F. Bonaccorso, F. Torrisi, G. Privitera, V. Nicolosi, T. Hasan, G. Savini, N. Pugno, A.C. Ferrari
Growth, synthesis techniques and integration methods
Spain
Ambient p-doping of CVD graphene
Alberto Boscá, D. López-Romero, J. Martínez, J. A. Garrido, F. Calle.
Applications (gaz sensors, composites, nanoelectronic devices...)
Institut de Minéralogie et de physique des milieux condensés (IMPMC)
France
Raman scattering in single layer MoS2: Phonon Bandwidths, zone edge phonons and 2D effects
Mohamed Boukhicha, Karim Gacem, Mykhaylo Antal, Matteo Calandra, Abhay Shukla
Other 2 dimensional materials
Korea Institute of Bukhvalov Advanced Study (KIAS)
Korea
Catalytic properties of Danil Boukhvalov imperfect graphene: first principles modeling
Blaszczyk
Bonaccorso
Boscá
Boukhicha
Bundesmann
University of Warsaw
Cambridge University
ISOM, E.T.S.I de Telecomunicación, UPM
Universität Regensburg
Bustero
TECNALIA
Candini
Centro S3 Istituto Nanoscienze CNR
Ceccato
Aarhus University
Spin relaxation in Germany graphene induced by adatoms
Chemistry of Graphene
Jan Bundesmann, and Klaus Richter
Quantum transport
Spain
Improvement of thermal conductivity in graphene reinforced cyanate ester resin
I Bustero, G. Atxaga, S. Florez, I. Gaztelumendi, M. A. Mendizabal, B. Perez
Applications (gaz sensors, composites, nanoelectronic devices...)
Italy
Hybrid Graphene ʹ Molecular Magnet Devices for Spintronics
A. Candini, C. Alvino, S. Klyatskaya, M. Ruben, W. Wernsdorfer and M. Affronte
Magnetism and Spintronics
Insights into the chemical modification Denmark of graphene using diazonium salts
Marcel Cecato, Mikkel Kongsfelt, Louis Nilsson, Bjarke Jørgensen, Liv Hornekær, Steen U. Pedersen and Kim Daasbjerg
Chemistry of Graphene
POSTERS
Giuseppe Valerio
Surname
3
Name
Name
Yasemin
Anadolu University
Country
Title
Authors
Topic
Turkey
Preparation of Graphene Sheets from Expandable Graphite and Their Utilization in Ceramic-Matrix Composites
Yasemin Celik and Ender Suvaci
Growth, synthesis techniques and integration methods
Sweden
Field-Effect Sensor Based on Graphene Thin Films Fabricated by Layer-by-Layer Stacking
Si Chen, Patrik Ahlberg, Xin-Dong Gao, Zhi-Bin Zhang, Shi-Li Zhang, Wencai Ren, and Huiming Cheng
Applications (gaz sensors, composites, nanoelectronic devices...)
Won Jin Choi, CheolSoo Yang, Jeong-O Lee
Growth, synthesis techniques and integration methods
Chen
Won Jin
Choi
Korea Research Institute of Chemical Technology
Korea
Surface modification and patterning of graphene using PDMSinterface bonding
Mairbek
Chshiev
Spintec, CEA/CNRS/UJFGrenoble, INAC
France
Magnetic insulator H. X. Yang, D. Terrade, proximity induced spin- X. Waintal, S. Roche, polarization in graphene M. Chshiev
Magnetism and Spintronics
Poland
Contactless magnetoresistance in large area CVD graphene grown on SiC substrates
Tymoteusz Ciuk, Jerzy Krupka, Cezariusz Jastrzebski, Jaroslaw Judek, Wlodek Strupinski, Serkan Butun, Ekmel Ozbay, and Mariusz Zdrojek
Magnetism and Spintronics
Quantized Charge Pumping in Graphene
M. R. Connolly, K. L. Chiu, S. Giblin, M. Kataoka, J. Fletcher, J. Griffiths, G. A. C. Jones, C. G. Smith, J. T. Janssen
Quantum transport
Tymoteusz
Ciuk
Institute of Microelectronics and Optoelectronics
National Physical Laboratory
United Kingdom
Malcolm
Connolly
Iris
Crassee
Intrinsic terahertz Université de Switzerland magnetoplasmons in Genève monolayer graphene
I. Crassee and A. B. Kuzmenko
Spectrocopies and microscopies
Cruz
TEMA Nanotechnology Research Portugal Division University of Aveiro
Synthesis of graphenebased nanocomposites as SERS substrates in biodetection
Sandra M.A. Cruz, Helena I.S. Nogueira and Paula A.A.P. Marques
Spectrocopies and microscopies
Poland
Scanning tunnelling spectroscopy investigations of chemical composition of graphene/Cu(111) interface
P. Dabrowski, I. Wlasny, Z. Klusek, W. Kozlowski, I.Pasternak, W.Strupinski, J.Baranowski
Spectrocopies and microscopies
Belgium
Boron and nitrogen doping from first principles
Xavier Declerck, Andrés R. BotelloMéndez, Aurélien Lherbier, and JeanChristophe Charlier
Spectrocopies and microscopies
France
Effect of negative differential conductance in graphene Esaki diodes: GNR or GNM?
V. Hung Nguyen, F. Mazzamuto, J. SaintMartin, A. Bournel, P. Dollfus
Quantum transport
Sandra
Pawel
Xavier
POSTERS
Celik
Institution
Uppsala University
Si
4
Surname
Philippe
Dabrowski
Declerck
Dollfus
Institute of Electronic Materials Technology
Université Catholique de Louvain
CNRS - Univ. Paris-Sud
Institution
Country
Title
Authors
Downing
University of Exeter
United Kingdom
Zero-energy states in graphene waveguides, quantum dots and rings
C. A. Downing, R. R. Hartmann, N. J. Robinson, D. A. Stone and M. E. Portnoi
Quantum transport
S. D. Ganichev, C. Drexler, P. Olbrich, M. M. Glazov, S. A. Tarasenko, J. Karch, M. Fehrenbacher, D. Weiss, J. Eroms, R. Yakimova, S. LaraAvila, S. Kubatkin, E. L. Ivchenko
Spectrocopies and microscopies
A.V. Eletskii, I.M. Inskandarova, A.N.Knizhnik, D.N. Krasikov
Quantum transport
Terahertz Radiation Germany Induced Edge Currents in Graphene
Drexler
University of Regensburg
Alexander
Eletskii
National Research Center "Kurchatov Institute"
Russia
Amanda
Ellis
Flinders University
Australia
Donats
Erts
University of Latvia
Latvia
Christoph
Tkatschenko
Stijn
Takeshi
Krzysztof
Jianhua
Fedor
Fias
Fujii
Universit채t Regensburg
VUB
Fuji electric corporation
Gajewski
Wroclaw University of Technology
Gao
National Institute for Materials Science (NIMS)
Molecular Dynamic Simulation of the Thermal Conductivity of Graphene and Graphene Oxide
Graphene Oxide FlowerAmanda V. Ellis like Microstructures from Carbon Nanotubes
Topic
Chemistry of Graphene
Application of Ge Donats Erts, Jelena Nanoelectromechanical Kosmaca, J.D. Holmes, nanowire mass sensor systems for graphene exfoliation Jana Andzane
Superlattice Effects on Transport in Graphene Germany and Graphene Nanoribbons
Fedor Tkatschenko, Jan Bundesmann, Viktor Kr체ckl, Dmitry Ryndik, and Klaus Richter
Quantum transport
Belgium
Aromaticity patterns in graphene nanoribbons
Stijn Fias, Francisco J. Martin-Martinez, Gregory Van Lier, Frank De Proft and Paul Geerlings
Chemistry of Graphene
Japan
Cu(111) epitaxial films on mica(001) substrate used for high quality graphene growth by chemical vapor deposition
T. Fujii, M. Sato, A. Takigawa and Y. Ichikawa
Poland
SPM investigations of electrical properties of graphene nanostructures on 6HSiC substrate
Krzysztof Gajewski, Daniel Kopiec, Magdalena Moczala, Adam Piotrowicz, Michal Zielony, Grzegorz Wielgoszewski, Teodor Gotszalk, Wlodzimierz Strupinski
Japan
Epitaxial Growth of Single- and Few-layer JianhuaGao, Keisuke Sagisaka, Nobuyuki Graphene on Pt(111) and Pd(111) Surfaces by Ishida, Daisuke Fujita Surface Segregation
Growth, synthesis techniques and integration methods
Spectrocopies and microscopies
Growth, synthesis techniques and integration methods
POSTERS
Charles
Surname
5
Name
Name
Ainara
Louis
Zahra
Rossella
Maria del Carmen
POSTERS
GarciaGallastegui
Gaudreau
Gholamvand
Giardi
Gimenez Lopez
Institution
Imperial College London
ICFO-The Institute of Photonic Sciences
Dublin City University
IIT (Italian Institute of Technology)
University of Nottingham
Gil
University of Goncalves Aveiro
Jesper
Goor Pedersen
karin
6
Surname
Kacper
Goss
Grodecki
Aalborg University
Country
United Kingdom
Spain
Ireland
Italy
United Kingdom
Portugal
Title
Authors
Graphene Oxide supported Layered Double Hydroxides for CO2 capture applications
Ainara GarciaGallastegui, Diana Iruretagoyena, Mohamed Mokhtar, Abdullah Asiri, Sulaiman N. Basahel, Shaeel A. AlThabaiti, Abdulrahman O. Alyoubi, David Chadwick, Milo S. P. Shaffer
Applications (gaz sensors, composites, nanoelectronic devices...)
Hybrid graphenequantum dot phototransistors with ultrahigh gain
Louis Gaudreau, Gerasimos Konstantatos, Michela Badioli, Johann Osmond, Maria Bernechea, F. Pelayo Garcia de Arquer, Fabio Gatti , Frank H. L. Koppens
Applications (gaz sensors, composites, nanoelectronic devices...)
Zahra Gholamvand, , Kieran Nolan, John Tobin, Anne Morrissey
Applications (gaz sensors, composites, nanoelectronic devices...)
R. Giardi, S. Porro, A. Chiolerio, F. Sordo, M. Sangermano, E. Celasco, A. Chiodoni
Applications (gaz sensors, composites, nanoelectronic devices...)
Self-assembly of a sulphur-terminated graphene nanoribbon within a single-walled carbon nanotube.
M. C. Gimenez-Lopez, A. Chuvilin, E. Bichoutskaia, T. W. Chamberlain, G. A. Rance, N. Kuganathan, J. Biskupek, U. Kaiser, A.N. Khlobystov
Growth, synthesis techniques and integration methods
New bioactive PMMAHydroxyapatite based bone cement reinforced with graphene oxide
Gil Gonçalves, Sandra M.A. Cruz, José Grácio, Paula A.A.P Marques,Cecilia Ramírez-Santillán, María Vallet-Regí, María-Teresa Portolés
Applications (gaz sensors, composites, nanoelectronic devices...)
Jesper Goor Pedersen and Thomas Garm Pedersen
Magnetism and Spintronics
Lapo Bogani, C. Cervetti, A. Cornia, E. U. Stützel, S. Rauschenbach, F. Luis, M. Dressel, K. Kern, M. Burghard
Magnetism and Spintronics
K. Grodecki, R. Bozek, A. Wysmolek, R. Stepniewski, W. Strupinski
Spectrocopies and microscopies
Hydrothermal synthesis of TiO2 nanotube/Graphene oxide composite and its application in photocatalytic purification of water Simultaneous in-situ graphene oxide reduction and UV curing of acrylic based formulations for inkjet printing.
Magneto-Optical Denmark Properties of Antidot Lattices
Physikalisches Institut Single Molecule Germany Universität Magnets on Graphene Stuttgart Institute of Electronic Materials Technology
Poland
Graphene formation on SiC (0001) surface steps by CVD process
Topic
Güryel
Institution
Country
Title
Free University of Brussels Vrije Universiteit Brussel
Belgium
Influence of Structural Defects and Chemical Functionalisation on the Songül Güryel Mechanical Properties of Graphene
United Kingdom
Owen
Guy
Swansea University
James
Hague
The Open University
Balazs
Chang-Soo
Hajgato
Han
Vrije Universiteit Brussel
Graphene Biosensors
United Kingdom
Gap engineering in atomically thin materials
J.P.Hague
Quantum transport
Belgium
Balázs Hajgató, Songül Computation of Intrinsic Güryel, Jean-Marie Blarion, Hans E. Mechanical Properties Nanoelectromechanical Miltner, Frank De of Double Layer systems Proft, Paul Geerlings, Graphene Yves Dauphin, Gregory Van Lier
United Kingdom
Towards a realistic model of nanographene Jack Baldwin, Richard Taylor and Y. Hancock - linking theory and experiment Electronic properties and STM images of N and B doped graphene
Luc
Henrard
University of Namur (FUNDP)
Belgium
Lisa
Hesse
Universität Regensburg
Stefanie
Heydrich
Universität Regensburg
Chi
Hussein
Huynh
Chang-Soo Han, Changhyun Kim, Junghee Park
Korea
Hancock
Laith
Direct transfer of graphene without the removal of a metal substrate using a liquid polymer
Korea University
KAIST
University of Freiburg
Monash University
Nanoelectromechanical systems
Applications (gaz sensors, composites, nanoelectronic devices...)
Yvette
Hong
Topic
Z. Tehrani, O.J. Guy, G. Burwell, S. Teixeira, S. Doak
The University of York
Seul Ki
Authors
Chemistry of Graphene
Magnetism and Spintronics
L. Henrard, S.-O. Guillaume, B. Zheng, J.-C. Charlier
Spectrocopies and microscopies
Orbital Magnetism in Germany graphene bulk and nanostructures
Lisa Hesse, Juergen Wurm, Klaus Richter
Magnetism and Spintronics
Photoluminescence in Germany Graphene Antidot lattices
S. Heydrich, D. Hutzler, J. Eroms, D. Weiss, T. Korn, C. Schüller
Korea
Chemical Analysis and Thermal Curing Effects Seul Ki Hong of CVD graphene during Transfer Pocess
Decorated Carbon Nanostructured Germany Electrodes for Biofuel Cell applications
Australia
Structured Graphene ʹ Spinnable CNT and Beyond
Spectrocopies and microscopies
Chemistry of Graphene
L. Hussein, F. Olcaytug, and G. Urban
Applications (gaz sensors, composites, nanoelectronic devices...)
Chi P. Huynh , Stephen C. Hawkins, Mark Hickey, Amanda Barnard and Tim Williams
Growth, synthesis techniques and integration methods
POSTERS
Songül
Surname
7
Name
Name
Mari
Adelina
Country
Title
Authors
Ijäs
Aalto University School of Science
Finland
Chemical modification of graphene with Cl
Mari Ijäs, Paula Havu, Ari Harju
Chemistry of Graphene
Ilie
University of Bath
United Kingdom
Surface Potential Variations in Graphene Induced by Crystalline Ionic Substrates
Gavin J. Jones, Asieh Kazemi, Ying Wu, Simon Crampin, Adelina Ilie
Spectrocopies and microscopies
Isic
Institute of Physics
Serbia
Richard
Jackman
University College London
United Kingdom
Josep Miquel
Frederic
Bernd
POSTERS
Institution
Goran
Seung Yol
8
Surname
Kendra Fong Yu
Jeong
Jornet
Joucken
Kaestner
Kam
Goran Isic, Borislav Plasmonic resonances in Vasic, Aleksandar DĂƚŬŽǀŝĐ͕ hƌŽƓ the infrared spectra of Ralevic, Angela nanostructured Beltaos, Marko graphene ^ƉĂƐĞŶŽǀŝĐ͕ ZĂĚŽƓ Gajic Fang Zhao, Thuong Thoung Nguyen, Mo Diamond as a platform Golsharafi, Suguru for supporting graphene Amakubo, Glenn C. Tyrrell, KP Loh and Richard B. Jackman
Topic
Spectrocopies and microscopies
Growth, synthesis techniques and integration methods
Highly Concentrated and Conductive Reduced Graphene Oxide Nanosheets by Monovalent Cation-pi interaction: Toward Printed Electronics
Seung Yol Jeong, Sung Hun Kim, Joong Tark Han, Hee Jin Jeong, and Geon-Woong Lee
Chemistry of Graphene
Prospects of Grapheneenabled Wireless Communications
Ignacio Llatser, Sergi Abadal, Raúl Gómez Cid-Fuentes, Josep Miquel Jornet, Albert Cabellos-Aparicio, Eduard Alarcón, Josep Solé-Pareta and Ian F. Akyildiz
Applications (gaz sensors, composites, nanoelectronic devices...)
Localized state and charge transfer in nitrogen-doped epitaxial graphene
Frederic Joucken, Yann Tison, Jerome Lagoute, Jacques Dumont, Damien Cabosart, Bing Zheng, Vincent Repain, Cyril Chacon, Yann Girard, Andres Rafael Botello-Mendez, Sylvie Rousset, Robert Sporken, JeanChristophe Charlier, and Luc Henrard
Spectrocopies and microscopies
PTB
Nanoscale dual-gating Germany of bilayer graphene on GaAs substrates
A. Müller, B. Kaestner, M. Friedemann, M. Woszczyna, K. Pierz, F. J. Ahlers, H. W. Schumacher
Quantum transport
National University of Singapore
Influence of Graphite Defect Density on Oxidation Behavior and Singapore Pi-electron Topology of Sub-stoichiometric Graphene Oxides
Zhi-Li Chen, Li-Hong Zhao, Roland G-S. Goh, Fong-Yu Kam, Jie Song, Chemistry of Graphene Wang-Zhi Chua, GeokKieng Lim and Lay-Lay Chua
Korea Electrotechnology Research Institute
NaNoNetworking Center in Catalunya (N3Cat)
University of Namur (FUNDP)
Korea
Spain
Belgium
Emine
Mikkel
Sven
Selma
Bijandra
Feodor
John
Istvan
Kang
Institution
Jeonju University
Kayhan
Technical University of Darmstadt
Klarskov
DTU Nanotech
Kochmann
Institute of Analytical Chemistry
Koghee
Kumar
Kusmartsev
Landers
Laszlo
University of Antwerp
INAC/CEA Grenoble
Loughborough University
Rutgers, The State University of New Jersey
Country
Title
Authors
Quantum Transport through Heterobilayers of Graphene Nanoribbon and Porphyrin Tape
Hong Seok Kang
Quantum transport
Emine Kayhan, Joerg J. Schneider
Applications (gaz sensors, composites, nanoelectronic devices...)
Mikkel B Klarskov, Timothy J Booth, Dirch H Petersen, Peter Bøggild
Applications (gaz sensors, composites, nanoelectronic devices...)
The Fluorescence Germany Properties of Graphene Oxide
Sven Kochmann, Alexander Zöpfl, Thomas Hirsch, Otto S. Wolfbeis
Spectrocopies and microscopies
Belgium
Merging and alignment of Dirac points in a shaken honeycomb optical lattice
Selma Koghee, Lih-King Lim, M.O. Goerbig, and C. Morais Smith
Other 2 dimensional materials
France
Epitaxial Graphene on Si face of SiC: A Comparative Study of Different Growth Conditions
B. Kumar, F. Duclairoir, L. Dubois, D. Rouchon, M. Paillet, J.-R. Hutzinger, A. Tiberj, A. Zahab, G. Lapertot, J.L. Sauvajol, G. Bidan, P. Maldivi
Growth, synthesis techniques and integration methods
United Kingdom
^ƚĂďůĞ ͞&ůĂƚ͍ &Žƌŵ ŽĨ Two-Dimensional Crystals: Could Graphene, Silicene, Germanene Be Minigap Semiconductors and Have Huge Magnetoresistance??
F. V. Kusmartsev, A. K͛,ĂƌĞ <͘ /͘ <ƵŐĞů͕ d͘ ,͘ Hewett, M. B. Gaifullin, F. A. Mamari, and O. E. Kusmartseva
Spectrocopies and microscopies
Investigation of Alumina/Graphene Oxide role in catalysis
John Landers, Ankush Biradar, Remie Yu, Daniel Mastrogiovanni, Eric Garfunkel, Chemistry of Graphene Tewodros Asefa, Arthur W. Chester, Alexander V. Neimark
Molecular dynamics simulation of carbon nanostructures
István László , Ibolya Zsoldos
Korea
Synthesis, Characterization and Gas Sensing Behaviour of Large Area Germany Continuous and Transparent Graphene Films by Chemical Vapor Deposition Micro four-point probe characterization of Denmark nanostructured graphene
United States
Budapest University of Technology and Economics, Hungary Department of Theoretical Physics
Topic
Growth, synthesis techniques and integration methods
POSTERS
Hong Seok
Surname
9
Name
Name
Bo W.
Petr
Lazar
University of Copenhagen
Palacky University
Country
Title
Graphene Oxide as a Mono Atomic Protection Layer for Molecular Denmark Electronics: A Quantative Structural Study
Authors
Søren Petersen, Magni Glyvradal, Robert &ĞŝĚĞŶŚĂŶƐ͛ů͕ ĂŶĚ Ž W. Laursen
Topic Applications (gaz sensors, composites, nanoelectronic devices...)
Czech Republic
Nature of Interaction of Graphene with Ag, Au, Pd Metals
Michal Otyepka, Jaroslav Granatier, Petr Lazar, Pavel Hobza
Belgium
Chemically Tunable Transport Phenomena of Functionalized Graphene
Nicolas Leconte, A. Lherbier, F. Varchon, P. Ordejon, D. Soriano, J.J. Palacios, S. Roche and J.-C. Charlier
Quantum transport
Gun-Do Lee, Euijoon Yoon, Cai-Zhuang Wang, Kai-Ming Ho
Growth, synthesis techniques and integration methods
Jae-Ung Lee, Duhee Yoon and Hyeonsik Cheong
Spectrocopies and microscopies
Magnetism and Spintronics
Chemistry of Graphene
UCL/IMCN/ NAPS
Gun-Do
Lee
Seoul National University
Korea
Jae-Ung
Lee
Sogang University
Korea
Lenz
Freiburg institute for advanced studies
Graphene with a spinGermany orbit super-lattice potential
Lucia Lenz and Dario Bercioux
Lenzer
University of Siegen
Ultrafast transient absorption and RamanGermany imaging studies of stacked graphene
Thomas Lenzer, Kawon Oum, Rainer Bornemann and Peter Haring Bolivar
Spectrocopies and microscopies
Li
University of Copenhagen
Solution-Processed Ultrathin Chemically Derived Graphene Films Denmark as Soft Top Contacts for Solid-State Molecular Electronic Junctions
Tao Li, Jonas Rahlf Hauptmann, Zhongming Wei, Søren Petersen, Thomas Bjørholm, Kasper Nørgaard, Bo Wegge Laursen
Applications (gaz sensors, composites, nanoelectronic devices...)
Qiang
Li
Aarhus University
Niclas
Lindvall
Lucia
Thomas
Tao
Ludwika
POSTERS
Laursen
Institution
Leconte
Nicolas
10
Surname
Andrzej
Chalmers University of Technology
Lipinska
Institute of Electronic Materials Technology
Lissowski
Society of the Polish Free University
Atomistic Processes of Grain Boundary Motion and Annihilation in Graphene ƐƚŝŵĂƚŝŽŶ ŽĨ zŽƵŶŐ͛Ɛ Modulus by Raman Spectroscopy on Biaxially Strained Graphene
Suspended Graphene Based Devices and Denmark Nanomechanical Properties Towards transfer-free Sweden fabrication of graphene NEMS
Qiang Li, Ying Fang, Mingdong Dong
Nanoelectromechanical systems
Niclas Lindvall, Jie Sun, Nanoelectromechanical Galib Abdul, and systems August Yurgens
Poland
Fabrication of graphene flakes via oxidationreduction method
L. Lipinska, J. Jagiello, M. Zdrojek, E. Talik, M. Andrzejczuk, M. Lewandowska, M. Chemistry of Graphene Mozdzonek, A. Aksienionek, K. Kielbasinski, E. Brzozowski, A. Strojny
Poland
Modelling graphene growth by atomistic simulation of 2D polycrystal crystallization - video
Andrzej Lissowski
Growth, synthesis techniques and integration methods
Tomas
J. Marcelo
Liu
Lofwander
Lopes
Institution Institut für Theoretische Physik, Universität Regensburg
Country
Minimal Tight-Binding Model for Quantum Germany Transport in Graphene Heterojunctions
Quantum transport
Paul-DrudeInstitute
Synthesis of nanocrystalline Germany graphene on Al2O3(0001) by molecular beam epitaxy
M. H. Oliveira Jr., T. Schumann, M. Ramsteiner, R. Hey, L. Geelhaar, J. M. J. Lopes, and H. Riechert
Growth, synthesis techniques and integration methods
Belgium
Mathieu
LPN CNRS
France
Aleksandar
Matkovic
Institute of Physics, University of Belgrade
Serbia
Jacek
Mazur
Institute of Non-Ferrous Metals
Poland
Wolfgang
Francesco
Mehr
Mercuri
Quantum transport
Sweden
VUB
Claire
Ming-Hao Liu, Jan Bundesmann, Klaus Richter
T. Lofwander, A. Bergvall, K. Berland, P. Hyldgaard, and S. Kubatkin
MartinMartinez
Francisco
Topic
Graphene nanogap for gate-tunable quantumcoherent singlemolecule electronics
Spain
LopezPolin
Authors
Chalmers University of Technology
Universidad Autonoma de Madrid
Guillermo
Title
IHP
CNR-ISMN
Guillermo López-Polín, Cristina GómezSpectrocopies and Navarro*, Francisco J. Guzmán-Vázquez, Julio microscopies Gómez- Herrero, Juan J. Saenz and Sacha Francisco J. MartinEdge functionalization Martinez, Stijn Fias, of graphene Gregory Van Lier, Chemistry of Graphene nanoribbons for Frank De Proft and electronic applications Paul Geerlings C. Mathieu, B. Lalmi, T. Effect of Oxygen O. Mentes, E. Spectrocopies and Adsorption on the Local Pallecchi, A. Locatelli, microscopies Properties of Epitaxial S. Latil, R. Belkhou and Graphene on SiC A. Ouerghi A. Matkovic, U. Ralevic, A. Beltaos, M. Spectroscopic M. Jakovljevic, G. Isic, Spectrocopies and ellipsometry B. Vasic, Ð Jovanovic, microscopies measurements of doped ͘ >ĂnjŝĐ͕ D͘ ^ŵŝůũĂŶŝĐ͕ graphene D. Vasiljevic-Radovic and R. Gajic Jacek Mazur, Adriana Application of Applications (gaz multilayer graphene for Wrona, Barbara sensors, composites, Juszczyk, Joanna modification of the Kulasa, Szymon nanoelectronic properties of composite Malara, Marian devices...) materials Czepelak
Measurement of reduced graphene oxide conductivity using Electrostatic Force Microscopy
Complementary hot carrier transistor with Germany vertical graphene base electrode for THz applications
Italy
Atomistic control of the properties of nanographenes and design of devices: insights from simulations
W. Mehr, J. Dabrowski, Ch. Scheytt, G. Lippert, Y.-H. Xie, M.C. Lemme, S. Vaziri, G. Lupina
Applications (gaz sensors, composites, nanoelectronic devices...)
Francesco Mercuri, Matteo Baldoni, Daniele Selli, Antonio Sgamellotti
Quantum transport
POSTERS
Ming-Hao
Surname
11
Name
Name
Adrien
Somayeh
Mohamadi
CRHEA-CNRS
University of Siegen
Moraes
State Universtity of Campinas
Eden
MoralesNarváez
Catalan Institute of Nanotechnology
Munarriz
Facultad de Ciencias Fisicas, Universidad Complutense de Madrid
Kazuo
Ashok
Mehdi
Christian
Zhenhua
POSTERS
Michon
Institution
Ana
Javier
12
Surname
Robin
Muramatsu
Incubation Alliance,Inc.
Nanjundan CEA Grenoble
NeekAmal
Teacher Training University
Neuen
Fraunhofer Institute for Algorithms and Scientific Computing (Fraunhofer SCAI)
Ni
Nicholas
Southeast University
Physics Dept, Oxford University
Country
France
Title
Authors
A. Michon, S. Vézian, D. Lefebvre, A. Tiberj, Structure and interface J.-R. Huntzinger, J. of graphene films grown Camassel, F. Cheynis, F. Leroy, P. Müller, L. on SiC using propaneLargeau, O. Mauguin, hydrogen-argon CVD T. Chassagne, M. Zielinski and M. Portail
Surface Modification of Graphene Thorough Germany Controlled Radical and Conventional Free Radical Polymerization Removal of oxidation debris from graphene oxide: influence on the Brazil formation of composites based on silver nanoparticles
Somayeh Mohamadi, Naser Sharafi-Sanjani, and Holger Schönherr Ana C. M. Moraes, Andréia F. de Faria, Diego Stéfani T. Martinez, Amauri J. Paula, Oswaldo L. Alves
Topic
Growth, synthesis techniques and integration methods
Applications (gaz sensors, composites, nanoelectronic devices...)
Chemistry of Graphene
Spain
Optical biosensors based on graphene
Eden Morales Narváez, Briza Pérez-López, Arben Merkoçi
Applications (gaz sensors, composites, nanoelectronic devices...)
Spain
Spin-dependent negative differential resistance in graphene superlattices
J. Munárriz, C. Gaul, F. Domínguez-Adame, P. Orellana, C. Mueller, A. V. Malyshev
Quantum transport
Kazuo Muramatsu , Kouichi Sutani , Masahiro Toyoda
Growth, synthesis techniques and integration methods
Japan
France
Iran
Development and Study of manufacturing method of few layers graphene dispersed solution for wet coating Facile synthesis of high quality metal free reduced graphene nanosheets from expandable graphite oxide
Effect of grain boundary M. Neek-Amal and F. on the buckling of M. Peeters graphene nanoribbons
Effects of nanostructures on Germany macroscopic physical properties of graphene layers China
United Kingdom
Nanjundan Ashok Kumar, Lionel Dubois, Serge Gambarelli, Chemistry of Graphene Florence Duclairoir and Gérard Bidan
Nanoelectromechanical systems
Christian Neuen, Jan Hamaekers
Chemistry of Graphene
Surface enhanced Raman scattering of graphene
Zhenhua Ni, Yingying Wang, Zexiang Shen
Spectrocopies and microscopies
Energy loss rates of hot Dirac fermions in epitaxial, exfoliated and CVD graphene under high magnetic fields
R.J. Nicholas, A.M.R. Baker, J.A. AlexanderWebber, T. Altebaeumer, D. McMullan, Cheng-Te Lin and Lain-Jong Li
Quantum transport
Name
Surname
Alain
Nogaret
Laszlo
Oroszlany
Silvio
Institution
Country
Title
Authors
University of Bath
United Kingdom
Tunneling Negative Differential Resistance in Flexible Silicone/Graphite Composites
A. Nogaret, S. Littlejohn, S. Crampin
Eötvös University Budapest
Hungary
Intraband electron László Oroszlány, focusing by a flat lens in Csaba Péterfalvi, József Cserti, Colin Lambert bilayer graphene
Quantum transport
Belgium
Graphene nanoribbons as low-bandgap donor Silvio Osella, David materials for organic Beljonne photovoltaic: Quantumchemical aided design
Spectrocopies and microscopies
Formation of Graphene M. F. Proença, R. F. Nanoribbons in Solution Araújo, F. Costa, A. J.
Osella
Chemistry of Novel Material
Paiva
Universidade do Minho
Portugal
Parvez
Max Planck Institute for Polymer Research
Nitrogen-doped Graphene and its Ironbased composite as Germany Efficient Electrocatalysts for Oxygen Reduction Reaction
Pasternak
Institute of Electronic Materials Technology
Graphene growth on Cu mono- and polycrystalline substrates
Topic Applications (gaz sensors, composites, nanoelectronic devices...)
M. C. Paiva, E. Cunha,
Maria
Chemistry of Graphene
Fernandes, M. A. Ferro
Maria
Soren
Peressi
University of Trieste, Physics Dep.
Petersen
Nano-Science Center, Department of Chemistry, University of Copenhagen
Carlo Antonio Pignedoli
Tiziana
Jana
Empa Materials Science and Technology
Poland
Italy
I. Pasternak, K. Grodecki, P. Dabrowski, I. Wlasny, Z. Klusek, W. Strupinski
Growth, synthesis techniques and integration methods
Hydroxyl Functional Groups on Pristine, Defected Graphene, and Maria Peressi, Nahid Ghaderi Graphene Epoxide: insights from first principles calculations
High Conductance, Large Area, Single Layer Denmark Graphenes from Graphene Oxide
Søren Petersen, Yudong He, Jiang Lang, Chemistry of Graphene Wenping Hu and Bo W. Laursen
C. A. Pignedoli, S. Blankenburg, J. Cai, P. Ruffieux, R. Jaafar, D. Passerone, X. Feng, K. Müllen, R. Fasel T. Polichetti, L. Lancellotti, E. Massera, M. L. Miglietta, F. Ricciardella, S. Graphene: different Romano, O. Tari, S. fabrication technologies Gnanapragasam and for solid state devices G. Di Francia R. Giorgi, T. Dikonimos, N. Lisi, E. Salernitano, S. Gagliardi, M. Falconieri
Graphene Nanoribbon Switzerlan Heterojunctions via d partial cyclodehydrogenation
Polichetti
ENEA
Italy
Poltierova Vejpravova
Institute of Physics AS CR, v.v.i.
Czech Republic
Magnetic and transport properties of graphene@MNPs hybrides
Chemistry of Graphene
J.P. Vejpravova, B. Bittova, M. Kalbac, J. Vlcek, J. Prokleska, K. Carva, A. Repko
Growth, synthesis techniques and integration methods
Applications (gaz sensors, composites, nanoelectronic devices...)
Magnetism and Spintronics
POSTERS
Iwona
Applications (gaz sensors, composites, nanoelectronic devices...)
13
Md Khaled
Md Khaled Parvez, Shubin Yang, Yenny Hernandez, Xinliang Feng, Klaus Müllen
Name
Andrei
Andrei
Stefano
Deborah
Pieter
Popov
Prezioso
Prezzi
Probaeys
Institute for Spectroscopy Russian Academy of Science Institute for Spectroscopy Russian Academy of Science Università dell´Aquila
CNR Nanoscience Insitute
Country
Title
Authors
Russia
Chiral graphene nanoribbon inside carbon nanotube: ab initio study
Andrei M. Popov, Irina V. Lebedeva, Andrey A. Knizhnik, Andrei N. Chemistry of Graphene Khlobystov, Boris V. Potapkin
Russia
Molecular dynamics simulation of selfretracting motion of graphene flakes
Andrei M. Popov, Irina V. Lebedeva, Andrey A. Nanoelectromechanical Knizhnik, Yurii E. systems Lozovik and Boris V. Potapkin
Italy
Reduced Graphene Oxide: fundamentals and applications.
Italy
Structure, Stability and Electronic Properties of Graphene Edges on Co(0001)
IMOMEC/IMEC & University Belgium Hasselt
Rabot
CEA
France
Julien
Rioux
University of Konstanz
Germany
Spectrocopies and microscopies
Growth, synthesis techniques and integration methods
Modularity of CMOScompatible synthesis of graphene by segregation methods
Caroline Rabot, Alexandru Delamoreanu, Aziz Zenasni, Patrice Gergaud
Growth, synthesis techniques and integration methods
Optical spin current injection in graphene
Julien Rioux, J.E. Sipe, and Guido Burkard
Mechanical stabilization of graphene aerogels by vulcanization with pure sulphur
Valentina Romeo1, Gianfranco Chemistry of Graphene Carotenuto1, Giovanni Marletta2
Molecular graphene of shungite
Natalia Rozhkova and Elena Sheka
Italy
Natalia
Rozhkova
Institute of Geology Karelian Research Cbtre RAS
Russia
Alexander
Rudenko
Hamburg University of Technology
Adsorption of cobalt on Germany graphene: a quantum chemical perspective
CIDETEC-IK4
Graphene produced by electrochemical exfoliation of graphite: electroanalytical properties
Spain
Spectrocopies and microscopies
DŝůŽƓ EĞƐůĄĚĞŬ͕ Andrew Taylor, &ƌĂŶƚŝƓĞŬ &ĞŶĚƌLJĐŚ͕ Otakar Frank, Martin <ĂůďĄē ĂŶĚ >ĂĚŝƐůĂǀ Kavan
IMCB-CNR
Ruiz
S. Prezioso, F. Perrozzi, M. Donarelli, F. Bisti, S. Santucci, L. Palladino, M. Nardone, E. Treossi, V. Palermo, and L. Ottaviano Deborah Prezzi, Daejin Eom, Kuang T. Rim, Hui Zhou, Michael Lefenfeld, Colin Nuckolls, Tony F. Heinz, George W. Flynn, and Mark S. Hybertsen
Topic
Low Temperature Graphene Growth Using Large Area LinearAntenna Microwave Plasma Enhanced CVD System
Romeo
Virginia
POSTERS
Popov
Institution
Caroline
Valentina
14
Surname
Magnetism and Spintronics
Chemistry of Graphene
A.N. Rudenko, F.J. Keil, M.I. Katsnelson, A.I. Chemistry of Graphene Lichtenstein Virginia Ruiz, Aintzane Ochoa, Pedro M. Carrasco, Ibon Odriozola, Germán Cabañero
Growth, synthesis techniques and integration methods
Name
Surname
Burkhard
Sachs
University of Hamburg
Theory of grapheneGermany boron nitride heterostructures
Samuels
University of Surrey
United Kingdom
Country
Eric Vinod
Sandana
Ecole Polytechnique
France
Adarsh
Sandhu
Toyohashi University of Technology
Japan
Manav
Peter
Andrea
Santos
Saxena
Indian Institute of Technology Kanpur
Schellenberg
Schlierf
University of Minho
ISOF Istituto Sintesi Organica e Fotoreattività
Shylau
Linkoping University
Jose Angel
SilvaGuillén
Centre d´Investigació en Nanociència i Nanotecnologia (CIN2)
Olga
M.V.Lomonosov Sinitsyna Moscow State University
Artsem
Organic and Metalloorganic Doping of Graphene
Authors B. Sachs, T. Wehling, M. Katsnelson, A. Lichtenstein
Alexander Samuels and J David Carey
Co Chang Seok Lee, Costel Sorin Cojocaru, Synthesis of conducting Waleed Moujahid, Bérengère Lebental, transparent few-layer Marc Chaigneau, Marc graphene directly on Châtelet, François Le Glass at 450°C Normand and Jean-Luc Maurice, Eric Vinod Sandana Ecofriendly Reduction of Y. Tanizawa, Sreejith Raveendran, T. Graphene Oxide Using Maekawa, D. Sakthi Extremophile Bacteria Kumar, A. Sandhu
Belgium
Reflectance of pristine and N-doped epitaxial graphene from THz to mid-IR
Cristiane N. Santos, Frédéric Joucken, Domingos de Sousa Meneses, Patrick Echegut, Jessica Campos-Delgado, Jean-Pierre Raskin, Robert Sporken, Benoît Hackens
India
Graphene: In Our Food Stuffs since Mesolithic Age
Manav Saxena, Sabyasachi Sarkar
Portugal
Efficient graphene preparation by combined intercalation ʹ exfoliation steps
Peter Schellenberg, César Bernardo, Hugo Gonçalves, Michael Belsley, José Alberto Martins, Cacilda Moura, Tobias Stauber
Topic
Chemistry of Graphene Applications (gaz sensors, composites, nanoelectronic devices...)
Growth, synthesis techniques and integration methods
Growth, synthesis techniques and integration methods
Spectrocopies and microscopies
Chemistry of Graphene
Growth, synthesis techniques and integration methods
Andrea SCHLIERF,
Italy
The interaction of Emanuele TREOSSI, pyrene derivatives with Huafeng YANG, Cinzia graphene nanoplatelets CASIRAGHI, Vincenzo
Chemistry of Graphene
PALERMO
Interacting electrons and spin-splitting in Artsem Shylau, Anton graphene and graphene Volkov, Igor Zozoulenko nanoribbons in the quantum Hall regime
Quantum transport
Spain
Electronic Transport Between Platinum Contacts Through Graphene/Nanotubes Structures
J.A. Silva-Guillén, F. D. Novaes, R. Rurali, P. Ordejón
Quantum transport
Russia
New promising pyrolytic graphite for micromechanical exfoliation of graphene
O.V. Sinitsyna, E.A. Khestanova, A.A. Antonov, I.G. Grigorieva, I.V. Yaminsky
Spectrocopies and microscopies
Sweden
POSTERS
Cristiane
Université catholique de Louvain (UCL)
Title
15
Alexander
Institution
Name
Jozef
Jangyup
Mo
Giorgio
Son
Song
CMT group, Departement Fysica, Universiteit Antwerpen Materials Science and Engineering, Yonsei University Loughboroug h University
Country
Title
Authors
Belgium
First-principles investigation of titanium and titanium dioxide adsorption on graphene
Jozef Sivek, O. Leenaerts, B. Partoens, and F. M. Peeters
Other 2 dimensional materials
Korea
In-situ Raman study on CVD-grown graphene microbridge under high current density
Jangyup Son, Minkyung Choi, Sangjin Kim, Sangho Lee, Sukang Bae, Byunghee Hong, InSang Yang, and Jongill
Spectrocopies and microscopies
Fabrication and Applications of Graphene in Loughborough University
Mo Song, Jie Jin, Dongyu Cai, Yue Lin, Xiao Wang and Rehman Rafiq
United Kingdom
Synthesis and Characterization of graphene oxyfluoride
Applications (gaz sensors, composites, nanoelectronic devices...)
Spectrocopies and microscopies
Spitalsky
Polymer Institute, Slovak Academy of Sciences
Slovakia
New Electric Conductive Polymeric Nanocomposites Based on Graphene
Strobl
CVD Equipment Corporation
United States
ϯ s 'ƌĂƉŚĞŶĞΡ Material Production Scale-up Process using Ni powders
Karlheinz Strobl, Mathieu Monville, Subarna Banerjee, Shish-sheng Chang
Growth, synthesis techniques and integration methods
Strupinski
Institute of Electronic Materials Technology
Poland
Graphene Epitaxy by Chemical Vapor Deposition on SiC
W.Strupinski1, K.Grodecki, R.Bozek, A.Wysmolek, R.Stepniewski and J.M.Baranowski
Growth, synthesis techniques and integration methods
Onejae
Sul
Korea Advanced Institute of Science and Technology
Korea
Controlling the Chirality ŽĨ 'ƌĂƉŚĞŶĞ͛Ɛ ĞĚŐĞƐ using Polarization Selective Laser Annealing
Onejae Sul, Milan Begliarbekov, Ken-Ichi Sasaki, Eui-Hyeok Yang, Stephan Strauf
Spectrocopies and microscopies
Nikodem
Szpak
Nikodem Szpak
Quantum transport
J.-M. Themlin, F.C. Bocquet, R. Bisson, J.M. Layet, and T. Angot2
Spectrocopies and microscopies
Cornelius Thiele, Alexandre Felten, Cinzia Casiraghi, Hilbert v. Löhneysen and Ralph Krupke
Growth, synthesis techniques and integration methods
Wlodzimierz
Jean-Marc
Cornelius
University DuisburgEssen
Themlin
IM2NP UMR CNRS 7334
Thiele
Karlsruhe Institute of Technology
Italy
Giorgio Speranza, Stefano Borini, Matteo Bruna, Barbara Massessi, Cristina Cassiago, Alfio Battiato, Ettore Vittone, Luigi Crema Zdeno Spitalsky, Marketa Ilcikova, Jan Kratochvila, Igor Krupa, Manuel Pedro Graca, Luis Cadillon Costa
Topic
FBK
Karlheinz
POSTERS
Sivek
Institution
Speranza
Zdeno
16
Surname
A sheet of graphene ʹ Germany quantum field in a discrete curved space
France
Reversible Formation and Hydrogenation of Deuterium-Intercalated Quasi-Free-Standing Graphene on 6HSiC(0001)
Electron-beam-induced Germany direct etching of Graphene
Applications (gaz sensors, composites, nanoelectronic devices...)
Pierre
Gerald V.
Romualdas
Andrey
Daniela
Toke
Title
Authors
Budapest University of Technology and Economics
Hungary
Excitations of bilayer graphene in the quantum Hall regime
Csaba Toke, Vladimir I. Fal'ko, Judit Sári
Experimental study of nucleation and growth mechanisms of graphene synthesized by Low Pressure Chemical Vapor Deposition on copper foil
Pierre Trinsoutrot , Caroline Rabot , Hugues Vergnes , Alexandru Delamoreanu, Aziz Zenasni, Brigitte Caussat
Growth, synthesis techniques and integration methods
Spectrocopies and microscopies
Magnetism and Spintronics
Troppenz
HelmholtzZentrum Berlin für Materialien und Energie GmbH
Strain analysis of CVD Germany graphene by in situ Raman spectroscopy
Gerald V. Troppenz, Marc A. Gluba, Jörg Rappich, Norbert H. Nickel
Trusovas
Center for physical sciences and technology
Graphite Oxide Reduction to Graphene Lithuania Applying Ultrashort Laser Pulses
Romualdas Trusovas, Gediminas Raciukaitis, Jurgis Barkauskas, Regina DĂǎĞŝŬŝĞŶĞ
Growth, synthesis techniques and integration methods
University of Bielefeld
A molecular route to 1 nm thick carbon nanomembranes Germany (CNMs) and graphene for functional applications
Andrey Turchanin and Armin Gölzhäuser
Growth, synthesis techniques and integration methods
MPI for Solid State Research
Investigation of excitonic Fano Germany resonances in graphene using optical spectroscopy
Daniela Ullrich, Patrick Herlinger, Dong-Hun Chae, Tobias Utikal, Harald Giessen, Jurgen Smet, and Markus Lippitz
Spectrocopies and microscopies
Spain
Effect of Topological Disorder and Spin-Orbit Coupling in the Transport Properties of Graphene
D. V. Tuan, D. Soriano, F. Ortmann, A. Kumar, M. F. Thorpe and S. Roche
Quantum transport
Paul Sabatier University
France
On the Interaction of Beryllium atoms with Graphene Nanostructures
V.G.Chilkuri, T. Leininger,S. Evangelisti,A.Monari
Walters
Haydale Ltd
United Kingdom
Wang
Shanghai Institue of Optics and Fine Mechanics
Turchanin
Ullrich
Chilkuri
Vijay Gopal
ICN
France
Topic
LGC
Van Tuan
Kangpeng
Country
Trinsoutrot
Dinh
Ian
Institution
China
Plasma production and functionalisation of Ian Walters 'ƌĂƉŚĞŶĞ ĂŶĚ 'EW͛Ɛ ďLJ plasma exfoliation
Ultrafast Nonlinear Optical Responses of 2D MoS2 Nanosheets
Kangpeng Wang, Jintai Fan, Long Zhang, Mustafa Lotya, Danny Fox, Hongzhou Zhang, Werner J. Blau, Jonathan N. Coleman, Jun Wang
Chemistry of Graphene
Applications (gaz sensors, composites, nanoelectronic devices...)
Other 2 dimensional materials
POSTERS
Csaba
Surname
17
Name
Name
Rune
John
Neil
Andrzej
Igor
Malgorzata
Zhong-Shuai
18
POSTERS
Weiping
Surname
Wendelbo
Williams
Wilson
Witowski
Wlasny
Institution
Abalonyx AS
University of Bristol
University of Warwick
Faculty of Physics, University of Warsaw
University of Lodz
Westpomeranian University of Wojtoniszak Technology in Szczecin
Wu
Max-Planck Institute for Polymer Research
Wu
Electrical Engineering Division, Engineering Department, University of Cambridge
Country
Title
Authors
Norway
A Robust, Scalable Process for Automated Production of Highly Dispersed Graphene Oxide and its use in transparent conductive coatings
Rune Wendelbo and Nils Berner
United Kingdom
Development of NanoComposites which John Williams, Sameer Rahatekar, Martin include Plasma Chemistry of Graphene Williams Functionalized Graphene Nanoplatelets
United Kingdom
The real graphene oxide revealed: stripping the oxidative debris from the graphene-like sheets
Neil R. Wilson, Priyanka A. Pandey, Joseph J. Lea, Matthew Chemistry of Graphene Bates, Ian A. Kinloch, Robert J. Young, and Jonathan P. Rourke
Poland
Characterisation of multilayer grapheene obtained by SiC sublimation on C surface by far infrared magnetospectroscopy and Raman spectroscopy
A.M. Witowski, R. Stepniewski, A. Wysmolek, A. Drabinska, J. Baranowski, K. Grodecki, W. Strupinski, M. Orlita, M. Potemski
Spectrocopies and microscopies
Poland
Scanning Tunneling Spectroscopy (STS) studies of Graphene-Au interactions
I. Wlasny, P. Dabrowski, , Z. Klusek, J. Slawinska, I. Zasada, W. Kozlowski, M. Wojtoniszak, E. Borowiak-Palen
Spectrocopies and microscopies
Poland
Novel method of graphite exfoliation towards synthesis of graphene
Malgorzata Wojtoniszak, Ewa Mijowska
Growth, synthesis techniques and integration methods
Zhong-Shuai Wu, Yi Sun, Long Chen, Khaled Parvez, Xinliang Feng,*and Klaus M端llen
Applications (gaz sensors, composites, nanoelectronic devices...)
United Kingdom
Gold decorated graphene flakes for large area transparent conductive films
W. Wu; T. Hasan; F. Torrisi; M. Zelazny; G. Privitera; F. Bonaccorso; A.C. Ferrari
Chemistry of Graphene
Zhenyuan Xia, Emanuele Treossi, Vincenzo Palermo
Chemistry of Graphene
Y. E. Yaish, E. M. Hajaj, O. Shtempluk, A. Razin, and V. Kochetkov
Nitrogen Doped Graphene-Supported Germany Fe3O4 Nanoparticles for Efficient Oxygen Reduction Reaction
Zhenyuan
Xia
ISOF-CNR
Italy
Nanoscale Comparison of graphite exfoliation by supramolecular, chemical and electrochemical methods
Yuval
Yaish
Technion
Israel
Chemical Potential of Inhomogeneous Single Layer of Graphene
Topic
Chemistry of Graphene
Quantum transport
Institution
Country
Title
Authors
Topic
China
Reaction Mechanisms of Chemical Reduction of Graphene Oxide by Yan Su, Ji Jun Zhao Sulfur-Containing Compounds膇再 DFT Study
ETRI
Korea
High Power Light Emitting Diode Operation with Graphene Transparent Electrode
Doo-Hyeb Youn, HongKyw Choi, Jae-Hyung Han, Young-Jun Yu, Seung-Hwan Kim, Sung-Ran Jeon, SungYool Choi, and ChoonGi Choi
Applications (gaz sensors, composites, nanoelectronic devices...)
Yu
Massachusetts Institute of Technology
United States
Current Saturation in Few-layer MoS2 FET
Lili Yu, Allen Hsu, Han Wang, Yumeng Shi, Jing Kong, Tomas Palacios
Other 2 dimensional materials
Recep
Zan
The University of Manchester
United Kingdom
Interaction of Metals with suspended Graphene observed by Transmission Electron Microscopy
Recep Zan, Ursel Bangert, Quentin Ramasse, Konstantin S Novoselov
Spectrocopies and microscopies
Zeila
Zanolli
Universite de Liege, Institut de Physique
Belgium
Quantum spin transport Zeila Zanolli, Giovanni Onida, Jean-in carbon chains with graphene-like contacts. Christophe Charlier
Quantum transport
Zarenia
University of Antwqerp
Belgium
Energy levels of quantum rings in bilayer graphene
Zebrev
National Research Nuclear University MEPHI
Su
Doo-Hyeb
Lili
Mohammad
Gennady
Yan
Youn
Dalian University of Technology
Russia
M. Zarenia, J. M. Pereira Jr., F. M. Peeters, and G. A. Farias
Two-dimensional charge relaxation in graphene: generalized Gennady Zebrev telegraph equations and pseudo-relativistic invariance
Xiaoyan
Zhang
Stratingh A novel way to prepare Institute for soluble graphene Chemistry and Zernike Netherlands through organic functionalization on Institute for graphene Advanced Materials
Shou-En
Zhu
Delft Graphene University of Netherlands nanomechanical Technology piezoresistive sensor
Shou-En Zhu, Victor E. Calado, Chao Zhang, G.C.A.M. Janssen
Ziatdinov
Tokyo Institute of Technology, Dept of Organic and Polymeric Materials
Maxim Ziatdinov, Shintaro Fujii, Koichi Kusakabe, Manabu Kiguchi, Takehiko Mori, and Toshiaki Enoki
Maxim
Japan
Visualization of electronic states along the boundaries of graphite nanoholes
Chemistry of Graphene
Applications (gaz sensors, composites, nanoelectronic devices...)
Quantum transport
Xiaoyan Zhang, Wesley R. Browne, Bart J. van Chemistry of Graphene Wees and Ben L. Feringa
Nanoelectromechanical systems
Spectrocopies and microscopies
POSTERS
Surname
19
Name
Gate Control of Nonlinear Characteristics in Chemically Doped Graphene Three-Branch Junction Device 1,2
2
Shaharin Fadzli Abd Rahman , Seiya Kasai , Abdul Manaf Hashim
3
1
Faculty of Electrical Engineering, Universiti Teknologi Malaysia, 81310 Skudai, Johor, Malaysia Graduate School of Information Science and Technology, and Research Center for Integrated Quantum Electronics, Hokkaido University, N14, W9, Sapporo 060-0814, Japan 3 Malaysia-Japan International Institute of Technology, Universiti Teknologi Malaysia, International Campus, 54100 Kuala Lumpur, Malaysia shaharinfadzli@fke.utm.my 2
Graphene is regarded as a promising channel material for switching device due to its ultra high carrier mobility. Furthermore, its unique properties such as ambipolar transport open up new possibility for novel device applications. Three-branch junction (TBJ) device, a nanodevice having a simple structure, exhibits unique nonlinear voltage transfer characteristics even at room temperature [1]. This nonlinear behaviour could also be observed in graphene TBJ device [2,3]. For graphene TBJ device, the polarity of the nonlinear characteristics depends on the conduction type of graphene channel, which could be controlled by gate voltage [2]. By changing gate bias condition and measurement configuration, graphene TBJ is expected to operate as an AND and OR logic gate. In this paper, we investigate the gate control of the nonlinear characteristics in chemically doped graphene TBJ. Figure 1 shows a schematic illustration and scanning electron microscope (SEM) image of the fabricated graphene TBJ. A single graphene flake was prepared by mechanical exfoliation of kish graphite and was placed on a 300-nm thick SiO2/p-Si substrate. Then, Ohmic electrodes were formed using sputtering of 35nm-thick PtPd alloy. Finally, the T-shaped TBJ structure was fabricated by means of electron beam lithography and oxygen plasma etching. The width of each TBJ branch, W, and the total length of right and left branches, L, were 200 nm and 1 Âľm, respectively. The fabricated device was immersed into Polyethyleneimine (PEI), an n-type dopant of graphene, to control the Dirac point (VDirac) and improve the mobility of graphene channel. For the typical operation of the TBJ device, input voltages, VR and VL, were applied at right and left branches, respectively, and voltage at center branch was measured as output voltage, Vout. In order to control the nonlinear characteristic of TBJ, gate voltage, VBG, was applied at the Si substrate. Figure 2(a) shows the measurement results when input voltages were applied in push-pull (VR=-VL) fashion. In general, the measured voltage transfer characteristics showed two shapes, namely, V-shape and bellshape. When the conduction type changed from n-type to p-type, the curve changed from the bellshape to the V-shape. The curvature of the observed nonlinear curve increased when VBG approached VDirac. We also measured voltage transfer characteristics when the input voltage was applied in pushfixed fashion, where left branch is grounded. Figure 2(b) shows the measured voltage transfer characteristics. A VBG-controlled nonlinear behaviour of the TBJ device was also observed in this measurement configuration. This nonlinear behaviour could be explained by the difference of number of carrier induced by VBG in right and left branches of TBJ. Based on a simple lumped equivalent circuit consists of two capacitances connected in series, the relationship between the curvature of nonlinear characteristics, ÄŽ, in Fig. 2(a) and VBG was given by the following equation.
D |
VBG
1 VDirac
This equation shows that the polarity and the curvature of the nonlinear characteristics could be controlled by applying VBG.
References [1] H. Q. Xu, Appl. Phys. Lett., 78 (2011) 2064. [2] A. Jacobsen, I. Shorubalko, L. Maag, U. Sennhouser, and K. Ensslin, Appl. Phys. Lett., 97 (2010) 032110. [3] R. Gockeritz, J. Pezoldt, and F. Schwierz, Appl. Phys. Lett., 99 (2011), 173111. Figures
VC 500 nm
Center branch
200 nm VR 1 Âľm
VL
Left branch
SiO2 Si
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VBG Figure 1. Schematic illustration and scanning electron microscope image of fabricated graphene TBJ device
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Figure 2. (a) Measured voltage transfer characteristics of graphene TBJ when input voltages were applied in pushpull fashion and (b) voltage transfer characteristics for measurement in push-fixed fashion.
Studies of Isocyanate oligomer mixed with Graphene Oxide Konstantinos Kouroupis-Agalou CNR, ISOF, Via Gobetti 101, 40129, Bologna, Italy k_agalou@isof.cnr.it Polymer composites have been used in the last decades in order to improve mechanical, thermal, 3 electrical and gas barrier properties . With the discovery of graphene, a new area of research has been established by mixing graphene with different polymers in order to create graphene-based polymer composite materials. Herein, we present a study of different composites based on Graphene oxide (GO) and the basic components of polyurethane (PU). It is essential to understand and study at nanoscale level the interactions that take place between graphene and polymers in order to develop and improve new composite materials where the exceptional properties of graphene are transferred from single sheet scale to bulk material properties. For this reason, in this study we will show how the basic components of PU can react when they are deposited on different substrates where ultra-thin layers of GO were deposited before. We know that Polyurethane is produced when Isocyanate and polyols are chemically reacting together. By replacing water with GO (which already contains water) should have some interesting properties. The study of the interactions between these two components was studied at the nanoscale by using scanning probe microscopy. The first preliminary results show that the Isocyanate oligomer in very low concentrations is adsorbed preferably on the GO sheets and seems to polymerize on them. In order to understand if this is real, a series of experiments with GO and Isocyanate solutions in different concentrations has been made. By examining the solutions we found that when GO is mixed with Isocyanate (dissolved in Toluene) an emulsion is created that could be stable for many weeks. This was explained because GO can act as an emulsifier and create sub-millimiter-sized organic solvent 4 droplets (toluene) that can be stable . The size of the toluene droplets was found to depend on the concentration of GO in water. Moreover, when we changed the concentration of GO, it was noticed 2 that GO was DFWLQJ DV DQ DPSKLSKLOH DQG *2ÂśV DPSKLSKLOLFLW\ FDQ be tuned by pH . References [1] Horacio J. Salavagione, Gerado Martinez, Garry Ellis, Physics and Applications on Graphene, 2011, 170-192. [2] Jaemyung Kim, Laura J. Cote, Franklin Kim, Wa Yuan, Kenneth R. Shull, Jiaxing Huang. J. Am. Chem. Soc. 2010, 8180-8186. [3] Hyunwoo Kim, Ahmed A. Abdala, and Christopher W. Macosko, Macromolecules, 2010, 65156530. [4] Kim, J. W. Lee, D. Shum, H. C. Weitz, D A. Adv. Matter. 2008, 20, 3239-3243.
CVD grown graphene evaluated with Raman and optical microscope Patrik Ahlberg, Si Chen, Shi-Li Zhang, Ulf Jansson and Zhibin Zhang
Solid State Electronics, the Ă&#x2026;ngstrĂśm Laboratory, Uppsala University, 75237, Uppsala, Sweden patrik.ahlberg@angstrom.uu.se This work has been aimed at developing a controllable process for the graphene growth via chemical vapour deposition (CVD) [1-3] using a home built thermal CVD system. Copper foil is used as catalytic substrates for graphene growth since it has been shown to be able to control the amount of layers of o graphene [1-4]. During graphene growth, temperature has been held constant at 1000 C and methane gas was used as feedstock, combined with argon and hydrogen gas it has been tested in the pressure range of 1-10 Torr. After graphene growth on Cu foil, a well-known process for transferring graphene from Cu foil to 300nm SiO2/Si was implemented [4-5]. Briefly the graphene on copper was coated with poly(methyl methacrylate) (PMMA) and cured in a refrigerator for at least 20 hours. The samples were subsequently soaked in an etchant consisting of sodium persulfate and water (1:5 wt), until the polymer film had been cleanly separated from the copper. The films are then cleaned in DI-water followed by transfer to SiO2/Si. The end product is cleaned with warm acetone to remove the PMMA. Raman characterization was conducted on a Renishaw RamaScope with a 514nm laser [6]. After transferring graphene it is found that some of the samples are dominated by single layer graphene (SLG) on Cu foil based on the Raman spectroscopic results measured at different spots. A good coverage of SLG over a whole sample has been the major challenge. The problem has been the result from the need to control and limit the amount of multi-layer graphene (MLG) present in the samples, while getting a good coverage. This graphene on copper foil is apparently distorted in a Raman spectrum due to the background signal originated from the Cu foil. This motivates why graphene has to be transferred to SiO 2 for Raman measurement. With graphene transferred onto the SiO2/Si the Raman results (i.e. the blue curve in Figure 1) indicate the high quality of the SLG sample with I2D/IG approaching 3 and little D-peak at 1360 -1 cm . When measured at different spot, some areas show Raman signal with features as red line in Figure 1 which possibly is resulted from the influence of residual polymer on SLG. The large portion of graphene film as single layer flake grown by CVD process can be corroborated by the images of optical microscope with a magnification of 5x-50x obtained with graphene transferred onto SiO2/Si (Figure 2). The photographs in Figure 2 show that the majority of graphene comprises SLG. As shown in figure 2-a, some darker flakes are randomly distributed and covering about 5% of the graphene film which are probably due to the formation of MLG in the film [3]. This can be compared with Figure 2-b where no such darker fields are visible (the visible black spots in figure 2-b are smaller and darker suggesting them to be PMMA residues). The difference between these two samples are a growth time of four hours for 2-a while only one and half hour for 2-b. Based on Figure 2, it can be assumed that SLG was preferably grown over the Cu foil in the CVD process, followed by the occurrence of growth of MLG. References 1. Sukang Bae et al, Roll-to-roll production of 30-inch graphene films for transparent electrodes, Nature Nanotechnology, vol 5, (2010), 574-578 [CVD, Cu, 1000] 2. Ivan Vlassiouk, Sergei Smirnov, Illia Ivanov, Pasqual Fulvio, Sheng Dai, Harry Meyer, Miaofang Chi, Dale Hensley, Panos Datskos, Nickolay Lavrik, Electrical and thermal conductivity of low temperature CVD graphene: the effect of disorder, Nanotechnology 22 (2011) 275716 [CVD, Cu, 1000] 3. Ivan Vlassiouk, Murari Regmi, Pasquale Fulvio, Sheng Dai,Panos Datskos, Gyula Eres and Sergei Smirnov, Role of Hydrogen in Chemical Vapor Deposition Growth of Large SingleCrystal Graphene, ACS Nano, Issue 7, (2011) 6069-6076 [CVD, Cu 1000, MLG flakes] 4. Xuesong Li, Yanwu Zhu, Weiwei Cai, Mark Borysiak, Boyang Han, David Chen, Richard D. Piner, Luigi Colombo, and Rodney S. Ruoff, Transfer of Large-Area Graphene Films for HighPerformance Transparent Electrodes, Nano Letters Vol 9 No 12, (2009) 4359-4363 [CVD, Cu, PMMA] 5. Alfonso Reina, Xiaoting Jia, John Ho, Daniel Nezich, Hyungbin Son, Vladimir Bulovic, Mildred S. Dresselhaus, and Jing Kong, Large Area, Few-Layer Graphene Films on Arbitrary Substrates by Chemical Vapor Deposition, Nano Letters, Vol 9 No 1 (2009), 30-35 [CVD, PMMA]
6. Andrea C. Ferrari, Raman spectroscopy of graphene and graphite: Disorder, electron±phonon coupling, doping and nonadiabatic effects, Solid State Communications 143 (2007) 47±57 Figures
Figure 1 Three different curves of graphene with different affecting backgrounds. The steepest curve is of, what probably is SLG, with residual PMMA on top of it. The middle curve is of MLG, on copper, indicated here as the shift in both the G and 2D peak. And the lowest one is of mostly SLG on SiO2
a
b
. Figure 2 Microscopic images of graphene at 50x magnifications, the size bar is 30µm.
AFM-studies of humidity dependence of friction in graphene and other 2D materials M. Ahlskog, M. Hokkanen, J. Lievonen Nanoscience Center, University of Jyväskylä, FI - 40014 Jyväskylä, Finland ahlskog@jyu.fi A. Helle, K. Holmberg VTT Technical Research Centre of Finland P.O.Box 1000 FI - 02044 VTT Finland Nanotribology is a subfield of nanoscience and tribology that investigates frictional and wear properties in nanoscale systems. Many applications of nanotechnology are plagued by frictional phenomena that arise from their inherently large surface-to-volume ratios. Research in the field of nanotribology is hence necessary to advance nanotechnology for the purposes of practical applications. The tip of an Atomic Force Microscope (AFM) provides a controllable single-asperity probe for direct nanoscale investigation of friction. Specialized AFM imaging modes exist for the study of frictional properties of surfaces. In Lateral Force Microscopy (LFM), the torsion of an AFM cantilever due to lateral forces is recorded to acquire the frictional profile of the surface. We have carried out nanotribological LFM measurements under the conditions of varying humidity with an environment-controlled AFM instrument [1]. In addition to measuring the mean values of lateral force (LF), force-displacement (F-Z) curves were obtained. The investigated surfaces have included lowfriction coatings graphene, molybdenum disulfide, (MoS2, Fig. 1) and diamond-like carbon, DLC [2] as well as hydrophilic- and hydrophobic-treated silicon [3]. For these coatings, our measurements suggest a trend of decreasing friction with decreasing humidity. For the Si samples, drastic difference emrges between hydrophilic and hydrophobically treated surfaces.
References [1] J. Lievonen, K. Ranttila, and M. Ahlskog, Review of Scientific Instruments, 78 (2007) 043703. [2] M. Hokkanen et al, unpublished. [3] J. Lievonen, Ph.D Thesis, Univ. of Jyväskylä, Dept. of Physics, Research Report 8 (2011). Figures
Figure 1. Lateral force measured as a function of relative humidity with three different scanning speeds for MoS 2-Ti.
Computer Simulation of Epitaxial Graphene Assembly on Silicon Carbide Surface with Using Semi-Empirical Quantum Chemistry Methods
N.I.Alekseyev, V.V.Luchinin, A.S.Ivanov
6W 3HWHUVEXUJ (OHFWURWHFKQLFDO 8QLYHUVLW\ ³/(7,´ XO 3URIHVVRUD 3RSRYD 6W -Petersburg, Russia.
The described simulation is an initial step in a series of calculation problems in the graphene area. They seek to formulate experimental conditions of SiC Epitaxial Graphene formation under which the probability of joints, "moire" superstructures and similar defects would be reduced to minimum. As a test for suitability of the approach the variants of reconstruction for Si (0001)- and Carbon (0001) faces, preceding the graphene synthesis, were researched. It was shown that the 0
simulation reproduces principal experimental results such as [6ÂĽ [ ÂĽ @5 reconstruction for the Si face as well as specific form of small-scale reconstructions on carbon face. As we know from experiments, the immediate synthesis of graphene from SiC is preceded 0
by a series of the surface reconstructions. I.g., the > ÂĽ [ ÂĽ @5 reconstruction is authentically observed for Si face [0001]Si. Formally, the origin of this reconstruction is associated with the formation of a matching superstructure, in which 6 centered hexagonal Si cells of Âł]LJ]DJ´-shaped sides of 0.308 nm size, having a period of 0.533 nm along the "zigzag" sides of the cell, correspond with 13 hexagonal graphene cells with an accuracy of ~ 1% [1]. At the same time this reconstruction is not the unique one. Therefore a task was formulated to analyze the possible reconstruction shape in the course of SiCĂ&#x17D;Graphene transformation with a viewpoint of the matrix energy code. The research apparatus were semi-empirical quantum chemistry methods CC. Molecular mechanics (MM)- optimized configurations were used as an initial approximation. For Si [0001] face an immediate object of the research was the reconstruction cells 0
> ÂĽ [ ÂĽ @5 and [nxn]. All the Si atoms within the cell were considered to be evaporated, except the only atom, located at the center of the cell (atom 1 in Fig.1). The simulation showed that the atom becomes a natural centre of the graphene cell mesh (a nucleus). Building material for the mesh are "sticking" carbon atoms (atoms 4 in Fig.1), remaining on the surface of the upper SiC layer after Si atoms are evaporated. However, the drift of such atoms drift towards the center of the cell only RFFXUV LI QÂ&#x2022; Otherwise the activation barrier for the drift is significantly higher than the evaporation energy for Si 0
atoms. So, the character of the > ÂĽ [ ÂĽ @5 reconstruction is reasoned without application to geometrical commensurability of the SiC- and the graphene cell size. In the case of the graphene formation at the carbon face the process is developed as a sequence of the surface reconstructions with a rather short spatial periods [2x2], [3x3]. CC methods allowed to get clear this sequence as well as to find the intermediate form of the before-graphene superstructures.
The driving force for the transformation on C-face is a large quantity of energy binding carbon atoms one with another. This fact reduces drastically the activation barriers for joining carbon atoms, despite the destruction of highly symmetrical structure of SiC. Some intermediate configurations with short-period reconstruction along the energyoptimal assembly path are shown in Fig.2.
References [1] K.V. Emtsev, A.Bostwick, K.Horn, et. al. Nature Materials. 2009. Vol 8 . P.203-210. Figures
Fig.1.
Fig.2. a
Fig.2. b
Figure captions
Fig.1. A fragment of the reconstructed SiC surface in a vicinity of the central Si atom. Sideway view. PM3 optimization. The Si atom (deep blue) in bonded with 3 carbon atoms (dark black lines of bonds, carbon atoms are grey, Si atoms in the underlayer are white circles).
Fig.2. a) a [2x2] reconstruction with the period of 5.832Ă&#x2026; (mismatch lattice with SiC~4%); C-atoms, sticking from the plane, are shown as tilted lines; carbon atoms are black, Si atoms are white. b) a [3x3] superstructure with the period of 9.096 Ă&#x2026; (mismatch is 0.85%).
Two-phonon scattering in graphene in the quantum Hall regime Arseny Alexeev1 and Mikhail Portnoi1,2 1
2
School of Physics, University of Exeter, Stocker Road, Exeter, EX4 4QL, United Kingdom International Institute of Physics, Av. Odilon Gomes de Lima, 1722, Capim Macio, CEP: 59078-400, Natal - RN, Brazil A.Alexeev@exeter.ac.uk
One of the most distinctive features of graphene is its huge inter-Landau-level splitting in experimentally attainable magnetic fields resulting in the room-temperature quantum Hall effect. We calculated the longitudinal conductivity due to two-phonon scattering in graphene in quantizing magnetic fields over a broad range of temperatures. The multi-phonon scattering mechanism [1] is known to be negligible for conventional two-dimensional systems under the quantum Hall conditions apart from exotic cases such as magneto-roton dissociation in phonon spectroscopy [2]. However, our calculations show that this mechanism dominates in the high-temperature quantum Hall regime in graphene, since at elevated temperatures the energy of an acoustic phonon with a wavevector comparable to the inverse magnetic length is much smaller than the temperature; therefore, a number of such phonons increases drastically. Single-phonon processes in pristine graphene in this regime remain suppressed due to momentum and energy conservation requirements. We show that the two-phonon scattering mechanism provides a significant error in Hall conductivity measurements and it is therefore the major obstacle in using graphene as a room-temperature quantum Hall standard of resistance.
The results of our calculations in two temperature regimes are shown in the figure below. In both panels we plot the temperature dependence of the pre-exponential factor in the phonon-induced dissipative conductivity V xx . The low temperature regime corresponds to k B T ! s / O , whereas the hightemperature regime is when k B T !! ! s / O , where
O
! eB is the magnetic length and s is the
speed of sound in graphene. In both cases we remain in the quantum Hall regime, which requires k B T !v F / O , where v F is the Fermi velocity in graphene. One can see that in the hightemperature regime the phonon-induced longitudinal conductivity is of the order of e 2 h , which is comparable to the disorder-induced contribution to V xx . References [1] V.N. Golovach and M.E. Portnoi, Phys. Rev. B 74, 085321 (2006). [2] V.M. Apalkov and M.E. Portnoi, Phys. Rev. B 66, 121303 (2002).
Figures
The pre-exponential factor in phonon-assisted dissipative conductivity in the low-temperature (top panel) and hightemperature (bottom panel) regime calculated for different Landau levels from n=0 to n=5 in a magnetic field B=10T.
Magnetoelectric and Flexomagnetic effects in hybrid C/BN nanostructures J. M. Pruneda, R. Martinez-Gordillo Centro de Investigacion en Nanociencia y Nanotecnologia (CIN2), Campus de la UAB, Bellaterra 08193, Spain miguel.pruneda@cin2.es It has been predicted that, under a corrugating distortion, two-dimensional non-centrosymmetric crystals become strongly polarized in the plane of the film, with a nonlinear electromechanical effect. Hence, polar hexagonal BN monoatomic sheets are expected to exhibit not only piezoelectricity but also an unusual flexoelectricity [1]. Surprisingly, similar corrugations would induce a gap opening in graphene [2]. The realization of hybrid C/BN monoatomic sheets [3] with potential half-metallic properties [4], opens the possibility to achieve multifunctional properties in a graphene-like structure, where electromagneto-mechanical properties can be tuned externally by application of an external perturbation (electric or magnetic fields, mechanical strains, etc). In this contribution we will present first-principles density functional calculations to show strong magnetoelectric (ME) effects in hybrid nanostructures made from C and BN domains with zigzag interfaces. This effect originates from the magnetic properties of graphene's zigzag edges and the dielectric properties of the latter, and is highly anisotropic because of the different properties of the C-B and C-N bonds. For nanotube geometries, the linear ME coefficient compares to that of prototypical Cr2O3, whereas for 2D monolayers, the surface ME coefficient is larger than the one predicted for graphene nanoribbons on silicon substrates. Band shifts and gap modulations (also seen in pure BN and C nanotubes) are observed and can give rise to an inversion of the spin of half-metallic states, which would allow for electric-field control of conducting spins at graphene nanoconstrictions embedded in BN sheets. Flexomagnetic effects will also be discussed for corrugated planes made out of BN and C strips with zigzag edges. References [1] I. Naumov, A. M. Bratkovsky, and V. Ranjan, Phys. Rev. Lett., 102 (2009) 217601. [2] I. Naumov and A. Bratkovsky, Phys. Rev. B 84 (2011) 245444. [3] L. Ci et. al. Nat. Mater. 9 (2010) 430. [4] J. M. Pruneda, Phys. Rev. B 81 (2010) 161409(R).
Different chemical approaches to produce graphene derivatives P. Álvarez, C. Botas, R. Santamaría, C. Blanco, M. Granda, R. Menendez Instituto Nacional del Carbón, CSIC, C/Francisco Pintado Fe, 26, 28011, Oviedo, Spain par@incar.csic.es The exfoliation of graphite to obtain graphene oxide (a procedure based on the Hummers, Brodie or Staudenmaier methods [1-3]) is nowadays the most widely applied top-down strategy for the preparation of graphenes of different quality, mainly due to its scalability, low cost and the availability of a considerable amount of bibliographic information on the process. By using specifically prepared graphites with controlled crystal properties from the same precursor, we have demonstrated experimentally that the crystalline structure of the starting graphite has a marked influence on the structure of graphene oxides and also on the average area of the sheets. Once the graphene oxide is obtained, a reduction process is required in order to produce thinner flakes through the elimination of the oxygen functional groups (heat treatment assisted with a mixture of Ar and hydrogen, hydrazine, etc) [4-5]. However, the reduction mechanisms and their effects on the properties of the final graphene materials have still not been clarified.
We present some of the results (in collaboration with IREC, UAL and ICMB) of our study on the effect of graphene oxide structure (degree of structural perfection, type of oxygen functional groups and location) on its behavior when is subjected to these types of reduction process: i) chemical reduction with hydrazine, ii) thermal reduction and iii) reduction with hydrogen.
In the case of reduction with hydrazine and hydrogen, we have experimentally proved the theoretical model of Gao et al. [6] who attributed a more effective deoxygenation to the oxygen functional groups located at the interior of the aromatic domains than to those located at the edges. The reduced graphene oxides exhibited a very different atomic structure and stacking tendency. The location of the remaining hydroxyl groups at the edges in one of the materials propitiated lateral interactions which brought about a substantial increase in the size of the sheets (Figure 1). Furthermore, in collaboration with ITQ, we have demonstrated that one way to restore the graphene structure after the chemical reduction of graphene oxide would be to reconstruct of the C sp2-hybridized bonds by using carbon monoxide.
The exfolation and thermal reduction of graphene oxides offers a simple and clean way to obtain graphenes. We have performed studies on the effect of temperature (from 120 º C to 2,400 º C), on the exfoliation/reduction behavior of the graphite oxide and have been identified the critical factors related to the efficiency and quality of the products (Figure 2). The results obtained show that the exfoliation temperature and the effectiveness of the thermal reduction are largely dependent on the chemical structure of the graphene oxide (type of functional groups and location) which in turn depends on the characteristics of the parent graphite. Moreover, it was found that the use of temperatures above 1000 ºC not only improve the structural order of the graphene sheets but also facilitate stacking, though they fail to promote the recovery of the interactions characteristics of the parent graphites (Figure 3).
Hydrogenation of the graphene oxides also proved to be an effective way to eliminate the oxygen functional groups. However, unlike thermal reduction, this methodology produces an increase of the C3
Sp carbon structure. We also determined the critical ranges of temperatures that produce the main
structural changes in the graphene oxide, being 150 and 200 ยบC the initial temperature of elimination of the main functional groups in the graphene and 500-600 ยบC when the re-ordering of the graphene layers occurs. References [1] Hummers W, Offeman R, J. Am. Chem. Soc., (1958) 1339. [2] Brodie BC, Ann. Chim. Phys., 59 (1860) 466. [3] Staudenmaier L, Ber Dtsch Chem Ges, 31 (1898) 1481. [4] Kaniyoor A, Baby TT, Ramaprabhu S, J. Mat. Chem., (2010) 8467. [5] Stankovich S, Piner R D, Chen X, Nguyen S T, Ruoff R S, J. Mat. Chem., 16 (2006) 155. [6] Gao X, Jang J, Nagase S, J. Physic. Chem. C, 114, (2010) 832-842. Figure1
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Figure 1: Example of the different composition of the graphene layer, measured by HRTEM (a and b), that conditioned the morphology of the partially reduced graphene upon reduction with hydrazine (SEM , TEM AFM images and C1s XPS results (c,e,f,d respectively) of the partially reduced graphene corresponding to the graphene oxide (a) and g,h,I,j corresponding to the precursor (b)). Figure2
Figure 3
Figure 2: XRD results showing the variation of Lc and d(002) values with temperature reduction of GO
Figure 3: TEM image of graphite oxide thermally reduced at 2400ยบC.
Advantage of few-layer graphene in comparison with graphene for applications I.V. Antonova, I.A. Kotin, V.Ya Prinz A.V.Rzanov Institute of Semiconductor Physics, /DYUHQWÂśHY Ave. 13, Novosibirsk, Russia antonova@isp.nsc.ru The discovery of graphene opens way towards unique device architectures, functionalities and physical phenomena. The early period of investigation into the interesting properties of graphene has greatly extended the range of future applications of the material. Turning to applications surprisingly revealed the fact that few-layer graphene often demonstrates even more exiting properties than graphene itself. It can be argued that the discovery of few-layer graphene provides a basis for development of unique graphene-based devices for future electronics. For example, A.J. Hong [1] reported memory applications for graphene. Graphene flash memory has the potential to exceed the performance of current flash memory technology by utilizing intrinsic properties of graphene such as high density of states, high work function, and low dimensionality. Graphene (SLG) and few-layer graphene (FLG, thickness ~ 5 nm) flash memory devices formed on CVD large-area graphene sheets and integrated into a floating gate structure are compared in [1]. While SLG devices exhibit a memory window width of a 9 XVLQJ D SURJUDP HUDVH YROWDJH RI Â&#x201C; 9 )/* GHYLFHV VKRZ a window width of ~ 6 V for the same program/erase voltage. In our study [2] we have compared responses of SLG and FLG to gas absorption and have demonstrated extremely high gas sensing properties of p-type FLG flakes. The change in SLG resistivity due to adsorbed ammonia molecules is normally below several (~ 4 %) per cents (see, e.g. [3]). We have found that the current response of FLG to ammonia adsorption was strongly dependent on FLG thickness, and it exceeded the current response of SLG E\ ¡ 7 orders of magnitude (Fig.1). A maximal response was found in FLG samples with thickness ~ 2 nm. FLG samples whose thickness was smaller than 3 nm demonstrated p-n conversion of flake conductivity. This finding suggested that our structures, at least those comprising less than ten graphene layers, were capable of showing a 7 strong increase in resistance (10 times, switching effect) when exposed to ammonia ambient for moderate times. Samples with flake thicknesses ranging in the interval from 3 to 10 nm also exhibited pronounced responses to ammonia, showing from slightly above one order of magnitude to 25 % change in resistivity. No conversion to n-type conductivity was found in those samples. The high response of FLG flakes to ammonia adsorption can be understood within the framework of a model that implies the formation of multiple p-n-p junctions during ammonia absorption at the grain boundaries in polycrystalline graphene flakes considering the fact that the interlayer screening length in the material was ~1 nm [4]. Thus, polycrystalline FLG flakes may prove useful in gas sensor applications. Growth of graphene is aimed at the development of graphene-based electronics. FLG has at least one layer that is not at the surface and not interacting with substrate, which is not susceptible to contamination and disorder due to processing [5]. Thus, grown graphene multilayers can be expected to be more important for application than monolayers. Moreover, a rich experience of graphite intercalation compounds can be used for creation of new hybrid materials or heterostuctures by means of intercalation of different substances into FLG. Intercalation ensures pure and two-side functionalization of graphene. A new interesting approach to creating a stable hybrid material with unexpected properties having considerable potential for future applications was developed in our group [6]. This approach is based on intercalation of the well-known polar solvent N-methylpyrrolidone (NMP) into few-layer graphene combined with heat treatment. Electrical properties of the new material proved to be dramatically different from those of graphene. We have included an anneal step into the fabrication process of our structures with the aim to stimulate the interaction of NMP with graphene sheets. Depending on process temperature, the obtained material could be produced with the following properties: a broad range of resistivity values (6 - 7 orders of magnitude, see Fig. 2) in combination with 2 3 a high carrier mobility, tunable band-gap (from 0 up to 3 - 4 eV), and sp or sp hybridization of carbon atoms. An additional tool to manage with resistance of the hybrid material was found to be treatment of hybrid structures in hydrofluoric acid vapor. Such treatment during few minutes led to a strong decrease in resistivity of the hybrid material. For FLG structures, HF treatment leads to a decrease of flake 2 4 7 resistivity by 10 ¡ 0 times and 10 x decrease of resistivity for graphene due to cleaning of the surfaces from NMP. So, HF treatment offers additional technological approach for managing with film properties. The revealed possibility to govern the surface conductivity by means of HF treatment gives us the way WR PRQLWRU WKH ORFDO SURSHUWLHV RI WKH QHZ PDWHULDO DQG WR FUHDWH VWUXFWXUHV RI WKH ÂľJUDSKHQH - atomically thin high-UHVLVWLYLW\ EDUULHUÂś W\SH, which offer an unique tool for 2D and 3D nanostructuring and for design of novel devices.
References [1]. A.J. Hong, E.B. Song, H.S. Yu, M.J. Allen, J. Kim, J.D. Fowler, J.K. Wassei, Y. Park, Y. Wang, J. Zou, R.B. Kaner, B.H. Weiller, K.L. Wang, ACS Nano, 10 (2011) 7812. [2]. I. Antonova, S. Mutilin, V. Seleznev, R. Soots, V. Volodin, V. Prinz, Nanotechnology, 22 (2011) 285502. [3]. F. Shedin, A.K. Geim, S.V. Morozov, E.W. Hill, P. Blake, M.I. Katsnelson, K.S. Novoselov, Nature Mat., 6 (2007) 652. [4]. Y. Sui, J. Appenzeller, Nano Lett., 9 (2009) 2973. [5]. W.A. de Heer, MRS Bulletin, 36 (2011) 632. [6]. I.V.Antonova, I.A.Kotin, R.A.Soots, V.A.Volodin, V.Ya.Prinz, in press.
8
10
6
I/Io, (arb.u.)
10
1 2 G
4
10
2
10
0
10
-2
10
0
2
4
6
8 10 d (nm)
800
900
Figure 1. Change in the current flowing through few-layer graphene flakes under NH3 absorption as a function of flake thickness for two ammonia sources, 1 and 2, used to organize the gaseous air/ammonia ambient in the present study. Source 2 was an ammonia source more intense in comparison with source 1. The point G indicates the 4-% response of graphene to ammonia adsorption known from Ref. [3].
Resistivity (Ohm.cm)
5
10 4 10 3 10 2 10 1 10 0 10 -1 10 -2 10 -3 10 -4 10
0 80 120 160 200 240 0 Temperature of HS creation ( C)
Figure 2. Resistivity as a function of temperature at which the hybrid structures (HS) were fabricated. The resistivity of the reference FLG is indicated in Fig. 2 as a point at zero temperature.
Charge transfer engineering in graphene nanoribbons using metallic contacts and organic adsorbed layers Chloé Archambault, Alain Rochefort Département de génie physique and Regroupement québécois sur les matériaux de pointe (RQMP), École Polytechnique de Montréal, C.P. 6079, Succursale Centre-ville, Montréal, Québec, H3C 3A7 chloe.archambault@polymtl.ca Graphene is a very promising material for electronics due to its extremely high electron mobility. Its twodimensional structure is especially well suited for the current microfabrication techniques. On the other hand, most electronic devices are not built from semi-metals such as graphene but rather from semiconductors. Fortunately, graphene can be made semiconducting after being engineered into nanoribbons [1, 2], and then can be incorporated into the fabrication of transistors [3, 4].
The electronic interactions between graphene nanoribbons (GNRs) and the indispensable metallic contacts remains a source of high interest since critical features at such a small length scale, for example the presence of metal induced gap states (MIGS), can have a dramatic impact on the final device performance. Beyond the effect of metallic electrodes, an interesting approach to control the electronic properties of GNRs through doping would be to use a self-assembled adsorbed layer (see Figure 1) [5]. In addition, this adlayer could also act as an additional conduction channel when properly connected to the electrodes.
Accordingly, we report the results of first principles density functional theory (DFT) calculations of finite GNRs in contact with Au and Pd electrodes. Our results make clear evidences of the molecular hybridization between frontier orbitals of GNRs and the metallic states. Moreover, we observe significant charge transfer from graphene to the electrode which extends into the nanoribbon beyond the zone located immediately under the contact. In the case of zigzag GNRs, the charge transfer is mainly accommodated by the ribbon's edge states (see Figure 2). By plotting the local density of states (LDOS) at the Fermi level (see Figure 3), we have evaluated the penetration length of MIGS. For the adsorbed organic layer, our first DFT calculations suggest that the chemical nature of the layer appears more important than the native structure (armchair, zigzag) of the GNRs on the magnitude of doping. References [1] K. Nakada, M. Fujita, G. Dresselhaus, and M. Dresselhaus, Phys. Rev. B, 54 (1996) 17954. [2] M. Han, B. Özyilmaz, Y. Zhang, and P. Kim, Phys. Rev. Lett., 98 (2007) 206805. [3] X. Wang, Y. Ouyang, X. Li, H. Wang, J. Guo, and H. Dai, Phys. Rev. Lett., 100 (2008) 206803. [4] Z. Chen, Y.-M. Lin, M. J. Rooks, and P. Avouris, Physica E, 40 (2007) 228. [5] A. Rochefort, and J. Wuest, Langmuir, 25 (2009) 210-215.
Figures
Figure 1: Terephthalic acid chain adsorbed on an armchair GNR between two metallic contacts.
Figure 2: Change in the electronic density (red for positive, blue for negative) of a zigzag GNR in contact with an Au (111) electrode.
Figure 3: Variation in the LDOS of an armchair GNR at Fermi energy upon addition of an Au (111) contact (represented by darker circles).
Emulsion mixing technique for preparation of poly(buthylacrylate/methylmethacrylate)/graphene electrically conductive composite films a
Alejandro Arzac , Radmila Tomovska a
a,b
Institute for Polymer Materials, POLYMAT, The University of Basque Country, P.O. Box 1072, 20018 Donostia-San Sebastian, Spain b
IKERBASQUE, Basque Foundation for Science, 48011 Bilbao, Spain radmila_tomovska@ehu.es
The water-borne polymeric dispersions are products with a huge number of applications that include adhesives, paints, paper coatings, additives forconcrete, tires, textile and leather industry, cosmetics and 1 biomedicine . They are usually produced using the polymerization in dispersed media techniques. To enhance the polymer properties, various inorganic nanoparticles with different sizes and aspect ratios have been added (for instance, silica and clay improved mechanical and thermal properties, while carbon 2-5 nanotubes (CNT) introduced electrical conductivity. In this work graphene nanoplatelets (GNP) have been used for preparation of hybrid graphene/polymer dispersions by emulsion mixing technique (blending of stable aqueous dispersions of GNP or modified graphene nanoplatelets (mGNP) with waterborne polymeric dispersion (latex)). The procedure consisted of low-energy sonication of the polymer and GNP aqueous dispersions. The polymer latex with 40 wt% solids content was produced by seeded semibatch emulsion polymerization of buthylacrylate/methylmethacrylate (BA/MMA) with 50/50 wt/wt. Stable aqueous dispersions of GNP (commercial, Sky Spring Nanomaterials) were prepared by employing ultrasonication to GNP dispersed in water or in 2% water solution of two types of surface active compounds (Sodium Cholate (SC) and Triton x200). It was determined that ultrasonication will additionally exfoliate graphene nanoplatelets and decreased their size, as it could be clearly be seen from TEM images of GNP before and after sonication, shown in Figure 1. mGNP were prepared by azide photochemistry. Poly(ethylene glycol) methyl ether azide was UV irradiated, that results in fast production of very recative nitrene radicals and their [2+1] cycloaddition to graphene -electronic system. In this way, poly(ethylene glycol) methyl ether was covalently attached to GNP surface, producing mGNP. The films were formed from the GNP/polymer or mGNP/polymer water dispersions by evaporation of water under standard atmospheric conditions. The electrical conductivity of the films was investigated as dependence on GNP concentration, varied from 0.1 wt % to 1 wt % related to polymers. The electrical conductivity of these films was determined by a four probe conductivity meter, obtaining conductivity for -1 concentrations above 0.4 wt% of GNP, reaching a maximum conductivity of 0.94 S.cm for 0.5wt% GNP stabilized by triton x200.
References 1. Y. Reyes, J.M. Asua, J Polym Sci Part A: Polym. Chem. 48 (2010) 2579. 2. J. Cheng, M. Chen, S. Zhou, L. Wu, J Polym Sci Part A: Polym. Chem, 44 (2006) 3807. 3. G. Diaconu, M. Micusik, A. Bonnefond, M. Paulis, J.R. Leiza, Macromolecules 42 (2009) 3316. 4. T.Wang, C.-H. Lei, D. Liu, M. Manea, J.M. Asua, C. Creton, A.B. Dalton, J.L. Keddie, Adv. Mater. 20 (2008) 90. 5. I. Jurewicz, P.Worajittiphon, A.A.K. King, P.J.Sellin, J.L.Keddie, A.B.Dalton, J. Phys. Chem. B 115 (2011) 6395.
Figures
(a)
(b)
Figure 1. TEM images of GNP (a) before and (b) after sonication.
Edge states and flat bands in graphene nanoribbons with arbitrary geometries 1,
2
2
3
A. Ayuela W. Jask贸lski , M. Pelc , H. Santos , and L. Chico 1
3
Donostia International Physics Center, San Sebastian, Spain 2 Instytut Fizyki UMK, Torun, Poland 3 Instituto de Ciencia de Materiales, Madrid, Spain swxayfea@sw.ehu.es
Graphene nanoribbons (GNR), stripes of nanometric widths cut from graphene, are the subject of a growing interest. They exhibit edge-localized states, which may play an important role in transport and magnetic properties. For instance, the magnetic properties of nanoribbons are directly related to the existence of localized edge states [1]. All these edge terminations have been experimentally identified by different techniques, such as scanning tunneling microscopy [2,3], high-resolution transmission electron microscopy [4], or atom-by-atom spectroscopy [5]. It is thus important to identify general edges and nanoribbons that present localized edge states, as well as their degeneracy and characteristics.
We prescribe general rules to predict the existence of edge states and zero-energy flat bands in graphene nanoribbons and graphene edges of arbitrary shape [6]. No calculations are needed. For the so-called minimal edges, the projection of the edge translation vector into the zigzag direction of graphene uniquely determines the edge bands. By adding nodes to minimal edges, arbitrarily modified edges can be obtained (Fig. 1); their corresponding edge bands can be found by applying hybridization rules of the extra states with those belonging to the original edge. Our prescription correctly predicts the localization and degeneracy of the zero-energy bands at one of the graphene sublattices, confirmed by tight-binding and first-principles calculations (Fig. 2). It also allows us to qualitatively predict the existence of E = 0 bands appearing in the energy gap of certain edges and nanoribbons. References [1] Y. W. Son, M. L. Cohen, and S. G. Louie, Nature 444, 347 (2006). [2] Y. Niimi, T. Matsui, H. Kambara, K. Tagami, M. Tsukada, and H. Fukuyama, Phys. Rev. B 73, 085421 (2006). [3] Y. Kobayashi, K.-I. Fukui, T. Enoki, and K. Kusakabe, Phys. Rev. B 73, 125415 (2006). [4] Z. Liu, K. Suenaga, P. J. F. Harris, and S. Iijima, Phys. Rev. Lett. 102, 015501 (2009). [5] K. Suenaga and M. Koshino, Nature 468, 1088 (2010). [6] W. Jask贸lski, A. Ayuela, M. Pelc, H. Santos, and L. Chico, Phys. Rev. B 83, 235424 (2011).
Figures
FIG. 1: Geometries of several modified zigzag graphene edges: (a) Bearded zigzag edge, composed of Klein defects; (b) a cape structure on a zigzag edge, obtained by bonding one extra atom to two adjacent Klein defects; (c) a cove edge; and (d) a periodic modified edge with a cape.
FIG. 2: (Color online) Localization of the wave functions corresponding to the E = 0 band at k = Ę&#x152; for 40(2,0) (left) and 40(4,0) (right) GNR with a cape structure at the edges. The corresponding edges are shown in Figs. 5(c) and 5(d), respectively. Only an edge and a few neighboring nodes in the GNRs unit cells are shown. Upper panel: Results obtained using tight-binding method. Bottom panel: Results of first-principles calculations. The dot diameter in the upper panel reflects the TB density at the nodes. No dot means that the wave function is exactly zero at this node.
Few layer graphene decorated with Pd nanoparticles: synthesis, characterisation and catalytic applications in the electrochemical oxidation of alcohols 1,2
1
1,3
4
5
Bruno F. Machado , Revathi R. Bacsa , Julien Beausoleil , Victoria Tishkova , M. Bellini , Andrea 5 5 5 1 Marchionni , Francesco Vizza , Claudio Bianchini , Philippe Serp 1 Laboratoire de Chimie de Coordination UPR CNRS 8241, composante ENSIACET, Toulouse University, 118 route de Narbonne, 31077 Toulouse Cedex 4, France 2 Laboratório de Catálise e Materiais (LCM), Laboratório Associado LSRE/LCM, Faculdade de Engenharia, Universidade do Porto, Rua Dr. Roberto Frias s/n, 4200-465 Porto, Portugal 3 Arkema, Groupement de Recherches de Lacq, BP 34, 64170 Lacq, France 4 &HQWUH G¶pODERUDWLRQ GHV PDWpULDX[ HW G¶pWXGHV VWUucturales CNRS, 29 rue Jeanne-Marvig, BP 4347, 31055 Toulouse, France 5 Istituto di Chimica dei Composti Organometallici (ICCOM-CNR),Via Madonna del Piano 10, 50019 Sesto Fiorentino, Italy revathi.bacsa@ensiacet.fr Due to their unique graphitized basal planar structure, chemically inert nature and high conductivity, graphene and graphene-based composite materials are among the most promising alternatives as 1,2
electrode materials in energy-related devices.
Graphene sheets decorated with metal nanoparticles
are typical examples of emerging metal-carbon composites that currently attract special research efforts 3
due to their enhanced potential for catalytic applications. One of the most interesting is related to the electrocatalytic activity in alcohol oxidation reactions for energy production where metal-decorated 4
graphene has been reported to be more efficient than any other commercially available material. In this communication, we present first results on the CVD synthesis of few layered graphene (FLG), the formation of metal-graphene composites and the testing of these materials in the electrochemical oxidation of ethanol, ethylene glycol, glycerol and 1,2-propanediol. FLG and FLG-MWCNT (multiwall carbon nanotubes) composites were prepared by ethylene CVD at 5,6
650°C.
TEM (Figure1a) and Raman spectra showed that the FLG sample contained few layered
graphene flakes with thickness (as determined by the broadening of the x-ray powder diffractograms) ranging from 0.5-8 nm. In order to achieve better metal dispersion, all carbon samples were treated with nitric acid to introduce surface oxygen functionalities that will help anchoring the metal precursor. A suspension of 1 g of sample was placed in a 250 mL flask containing 100 mL of THF and sonicated for 15 minutes prior to the addition of 0.15 g [Pd2(dba)3] precursor and left stirring at 40°C for 3 days. The solid product was filtered and dried under vacuum, and then it was ground and reduced at 300°C in hydrogen (20 vol. % in Ar) . After reduction the sample was stored under argon. The three samples were characterized via cyclic voltammetry (CV) with KOH 2M, Ethanol (EtOH) 10 wt. % KOH 2M, Ethylene glycol (EG) 10 wt. % KOH 2M, Glycerol (Gly) 10 wt. % and 1,2-propandiol (1,2Prop) 10 wt. % KOH 2M aqueous solutions. The reference electrode was Ag/AgCl/KClsat and all potentials were referred versus RHE. The working electrode was made as described below: each catalyst (about 40 mg) was dispersed in a solution of water and i-propanol (1.2:0.7 g) and sonicated for one hour; the resulting ink was dropped on a glassy carbon disk (about 5 mg) and a drop (2. ȝL) of a -2
solution of AS-4 Tokuyama ionomer was DGGHG 7KH PHWDO ORDGLQJ ZDV DERXW ȝJ FP . Figure 1 shows a TEM image of few layer graphene and (b) shows a graphene flake coated with Pd nanoparticles. A statistical count of the particle sizes for the samples yielded an average diameter of 8.4 nm for Pd/FLG and 8 nm for Pd/MWCNT. On the other hand, Pd/FLG-MWCNT showed an average particle size of 6.4 nm indicating the presence of a synergetic effect for the mixture of graphene and CNTs as a smaller Pd particle size was observed.
100 nm
Figure 1. TEM micrographs of (a) few layer graphene, and (b) Pd/FLG catalyst.
Pd/FLG catalyst showed the highest performance toward the oxidation of ethanol with a peak current of -2
-2
ca. 100 mA cm and a current at 0,5 V of ~ 10,5 mA cm . The best catalyst for the other alcohols (1,2propanediol, ethylene glycol and glycerol) was Pd/FLG-MWCNT with a very high values of current at 0.5 V and peak current densities. This improved performance is probably related to the presence of smaller Pd particles. These results are very promising, especially if compared to previously published results for 7
-2
Pd/CNT that showed a current density peak of about 34 and 47 mA cm toward ethanol and glycerol oxidation. CONCLUSIONS The synthesis and characterization of few layer graphene coated with Pd nanoparticles has been reported. The FLG-MWCNT composite substrate showed improved performances when compared to MWCNT or FLG substrates and this observation is related to the smaller particle size of the Pd on this substrate. References [1] X. Huang, Z. Yin, S. Wu, X. Qi, Q. He, Q. Zhang, Q. Yan, F. Boey, H. Zhang, Small 7 (2011) 1876. [2] V. Singh, D. Joung, L. Zhai, S. Das, S.I. Khondaker, S. Seal, Progress in Materials Science 56 (2011) 1178. [3] B.F. Machado, P. Serp, Catalysis Science & Technology 2 (2012) 54. [4] R. N. Singh, R. Awasthi, Catalysis Science & Technology 1 (2011) 778. [5] R. Bacsa, P. Serp, FR 11.03952 (2011). [6] J. Beausoleil, R. Bacsa, B. Caussat, P. Serp, FR 11.62270 (2011). [7] V. Bambagioni, C. Bianchini, A. Marchionni, J. Filippi, F. Vizza, J. Teddy, P. Serp, M. Zhiani, Journal of Power Sources 190 (2009) 241.
Development of a technique based on methylene blue for characterizing specific surface area of graphenes and other carbon nanostructures J. Baker, D. Gethin, WCPC, Swansea University, Swansea, United Kingdom J.A.BAKER.617807@swansea.ac.uk One of the greatest difficulties when developing methods to produce graphene is assessing the quality of the material produced. The specific surface area is one property that can be used and is particularly useful if the graphene is to be used in a catalytic application (such as the counter electrode in a dye solar cell) where the surface area gives an indication of the number of active sites available for catalysis. 2
The theoretical surface area of a single graphene sheet is 2630m per gram and that of a closed 2 1 single walled carbon nanotube is 1315m /g . However, when making graphene in quantities greater than single flakes, this theoretical surface area is difficult to attain due to the presence of multiple layer nanoplatelets as well other impurities. Nitrogen adsorption is the most commonly used method for assessing surface area with a number of commercially produced systems available for this purpose. This technique requires a bulk sample for 2 assessment of at least 5m and the samples must be in solid form. A methylene blue technique has the advantage of no minimum sample size and can directly measure samples dispersed in a liquid. 2
Previous methods for assessing the surface area of graphene by methylene blue have required the graphene to be suspended in ethanol and filtered and had some discrepancy with the nitrogen absorption measurement. This work develops the methylene blue method using water as the solvent and removing the need for filtration.
1.
Peigney, A., Laurent, C., Flahaut, E., Bacsa, R.R. & Rousset, A. Specific surface area of carbon nanotubes and bundles of carbon nanotubes. Carbon 39, 507-514 (2001).
2.
Mcallister, M.J. et al. Single sheet functionalisation by oxidation and thermal Expansion of Graphite. chem. materials 19, 4396-4404 (2007).
A generalised, tight-binding transport model description for random edge-defected ZGNRs Jack Baldwin and Y. Hancock Department of Physics, The University of York, Heslington, York, UK, YO10 5DD jb574@york.ac.uk, y.hancock@york.ac.uk The transport properties and magnetisation of edge-defected zig-zag graphene nanoribbons (ZGNRs) have been studied within the Landauer-BĂźttiker formalism [1] using a generalised tight-binding model that has been shown to be accurate against ab initio calculations [2]. The generalised tight-binding model includes up to third nearest-neighbour hopping and a mean-field Hubbard-U term, with a singleparameter set for armchair, zigzag and mixed-edge nanoribbon systems [2]. The interplay between the extended hopping, Hubbard-U and random edge-disorder on the coherent transport properties has been investigated for small-width ZGNRs of finite device length (Fig. 1a). Second nearest-neighbour hopping and the mean-field Hubbard-U have been shown to be essential for reproducing the electronic properties (band-gap and asymmetry) and magnetism in ZGNRs predicted by ab initio calculations [2]. Hence this work extends previous studies on ZGNRs that have used only a nearest-neighbour tightbinding model together with random edge-disorder [3-6], or ab initio methods to study systematic edgedisorder [7]. Two types of random, edge-disorder were investigated namely, weak-disorder and edge-vacancy defects. Weak-disorder was introduced by perturbing the on-site energy of the edge-atoms by a random amount within the range Âą|V| eV (see also Li et al. [3]), whereas edge-vacancy defects were added by random removal of the individual carbon atoms at the edge-sites in a manner that avoids the formation of unrealistic edge-structures (for example Klein defects) (Fig. 1b) [4]. Small-width ZGNR systems were chosen as previous reports have shown that the coherent transport properties of these systems are more sensitive to edge-defects [3]. We calculated ensemble averages for the transport properties using a minimum of 9 randomly generated defected systems in order to ensure good statistics in our results. Random edge-vacancies are found to decrease the calculated conductance for the ZGNR system (Fig. 2a). Significant differences in the conductance occurs about the Fermi energy (EF), where the HubbardU is seen to open up the transport gap. Against the extended model results for the ideal system, the Hubbard-U induced gap in the defected system is slightly smaller in width. This reduction in width arises from the perturbation obtained in the gap region from the extended hopping terms. Away from EF there is little difference between the extended model results for systems that are with or without the HubbardU (Fig. 2a). At higher energies, however, these differences become more significant, and in general, it is shown that the extended model acts to bolster the conductance results. Random, weak-disorder of the edge-atoms in ZGNR systems results in the formation of a transport gap, which increases in width as a function of the increasing value of the disorder, and has been explained in terms of the onset of Anderson localisation [3-6]. This, however, is not the only mechanism for gap formation in ZGNRs, and therefore we have also been interested to study the interplay between weakdisorder effects and the gap-forming properties of the Hubbard-U. Fig. 2(b) shows that Hubbard-U effects in the random, weak-disordered system results in a larger transport gap than that of the weakly disordered system described by the hopping terms alone, and that these effects are in fact additive resulting in an overall increase in the transport gap.
References [1] Datta, S., Cambridge Univ Pr, (1997). [2] Hancock, Y. et al., Physical Review B 81 (2010) 245402. [3] Li, T.C. and Lu, S.P., Physical Review B 77 (2008) 085408. [4] Cresti, A. and Roche, S., New Journal of Physics 11 (2009) 095004. [5] Mucciolo, E.R. et al., C.H., Physical Review B 79 (2009) 075407. [6] Cresti, A. et al., Nano Research 1 (2008) 361. [7] Huang, B. et al., Physical Review B 77 (2008) 153411. (a)
(b)
Fig. 1: (a) An ideal ZGNR device showing the dimensions of the ribbon used in this study and (b) a 10% vacancy edge-defected ZGNR device system. Coloured circles correspond to the local spin-polarisation results, where red refers to net spin-up and blue refers to net spin-down. (a)
(b)
Fig.2: Effect of the mean-field Hubbard-U and extended hopping terms on the transport properties of (a) a 10% random edge-vacancy defected ZGNR and (b) a random, weak-edge disordered ZGNR. Both types of disorder were applied to the ideal ZGNR system shown in Fig 1, with an ensemble average obtained for the results over a minimum of 9 random configurations. Here, t1,2,3 (t1) specifies the range of the nearest-neighbour hopping where 1, 2 and 3 denote first, second and third nearest-neighbour hopping, respectively. The parameters for the generalised, tight-binding model are t1 = 2.7, t2 = 0.20, t3 = 0.18 and U = 2.0 in units of eV. For defected systems with HubbardU, solid(dashed) lines correspond to spin-up(down). For all other cases, the results are spin-independent.
Graphene research in Lithuania with subsequent application in bioanalysis, energy storage and optical materials
1
1
1
2
Jurgis Barkauskas , $OGRQD %HJDQVNLHQơ , -XOLMD 5D]PLHQơ , *HGLPLQDV 5DþLXNDLWLV , Rasa 2 2 3DXOLXNDLWơ , 5DLPRQGD &HOLHãLnjWơ .
1 2
Vilnius University, Universiteto str. 3, LT-01513 Vilnius, Lithuania
Center for Physical Sciences and Technology, Savanoriu ave. 231, LT-02300 Vilnius, Lithuania jurgis.barkauskas@chf.vu.lt
Several research groups are active in the area of graphene, which are spread across Universities and Research Centers in Lithuania. They are working in close collaboration with each other as well as with national and European institutions to perform fundamental and applied research in graphene and related topics. In Vilnius University graphene is under investigation within several different research groups. At the Department of General & Inorganic Chemistry, Laboratory of Carbonaceous Materials has been established since 1992. Research in this laboratory is focused on: (i) Synthesis of carbon nanostructures (grapene, graphene oxide, SWNT, MWNT, nanocones, nanopipettes, nanorods, etc.) from various precursors; (ii) Modification the surface of carbon nanostructures with functional groups; (iii) Preparation of membranes from the synthesized carbon nanostructures; (iv) Examination of the synthesized carbon nanostructures; (v) Application of synthesized carbons in nanoelectronics; fuel cells; energy storage; bionanotechnology, etc.; (vi) Building up mathematical models having an aim to optimize the synthesis conditions, construction of sensors and other parameters; (vii) Investigation of synergism in micro-disperse compositions including various forms of carbon [1 ± 3]. Several different synthesis methods of graphene oxide as well as other pre-graphene phases are pursued in the Laboratory of Carbonaceous Materials. Product range includes both negative and positive charged particles with different functional groups. Graphene oxide and graphene membranes are prepared by using a modified protocol of filtration of water suspension; this protocol includes the elements of layerby layer assembly. The thickness of membranes can reach several nm. At the same Department in the Laboratory of Sol-Gel Chemistry graphene, graphite oxide and SWCNTs are studied in the role of promising material for saturable absorbers in laser mode locking [4] because of the fast recovery time, which covers a broad spectral range in the near infrared, and excellent chemical stability. Currently, fabrication of polymer composites based on polymer matrix for saturable absorbers is very popular. But due to nature of organic polymer matrix, these composites are unstable on higher temperatures or in high energy light expositions. Therefore a sol-gel process for the fabrication for inorganic (i.e. silica) or semi-organic (organic-modified silica) matrix can be a promising way to produce photo-chemically and thermally stable composites: thin/thick coatings, glasses and ceramics. Another research group from Vilnius University working with graphene is set up at the Institute of Biochemisty, Department of Bioanalysis. Main research activity at this department is design of
biosensors and bioanalytical systems and investigation of electron transport in the system electrodeenzyme. Carbon nanotubes and graphene-related materials are successfully used as electrode materials [4]. Using these materials Department of Bioanalysis focused their activity on creation of biosensors and biosensor arrays for the determination of physiologically important metabolites like glucose, ethanol, cholesterol, heavy metals, inhibitors, etc. Biosensor based analytical systems have been developed and implemented into medicine food quality control. At the present time research are focused mainly on (i) Creation of reagentless screen-printed biosensors and biosensor arrays; (ii) Creation of analytical systems for on line monitoring of biologically active compounds in food-stuffs and environment. To reach those objectives both modified graphene and carbon nanotubes are used as enzyme supports and electrode materials. Research is performed in tight collaboration with Italian, Swedish and German research centers on the basis of joint projects. Center fo Physical Sciences and Technology includes a few groups working in different departments that are involved in research and technology development by using graphene. Department of Nanoengineering in Center for Physical Sciences and Technology was established just in 2011. The mission of the Department is development of new nanoscale materials, structures and processes for life science, medicine and analytical applications. Graphene and/or graphene oxide is used to develop sensors and biosensors employing nanostructured materials. Role of graphene nanostructures in such sensors is to enhance an electrochemical signal as (bio)sensor response to an analyte. Project funded by the European Social Fund under the Global Grant measure, Project No. VP1-3.1-â00-07-K-01- ³1DQR- and Lasertechnology Application to the Investigation and Modification of Graphene and to the Development and Miniaturisation of (Bio)Sensors for Food Quality &RQWURO´ LV FDUULHG RXW LQ WKLV 'HSDUWPHQW DQG LW LV EDVHG RQ JUDSKHQH UHVHDUFKHV DQG DSSOLFDWLRQ WR miniaturised (bio)sensors. For this purpose, graphene is functionalised to improve its immobilisation ability on an electrode surface and affinity to biologically labile molecules. Further, immobilised graphene
is
characterised
applying
optical
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methods.
Reproducibility
investigations of such nanostructure formations has a great importance in these researches. The optimised protocols for graphene nanostructure formation will be applied to miniaturisation of electrochemical (bio)sensing devices. Activities at Department of Laser Technologies cover localized reduction of graphite oxide to graphene by laser irradiation, formation of heat and electro-conducting circuits [4] as well as modification and structuring deposited graphene layers for sensors.
References
[1] R. Rimeika, J. Barkauskas, D. ýLSO\V $SSOLHG 3K\V. Lett. 99 (2011) 051915. [2] - %DUNDXVNDV , 6WDQNHYLÞLHQƥ - 'DNãHYLÞ $ 3DGDUDXVNDV Carbon 49 (2011) 5373. [3] J. Razumiene, V. Gureviciene, E. Voitechovic, J. Barkauskas, V. Bukauskas, A. Setkus. J. Nanosci. Nanotechnol. 11 (2011) 9003. [4] J. Razumiene, A. Vilkanauskyte, V. Gureviciene, J. Barkauskas, R. Meskys, V. Laurinavicius. Electrochimica Acta 51 (2006) 5150. [5] R. Trusovas, G. 5DÞLXNDLWLV - Barkauskas, R. 0DüHLNLHQƥ. J. Laser Micro/Nanoeng. 7 (2012) 4953.
In-situ Characterization of Graphene Growth Bernhard C. Bayer, Robert S. Weatherup, Piran R. Kidambi, Stephan Hofmann Department of Engineering, University of Cambridge, Cambridge, CB3 0FA, UK bcb25@cam.ac.uk
Chemical vapor deposition (CVD) is the most promising technique for scalable and economical monoand few-layer graphene (M-/FLG) growth, a key requirement for future device applications. The current understanding of the M-/FLG growth processes, however, is very limited. Key questions remain open, such as what M-/FLG quality can be achieved with CVD, in particular, if for cost effectiveness sacrificial poly-crystalline metal films/foils and less stringent vacuum/CVD process conditions are used. We study M-/FLG CVD by complementary in situ probing under realistic process conditions with the aim of revealing the dominant growth mechanisms. Here we focus on time-resolved in-situ X-ray photoelectron spectroscopy and in-situ X-ray diffractometry of model polycrystalline Ni catalyst films [1,2]. We show that M-/FLG growth occurs during isothermal hydrocarbon exposure and is not limited to precipitation upon cooling. We show that growth is however also not limited to a pure surface process as we find significant dissolution of carbon into the bulk and sub-surface of the metallic Ni catalyst. A coherent growth model is established based on our insights from surface chemistry and structural evolution. Alloying Ni with Au is found to allow low temperature (<450ยบC) CVD of predominantly monolayer (>74%) 2
JUDSKHQH ILOPV ZLWK DQ DYHUDJH ' * SHDN UDWLR RI a DQG GRPDLQ VL]HV LQ H[FHVV RI ศ P [1]. Au alloying drastically lowers the graphene nucleation density, allowing more uniform and controlled growth at CMOS compatible temperatures.
References [1] R. S. Weatherup, B. C. Bayer, R. Blume, C. Ducati, C. Baehtz, R. Schlรถgl, S. Hofmann, Nano Lett., 11 (2011) 4154. [2] R. S. Weatherup, B. C. Bayer, R. Blume, C. Baehtz, P. R. Kidambi, M. Fouquet, C. T. Wirth, R. Schlรถgl, S. Hofmann, ChemPhysChem, (2012), doi:10.1002/cphc.201101020.
Superconductivity in Two-dimensional Crystals 1
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S. J. Bending , M. S. El Bana , David Hudson , Saverio Russo
3
1
Department of Physics, University of Bath, Claverton Down, Bath BA2 7AY, UK. 2 Department of Physics, Ain Shams University, Cairo, Egypt. 3 Physics, College of Eng., Math. & Phys. Sciences, University of Exeter, Exeter EX4 4QL, UK s.bending@bath.ac.uk Since the first isolation of graphene in 2004 several theoretical [1] and experimental [2,3] works have addressed the problems of superconductivity and the superconducting proximity effect in it. Here we describe our experiments in this field that include studies of both the proximity effect in single and fewlayer graphene flakes as well as the superconducting transition in few unit cell chalcogenide flakes. Graphene structures with superconducting Al electrodes have been realised by micromechanical cleavage techniques on Si/SiO2 substrates. Devices show good normal state transport characteristics, efficient back-gating of the longitudinal resistivity, and low contact resistances. Proximity-induced critical currents are being investigated in devices with junction lengths in the range 250-750nm, and characterised as a function of temperature, back gate voltage (carrier type ± electron/hole) and magnetic field. In addition this work has been extended to investigations of two-dimensional superconducting crystals of NbSe2 produced using the same techniques used for graphene except with normal Au electrodes. While very thin NbSe2 flakes do not appear to conduct, slightly thicker flakes are superconducting with a Tc that is only slightly depressed from the bulk value (7.2K). The resistive transition shows a rather sharp high temperature transition to about 50% of the normal state resistance followed by a much broader tail-like transition to zero resistance at very low temperatures, and exhibits a dependence on back-gate voltage, albeit a relatively weak one. The behaviour of several flakes has been characterized as a function of temperature and applied field. The sharp high temperature transition could be indicative RI WKH IRUPDWLRQ RI VXSHUFRQGXFWLQJ RUGHU LQ LQKRPRJHQHRXV µSXGGOHV¶ ZKLOH WKH ORZ WHPSHUDWXUH WDLO LV reminiscent of a Berezinskii±Kosterlitz±Thouless (BKT) transition due to the binding/unbinding of vortexantivortex pairs. Our results will be analysed in terms of available theories for these phenomena. References [1] M. V. Feigel'man et al., Solid State Communications 149, 1101 (2009). [2] H. B. Heersche et al., Nature 446, 56 (2007). [3] A. Kanda et al., Physica C 470, 1477 (2010).
Large area Micro- and Nanostructuring of Graphene on various Substrates using Nanoimprint Lithography 1
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Iris Bergmair , Wolfgang Hackl , Maria Losurdo , Maria Giangregorio , Giovanni Bruno , Christian 3 3 3 4 5 Helgert , Thomas Pertsch , Ernst-Bernhard Kley , Thomas Mueller , Thomas Fromherz and Michael 1 Muehlberger 1
Functional Surfaces and Nanostructures, Profactor GmbH, Im Stadtgut A2, 4407 Steyr-Gleink, Austria iris.bergmair@profactor.at 2 Institute of Inorganic Methodologies and of Plasmas-CNR, via Orabona, 4, 70126 Bari, Italy 3 Institute for Applied Physics, Friedrich-Schiller-Universität Jena, Max Wien Platz 1, 07743 Jena, Germany 4 Institute of Photonics, Vienna University of Technology, Gußhausstraße 25-29, 1040 Wien 5 Institute of Semiconductor and Solid State Physics, Johannes Kepler University of Linz, Altenbergerstr. 69, 4040 Linz, Austria
In this work we demonstrate the micro- and nanostructuring of graphene using UV-based Nanoimprint Lithography (NIL) on nickel, copper or silica substrates. Exfoliated [1] as well as chemical vapor deposited (CVD) graphene [2][3] was used to demonstrate that our technique is suitable for large-area patterning up to 1 x 1 cm². Feature sizes down to 20 nm were achieved by a wafer-scale process which opens up new possibilities for low-cost and high-throughput manufacturing of graphene-based devices for high frequency applications [4], graphene optoelectronics [5], [6] photonics [7], plasmonics [8]. The most frequently reported method to structure graphene is e-beam lithography [9] with a low throughput. NIL allows fast nanopatterning of structures on large areas and is therefore a suitable technique for future mass production. In the last years few approaches have been started to achieve structured graphene using NIL. Liang et al. have reported a method using exfoliation of graphene layers with a patterned graphite stamp [10] and electrostatic assisted exfoliation [11]. Moreover, first steps were undertaken by the same group to achieve nanopatterned graphene by thermal NIL on top of electrostatically exfoliated graphene flakes and subsequent oxygen-assisted etching [12]. One drawback of all these methods is the dependency on random graphene flakes which furthermore were subject to a varying number of layers. Our work represents the first comprehensive investigation of a potentially low-cost, direct imprint process capable of achieving large areas of micro- and nanostructured graphene showing a UV-based NIL process (Figure 1) on exfoliated graphene (Figure 2), on CVD graphene on nickel and copper substrates over 1 x 1 cm² [2], [3] (Figure 3) and patterning of CVD graphene transferred from copper onto silica (Figure 4). For the results shown here a two layer resist system (LOR1A and mr-UVCur06) was spin coated on a graphene substrate (Figure 1(a)). The mr-UVCur06 is structured using UV-based NIL on 2.5 x 2.5 cm² and the pattern is transferred to the substrate by reactive ion etching using oxygen (Figure 1(b)). Afterwards the LOR1A is dissolved in a developer such that the structured graphene layers remain (Figure 1(c)). The processed graphene films 3
2
show electron mobilities of up to 4.6 10 cm /Vs, which confirms them to exhibit state-of-the-art electronic quality. th
The authors acknowledge funding by the European Community's 7
Framework Programme under
grant agreement 228637 (NIM_NIL: www.nimnil.org). The Austrian authors acknowledge additional
funding by the NILgraphene project within the NILaustria research project cluster. References [1] K.S. Novoselov et al., Science 306 (2004) 666. [2] M. Losurdo et al., J. Phys. Chem. C 115 (2011) 21804. [3] M. Losurdo et al., Phys. Chem. 13 (2011) 20836. [4] Y.M. Lin et al., Science 327 (2010) 662. [5] T. Mueller et al., Nature Photon. 4 (2010) 297. [6] M. Liu et al., Nature 474 (2011) 64. [7] A. Vakil and N. Engheta, Science 332 (2011) 1291.
[8] F. Koppens et al., Nano Lett. 11 (2011) 3370. [9] A.K. Geim et al., Nature Mater. 6 (2007), 6, 183. [10] X. Liang et al., Nano Lett, 7 (2007) 3840. [11] X. Liang et al., Nano Lett, 9 (2009) 467. [12] X. Liang et al., Nano Lett. 10 (2010) 2454.
Figures
Figure 1: Schematic drawing of UV-based NIL structuring process of graphene using (a) spin coating of resists, (b) Imprinting and etching, (c) lift-off of resists and remaining patterened graphene.
Figure 2: Structured graphene with feature sizes down to 20 nm (dark area) and a period of 600 nm in either lateral direction.
Figure 3: Microstructured graphene on copper substrate with a patterned area of 1 x 1 cm².
Figure4: CVD graphene transferred on silica and microstructured by UV-based NIL.
Assembling graphene with diamond as novel platforms for biointerfacing and photovoltaics P. Bergonzo, J.A. Garrido, K.P. Loh
CEA, LIST, Diamond Sensors Laboratory, F-91191 Gif-sur-Yvette, France.Organization, philippe.bergonzo@cea.fr
Recent advances in the fabrication of diamond and graphene have established these carbon materials as the most promising candidates on the roadmap for the next generation building blocks for the electronics industry. Here we consider processes and advantages in the integration of both these materials into a common platform for biointerfacing with neural tissues as well as for photovoltaics. The first structures considered here address neurointerfacing devices combining diamond and graphene microelectrode array networks. These assemblies can be used in applications aiming to repair the nervous system following an accident or disease, notably to correct the loss or impairment of eyesight (through retinal degeneration) or hearing (through damaged cochlea). Traumatic spinal injuries, drug-resistant epilepsies, psychiatric disorders and chronic neurodegenerative pathologies can also be treated (in the cortex) with such reconstructive approaches. There is a need to create better implantable devices through the use of improved interfacing between the electronic implants and living cells. For such, the involvement of highquality, low-cost, carbon-based materials constitutes a real breakthrough. Diamond and graphene are well-adapted for use in medical implants, because they (i) offer a wide range of electronic properties (metal, semiconductor and insulator), (ii) are bio-inert and (iii) are physically robust. In the case of diamond, notable advances included the fabrication of microelectrode arrays (MEAs) for cell and tissue interfacing with ‘soft’ diamond implants (diamond on polymeric materials). Devices were evaluated in laboratory animals for retinal stimulation demonstrating their robustness. These diamond implants considerably reduced the formation of glial scar tissues, enabled stimulation currents to be raised by more than one order of magnitude before causing visible chemical alteration, and enabled long lasting operation with reduced biofouling. The introduction of atomic layers of graphene onto diamond surfaces is expected to improve the device performance, while providing highly sensitive recording capabilities and maintaining the advantages of diamond implants, including operational stability. In parallel, for photovoltaic applications, there are fundamental reasons why integrating diamond and graphene, along with organic or inorganic photoactive materials, can be advantageous in many applications. The diamond – graphene – organic interfaces present unique properties such as energy offsets that favour fast charge transfer for photovoltaics, and it combines high carrier mobility, ultrahigh thermal conductivity with thermal and chemical robustness. One of the point to improve to take advantage of structures targeting photochemical properties of graphene/quantum dots is to be able to couple them efficiently on a transparent substrate that provides electrical conductivity without altering the all-carbon nature of the graphene interface. Here we aim at designing, fabricating and assembling solar devices on a unique all carbon electronics platform, based on graphene sheets hybridised on conducting and transparent diamond electrodes.
Direct imaging of atomic-scale ripples in few-layer grapheme 1
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Sagar Bhandari , Wei L. Wang , Robert Westervelt , Efthimios Kaxiras 2 School of Engineering and Applied Sciences, Department of Physics, Harvard University, Cambridge, MA 02138, U.S.A sbhandar@fas.harvard.edu
1
Graphene has been touted as the prototypical two-dimensional solid of extraordinary stability and strength. However, its very existence relies on out-of-plane ripples as predicted by theory [1] and confirmed by experiments [2,3]. Evidence of the intrinsic ripples has been reported in form of broadened diffraction spots in reciprocal space [2,3], in which however all spatial information is lost. Here we show direct real-space images of the ripples in a few-layer graphene (FLG) membrane resolved at the atomic scale using monochromated aberration-corrected transmission electron microscopy (TEM). The thickness of FLG amplifies the weak local effects of the ripples, resulting in spatially varying TEM contrast that is unique up to inversion symmetry. We compare the characteristic TEM contrast with simulated images based on accurate first-principles calculations of the scattering potential. Our results characterize the ripples in real space, and suggest that such features are likely common in ultra-thin materials, even in the nanometerthickness range. References: [1] Nelson, D. R. & Peliti, L. Fluctuations in membranes with crystalline and hexatic order. J PhysParis, 48, (1987) 1085-1092. [2] Meyer, J. C. et al. The structure of suspended graphene sheets. Nature, 446 (2007) 60-63. [3] Lee, C., Wei, X. D., Kysar, J. W. & Hone, J. Measurement of the elastic properties and intrinsic strength of monolayer grapheme, Science, 321 (2008) 385-388.
Plasmonic gold nanoparticle deposition on pristine and functionalized graphene Giuseppe V. Bianco, Maria M. Giangregorio, Maria Losurdo, Pio Capezzuto, Giovanni Bruno, Institute of Inorganic Methodologies and of Plasmas, IMIP-CNR, via Orabona 4, 70126 Bari, Italy giuseppevalerio.bianco@cnr.it Nowadays, there is no doubt that materials of the graphene family (single layer graphene, SLG, few layer graphene, FLG, graphane, graphene nanoribbon, GNRs, reduced graphene oxides, RGO, graphene oxide, GO, epitaxial graphene, EG) can be successfully employed in a variety of hybrid structures that take advantage of their unique optical and electronic properties. It is expected, among others, that the marriage between graphene and plasmonic gold nanoparticles (AuNPs) can yield novel functional nanostructured materials which can be fully exploited in a variety of applications spanning from optoelectronics, such as organic photovoltaics and light emitting devices, to sensors, such as bio-and gas sensors also in SERS devices. In this contribution we prepare AuNPs/graphene hybrids by combining different approaches as detailed: x three main methods have been used to fabricate grapheneÂąAuNPs composites: - direct vapor growth of AuNP by PVD, both sputtering and evaporation on the graphene surface - solution growth of AuNP by reduction of HAuCl4 on graphene. - anchoring of solution gold nanoparticles on functionalized graphene. x The graphene has been obtained by both the Cu catalyzed CVD and the Si sublimation of SiC. -The CVD graphene sheets were obtained by CH 4/H2 at 900 °C on Cu foil and the standard transferring method using FeCl3 and PMMA on 300 nm SiO2/Si substrate [1]. -The Epitaxial graphene is produced on 6H-SiC by Si sublimation in a UHV reactor (Base -8 vacuum better than 10 torr) at 1600°C.[2] We have identified few-layer (<5 layers) graphene sheets using spectroscopic ellipsometry and attenuation SiC Raman peak method. Raman spectroscopy has been used to fingerprint the graphene layer quality, and some representative spectra are shown in Fig. 1. The aim is at engineering both optical properties, i.e., transparency, plasmon resonance energy and amplitude, and electrical properties, i.e., conductivity and carrier density by exploiting the charge transfer processes in the Au NPs/graphene hybrids. Here we show for the first time the dynamic of the surface plasmon resonance (SPR) of AuNPs/graphene/ obtained by Au sputtering onto EG and CVD-graphene, allowing tailoring the SPR energy and amplitude to the target application (SERS-sensor or PV) (Figure 2). For this kind of hybrids, an upshift for both the G and 2D peaks of graphene is observed which is consistent with a p-type doping of graphene as reported by others [3]. We present the correlation existing between the Au NPs size and graphene p-type doping mutual affecting the SPR characteristics. A comparative discussion about Au NPs effects and doping realized by the wet HAuCl4 chemistry will be presented. Furthermore, we also investigated and present data on the assembling of colloidal Au NPs on molecular functionalized graphene. Specifically, in order to allow uniform coating of AuNP on the pristine graphene, functionalization is necessary to induce uniform surface groups as active anchoring sites. We discuss the effect of various functionalizing molecules including CTAB, aliphatic (dodecanthiol) and aromatic (BZT) the difference in between the categories being the additional coupling of the aromatic system of the functionalizing layer with graphene. Specifically, the Benzyl thiol (BZT) is an excellent candidate for selective functionalization of graphene surfaces owing to its planar conjugated ring (BZ) system and its thiol (-SH) functionality. While BZ partition adheres to gUDSKHQH OLNHO\ YLD Ę&#x152;-Ę&#x152; VWDFNLQJ and hydrophobic forces, the terminal thiol group reacts with gold to strong covalent AuS bond. The Raman analysis on those systems shows that the aromatic BZT link between the graphene and the Au NPs enhances the charge transfer and p-type doping of graphene. We also observe an enhanced SERS effect of both graphene modes and BZT mode providing evidence of an effective coupling of the SPR of Au NPs. On the other hand, in the case of the CTAB linker, there is no effective charge transfer to Au NPs and n-type molecular doping by the CTAB self-assembled monolayer occurs confirmed also by the downshift of the G-band. Thus, the plasmonic properties of Au NPs and doping effects can be tailored independently. References [1] X. Li, W, Cai, J. An, S. Kim, J. Nah, D. Yang, L. Colombo, R.S. Ruoff, Science 324 (2009) 1312. [2] C. Berger, Z. Song, T. Li, X. Li, A.Y. Ogbazghi, R. Feng, Z. Dai, A.N. Marchenkov, E.H. Conrad, P.N. First, W.A. de Heer, J. Phys. Chem. B, 108 (2004) 19912. [3] F. Schedin, E. Lidorikis, A. Lombardo,V. G. Kravets, A. K. Geim, A. N. Grigorenko, K. S. Novoselov, A. C. Ferrari, ACS Nano, 4 (2010), pp 5617Âą5626 [4] S. Shivaraman et al. J. Electron. Mater. 38 (2009) 725
Contribution: Oral
CVD Graphene on SiO2 2D
Epitaxial Graphene on SiC 1594 cm -1 2707cm -1
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Figure 1: Raman spectra of (a) EG on C-face SiC by the SiC sublimation method; the inset shows the difference spectrum between SiC substrate and EG/SiC and of (b) on Cu-foil by CVD from CH4-H2. For EG/SiC the attenuation of substrate Raman intensity is S=0.88 corresponding to estimated thickness = 4 layers [4]
Au NP/Graphene/SiC 2.6
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Figure 2: Real time dynamics of the surface plasmon resonance (SPR) peak of Au nanoparticles deposited on epitaxial graphene/SiC and on CVD graphene transferred on Al 2O3 monitored by ellipsometric spectra of the imaginary part of the dielectric function. The AFM morphologies show the different aspect ratio of the AuNPs grown on the different graphene.
SERS of BZT Ȝ laser 632 nm Matching SPR of Au NPs
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Figure 3: Colloidal Au NPs linked to graphene through the BZT self assembled layer; the Raman spectra show the SERS enhancement of the graphene modes (green laser at 532 nm) and the SERS enhancement of the BZT thiol modes (red laser at 633 nm) matching Au NPs SPR.
Interaction of epitaxial graphene with SiC substrate studied by Raman spectroscopy 1,2
K.Grodecki
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, J.A.Blaszczyk , A.Drabinska , W.Strupinski , A.Wysmolek 5 6WÄ&#x160;SQLHZVNL , and J. M. 1,2 Baranowski 1
Faculty of Physics, University of Warsaw, Poland Institute of Electronic Materials Technology, Warsaw, Poland 3 Institute of Microelectronics and Optoelectronics, Warsaw University of Technology, Poland 4 Institute of Heat Engineering, Warsaw University of Technology, Poland 2
The micro-Raman spectroscopy was used to investigate interaction of two types of epitaxial graphene with 4H-SiC(0001) substrates. The first type of graphene structures were grown using classical Si sublimation from SiC. The second type of graphene was obtained by chemical vapor deposition (CVD) of carbon from propane on SiC substrate. The crucial element of the later technique is Si sublimation blocking by controlling argon pressure in the reactor . Two type of experiments have been performed using micro- Raman spectroscopy. The first experiment based on micro-Raman maps of 2D band with submicron spatial resolution. Micro-Raman maps performed on 2x2mm area (100 points) for the CVD sample, of one monolayer thickness, -1 -1 showed that the average energy of 2D band is of about 2695cm , thus only 15cm blue shifted in comparison to freestanding graphene. On the other hand for the sublimated sample of monolayer -1 -1 thickness, the average 2D band energy was of about 2740cm , which is strongly blueshifted (60cm ) with respect to the freestanding material. Thus, the blueshift for sublimated graphene is much larger than for the CVD samples. That suggests that strain in the graphene obtained by sublimation method is much stronger. The second type of experiments were focused on temperature dependence of the 2D Raman 0 0 band. The measurements were performed in the temperature range between 20 C to 150 C. The obtained results showed that the 2D band position is red shifting with the temperature increase for both kind of graphene samples and the observed shift rate depend on graphene thickness. For the -1 0 sublimated graphene the temperature induced shift of the 2D band varied from -0.11 to -0.17 cm / C. On the other hand, for the CVD grown graphene temperature induced shift of the 2D band varied -1 0 from -0.034 (close to the value obtained for freestanding material) to -0.095cm / C. These results confirm that graphene grown by CVD technique interacts much weaker with the SiC substrate, and is not so strongly pinned to SiC surface as graphene grown by sublimation technique. This work has been partially supported by Polish Ministry of Science and Higher Education projects 670/N- ESF-EPI/2010/0 and 671/N- ESF-EPI/2010/0 within the EuroGRAPHENE programme ÂŤEPIGRATÂťof the European Science Foundation and Didactic Development Program of the Faculty of Power and Aeronautical Engineering of the Warsaw University of Technology.
Single Molecule Magnets on Graphene 1
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Lapo Bogani , C. Cervetti , A. Cornia , E. U. StĂźtzel , S. Rauschenbach , 4 1 2 2 F. Luis , M. Dressel , K. Kern , M. Burghard 1
1. Physikalisches Institut, Universität Stuttgart, Pfaffenwaldring 57, 70550 Stuttgart, Germany 2
3
Max Planck Institut fĂźr FestkĂśrperforschung, Heisenbergstr. 1, 70569 Stuttgart, Germany
Dipartimento di Chimica, UniversitĂ di Modena e Reggio Emilia, Via Campi 183, 41100 Modena, Italy 4
Instituto de Ciencia de Materiales de AragĂłn, E-50009 Zaragoza, Spain lapo.bogani@pi1.physik.uni-stuttgart.de
Carbon nanostructures are widely recognized as the roots of future electronics showing remarkable mechanical, transport and chemical properties. Graphene in particular is attracting increasing interest being the set of exotic phenomena i.e. non-local quantum effects, evanescent wave transport and anomalous Hall effect, which only now start to be deepened and could pave the way for unexpected applications. $GGLWLRQDOO\ JUDSKHQHÂśV WZR-dimensional electron gas is directly exposed to external agents, so that multifunctional nanodevices created by chemical functionalization can be envisaged. However what governs the assembly of macromolecules on graphene and the resulting effect on the hybrids is still largely unknown. Here we present hybrid nanodevices made of different types of graphene and magnetic molecular clusters exploiting non covalent self-assembly. The properties of the hybrids are investigated by means of AFM, Matrix-DVVLVWHG 72)06 5DPDQ Č?-SQUID and Transport measurements. We show that the resulting molecular structures can be governed by the deposition conditions and that the nature of the graphene surface plays a fundamental role. Eventually, using residual molecular mobility we create molecular random networks on graphene, demonstrating self-reorganization under external stimuli, and we show how the resulting organization affects the magnetization dynamics of the hybrids.
References [1] L. Bogani et. al. Nature Materials 7, 179 (2008) [2] L. Bogani et. al. Angew. Chem. 121, 760 (2009) [3] M. Burghard et. al. Adv. Materials 21, 2586 (2009) [4] A. Cornia et. al. Angew. Chem. 116, 1156 (2004) [5] D. Gatteschi et. al. Angew. Chem. 42, 268 (2003) [6] H. Shin et. al. Adv. Funct. Mat. 19, 1987 (2009) Figures
Fe4 SMM grafted on graphene.
Exfoliation and Sorting of Graphite flakes and inorganic two-dimensional materials 1
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F. Bonaccorso , F. Torrisi , G. Privitera , V. Nicolosi , T. Hasan , G. Savini , N. Pugno , A.C. Ferrari 1
1
Department of Engineering, University of Cambridge, JJ Thomson avenue, Cambridge CB3 0FA, UK 2 School of Chemistry, School of Physics & CRANN Trinity College Dublin Dublin 2 Ireland 3 Dept. of Structural Engineering, Corso Duca degli Abruzzi 24, 10129 Torino, Italy fb296@cam.ac.uk
Liquid-phase exfoliation of graphite [1] is a promising tool for mass production of single and multi-layer graphene flakes, as well as inks [2], thin films [1], and composites [3,4]. Here we report high yield production of graphene via low power sonication of graphite in sodium deoxycholate (SDC) followed by ultracentrifugation. There are two main approaches to ultracentrifugation: sedimentation-based separation (SBS) and isopycnic separation. The former discriminates particles by their difference in mass. The latter exploits density differences between particles in a density gradient medium [5,6]. Our results suggest that graphite exfoliation via sonication produces flakes with lateral sizes increasing with the number of layers. We thus exploit SBS to separate graphite flakes by number of layers [8]. TEM and Raman spectroscopy indicate that ~65% of the flakes produced by SBS are monolayer with average 2 size ~600nm [9,10]. Isopycnic separation allows us to obtain larger flakes than SBS. This requires the creation of density differences between flakes with different number of layers. Surfactants provide this density variation [11]. In this case, sorting is strongly dependent on the surface/volume ratio and the coverage and clustering of the surfactant molecules. SDC is the most effective surfactant for exfoliation and sorting of graphite flakes, with ~60% of the flakes in the topmost fraction being monolayers, with 2 average size Č?P . Ultracentrifugation can also be used to sort nanodiamonds in terms of shape and dimensions, and can also be applied to inorganic layered materials, such as Boron Nitride, Tungsten Disulfide, Molybdenum Disulfide, etc.[12]. References [1] Y. Hernandez et al. Nature Nano, 3 (2008) 563. [2] F. Torrisi, et al., arXiv:1111.4970v1 (2011). [3] T. Hasan, et al., Adv. Mater. 21 (2009) 3874. [4] T. Hasan, et al. Phys. Status Solidi B 247 (2010) 2953. [5] M. S. Arnold et al., Nat. Nano 1 (2006) 60. [6] F. Bonaccorso et al., Journal of Physical Chemistry C 114 (2010) 17267 [7] R. W. Fox, A. T. McDonald, P. J. Pritchard, Introduction to Fluid Mechanic, Wiley, 6 Edition (2003) ISBN 0471202312 [8] F. Bonaccorso et al. submitted (2012). [9@ 2 0 0DUDJRÂś et al., ACS Nano, 4 (2010) 7515. [10] F. Bonaccorso et al., Nature Photonics 4 (2010) 611. [11] A. A. Green et al, Nano Lett. 9 (2009) 4031. [12] J.N. Coleman et al. Science 331 (2011) 568.
Ambient p-doping of CVD graphene Alberto Boscá, D. López-Romero, J. Martínez, J. A. Garrido, F. Calle. ISOM, Instituto de Sistemas Optoelectrónicos y Microtecnología, E.T.S.I de Telecomunicación, Universidad Politécnica de Madrid, 28040 Madrid, Spain albosca@isom.upm.es CVD graphene is a versatile material, which high mobility and large area make it very promising for electronics, touchscreens, optoelectronics, etc. One of the drawbacks of this material is that the 1
transference process always leaves some polymer residues . Among other, these defects contribute to shift the Dirac point from 0 V to high voltage. There are several methods for removing those residues, 2
such as heating with Ar/H2 flux, oxygen plasma treatment or cleaning by an electrical current . In this work, we use the latter procedure of current cleaning in vacuum conditions, and reduce the Dirac point voltage, from high values (above 90 V) to 40 V. We also quantify the p-doping that occurs while removing the sample from vacuum. The initial material is CVD graphene transferred to SiO 2/doped Si substrate. We use Hall bar structures because they have large area, what is useful for measuring how gases affect the material (fig. 1). The device is then patterned using optical lithography and reactive ion etching (RIE) oxygen plasma. After this step, in some of the samples we deposit a thin layer of aluminum (4 nm thick) via e-beam evaporation under low vacuum conditions. The aluminum is oxidized in ambient conditions after removing the samples from the low vacuum chamber, resulting in an AlOx layer. Better contrast between graphene and the substrate is achieved after this process. Pads lithography and metal deposition (5 nm Ti/ 50 nm Au) is done for the samples without cap layer. For the samples with cap layer, diluted KOH solution is used after the pads lithography for removing the aluminum oxide layer before the metal deposition. This way we assure that we have a good contact surface. Finally we have two types of sample, very similar except for the cap layer. The doped Si substrate will be used as back gate in both types of samples. A Janis probe station is used for measuring the different samples at room temperature in vacuum (10
-4
Torr). A 4145B semiconductor parameter analyzer with ICS software is used for programming the time domain measurements as well as the current induced cleaning. A comparison of I /VG characteristics before and after the current cleaning for two different samples is shown in fig. 2. Once the sample is cleaner, the chamber is exposed to the normal atmosphere. The Dirac point voltage shift is measured in different times. Fig. 3 shows a comparison between capped and uncapped samples. The shift increases very fast until it reaches a saturation limit. A fit of the data to exponential decay-like functions provides time constants of around 3 minutes for the uncapped layer sample, and 10 minutes for the capped sample, showing that it is more difficult for the gases to contact with graphene when this kind of cap layer is used. More work is underway to optimize the cap layer (e.g., increasing the thickness of AlOx), so that graphene may keep the same behavior in vacuum and open air.
Acknowledgements are due to the Spanish Ministery of Science and Innovation for support by project TEC2010-19511. References [1] Lin, Y.-C. et al, Nano Letters, 12 (2012) 414-419. [2] Moser, J. et al, Applied Physics Letters, 91 (2007) 3.
Figures
Fig.1: Device layout
Fig. 2: Comparison before and after the current annealing for the two types of samples
Fig.3: Dirac Point shift produced by atmospheric p-doping
Raman scattering in single layer MoS2: Phonon bandwidths, zone edge phonons and 2D effects.
Mohamed Boukhicha, Karim Gacem, Mykhaylo Antal, Matteo Calandra, Abhay Shukla UniversitĂŠ Pierre et Marie Curie-Paris 6, CNRS-UMR7590, Institut de MinĂŠralogie et de Physique des Milieux CondensĂŠs BoĂŽte Courrier 115, 4 Place Jussieu, 75252 PARIS cedex 05, Paris, France
med.boukhicha@yahoo.fr
The discovery of graphene has opened up a wide domain of investigation related to the properties and characteristics of two dimensional materials. Raman scattering has been particularly useful in the case of graphene for identifying the number of layers as well as for studying effects such as electronphonon coupling, doping, and the formation and concentration of defects. Molybdenum disulfide is a dichalcogenide that can be prepared in single or few layer configurations by the anodic bonding method [1,2], (Figure1). These samples are stable and insensitive to chemical changes so that fundamental vibrational properties can be investigated using Raman scattering. In this work we measure phonons in single, few layer and bulk MoS2 using single and multiple phonon Raman scattering (Figure2). We confirm that the variation of E2g and A1g Raman shifts seen by Lee et al. [3] is a reliable measure of the number of layers in this material. We compare experiments with ab-initio phonon calculations and address some open questions concerning two phonon Raman scattering in MoS2 and subtle differences between the bulk and monolayer phonon spectra.
[1]Shukla, A.; Kumar, R.; Mazher, J.; Balan, A. Graphene Made Easy: High Quality, Large-Area Samples. Solid State Commun. 2009, 149, 718¹ 721 [2]Balan, A.; Kumar, R.; Boukhicha, M.; Beyssac, O.; Bouillard,J.-C.; Taverna, D.; Sacks, W.; Marangolo, M.; Lacaze, E.;Gohler, R.; et al . Anodic Bonded Graphene. J. Phys. D-Appl.Phys. 2010, 43, 374013 [3]C. Lee ; Hugen Yan ; Louis E.Brus; Tony F.Heinz ; James HRQH 6XQPLQ 5\X ³$QRPDORXV /DWWLFH Vibrations of Single-and Few-/D\HU 0R6 ´ $&6 QDQR QR ¹2700.
Figure1: AFM image of 1, 2 and 3 layers of MoS2.
Figure2: E2g and A1g lines evolution as a function of
number of layers.
Catalytic properties of imperfect graphene: first principles modeling Danil Boukhvalov School of Computational Sciences, Korea Institute for Advanced Study (KIAS), Seoul 130-722, Korea danil@kias.re.kr Recent experimental results evidence unusual catalytic activity of various forms of imperfect graphenes. In our works we have considered three different types of experimentally observed reactions over graphene substrate: (i) oxygen reduction reaction over nitrogen-doped graphene [1]; (ii) oxidation and hydration of alcohols and other compounds over graphene oxide (GO) [2] and (iii) hydration of magnesium over carbon coated Fe and Ni nanoparticles [3].
For the oxygen reduction reaction we performed a modeling of step-by-step oxygen reduction reactions over pure and N-doped graphene for the case of 2 and 4% of nitrogen content [4]. Results of our calculations (see Fig. 1) evidence that 4% N-doped graphene is better than conventional Pt-catalysts from the energetics point of view. Corrugation of N-doped graphene sheets provides diminishment of the oxygen load.
Based on experimental results about significant reduction of GO during oxidation or hydration of various chemical species we propose the model of catalytic properties of this compound. Based on the results of calculations we found that this reactivity stemmed from the transfer of hydrogen atoms from the RUJDQLF VXEVWUDWH WR WKH *2 VXUIDFH ,Q SDUWLFXODU HSR[LGH JURXSV GHFRUDWLQJ *2ÂśV EDVDO SODQH ZHUH ring-opened, resulting in the formation of vicinal diols, followed by dehydration. Consistent with the experimentally observed dependence of this chemistry on molecular oxygen, our computations revealed that the partially reduced catalyst was able to be recharged by oxygen, allowing for catalyst turnover. Functional group-free carbon materials, such as graphite, were calculated to have substantially higher reaction barriers, indicating that the high chemical potential and rich functionality of GO are necessary for the observed reactivity. [5]
Combined soft X-ray and theoretical exploring of carbon coated transitional metals nanoparticles demonstrate that the carbon shells of these materials are the multilayer graphene with significant amount of Stone-Wales (SW) defects [6]. Based on our previous computational results about enhancement of the chemical activity of graphene with SW defects [7] we performed systematic survey of the perfect and imperfect (with SW defects) graphene monolayer and trilayers over transitional metal substrates and without its. From the calculated chemisorption energies (see Fig. 2) we can conclude that graphene multilayer with SW defects and all types of graphene monolayers over transitional metal substrate is feasible for catalysis. Based on these results we can predict that graphene on Cu(111) and epitaxial graphene on SiC with established there defects [8] could be also used as catalysts for hydrogen dissociation required for the hydration of magnesium.
Based on aforementioned experimental results and our calculations we can conclude that in contrast to rather chemically inert perfect graphene and high chemical activity of fullerenes and similar compounds LPSHUIHFW JUDSKHQH ZLWK ³PRGHUDWH´ FKHPLFDO DFWLYLW\ OLJKW ZHLJKW DQG ODUJH VXUIDFH DUHD Ls very prospective materials for carbon based catalysis.
References [1] S. Yang, X. Feng, X. Wang, K. MĂźllen Angew. Chem. Int. Ed. 50 (2011) 5339; Z. H. Sheng, L. Shao, J.-J. Chen, W.-J. Bao, F.-B.Wang, X.-H. Xia ACS Nano 5 (2011) 4350; L. Qu, Y. Liu, J.-B. Baek, L. Dai, L. ACS Nano, 4 (2010) 1321. [2] D. R. Dreyer, H.-P. Jia, C. W. Bielawski, C. W. Angew. Chem. Int. Ed. 49 (2010) 6813. [3] D. W. Boukhvalov et al., (submitted). [4] D. W. Boukhvalov and Y.-W. Son Nanoscale 4 (2012) 417. [5] D. W. Boukhvalov, D. R. Dreyer, C. W. Bielawski and Y.-W. Son (submitted). [6] V. R. Galakhov, A. Buling, M. Neumann, N. A. Ovechkina, A. S. Shkvarin, A. S. Semenova, M. A. Uimin, A. Ye. Yermakov, E. Z. Kurmaev, O. Y. Vilkov, and D. W. Boukhvalov J. Phys. Chem. C 115 (2011) 24615. [7] D. W. Boukhvalov and M. I. Katsnelson Nano Letters 8 (2008) 4373. [8] D. W. Boukhvalov and C. Virojanadara Nanoscale (2012) in press. Figures
Figure 1 Free energies diagrams for oxygen reduction reactions over platinum, pure and NÂądoped graphenes.
Figure 2 Calculated chemisorption energies for step-by-step hydration of perfect and imperfect graphenes (free standing and over various transition metal substrates).
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Improvement of thermal conductivity in graphene reinforced cyanate ester resin I Bustero, G. Atxaga, S. Florez, I. Gaztelumendi, M. A. Mendizabal, B. Perez, TECNALIA, Mikeletegui, 2, San Sebastián, Spain Izaskun.bustero@tecnalia.com The advantages of composites such as light weight, corrosion resistance and easy processing make them a good material in several applications, in particular in aerospace sector. There is a big interest to increase thermal conductivity of polymer based material to be used in light weight high performance thermal management systems. In particular cyanate ester resins have been touted as potential material for structural aerospace applications. These matrices have outstanding physical properties such as low water absorption and outgassing, excellent mechanical properties, dimensional and thermal stability, low ionic contaminant concentration, good dielectric properties, and have the benefit of being single component materials. The thermal conductivity of polymers has been traditionally improved by the addition of thermally conductive fillers. High filler content is usually needed to achieve the required thermal conductivities that leads to a significant increase of the weight of the composite and represents a significant processing challenge. Carbon based filler appear to be the best candidates when lightweight is required. Nevertheless the results obtained up to now with carbon nanotubes (CNTs) are relatively low compared with expectations, According to the bibliography [1] cyanate ester composites with MWNT content up to ±1
±1
±1
25% wt exhibit a slight increase of the conductivity from 0.2 Wm K to 0,4 Wm K. In the past few years an important progress has been made in the developing of graphene based composites. Graphene possesses similar mechanical properties as CNT but has superior electrical and thermal properties. In this work two types of graphene have been incorporated into a cyanate ester resin LTM123 provided by ACG Company to enhance its thermal properties and have been compared with two ceramic fillers, micro and nanoBN. These fillers were added gradually to a fixed amount of cyanate ester resin up to its maximum acceptable load. Sample thermal conductivity was measured by Hot Disk sensors TPS 2500 S and associated software at room temperature. The best results have been achieved with a 14% wt of graphene GR122, the thermal conductivity of the ±1
±1
resin has been increased from 0.2 Wm K
±1 ±1
to 1 Wm K ±1
whereas the same concentration of graphene
±1
SG8 only increases the conductivity up to 0,4 Wm K . Significant differences have been also found in their processability and dispersability in the resin as consequence of the different quality of the graphene as it can be observed in the SEM images. The results show that graphene are more efficient filler materials for increasing the thermal conductivity of cyanate resin than conventionally used fillers.
References [1] Vincent Calard, Celeste Pereira, Antonio Vavouliotis, Stefan Forero, Laurent Pambaguian, Fellicitas Hepp, Proceedings "3rd Vienna International Conference on Nano-Technology" (2009)
Filler reference
Filler load in samples (%wt)
Thermal conductivity W/Km
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0
0,2
nanoBN (BN nanopowder (PLASMACHEM))
26.2
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100
microBN (S12- ESK)
12.6
0,5
150
Improvement (%)
14 Graphene Avanzare SG8 0,4 100 (AIGMO-179) Graphene 5.5 0,5 150 Avanzare GR122 (AIGMO-179) Graphene 14 1 400 Avanzare GR122 Table 1. Thermal conductivity values of different fillers reinforced cyanate ester resin Figures a)
b)
Figure 1. SEM images of graphene Avanzare ( a) SG8 and (b) GR122 (courtesy of Avanzare) a)
b)
Figure 2. SEM images of nanocomposite 14% wt Graphene Avanzare ( a) SG8 and (b) GR122 in cyanate ester
Hybrid Graphene – Molecular Magnet Devices for Spintronics 1
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A. Candini , C. Alvino , S. Klyatskaya , M. Ruben , W. Wernsdorfer and M. Affronte 1
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S3, Istituto Nanoscienze – CNR, via Campi 213/a 41125 Modena, Italy. Dipartimento di Fisica, Università di Modena e Reggio Emilia, via Campi 213/a 41125 Modena, Italy 3 Institute of Nanotechnology (INT), Karlsruhe Institute of Technology (KIT), 76344 EggensteinLeopoldshafen, Germany 4 Institut Néel, associé à l’Université Joseph Fourier, CNRS, BP166, 25 Av des Martyrs, 38042 Grenoble Cedex 9, France andrea.candini@nano.cnr.it
Spintronic devices interconvert spin to charge information and nanostructured carbon systems turn out to be a unique platform to build hybrid devices at the molecular scale. Here, we focus on graphene based devices used as highly sensitive magnetometers. The key idea is to realize novel spintronic nano-architectures where the electrical current is controlled by the quantum properties of few singlemolecule magnets grafted on top of the graphene layer. As a first step, we explored different strategies to graft molecular nanomagnets on carbon surfaces. For example, we developed a two-steps procedure using Sulfur-terminated amine SAM to electrostatically attach antiferromagnetic rings on HOPG [1]. Alternatively, we employed TbPc2 molecules appositely functionalized with pyrene groups that selectively graft to graphene on SiO2 [2]. By means of microRaman spectroscopy and Atomic Force Microscopy we studied the coupling interaction between the two materials, that is carried mainly by the ı -systems, preserving the integrity of the molecules and the intrinsic properties of graphene[2]. Secondly, we characterized the low temperature magnetoconductance of pristine graphene devices founding that below 1 K a magnetic hysteresis appears in the signal, when the field is swept at high enough rates (dB/dt > 10 mT) [3]. The magnetic signal does not depend on the size nor on the transport regime of the device. We attribute the origin of these hysteresis loops to the magnetization reversal of paramagnetic centers in graphene, which might originate from structural defects in the graphene layer, most probably vacancies [3]. Finally, we present the design and the realization of hybrid devices made by graphene nanoconstrictions with sizes down to 10 nm, fabricated by Electron Beam Lithography and plasma etching, decorated with TbPc2 magnetic molecules. The magnetization reversal of the molecules in proximity with graphene is detected by the magnetoconductivity of these hybrid devices, which shows the uniaxial magnetic anisotropy typical of the TbPc2 SMMs. Our results depict the behaviour of multiple-field-effect nano-transistors with sensitivity at the single-molecule level [4][5]. References [1] A. Ghirri et al., Advanced Functional Materials 20, (2010), 1552-1560. [2] M. Lopes, A. Candini et al., ACS Nano 4, (2010), 7531-7537. [3] A. Candini, et al., Physical Review B 83, (2011), 121401 [4] A. Candini, et al., Nano Letters 11, (2011) 2634-2639.
Insights into the chemical modification of graphene using diazonium salts 1
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Marcel Cecato , Mikkel Kongsfelt , Louis Nilsson , Bjarke Jørgensen , Liv HornekÌr , Steen U. 1 1 Pedersen and Kim Daasbjerg 1
Department of Chemistry, Aarhus University, 8000 Aarhus C, Denmark Department of Physics and Astronomy and Interdisciplinary Nanoscience Center (iNANO), Aarhus University, 8000 Aarhus C, Denmark mceccato@chem.au.dk
2
Graphene is an intensively investigated material due to its engaging properties like high mechanical strength, conductivity and optical transparency. Modifications of graphene resulting in defects in the 2
otherwise perfect sp -character of the graphene have been shown to open up a bandgap[1]. This result makes graphene particularly interesting for new, highly efficient electronic devices. Such graphene modifications will enable scientists to engineer the properties of this material in order to attend to specific purposes. However, this ambition requires a broad spectrum of tools. One of these might be the use of aryl diazonium salts as grafting agents. Generally, a diazonium salt is reduced by a one-electron reduction liberating nitrogen gas and forming a reactive aryl radical which immediately reacts with the surface [2]. The reduction of the diazonium salt is such a propitious process, that on many substrates it will appear spontaneously [3]. It has been shown, both theoretically and experimentally, that graphene, in different forms, can be grafted by the use of diazonium salts [4-7]. Thus, the conditions in which the graphene is produced will have an effect on its reactivity. Indeed, the reactivity of epitaxial graphene on different substrates might largely differ and the number of graphene layers was shown to be important [6]. In this contribution we discuss the reactivity of 4-nitrobenzenediazonium on epitaxial graphene. Epitaxial graphene was grown on single crystal silicon carbide by vacuum graphitization [8-9], and then grafted by immersion into a solution of 4-nitrobenzenediazonium salt for different grafting time periods. As a result, the surface was modified [Figure 1] by spontaneous grafting of the aryl radicals. The high resolution XPS spectrum for N1s [Figure 2] shows the presence of several types of nitrogen. Undoubtedly, the N1s peak at 406 eV corresponds to that of nitrogen in a nitro group [10]. The two minor peaks around 400 eV could originate from partially reduced nitro groups (i.e. hydroxylamino or amino groups) [10-11] or partially reduced azo groups [11-13]. Thereafter, the samples were further analyzed with STM and, considering the grafting times applied, it was evident that only a moderate coverage of the graphene in small islands was achieved in agreement with earlier results on HOPG surfaces[14]. These results indicate that the grafting of graphene is initiated randomly, maybe at defects, or by dimer formation as depicted by Jiang et al. [5].
.References [1] R. Balog, B. Jørgensen, L. Nilsson, M. Andersen, E. Rienks, M. Bianchi, et al., Nature Mater., 9 (2010) 315. [2] M. Delamar, R. Hitmi, J. Pinson, J. M. Saveant, J. Am. Chem. Soc., 114 (1992) 5883. [3] M.P. Stewart, F. Maya, D.V. Kosynkin, S.M. Dirk, J.J. Stapleton, C.L. McGuiness, et al., J. Am. Chem. Soc., 126 (2004) 370. [4] E. Bekyarova, M.E. Itkis, P. Ramesh, C. Berger, M. Sprinkle, W. A. de Heer, et al., J. Am. Chem. Soc., 131 (2009) 1336. [5] D.-e Jiang, B. G.Sumpter, S. Dai, Phys. Chem. B, 110 (2006) 23628. [6] R. Sharma, J. H. Baik, C. J. Perera, M. S. Strano, Nano Lett., 10 (2010) 398. [7] J. M. Englert, C. Dotzer, G. Yang, M. Schmid, C. Papp, J. M. Gottfried, H.-P. Steinrück, E. Spiecker, F. Hauke, A. Hirsch, Nature Chem., 3 (2011) 279. [8] C. Berger, Z. Song, T. Li, X. Li, A. Y. Ogbazghi, R. Feng, et al., Phys. Chem. B, 108 (2004) 19912. [9] C. Berger, Z. Song, X. Li, X. Wu, N. Brown, C. Naud, et al., Science (New York, NY), 312 (2006) 1191. [10] A. Adenier, E. Cabet-Deliry, A. Chaussé, S. Griveau, F. Mercier, J. Pinson, et al., Chem. Mater., 17 (2005) 491. [11] C. Saby, B. Ortiz, G.Y. Champagne, D. Bélanger, Langmuir, 13 (1997) 6805. [12] P. Doppelt, G. Hallais, J. Pinson, F. Podvorica, S. Verneyre, Chem. Mater., 19 (2007) 4570. [13] D. M. Shewchuk, M. T. McDermott, Langmuir, 25 (2009) 4556. [14] A. H.Holm, R. Møller, K. H. Vase, M. Dong, K. Norrman, F. Besenbacher, et al., New J. Chem., 29 (2005) 659. Figures
Figure 2.Schematic illustration of the grafting of 4-nitrobenzenediazonium salt on an epitaxial graphene surface.
Figure2. High resolution N1s XPS spectrum showing at least three nitrogen species at 406 eV, 402 eV and 399 eV. 2 B: STM image of nitro grafted epitaxial graphene on SiC, scale 500 x 500 Å .
Contribution (Poster)
Preparation of Graphene Sheets from Expandable Graphite and Their Utilization in CeramicMatrix Composites Yasemin Çelik, Ender Suvacı Anadolu University, Department of Materials Science and Engineering, Eskiúehir, Turkey ybozkaya@anadolu.edu.tr
Graphene is a promising material in the field of composites due to their unique electrical, thermal and mechanical properties. It has attracted researchers’ attention especially in polymer-matrix composites. However, the number of studies which utilize graphene in ceramic-based composites is limited [1-3]. Chemical exfoliation routes provide high yield of graphene for the composite applications. Expandable graphite is a widely used precursor material for the preparation of graphene sheets by chemical exfoliation. It can be easily prepared by intercalation of various species, such as sulphuric acid, between the layers of readily available graphite flakes [4]. In the present study, graphene sheets were prepared from expandable graphite and then utilized for the production of graphene/Al2O3 composite systems. The expandable graphite was kindly provided by Asbury Carbons (Expansion ratio: 307, Grade 3772, Lot 7335-3). After drying in a vacuum oven, it was subjected to a heat treatment at 900qC for 2 min. The as-obtained expanded graphite was then exfoliated into graphite/graphene sheets in 1-methyl-2-pyrrolidone (NMP, Merck-Emplura) by a low power sonication and large graphitic flakes were removed by a subsequent centrifugation at 600 rpm for 45 min. The resulted graphene sheets were incorporated into an D-Al2O3 powder (Taime TM-DAR) and sintered by spark plasma sintering at 1300-1500qC. Phase analyses of the expanded and exfoliated graphite samples and graphene/Al2O3 composites were performed by X-ray diffractometer (XRD, Rigaku Rint 2200, Tokyo, Japan) with CuKĮ radiation. The morphology of these samples was examined by field-emission-gun scanning electron microscopy (FEG-SEM, Zeiss Supra 50 VP). Besides the critical parameters in the production of graphene sheets from expandable graphite, the electrical conductivity and mechanical strength of the prepared graphene/Al2O3 composites will also be discussed in this presentation.
Contribution (Poster) References [1] Wang, K., Wang, Y., Fan, Z., Yan, J. and Wei, T., Preparation of graphene nanosheet/alumina composites by spark plasma sintering, Mater. Res. Bull. 46 (2011) 315-318 [2] Fan, Y., Wang, L., Li, J., Li, J., Sun, S., Chen, F., Chen, L., Jiang, W., Preparation and electrical properties of graphene nanosheet/Al2O3 composites, Carbon 48 (2010) 1743-1749 [3] Walker , L.S., Marotto, V.R., Rafiee, M.A., Koratkar, N., and Corral, E.L., Toughening in graphene ceramic composites, Acsnano, 5[4] (2011) 3182-3190 [4] Gu, W. et al., Graphene sheets from worm-like exfoliated graphite, J. Mater. Chem., 19 (2009) 33673369
Field-Effect Sensor Based on Graphene Thin Films Fabricated by Layer-by-Layer Stacking Si Chen, Patrik Ahlberg, Xin-Dong Gao, Zhi-Bin Zhang, and Shi-Li Zhang Division of Solid-State Electronics, Department of Engineering Sciences, The Ångström Laboratory, Uppsala University, P.O. Box 534, SE-751 21 Uppsala (Sweden) si.chen@angstrom.uu.se Wencai Ren and Huiming Cheng Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, P. R. China Graphene based field-effect transistors and electrochemical electrodes have recently attracted a great 2
deal of attention [1,2]. Graphene is a two-dimensional sheet comprising sp ±hybrided carbon atoms. With its charge transport properties being extremely sensitive to local environment, graphene is considered a promising channel material for ion and biomolecule sensing in electrolyte. Unfortunately, large-scale fabrication of uniform graphene remains a challenge at the present. Graphene sheets derived from widely used approaches, such as exfoliation of graphite and vapor phase growth, are normally randomly located, with a wide distribution of electronic properties. As a result, great efforts are needed for construction of biosensors based on the available graphene. Furthermore, large diversities in sensor performance also make it difficult to interpret the results from different sensing experiments.
In 2010, Kim and colleagues reported a layer-by-layer stacking technology which can produce large scale (30 inch) graphene thin films with a controlled number of layers by chemical vapor deposition on a Cu substrate [3]. Excellent uniformity and stability make these films promising channel materials in field effect sensing applications. Here we use the same approach to fabricate graphene films on plastic substrate and subsequently construct a field-effect transistor (FET) as shown in Figure 1(a). Ion sensing properties of the graphene FET (GFET) are evaluated by means of current-voltage (IDS-VGS) and capacitance-voltage (C-V) methods. Schematic representations of the experimental arrangement for the characterization schemes are shown in Figure 1(b) and Figure 3(a).
The derived films comprise 4 stacking layers in average with a transmittance of 85% and a sheet resistance of about 300 ohm/Ƒ. As found in Figure 2(a), the IDS-VGS curves of the GFET in different NaCl solutions show similar an Ion/Ioff ratio of 1.5. Furthermore, a 150-mV negative shift of the minimum conductance point is observed when increasing NaCl concentration from 0.01 to 10 mM. However, the +
IDS-VGS shift is negligible when changing the pH value of electrolyte, i.e., [H ], as seen in Figure 2(b). The C-V curves measured in different NaCl solutions are depicted in Figure 3(b). The electrical double layer capacitance, i.e., CEDL, can be viewed as a Stern layer capacitance (CStern) and a diffuse layer capacitance (CD) connected in series and calculated as 1/CEDL=1/CStern+1/CD [4]. As NaCl concentration increases, the diffuse layer becomes more compact. Consequently, CD, CEDL and the gate capacitance (CG) should increase. However, the measured CG of GFET increases when increasing NaCl concentration from 0.01 mM to 0.1 mM, and then decreases when the concentration further increases from 1 mM to 10 mM. The results indicate that the NaCl solution contains ions that can specifically absorbed to graphene surface and thereby modifying CStern. As shown in Figure 3(c), a negative shift of the C-V curve by increasing NaCl concentration demonstrates that the specifically absorbed species are +
+
positively charged, i.e., Na ions. On the contrary, H can neither specifically adsorb to graphene surface nor chemically react with the functional groups on a graphene surface. Thus, the GFET is +
insensitive to H in this case.
References [1] Y. Ohno, K. Maehashi, K. Matsumoto, Biosensors & Bioelectronics 26 (2010) 1727-30. [2] W. Yang, K. R. Ratinac, S. P. Ringer, P. Thordarson, J. J. Gooding, and F. Braet, Angew. Chem. Int. Ed. 49 (2010) 2114-2138. [3] S. Bae, H. Kim, Y. Lee, X. Xu, J.-S. Park, Y. Zheng, J. Balakrishnan, T. Lei, H. R. Kim, Y. I. Song, Y.J. Kim, K. S. Kim, B. Ă&#x2013;zyilmaz, J.-H. Ahn, B. H. Hong, and S. Iijima, Nat. Nanotech. 5 (2010) 574-578.[4] [4] V. S. Bagotsky, Fundamentals of Electrochemistry, 2nd ed.; John Wiley & Sons, Inc.: Hoboken, NJ, 2005. Figures
Figure 1 (a) Photo pictures of graphene films transferred onto a PET substrate (upper) and a GEFT with PDMS container and manually coated silver electrodes (lower), RE: Ag/AgCl reference electrode, (b) schematic representation of IDS-VGS measurement arrangement.
Figure 2 (a) IDS-VGS and IGS-VGS (gate leakage current) curves measured in different NaCl solutions, insert: shift of the minimum conductance point as a function of NaCl concentration, and (b) IDS-VGS curves measured in 0.1 M NaCl solution with different pH values, all at VDS=0.1 V.
Figure 3 (a) Schematic representation of the C-V measurement arrangement, CE: platinum counter electrode, (b) measured and (c) vertically shifted C-V curves in different NaCl solutions, insert: shift of the minimum capacitance point as a function of NaCl concentration, and (d) C-V curves measured in 0.1 M NaCl solution with different pH values.
Surface modification and patterning of graphene using PDMS-interface bonding Won Jin Choi, Cheol-Soo Yang, Jeong-O Lee* Korea Research Institute of Chemical Technology, Thinfilm Material Research Group, Daejeon, Korea jolee@krict.re.kr
There has been considerable interest in graphene patterning with feature sizes from 100 ȝP to sub10nm, since graphenes of sub-10nm scale widths have possibilities for room temperature transistor applications due to a large electronic band gaps (1-3), while graphenes of ȝP scale widths have been focused on electrodes applications due to their flexibility, transparency and exceptional electronic conductivities (4-6). Such graphene patterns have been fabricated by a conventional lithography and dry etching. Here we present a convenient and scalable method for graphene patterning by soft lithographic technique using only PDMS (poly-dimethylsiloxane) without any chemical agent. So far, graphene patterning using nanoimprinting method has been hampered by chemically inert surface of graphene. Our key strategy for the patterning of graphene using PDMS is a three-step sequence involving the PDMS diffusion, modification of PDMS surface to hydroxyl-terminated self-assembled layer and tailoring the graphene utilizing PDMS-interface bonding. Our approach, unlike earlier reported dry etching-based methods, is based on mechanically tearing the graphene. This facile and scalable method can be a platform process from 100 ȝP ZLGWK IRU HOHFWURGH WR VXE-10nm width for transistor applications.
References [1] A.Sinitskii & J.M. Tour. J.Am.Chem.Soc, 132 (2010), 14730-14732. [2] Bai, J.; Zhong, X.; Jiang, S.; Huang, Y.; Duan, X., Nature Nanotechnolgy, 5 (2010), 190. [3] Son, Y. W.; Cohen, M. L.; Louie, S. G., Phys. Rev. Lett. 97 (2006), 216803. [4] Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Zhang, Y.;Dubonos, S. V.; Grigorieva, I. V.; Firsov, A. A, Science, 306 (2004) 666-669. [5] Eda, G., Fanchini, G. & Chhowalla, M., Nature Nanotechnology, 3 (2008), 270-274. [6] Lee, C., Wei, X., Kysar, J. W. & Hone, J. Science, 321 (2008), 385-388.
Figures
Graphene patterning using PDMS without any chemical agent.
Contactless magnetoresistance in large area CVD graphene grown on SiC substrates
1
1
2
2
3
Tymoteusz Ciuk , Jerzy Krupka , Cezariusz Jastrzebski , Jaroslaw Judek , Wlodek Strupinski , Serkan 4 4 2,a Butun , Ekmel Ozbay , and Mariusz Zdrojek 1
Institute of Microelectronics and Optoelectronics, Warsaw University of Technology, Koszykowa 75, 00-662 Warsaw, Poland 2 Faculty of Physics, Warsaw University of Technology, Koszykowa 75, 00-662 Warsaw, Poland 3 Institute of Electronic Materials Technology, Wolczynska 1335, 01-919 Warsaw, Poland 4 Department of Physics, Department of Electrical and Electronics Engineering, Nanotechnology Research Center, Bilkent University, Bilkent, 06800 Ankara, Turkey a) Electronic mail: zdrojek@if.pw.edu.pl
We present contactless measurements of magnetoresistance (MR) of large area graphene films grown by 1 2,3 CVD method on semi-insulating SiC substrates . For this purpose we propose microwave technique using single post dielectric resonator operating at frequency about 13.5 GHz was used. Figure 1a and 1b shows details of the dielectric resonator we used in the experiment. Microwave measurements results have been compared with results obtained with classical DC methods showing good agreement and proves usefulness of microwave technique in magnetoresistance studies (see Fig 1b and 1d for samples details). Experiments have been performed at 4.2K in magnetic fields in the range from 0 to 7 T. Significant differences in magnetoresistance have been observed depending on the orientation and crystal structure of SiC substrates (see Fig 2). Wafer-scale epitaxial graphene was grown on three semi-insulating SiC surfaces: 4H-Si, 6H-Si, and 4H-C. Here, symbols 4H and 6H stand for silicon carbide poly types, labels Si and C denote polarities 16 -2 2 (faces). Studied samples have carrier concentrations above 3.8x10 m and mobility below 0.16m /Vs. The details of the experimental technique and interpretation of magnetoresistance will be discussed in the contribution. References: 1
W. Strupinski, K. Grodecki, A. Wysmolek, R. Stepniewski, T. Szkopek, P.E. Gaskell, A. Gruneis, D. Haberer, R. Bozek, J. Krupka, and J.M. Baranowski, Nano Lett. 11, 1786 (2011). 2 J. Krupka and W. Strupinski, Appl. Phys. Lett. 96, 082101 (2010). 3 J. Krupka, W. Strupinski, and N. Kwietniewski, Journal of Nanoscience and Nanotechnology 11(3), 3358 (2011).
FIG. 1. (color online) (a) Photograph and (b) schematic of 13.5 GHz single post dielectric resonator intended for contactless sheet resistance and magneto-resistance measurements. Draft of the sample for AC (c) and
DC (d) experiment. The electric contacts (Ti/Au) are placed at the middle of each edge of the sample. The opposite electrode are located 8mmfrom each other.
FIG. 2. (color online) Magnetoresistance for three large area graphene films grown on semi-insulating SiC with different poly-types and polarities. Insets emphasize negative relative magnetoresistance for low magnetic fields. Temperature equals 4.2K.
Quantized Charge Pumping in Graphene 1,2
2
1
1
1
2
2
M. R. Connolly , K. L. Chiu , S. Giblin , M. Kataoka , J. Fletcher , J. Griffiths , G. A. C. Jones , C. G. 2 2 Smith , J. T. Janssen 1
National Physical Laboratory, Hampton Road, Teddington TW11 0LW, UK Cavendish Laboratory, Department of Physics, University of Cambridge, Cambridge, CB3 0HE, UK mrc61@cam.ac.uk
2
The ability to control the transfer of single Dirac Fermions around a graphene circuit would enhance graphene's impact in a number of key sectors, including digital electronics, quantum information processing, and quantum metrology. In this presentation we describe the fabrication and operation of a graphene charge pump which achieves this by the adiabatic transfer of single charges between two series coupled localized states in a graphene nanostructure. The application of phase-shifted radiofrequency signals to all-graphene side-gates capacitively coupled to the nanostructure results in a single charge being transferred between the states each cycle, and generates a net current equal to the fundamental electronic charge times the drive frequency [1]. We discuss the role played by the geometry of the nanostructure on pumping and the experimental conditions required to achieve good pump performance. We also explore the quantization accuracy and robustness of the pumped current as a function of drive frequency. Quantized pumping is observed up to frequencies of several GHz, which, being much higher than previously achieved using conventional metallic or semiconductor adiabatic pumps, opens new vistas for adiabatic charge pumps in applications requiring high-frequency manipulation of single charges. References [1] Pothier, H., Lafarge, P., Urbina, C., Esteve, D., Devoret, M. H., Europhysics Letters, 17(3) (1992) 249-254.
Intrinsic terahertz magnetoplasmons in monolayer graphene I. Crassee and A. B. Kuzmenko UniversitÊ de Genève, Quai Ernest Ansermet 24, Genève, Switzerland iris.crassee@unige.ch We present experimental evidence for intrinsic terahertz plasmonic absorption in epitaxial monolayer graphene on the Si-face of SiC [1]. The plasmon originates from naturally present nanoscale defects, such as substrate terraces and wrinkles. In magnetic field, the plasmon peak splits in two modes, showing the characteristic signatures of magnetoplasmons observed previously in spatially confined two-dimensional electron gases [2]. We found that the plasmon dramatically changes the magnetooptical response of graphene. Particularly, the Faraday angle is maximum close to the plasmon frequency and therefore the rotation can be controlled not only by magnetic field but also by the plasmon frequency. Our results show that graphene is a unique material combining excellent plasmonic properties with very strong magneto-optical effects. References [1] I. Crassee et al., Submitted, (2012). [2] S.J. Allen, H.L. Stromer and J.C.M. Hwang. Phys. Rev. B, 28 (1983) 4875.
Synthesis of graphene-based nanocomposites as SERS substrates in biodetection 1,2
2
Sandra M.A. Cruz , Helena I.S. Nogueira and Paula A.A.P. Marques
1
1
TEMA-NRD, Mechanical Eng. Department, University of Aveiro, 3810-193 Aveiro, Portugal 2 CICECO, Department of Chemistry, University of Aveiro, 3810-193 Aveiro, Portugal sandracruz@ua.pt
The detection of biomarkers as genes and proteins of some pathologies like cancer should be made as early as possible to allow their treatment. Surface-enhanced Raman spectroscopy (SERS) is a technique capable of rapid label-free identification of small target analytes because of the high VHQVLWLYLW\ DQG WKH ³ILQJHUSULQWLQJ´ DELOLW\ WR SURGXFH GLVWLQFW VSHFWUD Irom molecules with similar structure and function. In addition, the preparation of samples uses a minimum quantity of expensive reagents and takes much less time than other techniques such as polymerase chain reaction (PCR) or immunoassays[1]. Substrates used in SERS are typically composed by metal nanoparticles, in particular Ag and Au, due to the strong enhancement that may be induced in the Raman signal of surface adsorbed molecules. Graphene (Gra), a 2D monolayer of sp² bonded carbon atoms in a dense honeycomb crystal structure, may induce by itself a Raman enhancement effect on adsorbed molecules [2], also the association of noble metal nanoparticles and Gra can produce an effective SERS substrate, as we have demonstrated in a previous work [3]. Our aim is to produce noble metal/graphene nanocomposites in a controlled manner in order to fine-tune their SERS activity and make them specific to certain molecules identification and quantification. In this way, we aim to contribute to the development of manageable SERS sensors for the selective detection of biomolecules in targeted research. In this communication we present results on the synthesis optimization of gold and silver/graphene nanocomposites towards SERS activity. The experimental procedures were modified to our purpose, by changing some parameters as metal and graphene oxide concentration and reduction agent composition. Generally, HAuCl4 or AgNO3 were respectively added to graphene oxide (GO) aqueous suspensions and stirred for 30 minutes, in order to promote electrostatic interactions between the oxygen functional groups of GO and the metal ions. The reducing agent was then introduced and the reaction followed under different conditions. The reducing agent composition was varied according to the literature methods (Table). The in situ metal reduction allows the simultaneous metal nanoparticles formation on the graphene sheets surface and the reduction of graphene oxide, producing a uniform substrate. The samples were then characterized by UV-visible, SEM and HRTEM. SERS spectra were recorded under the same conditions using thiosalicylic acid (SH) as analyte, in a Bruker RFS100/S FTRaman spectrometer with a 1064 nm laser. Figure shows that methods 1 and 2 produce efficient SERS substrate. We found that by changing the synthetic conditions it is possible to modulate the amount, size and morphology of the metal nanoparticles grown at the graphene sheet. Also interestingly was that the SERS signal depends on the reducing agent used. References [1] K. C. Bantz, A. F. Meyer, N. J. Wittenberg, H. Im, O. Kurtulus, S. H. Lee, N. C. Lindquist, S. H. Oh and C. L. Haynes, Phys Chem Chem Phys 13 (2011), p. 11551. [2] X. Ling, L. Xie, Y. Fang, H. Xu, H. Zhang, J. Kong, M. S. Dresselhaus, J. Zhang and Z. Liu, Nano Letters 10 (2009), p. 553. [3] G. Goncalves, P. A. A. P. Marques, C. M. Granadeiro, H. I. S. Nogueira, M. K. Singh and J. Gracio, Chemistry of Materials 21 (2009), p. 4796. [4] X. S. Zhao, J. Z. Ma, J. T. Zhang, Z. G. Xiong and Y. Yong, Journal of Materials Chemistry 21 (2011), p. 3350. [5] Y.-K. Yang, C.-E. He, W.-J. He, L.-J. Yu, R.-G. Peng, X.-L. Xie, X.-B. Wang and Y.-W. Mai, Journal of Nanoparticle Research 13 (2011), p. 5571. [6] X. Fu, F. Bei, X. Wang, S. O'Brien and J. R. Lombardi, Nanoscale 2 (2010), p. 1461.
Table
Table± Methods used for Au and Ag nanocomposite synthesis. Method
Reducing agent
[3]
Tri-Sodium citrate
[4]
Glucose and aq. ammonia sol. (30 wt.%)
[5]
DMSO (in substitution of DMF, literature)
[6]
Ethylene Glycol
1 2 3 4
Composite
SERS signal
AuGO1 AgGO1 AuGO2 AgGO2 AuGO3 AgGO3 AuGO4 AgGO4
yes yes yes yes no no no no
Figure
Raman Intensity
AuGO1_SH AgGO1_SH AgGO2_SH AuGO2_SH SH
1700 1600 1500 1400 1300 1200 1100 1000
900
800
-1
wavenumber (cm ) Figure ± SERS spectra of thiosalicylic acid using the composites synthesized by method 1 and 2. Acknowledgements: S.M.A. Cruz thank FCT for the PhD grant (SFRH/BD/68598/2010)
Scanning tunnelling spectroscopy investigations of chemical composition of graphene/Cu(111) interface P. Dabrowski a
a,b
a
a
a
b
b
, I. Wlasny , Z. Klusek , W. Kozlowski , I.Pasternak , W.Strupinski , J.Baranowski
b
Solid States Physics Department, University of Lodz,Pomorska 149/153, Lodz 90-236, Poland b
Institute of Electronic Materials Technology, Wolczynska 133, Warsaw 01-919, Poland paweldabrowski@std2.phys.uni.lodz.pl
Graphene is two-dimensional allotrope form of carbon. It possesses unique conical dispersion relation in proximity of the Dirac points, which results in massless Dirac fermion character of charge carriers. This leads to numerous amazing properties of graphene like quantum Hall effect, ballistic transport of electrons, electronic spin transport, micron scale coherence length or single electron tunneling.
Graphene physisorbed on metals regains its conical band structure in proximity of Dirac points. It has been shown, however, that it is doped by the charge carrier transfer from the substrate, which is exhibited by shift of Fermi level. Specifically, in case of graphene on Cu(111) substrate this shift has been reported to amount to about -0.4 eV [1].
Recently, it has been demonstrated that air exposure of graphene on Cu(100) surface results of oxygen intercalation between those two materials, which leads to increase of Fermi level shift up to -0.6 eV. On graphene on Cu(111) this effect is not observed, suggesting that graphene layer protects that substrate from air exposure [2].
We report on scanning tunneling spectroscopy (STS) investigations of chemical composition of graphene deposited on Cu(111) monocrystal substrate. Our sample was prepared by the CVD growth o
technique. The copper substrate was annealed in a H2/N2 gas mixture at 1020 C. Carbonization was o
conducted at 1020 C and propane gas was used as a carbon precursor. After deposition, the copper substrate with graphene films was cooled down to a room temperature in argon atmosphere.
We show that graphene monolayer prevents not only intercalation of oxygen during atmosphere exposure, but also prevents formation of compounds usually formed on surface of copper, like copper(II) carbonate and copper(II) hydroxide. Our STS measurements show that air exposed graphene on Cu(111) exhibits characteristic Fermi level shift value, however surface not protected by graphene shows signs of degradation. What is more, degraded Cu(111) surface is present also in proximity of graphene (Fig. 1). This leads to change of electronic structure of both substrate and graphene around its edges.
This work was financially supported by Polish Ministry of Science and Higher Education in the frame of grant N N202 204737.
References [1] L. Gao, J. R. Guest, N. P. Guisinger, Nano Letters, 10 (2011) 3512-3516. [2] A. L. Walter, S. Nie, A. Bostwick, K. S. Kim, L. Moreschini, Y. J. Chang, D. Innocenti, K. Horn, K. F. McCarty, E. Rotenberg, Pysical Review B, 84 (2011) 195443 Figures a)
b)
c)
Fig. 1 a) 200 nm x 200 nm STM topography of graphene ± substrate interface. b) Conductance map of the investigated sample (blue, yellow, red denote low, medium and high conductance, respectively). Areas of medium conductance represent graphene on uncontaminated substrate, orange areas are graphene on contaminated copper, blue areas are contaminated substrate. c) STS curves from respective areas: black curve ± graphene on uncontaminated substrate, red ± graphene on contaminated substrate, green ± contaminated copper.
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Effect of negative differential conductance in graphene Esaki diodes: GNR or GNM? V. Hung Nguyen, F. Mazzamuto, J. Saint-Martin, A. Bournel, P. Dollfus Institute of Fundamental Electroniucs, CNRS, Univ. Paris-Sud, UMR 8622, Orsay, France philippe.dollfus@u-psud.fr The effect of negative differential conductance (NDC) has been exploited for high frequency applications in many conventional semiconductor devices [1]. These last years, this effect has been also investigated in several graphene structures [2-8]. However, due to the lack of bandgap in 2D graphene systems [2-5] or to the influence of edge defects in narrow graphene nanoribbons (GNR) [7-8], it was shown that the NDC effect is generally weak in these structures. Recently, we have demonstrated that a strong NDC effect can be achieved in graphene PN junctions [9], and especially in GNR heterostructures with a peak-to-valley current ratio (PVR) higher than one thousand at room temperature [10]. To obtain such a strong effect, a large energy bandgap is an essential ingredient, which can be achieved, e.g., by cutting graphene into armchair GNRs or by patterning graphene nanomeshes (GNMs) where the size of nanoholes and the distance between them can be controlled down to the sub 10-nm scale [11]. Indeed, an increase of bandgap reduces strongly the valley current and may result in an increase of the PVR of NDC as proved in ref. [9]. However, such a large bandgap also results in the appearance of evanescent states in the transition region between p-doped and n-doped zones, which reduces the peak current and makes it very sensitive to the transition length. By taking advantage of the possibilities of bandgap engineering in armchair GNRs or GNMs, we demonstrate that the NDC effect can be improved significantly in GNR PN heterojunctions wherein the bandgap is large in the two junction sides while it is small in the transition region. This configuration may be obtained in GNRs as the T-shape GNR schematized in Fig. 1. Actually, in such heterostructures the interband tunneling in the peak current regime is enhanced while the valley current is maintained small thanks to the large bandgap in the doped regions. It is illustrated in the DOS and the transmission coefficient plotted in Fig.2 for a uniform-width armchair GNR and a T-shape GNR of width characterized by the numbers of dimmers MC = 21 (EG = 0.37 eV) in the doped regions and MD = 29 (EG = 0.062 eV) in the transition region. In particular, Fig.2 shows that while the evanescent states around the neutral point in the transition region are observed clearly in the simple junction (left), they are not visible in the T-junction (right). It leads to the enhancement of the interband tunnelling in the latter case, as shown in the plot of transmission probability. The resulting I-V characteristics is displayed for three junctions in Fig. 3. While the valley current is always small, the peak current and hence the PVR increase significantly when reducing EG. For instance, the PVR is about 1100 and 2300 for MD = 33 (EG = 0.233 eV) and MD = 29 (EG = 0.062 eV), respectively. Additionally, it is weakly sensitive to the transition length and to the edge disorder (not shown). However, though efficient, these GNR configurations still raise a technological challenge for their manufacturing and the current is small in single GNR. Alternatively, GNM structures (Fig. 4) may provide similar effects on large sheets with high current [12]. In GNM/pristine graphene/GNM heterostructures wherein the bandgap (of pristine graphene) in the transition region is zero, a PVR as high as few hundreds can be osberved, with a weak dependence on the transition length (Fig. 5). In this structure, the size of nanoholes and the distance between them are 3 nm and 5 nm respectively, which is not far from those already achieved experimentally [11]. These NDC effects will be discussed and compared in both cases of GNR and GNM nanostructuring.
References [1] H. Mizuta and T. Tanoue, The Physics and Applications of Resonant Tunneling Diodes, Cambridge University Press, Cambridge (1995). [2] V. Nam Do, V. Hung Nguyen, P. Dollfus, and A. Bournel, J. Appl. Phys. 104 (2008) 063708. [3] H. Chau Nguyen and V. Lien Nguyen, J. Phys.: Condens. Matter 21 (2009) 045305. [4] V. Hung Nguyen, A. Bournel, V. Lien Nguyen, and P. Dollfus, Appl. Phys. Lett. 95 (2009) 232115. [5] G. Fiori, IEEE Electron Device Lett. 32 (2011) 1334. [6] Z. F. Wang et al., Appl. Phys. Lett. 92 (2008) 133114. [7] G. Liang, S. B. Khalid and K.-T. Lam, J. Phys. D: Appl. Phys. 43 (2010) 215101. [8] V. Nam Do and P. Dollfus, J. Appl. Phys. 107 (2010) 063705. [9] V. Hung Nguyen, A. Bournel, and P. Dollfus, J. Appl. Phys. 109 (2011) 093706. [10] V. Hung Nguyen et al. Appl. Phys. Lett. 99 (2011) 042105. [11] J. Bai, X. Zhong, S. Jiang, Y. Huang, and X. Duan, Nat. Nanotechnol. 5 (2010) 190. [12] V. Hung Nguyen et al., Nanotechnol. 23 (2012) 065201 Figures (a)
N-doped
Transition
P-doped (c)
Thermionic EFG (b)
Gap(MC)
U0-qV
MD
Interband tunneling
Gap(MC)
EFR
Thermionic
MC
Figure 1. Schematic view of (a) armchair pn junction and (b) T-like pn junction with reduced bandgap in the transition region. (c) Schematic band profile of a graphene pn junction with uniform bandgap. MC = MD = 21
MC = 21 / MD = 29 5
MC = 21 / MD = 29
Current (mA)
4 3 2 MC = 21 / MD = 33 1
Simple TͲshape
0
Figure 2. Local DOS of simple (MC = MD) and T-junctions (MD > MC and (right) corresponding transmission probability.
Figure 4. Schematic view of GNM Esaki diode.
MC = MD = 21 0
0.1
0.2 0.3 Bias voltage (V)
0.4
0.5
Figure 3. I-V characteristics (a) of the T-junctions with different indexes MD. (L = 10.2 nm).
Figure 5. I-V characteristics of GNM devices with different transition lengths.
Acknowledgments: This work was supported by the ANR (projects NANOSIM_GRAPHENE and MIGRAQUEL).
Zero-energy states in graphene waveguides, quantum dots and rings
C. A. Downing
1
(1)
, R. R. Hartmann
(1)
, N. J. Robinson
(1,2)
, D. A. Stone
(1)
and M. E. Portnoi*
(1,3)
School of Physics, University of Exeter, Exeter, United Kingdom
2
Rudolf Peierls Centre for Theoretical Physic, University of Oxford, Oxford, United Kingdom
3
International Institute of Physics, Universidade Federal do Rio Grande do Norte, Natal, Brazil *m.e.portnoi@exeter.ac.uk
There is a widespread belief that electrostatic confinement of graphene charge carriers, which resemble massless Dirac fermions, is impossible as a result of the Klein paradox. We show that full confinement is indeed possible for zero-energy states in pristine graphene. We present exact analytical solutions for the zero-energy modes of two-dimensional massless Dirac fermions confined within a smooth onedimensional potential V x D / cosh E x [1]. This potential provides a reasonable fit for the potential profiles of existing top-gated graphene structures [2-5]. A simple relationship between the characteristic strength and the number of modes within the potential is found. An experimental setup is proposed for the observation of these modes (see Fig.1). Thus, we present a solution to obtaining on/off behavior within graphene, a major obstacle to device realization. A new numerical method, [6] based on the variable-phase method [7] has been developed to find the number of fully confined zero-energy modes in any smooth potential, decaying at large distances faster than the Coulomb potential. The method allows one to reformulate the Dirac-Weyl equation governing the charge carriers in graphene into a nonlinear, first-order differential for reflection coefficient. The method is numerically efficient and can easily be used to evaluate the conductivity of a channel formed by a realistic top-gate potential.
We also show that full confinement is possible for zero-energy states in electrostatically-defined quantum dots and rings with smooth potential profiles. Again, analytic solutions are found for a class of model potentials [8]. These exact solutions allow us to draw conclusions on general requirements for the potential to support fully confined states, including a critical value of the potential strength and spatial extent. We have also developed our numerical method so that we can handle two-dimensional potentials and so treat scattering problems. The implications of fully-confined zero-energy states for STM measurements and minimal conductivity in graphene are discussed.
This work was supported by EPSRC (C.A.D., R.R.H. and N.J.R.), the EU Initial Training Network Spinoptronics (Grant No. FP7-237252) and FP7 IRSES projects SPINMET (FP7-246784), TerACaN (FP7-230778) and ROBOCON (FP7-230832).
References [1] R. R. Hartmann, N. J. Robinson, and M. E. Portnoi, Phys. Rev. B 81, 245431 (2010). [2] R. V. Gorbachev, A. S. Mayorov, A. K. Savchenko, D. W. Horsell, and F. Guinea, Nano Lett. 8, 1995 (2008). [3] G. Liu, J. Velasco, Jr., W. Bao, and C. N. Lau, Appl. Phys. Lett. 92, 203103 (2008). [4] A. F. Young and P. Kim, Nat. Phys. 5, 222 (2009). [5] J. R. Williams, T. Low, M. S. Lundstrom, and C. M. Marcus, Nature Nanotechnology 6, 222 (2011). [6] D. A. Stone, C. A. Downing, and M. E. Portnoi, arXiv: 1112.4034 (2011). [7] P. M. Morse and W. P. Allis, Phys. Rev. 44, 269 (1933). [8] C. A. Downing, D. A. Stone, and M. E. Portnoi, Phys. Rev. B 84, 155437 (2011).
Figures
Fig. 1: (a) A schematic diagram of a Gedanken experiment for the observation of localized modes in graphene waveguides, created by the top gate (VTG). The Fermi level is set using the back gate (V BG WR EH DW İF = 0. (b) The HOHFWURVWDWLF SRWHQWLDO FUHDWHG E\ WKH DSSOLHG WRS JDWH YROWDJH 7KH SODQH VKRZV WKH )HUPL OHYHO SRVLWLRQ DW İ F = 0.
Molecular Dynamic Simulation of the Thermal Conductivity of Graphene and Graphene Oxide 1
1
1
2
A.V. Eletskii , I.M. Inskandarova , A.N.Knizhnik , D.N. Krasikov 1
1DWLRQDO 5HVHDUFK &HQWHU ³.XUFKDWRY ,QVWLWXWH´ 0RVFRZ 5XVVLD 2
KINTECH LAB, Moscow, Russia
The molecular dynamic simulation of the thermal conductivity of one layer and few layer graphene, and graphene oxide has been performed. As the analysis has indicated the main factors which can influence thermal conductivity of graphene paper materials include (1) interplane energy transfer in layered material, (2) intraplane thermal conductivity of each layer, (3) interlayer interaction. We found that for typical sizes of flakes in reduced graphene oxide (GO) about 10 m one can expect that interplane energy transfer will not limit the thermal transport, while intraplane thermal conductivity of ideal graphene planes can exceed 3000 W/m K. However, large concentration of oxygen defects in reduced graphene oxide (about 10 at%) should results in strong reduction of intraplane thermal conductivity: calculated critical defect density are about 0.1-1% at%. Thus based on the current knowledge of the structure of reduced graphene oxide one can not assume large thermal conductivity of GO-based materials. More detail analysis of reduced GO structure should be done to understand significance of these defects on thermal transport. Moreover, we show that thermal conductivity of layered materials can be limited by interplane interactions, which results in additional phonon scattering. To prevent these additional scattering one needs to apply careful engineering of interplane structure and direct calculations of influence of this structure on thermal transport in layered materials such as graphene paper.
Graphene Oxide Flower-like Microstructures from Carbon Nanotubes Amanda V. Ellis Flinders Centre for Nanoscale Science and Technology, School of Chemical and Physical Sciences, Flinders University, Sturt Road, Bedford Park, Adelaide, SA 5042, Australia amanda.ellis@flinders.edu.au 1
To date, graphene has been prepared by a range of methods, including hydrothermal, oxygen plasma treatment2 and electrochemical exfoliation of graphite in ionic liquids.3 However, the most facile large scale method of GO production is the prolonged ultrasonication of graphite or carbon nanotubes (CNTs) in various solvents.4,5 Using this technique the exfoliation of graphite or unzipping of CNTs can produce aromatic carbon sheets which are functionalized with -OH, -COOH and epoxy moieties, called graphene oxide (GO).6 Significantly, select solvents such as N-methylpyrrolidone (NMP) exhibit strong interactions with sp2 carbon nanostructures, including CNTs7 and graphene.8Therefore exfoliation or unzipping is possible because the energy required for this is balanced by the solvent±graphene interaction for solvents whose surface energies match that of graphene.8 By capitalizing on this interaction, this work produced GO by the unzipping of single-wall carbon nanotubes (SWCNTs) in NMP. Further we will report on the controlled generation of graphene oxide (GO) flower-like microspheres (Figure 1) by selfassembly of solutions of GO in N-methylpyrrolidone (NMP) on glass and silicon slides and propose a mechanism for this self-assembly.9 Firstly, the GO precursor was prepared in a simple, easily scalable process employing the ultrasonication of SWCNTs at ambient conditions. We found that GO was not obtained by this approach when the concentration of SWCNTs in NMP exceeds 0.3 mg/mL. The GO microspheres were characterized using Raman scattering spectroscopy which showed a disorder-induced first order VFDWWHULQJ RU ³' -VFDWWHULQJ´ EDQG10 at approximately 1447 cm -1 related to the direct result of disorder induced graphene edge effects,10 or in this case the formation of C=O moieties. UV-Visible spectroscopy showed that the GO in NMP exhibited a blue luminescence at 406 nm (3.1 eV) when 11 excited at 300 nm. Eda et al. calculated the band gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of a single benzene ring to be approx. 7 eV, which decreased down to approx. 2 eV for a cluster of 20 aromatic rings. The GO mono-layers reported in this work have a band gap of around 3.1 eV suggesting that they have sp2 domains that are 3 approximately 15 aromatic rings surrounded by sp localized states. Lastly, the GO microspheres were tested for a photoelectric response when deposited onto n-type silicon (Figure 1). Results indicated that even though the fill factor (~0.28) was low a short circuit photocurrent density of 8 PA and an open circuit photovoltage of 65 mV was observed. It is also expected that the morphology of the GO flowers will provide outstanding light trapping functionality compared to flat graphene/GO sheets. In light of this and the recent reports of graphene-on-silicon solar cells a more detailed investigation of the light harvesting ability of this material is warranted. Furthermore, the successful fabrication of these GO hierarchical morphologies provides more complex forms with extremely high surface-to-volume ratios for potentially new catalytic or scaffolding applications. References [1] Pan, D.; Zhang, J.; Li, Z.; Wu, M. Advanced Materials 22 (2010) 734. [2] Gokus, T.; Nair, R. R.; Bonetti, A.; Bohmler, M.; Lombardo, A.; Novoselov, K. S.; Geim, A. K.; Ferrari, A. C.; Hartschuh, A. ACS Nano 3 (2009) 3963. [3] Lu, J.; Yang, J. X.; Wang, J.; Lim, A.; Wang, S.; Loh, K. P. ACS Nano 3 (2009) 2367. [4] McAllister, M. J.; Li, J. -L.; Adamson, D. H.; Schniepp, H. C.; Abdala, A. A.; Liu, J.; Herrera-Alonso, M.; Milius, D. L.; Car, R.; 3UXG¶KRPPH, R. K.; Aksay, I. A. Chemistry Materials 19 (2007) 4396. [5] Valles, C.; Drummond, C.; Saadaoui, H.; Furado, C. A.; He, M.; Roubeau, O.; Ortolani, L.; Monthioux, M.; Penicaud, A. Journal of the American Chemical Society 130 (2008) 15802. [6] Eda, G.; Lin, Y.-Y.; Mattevi, C.; Yamaguchi, H.; Chen, H. -A.; Chen, I-S.; Chen, C. -W.; Chhowalla, M. Advanced Materials 22 (2009) 505. [7] Hasan, T.; Scardaci, V.; Tan, P.; Rozhin, A. G.; Milne, W. I.; Ferrari, A. C. Journal of Physical Chemistry C 11 (2007) 12594. [8] Hernandez, Y.; Nicolosi, V.; Lotya, M.; Blighe, F. M.; Sun, Z.; De, S.; Eda, I. T.; Fanchini, G.; Chhowalla, M. Nature Nanotechnology 3 (2008) 563. [9] Vogt, A. P.; Gibson, C. T.; Tune, D. D.; Bissett, M. A.; Voelcker, N. H.; Shapter, J. G.; Ellis, A.V. Nanoscale 3 (2011) 3076.
[10] Gupta, A.; Chen, G.; Joshi, P.; Tadigadapa, S.; Eklund, P. C. Nano Letters 6 (2006) 2667. [11] Eda, G.; Lin, Y. -Y.; Mattevi, C.; Yamaguchi, H.; Chen, H. -A.; Chen, I. -S.; Chen, C. W.; Chowalla, M. Advanced Materials 21 (2009) 1. Figures
Figure 1: Left - Scanning electron micrograph of self-assembled graphene oxide deposited from a solution of graphene oxide in N-methylpyrrolidone (NMP) on n-type silicon; Middle Âą optical image under white light in a 2 Raman microscope; and Right - current density (mA/cm ) versus voltage (mV) curves obtained from the selfassembled graphene oxide on n-type silicon.
Application of Ge nanowire mass sensor for graphene exfoliation 1
1
2
Donats Erts , Jelena Kosmaca , J.D. Holmes , Jana Andzane
1
1
Unversity of Latvia, Raina blvd. 19, Riga, LV-1586, Latvia Department of Chemistry, National University Ireland, Cork, Ireland Donats.Erts@lu.lv
2
Graphene is a 2D allotrope of carbon which can be derived from graphite. It plays an important role as a novel material, which is coming to replace silicon compounds from existing devices in nanoelectronics. Although best quality graphene is obtained by mechanical exfoliation of pyrolitic graphite, thickness of individual graphene samples layers has to be determined and after that samples with appropriate layers can be used for applications [1].
Here we demonstrate both simple and effective method for graphene sample selection from the pyrolitic graphite surface and manipulation with it by using a germanium nanowire as nanoelectromechanical tool. At the same time number of graphene layers of the selected sample is determined in easy and precise way before placing graphene sample on desired position on the surface.
Experimental equipment includes nanomanipulation system, which is staged inside scanning electron microscope. It supports the control and visualization of graphene sample choosing and spotting as well as other actions we want to do.
Single clamped germanium nanowires with resonance frequencies in the range from kHz up to MHz are used as nanoelectromechanical mass sensors. Oscillation of Ge nanowire is excited by oscillating electrostatic field applied between germanium nanowire and counter electrode. Nanowire resonance is observed visually in SEM images (Figure 1a). Due to a high adhesion between germanium nanowire and graphitic structures, graphene sample can be cleaved from the graphite surface involving just tiny contact area (less than 1% of graphene sample surface).
The effective mass of the selected graphene sample, placed on the end of cantilevered nanowire (Figure 1b,c), is calculated from the difference between resonance frequencies of free and loaded nanowire [2]:
,
where is mass of a uniform cantilever, is the constant for first harmonic, and are resonant frequencies of empty and loaded cantilever. For a particle of mass , attached at a distance x from the base of the cantilever, the effective mass is given by
.
The number of graphene layers in the sample is calculated from the graphene sample mass and surface area. Surface area of graphene sample is determined from SEM images. For example, mass of . -16
sample shown in Fig. 1c is 2 1
g and correspond to 2-3 graphene monolayers.
Selected grafene samples with certain number of layers can be placed in defined positions on the substrate and may be used for integration of graphene with different complex structures.
References [1] A.K. Geim, Graphene: Status and Prospects, Science, 324, 5934, (2009). [2] J. Zhou et al, Nanowire as pico-gram balance at workplace atmosphere, Solid State Communications, 139 (2006). Figures
Figure 1. Images of Ge nanowire at resonance frequency (a), and graphene samples at the end of Ge nanowire
afrer cleaving from the graphite surface (b,c).
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Aromaticity patterns in graphene nanoribbons
Stijn Fias, Francisco J. Martin-Martinez, Gregory Van Lier, Frank De Proft and Paul Geerlings Free University Brussels (VUB), Research Group General Chemistry (ALGC), Pleinlaan 2, B-1050, Brussels, Belgium sfias@vub.ac.be
We analyse the geometry, electronic structure and aromaticity of graphene nanoribbons (GNRs) and carbon nanotubes (CNTs) through a series of delocalisation and geometry analysis methods.[1] In particular, the Six Centre Index (SCI) [2-3] is found to be in good agreement with the Mean Bond Length (MBL) and Ring Bond Dispersion (RBD) geometry descriptors. Based on Density Functional Theory (DFT) periodic calculations, the type of edge and the width of the GNRs are found to be the factors determining the aromaticity pattern. Unlike zigzag GNRs, which are geometrically uniform, armchair GNRs present distinct geometrical patterns depending on their width. For this last class of GNRs, three distinct classes of aromaticity patterns have been found, the so-called incomplete-Clar (i-C), Clar (C) and KekulĂŠ (K) class, appearing periodically as the width of the ribbon is increased. The appearance of such distinct aromaticity distribution is explained within the IUDPHZRUN RI WKH &ODUÂśV 6H[WHW 7KHRU\ 7KH SCI is found to be very useful tools for easily analysing the aromaticity in graphite-like structures such as GNRs and CNTs. It has been shown that delocalisation indices like the SCI and the bond order are able to probe the aromaticity distribution starting from uniform geometries without any optimisation procedure. In this way, these delocalisation indices are able to predict the bond length distribution throughout the nanosystem without the burden of a geometry optimization. In this way, the delocalisation analysis methods are shown to be very fast and reliable tools for easily analysing the aromaticity in carbon nanosystems.
References [1] Martin-Martinez, F. J.; Fias. S.; Van Lier, G.; De Proft, F.; Geerlings, P. Chem. Eur. J. Accepted. [2] Bultinck, P.; Ponec, R.; Van Damme, S. J. Phys. Org. Chem., 18 (2005) 706-718. [3] Bultinck, P.; Fias, S.; Ponec, R. Chem.-Eur. J. 12 (2006) 8813-8818.
Figures
MBL and SCI representations for the three classes of armchair GNRs. Percentages in SCI are related to benzene (100%). Arrow in the left bottom part of the figure indicates the periodic direction.
Cu(111) epitaxial films on mica(001) substrate used for high quality graphene growth by chemical vapor deposition T. Fujii, M. Sato, A. Takigawa and Y. Ichikawa Corporate R&D Headquarters, Fuji Electric Co., Ltd., Hino, Tokyo 191-8502, Japan fujii-takesi@fujielectric.co.jp Graphene attracts much attention as a wonder material to overcome the limit of traditional materials in the field of electronics, energy storage, thermal management, MEMS and so on. In particular, application of transparent conductive film (TCF) is expected to adapt for solar cells, flat panel displays, and touch panels, because it allows us the high flexibility, cost efficiency and rare-metal free compared with Indium tin oxide. Recently, large-area with high quality graphene grown on transition metal substrates by chemical vapor deposition and transferring to arbitrary substrates have been reported in some research groups to prepare the graphene TCF
1-3)
. However, the sheet
resistance of graphene TCF deposited by CVD was not high enough to use for solar cells application. For further improvement of the quality of graphene, the crystallinity of Cu substrates such as symmetry and single crystallization must be needed. Reddy et. al. explored a single crystal Cu(111) thin film deposited on basal-plane sapphire by evaporation as a substrates, and obtained low defect graphene 4)
on them . To find a feasible way, in this study, we report that epitaxially grown Cu(111) films on synthetic mica (KMg3(AlSi3O10)F2) substrates are excellent to avoid defect creation in thin film Cu. Because has a low surface energy and the pit density could be reduced in the thin film Cu deposited on it. In addition, large-area mica substrates are easily obtained and are reusable by cleaving easily the Cu deposited surface layer at a cleavage plane of mica. These are advantages for industrialization of large-DUHD 7&)ÂśV For the growth of graphene, we used 10 cm x 10 cm synthesized mica(001) with a thickness of 0.5mm as a substrate. Cu film with 530 nm in thickness was deposited on the cleaved surface of the mica(001) by EB evaporation at 600 {C. The Cu deposited mica(001) substrate was loaded into a CVD reactor, and it was annealed for 30 min at 800 {C under the hydrogen atmosphere of 0.22 Torr for 30 min. After that, the substrate temperature was increased to 1000 {C, and turned off the hydrogen gas, then CH4 gas was introduced at a flow rate of 20 sccm. Growth of graphene was carried out for 30 min at a gas pressure 30 Torr. In order to characterize the crystalline structure of Cu(111)/mica(001), we performed X-ray diffraction (XRD) and electron backscatter diffraction (EBSD) measurements. XRD results of the Cu thin film after CVD show the Cu(111) and Cu(222) reflections with higher-order spectra from the mica(001) substrate. The lattice constant of Cu (111) was 0.2085 nm, which is in good agreement with that of bulk single crystalline. The in-plane distribution of the crystal orientation of deposited Cu examined by EBSD shows the orientation of the deposited Cu was (111) over the surface, but we observed the boundaries attributed to twin crystalline structure consisting of ABC and ABA stacking structures in fcc., The average domain size surrounded by the boundaries are 600
m or larger,
however, it is large enough to grow graphene. Moreover, AFM study revealed the Cu(111) film on Mica(001) after depositing at 1000{C maintained an atomically flat terrace with step bunching structure. Thus, it is expected to be an excellent template for growth of graphene. The Raman spectra of the graphene on the Cu(111) film at three different measurement points -1
are shown in Fig. 1. Raman spectra of the graphene have no D peak (1358cm ), and IG/I2D, is smaller than unity. These results indicate that the graphene is almost mono-layer with no defects. Moreover, Raman mapping show that the full width at half maximum of IG at the twin boundary is larger than that on the other area. This result suggests that the six-membered rings are distorted at the twin boundary
of Cu(111) to solve the discontinuity of each domain of graphene without introducing defects such as dangling bonds and/or irregular bonds. From these results, Cu(111)/mica(001) is promising substrate for a high quality and large-area graphene growth in the Cu-based CVD method. This work was supported in part by the New Energy and Industrial Technology Development Organization (NEDO) under the Ministry of Economy, Trade and Industry (METI) of Japan.
References [1] K. S. Kim, et al., Nature, 457 (2009) 706. [2] X. Li, et.al., science, 324, (2009) 1312.; X. Li, et.al., Nano Letters, 9 (2009) 4359. [3] S. Bae, et.al., Nat. Nanotech., 5, (2010) 574-578. [4] K. M. Reddy, et.al., Appl. Phys. Lett., 98 (2011) 113117 - 113117-3. Figures Fig. 1
Intensity (a.u.)
graphene/Cu(111)/Mica(001) G
2D
Point 1 Point 2 Point 3
1200
1600 2000 2400 Raman shift (cm-1)
2800
Figure caption
Fig.1 Raman spectra of the grahene/Cu(111)/Mica(001) measured at three different points by using 488 nm excitation wavelength.
SPM investigations of electrical properties of graphene nanostructures on 6H SiC substrate 1)
1)
1)
1)
1)
Krzysztof Gajewski , Daniel Kopiec 0DJGDOHQD 0RF]DĂĄD , Adam Piotrowicz 0LFKDĂĄ =LHORQ\ , 1) 1) 2) Grzegorz Wielgoszewski , Teodor Gotszalk , :ĂĄRGHN 6WUXSLÄ&#x201D;VNL 1) :URFĂĄDZ 8QLYHUVLW\ RI 7HFKQRORJ\, Faculty of Microsystem Electronics and Photonics, ul. Z. Janiszewskiego 11/17, PL- :URFĂĄDZ, Poland 2) Institute of Electronic Materials Technology, Wolczynska 133, PL-01919 Warsaw, Poland krzysztof.gajewski@pwr.wroc.pl Among techniques used for graphene investigation, those based on scanning probe microscopy (SPM) play an important role as they give the opportunity to perform localized experiments in nanometre scale [1]. This may be especially interesting if the electrical properties are considered, as a wide variety of electrical SPM-based techniques have already been developed [2].
Many methods of graphene production have been considered to date, including mechanical exfoliation and chemical or thermal decomposition of silicon carbide (SiC). However, CVD techniques are most promising in terms of suitability for commercial production, although the electrical properties of CVD graphene are not as good as those of graphene fabricated using mechanical exfoliation. With use of CVD techniques, graphene may be produced on many types of substrates, including platinum, nickel, ruthenium, iridium and cobalt [3]. Epitaxy on SiC substrates may be one of the most interesting methods due to better quality of graphene grown on this material in comparison with the others [4].
In this paper we present our investigations of the CVD-grown graphene on the 6H-SiC substrate. Graphene monolayer was present in the entire investigated surface. Electrical properties of the graphene were characterized with use of the following SPM techniques: conductive atomic force microscopy (C-AFM), Kelvin probe force microscopy (KPFM) and scanning tunnelling microscopy (STM). In these techniques conductive probe approach the investigated surface and interact with it. Those interaction in result give us possibility to record data about current (C-AFM) or surface potential (KPFM). C-AFM and STM images were obtained using home-made microscopes. Current-voltage spectroscopy curves and current-load force spectroscopy curves were also recorded. We observe that electrical properties of the graphene strongly depend on substrate surface, especially at the edges of SiC terraces. Spectroscopy data show us, that electrical conductivity increases with higher values of the load force. Moreover, the contact between metallic tip and graphene on SiC exhibits tunnelling properties. KPFM measurements confirm the observations Âą on the edges, where strong distortion of the graphene material occurs, we observed the potential increase.
References [1] U. Starke, C. Riedl, J. Phys.: Condens. Matter, 21 (2009) 134016 [2] A. Avila, B. Bhushan, Crit. Rev. Solid State Mater. Sci., 1 (2010) 38 [3] C. Soldano, A. Mahmood, E. Dujardin, Carbon, 8 (2010) 2127 [4] W. Strupinski, K. Grodecki, A. Wysmolek, R. Stepniewski, T. Szkopek, P. E. Gaskell, A. Grüneis, D. Haberer, R. Bozek, J. Krupka, J. M. Baranowski, Nano Lett., 11 (2011) 1786 Figures
a)
b)
Topography (a) and current (b) images of the CVD graphene grown on 6H-SiC. C-AFM image, scan area of 3.25 µm × 3.25 µm, sample bias: 0.5 V, load force: 30 nN
Potential image overlaying topography image of the CVD graphene on 6H-SiC.
STM image of moiré patterns of the CVD graphene grown on 6H-SiC
Terahertz Radiation Induced Edge Currents in Graphene a
a
a
b
b
a
S. D. Ganichev , C. Drexler , P. Olbrich , M. M. Glazov , S. A. Tarasenko , J. Karch , M. a a a c c d b Fehrenbacher , D. Weiss , J. Eroms , R. Yakimova , S. Lara-Avila , S. Kubatkin , E. L. Ivchenko a
Terahertz Center, University of Regensburg, 93040 Regensburg, Germany A.F. Ioffe Physical-Technical Institute, Russian Academy of Sciences, 194021 St. Petersburg, Russia c Department of Physics, Chemistry and Biology, Linköping University, S-58183 Linköping, Sweden d Chalmers University of Technology, S-41296 Göteborg, Sweden
b
sergey.ganichev@physik.uni-regensburg.de
Abstract²We report on the observation of edge photocurrents in graphene layers excited by polarized terahertz (THz) radiation. We demonstrate that shining laser radiation on the boundary of unbiased monolayer graphene at room temperature results in a direct electric current along the sample edge. The current contains a substantial contribution which is solely driven by the light's helicity and reverses its direction upon switching the helicity sign.
Of the many studies of graphene, a substantial portion is devoted to the physics of graphene edges. In transport experiments edge effects are usually masked by bulk properties, nonetheless the graphene edges are expected to play a crucial role in the electronic properties of graphene-based nanoscale devices. Here, we present an opto-electronic method to uniquely distinguish edge from bulk scattering by exploring edge photocurrents in graphene samples illuminated by terahertz (THz) radiation. For circularly polarized light the edge current is observed to form a vortex winding around the edges of the square-shaped
samples.
Its
direction
reverses
upon
switching the radiation helicity from left- to right-handed. Evidently, the photocurrent is caused by the local symmetry breaking at the sample edges resulting in an asymmetric scattering of carriers driven by the radiation electric field. It gives rise to a directed electric current along the sample boundary in a narrow stripe of width comparable to the mean free path. We show that the photocurrent measurements provide direct access to electron transport at the graphene edges and allow mapping the variation of scattering times along the edges. The currents are generated employing THz circularly polarized radiation of a cw methanol laser operating at a wavelength of 118 µm as well as a pulsed ammonium high power laser delivering the wavelengths 90, 148 and 280 µm. Two types of samples were studied: small-area graphene flakes with typical sizes of 10 to 30 µm, prepared by mechanical exfoliation technique, and large-area graphene epitaxially grown on a 4H-SiC(0001) substrate with a size of 2
3x3 and 5x5 mm .
Figure: Spatial distribution of the circular SKRWRFXUUHQW 7KH FXUUHQW UHYHUVHV LW¶V sign for two parallel edges and vanishes in between. Inset: Experimental geometry of the setup and detailed spatial resolution of the peak, respectively.
Normal incident illumination of the edge of unbiased large-area samples between any pair of contacts results in a photocurrent. By contrast, if the laser spot is moved toward the center the signal vanishes. The detected signal depends strongly on the radiation polarization. In particular, the photocurrent has opposite direction for right- and left-handed circularly polarized radiation. To prove that the photocurrent is caused by illuminating the graphene edges, we scanned the laser spot across the sample along the y-axis. The signal was picked up from a pair of contacts at the sample top and bottom edges aligned along the x-axis. The experimental geometry and the photocurrent versus the spot position are shown in Fig. 1. The current reaches its maximum for the laser spot centered at the edge and rapidly decays with the spot moving. A comparison of JA(y) with the independently recorded laser profile (lines) shows that the signal just follows the Gaussian intensity profile. Investigating the current excited by circularly polarized radiation for different pairs of contacts we observed that it forms a vortex around the sample edge. We note, that at oblique incidence of radiation, the edge current is superimposed with the interior circular ac Hall effect [2]. Microscopically the photocurrent is caused by the local symmetry breaking at the sample edges resulting in an asymmetric scattering of carriers driven by the radiation electric field. It gives rise to a dc electric current along the sample boundary in a narrow stripe of the width comparable with the mean free path. The experimental observations are well described by the microscopic theory of edge currents developed in the framework of the Boltzmann kinetic equation. By expanding the electron distribution function up to second order in the ac electric field of THz radiation we derive the analytical equation for the total electric current resulting from the action of the electric field accompanied by diffusive electron scattering at the sample edges. Only carriers within the distance of the mean free path to the edge contribute to the current, limiting the considerations to one dimension. Comparison of the experimental observations with theory reveals that the edges of n-type graphene layers exhibit p-type conductivity. This result is in agreement with results from scanning photocurrent microscopy [3] and spatially resolved Raman measurements [4]. Our observations clearly demonstrate that illuminating monolayer graphene edges with polarized terahertz radiation at normal incidence results in a dc edge current. The effect is directly coupled to electron scattering at the graphene edges and vanishes in bulk graphene. While the circular edge photocurrents should exist in any two-dimensional charge carrier system the specific properties of graphene, i.e., the high velocity of massless Dirac fermions, facilitate the experimental observation. Our results suggest that the circular photocurrents can be effectively used to study edge transport in graphene even at room temperature.
References
[1] J. Karch, C. Drexler, P. Olbrich, M. Fehrenbacher, M. Hirmer, M .M. Glazov, S. A. Tarasenko, E. L. Ivchenko, B. Birkner, J. Eroms, D. Weiss, R. Yakimova, S. Lara-Avila, S. Kubatkin, M. Ostler, T. and S. D. Ganichev, Phys. Rev. Lett. 107 (2011) 276601. [2] J. Karch, P. Olbrich, M. Schmalzbauer, C. Zoth, C. Brinsteiner, M. Fehrenbacher, U. Wurstbauer, M. M. Glazov, S. A. Tarasenko, E. L. Ivchenko, D. Weiss, J. Eroms, R. Yakimov, S. Lara-Avila, S. Kubatkin, and S. D. Ganichev, Phys. Rev. Lett. 105 (2010) 227402. [3] E.J.H Lee, Helin Cao, Wei Wu, Qingkai Yu, and Yong P. Chen, Nature Nano. 3 (2008) 486. [4] S. Heydrich, M. Hirmer, C. Preis, T. Korn, J. Eroms, D. Weiss, and C. Sch端ller Appl. Phys. Lett. 97 (2010) 043113.
Epitaxial Growth of Single- and Few-layer Graphene on Pt(111) and Pd(111) Surfaces by Surface Segregation *
1
2
2
JianhuaGao, Keisuke Sagisaka, Nobuyuki Ishida, Daisuke Fujita 1
2
International Center for Young Scientists (ICYS) 2
Advanced Nano Characterization Unit (ANCU)
National Institute for Materials Science (NIMS) 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047 GAO.Jian-Hua@nims.go.jp 2
Graphene, the typical sp ±bonded two dimensional materials with honeycomb crystal structure, are attractive in fundamental research because of their remarkable properties. High quality graphene over large-scale can provide new opportunities in potential applications. Therefore, synthesis of high-quality graphene is highly desirable. It is known that 3-D growth of graphitic carbon can occur on the metal surfaces at elevated temperatures through surface segregation and precipitation. Here we report the fabrication of single-layer graphene on carbon-doped single-crystal Pt(111) (0.05 %) and Pd(111) (0.5 %) substrate. It is found that uniform single-layer graphene islands about 50µm formed on Pt(111) surfaces. Continuous, wafer-scale, single-layer graphene can be achieved on Pd(111) surfaces by adjusting the experimental parameters. The atomic structure of graphene islands has been investigated by scanning tunneling microscopy (STM), which exhibit hexagonal atomic lattice and morié pattern for Pt(111) and Pd(111) substrates, respectively. The present synthesis can provide a novel technique for large-scale graphene fabrication used for fundamental research and potential applications. References [1] D. Fujita and K. Yoshihara, J. Vac. Sci. Technol. A, 12, 2134 (1994). [2] D. Fujita, M. Schleberger and S. Tougaard, Surf. Sci. 331-333, 343 (1995). Figures
Fig.1 AES map images of single-layer graphene by surface segregation. CKLL image of single-layer graphene islands on Pt(111) (a) and Pd(111) (b) substrate.
Graphene Oxide supported Layered Double Hydroxides for CO 2 capture applications Ainara Garcia-Gallasteguia,b, Diana Iruretagoyenad, Mohamed Mokhtarc, Abdullah Asiric, Sulaiman N. Basahelc, Shaeel A. Al-Thabaitic, Abdulrahman O. Alyoubic, David Chadwickd, Milo S. P. Shaffera a
Department of Chemistry, Imperial College London, South Kensington Campus, London SW7 2AZ, UK b Bio Nano Consulting, 338 Euston Road London NW1 3BT, UK c Department of Chemistry, Faculty of Science, King Abdulaziz University, Jeddah 21589, Saudi Arabia d Department of Chemical Engineering, Imperial College London, South Kensington Campus, London SW7 2AZ, UK a.garcia-gallastegui@imperial.ac.uk, ainara.garcia@bio-nano-consulting.com, m.shaffer@imperial.ac.uk
Layered double hydroxides (LDHs), also known as hydrotalcite-like compounds, belong to a large class of synthetic two-dimensional (2D) nanostructured anionic clays. Their structure is composed of positively charged brucite-like Mg(OH)2 layers in which a fraction of divalent cations, octahedrally coordinated by hydroxyls, are partially substituted by trivalent cations. The excess of positive charge is balanced by intercalated anions. The potential of LDHs as CO2 adsorbent materials open an attractive alternative to the current carbon capture and storage (CCS) technologies. LDHs require less energy to be regenerated and show better multi-cycle stability than other potential CO2 solid adsorbents (e.g. calcium oxides). Despite these positive adsorption properties, LDHs commercial use is limited because they possess a relatively low CO2 adsorption capacity. Recently, we have observed that the CO2 adsorption performance of LDHs is considerably enhanced by supporting them onto oxidised multiwalled carbon nanotubes (MWNT). [1] Following a similar strategy, here we have used graphene as an ideal atomic-thick 2D material to, in principle, maximize the contact area with the 2D LDHs and in turn enhance the CO2 uptake capacity of the assembly.
Specifically, LDH nanoparticles were precipitated directly onto graphene oxide (GO) and the dependence of the structural and physical properties of the Mg-Al LDH have been studied, using electron microscopy, X-ray diffraction, thermogravimetric analysis (TGA), and BET surface area measurements. After a thermal decomposition, layered double oxides (LDO) are obtained with the basic sites required for the CO2 adsorption. In order to study the effect of the GO content in a LDO/GO hybrid, a range of samples with different proportions of LDO were prepared and fully characterised. It was found that the CO2 adsorption capacity and multi-cycle stability of the LDO were both increased when supported onto GO due to an enhanced particle dispersion and gas accessibility.
References [1] Garcia-Gallastegui A., Iruretagoyena D., Mokhtar M., Asiri A., Basahel S. N., Al-Thabaiti S. A., Alyoubi A. O., Chadwick D., Shaffer M. S. P. Submitted (2012).
Scheme 1. Schematic representation of the synthesised LDH/GO hybrids.
Hybrid graphene-quantum dot phototransistors with ultrahigh gain Louis Gaudreau, Gerasimos Konstantatos, Michela Badioli, Johann Osmond, Maria Bernechea, F. Pelayo Garcia de Arquer, Fabio Gatti , Frank H. L. Koppens ICFO Âą The Institute of Photonic Sciences, Mediterranean Technology Park Av. Carl Friedrich Gauss, num. 3, 08860 Castelldefels (Barcelona), Spain louis.gaudreau@icfo.es Graphene has recently emerged as a new platform for opto-electronic applications and photodetection due to its broad spectral bandwidth and its high carrier mobility at room temperature [1, 2]. Graphene small size and ease of fabrication makes it a good candidate for integration into low power photodetection architectures with, for example, complementary metal oxide semiconductor (CMOS) technology. So far, graphene based photodetectors have been limited by its weak light absorption (~2.3%)[3] and small photodetection area, and they have not shown gain. We present a novel photodetection scheme based on a hybrid graphene-quantum-dot phototransistor that shows ultra-high 8
3
gain of 10 at room temperature while applying electric fields as low as 10 V/cm. In this device, we combine the favorable electronic and optical qualities of graphene with the unique properties of colloidal quantum-dots, such as bandgap tunability via the quantum confinement effect and high absorption coefficients [4]. This super-sensitive device can detect power in the fW regime throughout VIS/NIR range, paving the way towards chip-integrated single photon detection. In addition, its sensitivity is fully tunable by electrostatic gating.
References [1] K.S. Novoselov et al., Science, 306 (2004) 666. [2] F. Bonaccorso, Z. Sun, T. Hasan, A. C. Ferrari, Nature Phot. 4 (2010) 611. [3] R. R. Nair, et al., Science 320 (2008) 1308. [4] G. Konstantatos et al, Nature, 442 (2006) 180.
Hydrothermal synthesis of TiO2 nanotube/Graphene oxide composite and its application in photocatalytic purification of water a
b
a
c
Zahra Gholamvand , , Kieran Nolan , John Tobin , Anne Morrissey a School of Biotechnology, Dublin City University, Dublin 9, Ireland. b School of Chemical Sciences, Dublin City University, Dublin 9, Ireland c Oscail, Dublin City University, Dublin 9, Ireland Email: Zahra.gholamvand2@mail.dcu.ie The degradation of organic pollutants in water such as detergents, dyes, pesticides, herbicides and pharmaceuticals by the photocatalytic semiconductor TiO 2 has attracted extensive attention in recent decades [1,2]. TiO2 nanotube (TNT) is considered as a modified structure in photocatalysis owing to its special electronic and mechanical properties, high photocatalytic activity, large specific surface area and high pore volume [3,4]. The aim of this study is to increase the photocatalytic efficiency of TNTs in combination with Graphene to purify water and wastewater. Graphene and graphene oxide (GO) are showing promise to turn into an alternative photocatalyst support material due to their planar structure, having functional groups, large surface area, and high transparency [5,6]. In the present work TNTs were prepared via hydrothermal method from commercial TiO 2 P25 as a 3
starting material. 3 g of P25 was introduced to a Teflon-lined autoclave containing 70 cm 10M NaOH and heated at 130 C for 24 h. After the heat treatment the obtained precipitate was washed several times with 0.1M HCl and distilled water. TEM image of as synthesized TNTs are shown in figure 1. The as-received TNTs were dispersed in graphene oxide aqueous solution, produced from graphite via PRGLILHG +XPPHU¶V PHWKRG DQG Zere ultrasonicated for 1 hour. The post-treatment of TNT/GO composite was carried out by at temperatures of between 300± 600 C.
Pure TNTs and TNT
composites has been characterized by Fourier transform infrared spectroscopy, Raman spectroscopy, UV-vis diffuse reflectance spectroscopy, scanning and transmission electron microscopes, X-ray diffraction and BET analysis. Famotidine and Amoxicillin were chosen as sample pharmaceutical pollutants and their concentration was measured using HPLC techniques after certain UV exposure intervals in a UV immersion well reactor shown in figure 2. The Photocatalytic experiments data show that this composite has a higher photocatalytic activity than bare TNT and commercial TiO2. The advantage of using graphene is the ability to remove hazardous FRPSRXQGV FRQWDLQLQJ EHQ]HQH ULQJV LQ ZDWHU DQG ZDVWHZDWHU WKURXJK VWURQJ ʌ-ʌ LQWHUDFWLRQ DQG KLJK affinity to functional groups such as oxygen epoxies, carbonyl (=CO), hydroxyl (-OH) and phenols attached to both sides.
Keywords: Graphene oxide, Graphene, Adsorption, pharmaceuticals, HPLC
References [1] Yang,Hai; An,Taicheng; Li,Guiying; Song,Weihua; Cooper,William J.; Luo,Haiying; Guo,XindongAuthors, Journal of hazardous materials, vol. 179, Issue1-3 (2010) 834-839 [2] Keane,David; Basha,Shaik; Nolan,Kieran; Morrissey,Anne; Oelgemöller,Michael; Tobin,John, Catalysis Letters, Vol.141, Issue 2 (2011) 300-308 [3] Awitor,K.O.; Rafqah,S.; Géranton,G.; Sibaud,Y.; Larson,P.R.; Bokalawela,R.S.P.; Jernigen,J.D.; Johnson,M.B. Journal of Photochemistry and Photobiology A: Chemistry, Vol.199, Issue2-3 (2008) 250254 [4] Wong,Chung Leng; Tan,Yong Nian; Mohamed,Abdul Rahman, Journal of environmental management, Vol.92 Issue7 (2011)1669-680 [5] Singh,Virendra; Joung,Daeha; Zhai,Lei; Das,Soumen; Khondaker,Saiful I.; Seal,Sudipta, Materials Science, Vol.56 Issue8 (2011)1178-1271 [6] Sreeprasad,T.S.; Maliyekkal,Shihabudheen M.; Lisha,K.P.; Pradeep,T., Journal of hazardous materials, Vol.186 Issue1 (2011)921-931. Figures
Figure1. TEM image of TiO 2 nanotubes synthesised hydrothermally at 130 °C for 24 hours
Figure 2. Schematic view of the Photocatalytic reactor used for photocatalytic water treatment
Simultaneous in-situ graphene oxide reduction and UV curing of acrylic based formulations for inkjet printing 1
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R. Giardi , S. Porro , A. Chiolerio , F. Sordo , M. Sangermano , E. Celasco , A. Chiodoni
1
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Italian Institute of Technology (IIT), Center for Space Human Robotics, Turin, Italy Politecnico di Torino, Dipartimento Scienza Applicata e Tecnologia (DISAT), Turin, Italy
2
Rossella.Giardi@polito.it Because of the high specific surface area, good chemical stability, and outstanding electrical and thermal conductivity, graphene is predicted to be an excellent electrode material candidate for energyconversion/storage systems [1]. The nanoscale dimensions of graphene-based materials enable them to be solubilized and transferred into inks, which can be exploited for direct printing of electrodes using additive technology, such as inkjet printing [2]. Inks formulation for application to printable electrodes requires large quantities of material, which can be synthesized by wet chemical routes, such as exfoliation of graphene oxide (GO) from bulk graphite [3]. GO may be subsequently reduced to reduced graphene oxide (RGO) using several methods [4-6]. UV irradiation is a promising method for reducing GO, which allows the simultaneous photopolymerization of acrylic resin matrices, such as poly(ethylene glycol) diacrylate (PEGDA), which can be used in the formulation of inks [7]. This work explores the possibility of introducing GO water solutions into a PEGDA matrix, thus creating a conductive printable ink containing UV-reduced GO sheets, of variable viscosity depending on water content. Commercial GO was purchased from Cheap Tubes Inc. (USA). Starting from a dispersion of GO in deionised water with a photoinitiator, GO/PEGDA nanocomposite samples were prepared as thin films adding the GO dispersion to PEGDA, and subsequently polymerized by irradiation with UV light. I-V characteristics of the irradiated films were measured by means of standard two point contact and resistivity was computed comparing different compositions in order to assess the dispersion effect. The same solutions of GO dispersed in deionised water with photoinitiator were deposited onto a silicon wafer, and subsequently irradiated with UV light. Pristine GO solutions and irradiated samples were analysed by X-ray photoelectron spectroscopy (XPS) and electron energy loss spectroscopy (EELS), in order to evaluate the reduction of oxygen containing groups. XPS measurements (Figs. 1-2) show that after few minutes of UV irradiationGO was visibly reduced, with a significant decrease of both height and area of C=O and O-C=O peaks, relatively to C-C peak. This tendency was also confirmed by EELS. This behavior was confirmed when GO-aqueous dispersion was added to photocurable PEGDA resin. It was shown an increase in conductivity of PEGDA films 7
containing GO-aqueous by one order of magnitude, from (5.4Âą0.5) Ă&#x2014; 10 Â&#x;FP 3(*'$ 89-cured in 6
air) to (1.7Âą0.2) Ă&#x2014; 10 Â&#x;FP 3(*'$ FRQWDLQLQJ an actual content of 1 wt% of GO, UV treated in air). In order to control the printability of an ink, it is essential to be able to tailor the viscosity. Here we show the possibility of formulating a conductive ink containing UV-reduced graphene oxide, based on PEGDA-water solutions of controllable viscosity. Future work will involve inkjet printing tests of conductive patterns and electrical/morphological characterization of the printed geometries.
References [1] J. Su, M. Cao, L. Ren, C. Hu, J. Phys. Chem. C, 115 (2011) 14469. [2] D.S. Hecht, R.B. Kaner, MRS Bulletin, 36 (2011) 749. [3] D.C. Marcano, et al., ACS Nano, 4 (2010) 4806. [4] H.W. Tien, et al., Carbon, 49 (2011) 1550. [5] S. Gilje, et al., Adv. Mater., 22 (2010) 419. [6] I. Jung, et al., J. Phys. Chem. C, 113 (2009) 18480. [7] M. Sangermano, et al., Macromol. Mater. Eng., 296 (2011) 401.
Figures
Fig. 1: XPS spectrum of GO before UV curing.
Fig. 2: XPS spectrum of RGO after UV curing.
Self-assembly of a sulphur-terminated graphene nanoribbon within a single-walled carbon nanotube. 1
2-3
1
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M. C. Gimenez-Lopez , A. Chuvilin , E. Bichoutskaia , T. W. Chamberlain , G. A. Rance , N. 1 4 4 1 Kuganathan , J. Biskupek , U. Kaiser , A.N. Khlobystov *. 1
School of Chemistry, University of Nottingham, University Park, Nottingham NG7 2RD, UK, CIC nanoGUNE Consolider, Tolosa Hiribidea 76, E-20018, Donostia-San Sebastian, Spain, 3 4 IKERBASQUE, Basque Foundation for Science, 48011 Bilbao, Spain, Central Facility of Electron Microscopy, Group of Electron Microscopy of Materials Science, Ulm University, Albert-Einstein-Allee 11, D-89081 Ulm, Germany. 2
pczank@exmail.nottingham.ac.uk or pczmdcd@exmail.nottingham.ac.uk
Graphene nanoribbons (GNRs) possess many of the unique properties of the parent material but with the added advantage of the ability to tune these properties through modification of the nanoribbon width and its edge structure.
1,2
As a result, the potential applications of GNRs in electronic devices extend well
beyond those of graphene,
3-5
which has stimulated a significant current effort in the preparation of 6-8
GNRs. Until very recently most approaFKHV ZHUH EDVHG RQ D ³WRS-GRZQ´ PHWKRGRORJ\
offering only a
OLPLWHG GHJUHH RI FRQWURO RYHU WKH *15œV DWRPLF VWUXFWXUH EXW D UHFHQW EUHDNWKURXJK LQ WKH ³ERWWRP-XS´ direction has enabled assembly of GNRs with well-defined atomic structures utilising molecules as the 9
carbon source. As the structure of a GNR appears to be strictly determined by the structure of the initial molecules, this method requires the careful preparation and pre-organisation of the molecular precursors on atomically flat surfaces under precisely controlled ultra-high vacuum conditions.
10
In our study , we demonstrate that a GNR can self-assemble spontaneously from a random mixture of elements within a single-walled carbon nanotube (SWNT) that is utilised as both the reaction vessel and a 1D template for the growth of GNRs. We identify the two key principles that lead to the formation of GNR: (i) 1D confinement at the nanoscale ensures propagation of GNR only in one dimension, and (ii) incorporation of heteroatoms into the predominantly carbon-based elemental feedstock leads to the termination of dangling bonds, stabilising the structure and making the otherwise unstable nanoribbons thermodynamically viable over other possible forms of carbon.
The resultant structure GNR@SWNT represents a new, unexpected hybrid form of carbon, with potentially exciting functional properties. The width of the nanoribbon is strictly determined by the diameter of SWNT-nanoreactor, while its elemental composition is controlled by the molecular precursors loaded into the nanotube. Our electron microscopy imaging reveals remarkable structural and dynamic behaviour of the GNR@SWNT system, including elliptical distortion of the nanotube, helical twist and the screw-like motion of the nanoribbon along the nanotube. These unexpected effects offer new mechanisms for controlling properties of carbon nanomaterials, such as electronic band gap and concentration of charge carriers. A recent study indicated that hydrogen-terminated nanoribbons 11
(H-GNR) can also be formed inside nanotubes by pyrolysis of polyaromatic hydrocarbons. .
References [1] Geim, A.K., Science, 324, (2009) 1530. [2] Berger, C., Song, Z., Li, X., Wu, X., Brown, N., Naud, C., Mayou, D., Li, T., Hass, J., Marchenkov, A.N., Conrad, E.H., First, P.N., de Heer, W.A. , Science, 312, (2006) 1191. [3] Yang, L., Cheol-Hwan, P., Son, Y-W, Cohen, M.L . Louie, S.G.. Phys. Rev. Lett., 99, (2007) 186801. [4] Barone, V., Hod, O., Scuseria, G.E., Nano Lett., 6, (2006) 2748. [5] Novoselov, K.S., Geim, A.K., Morozov, S.V., Jiang, D., Zhang, Y., Dubonos, S.V., Grigorieva, I.V., Firsov, A.A., Science, 306, (2004) 666. [6] Kosynkin, D.V., Higginbotham, A.L., Sinitskii, A., Lomeda, J.R., Dimiev, A., Price, B.K., Tour, J.M., Nature, 458, (2009) 872. [7] Elias, A.L., Botello-Mendez, A.R., Meneses-Rodriguez, D., Gonzalez, V.J., Ramirez-Gonzalez, D., Ci, L., Muñoz-Sandoval, E., Ajayan, P.M., Terrones, H., Terrones, M., Nano Lett., 10, (2009) 366. [8] Jiao, L., Wang, X., Diankov, G., Wang., H., Dai, H., Nature Nanotechnol., 5, (2010) 321. [9] Cai, J., Ruffieux, P., Jaafar, R., Bieri, M., Braun, T., Blankenburg, S., Muoth, M., Seitsonen, A.P., Sleh, M., Feng, X., Müllen, K., Fasel, R., Nature 466, (2010) 470. [10] Chuvilin, A., Bichoutskaia, E., Gimenez-Lopez, M.C., Chamberlain, T.W, Rance, G.A., Kuganathan, N, Biskupek, J., Kaiser, U. and Khlobystov, A.N., Nature Materials. 10, (2011) 687. [11] Talyzin, A.V., Anoshkin, I.V., Krasheninnikov, Nieminen, R.M., Nasibulin, A.G., Jiang, H., Kauppinen, E.I., Nano Lett., 11, (2011) 4352.
Figures a
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1 nm
b
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f
@SWNT 1
@SWNT
+ 2
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heat S-GNR@SWNT or e-beam
e
@SWNT 2
Carbon nanotubes serve as containers and nanoreactors for molecules. Functionalised fullerenes 1 (a) bearing an organic group with sulphur atoms on their surface are spontaneously and irreversibly encapsulated into a SWNT (b) due to the strong van der Waals interactions between the fullerene cage and the interior of the hostnanotube. Under prolonged exposure to the e-beam, the functional groups and the fullerene cages decompose and re-assemble into a nanoribbon inside a (14,5)-SWNT (c). Sulphur atoms terminating the edges of the nanoribbon appear as chains of dark atoms (c ± an experimental AC-HRTEM image, e ± a model of S-GNR@SWNT and d ± an image simulated from the model). Sulphur-terminated nanoribbons can be also formed from other sulphurcontaining organic molecules, such as tetrathiafulvalene (TTF) 2 or a mixture of TTF 2 and C60 3 inserted in nanotube (f) and decomposed at high temperature (over 1000 oC) or under e-beam radiation. (In structural diagrams atoms of sulphur, oxygen, nitrogen and carbon are coloured in yellow, red, blue and grey respectively; atoms of hydrogen in structural diagram of 1 are omitted for clarity).
New bioactive PMMA-Hydroxyapatite based bone cement reinforced with graphene oxide a
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Gil Gonçalves , Sandra M.A. Cruz , José Grácio , Paula A.A.P Marques , b c,d b Cecilia Ramírez-Santillán , María Vallet-Regí , María-Teresa Portolés a
Nanotechnology Research Division, Center for Mechanical Technology & Automation, University of Aveiro, 3810193 Aveiro, Portugal. b Department of Biochemistry and Molecular Biology I, Faculty of Chemistry, Universidad Complutense, 28040Madrid, Spain c Department of Inorganic and Bioinorganic Chemistry, Faculty of Pharmacy, Universidad Complutense, 28040Madrid, Spain d Networking Research Center on Bioengineering, Biomaterials and Nanomedicine, Madrid, Spain ggoncalves@ua.pt
The technologies associated with production of graphene have evolved greatly in recent times and there is no doubt that graphene has risen as a shining star in the horizon on the path of the scientists 1, 2 searching for new materials for future electronic and composite industry. One of the most interesting applications is the use of graphene as a mechanical reinforcing agent in polymer matrices. The ease chemical manipulation of graphene surface makes it one of the most attractive reinforcements of 3 polymer matrixes, since it is possible to match the interfaces to achieve a seamless integration of all 4 constituents at atomic level. Polymethylmethacrylate (PMMA) bone cement is widely used for prosthetic fixation in orthopaedic surgery; however, the interface between bone and cement is considered a weak zone. The addition of hydroxyapatite (HA) enhances the connection to the bone since HA is the main inorganic constituent of bone tissue. However, its addition to the PMMA cement formulation lowers the mechanical properties of the composite. One possibility to overcome this problem can be the addition of a second nano-filler that works as a mechanical reinforcement. In the present study graphene oxide (GO) was added to a composite matrix of PMMA/HA bone cement in the concentration range between 0.01 and 1.0 wt%. The preparation method consisted in the addition of the nano-fillers to the solid fraction of the cement followed by homogenization in aqueous suspension, that was then freeze granulated to maintain the uniform distribution of all the components and dried by liophilization. The nanocomposite cements were then prepared by the traditional technique of mixing solid and liquid phases promoting the PMMA in situ radical polymerization, keeping the solid to liquid concentrations relation of the traditional bone cement unchanged. The nanocomposite materials were then chemically and structurally characterized showing a very good distribution of the fillers inside the polymeric matrix (Figure 1). Concerning the mechanical testing, the initial results, although positive, were not as high as expected. This has prompted us to realize further studies concerning the determination of the chain size distribution and chemical structure of the polymer extracted from the composite. The data indicate that GO has an active intervention during the PMMA radical polymerization by acting as radical scavenger during the PMMA polymerization reaction due to the delocalized ʌ-bonds. As a consequence, there is inhibition and retardation of the polymerization, which reflects on the final mechanical properties of the nanocomposite. In order to suppress this drawback we decided to increase the radical concentration added during the PMMA bulk polymerization to overcome the percentage of radicals inactivated by GO. We found that by doubling the initial radicalar agent concentration, the mechanical properties of the final nanocomposites show an enormous increase, being the best results obtained for a GO reinforce of 0.5 wt%. Considering the final envisaged application of the novel nanocomposite as bone cement, in vitro bioactivity and biocompatibility studies were performed. The ability of this nanocomposite to promote the growth of a calcium phosphate layer at their surface in a simulated body fluid (SBF) was demonstrated. We show the importance of the HA presence in the nanocomposite to promote the bioactive behaviour. The biocompatibility of this material was evaluated in vitro with mouse L929 fibroblasts and human Saos-2 osteoblasts cultured for 3 days in contact with smooth and rough surfaces of disks prepared with
the novel composite. Both cell types adhere and grow on all these surfaces with high cell viability assessed by propidium iodide exclusion (80-85% in fibroblasts and 90% in osteoblasts) and low apoptosis levels evaluated by flow cytometric analysis of subG1 phase (1% in fibroblasts and 4% in osteoblasts). The morphology of L929 fibroblasts and Saos-2 osteoblasts cultured on disks was analyzed by Scanning Electron Microscopy (SEM) and Confocal Microscopy (CM). Both cell types colonize the disk surfaces exhibiting their characteristic morphology with a distinctive actin network and no apoptotic nuclei. Figure 2 shows the morphology of L929 fibroblasts by SEM (2a) and Saos-2 osteoblasts by CM (2b) after culture on this novel material. In conclusion, PMMA based bone cements reinforced with GO are potential candidates to be used as bone cements with augmented mechanical properties and appropriated biological behaviour.
References [1] Singh, V.; Joung, D.; Zhai, L.; Das, S.; Khondaker, S. I.; Seal, S. Prog. Mater. Sci. 56 (2011) 1178. [2] Huang, X.; Yin, Z.; Wu, S.; Qi, X.; He, Q.; Zhang, Q.; Yan, Q.; Boey, F.; Zhang, H. Small, 7 (2011) 1876. [3] Allen, M. J.; Tung, V. C.; Kaner, R. B. Chem. Rev., 110 (2010) 132. [4] Goncalves, G.; Marques, P.; Barros-Timmons, A.; Bdkin, I.; Singh, M. K.; Emami, N.; Gracio, J. J. Mater. Chem. 20 (2010), 9927.
Figures:
Figure1: Photograph of nanocomposite specimen prepared with 0.5 wt% of GO. SEM images show a good integration of the fillers with the polymer.
Figure 2: Morphology of L929 fibroblasts (a) and Saos-2 osteoblasts (b) cultured on nanocomposite disks evaluated by Scanning Electron Microscopy and Confocal Microscopy respectively.
Magneto-Optical Properties of Antidot Lattices Jesper Goor Pedersen, Thomas Garm Pedersen Aalborg University, Skjernvej 4A, DK-9220 Aalborg East, Denmark jgp@nano.aau.dk Periodic modulations of graphene, in the form of graphene antidot lattices, have been shown to produce sizable band gaps in the otherwise semimetallic graphene [1]. Such antidot lattices may be realized via, e.g., actual perforations of the graphene sheet [2], or using hydrogen adsorption [3]. We present theoretical results regarding the magneto-optical properties of such structures. The individual antidots may be modelled in a variety of ways. As a first approximation, the effect of the antidots may be included by directly adding a band gap to graphene via a mass term in the Dirac equation (DE) or, equivalently, adding a staggered potential in a tight-binding (TB) description of graphene. We have compared the results of these two methods, and find that the inclusion of overlap between neighbouring pi-orbitals in a TB treatment has pronounced consequences for the optical Hall conductivity, that are entirely missed in a DE approach [4]. Further, we find that a sufficiently large band gap quenches the effect of the magnetic field. To more accurately model the individual antidots, we have studied a DE model of a single graphene antidot, modelled via a circularly symmetric position-dependent mass term, in a magnetic field [5]. Here, analytical expressions can be derived for the spinor eigenstates, which in turn can be used to formulate an eigenvalue condition for the spectrum. In the limit of an infinite mass term, corresponding to, e.g., perforations in the graphene sheet, we present approximate analytical expressions for the energies. The resulting density of states exhibits a very rich structure, when the radius of the antidot is of the order of the magnetic length. The eigenstate stemming from the zeroth Landau level of ordinary graphene is localized predominantly at the edge of the antidot, regardless of the size of the mass term. By simulating STM measurements, we discuss the possibility of experimentally probing such localized states. Due to the nature of the spectrum at large negative values of the angular momentum, STM measurements will only indicate localized states for very specific, narrow energy ranges. To treat the antidot lattices more accurately, we need to consider the exact atomic structure of the antidots. This is accomplished in a TB model, where a magnetic field is included via a Peierls substitution. However, the magnetic phase added to the overlap integrals in the TB model forces the calculations to be performed on a magnetic supercell which may contain several houndred antidot lattice unit cells. This results in structures with more than one million carbon atoms, for which direct diagonalization methods are unsuitable. Instead, we derive recursive relations for the diagonal optical conductivity and the density of states using recursive GreenÂśs functions, including periodic boundary conditions in both directions [6]. Using these methods, we discuss unique signatures of the graphene antidot lattices that are not captured in the simpler models. References [1] T. G. Pedersen, C. Flindt, J. Pedersen, N. A. Mortensen, A.-P. Jauho, and K. Pedersen, Physical Review Letters, 100 (2008) 136804; J. A. FĂźrst, J. G. Pedersen, C. Flindt, N. A. Mortensen, M. Brandbyge, T. G. Pedersen, and A.-.P Jauho, New Journal of Physics, 11 (2009), 095020 [2] R. Balog et al., Nature Materials, 9 (2010), 315. [3] J. Bai, X. Zhong, S. Jiang, Y. Huang, and X. Duan, Nature Nanotechnology, 5 (2010), 190; M. Kim, N. S. Safron, E. Han, M. S. Arnold, and P. Gopalan, Nano Letters, 10 (2010), 1125. [4] J. G. Pedersen and T. G. Pedersen, Physical Review B, 84 (2011), 115424. [5] J. G. Pedersen and T. G. Pedersen, Physical Review B, 85 (2012), 035413. [6] J. G. Pedersen and T. G. Pedersen, in preparation.
Figures
Gapped graphene in a magnetic field. Optical Hall conductivity in units of the DC graphene conductivity. Results are shown for a magnetic field B = 78 T and for four different values of the mass term, all calculated using the TB model with overlap. The chemical potential is in each case fixed at the lowest Landau level. Full (dashed) lines indicate the real (imaginary) part of the conductivity. The inset shows a closer view of the first resonance. The results of the TB model without overlap are shown by thin, black lines, illustrating the slight discrepancies between the models in this regime. The DE results are practically identical to those of the TB model without overlap.
Single graphene antidot in a magnetic field. (left) Radial probability distribution corresponding to the zeroth Landau level (LL0) and the two states energetically above (n=1) and below (n=-1) the LL0 level. Results are shown for two different values of the mass term in units of the cyclotron energy. Note the unique behavior of the LL0 state, which remains localized near the antidot edge at r=2, measured in units of the magnetic length. (right) Simulated STM measurements, showing the tunneling current as a function of position for four different values of the Fermi energy.
Graphene antidot lattice in a magnetic field. Density of states calculated for a structure containing roughly 1.2 million carbon atoms. The lower-energy features are well described by a gapped graphene model.
Graphene formation on SiC (0001) surface steps by CVD process 1,2
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K. Grodecki , R. Bozek , A. Wysmolek , R. StĊpniewski , W. Strupinski , 1,2 and J. M. Baranowski 1
Institute of Electronic Materials Technology, Wolczynska 133, 01-919 Warsaw, POLAND 2 Institute of Experimental Physics, Physics Department University of Warsaw, Hoza 69, 00-891 Warsaw, POLAND
Understanding of formation processes of epitaxial graphene as well as its interaction with SiC substrates is still very limited. In this communication we show that new and valuable information about physical properties of epitaxial graphene can be obtained from mutual studies including Atomic Force Microscopy (AFM), Scanning Kelvin Probe Microscopy (SKPM) and confocal micro-Raman spectroscopy, with submicron spatial resolution. Combination of these techniques allows for direct comparison of graphene morphology, thickness as well as strain state on terraces, step edges and macrosteps present at SiC substrates. Here we focus our attention on the graphene structures grown by Chemical Vapor Deposition (CVD) on 4H-SiC(0001) substrates, using propane as a source of carbon [1]. Typical AFM images obtained for graphene grown on 4H-SiC(0001) on-axis substrates show well resolved, nearly flat terraces of width of about 1 ± 10 m separated by macrosteps (typically 5nm high). Micro-Raman experiments performed on the same samples show that graphene on the terraces is very uniform and almost unstrained. The average position of Raman 2D line in these -1 -1 regions is between 2690 ±2700cm , whereas the half-width of the 2D line is of about 35-40cm , what -1 indicates a good quality of the graphene. On the other hand the 2D line is blue shifted to 2730cm on the step edges, what indicates strong compressive strained in these regions. Interestingly, the integrated intensity of the 2D line on the step edges is about twice larger than on terraces, what would be interpreted in terms of formation of thicker graphen. This result is consistent with the SKPM measurements which revealed different electric potential on the flat terraces and the step edges. These results are closely related to the observation of erosion of step front observed by AFM. The erosion spreads at distance 200-300 nm from the step edge and leads to the formation of sleeplike structures of 1nm high. In the eroded regions wrinkles are observed, what also indicates formation of thicker graphene multilayer. In summary, our results strongly suggest that in the CVD technique, similarly to the sublimation method, step bunching plays important role in the graphene formation. Apparently, graphene nucleates at the SiC macrosteps edges and grow laterally over the terraces regions above steps. *This work has been partially supported by Polish Ministry of Science and Higher Education projects 670/N- ESF-EPI/2010/0 and 671/N- ESF-EPI/2010/0 within the EuroGRAPHENE programme «EPIGRAT»of the European Science Foundation. [1] Graphene Epitaxy by Chemical Vapor Deposition on SiC, W. Strupinski, K. Grodecki; A.Wysmolek, R. Stepniewski, T. Szkopek, P. E. Gaskel, A. Gruneis, D. Haberer., R. Bozek, J. Krupka, and J. M. Baranowski Nano Letters 1, 1786 (2011)
Influence of Structural Defects and Chemical Functionalisation on the Mechanical Properties of Graphene a
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Songül Güryel , Balázs Hajgató , Jean-Marie Blarion , Hans E. Miltner , Frank De Proft , a b a Paul Geerlings , Yves Dauphin , Gregory Van Lier a) Free University of Brussels - Vrije Universiteit Brussel (VUB), Pleinlaan 2, 1050 Brussels, Belgium b) SOLVAY S.A., Innovation Center, rue de Ransbeek, 310, 1120 Brussels, Belgium sguryel@vub.ac.be In recent years graphene has gained significant attention. In particular, its excellent mechanical properties are an important advantage for the practical applications of graphene. These mechanical properties have extensively been investigated, and in particular, the Young’s Modulus has been predicted using a range of experimental and theoretical approaches. Many theoretical methods have been used to predict the mechanical properties of graphene. In particular, a supermolecular approach can be used, studying a finite graphene sheet. Using this approach with ab initio calculations has already allowed us to predict the Young’s modulus of graphene 1
as 1.11 TPa. The mechanical properties of pristine graphene can also be investigated by periodic calculations on infinite graphene sheets, and a value of 1.029 TPa was calculated at 0.3% elongation.
2
Nonetheless, stress-strain curves are seldom reported, and other mechanical properties such as the bending modulus were not investigated up to now. Structural defects in the graphene lattice can affect the electronic properties of graphene in unexpected ways, and harnessing the influence of these defects may be one method to control both the mechanical strength and electrical properties of a material. Defects change essentially not only the electronic properties but also the chemical properties of graphene. Real graphene systems contain structural defects and in experimental studies graphene oxides or reduced graphene oxides are often used. In addition, the influence of these defects on the mechanical properties is unclear. In this study, structural defects occurring in real graphene systems are considered by modeling single vacancies and oxygenated vacancies, and the Young’s modulus has been predicted by using semiempirical and Density Functional Theory (DFT) methods.
3
For a finite graphene sheet within a
supermolecular approach, the internal forces are calculated and the Young’s modulus predicted when external strain is applied on the system. These results are in a good agreement with theoretical and experimental results from the literature. In addition, the influence of the presence of a single vacancy, as well as for oxygenation of a vacancy, on the mechanical properties of graphene has been analysed. Our results indicate that the presence of structural defects in the system will stiffen the system upon low strain, but reduces the elastic limit from about 20% strain for pristine graphene to less than 10% strain when defects are present.
References [1] G. Van Lier, C. Van Alsenoy, V. Van Doren, P. Geerlings, Chem. Phys. Lett. 326 (2000) 181–185. [2] K. N. Kudin, G. E. Scuseria, B. I. Yakobson, Phys. Rev. B, 64 (2001) 235406. [3] S. Güryel, B. Hajgató, Y. Dauphin, J.-M. Blairon, H.E. Miltner, F. De Proft, P. Geerlings, G. Van Lier, submitted for publication (2012). Figure
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Graphene sheet containing a single vacancy (indicated in red), and elongated at two anchor atoms on each side of the sheet in the direction of the arrows (1), together with a colour coding of the geometry, depicting the deformation of the system upon 20% induced strain.
Graphene biosensors for disease biomarkers Owen J. Guy, Z. Tehrani, G. Burwell, S. Teixeira, S. Doak College of Engineering, Swansea University, Singleton Park, Swansea, UK o.j.guy@swansea.ac.uk Keywords: Biosensor, Biomarker, Generic biosensor, Functionalisation, Epitaxial graphene
The development of miniaturized systems for detection of disease biomarkers, at clinically relevant concentrations in biological samples, is key in the early diagnosis and monitoring of diseases. This paper presents the development of novel antibody functionalized epitaxial graphene devices for biosensing applications. This is the first reported example of an epitaxial graphene biosensor. A generic biosensor technology has been developed that is capable of detecting sub-nano molar concentrations of biological molecules. Growth of multilayer epitaxial graphene, on silicon carbide (SiC) substrates, has been performed under ultra high vacuum (UHV) conditions and under high-temperature / high vacuum growth condition. The graphene layers grown in this work have high structural integrity and exist as a continuous layer, extending over the terraced SiC substrate. In monolayer epitaxial graphene, the electron transport properties are dominated by the graphene-SiC interface layer. Multilayer graphene is less influenced by the substrate and has therefore been used in the fabrication of these sensors. The process for multilayer graphene grown has been investigated using X-ray Photoelectron Spectroscopy (XPS). Silicon Carbide (SiC) has been discovered to be a suitable substrate for graphene growth [1, 2]. During annealing at temperatures of between 1100ºC and 1700ºC the SiC surface reconstructs itself, with silicon atoms subliming and leaving behind a layer, or multiple layers, of epitaxial graphene [3]. Epitaxial JUDSKHQHœV VXSHUE HOHFWURQLF SURSHUWLHV KLJK FDUULHU PRELOLWLHV DQG UHSRUWHG ELRFRPSDWLELOLW\ >4], and substrate-inferred processability make it ideal for fabrication of nano-scale electronics and sensors. Few-layer epitaxial graphene (FLEG) has been grown on silicon carbide substrates and patterned into resistor channel devices (Fig. 1). The channels have subsequently been electrochemically functionalised with antibody bio-receptors. A generic electrochemical surface functionalisation chemistry, which can be used to attach a variety of ³ELR-UHFHSWRUV´ WR JUDSKLWLF VXUIDFHV KDV EHHQ GHYHORSHG The novel electrochemical method for attachment of antibodies to epitaxial graphene/SiC surfaces using chemical functionalisation of graphene with nitro groups and subsequent reduction to an amine, has been monitored using X-ray Photoelectron Spectroscopy (XPS) (Fig. 2). Subsequent attachment of a fluorescently labeled antibody to the graphene surface has been confirmed using fluorescence microscopy, in the first known biofunctionalisation of epitaxial graphene on SiC (Fig. 3). This change can be detected as an electrical signal from the biosensor, enabling highly sensitive detection of biomarker analytes. The electrochemical functionalisation technology reported in this paper is a generic platform for the attachment of any number of antibodies or other bioreceptor molecules. Several antibodies including those targeted against Beta-actin, 8-OHdG and troponin have been covalently attached to graphite and to graphene. Attachment has been verified using laser scanning confocal fluorescence microscopy (LSCM) and atomic force microscopy (AFM). AFM shows an increase in surface roughness from 1nm before functionalisation to around 2nm after antibody attachment. Fluorescence measurements were conducted by using a labeled secondary antibody to the surface bound primary antibody. The
functionalisation technology has been integrated with a graphene electronic device to fabricate a prototype biosensor which has been used to detect nM concentrations of the oxidative stress biomarker 8-OHdG. 7KH ELRVHQVRUV ZRUN RQ WKH SULQFLSOH RI D WDUJHW GLVHDVH ELRPDUNHU ELQGLQJ ZLWK D ³ELR-UHFHSWRU´ attached to the graphene surface, yielding a change in the surface charge density. This change can be detected as an electrical signal from the biosensor device. The device itself consists of a conductive graphene channel ¹ IXQFWLRQDOLVHG ZLWK WKH ³ELRUHFHSWRU´ *UDSKHQH QDQR-channel sensors have the potential for much greater sensitivity to biomarkers than traditional bioassays because of their high signal-to-noise ratios (S:N). The results from a specific sensor, fabricated by functionalising a graphene nanochannel surface with an antibody bioreceptor, indicative of oxidative stress and prostate cancer risk, will be presented.
[1] O.J. Guy et al. J. Appl. Surf. Sci. ¾,QYHVWLJDWLRQ RI WKH +-6L& VXUIDFHœ $SSO Surf. Sci. 254, p.80988105, 2008. [2] C. Berger et al., Science 312, p.1191, 2006. [3] A. Castaing, O.J. Guy, M. Lodzinski, 6 3 :LONV ³,QYHVWLJDWLRQ RI *UDSKHQH *URZWK RQ +-6L&´ Mater. Sci. Forum 615-617, 223-226, 2009. [4] X. Sun, Z. Liu, K. Welsher, J.T. Robinson, A. Goodwin, S. Zaric, H. Dai, Nano Res. 1: 203, p.212, 2008.
Fig. 1: (a) Schematic of graphene devices and (b) SEM image of epitaxial graphene microchannel device. 25000 Intensity
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Fig. 2: XPS spectrum of nitrobenzene functionalized graphene surface. Inset: N 1s peak, conversion of nitro to amino group upon subsequent electrochemical reduction to aniline.
Epitaxial Graphene selectively functionalized with a Quantumdot labeled antibody.
Fig. 3: Laser scanning confocal fluorescence micrograph of epitaxial Graphene selectively functionalized with a Quantum-dot labeled antibody.
Gap engineering in atomically thin materials James P. Hague Department of Physical Sciences, The Open University, Walton Hall, Milton Keynes, MK7 6AA, UK J.P.Hague@open.ac.uk A perturbative theory is presented for the strong enhancement of graphene-on-substrate bandgaps by attractive interactions mediated through phonons in a polarizable superstrate. By constructing a set of self-consistent eqautions, it is demonstrated that gaps of up to 1eV can be formed for experimentally achievable values of electron-phonon coupling and phonon frequency [1]. As shown in figure 1, gap enhancements computed using perturbation theory range from 1 to 4, indicating possible benefits to graphene electronics through greater bandgap control for digital applications, through the relatively simple application of polarizable materials. Additionally, polaron spectral functions are computed for heavily doped graphene-on-substrate systems using the diagrammatic quantum Monte Carlo technique to investigate the effects of interaction on spectral functions when the symmetry between graphene sub-lattices is broken by a substrate [2]. Several polaronic features are visible, including band-flattening and changes in particle lifetimes. The difference between energies on each sub-lattice increases with coupling, indicating an augmented transport gap at the K point, while the spectral gap decreases slightly (as shown in figure 2). In the absence of a gap, additional flattening is found around the K point. I also discuss the effects of substrates on atomically-thin graphene-like materials such as boron nitride (BN) and silicene. To cope with more complicated momentum-dependent interactions that may be formed between electrons and phonons within monolayers, I reintroduce momentum dependence to the perturbation theory. Gap enhancements are reassessed, and useful forms of the interaction for gap tuning are identified.
References [1] J.P.Hague. Tunable graphene band gaps from superstrate-mediated interactions. Phys. Rev. B, 84 (2011) 155438. [2] J.P.Hague. Polaron effects in heavily doped graphene on substrates. arXiv:1107.2507 (2011) Figures
Figure 1: Substrate gap enhancement vs superstrate mediated electron phonon coupling O. ' is the non interacting gap, T the temperature, t the hopping and : the phonon frequency.
Figure 2: Spectral function, A(E), for heavily doped graphene at the K point.
Computation of Intrinsic Mechanical Properties of Double Layer Graphene a
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BalĂĄzs HajgatĂł , SongĂźl GĂźryel , Jean-Marie Blarion , Hans E. Miltner , Frank De Proft , a b a Paul Geerlings , Yves Dauphin , Gregory Van Lier a) Free University of Brussels - Vrije Universiteit Brussel (VUB), Pleinlaan 2, 1050 Brussels, Belgium b) SOLVAY S.A., Innovation Center, rue de Ransbeek, 310, 1120 Brussels, Belgium hajgato@vub.ac.be Until the second trimester of the late century only two ordered forms of carbon were known to scientists, namely diamond, with its perfect crystal structure, and graphite, also crystalline but black and flaky and not at all transparent. Besides those ordered forms, also coal, coke, soot, lampblack, and the many kinds of charcoal were known. The graphite structure reflects its properties, since it is made up of sheets of carbon atoms arranged in a hexagonal lattice, like a honeycomb of fused benzene rings, and with weak bonding between adjacent sheets. This means that graphite easily forms flakes where the sheets can slide over each other, providing use of graphite as a lubricant, and resulting good electrical conductivity. But it is only in 2007 that researchers in Manchester found a way to mechanically peel 1
single two-dimensional sheets from three-dimensional graphite crystals . Graphene is the name given to this flat monolayer of carbon atoms tightly packed into a two-dimensional honeycomb lattice. 1
Since the first experimental analysis , graphene has recently gained significant attention. In particular, its excellent mechanical properties are an important advantage for the practical applications of graphene. These mechanical properties have extensively been investigated, and in particular, the <RXQJÂśV 0RGXOXV Kas been predicted using a range of experimental and theoretical approaches. On the experimental side, by ultrasonic, sonic resonance, and static test methods, Blakslee et. al.
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annealing under compressive stress. Frank et al. measured the modulus for a stack of graphene sheets (less than five layers) to be 0.5 TPa using an atomic force microscope. More recently, by nanoindenting the centre of a free-standing monolayer graphene membrane with an atomic force 4
microscope, Lee et al. PHDVXUHG WKH <RXQJÂśV PRGXOXV DV 73D DVVXPLQJ WKH WKLFNQHVV RI graphene to be 0.335 nm. Many theoretical and computational studies have also been performed to investigate the mechanical 5
properties of graphene, for example, the pioneering study by Van Lier et al. , using super-molecular approach. There are numerous theoretical studies using a super-molecular approach, to calculate 6
mechanical properties, however the number of infinite (periodic) calculations is very scarce . Stressstrain curves are seldom reported, and other mechanical properties for example bending modulus were not investigated up to now. ,Q WKLV VWXG\ WKH <RXQJÂśV and bending moduli of single and double layer graphene have been theoretically investigated using Periodic Boundary Condition (PBC) Density Functional Theory (DFT) with the PBE, HSEh1PBE, and M06L functionals in conjunction with the 6-31G* and the 3-21G basis sets. The unit-cell size and shape dependence as well as the directional dependencies of the mechanical properties have been also investigated. The calculated stretching and bending strain-stress curves are also reported (see figures).
References [1] A. K. Geim, and K. S. Novoselov, Nature Materials, 6 (2007) 183±191. [2] O. L. Blakslee, D. G. Proctor, E. J. Seldin, G. B. Spence, T. Weng, J. Appl. Phys. 41 (1970) 3373± 3382. [3] I. W. Frank, D. M. Tanenbaum, A. M. van der Zande, P. L. McEuen, J. Vac. Sci. Technol. B 25 (2007) 2558±2561. [4] C. Lee, X. Wei, J. W. Kysar, J. Hone, Science 321 (2008) 385±388. [5] G. Van Lier, C. Van Alsenoy, V. Van Doren, P. Geerlings, Chem. Phys. Lett. 326 (2000) 181±185. [6] K. N. Kudin, G. E. Scuseria, B. I. Yakobson, Phys. Rev. B, 64 (2001) 235406. Figure
Stress-Strain curves for double-layer graphene, using all employed methods (only 3-21G basis set). Both the errors between the minimum and maximum in percentage and the number of different levels of theoretical calculations (# DATA) are displayed on the right axis.
Direct transfer of graphene without the removal of a metal substrate using a liquid polymer Chang-Soo Han, Changhyun Kim, Junghee Park Korea University, Anam Seongbuk, Seoul, 136713, Korea cshan@korea.ac.kr We report a facile and effective method for the direct and controlled transfer of multilayer graphene from a Co substrate to polydimethylsiloxane (PDMS). Liquid PDMS was used for conformal contact with graphene, and the amount of curing agent was varied to control the surface energy of the PDMS. When the Co substrate was detached from the solid PDMS, graphene was transferred to the PDMS. The number of graphene layers transferred depended on the mixing ratio of the curing agent. Moreover, the morphology of the Co substrate was also transferred to the PDMS surface. This method enables the transfer of graphene on a wavy and uneven surface. Figure 1 illustrates the process of graphene transfer using liquid PDMS. First, we placed multilayer graphene on a Co substrate in the rectangular frame, leaving space around the perimeter. Next, liquid PDMS and the curing agent were poured into the frame. To remove bubbles from the liquid PDMS, the sample was evacuated in a vacuum chamber for 10 min. After hardening for 60 min at 60°C in air, the solid PDMS was carefully separated from the frame. Finally, the Co substrate was manually detached from the PDMS. An advantage of using this process is that the Co substrate can be reused. We considered that graphene transfer may have occurred via three types of adhesion: PDMS±graphene, graphene±graphene, and graphene±Co substrate. If PDMS±graphene had a greater adhesion force than graphene±graphene, graphene would be transferred to PDMS. Under this assumption, the amount of curing agent should control the adhesion force between PDMS and graphene. Moreover, solutionphase PDMS exhibited a strong adhesion force due to fully conformal contact with the graphene surface. When the mixing ratio of the curing agent was varied, the adhesion force between PDMS and graphene changed, allowing control of the number of graphene layers transferred.
To characterize graphene before and after transfer, Raman spectra (Fig. 3) were collected for the Co substrate before and after transfer, and for the PDMS surface after transfer. The D peak at 1360 cm±1 provides information on graphene defects during synthesis and transfer. The graphene samples on the Co substrate showed few defects before transfer. The D peaks for graphene on PDMS were similar before and after transfer. Thus, the intrinsic structure of graphene was preserved during transfer to PDMS. Additionally, the position, shape, and intensity of G and 2D peaks were used to determine the number of graphene layers. In our experiments, the four samples of graphene synthesized on the Co substrate were multilayer graphene. After graphene transfer, the intensity and shape of the Raman peaks were distinct compared to the peaks of pristine graphene. The positions of the 2D and G mode peaks did not change, indicating that graphene was transferred from the Co substrate without structural changes. References [1] Changhyun Kim, Ju Yeon Woo, Kyungnam Kim, Jinwoong Choi, Junghee Park and Chang-Soo Han, Scripta Merialia, 66 (2012) 535.
Figures (a)
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Figure 1. Transfer of graphene from a Co substrate using liquid PDMS. (a) The substrate was placed on a metal box frame. (b) After the curing agent and PDMS were mixed together, the liquid was poured into the box. (c) After curing, PDMS was separated from the box, and (d) the Co substrate was detached from PDMS.
Figure 2. Raman spectra of graphene on the Co substrate before transfer (left), after transfer (middle), and on PDMS after the transfer for four samples (right).
Towards a realistic model of nanographene – linking theory and experiment Jack Baldwin, Richard Taylor and Y. Hancock Department of Physics, The University of York, Heslington, York, UK, YO10 5DD y.hancock@york.ac.uk Accurate modelling of nanographene requires large-scale calculations that take into account realistic device features and system-sizes. Such calculations, however, are mostly beyond the computational reach of density functional theory simulations, and hence more efficient and accurate methods of simulation are actively being sought.
In this work, we have chosen as our starting point, a generalised tight-binding (TB) transport model for nanographene [1], which has been shown to faithfully reproduce density functional theory results (Figs. 1 & 2). The TB model is computationally tractable and can be efficiently scaled to calculate experimentally relevant nanoscale structures. The next step in our model development has been to produce a graphic user interface for graphene (GRUI) (Fig. 3(a)), which has enabled a direct link between theory and experiment using structural input from TEM and STM images (Fig. 3(b)). In linking theory to experiment we aim to develop greater accuracy simulations by direct comparison of our model output (for example, density of states and transport calculations) with STM tunnelling current results, as well as experimental transport measurements. Such comparisons will enable us to parameterise our TB model directly against experiment rather than to ab initio results, as well as provide a means to easily include realistic structural features (defects, strain, vacancies, etc) and system sizes. During this talk I will demonstrate the use of the GRUI and highlight its potential to facilitate the development of greater accuracy models, therefore enabling our physical understanding of nanoscale graphene. References [1] Hancock, Y. et al., Physical Review B 81 (2010) 245402. [2] Huang, B. et al., Physical Review B 77 (2008) 153411. [3] Girit, Ç.Ö., et al., Science 323 (2009) 1705. 0.24
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Fig. 1: The generalised TB model is comprised of up to third nearest-neighbor hopping and a mean-field Hubbard-U term. The graph shows the calculated magnetic moment per edge-atom that has been obtained using this model at three systematic edge-vacancy concentrations corresponding to the structures shown in Fig. 2. Comparison with Fig. 1(g) of Ref [2] demonstrates the high-level of accuracy of the TB model against ab initio calculations.
Fig 2: Zig-zag nanoribbon structures showing the computed local net spin-polarisations as obtained from the generalisd TB model. Here red (blue) refer to a net spin-up (spin-down) and green refers to a zero local spinpolarisation. The structures correspond to the calculated local magnetic moment per atom (Fig. 1) and from left to -1 -1 -1 -1 right are for 0.0Å , 0.034Å , 0.068Å and 0.136Å symmetric defected systems, respectively.
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Fig 3: (a) Screen-shot of the graphic user interface for graphene systems (GRUI) and (b) use of the GRUI showing the overlay of the graphene-mesh onto the experimental TEM result of Girit et al. [3]. The mesh can be positioned over the experimental images and the individual mesh-atoms can be moved so that they sit directly over the experimental result. The GRUI then automatically re-parameterises the TB model to take into account the local atomic perturbations. Experimental image from Girit, Ç.Ö., et al., Science 323 (2009) 1705. Reprinted with permission from AAAS.
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Orbital Magnetism in graphene bulk and nanostructures Lisa Hesse, Juergen Wurm, Klaus Richter Institut f端r Theoretische Physik I, Germany lisa.hesse@physik.uni-regensburg.de
We study the magnetic response of finite and bulk graphene structures due to orbital motion of the charge carriers. Besides a semiclassical approach we use exact quantum mechanical calculus within the Dirac formalism to derive different analytic expressions for the magnetic susceptibility of extended systems at various field regimes. This allows us to study on the one hand edge effects which are accessible through our semiclassical treatment but also to gain profound knowledge of the importance of bulk effects in finite systems. In order to provide an independent confirmation of the theory we also perform numerical calculations on graphene nanostructures based on a tight-binding approximation.
Photoluminescence in Graphene Antidot lattices
S. Heydrich, D. Hutzler, J. Eroms, D. Weiss, T. Korn, C. Schüller
Institut für Experimentelle und Angewandte Physik, Universität Regensburg, 93040 Regensburg, Germany stefanie.heydrich@physik.uni-regensburg.de
Since its experimental isolation, graphene has generated a lot of interest, even spawning research into other two-dimensional materials like MoS2. While semi-conducting MoS2-singlelayers naturally emit photoluminescence (see e.g. [1]), this is not expected for graphene due to its gapless bandstructure. However, as was reported previously [e.g. 2,3], femtosecond pulsed laser excitation creates an electron-hole plasma in graphene which emits a broadband luminescence. Here, we present recent observations of the behavior of photoluminescence in nanostructured as well as in pristine graphene. The structure consists of a regular antidot pattern. It has been written by electron beam lithography and was etched using reactive ion etching with oxygen as reactive gas. Distance of the antidots is 100nm. We utilize fast, high-resolution scans to map graphene flakes on Si/SiO2-substrates. Thus, a luminescence intensity image of both the flake and its structured areas is created. In structured areas, the absolute intensity of the photoluminescence is smaller than in pristine flakes due to parts of the graphene having been etched away. However, the observed signal in the patterned flakes is larger than expected from the mere amount of illuminated graphene. This enhancement is roughly 1/15 of the expected signal in single layer and 1/7 in bilayer graphene. A possible explanation could be additional charge carriers at the antidot edges [4] causing this increase in luminescence. Financial support by the DFG via GRK 1570 is gratefully acknowledged.
References
[1] T. Korn, S. Heydrich, M. Hirmer et al., Appl. Phys. Lett. 99, 102109 (2011). [2] R.J. Stöhr, R. Kolesov, J. Pflaum et al., Phys. Rev. B 82, 121408(R) (2010). [3] C.H. Lui, K.F. Mak, J. Shan et al., Phys. Rev. Lett. 105, 127404 (2010). [4] S. Heydrich, M. Hirmer, C. Preis et al., Appl. Phys. Lett. 97, 043113 (2010).
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R1 AD1
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Fig. 1: (a) Microscope image of graphene flake on Si/SiO2. Patterned areas AD1(single layer) and AD2 (lower part single layer, upper part bilayer) are denoted by white rectangles. Reference areas R1 (single layer) and R2 (bilayer) were left unstructured. (b) SEM image of a part of area AD1. The graphene appears dark gray, the lighter, regular pattern of the circular antidots where the graphene has been etched away is clearly visible. Distance of the antidots is 100nm. (c) Intensity of PL scan of the flake depicted in (a). Scanned area is marked by dashed black rectangle in (a). Clearly, the intensity is lower in the patterned areas AD1 and AD2 than in the reference region R1.
surface ratio: (expected PL intensity)
PL intensity ratio: experimental value
AD1/R1
80%
85%
AD2/R2
68%
80%
Fig. 2: Table comparing expected values and experimental values from the scan shown in Fig. 1. The expected values are taken to be the surface ratio between graphene and antidots, that is substrate. In unpatterned areas, this ratio is 100%. Experimental values in patterned areas are compared to the experimental value of the reference areas, which are taken to be 100%. All values are mean values taken from the scan.
Chemical Analysis and Thermal Curing Effects of CVD graphene during Transfer Process
hsk310@kaist.ac.kr
References
Figures
50000
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40000 30000 20000
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20000
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60000
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70000
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Binding Energy (eV)
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<Mechanically exfoliated graphene>
<CVD graphene on metal substrate>
<CVD graphene on dielectric>
800
Mobility [cm2/Vs]
Counts / s
25000
Raw Intensity Background C=C C-OH C=O O=C-OH
80000
Hall effect meaurement Graphene FET
<Graphene FET structure> Drain
Source
750 700 650
O=C-OH removing
600 550
No annealing
HV anneal Ar anneal H2 anneal
Post Process
Back Gate
Decorated Carbon Nanostructured Electrodes for Biofuel Cell applications L. Hussein
1,2*
1
, F. Olcaytug , and G. Urban
1,2
1
Department of Microsystems Engineering (IMTEK), Laboratory for Sensors, University of Freiburg, Germany 2 Freiburg Materials Research Centre (FMF), University of Freiburg, Germany *E-Mail: laith.hussein@fmf.uni-freiburg.de
One-compartment biofuel cells are considered as an attractive power sources and they are in high demand for small biomimetic-based medical devices [1].
This kind of cells (Fig. 1) needs very active and tolerant catalysts which are highly-important for mixedreactant system applications. Therefore, Buckypapers (BPs) were fabricated from commercially available carbon nanotubes (CNTs) (Fig. 2a) and decorated with redox enzymes (Fig. 2b) [1-4]. BP is mesoporous, highly conductive, flexible and mechanically stable material [2]. A high enzyme molecules loading on BP can be achieved due to its large mesoporous surface area which enhances also the utilization and electroactive surface area (i.e. coverage effect).
The enzymatic cathodes based on bilirubin oxidase or laccase [4] decorated-BP electrodes showed significant enhancements of bioelectrocatalytic performance with direct electron transfer for molecular dioxygen reduction reaction [3,4].
Using such three-dimensional electrodes based on BP or redox hydrogel matrix can solve the problems associated with the use of traditional two-dimensional electrodes. The performance of the cells was studied depending on the operation conditions. The highest power output of 26 ¾W cm-2 at + 0.20 V was achieved in O2-saturated solution (pH 7.2 at 37 °C) with a physiological glucose concentration of 5 mM. Moreover, in contrast to the literature, our bioelectrochemical system seems to be more effective and reproducible in O2-saturated phosphate buffer solution than air [5].
Furthermore in this contribution, our progress on synthesizing novel carbon nanostructures by inductively coupled radio frequency (RF) plasma enhanced chemical vapour deposition (PECVD) will be presented as well. The resulting vertically-aligned carbon nanostructures (CNs) of 2D carbon nanowalls (Fig. 3a) and 1D nanofibers (Fig. 3b) can be controlled and grown on different surfaces. These nanostructures can accelerate the bioelectrocatalytic reactions by facilitating the mass transport and thus the rapid diffusional fluxes of reactants and products to internal surfaces. References (1) L. Hussein, Y. Feng, A. Habrioux, K. Servat, B. Kokoh, N. Alonso-Vante, G. Urban, M. Krueger, Transducers 2009, pp. 2254. (2) L. Hussein, G. Urban, M. Krueger, Phys. Chem. Chem. Phys., 2011, 13, 5831. (3) L. Hussein, Y. J. Feng, N. Alonso-Vante, G. Urban, M. Krueger, Electrochimica Acta, 2011, 56, 7659. (4) L. Hussein, S. Rubenwolf, F. von Stetten, G. Urban, R. Zengerle, M. Krueger, S. Kerzenmacher, Biosensors and Bioelectronics, 2011, 26, 4133. (5) N. Mano, N, F. Mao, A. Heller, Journal of American Chemical Society 2003, 6588.
Figure 1: Schematic configuration of the membraneless glucose fuel cell (DGFC), employing glucose and oxygen as a fuel and an oxidizer, respectively. RE represents reference electrode.
Figure 2: SEM a) and TEM b) images of the laccase decorated-BP electrode.
Figure 3: SEM images of different vertically-aligned carbon nanostructures: (a) carbon nanowalls, (b) carbon nanofibers.
Structured Graphene Âą Spinnable CNT and Beyond Chi P. Huynh
1, 2
1
1
1
, Stephen C. Hawkins , Mark Hickey , Amanda Barnard and Tim Williams
3
1. CSIRO Materials Science and Engineering, Clayton, Victoria 3168, Australia 2. Department of Materials Engineering, Monash University, Clayton Victoria 3168, Australia 3. Monash Centre for Electron Microscopy, Monash University, Clayton Victoria 3168, Australia Chi.Huynh@csiro.au Graphene, a hexagonal lattice of carbon only a single atom thick, is attracting immense interest due to its exceptional properties of electrical conductivity, thermal stability, and mechanical strength as well as its unique electronic properties. Many potential applications such as nano-scale electronic devices, sensors and interconnects need graphene structures with well-controlled feature size and tuneable electronic properties, such as arrays of graphene Âľnano-ribbonsÂś Âľnano-JULGVÂś DQG ÂľQDQRVWUXWVÂś DV a means of assembling the new nano- materials. Also required are structures to increase surface area or structural stability of graphene films, for example to produce dye-sensitised and flexible-organic solar cells. A range of methods have been used to produce patterned or structured graphene, including photolithography, direct laser writing, micro-contact printing as well as the direct growth of graphene 1 on pre-patterned catalyst substrates. For example Kim et al evaporated Ni through a mask on to a Si/SiO2 substrates to make circular- and strip-patterned graphene for integrated devices. We have developed a simple and efficient process to produce freestanding graphene membranes patterned and reinforced with highly aligned Âľ'LUHFWO\ 6SLQQDEOH &arbon Nanotube '6&17 ZHEÂś also produced in our laboratory. The DSCNT are specially grown CNT forests that can be drawn (spun) directly from the growth substrate as a continuous web which can be used directly or twisted into a yarn. The web provides the carbon for graphene growth as well as reinforcing and patterning the surface structure. We have also used commercial Cu TEM grids with different grid patterns alone and in conjunction with DSCNT and copper foil to create a range of self supporting graphene (Figure 1) structures. 2
The DSCNT was prepared according to our published methods and, from this; up to six layers of highly aligned web was laid down on a pre-annealed copper substrate. Graphene was grown using a 3 CH4 as the carbon source . It is also possible to adjust conditions to utilise carbon from the DSCNT structure, and this method is being developed further. Commercial Cu TEM grids were annealed either in direct contact with the copper substrate or placed on top of the DSCNT layer prior to Graphene synthesis (Figure 1). 4
Raman spectroscopy of graphene produces two major peaks and one minor . The D peak at 1350 -1 cm is associated with disorder and is very weak in a single graphene sheet away from edges. The G -1 peak at ~1580 cm is usually weak and narrow for single layer graphene, growing with number of Âą1 layers to be the largest peak in graphite. The 2D band at ~2700 cm is very sensitive to the number of graphene layers in the sample, broadening and developing a second peak at higher wavenumbers. There is some correlation in the intensity of G/ 2D peak height with number of layers up to about 5. We find by Raman (514 nm) (figure 2) that from our process with methane but without the DSCNTs, we grow about three layers of graphene. This is confirmed by TEM (Figure 3c & 3d) which shows three or 4 layers of graphene in the plain graphene sample. Raman analysis (Fig 2) of the graphene structures with DSCNTs show more graphite character as they are about 7 shells thick and so appear more disordered. Removal of excess (XS) DSCNTs gives a Raman spectrum more comparable to the pure graphene. The DSCNT web is seen (Fig 3a & 3b) to have blended into the graphene surface structure and modelling and further analysis are being conducted to understand the geometry of this connection. Placement of Cu TEM grids on a copper substrate followed by annealing (Fig 1) results in the grid being firmly bonded to the substrate. Where the grid edges are in close contact, a smooth boundary transition to the substrate results, whereas if the grid edge is separated, it remains distinct. A TEM grid placed on a layer of DSCNT and annealed suffers major distortion. Graphene grown on the TEM / copper substrate follows the geometry of the combined structure.
References 1) Kim, K. S.; Zhao, Y.; Jang H.; Lee, S. Y.; Kim, J. M.; Kim K. S.; Ahn, J. ; Kim P.; Choi J.; Hong, B. H. Nature letter, Vol 457, (2009) 2) Huynh, C. P.; Hawkins, S. C. Carbon 2010, 48, 1105±1115 3) Li, X.; Cai, W.; An,J.; Kim, S.; Nah, J.; Yang, D.; Piner, R.; Velamakanni, A.; Jung,I.; Emanuel Tutuc, E.; Banerjee, S. K.; Colombo, L.; Ruoff, R. S.; Science, Vol 324 4) Ferrari, A. C.; Meyer, J. C.; Scardaci, V.; Casiraghi, C.; Lazzeri, M.; Mauri, F.; Piscanec, S.; Jiang, D.; Novoselov, K. S.; Roth, S.; Geim, A. K. Phys. ReV. Lett. 2006, 97 (18), 187401±4 Figures
Fig.1 - Showing spinnable CNTs and Cu grids on top of Cu foils.
Fig. 2 ± Raman spectra of spinnable CNT , WebGrown spinnable CNT graphene and graphene on Cu foil
Fig.3- SEM of DSCNT crossed web (A), Cu substrate coated with graphene that has been patterned by CNT during growth (B, CNT has been removed), TEM of 10 nm diameter CNT and 3 layers graphene (C) and TEM of graphene footprint of CNT (D). The wavy circular patterns are typical µJKRVWLQJ¶ RI WKH &17 ZHEV
Chemical modification of graphene with Cl Mari Ijäs, Paula Havu, Ari Harju Department of Applied Physics, Aalto University School of Science, FI-00076, Espoo, Finland mari.ijas@aalto.fi
Chemical functionalization of graphene is an attractive and versatile means to modify its electronic properties to suit the intended applications. For example, a band gap can be induced through introduction of hydrogen [1], or fluorine [2] atoms onto the carbon network. Both for hydrogen and fluorine, theoretical calculations [3,4] agree well with the experimental results, although one-sided attachment of hydrogen on free-standing graphene should not be energetically stable. In the case of the next halogen species, chlorine, theoretical approaches [5], however, predict no covalent binding between the chlorine atoms and pristine graphene, whereas a recent experiments report signals associated with covalent C-Cl bonds [6,7]. Our density-functional theory calculations do not show covalent C-Cl bonding between chlorine and graphene, not even in the case where the van der Waals interaction and the presence of a silicon dioxide substrate are taken into account. Of the four considered surface terminations, there is a weakly stable chlorinated graphene structure only on the hydroxylterminated surface. This is similar to the case of graphene hydrogenation on SiO2 [8]. We are able to provide an alternative explanation to the experiments. Using an ab initio thermodynamics approach, we show that the experimental observations may be explained by the formation of graphene segments with chlorine-containing edges [9]. We calculate the energetic stability of a large number of edge terminations with different degree of chlorination, both in terms of edge formation energy and chlorine binding energy. We also consider both reaction conditions with atomic chlorine, and ambient conditions with molecular chlorine as the reference state. The low-energy structures are compatible with the lowest-energy hydrogenated edge structures [10]. Chlorinecontaining armchair edges are, in general, more stable than their zigzag-based counterparts, and most importantly, even in ambient conditions, some of them are more stable than chlorine atoms adsorbed onto the basal plane of graphene. a)
b)
Figure 1: (a) Chlorine on freestanding graphene (b) chlorinated graphene on the O-WHUPLQDWHG ÄŽ-quartz surface.
Contribution (Oral/Poster/Keynote) a)
b)
c)
d)
Figure 2: The stability of different chlorine-containing graphene edges. a) prisine graphene not taken into account b) pristine graphene region included. The superscripts a and z refer to armchair- and zigzag- type edges. c) The lowest-energy chlorinated zigzag edge [H-Cl-HCl] and d) the lowest-energy chlorinated armchair edge [HCl (cis)] in vacuum conditions. References [1] D. C. Elias et al., Science 323 (2009) 610. S. Ryu et al., Nano Lett. 8 (2008) 4597. [2] J. T. Robinson et al., Nano Lett. 10 (2010) 3001. R. R. Nair et al., Small 6 5 =ERĜLO HW al., Small 6 (2010) 2885. [3] J. O. Sofo, A. S. Chaudhar and G. D. Barber, Phys. Rev. B 75 (2007) 153401. [4] O. Leenaerts, H. Peelaers, A. D. Hernández-Nieves, B. Partoens, F. M. Peeters, Phys. Rev. B 82 (2010) 195436. [5] M. Klintenberg, S. Lebègue, M. I. Katsnelson, O. Eriksson, Phys. Rev. B 81 (2010) 085433. P. V. C. Medeiros, A. J. S. Mascarenhas, F. de Brito Mota, C. M. C. De Castilho, Nanotechnology 21 (2010) 485701. [6] B. Li, L. Zhou, D. Wu, H. Peng, K. Yan, Y. Zhou, Z. Liu, ACS Nano 5 (2011) 5957. [7] J. Z. Wu, L. Xie, Y. Li, H. Wang, Y. Ouyang, J. Guo, H. Dai, J. Am. Chem. Soc. 133, (2011) 19668. [8] M. Ijäs, P. Havu, A. Harju, P. Pasanen, Phys. Rev. B 84, (2011) 041403(R). P. Havu, M. Ijäs, A. Harju, Phys. Rev. B 84, (2011) 205423. [9] M. Ijäs, P. Havu, A. Harju, Phys. Rev. B 85, (2012) 035440. [10] T. Wassmann, A. P. Seitsonen, A. M. Saitta, M. Lazzeri, F. Mauri, Phys. Rev. Lett. 101, (2008) 096402. T. Wassmann, A. P. Seitsonen, A. M. Saitta, M. Lazzeri, F. Mauri, J. Am. Chem. Soc. 132, (2010) 3440.
Surface Potential Variations in Graphene Induced by Crystalline Ionic Substrates Gavin J. Jones, Asieh Kazemi, Ying Wu, Simon Crampin, Adelina Ilie* Centre for Graphene Science & Department of Physics, University of Bath, Bath BA2 7AY, UK a.ilie@bath.ac.uk Controlling and modulating the surface potential of the graphene sheet is important for producing onsheet junctions and superlattices. Such electronic structures are predicted to play an important role in building devices that exploit the novel Dirac nature of carriers in graphene, such as electron guides [1] and electron-beam supercollimators [2]. Graphene regions with different doping levels and, hence, surface potentials have to date been produced by electrostatic gates [3] or through chemical functionalization [4], both strategies requiring complex top-down lithographic procedures. We used Electrostatic Force (EFM) and Kelvin Probe (KP) microscopies to investigate few-layer graphene (FLG) domains on top of ionic crystals [5]. Experiments are supported by Density Functional and model calculations of graphene on stepped surfaces. Step edges, pits and protrusions within the ionic surface create sizeable and local perturbations of the surface potential of graphene overlayers. These were within the eV range in FLG with up to three layers, and become considerably screened in thicker layers. Such nanostructures could pave the way towards bottom-up creation of on-sheet p-n junctions and superlattices, as well as provide a test bed for studying local screening in graphene. References [1]. J. R. Williams, T. Low, M.S. Lundstrom and C.M. Marcus: Nat. Nanotech. 6 (2011) 222. [2]. C.-H. Park, Y.-W. Yang, M.L. Cohen, S.G. Louie: Nano Lett 8 (2008) 2920. [3]. J.R. Williams, L.DiCarlo, C.M. Marcus: Science 317 (2007) 638. [4]. H.-C. Cheng, R.-J. Shiue, C.-C. Tsai, W.-H. Wang, and Y.-T. Chen: ACS Nano 5 (2011) 2051. [5] G. J. Jones, A. Kazemi, S. Crampin, M. Philips, A. Ilie: Applied Physics Express (in press).
Figures
. FLG flake on a terraced ionic surface investigated by KP microscopy. (a) KP of the bare ionic substrate. Steps induce sharp variations in the surface potential. (b) Topography, with boundaries of FLG domains and substrate highlighted: green, substrate regions; blue, four-layer graphene; bi-layer graphene, between the two. (c) Amplitude image of the FLG. Labeled representative features: step edges (triangle), pits (cross), protrusions (encircled. (d) Surface potential image corresponding to (c). Bilayer domains are more strongly perturbed by the underlying nanostructures of the substrate than the 4-layer ones. (e) Surface potential originating from edges (crosses), and pits/protrusions (circles/squares), as a function of number of graphene layers, measured with the tip several nm away from surface. Band around 0 marks the noise level of the KP measurement.
Plasmonic resonances in the infrared spectra of nanostructured graphene 1
1
1
1
1
*RUDQ ,VLĂź , %RULVODY 9DVLĂź $OHNVDQGDU 0DWNRYLĂź 8URĂŁ 5DOHYLĂź , Angela Beltaos , Marko 2 1 6SDVHQRYLĂź 5DGRĂŁ *DMLĂź Institute of Physics, Pregrevica 118, 11080 Zemun (Belgrade), Serbia ICFO-Institut de CiĂŠncies Fotoniques, Mediterranean Technology Park, Castelldefels (Barcelona), 08860, Spain isicg@ipb.ac.rs Surface plasmons can be understood as electromagnetic waves bound to a surface between a conductor and insulator. In last two decades, they became one of the focal points in photonics due to their ability to support strong electromagnetic fields confined at the conductor-insulator interface well below the diffraction limit. Graphene has been seen as an interesting ground for plasmonics in the infrared (Jablan et al., 2009, Koppens et al., 2011) owing to its thinness which offers the possibility of changing its carrier density over several orders of magnitude by means of a gate voltage (Novoselov et al., 2004). While possible in theory, in practice plasmons in graphene are not expected at frequencies above mid-infrared (e.g. in the visible spectrum) due to the limited carrier concentration that can be generated by a gate voltage before the SiO2 layer that supports graphene breaks down. The field of graphene plasmonics is currently receiving a lot of attention in the literature (Nikitin et al., 2012, Thongrattanasiri et al., 2012) because plasmonic resonances in nanostructured graphene provide means to electrically modulate the light transmission and reflection through the thin graphene layer (Ju et al., 2011). In this theoretical study the experimentally relevant case of graphene on top of a few hundred nanometer thick SiO2 layer on a doped silicon substrate is considered. The investigated structures include the array of graphene nanoribbons and graphene dots and antidots arranged in a rectangular lattice. The signature of plasmonic resonances in the infrared reflectance spectra and its dependence on carrier concentration and presence of few nanometer thick adsorbed water layer is discussed.
This work is supported by the Serbian Ministry of Education and Science projects OI171005 and III45018 and the EU FP7 NIM_NIL Project (grant agreement no 228637, www.nimnil.org). References [1] 0 -DEODQ + %XOMDQ 0 6ROMDĂžLĂź, Phys. Rev. B, 80 (2009) 245435. [2] F.H.L. Koppens, D.E. Chang, F.J. Garcia de Abajo, Nano Lett., 11 (2011) 3370. [3] K.S. Novoselov, A.K. Geim, S.V. Morozov, D. Jiang, Y. Zhang, S.V. Dubonos, I.V. Grigorieva, A.A. Firsov, Science, 306 (2004) 666. [4] A. Yu Nikitin, F. Guinea, F.J. Garcia-Vidal, L. Martin-Moreno, Phys. Rev. B, 85 (2012) 081405. [5] S. Thongrattanasiri, F.H.L. Koppens, F.J. Garcia de Abajo, Phys. Rev. Lett., 108 (2012) 047401. [6] L. Ju, B. Geng, J. Horng, C. Girit, M. Martin, Z. Hao, H.A. H.A. Bechtel, X. Liang, A. Zettl, Y.R. Shen, F. Wang, Nature Nanotech., 6 (2011) 630.
Diamond as a platform for supporting graphene 1
1
1
1
1,2
3
Fang Zhao , Thuong Thoung Nguyen , Mo Golsharafi , Suguru Amakubo , Glenn C. Tyrrell , KP Loh and Richard B. Jackman 1
1
London Centre for Nanotechnology and Department of Electronic and Electrical Engineering, University College London, 17-19 Gordon Street, London, WC1H 0AH, UK
2
Applied Scintillation Technologies Ltd, Roydonbury Industrial Estate, Harlow, Essex, CM19 5BZ, UK 3
Department of Chemistry, National University of Singapore, 3 Science Drive, Singapore 117543 r.jackman@ucl.ac.uk
Inevitably the substrate that attaches to a material such as graphene will influence the electronic properties of the carbon monolayer. The majority of reports concerning the electronic properties of chemical vapour deposited (CVD) graphene that has been transferred to an insulating substrate for electronic characterization concern the use of SiO 2-Si, due to the widespread availability of this material system. A more ideal platform for the graphene could be another carbon form such as diamond, and indeed favourable reports of the use of Diamond-like carbon (DLC) [1] and diamond itself [2] have recently appeared. We initiated a study on the use of both single crystal and thin film nanocrystalline diamond, with a view to investigating the influence of the surface terminating groups on the diamond surfaces on the subsequent electronic properties of the deposited graphene layer. The initial results from this work, in terms of Hall effect measurements and Impedance Spectroscopy are reported here.
CVD produced graphene (Cu-foil) was transferred onto single crystal diamond (100) substrates that had been previously subjected to differing chemical and plasma treatments to lend them differing surface terminating groups. Diamond with monolayer attachments of H, O, F and N were investigated, in all cases leading to a p-type system once graphene had been deposited. Stark differences in the electrical character of the resultant graphene-diamond heterostructure were onbserved. For example, in table 1, it can be seen that higher carrier mobility values cannot be simply associated with lower carrier densities (as they are in conventional semiconductor systems). Rather, each chemisorbed species give rise to a unique character. In the case of H terminations, the maximum mobility arose, allied to the lowest carrier concentration, but the carrier concentration rose noticeably for O terminations, with modest decrease in mobility.
In contrast, a similar carrier concentration was observed when N-
terminations were charactersied, but a sharp decline in mobility was associated with this heterostructure. In the case of F-terminations, the mobility decreased even further, but the carrier concentration became extremely high. These results will be discussed in terms of the possible surfacetransfer effects that may be occurring within the diamond-terminating group-graphene heterostructures, and the potential use of this approach for engineering tunable electrical properties. References [1] High-frequency, scaled graphene transistors on diamond-like carbon. Yanqing Wu, Yu-ming Lin, Ageeth A. Bol, Keith A. Jenkins, Fengnian Xia, Damon B. Farmer, Yu Zhu & Phaedon Avouris Nature 472, 74Âą78 (2011)
2
> @ 続Graphene-on-Diamond Devices with Increased Current-Carrying Capacity: Carbon sp -on-sp
3
Technology, Jie Yu, Guanxiong Liu, Anirudha V. Sumant, Vivek Goyal, and Alexander A. BalandinNano Lett.,Pub: February 13, 2012 (Letter), DOI: 10.1021/nl204545q
Sample
Temperature (Kelvin)
TABLE 1. Sheet resistivity (Ohm/sq)
Graphene on HSCD Grpahene on OSCD Grpahene on NSCD Grpahene on FSCD
298
Mobility (cm2/Vs)
4392
Carrier concentration (cm-3) 6.44E+12
298
1345
2.79E+13
195.54
298
12000
2.37E+13
18.63
298
11716
5.12E+14
1.04
220.86
Highly Concentrated and Conductive Reduced Graphene Oxide Nanosheets by Monovalent Cation-pi interaction: Toward Printed Electronics Seung Yol Jeong, Sung Hun Kim, Joong Tark Han, Hee Jin Jeong, and Geon-Woong Lee* Graphene Hybrid World Class Laboratory, Nano Carbon Materials Research Group, Korea Electrotechnology Research Institute (KERI), Changwon, 641-120 (Korea) syjeong@keri.re.kr We introduce a novel route to preparing highly concentrated and conductive reduced graphene oxide (RGO) in various solvents by monovalent cation-pi interaction. Previously, the hydrophobic properties of high-quality RGO containing few defects and oxygen moieties have precluded the formation of stable [1,2]
dispersion in various solvents.
+
+
Cation-pi interaction between monovalent cations, such as Na or K ,
2
and six-membered sp carbons on graphene were achieved by simple aging process of graphene oxide (GO) nanosheets dispersed in NaOH or KOH solution. The noncovalent binding forces introduced by 2
the cation-pi interactions were evident from the chemical shift of the sp peak in the solid
13
C NMR
spectra. Raman spectra and the I-V characteristics also demonstrated the interactions in terms of the 2
presence of n-type doping effect due to the adsorption of cations with high electron mobility (39 cm /Vs) as shown in Figure 1. The RGO film prepared without a post-annealing process displayed superior electrical conductivity of 97,500 S/m at a thickness of 1.7 um. Moreover, mass production of GO paste with a concentration as high as 20 g/L was achieved by accelerating the cation-pi interactions with densification process. Our strategy can facilitate the development of large scalable production methods for preparing printed electronics made from high-quality RGO nanosheets. Mass production of an easily-dispersible cation-pi interacting graphene oxide (CIGO) powder was achieved by improving the effective interactions of alkali ions with a pi system as shown in Figure 2. The cation-pi interactions were accelerated by boiling a GO dispersion in KOH, which increased the number 2
of sp carbons upon mild deoxygenation. Subsequently, the CIGO solution was concentrated by rotary evaporation, then centrifuged three times to remove residual alkali metal ions. The viscosity provided a physical indication of the quality and dispersibility of the paste. The CIGO paste revealed a high 5
viscosity of 10 mPas at a 10
Âą2
sec
Âą1
shear rate under ambient conditions, which is comparable to the
viscosity of conventional silver paste.
[3]
In contrast, condensation of the non-interacting GO was
prevented by aggregation. These results demonstrate that monovalent cation-pi interactions are crucial for stabilizing a dispersion of highly concentrated GO and RGO with high quality. References [1] W. Gao, L. B. Alemany, L. Ci, P. M. Ajayan, Nat. Chem. 1 (2009) 403. [2] S. Stankovich, R. Piner, X. Chen, N. Wu, S. T. Nguyen, R. S. Ruoff, J. Mater. Chem. 16 (2006) 155. [3] S. P. Wu, L. Q. Zheng, Q. Y. Zhao, X. H. Ding, Colloids and Surfaces A: Physicochem. Eng. Aspects 372 (2010) 120.
Figures Figure 1
Figure 2
Figure caption Figure 1 Highly concentrated and conductive RGO nanosheets were prepared by introducing monovalent cation-pi + + 2 interactions between Na or K ions and six-membered sp carbons on graphene. I-V characteristics demonstrate the interactions in terms of the presence of n-type doping effect due to the adsorption of cations with high electron 2 mobility (39 cm /Vs).
Figure 2 Mass production of highly concentrated CIGO and its application to printed electronics. a) Production of foam-like CIGO structures after a freeze-drying process (red dashed arrow: SEM image of the CIGO, the scale bar indicates 100 um). b) highly concentrated CIGO paste, prepared by additional condensation processes, to yield a high viscosity (inset: CIGO film formation using bar-coating method). c) Bar-coated CIRGO on a PET substrate (inset: uniform coating on PET and a patterned CIRGO on PDMS substrate). d) Viscosity measurements of the highly concentrated CIGO paste.
Localized state and charge transfer in nitrogen-doped epitaxial graphene 1*
2
2
1
3*
Frederic Joucken , Yann Tison , Jerome Lagoute , Jacques Dumont , Damien Cabosart , Bing 3 2 2 2 3 2 Zheng , Vincent Repain , Cyril Chacon , Yann Girard , Andres Rafael Botello-Mendez , Sylvie Rousset , 1 3 1 Robert Sporken , Jean-Christophe Charlier , and Luc Henrard 1
Research Center in Physics of Matter and Radiation (PMR), University of Namur (FUNDP), 61 Rue de Bruxelles, 5000 Namur, Belgium 2 Laboratoire Materiaux et Phenomenes Quantiques, UMR7162, Universite Paris Diderot Paris 7, Sorbonne Paris Cite, CNRS, UMR 7162 case courrier 7021, 75205 Paris 13, France 3 Universite catholique de Louvain (UCL), Institute of Condensed Matter and Nanosciences (IMCN), 1/6 Place L. Pasteur, 1348 Louvain-la-Neuve, Belgium *two presenting authors Contact : fjoucken@fundp.ac.be Nitrogen-doped epitaxial graphene grown on SiC(000-1) was prepared by exposing the surface to an atomic nitrogen flux. The atomic nitrogen was produced by a remote RF plasma source. The exposure results in the creation of nitrogen-related defect sites in the graphene lattice, the concentration of which depends on the exposure time (the settings of the plasma are held constants for each sample). Using Scanning Tunneling Microscopy (STM) and Spectroscopy (STS), supported by Density Functional Theory (DFT) calculations, the simple substitution of carbon by nitrogen atoms has been identified as the most common doping configuration, as in ref. [1] (although the production method is different). High resolution images reveal a reduction of local charge density on top of the nitrogen atoms, indicating a charge transfer to the neighboring carbon atoms. For the first time, local STS spectra clearly evidenced the energy levels associated with the chemical doping by nitrogen, localized in the conduction band. To highlight this state, the tunneling spectra above the defect and above the graphene have been taken at FRQVWDQW WLSœV KHLJKW 7KLV DOORZHG to unambiguously identify the donor-state of the substitutional nitrogen. The shift of the Dirac point and the n-doping level associated have been estimated, giving a charge transfer of 0.8 electron by dopant atom. However, STS should be completed by a complementary technique (as ARPES) to confirm this point as it is known that the tunneling spectra of graphene around the Fermi level are perturbed by the absence of a phonon-induced channel (ref [2]). Various other nitrogen-related defects have been observed. The bias dependence of their topographic signatures demonstrates the presence of structural configurations more complex than substitution as well as hole-doping. Additional work is being carried out to acquire the STS signature of those more complex defects in order to identify them confidently. [1] Liuyan Zhao, Rui He, Kwang Taeg Rim, Theanne Schiros, Keun Soo Kim, Hui Zhou, Christopher GutiÊrrez, S. P. Chockalingam, Carlos J. Arguello, Lucia Pålovå, Dennis Nordlund, Mark S. Hybertsen, David R. Reichman, Tony F. Heinz, Philip Kim, Aron Pinczuk, George W. Flynn, Abhay N. Pasupathy, ³Visualizing Individual Nitrogen Dopants in Monolayer Graphene´ 6FLHQFH
[2] Y. Zhang, V. W. Brar, F. Wang, C. Girit, Y. Yayon, M. Pan-lasigui, A. Zettl, and M. F. Crommie, Giant ³phonon-induced conductance in scanning tunneling spectroscopy of Gate-tunable graphene´, Nat. Phys. 4, 627 (2008).
Nanoscale dual-gating of bilayer graphene on GaAs substrates A. M端ller, B. Kaestner, M. Friedemann, M. Woszczyna, K. Pierz, F. J. Ahlers, H. W. Schumacher Physikalisch-Technische Bundesanstalt, Bundesallee 100, 38116 Braunschweig, Germany bernd.kaestner@ptb.de The implementation of nanodevices into graphene has been an interesting but challenging topic. In particular, the lack of an energy gap is an obstacle for depleting carriers. A gap is at least required locally for gate defined devices out of a larger two-dimensional electron gas. An energy gap may be introduced by lateral confinement [1] or by breaking the inversion symmetry [2]. The former method leads to dimensions in the nm range which are hardly controllable lithographically and it causes large carrier-mobility degradation. Breaking the inversion symmetry, on the other hand, may be readily achievable in bilayer graphene [3] by applying an electric field perpendicular to the carbon atom planes (dual gating). However, a high sample quality over the dimensions of the gate metal is required to reduce the residual carrier concentration n0 at the charge neutrality point. In this work we explore nanoscale dual-gating of bilayer graphene exfoliated on GaAs-substrates.
Exfoliation of bilayer graphene has been achieved on an insulating 300 nm thick GaAs-AlAs multilayer system [4]. The conducting GaAs substrate acts as back-gate. Top-gates have been fabricated using a sputtered layer of 17 nm thick Al2O3 as gate insulator on top of a thin Al precursor [5]. We will present measurements using large area top-gates to demonstrate that a high relative dielectric constant of about 8 as well as low-hysteresis performance has been achieved. A series of four 100 nm wide Ti-Au top-gates define an island of about 200 nm in diameter. The widths of the graphene flakes were at least 1 m in all fabricated devices.
The diagram in Figure 1 shows the measured current ISD through the device as function of bias voltage VSD across the device. As the temperature is lowered from 4 K to 300 mK a transport gap opens without application of gate voltages. We employ finite element simulations to discuss the possibility of perpendicular electric fields resulting from the (grounded) top-gate metals straining the piezoelectric GaAs substrate. Variation of the back-gate voltage VBG tunes the transport gap in an oscillatory fashion, as seen in Figure 2. The influence of the top-gates within the range of negligible leakage currents was limited. Therefore the current path through the island could not be unambiguously be verified. However, the oscillations indicate the formation of a localized transport path within the much wider graphene flake. References [1] M. Y. B. Han et al., Phys. Rev. Lett., 104, 056801 (2007). [2] G. Giovanetti et al., Phys. Rev. B, 76, 073103 (2007). [3] J. B. Oostinga et al., Nature Materials 7, 151 (2008). [4] M. Friedemann et al., Appl. Phys. Lett. 95, 102103 (2009). [5] M. Friedemann et al., arXiv:1110.1535v1[cond-mat/mes-hall] (2011).
Figures
Figure 1: Temperature dependent ISD-VSD characteristic
Figure 2: ISD-VSD characteristic for varying back-gate voltage VBG
Influence of Graphite Defect Density on Oxidation Behavior and S-electron Topology of Substoichiometric Graphene Oxides b)
b)
b)
a)
a)
Zhi-Li Chen , Li-Hong Zhao , Roland G-S. Goh , Fong-Yu Kam , Jie Song b,d) a,b) Kieng Lim and Lay-Lay Chua
,
c)
Wang-Zhi Chua , Geok-
a
Department of Chemistry, National University of Singapore, Lower Kent Ridge Road, S117543, Singapore b Department of Physics, National University of Singapore, Lower Kent Ridge Road, S117542, Singapore c Hwa Chong Institution, 673 Bukit Timah Road, S269735, Singapore d DSO National Laboratories, 20 Science Park Drive, Science Park I, S118230, Singapore a0030326@nus.edu.sg; chmcll@nus.edu.sg Five commercial sources of natural (G1 and G3) and synthetic graphites (G2, G4 and G5) were investigated to study the role of defect density on their oxidative chemistry and properties of the functionalized graphene derivatives that are potentially suitable for optical or electronic applications. The starting graphites were characterized by Raman spectroscopy to yield the average mosaic size, and Fourier-transform infrared spectroscopy (FTIR) to yield the hydrogen concentration. These graphites were 1 2, 3 subjected to a Staudenmaier oxidation , and the sub-stoichiometric graphene oxides (sub-GOx) obtained were characterized via thermogravimetry analysis (TGA) to evaluate their oxidation state. The latter reveals the amount of labile oxygen groups in the sub-GOx to vary as G1 < G2 ± G4 < G5, in broad agreement with both elemental analysis and Raman results. $ PHWKRG WR ³ILQJHUSULQW´ WKH 5DPDQ G-band was developed and used to evaluate their S-electron topology. We show that the more defective graphites are more readily oxidized to give smaller nano4, 5 graphene domains. These also tend to possess a higher non-benzenoid character. The S-electron topologies of benzenoid and non-benzenoid systems were also studied using quantum-chemical calculations at the semi-empirical PM3-MNDO level. From the gathered findings, the most defective sub-GOx exhibits a higher activation energy for both electron and hole mobilities than the least defective sub-GOx, which gives sub-meV activation energies below 25 K. This suggests that the less defective G1 has given rise to a better sub-GOx with higher benzenoid character that can be thermally re-graphenized to a better graphene material with more extensive band-like transport. In contrast, more oxidized GOx re-graphenized to less perfect graphene material to give hopping transport. Therefore the defect type and density in the starting graphite play a crucial role in its chemistry that ultimately controls the electronic and optical properties of the derived functionalized graphenes.
References [1] [2] [3] [4] [5]
Staudenmaier, L., Ber. Dtsch. Chem. Ges., 31 (1898) 1481-1499. Wang, S.; Chia, P.-J.; Chua, L.-L.; Zhao, L.-H.; Png, R.-Q.; Sivaramakrishnan, S.; Zhou, M.; Goh, R. G.-S.; Friend, R. H.; Wee, A. T.-S.; Ho, P. K.-H., Adv. Mater. , 20 (2008) 3440-3446. Lim, G.-K.; Chen, Z.-L.; Clark, J.; Goh, R. G.-S.; Ng, W.-H.; Tan, H.-W.; Friend, R. H.; Ho, P. K.-H.; Chua, L.-L., Nat. Photonics, 5 (9) (2011) 554-560. Watson, M. D.; Fechtenkötter, A.; Müllen, K., Chem. Rev., 101 (2001) 1267-1300. Stein, S. E.; Brown, R. L., J. Am. Chem. Soc., 109 (1987) 3721-3729.
Figures
G band
(a)
Intensity
G5
(b)
D band
G4 G3 G2 G1
1100
1300 1500 1700 Raman shift (cm -1)
Figure 1a. Raman spectroscopy of graphite G1ÂąG5 obtained from commercial sources used for synthesis of graphene oxide. 2 Figure 1b. Transmission FTIR spectra of graphite G1ÂąG5 dispersed in KBr pellet showing sp QCÂąH stretch at 2972 Âą1 cm . Vibration arises from residual CÂąH (internal voids or fissions of graphene sheet) due to imperfect graphenization or impurities which further limit the lateral coherence and size of the mosaics.
002d_sp1_GO_H2Ocheck_p
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002d_sp1_GO_H2Ocheck_p
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4min
11min
-log(Transmittance)
-log(Transmittance)
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11min-50ÂşC 11min-75ÂşC
11min-50ÂşC 11min-75ÂşC
11min-100ÂşC
11min-100ÂşC
800 1000 1200 1400 1600 Wavenumbers(cm -1)
3000 3500 4000 Wavenumbers(cm -1)
Figure 2a. Thermogravimetry of the sub-*2[ RI * ĂG5 obtained by modified Staudenmaier oxidation. The samples were equilibrated in N2 glove box for 3 days before the analysis. The weight thermograms are plotted on the left axis and the derivative weight thermograms on the right. Figure 2b. In-situ FTIR spectroscopy of a neat film of sub-GOx (from G1) on intrinsic Si wafer. The progressive -1 -1 pumping and heating time of a thin film of sub-GOx (i) showing the OH bend at 1325 cm , C-OH stretch at 1200 cm Âą1 -1 and epoxy stretches at 1090, 950, 860 cm , (ii) showing the OH stretch at 3745cm .
(a)
2.0
(ci)
(b)
G1
-log(Transmittance)
1.5
(cii)
G3 G2 1.0 G4
0.5 G5
0.0 0
1000 2000 3000 4000 5000 6000 7000 8000 Wavenumber (cm -1)
Figure 3a. Raman spectroscopy of sub-GOx of G1ÂąG5 obtained by modified Staudenmaier oxidation. G band is -1 -1 -1 -1 -1 fitted with several Gaussian spectra with peak centre at 1540 cm , 1568 cm , 1580 cm 1592 cm and 1642 cm . The spectra have been offset for clarity. Figure 3b. Transmission FTIR spectra of sub-GOx dispersed in KBr pellet. The broad absorption is due to SÂąS* electronic transition of the graphenites. The intensity of this background decreases across G1 > * Ă* ! * LQ agreement with the oxidation trend. Figure 3c. Dependence of calculated vibration frequencies on type of termination edges in the S-electron network for (i) benzenoid and (ii) non-benzenoid structure.
Quantum Transport through Heterobilayers of Graphene Nanoribbon and Porphyrin Tape Hong Seok Kang Department of Nano & Advanced Materials, Jeonju University, Hyoja-dong, Wansan-ku, Chonju, Chonbuk 560-737, SOUTH KOREA hsk@jj.ac.kr Using the first principles calculation, we have shown that heterobilayers can be formed between armchair graphene nanoribbon (GNR) and zinc-porphyrin tapes (Zn-PPTs) or ruthenium-porphyrin tapes (Ru-PPTs). The PPTs investigated include triply lined (TL) and doubly linked (DL) PPTs. In addition, we have also investigated electronic structures and conductances of these heterobilayers. The bilayer involving the DL Zn-PPT is more stable than its TL correspondents due to stronger electronic coupling, which can be ascribed to the similar dispersion relations of the free-standing GNR and the DL PPT around the Fermi level. Specifically, the bilayer formation between the GNR and the DL Ru-PPT is especially strong Consequently, the bilayer formation of TL Zn-PPT with GNR turns it into a metal, while its DL correspondent remains semiconducting but exhibit increased on-current at an appropriate gate voltage. Our calculation of the band gap of the GNR as a function of the ribbon width also shows that the band gap oscillation is reduced upon bilayer formation with DL Zn-PPT. Furthermore, armchair GNRs exhibit highly spin-polarized transport by forming heterobilayers. As a low bias, the spin polarization of the DL bilayer is higher than that of the bilayer involving the TL Ru-PPT, and its linear response conductance will be perfectly spin-polarized.
References [1] Meriles, C. A.; Sakellariou, D.; Heise, H.; Moule, A. J.; Pines, A. Science, 293 (2001) 79. [2] Tsuda, A.; Nakamura, Y.; Osuka, A. Chem. Commun. (2003) 1096. Figure 1.
The molecular structures for configuration Z of the heterobilayer of 16-aGNR-(TL Ru-PPT) (a), and that for configuration Z of the heterobilayer of 16-aGNR-(DL Ru-PPT).
Figure 2.
The band structure of 14-GNR-(DL Ru-PPT) for the majority spin (a) and the minority spin (b) obtained from the LDA calculation.
Figure 3.
The zero-bias transmittances of 14-GNR-(DL Ru-PPT) for the majority spin (a) and the minority spin (b) obtained from the LDA calculation.
Synthesis, Characterization and Gas Sensing Behaviour of Large Area Continuous and Transparent Graphene Films by Chemical Vapor Deposition Emine Kayhan, Joerg J. Schneider Technische Universität Darmstadt Eduard-Zintl-Institut Fachbereich Anorganische Chemie, Petersenstrasse 18 D-64287, Darmstadt, Germany dilsademine@gmail.com Graphene, as two dimensional flat carbon material has attracted intense scientific interest due to its 1 2 3 exceptional electrical , mechanical and chemical behavior over the last couple of years. This strictly two4 5 dimensional (2D) material has potential applications in transistors, flexible electronics and gas sensors . The fabrication of high-quality, large-area graphene is crucial for exploiting its electronic properties. TEM and Raman data indicate that square centimeters sized samples of single- and few-layer graphene are manufactured with a chemical vapor deposition (CVD) technique on Ni-evaporated silicon substrate by optimizing the thickness of the catalyst metal, temperature as well as mixing ratio of precursor gases CH4 and H2. Sheet and contact resistances of the graphene film were determined. Gas sensor measurements reveal interesting response of graphene towards hydrogen. References [1] A. K. Geim and K. S. Novoselov, Nature Mat., 6 (2007) 183-191. [2] C. Lee, X. Wei, J. W. Kysar and J. Hone, Science, 321 (2008) 385±388. [3] D. C. Elias, R. R. Nair, T. M. G. Mohiuddin, S. V. Morozov, P. Blake, M. P. Halsall, A. C. Ferrari, D. W. Boukhvalov, M. I. Katsnelson, A. K. Geim, K. S. Novoselov, Science, 323 (2009) 610±613. [4] S. Bae, H. Kim, Y. Lee, X. Xu, J-S. Park, Y. Zheng, J. Balakrishnan, T. Lei, H. R. Kim, Y. Il Song, Y-J. Kim, K. S. Kim, B. Özyilmaz, J.-H. Ahn, B. H. Hong and S. Iijima, Nature Nanotechnology, 5 (2010) 574± 578. [5] R. K. Joshi, H. Gomez, F. Alvi and A. Kumar, J. Phys. Chem. C, 114 (2010) 6610±6613.
Micro four-point probe characterization of nanostructured graphene Mikkel B Klarskov, Timothy J Booth, Dirch H Petersen, Peter Bøggild DTU Nanotech, Technical University of Denmark, Ă&#x2DC;rsteds Plads 345B, DK-2800 Kgs. Lyngby, Denmark mikkel.olesen@nanotech.dtu.dk
The typical approach for investigating the electronic properties of graphene involves several mutually aligned lithographic steps to define the shape of the graphene and for metallic contacts [1-3]. Lithography must be aligned manually to micromechanically cleaved graphene, although graphene from chemical vapor deposition (CVD) and epitaxial growth are less restrictive. The lithographic steps are usually time consuming and it is difficult to avoid contamination of the graphene with resist residues, or damage to the graphene, which modifies the properties with respect to pristine graphene [4-5].
In this work [6], we present electrical measurements on single and few-layer graphene cleaved from naturally occurring large flake graphite onto silicon dioxide (SiO 2), using repositionable micro four-point probes (M4PP), and compare with results obtained through standard lithographic techniques. The electronic properties measured in this way are comparable to those obtained with lithographic electrodes, yet M4PP allow for fast characterization of as-fabricated graphene without exposing the graphene to chemicals or electron beam irradiation, or modifying the graphene after production. Moreover it provides the possibility of mapping the graphene surface with high spatial resolution.
In M4PP measurements we observe a hysteresis in the charge neutrality point depending on the rate of the gate sweep, which we ascribe to charge trapping in the SiO2 [2-3,7-10], with a maximum in the observed hysteresis at 0.8 V/s. The graphene appears to be very resistant to mechanical scratches from the probes; however, as expected, near the critical current densities [11] of graphene at the point shaped electrodes will lead to irreversible damage, typical at 10
8
2
A/cm . For measurements on
nanostructured graphene, micromechanical cleaved graphene is etched in a Hall bar geometry, and measurements on the sample with both fixed electrodes and M4PP are performed. In common with previous reports, [12-15] the nanostructuring of the graphene leads to a measurable perturbation of the electronic properties with respect to the pristine graphene.
The micro four-point probes can be used to perform local, non-destructive measurements on graphene in a far shorter timescale than using fixed lithographic contacts. The sample size is only limited by the probe pitch, and characterization is possible between many process steps. Micro four-point probes can therefore be used as an in-line verification tool for process monitoring in graphene production, as well as providing a unique possibility of probing and mapping the electronic transport properties of graphene.
References [1] Novoselov K S, Geim A K, Morozov S V, Jiang D, Zhang Y, Dubonos S V, Grigorieva I V, Firsov A A, Science, 306 (2004) 666-9 [2] Lafkioti M, Krauss B, Lohmann T, Zschieschang U, Klauk H, Klitzing K, Smet J H, Nano Lett. 10 (2010) 1149-53 [3] Lohmann T, Klitzing K, Smet J H, Nano Lett. 9 (2009) 1073-9 [4] Jacobsen A, Koehler F M, Stark W J, Ensslin K, New J. Phys. 12 (2010) 125007 [5] Ishigami M, Chen J H, Cullen W G, Fuhrer M S, Williams E D, Nano Lett. 7 (2007) 1643-8 [6] Klarskov M B, Dam H F, Petersen D H, Hansen T M, LÜwenborg A, Booth T J, Schmidt M S, Lin R, Nielsen P F, Bøggild P, Nanotechology, 22(44) (2011) 445702 [7] Dan Y, Lu Y, Kybert N J, Luo Z, Johnson A T C, Nano Lett., 9 (2009) 1472-5 [8] Joshi P, Romero H E, Neal A T, Toutam V K Tadigadapa S A, J. Phys: Condens. Matter, 22 (2010) 334214 [9] Liao Z, Han B, Zhou Y, Yu D, J. Chem. Phys., 133 (2010) 044703 [10] Wang H, Wu Y, Cong C, Shang J, Yu T, ACS Nano, 4 (2010) 7221-8 [11] Murali R, Yang Y, Brenner K, Beck T, Meindl J D, Appl. Phys. Lett., 94 (2009) 243114 [12] Bai J, Zhong X, Jiang S, Huang Y, Duan X, Nat. Nanotech., 5 (2010) 190-4 [13] Sinitskii A, Tour J M, J. Am. Chem. Soc., 132(42) (2010) 14730-2 [14] Safron N S, Brewer A S, Arnold M S, Small, 7(4) (2011) 492-8 [15] Liu L, Zhang Y, Wang W, Gu C, Bai X, Wang E, Adv. Mat. 23(10) (2011) 1246-51 Figures
Fig. 1. Scanning electron microscope image of a micro four-point probe approaching graphene and graphite on silicon dioxide.
Fig. 2. Schematic for measurement on nanostructured graphene, which includes a comparison of fixed and micro four-point probes measurements on the same graphene sample.
The Fluorescence Properties of Graphene Oxide Sven Kochmann, Alexander Zöpfl, Thomas Hirsch, Otto S. Wolfbeis Institute of Analytical Chemistry, Universitätsstrasse 31, 93053 Regensburg, Germany sven.kochmann@chemie.uni-regensburg.de
Graphene oxide (GO) is an intermediate on the route to chemically derived graphene, and it is easily synthesized. Its chemical structure is heterogeneous and consists of both, large areas of conjugated 2
sp -systems and various electronically isolated oxygen containing functionalities [1]. Contradictory optical properties have been found. A band gap which enables photoluminescence is introduced into GO with increasing oxidation level [2, 3]. Due to the remaining large
-electron system, it can also act
as a quencher of fluorescence [4]. GO is well dispersible in water and therefore of interest in many (bio)analytical applications. By tuning its size, oxidation level, number of layers and additional chemical functionalization it is possible to create tailored materials with specific optical properties.
We have systematically studied the multiple fluorescence of GO and its dependence on excitation wavelength, emission wavelength and pH value in the near-UV and visible (Fig. 1). The changes in fluorescence observed with alterations in pH do not depend on the excitation/emission wavelength. This may be useful for designing a probe compatible with many kinds of optical equipment (light sources, filters, etc.).
GO turned out to be a good quencher of the fluorescence of organic dyes, that are widely used in biological applications. Quenching was investigated in the presence of different concentrations of GO (Fig. 2a, b). The results were compared to models for dynamic or static quenching, but none of them matches the observations. However, the ¶6SKHUH RI DFWLRQ¶ model turned out to adequately describe the quenching mechanism (Fig. 2c).
The understanding of the mechanism in fluorescence quenching of GO allowed to take advantage of these findings in several optical applications: The material was applied to an improved visualization of graphene under the microscope and resulted in a better contrast. This can help to identify the number of graphene layers and to obtain information on the uniformity of graphene films. GO also was used as a surface substrate in Raman spectroscopy, enabling the acquisition of Raman data of even strongly fluorescent samples due to the suppression of the fluorescence. This provides an additional way for the characterization of fluorophores.
The research was supported by Deutsche Forschungsgesellschaft (GRK 1570).
References [1]
Allen MJ, Tung VC, Kaner RB (2010) Honeycomb carbon: a review of graphene. Chem Rev 110(1):132±145. doi:10.1021/cr900070d
[2]
Luo Z, Vora PM, Mele EJ, Johnson ATC, Kikkawa JM (2009) Photoluminescence and band gap modulation in graphene oxide. Appl Phys Lett 94(11):111,909±3. doi:10.1063/1.3098358
[3]
Bonaccorso F, Sun Z, Hasan T, Ferrari AC (2010) Graphene photonics and optoelectronics. Nat Photon 4(9):611±622. doi:10.1038/nphoton.2010.186
[4]
Liu Y, Liu C-Y, Liu Y (2011) Investigation on fluorescence quenching of dyes by graphite oxide and graphene. Appl Surf Sci 257: 5513. doi:10.1016/j.apsusc.2010.12
Figures
Figure 1:
Excitation-emission matrices of graphene oxide at pH values 1.0, 4.0, 7.1 and 10.1. Figure 2:
5(6)-&DUER[\IOXRUHVFHLQ ȝ0 DW S+ with increasing concentrations of GO, examined under visible light (a) and UV light (b). The Stern-Volmer plot (c) shows the experimental data and theoretical fitting according to the two models of combined quenching (dashed line) and sphere of action (solid line).
Epitaxial Graphene on Si face of SiC: A Comparative Study of Different Growth Conditions 1
1
1
5
4
4
4
B. Kumar, F. Duclairoir, L. Dubois, D. Rouchon, M. Paillet, J.-R. Hutzinger, A. Tiberj, A. Zahab, 2 4 3 1 G. Lapertot, J.-L. Sauvajol, G. Bidan, P. Maldivi
4
1
Service de Chimie Inorganique et Biologique, CEA/INAC, Grenoble, 38 054, France Service de Physique, Statistique, Magnétisme et Supraconductivité, CEA/INAC, Grenoble, 38 054, France 3 INAC/DIR, CEA, Grenoble, 38 054, France 4 Laboratoire Charles Coulomb, UM2, Montpellier, 34 095, France 5 CEA-Leti-MINATEC, Campus/DTSI/SCMC, Grenoble, 38 402, France
2
Contact: bijandra.kumar@cea.fr Graphene, a two-dimensional sheet of carbon atoms arranged in a hexagonal lattice has attracted scientific community attention intensively due to its unique structural and electrical properties, particularly in the field of microelectronics. Mainly graphene could be obtained from the QDWLYH VRXUFH RI JUDSKHQH ³WRS GRZQ´ RU VRXUFH RI FDUERQ ³ERWWRP XS´ DSSURDFKHV ZKLFK LQFOXGH exfoliation from bulk graphite, chemical reduction of exfoliated graphite oxide, growth on SiC by silicon desorption, chemical vapor deposition on metal surface and metal surfaces with carbon solubility and carbon diffusivity. In order for graphene to become a viable technology, production on large scale and high quality material are two prerequisites therefore graphene growth by SiC sublimation represent an attractive route for large scale synthesis and compatibility with microelectronics technology. Very large areas of graphene can be obtained with the SiC sublimation method, therefore this technique represents an attractive route [1, 2]. However, at the moment the booming of this material is notably hampered by the low homogeneity of the number of layers all over the SiC substrate. The studies presented in this poster are related to the optimization of growth of graphene on the Si face of 6H-SiC [3]. A comparative study of graphene growth under high vacuum or in inert media at atmospheric pressure will be discussed. In addition, evolution of surface morphology induced by annealing in presence of hydrogen at high temperature 1600°C (3°C/min) for 30 min and effect of such surface phenomena on graphene growth will be illustrated. In light of Raman and AFM studies (Fig. 1 and 2 respectively) conducted on a series of samples, it could be established that the annealing step combined to graphene growth under inert atmosphere at ambient pressure yields high quality graphene in terms of homogeneity, surface roughness and domain size on large areas. Such protocol was further investigated and the impact of sublimation time on graphene quality was studied. These experiments allowed achieving an optimized graphene synthesis protocol on Si face of SiC and hence obtaining quite large monolayer graphene domains (10 micron). In concern to the carrier mobility, it is highly dependent on graphene homogeneity and domain size. The recent reports state that samples with larger homogeneous graphene domains have two or three fold higher carrier mobility compare to samples exhibiting smaller graphene domains as surface steps and changes in layer thickness are key sites which seriously degrades transport in epitaxial graphene films on SiC [4]. Therefore in light of present experimental studies we can postulate that homogeneous graphene growth in optimal condition could also have high carrier mobility. The preliminary electrical measurements recorded on these graphene samples will be presented.
References: [1] Luxmi, N. Srivastava, R. M. Feenstra, P. J. Fisher, J. Vac. Sci. & Tech.B: Microelectronics and Nanometer Structures 2010, 28, C5C1. [2] W. Deheer, C. Berger, X. Wu, P. First, E. Conrad, X. Li, T. Li, M. Sprinkle, J. Hass, M. Sadowski, Solid State Comm 2007, 143, 92-100. [3] K. V. Emtsev, A. Bostwick, K. Horn, J. Jobst, G. L. Kellogg, L. Ley, J. L. McChesney, T. Ohta, S. A. Reshanov, J. Röhrl, E. Rotenberg, A. K. Schmid, D. Waldmann, H. B. Weber, T. Seyller, Nat Mater 2009, 8, 203-207. [4] Shuai-Hua Ji, J. B. Hannon, R. M. Tromp, V. Perebeinos, J. Tersoff, F. M. Ross, Nat Mater 2009, 8, 203-207.
Intensity (arb. units)
Raman spectra G-Peak 2D-Peak
1200 1400 1600 1800 2000 2200 2400 2600 2800 ¬1
Raman shift (cm )
Figure 1: Raman spectra of graphene grown on Si face (annealed under H2, growth at high T°C for 30 min).
Figure 2: AFM image of grapheme grown on Si face (annealed under H2, growth at high T°C for 30 min).
$ 6WDEOH Âł)ODWĆ&#x17D; )RUP RI 7ZR-Dimensional Crystals: Could Graphene, Silicene, Germanene Be Minigap Semiconductors and Have Huge Magnetoresistance?? F. V. Kusmartsev $ 2Âś+DUH K. I. Kugel, T. H. Hewett, M. B. Gaifullin, F. A. Mamari, and O. E. Kusmartseva Physics Department, Loughborough University, LE11 3TU, UK F.Kusmartsev@lboro.ac.uk The discovery of a flat two-GLPHQVLRQDO FU\VWDO NQRZQ DV JUDSKHQH KDV FRQWUDGLFWHG /DQGDXĂ3HLHUOVĂ 0HUPLQĂ:DJQHU arguments that there is no stable flat form of such crystals. Here, we show that the ÂłIODW´ VKDSH RI graphene arises due to a microscopic buckling at the smallest possible interatomic scale. We show that the graphene, silicene, and other two-dimensional crystals are stable due to transverse short-range displacements of appropriate atoms. The distortions are small and form various patterns, which we describe in a framework of Ising model with competing interactions. We show that when temperature decreases, two transitions, disorder into order and order into disorder, arise. The ordered state has a form of stripes where carbon atoms are shifted regularly with respect to the plane. The flat JUDSKHQH VLOLFHQH RU JHUPDQHQH SODQHV ORRN OLNH D PLFURVFRSLF ÂłZDVKERDUG´ ZLWK WKH wavelength of DERXW FRXSOH RI LQWHUDWRPLF VSDFLQJ RI DSSURSULDWH VXEODWWLFHV ZKLFK IRU JUDSKHQH LV DERXW Ă c At lower WHPSHUDWXUHV WKH RUGHUHG VWDWH WUDQVIRUPV LQWR D JODVV %HFDXVH RI XSĂGRZQ DV\PPHWU\ LQ buckled graphene, silicene and other two-dimensional crystals deposited on substrate, a minibandgap may arise. We derive a criterion for the minigap formation and show how it is related to the buckling and WR WKH JUDSKHQHĂVXEVWUDWH LQWHUDFWLRQ %HFDXVH RI WKH EDQGJDS WKHUH PD\ DULse new phenomena and in particular a rectification of ac current induced by microwave or infrared radiation. We show that the amplitude of direct current arising at wave mixing of two harmonics of microwave electromagnetic radiation is huge. Moreover, we predict the existence of miniexcitons and a new type of fermionic minipolaritons whose behavior can be controlled by the microwave and terahertz radiation. Moreover, using enhanced Raman microscope mapping we found graphene nanodomes which exist in graphite or any substrate where it was placed. In particular, we found a highly inhomogeneous network of graphene nanodomes on graphite. Such network of the graphene nanodomes is responsible for extraordinary magnetoresistance which we have measured. Our theory described the experiments well and revealed that the magnetoresistance depends most strongly on the density of these nanodomes, causing a huge effect at their optimal density. Indeed when graphene covers 46% of the surface we found a largest linear magnetoresistance that reaches up to 200% even in small magnetic fields even below 0.5T. The analysis is very general and applicable to any two phase graphene systems in arbitrary magnetic field. The discovery of inhomogeneous graphene nanodomes in graphite and other substrates provides a possibility for many potential practical applications, especially due to the huge magnetoresistance observed at low magnetic fields. Using the effect we designed simple working devices as magnetic sensors.
References [1] $ 2Âś+DUH F. V. Kusmartsev, and K. I. Kugel, dx.doi.org/10.1021/nl204283q | Nano Lett., 2012, 14,1-6 Figure 1
a)
b)
Figure caption: A microscopic image of the samples with four-point probe Van der Pauw configuration: 2 2 a) dimension ~2.64x0.16 mm ; b) dimension ~1.33x0.87 mm ) Figure 2
Figure caption: Raman spectral lines of graphene nanodomes detected on a surface of HOPG sample with mosaicity 0.4°¹0.1° and deposited on Si/SiO2 substrate, excited by the 514.5 nm line laser. Figure 3
a)
b)
Figure caption: a) Optical image of the flake and its Raman mapping tomography: the Nanodomes of graphene (in light color) are formed on a surface of graphite (in dark blue); Some nanodomes are indicated by white arrows. b) Magnetoresistance associated with the formation of nanodomes as a function of magnetic field for five different temperatures. Dots are experiments, the theoretical fitting of the experiments are described by solid curves.
Contribution (Poster)
Investigation of Alumina/Graphene Oxide role in catalysis John Landers, Ankush Biradar, Remie Yu, Daniel Mastrogiovanni, Eric Garfunkel, Tewodros Asefa, Arthur W. Chester, Alexander V. Neimark Rutgers, The State University of New Jersey, Piscataway NJ, United States jolander@rci.rutgers.edu Mesoporous alumina has long been utilized as a catalyst support in heterogeneous catalysis owing to its durability, tunability of its pore size distribution and ability to deposit precious metal nanoparticles into its pores(1, 2). However nanoparticle deposition is so far not ideal and there are continuing efforts to synthesize materials which can stabilize nanoparticles that lead to higher yields and selectivity. A second class of materials such as active carbon and carbon nanotubes have also been widely used due to their large surface area and outstanding electronic conductivity(3-5). However, these materials are usually impeded by their inaccessible pore structure. One material that has emerged over the past decade has been that of graphene oxide. Safe and environmentally friendly to synthesize, graphene oxide can be easily mixed with an alumina by a sol-gel method to produce tunable pore structures ideal for nanoparticle deposition. The presence of functional groups and defects on the graphene oxide sheets enhances the substrate nanoparticle interaction which can lead to a better dispersion uniformity and higher selectivity and yield(6-8). Moreover, the density and type of defect can be controlled by the chosen reduction method, thus allowing for finer control of the dispersion and size of the metal nanoparticles(9-13). Herein we report a new hybrid catalyst support which combines the optimal pore size features of mesoporous alumina with the desired ability of graphene oxide to anchor and stabilize precious metal nanoparticles for heterogeneous hydrogenation catalysis. Furthermore the particle size, shape and roughness of the novel catalyst support can be tailored by varying the starting material composition which enables control of the metal deposition and subsequential catalytic reaction.
References [1] J. S. Beck et al., J. Am. Chem. Soc. 114 (1992) 10834. [2] A. Taguchi, F. Schuth, Microporous Mesoporous Mat. 77 (2005) 1. [3] F. Rodriguez-Reinoso, Carbon 36 (1998)159. [4] P. Serp, M. Corrias, P. Kalck, Appl. Catal. A-Gen. 253 (2003) 337. [5] P. Serp, E. Castillejos, ChemCatChem 2 (2010) 41. [6] D. H. Lim, J. Wilcox, J. Phys. Chem. C 115 (2011) 22742. [7] R. F. Nie et al., Carbon 50 (2011) 586. [8] F. Banhart, J. Kotakoski, A. V. Krasheninnikov, ACS Nano 5 (2011) 26. [9] S. Moussa, V. Abdelsayed, M. S. El-Shall, Chem. Phys. Lett. 510 (2011) 179. [10] S. Moussa, A. R. Siamaki, B. F. Gupton, M. S. El-Shall, ACS Catal. 2 (2011) 145. [11] H. M. A. Hassan et al., J. Mater. Chem. 19 (2009) 3832. [12] C. Gomez-Navarro et al., Nano Lett. 10 (2011) 1144. [13] M. Acik et al., ACS Nano 4 (2011) 5861.
Contribution (Poster) Figures
Figure caption: Hydrogenation of styrene in a pressure reactor with 0.5% wt loading of Pd nanoparticles deposited onto the graphene oxide/alumina hybrid catalyst support. The y-axis depicts the pressure drop relative to the final pressure. The inlet displays four SEM images of varying starting composition without Pd. A-C refers to a (wt/wt) ratio of graphene oxide solution to alumina powder of 2:1, 1:1 and 1:2 all with 1 M HCl. D refers to 1:2 mixture ZLWKRXW DFLG 6FDOH EDU LQ HDFK LV Č?P
Molecular dynamics simulation of carbon nanostructures 1
IstvĂĄn LĂĄszlĂł , Ibolya Zsoldos 1
2
Department of Theoretical Physics, Institute of Physics, Budapest University of Technology and Economics H-1521 Budapest, Budafoki Ăşt 8., Hungary
2
Faculty of Technology Sciences, SzĂŠchenyi IstvĂĄn University, H-9026 *\Ä&#x2DC;U (J\HWHP WpU +XQJDU\ Laszlo@eik.bme.hu
Molecular dynamics calculations can reveal the physical and chemical properties of various carbon nanostructures or can help to devise the possible formation pathways. In our days the most well known carbon nanostructures are the fullerenes and the nanotubes. They can be thought of as being formed from graphene sheets, i.e. single layers of carbon atoms arranged in a honeycomb lattice. Usually the nature does not follow the mathematical constructions. An ideal nanotube can be thought of as a hexagonal network of carbon atoms that has been rolled up to make a cylinder. There is not any theory of carbon nanotube formation which is based on this construction. Although the first time the C60 and C70 were constructed by laser irradiated graphite, the fullerene formation theories are based on various fragments of carbon chains, and networks of pentagonal and hexagonal rings. In the present talk various formation pathways for carbon nanostructure formations will be studied in the frame work of molecular dynamics calculations. Suggestions will be given for practical experimental realizations.
Graphene Oxide as a Mono Atomic Protection Layer for Molecular Electronics: A Quantative Structural Study 1
2
2
Søren Petersen , Magni Glyvradal , 5REHUW )HLGHQKDQVœO , and Bo W. Laursen
*1
1) Nano-Science Center & Institute of Chemistry, University of Copenhagen, Copenhagen, Denmark 2) Nano-Science Center & Niels Bohr Institute, University of Copenhagen, Copenhagen, Denmark bwl@nano.ku.dk
Solving the interface problem between the organic and inorganic layers in molecular electronics have 1
been the focus of much research the last few decades. Here we investigate monolayer graphene oxide (mGO) as a protecting blocking layer between organic thin films and vapor deposited Ti/Al metal top 2
electrodes. Langmuir-Blodgett (LB) films of cadmium(II)behenate were used as a model systems for fragile organic thin films. The mGO protected LB films were examined with atomic force microscope (AFM) and X-ray reflectivity (XRR), with and without the metal top electrodes. The use of XRR allow for a detailed depth profiling of the organic film and mGO layer below the metal top layers. We find that the structure of the mGO protected LB films is perfectly preserved, contrary to that of unprotected films where it is well documented that the metal deposition completely destroys the structure of the two first 2
LB layers. This study provides detailed structural evidence for the efficient use of mono atomic layers of GO as protective interlayer for molecular thin films in optoelectronic devices.
1) Akkerman, H. B.; de Boer, B. J. Phys. Condens. Matter 2008, 20. 2) Cote, L. J.; Kim, F.; Huang, J. X. J. Am. Chem. Soc. 2009, 131, 1043. 3) Hansen, C. R.; Sorensen, T. J.; Glyvradal, M.; Larsen, J.; Eisenhardt, S. H.; Bjornholm, T.; Nielsen, M. M.; Feidenhans'l, R.; Laursen, B. W. Nano Lett. 2009, 9, 1052.
Figure 1 Deposition scheme of fatty acid metal salt-mGO hybrid film. Four layer Y-type LB-films of cadmium(II)behenate was first deposited on cleaned Si-wafers. Secondly, while the four layer film was kept submerged, the water surface was thoroughly cleaned for lipids followed by deposition of mGO. The mGO-film was then transferred by vertical deposition. Lastly, the film was covered by metal thin films.
Figure 2. Left: XRR data and fits for i) four-layer cadmium(II)behenate + GO LB-film on Si/SiO2 (red) ii) four-layer cadmium(II)behenate + GO LB-film with 50 Ă&#x2026; Ti + 100 Ă&#x2026; Al on Si/SiO2 (blue). The circles represent the data while the solid lines corresponds to the modeled fits. The red curve has been offset for clarity. Right: The electron density 2+ profile resulting from the fits on the left. The barrier effect from the mGO is here clearly visible from the intact Cd peaks. The slight increase in electron density is attributed to the natural occurring holes in the top lipid film.
Molecular dynamics simulation of self-retracting motion of graphene flakes 1
Andrei M. Popov, Irina V. Lebedeva,
2,3
Andrey A. Knizhnik, 2,3 Potapkin
2,3
Yurii E. Lozovik
1,4
and Boris V.
1
Institute of Spectroscopy, Fizicheskaya Street 5, Troitsk, Moscow Region 142190, Russia National Research Centre "Kurchatov Institute", Kurchatov Square 1, Moscow 123182, Russia 3 Kintech Lab Ltd., Kurchatov Square 1, Moscow 123182, Russia. 4 Moscow Institute of Physics and Technology, Institutskii pereulok 9, Dolgoprudny, Moscow Region 141700, Russia am-popov@isan-troitsk.ru 2
Graphene exhibiting extraordinary electrical and mechanical properties holds great promise for the use in nanoelectromechanical systems (NEMS). Recently a self-retracting motion of graphite microflakes, i.e. retraction of graphite flakes back into graphite stacks on their extension arising from the van der Waals attraction between the flakes, was observed [1]. An idea of the oscillator based on the telescopic oscillation of graphene flakes was suggested [1]. We perform atomistic simulations to analyze perspectives of using telescopic motion of graphene flakes in NEMS.
Our calculations show that the potential relief of interlayer interaction energy for the extended flake with the commensurate orientation (Fig. 1) has high potential energy barriers (Fig. 2). Therefore, if such a flake is placed near a local energy minimum, it is not able to start the self-retracting motion. If the flake initially has enough energy to start the self-retracting motion, this motion is slowed down by the corrugation of the potential energy relief. However, upon rotation of the flake to incommensurate states (Fig. 2), the potential energy relief becomes smooth. Thus, rotation of the flake to incommensurate states should facilitate its retraction.
Molecular dynamics simulations demonstrate that the self-retracting motion of the extended flake with the initial commensurate orientation proceeds in diverse ways even at almost the same initial position (see Fig. 3, movies are also available). The extended flake can retain its commensurate orientation and be locked near a local energy minimum or retract slowly. However, in majority of the simulations, fast retraction of the flake accompanied by its rotation to incommensurate states is observed. Analysis of the flake trajectories reveals that rotation of the extended flake with the initial commensurate orientation to incommensurate states occurs when the flake passes potential energy hills. In fact, the force of interlayer interaction is applied to atoms of the flake only in the overlap area between the neighbour flakes. Therefore, the force acting on the extended flake in the direction perpendicular to the direction of extension exerts a torque inducing rotation of the flake to incommensurate states.
One-two damped telescopic oscillations of the extended flake are very scarce. Therefore, as opposed to carbon nanotube walls which can perform telescopic oscillations with the Q-factor ~ 100-1000 [2], graphene flakes are not suitable for the use in oscillators. On the other hand, this result means that graphene flakes are perfect for the use in fast-responding electromechanical memory cells.
It should also be noted that even if the extended flake is fixed in the incommensurate state, the Q-factor of the telescopic oscillations of the flake does not exceed 3. Therefore, the absence of telescopic oscillations for graphenes flake is related to two factors: (1) high barriers to motion of the extended flake with the commensurate orientation, and (2) high dynamic friction force.
References [1] Q. Zheng, B. Jiang, S. Liu, Yu. Weng, L. Lu, Q. Xue, J. Zhu, Q. Jiang, S. Wang, L. Peng, Phys. Rev. Lett., 100 (2008) 067205. [2] I. V. Lebedeva, A. A. Knizhnik, A. M. Popov, Yu. E. Lozovik, B. V. Potapkin, Nanotechnology, 20 (2009) 105202. Figures
Fig. 1. (a) Commensurate ( M 0 , left) and incommensurate ( M 15 , right) states of graphene flakes. (b) Schematic representation of the telescopic motion of a graphene flake (system under consideration).
Fig. 2. Calculated interaction energy (in eV) of the graphene flakes of 34 Å x 34 Å size (446 atoms) at the equilibrium interlayer spacing 3.4 Å as a function of the relative position x, y (in Å; axes x and y are chosen along the armchair and zigzag directions, respectively) of the center of mass of the extended flake at the rotation angles of the flake (a) M 0 and (b) M 30 . The equipotential lines are drawn with a step of 0.6 eV. The energy is given relative to the global energy minimum.
Fig. 3. Different types of calculated dependences of displacement x (in Å) of the flake with the initial commensurate orientation extended in the armchair direction on time t (in ps) at temperature 300 K.
Chemically Tunable Transport Phenomena of Functionalized Graphene Nicolas Leconte, A. Lherbier, F. Varchon, P. Ordejon, D. Soriano, J.J. Palacios, S. Roche and J.-C. Charlier UniversitĂŠ catholique de Louvain, IMCN, NAPS-ETSF, Chemin des ĂŠtoiles 8 bte L7.03.01, 1348 Louvain la Neuve, Belgium nicolas.leconte@uclouvain.be We present an ab initio multiscale study and quantum transport simulations using the Kubo formalism [1] of chemically modified graphene based materials, whose properties are tuned by changing the density and nature of grafted molecular units. Depending on the nature of the introduced molecular bonding different conduction mechanism are obtained, including transition from weak to strong Anderson localization [2,3], as well as spin-dependent phenomena [4] and magnetoresistive fingerprints [5]. References [1] H. Ishii, F. Triozon, N. Kobayashi, K. Hirose, and S. Roche, C. R. Physique 10 (2009) 283 [2] N. Leconte, J. Moser, P. Ordejon, H. Tao, A. Lherbier, A. Bachtold, F. Alsina, C.M. Sotomayor Torres, J.-C. Charlier, and S. Roche, ACS Nano 4 (2010) 4033-4038 [3] N. Leconte, A. Lherbier, F. Varchon, P. Ordejon, S. Roche, and J.-C. Charlier 84 (2011) 235420 [4] N. Leconte, D. Soriano, S. Roche, P. Ordejon, J.-C. Charlier, and J.J. Palacios, ACS Nano 5 (2011) 3987-3992 [5] D. Soriano, N. Leconte, P. Ordejon, J.-C. Charlier, J.J. Palacios, and S. Roche, Phys. Rev. Lett. 107 (2011) 016602 Figures
Different localization behavior depending on the type of intrinsic magnetic ordering of hydrogen atoms on graphene. All atoms distributed on the same lattice (ferromagnetic ordering) do not localize charge carriers. Atoms randomly distributed over both sub-lattices (antiferromagnetic ordering) induce quantum localization effects.
Atomistic Processes of Grain Boundary Motion and Annihilation in Graphene 1
1
2
2
Gun-Do Lee , Euijoon Yoon , Cai-Zhuang Wang , and Kai-Ming Ho 1
Department of Materials Science and Engineering, Seoul National University, Seoul 151-742, Republic of Korea 2 Ames Laboratory-USDOE and Department of Physics, Iowa State University, IA 50011, USA gdlee@snu.ac.kr
To realize graphene electronics, it is essential to fabricate high-quality graphene on a large scale. Much effort has been devoted to the development of methods for the mass production of graphene. It has been shown that CVD growth of graphene on Cu [1] foil enables the mass production of high-quality single-layer graphene through the roll-to-roll method [2]. Generally, graphene films fabricated by CVD growth on metal surfaces are polycrystalline, composed of fine grains and grain boundaries (GBs). Although some theoretical studies have suggested that GBs can cause abnormal strength in graphene and open a transport gap, which is desirable for graphene-based nanoelectronic devices, many experimental studies have shown that GBs act as defects in graphene, impeding electronic transport, lowering thermal conductivity, and weakening mechanical properties. To realize the potential of graphene in â&#x20AC;&#x2DC;carbon-basedâ&#x20AC;&#x2122; electronics, it is highly desirable to achieve better control over the nucleation of individual graphene grains and to avoid GBs in fabricated graphene devices. Recently, we investigated the motion of GB at high temperature by performing tight binding molecular dynamics (TBMD) simulations and also carried out ab initio total energy calculations to verify the TBMD results of the formation energies and energy barriers for important configurations and their reconstructions. The TBMD simulations are performed using a modified environment-dependent tight-binding (EDTB) carbon potential [3], which was modified from the original EDTB carbon potential [4]. This modified EDTB carbon potential has been successfully applied to investigations of various defect structures in graphene and carbon nanotubes [5-7] In this study, we found that meandering structures of GB is energetically more favorable than other structures, in excellent agreement with experiments [8,9]. In TBMD simulation, sequential evaporation of dimers and Stone-Wales (SW) transformations cause the motion of GB toward armchair grain, resulting in the decrease of distance between two GBs. In spite of the evaporation of dimers, the structure of GB keeps the distinctive meandering structure through sequential SW transformations. In the TBMD simulation for the process of dimer evaporation, the erection and evaporation of dimers are observed after the formation of adatoms due to the bond breaking. In TBMD simulation, two approaching GBs also merge and annihilate through sequential evaporation of carbon dimers and SW transformations. It is interesting to note that various GB structures observed in experiments are also found in our TBMD simulation. These results elucidate the atomic scale mechanism for annihilation of GB which would occur in a high temperature annealing experiment. It is also expected to be found in experiments such as TEM studies. These results will shed interesting light on the fabrication of high-quality graphene nanoelectronic devices through the control of GB dynamics. References [1] Li, X.; Cai, W.; An, J.; Kim, S.; Nah, J.; Yang, D.; Piner, R.; Velamakanni, A.; Jung, I.; Tutuc, E.; Banerjee, S. K.; Colombo, L.; Ruoff, R. S. Science 2009, 324, (5932), 1312-1314. [2] Bae, S.; Kim, H.; Lee, Y.; Xu, X.; Park, J.-S.; Zheng, Y.; Balakrishnan, J.; Lei, T.; Ri Kim, H.; Song, Y. I.; Kim, Y.-J.; Kim, K. S.; Ozyilmaz, B.; Ahn, J.-H.; Hong, B. H.; Iijima, S. Nat Nano 2010, 5, (8), 574-578. [3] Lee, G.-D.; Wang, C. Z.; Yoon, E.; Hwang, N.-M.; Ho, K. M. Physical Review B 2006, 74, (24), 245411. [4] Tang, M. S.; Wang, C. Z.; Chan, C. T.; Ho, K. M. Physical Review B 1996, 53, (3), 979.
[5] Lee, G.-D.; Wang, C. Z.; Yoon, E.; Hwang, N.-M.; Kim, D.-Y.; Ho, K. M. Phys Rev Lett 2005, 95, (20), 205501. [6] Lee, G.-D.; Wang, C. Z.; Yoon, E.; Hwang, N.-M.; Ho, K. M. Physical Review B 2010, 81, (19), 195419. [7] Lee, G.-D.; Wang, C. Z.; Yoon, E.; Hwang, N.-M.; Ho, K. M. Physical Review B 2010, 81, (19), 195419. [8] P. Y.; Ruiz-Vargas, C. S.; van der Zande, A. M.; Whitney, W. S.; Levendorf, M. P.; Kevek, J. W.; Garg, S.; Alden, J. S.; Hustedt, C. J.; Zhu, Y.; Park, J.; McEuen, P. L.; Muller, D. A. Nature 2011, 469, (7330), 389-392. [9] Kim, K.; Lee, Z.; Regan, W.; Kisielowski, C.; Crommie, M. F.; Zettl, A. ACS Nano 2011, 5, (3), 21422146. Figures
Figure 1: Snapshots from TBMD simulations for graphene GBs. The distance between two GBs become smaller as the simulation goes on. The yellow and blue colors indicate carbon atoms and bonds on hexagonal rings and nonhexagonal rings, respectively. The red colors indicate evaporated dimers. Red solid circles indicate dimers which will undergo SW transformation at next step.
Figure2: Snapshots from TBMD simulations for dimer evaporation. Numbers show the trajectory of identical atoms.
Estimation of Young¶s Modulus by Raman Spectroscopy on Biaxially Strained Graphene Jae-Ung Lee, Duhee Yoon, Hyeonsik Cheong Department of Physics, Sogang University, Seoul 121-742, Korea hcheong@sogang.ac.kr The Young¶s modulus of single layer graphene is estimated by measuring the strain applied by a pressure difference across graphene membranes using Raman spectroscopy. The mechanical properties of graphene are interesting research subjects because its Young's modulus and strength are extremely high. Values of ~1 TPa for the Young's modulus have been reported [1,2]. Raman spectroscopy is a very powerful tool for investigating the intrinsic properties of graphene. Especially, the Raman spectrum of graphene is very sensitive to the change of mechanical deformation. Since the graphene membrane is impermeable to any gas [3], it is possible to apply strain on graphene by applying a net gas pressure across a suspended graphene. Several groups have studied such graphene balloons [4]. We directly measured the strain induced on the pressurized graphene balloons by Raman spectroscopy. Graphene samples were prepared on the pre-patterned silicon substrates covered with 300-nm thick SiO2 layer. The substrates were patterned by round holes with various diameters by photolithography and dry etching. The depth of the holes is ~5 m, and the diameters are 2.0, 3.1, 4.2, 5.3, 6.4 and 7.3 µm. The graphene samples were prepared directly on the cleaned substrate by mechanical exfoliation from natural graphite flakes. The samples were placed into a chamber, and a pressure difference was applied across the graphene membrane by evacuating the chamber. This pressure difference makes the graphene membrane bulge upward like a balloon. By measuring the shifts of the Raman G and 2D bands, we estimated the amount of strain on the graphene membrane. To estimate the amounts of strain from the Raman spectrum, we use the reported value of the Gr¾neisen parameters [5,6]. We estimated the biaxial strain on the center of graphene membrane to be about 0.19%. The strain at the center of the hole tends to increase as a function of the diameter of the hole. By comparing the strain estimated from the Raman measurements with numerical simulations based on the finite element method, we obtained the Young's modulus of graphene.
References [1] Changgu Lee, Xiaoding Wei, Jeffrey W. Kysar and James Hone, Science, 321 (2008) 385 [2] Steven P. Koenig, Narasimha G.Boddeti, Martin L. Dunn and J.Scott Bunch, Nature Nanotechnology, 6 (2011) 543 [3] J. Scott Bunch, Scott S. Verbridge, Jonathan S. Alden, Arend M. van der Zande, Jeevak M. Parpia, Harold G. Craighead and Paul L. McEuen, Nano Letter, 8, (2008) 2458 [4] Jacob Zabel, Rahul R. Nair, Anna Ott, Thanasis Georgiou, Andre K. Geim, Kostya S. Novoselov and Cinzia Casiraghi, Nano Letter, 12 (2011) 617 [5] Mohiuddin et al. Physical Review B, 79 (2009) 205433 [6] Duhee Yoon, Young-Woo Son and Hyeonsik Cheong, Physical Review Letter, 106 (2011) 155502
Figures
Schematic diagram of experimental setup.
Graphene with a spin-orbit super-lattice potential Lucia Lenz and Dario Bercioux Freiburg institute for advanced studies, Germany lucia.lenz@frias.uni-freiburg.de
We study the band-structure of graphene in the Dirac approximation in the presence of periodically modulated spin-orbit interactions. We show that, when the lattice momentum is along the modulation direction, the band-structure is given by two separate equations related to the different spin states under the potential. These equations can be derived by considering the case where either the two modes corresponding to one spin state or the two modes corresponding to the other spin state are available under the potential. This is in sharp contrast to the case where there is a finite angle between the lattice momentum and the modulation direction. Here, the band-equations cannot be separated and we evaluate the band-structure numerically.
Ultrafast transient absorption and Raman-imaging studies of stacked graphene Thomas Lenzer,1 Kawon Oum,1 Rainer Bornemann2 and Peter Haring Bolivar2 1
Universität Siegen, Physikalische Chemie, Adolf-Reichwein-Str. 2, 57076 Siegen, Germany 2 Universität Siegen, Zentrum für Sensorsysteme (ZESS) and Institut für Höchstfrequenztechnik und Quantenelektronik (HQE), 57068 Siegen, Germany lenzer@chemie.uni-siegen.de
Graphene / ionic liquid (IL) based quasi-solid state electrolytes may serve as promising replacements for volatile organic solvent electrolytes in dye-sensitized solar cells (DSSCs). Surprisingly little is known regarding their photophysical properties and dynamics of dye molecules after photoexcitation and the role of graphene in such complex environments. Moreover, information on the structure and dynamics at the IL-graphene interface is significantly lacking. In this contribution, we present our preliminary results for the ultrafast dynamics of stacked graphene films by using femtosecond broadband transient absorption experiments in the UV-VIS region with a time-resolution down to ca. 50 fs, ultrafast UV-pump / near-IR-probe techniques, and Raman-imaging microscopy. Raman-imaging allowed mapping out the number of graphene layers with high spatial resolution. In the time-resolved experiments, we varied the experimental conditions, such as number of graphene layers, pump/probe wavelengths, pump laser fluence, and detection time scale from the sub-picosecond range to 1 ns. The current results are consistent with ultrafast carrier relaxation dynamics on different timescales in the valence and conduction band upon excitation by the femtosecond laser pulse (components in the < 150 fs and ca. 1 ps range, and a long-lived one still visible at 1 ns). We hope this study will provide background information for a better understanding of the photoinduced dynamics of graphene in complex environments such as graphene / ionic liquid (IL) based DSSCs.
Acknowledgement Authors gratefully acknowledge Daniel Neumaier and Martin Otto (AMO GmbH, Aachen, Germany) for providing the graphene sample for these investigations and fruitful discussions with Sergey I. Druzhinin. Figures (a)
(b)
Normalized 'OD
1
increasing pump fluence
0.1
(pump/probe = 430 / 860 nm)
0.01
0.001
0
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Time / ps Figure 1. (a) Ultrafast transient absorption signals of a stacked graphene film on quartz/PMMA for different pump fluences of the excitation pulse at 430 nm; (b) Raman-imaging of the same sample (pump = 532 nm, size = 2 512x512 Pm , recorded in 1Pm step size).
Suspended Graphene Based Devices and Nanomechanical Properties Qiang Li, Ying Fang, Mingdong Dong Interdisciplinary Nanoscience Center (iNANO), Aarhus University, DK-8000 Aarhus C, Denmark qiang@inano.au.dk Graphene is attracting tremendous interests due to its superb physical properties and future promise in nanoelectronics. In particular, owing to its one atomic thickness and ultra high carrier mobility, grapheme field effect transistors has been proposed as a promising candidate for sensitive and label free detection of chemical and biological species. We have studied on the performance improvement of graphene devices by suspending them in aqueous solution through a novel in situ etching technique. Our results show that, owing to concomitantly increased transconductance and decreased noise level by removal of the oxide, the signal-to-noise ratios of suspended graphene nanodevices were improved in low-frequency regime for both hole and electron carriers compared with those supported on SiO2 substrates. We have also studied the nanomechanical properties of suspended graphene by using atomic force microscopy. References [1] Cheng, Z. G.; Li, Q.; Li, Z. J.; Fang, Y, Nano Lett 2010, 10 (5), 1864-1868 [2] Li, Q.; Cheng, Z.G.; Li, Z. J.; Fang, Y.; Wang, Z.H., Chin Phys B 2010, 19 (9), 097307 [3] Li, Q.; Zhang, S.; Song, J.; Dong, M.D., manuscript in prepare
Suspended Graphene Based Devices and Nanomechanical Properties Qiang Li, Ying Fang, Mingdong Dong Interdisciplinary Nanoscience Center (iNANO), Aarhus University, DK-8000 Aarhus C, Denmark qiang@inano.au.dk Graphene is attracting tremendous interests due to its superb physical properties and future promise in nanoelectronics. In particular, owing to its one atomic thickness and ultra high carrier mobility, grapheme field effect transistors has been proposed as a promising candidate for sensitive and label free detection of chemical and biological species. We have studied on the performance improvement of graphene devices by suspending them in aqueous solution through a novel in situ etching technique. Our results show that, owing to concomitantly increased transconductance and decreased noise level by removal of the oxide, the signal-to-noise ratios of suspended graphene nanodevices were improved in low-frequency regime for both hole and electron carriers compared with those supported on SiO2 substrates. We have also studied the nanomechanical properties of suspended graphene by using atomic force microscopy. References [1] Cheng, Z. G.; Li, Q.; Li, Z. J.; Fang, Y, Nano Lett 2010, 10 (5), 1864-1868 [2] Li, Q.; Cheng, Z.G.; Li, Z. J.; Fang, Y.; Wang, Z.H., Chin Phys B 2010, 19 (9), 097307 [3] Li, Q.; Zhang, S.; Song, J.; Dong, M.D., manuscript in prepare
Towards transfer-free fabrication of graphene NEMS Niclas Lindvall, Jie Sun, Galib Abdul, and August Yurgens Department of Microtechnology and Nanoscience, Chalmers University of Technology, SE-41296 Gothenburg, Sweden niclas.lindvall@chalmers.se Many exciting applications of graphene are based on combining several of the extraordinary properties of this material. Graphene shows great potential in nanoelectromechanical systems (NEMS) where its large mechanical strength, flexibility and electrical conductivity make it a unique material. Adding a very low mass and true two-dimensional nature, it may find its applications in several new types of sensor devices.
Recently, there has been large progress in the large-scale synthesis of graphene grown by chemical vapor deposition (CVD) [1-5]. This material exhibits excellent mechanical- and electronic properties and suspended graphene resonators have been demonstrated and characterized. However, the fabrication involves the transfer of graphene from its copper catalyst to the target substrate by a wet chemical etching method. This transfer procedure introduces metal residues, wrinkles and holes in the graphene. Due to these issues, there is a need for a transfer-free fabrication method for suspended graphene structures grown by CVD.
We fabricate suspended graphene without the need for graphene transfer. Graphene can be grown either catalytically on thin copper films, or non-catalytically directly on a dielectric substrate. A typical fabrication procedure includes the growth of graphene by CVD followed by the deposition of Au/Ti electrodes defined by e-beam lithography (ebl) and graphene etching using oxygen plasma. The structures are made suspended by chemical etching of the top substrate layer (copper or silicon dioxide) and are critically point dried.
We perform electrical-, mechanical- and optical characterization as well as scanning- and transmission electron microscopy to verify the graphene properties. Scanning electron micrographs of typical devices are shown in Figure 1. While its electronic properties are inferior to those of graphene grown on high quality copper foils, it shows remarkable mechanical strength. The sheet resistivity is typically in the order of 100 kâ&#x201E;Ś, making it, so far, unsuitable for high-frequency devices. However, with further improvement of the material quality, this work constitutes steps forward towards transfer-free fabrication of graphene NEMS grown by CVD.
References [1] J. Sun, N. Lindvall, M. T. Cole, K. B. K. Teo, and A. Yurgens, "Large-area uniform graphene-like thin films grown by chemical vapor deposition directly on silicon nitride", Appl Phys Lett vol. 98, p. 252107, 2011. [2] J. Sun, N. Lindvall, M. Cole, K. Angel, T. Wang, K. Teo, D. Chua, J. Liu, and A. Yurgens, "Low Partial Pressure Chemical Vapor Deposition of Graphene on Copper", IEEE Trans Nanotechnol, In press. [3] J. Sun, M. T. Cole, N. Lindvall, K. B. K. Teo, and A. Yurgens, "Noncatalytic chemical vapor deposition of graphene on high-temperature substrates for transparent electrodes", Appl Phys Lett, 100, p. 022102, 2012. [4] Y. Fu, B. Carlberg, N. Lindahl, N. Lindvall, J. Bielecki, A. Matic, Y. Song, Z. Hu, Z. Lai, L. Ye, J. Sun, Y. Zhang, Y. Zhang, and J. Liu, "Templated Growth of Covalently Bonded Three-Dimensional Carbon Nanotube Networks Originated from Graphene", Adv Mater, In press. [5] J. Sun, N. Lindvall, M. T. Cole, T. Wang, T. Booth, P. Boggild, K. B. K. Teo, J. Liu, and A. Yurgens, "Controllable chemical vapour deposition of large area uniform nanocrystalline graphene directly on silicon dioxide", J Appl Phys, In press.
Figures
Figure 1. (a) Side view and (b) top view scanning electron micrographs of suspended graphene membranes fabricated with a transfer-free chemical vapor deposition method. The graphene is suspended over a silicon dioxide substrate between two gold electrodes.
Fabrication of graphene flakes via oxidation-reduction method / /LSLÄ&#x201D;VND1 - -DJLHĂĄĂĄR1, M. Zdrojek2, E. Talik3, M. Andrzejczuk4, M. Lewandowska4, 0 0RÄŞGÄŞRQHN1, A. Aksienionek1 . .LHĂĄEDVLÄ&#x201D;VNL1, E. Brzozowski1, A. Strojny1. 1)
Institute of Electronic Materials Technology :yOF]\Ä&#x201D;VND -919 Warsaw, Poland 2) Warsaw University of Technology, Faculty of Physics, 00-662 Warsaw, Poland 3) Institute of Physics, University of Silesia, Uniwersytecka 4, 40-007 Katowice, Poland 4) Faculty of Science and Engineering, Warsaw UnivHUVLW\ RI 7HFKQRORJ\ :RĂĄRVND 02-507 Warsaw, Poland Ludwika.Lipinska@itme.edu.pl
Chemical methods are proved to be an efficient way for mass production of graphene dedicated to broad set of applications, for example for various type of composites. There are many strategies to achieve efficient wet exfoliation of graphite to very thin flakes. Among them the oxidation-reduction way is the closest for scaling up. Importantly, the intermediate product-graphene oxide GO is also very promising material with high application potential. In this work, we have succeeded in preparing single layer graphene oxide flakes by mild oxidation of expanded graphite. The GO sheets were investigated by Raman spectroscopy. The thickness was measured by atomic force microscopy (AFM). The size and surface morphology of flakes were probed by scanning electron microscopy (SEM) and transmission scanning electron microscopy (STEM) presented in Figures 1 and 2. Obtained graphene oxide was used to prepare polysiloxane composite. We showed that addition of only 0.5 % GO significantly enhances the thermal conductivity of the composite. The next step was the reduction of graphene oxide in order to improve electrical conductivity of the composite. We used several chemical reducing agents: sodium borohydride, ascorbic acid, sodium citrate, formic acid, benzylamine. Products of reactions - reduced graphene oxide (rGO) samples were characterized by infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS). Finally electrical measurements were performed by testing conductivity between dense spaced planar electrodes covered by rGO flakes. The principles and details will be presented on the poster. It turned out that conductivity of rGO strongly depends on used reducing agents and reaction conditions. The best results were obtained by two step reduction and synergistic reduction by two agents.
Figure 1. SEM image of GO flake
Figure 2. STEM picture of part of GO flake
Modelling graphene growth by atomistic simulation of 2D polycrystal crystallization - video Andrzej Lissowski Computer Section, Society of the Polish Free University, 7/38 Słupecka Str. 03-309 Warsaw, Poland aliss5h7@engineering.com John Werner Cahn first considered close-packed 5-, Hexa-, 7-gons (5H7) in 1965 (5/7 edge dislocations ED) and in lecture “Euler theorem and penta-hepta defects” during MIT 1970 conference “Shaping of tissue by deviation from hexagonal close-packing of cells”, triple junctions (TJ) of grain boundaries (GB) in 2D polycrystal were discussed. In our with Herbert Gleiter paper: “Rearrangement of atoms during GB migration” (Z. Metallkunde 1971) our work with Cahn about coincident GB was announced. Our work with Cahn continued after our lecture “Crystallization as rearrangement of GB and Ed by Voronoi centroidal iterations” at Symp. Phys.Chem. Comp.Films 1978. Modelling extends Boris Yakobson ideas.
Two frames from presented video of atomistic simulaton, mainly Voronoi centroidal iterations from random but uniform initial distributions, dynamically expose ED, 5H7, GB, two opposite (containing additional 5 or 7 disclination) TJ alternating along GB. Also were explored and applied Phase Field Crystals (after Landau, Cahn, Elder), conformal circle-packing, Ricci combinatorial curvature flow ... Crystallization of large 2D bicrystals, with sigma 7 (like Ising kagome phase transition) and sigma 13, 19 ... coincident GB, was presented many times in 2002. 2D polycrystals are emerging as not similar to commonly considered 2D close-packed grains with closed loops of GB, such as seen in 2D crossections of 3D polycrystals and in von Neumann-Mullins grain growth model. Rather more rich configurations of GB, TJ, disclinations, ED, vacancies, interstitials and other 5H7 could be expected in true 2D polycrystals. Cit. from our abstract at Int. Congress of Mathematicians 1983: "Two alternating TJ of GB allows only their even loops." Proof reminds simplest fundamental chirality of oriented clockwise and counter-clockwise triangles. This suggests very fundamental nature and importance of many implications. Two-dimensionality is not a poor "Flatland", but very rich 2D "Glasperlenspiel". 2D is stronger then 3D in structural changes: 5H7 recrystallization, curvatures, topological defects, quasicrystals, phase transitions, percolation ... Life is possible because biomembrane 2D reactions (5H7 changes in lipid rafts, clathrin coated pits ...) surpass surrounding 3D changes. Orientational pinwheels from visual cortex are 2D vortices and anti-vortices, like 5 and 7 disclinations (Gaussian curvature quanta) in 5H7. Very rich, but still poorly understood (graphene promises breakthrough) 5H7 patterns offer enormous power for best coding and control in biology and many other fields with 2D close-packing.
Minimal Tight-Binding Model for Quantum Transport in Graphene Heterojunctions
Ming-Hao Liu (ddd), Jan Bundesmann, Klaus Richter Institut für Theoretische Physik, Universität Regensburg, D-93040 Regensburg, Germany minghao.liu.taiwan@gmail.com A real-space Green’s function formalism based on a minimal tight-binding model (TBM) is adopted to efficiently simulate ballistic transport in graphene heterojunctions. The basic idea is to make use of the Bloch theorem along the transverse dimension of the bulk graphene (see Fig. 1), which greatly reduces the computation load and hence allows experimental sizes in the longitudinal dimension. Numerically, we will show [1] (i) the consistency of our TBM calculations with the existing results based on the effective Dirac theory for chiral tunneling through pnp junctions in monolayer graphene (MLG) and in bilayer graphene (BLG) (see Fig. 2), and (ii) spin-dependent tunneling through pn junctions in MLG with Rashba spin-orbit coupling (see Fig. 3). Extending the length scale to the experimental size and taking into account the realistic charge density profile due to gating, we further show (iii) good agreement of our minimal TBM, without free parameters, with the recent ballistic Klein tunneling experiment by Young and Kim [2].
[1] M.-H. Liu, J. Bundesmann, and K. Richter, Phys. Rev. B 85, 085406 (2012). [2] A. F. Young and P. Kim, Nat. Phys. 5, 222 (2009). [3] M. I. Katsnelson, K. S. Novoselov, and A. K. Geim, Nature Physics 2, 620 (2006).
FIGURES
eiky W eiky W eiky W eiky W eiky W eiky W eiky W eiky W eiky W eiky W eiky W b
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FIG 1. Schematic of a minimal tight-binding model that simulates a bulk MLG up to the nearest neighbor hoppings.
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FIG 2. Tunneling through a barrier for (a), (b) MLG with Fermi energy EF = 81.6 meV and (c), (d) BLG with EF = 17.1 meV. In (b), red (light gray) and blue (dark gray) curves correspond to V0 = 196.8 meV and V0 = 280.3 meV, respectively. In (d), red (light gray) and blue (dark gray) curves correspond to V0 = 48.7 meV and V0 = 100.7 meV, respectively. In both cases the barrier width is D = 100 nm and the incoming Fermi wave vector is kF = 2π/50 nm−1 , as considered in [3].
(EF , tR , V0 ) = (60, 30, 0) µeV
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Prospects of Graphene-enabled Wireless Communications 1
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Ignacio Llatser , Sergi Abadal , Raúl Gómez Cid-Fuentes , Josep Miquel Jornet , 1 1 1 1,2 Albert Cabellos-Aparicio , Eduard Alarcón , Josep Solé-Pareta and Ian F. Akyildiz 1
NaNoNetworking Center in Catalunya (N3Cat) Universitat Politècnica de Catalunya Barcelona, Spain E-mail: {llatser,abadal,rgomez,jmjornet,acabello,pareta,ian}@ac.upc.edu, eduard.alarcon@upc.edu 2
Broadband Wireless Networking Laboratory School of Electrical and Computer Engineering Georgia Institute of Technology, Atlanta, Georgia 30332, USA E-mail: {jmjornet,ian}@ece.gatech.edu
Graphene has recently attracted the attention of the research community due to its novel mechanical, thermal, chemical, electronic and optical properties. Due to its unique characteristics, graphene has given rise to a plethora of potential applications in many diverse fields, ranging from ultra high-speed transistors to transparent solar cells. Among these, a particularly promising emerging field is graphene-enabled wireless communications. Wireless communications among nanosystems cannot be achieved by simply reducing the size of classical metallic antennas, since that would impose the use of very high resonant frequencies in the optical range. Due to the expectedly very limited power of nanosystems, the low mobility of electrons in metals when nanometer scale structures are considered, and the challenges in implementing a nanotransceiver able to operate at this extremely high frequency, the feasibility of wireless communications at the nanoscale would be compromised if this approach were followed. Moreover, scaling down metallic antennas to a size of just a few micrometers would make them non-resonant and hence dramatically reduce their antenna efficiency. However, due to its groundbreaking properties, graphene is seen as the enabling technology to implement wireless communications among nanosystems. Indeed, graphene-based nano-antennas, or graphennas, just a few micrometers in size are envisaged to radiate electromagnetic waves in the terahertz band [1], at a dramatically lower frequency and with a higher radiation efficiency with respect to their metallic counterparts. Moreover, the progress in the development of graphene-based components shows that the high electron mobility of graphene makes it an excellent candidate for ultrahigh-frequency applications [2]. Recently-published work demonstrates the great potential of graphenebased ambipolar devices for analogue and RF circuits, such as LNAs, mixers and frequency multipliers [3,4]. Indeed, a graphenna supports the propagation of tightly confined Surface Plasmon Polariton (SPP) waves [5]. Due to their high effective mode index, the propagation speed of SPP waves can be up to two orders of magnitude below the EM wave propagation speed in vacuum. The consequences of this effect are twofold. On the one hand, it reduces the resonant frequency of the graphenna, enabling the use of much lower frequencies than the ones expected for such a small antenna. For instance, a graphenna with a size of a few micrometers is expected to resonate in the terahertz band (see Figure 2). The lower radiation frequency of graphennas results in a lower channel attenuation and less strict requirements for the transceiver. On the other hand, however, the mismatch between the EM wave propagation speed in the graphenna and the free space also reduces their radiation efficiency. Despite these challenges, graphene is seen as the enabling technology to implement wireless communications at the nanoscale. In consequence, graphennas have the potential to enable wireless communications among nanosystems. Communication and information sharing among nanosystems, in its turn, will allow the implementation of nanonetworks [6], i.e., networks of interconnected nanosystems, which are envisaged to create new applications in diverse fields. Therefore, nanonetworks will enhance the capabilities of individual nanosystems both in terms of complexity and range of operation, leading to the development of a novel networking paradigm. Figure 1 shows a conceptual diagram of a nanonetwork, consisting of a group of nanosystems communicating wirelessly with each other and with the Internet. Similarly to the way in which communication among computers enabled revolutionary applications such as the Internet,
nanonetworks are envisaged to enable a large amount of long-awaited applications that will change the way in which society interacts and understands technology. These include, amongst others: Ubiquitous Computing, which has been GHILQHG DV ³machines that fit the human environment instead of forcing humans to enter theirs´ ,Q WKLV FRQWH[W QDQRV\VWHPV ZLOO EH LQWHJUDWHG LQWR everyday objects and activities, and humans will communicate and interact with them. As an example, environmental sensors (such as light or temperature) could be interconnected with biometric monitors woven into clothing in such a way that the environment automatically adapts to the user needs. Programmable Matter, where many nanosystems will communicate among them to form tangible 3D objects that a user can interact with. This programmable matter will have the ability to change its physical properties, such as its shape, density and color, based on user input or autonomous sensing. For instance, programmable matter can provide tangible, interactive forms to information, so that a user can experience virtual objects and environments as if they were real. Wireless NanoSensor Networks (WNSNs) [7], i.e., networks of small sensors that can cooperatively measure magnitudes with unprecedented nanoscale accuracy and transmit this information to a central hub. For example, researchers have already built nanosensors able to measure physical characteristics of structures just a few nanometers in size, chemical compounds in concentrations as low as one part per billion, or the presence of biological agents such as virus, bacteria or cancerous cells. However, a single nanosensor is not enough to implement applications such as novel countermeasures against biological and chemical attacks at the nanoscale or advanced intra-body health monitoring and drug delivery systems. In order to enable these applications, collaboration among a swarm of these nanosensors by means of a WNSN will be needed. We envisage that graphene-enabled wireless communications, and nanonetworks in general, will have a great impact in almost every field of our society, ranging from healthcare to industrial or environmental protection, and many research efforts are required for the development of this novel networking paradigm. References [1] J. M. Jornet, I. F. Akyildiz, Proc. European Conference on Antennas and Propagation (2010). [2] M. C. Lemme, Solid State Phenomena 156-158 (2010), pp. 499¹509. [3] H. Wang, A. Hsu, J. Wu, J. Kong, T. Palacios, IEEE Electron Device Letters 31 (2010), pp. 906¹908. [4] S. Koswatta, A. Valdes-Garcia, M. Steiner, Y. Lin, P. Avouris, IEEE Transactions on Microwave Theory and Techniques 59 (2011), pp. 2739¹2750. [5] I. Llatser, C. Kremers, D. N. Chigrin, J. M. Jornet, M. C. Lemme, A. Cabellos-Aparicio, E. Alarcón, Proc. European Conference on Antennas and Propagation (2012). [6] I. F. Akyildiz, F. Brunetti, C. Blåzquez, Computer Networks 52 (2008), pp. 2260¹2279. [7] I. F. Akyildiz, J. M. Jornet, Nano Communication Networks 1, (2010), pp. 3¹19. Figures
Figure 2: Dependence of the absorption cross section of a graphene-based nano-patch antenna as a function of its width. The antenna length is L = 5 Č?m. The plots correspond to infinite, 10 Č?m, 5 Č?m, 2 Č?m and 1 Č?m wide patches (right to left). Figure 1: Conceptual diagram of a nanonetwork. The upper left corner shows a magnified individual nanosystem.
Graphene nanogap for gate-tunable quantum-coherent single-molecule electronics T. Löfwander, A. Bergvall, K. Berland, P. Hyldgaard, and S. Kubatkin Dep. of Microtechnology and Nanoscience ± MC2, Chalmers University of Tech., Göteborg, Sweden tomas.lofwander@chalmers.se
We present atomistic calculations of quantum coherent electron transport through fulleropyrrolidine terminated molecules bridging a graphene nanogap. We predict that three difficult problems in molecular electronics with single molecules can be solved by utilizing graphene contacts: (1) a back gate modulating the Fermi level in the graphene leads facilitates control of the device conductance in a transistor effect with high on-off current ratio; (2) the size mismatch between leads and molecule is avoided, in contrast to the traditional metal contacts; (3) as a consequence, distinct features in charge flow patterns throughout the device are directly detectable by scanning techniques. We show that moderate graphene edge disorder is unimportant for the transistor function.
References [1] A. Bergvall, K. Berland, P. Hyldgaard, S. Kubatkin, and T. Löfwander, Phys. Rev. B 84 155451 (2011). Figures
Figure 1. Left panel: Geometry of the transistor. Right panel: Transistor effect as seen in the transmission function versus back gate voltage and energy. A molecular level at the Fermi energy of the graphene leads (Vg=0) gives high transmission. Tuning the Fermi level of the leads so that the Dirac point coincides with the molecular level, leads to quenching of the transmission.
Measurement of reduced graphene oxide conductivity using Electrostatic Force Microscopy Guillermo López-Polín, Cristina Gómez-Navarro*, Francisco J. Guzmán-Vázquez, Julio GómezHerrero, Juan J. Saenz and Sacha Gómez M. Universidad Autónoma de Madrid, Campus Cantoblanco, Madrid, Spain guillermo.lopezpolin@uam.es In the route of transferring the extraordinary properties of graphene to real applications one of the most promising means for obtaining graphene in large amounts is the oxidation-reduction process of graphite [1]. The main advantages of this procedure lie in its scalability, cost effective and the high yield of monolayers obtained via this route. Deoxygenation of the initially insulating Graphene Oxide (GO) sheets leads to a partial recovery of the original conductivity of graphene[2]. Due to the different starting materials and procedures used in the preparation of Reduced Graphene Oxide (RGO) layers a large dispersion of conductivity can be found in literature[3]. The standard process to characterize the conductivity of micrometer sized films usually involves tedious lithography procedures that impede a quick estimation making highly desirable to develop a fast and non invasive method for characterizing the conductivity of single layers.
In this work we report experimental results showing that Electrostatic Force Microscopy (EFM) can be used as a contactless method to accurately distinguish between monolayered RGO sheets with different conductivities in the range of 0-3 S/m on an insulating substrate. Comparing EFM and conventional conductivity measurements we find that the electrostatic interaction between RGO sheets and a metalized AFM tip is strongly dependent on the conductivity of the layers. Using the reported method, it is possible to evaluate the conductivity of RG0 sheets in less than 1 hour, without the need of electrical contacts.
We also report theoretical modeling of the EFM signal[4] in our experimental set-up which allows us determining İeff of the RGO layers from the experimental data. This effective permittivity is found to span from 5, for the oxidized (insulating) case to 2000 for the more conductive layers.
We will also discuss how the sensitivity of EFM to high conductivities is due to the combination of the extreme thinness and high conductivity with low density of charge carriers of RGO layers.
References [1] Stankovich, S. et al., Journal Of Materials Chemistry, 16 (2006) 155. [2] Gomez-Navarro, C. et al., Nano Letters, 7 (2007) 3499. [3] Dai, B. et al., Nano Research, 4 (2011) 434. [4] Sacha, G. M.,et al., Journal of Applied Physics, 101 (2007) 024310.
Figures
Figure caption: AFM topographic image and electrostatic force image of a region comprising a gold marker (feature on the down-left corner), a reduced graphene oxide layer and a SiO2/Si substrate.
Edge functionalization of graphene nanoribbons for electronic applications Francisco J. Martin-Martinez, Stijn Fias, Gregory Van Lier, Frank De Proft and Paul Geerlings Vrije Universiteit Brussel (VUB), Research Group General Chemistry (ALGC), Pleinlaan 2, B-1050, Brussels, Belgium fmartinm@vub.ac.be Graphene nanoribbons (GNRs) have come into the picture as promising candidates for the design of electronic devices, due to their finite width and distinct electron confinement. As a matter of fact, the lateral quantum confinement in GNRs opens a band gap in a useful range for electronic and solar cells applications, with a sufficient on/off ratio for semiconducting devices. According to previous studies on GNRs, widths below 3 nm should be reached to obtain band gaps that fall in the same range of the current worldwide-used semiconductors materials like Si or GaAs. [1] Furthermore, a periodicity in the band gap exists as the width of the GNR is increased,[2] and the aromaticity distribution along the GNR could be crucial to understand the basic electronic properties of the GNRs. As already described in literature, three different types of aromaticity patterns have been found for armchair GNRs depending on their width. [3] In this work we study the aromaticity of GNRs using the Six Centre Index [4] (SCI) and the Mean Bond Length (MBL) geometry descriptor. [5] These patterns are similar to the patterns of armchair carbon nanotubes depending on the length.[6],[7] Each pattern appears periodically every three steps as the width of the GNR is increased, closely related to the same periodicity found in the energy band gap. [8] The reasons for the appearance of such patterns are proposed within the framework of the Clar sextet theory. Fluorine, Oxygen and Hydrogen edge functionalization are considered in order to perform a rational tuning of both the aromaticity patterns and the band gap. Even more, a way to induce aromaticity patterns and therefore suitable band gap in zigzag GNRs is shown. The close relation between aromaticity distribution and electronic properties of these materials is demonstrated and the possible application for electronics is discussed. References [1] Barone, V.; Hod, O.; Scuseria, G. E. Nano Lett. 6 (2006) 2748. [2] Son Y-W.; Cohen, M. L.; Louie, S. G. Phys. Rev. Lett. 97 (2006) 216803. [3] Baldoni, M.; Sgamellotti, A.; Mercuri, F. Org. Lett. 9 (2007) 4267. [4] Bultinck P.; Fias, S.; Ponec, R. Chem. Eur. J. 12 (2006) 8813. [5] Martin-Martinez, F. J.; Melchor, S.; Dobado J. A. Org. Lett. 10 (2008) 1991. [6] Matsuo, Y.; Tahara, K.; Nakamura, E. Org. Lett., 5 (2003) 3181. [7] Martin-Martinez, F. J.; Melchor, S.; Dobado J. A. Phys. Chem. Chem. Phys. 13 (2011) 12844. [8] Martin-Martinez, F.J.; Fias. S.; Van Lier, G.; De Proft, F.; Geerlings, P. Chem. Eur. J. in press (2012).
Figure
Band gap of GNRs depending on the width, and detail of the relation between the periodicity in the band gap and the aromaticity patterns. Only the central part of the ribbon is represented here.
Effect of Oxygen Adsorption on the Local Properties of Epitaxial Graphene on SiC 1
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C. Mathieu , B. Lalmi , T. O. Mentes , E. Pallecchi , A. Locatelli , S. Latil , R. Belkhou and A. Ouerghi
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1. CNRS- Laboratoire de Photonique et de Nanostructures, Route de Nozay, 91460 Marcoussis, France 2. Synchrotron SOLEIL, Saint-Aubin, BP48, F91192 Gif sur Yvette Cedex, France 3. Sincrotrone Trieste ELETTRA, Area Science Park, 34149 Basovizza, Trieste, Italy 4. CEA Saclay, DSM IRAMIS SPCSI, F-91191 Gif Sur Yvette, France claire.mathieu@lpn.cnrs.fr
Since its discovery, graphene has attracted tremendous interest and its unusual properties make it a promising candidate for future electronic and optic applications [1, 2]. Along the view of designing graphene-based devices, many efforts are devoted to the achievement of large scale graphene patterning in a reproducible way with controlled structural quality. Among the various ways to produce graphene [3], the growth of graphene layer on silicon carbide (SiC) is a very promising method for homogeneous large scale production with a high crystalline quality, as it has recently been demonstrated [4]. On the Si face, the first carbon layer present a honeycomb structure, which has no graphitic electronic properties, due to one-third of the carbon atoms that are bonded to the Si atoms of the substrate [5]. This layer therefore acts as a buffer layer and allows the next carbon layer, i.e. first graphene layer, to behave, from an electronic point of view, as an isolated graphene sheet. However, the Si dangling bonds that remain below this buffer layer deteriorate the carrier mobility, due to the high intrinsic electron doping. Some recent experiments ± LEEM, XPS, and EELS principally ± have demonstrated that the graphene ʌ EDQG RI WKH LQWHUIDFH OD\HU FDQ EH UHVWRUHG E\ WKH LQVHUWLRQ D WKLQ oxide layer between this interface and the SiC substrate [6].
We have followed, from local point of view, the structural and electronic modifications of epitaxial graphene, for different steps of the oxidation process. This has allowed us to finely study the transition of the buffer layer toward a partially decoupled graphene-like sheet, by a controlled in-situ exposition to oxygen. We demonstrate in the present contribution that the oxygen can partially decouple the buffer layer from the substrate and hence reduce the intrinsic electron doping, which leads in turn to the partial transformation of the buffer layer into a graphene-like one. As a result, the oxygen reacts with the Si dangling bonds, reducing the charge transfer from the substrate to the graphene layer, and also intercalates between the carbon layers.
References [1] K. S. Novoselov, Z. Jiang, Y. Zhang, S. V. Morozov, H. L. Stormer, U. Zeitler, J. C. Maan,G. S. Boebinger, P. Kim, A. K. Geim, Science, 315 (2007) 1379. [2] K. S. Novoselov, A. K. Geim, S. V. Morozov, D. Jiang, Y. Zhang, S. V. Dubonos, I. V. Grigorieva, A. A. Firsov, Science, 306 (2004) 666. [3] V. Singh, D. Joung, L. Zhai, S. Das, S. I. Khondaker, S. Seal, Prog. Mater. Sci., 56 (2011) 1178. [4] C. Berger, X. Wu, P. N. First, E. H. Conrad, X. Li, M. Sprinkle; J. Hass, F. Varchon, L. Magaud, M. L. Sadowski, Adv. Solid State Phys., 47 (2008) 145. [5] K. V. Emtsev, F. Speck, Th. Seyller, L. Ley, and J. D. Riley, Phys. Rev. B, 77 (2008) 155303. [6] S. Oida, F. R. McFeely, J. B. Hannon, R. M. Tromp, M. Copel, Z. Chen, Y. Sun, D. B. Farmer, J. Yurkas, Phys. Rev. B, 82 (2010) 041411.
Band dispersion as a function of k// around the K point of the first BrillRXLQ ]RQH REWDLQHG E\ Â&#x2014;$53(6 DW KČ&#x17E; eV, performed before oxidation and after each oxidation step. The Fermi level and the Dirac point are superimposed on the images. The charge density is also indicated for every step of the oxidation process.
Spectroscopic ellipsometry measurements of doped graphene 1
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Institute of Physics, University of Belgrade, Pregrevica 118, PO Box 68, 11080 Belgrade, Serbia Centre of Microelectronic Technologies and Single Crystals, a Division of the Institute of Chemistry, Technology and Metallurgy, University of Belgrade, NjeJRĂŁHYD 12, 11000 Belgrade, Serbia amatkovic@ipb.ac.rs
Graphene, a single layer of carbon atoms packed tightly into a honeycomb lattice, has been attracting wide attention due to its unique electronic properties [1]. Through its optical response in the visible and ultraviolet (UV) ranges, we can obtain information on the electronic structure in the nonlinear part of its dispersion relation. We investigate the optical response of graphene via spectroscopic ellipsometry [2,3] in the UV and visible ranges. Graphene samples are obtained by micromechanical exfoliation of natural graphite on the Si/SiO2 substrate. One of the samples is shown in figure 1. Furthermore, gold contacts are made by a UV photolithography process on some of the samples. These contacts allow us to externally control the doping level of the sample during the measurements. The optical conductance of graphene is described by a Fano model [4-6]. The parameters of this model are extracted from our spectroscopic ellipsometry measurements (figure 2), and the complex refractive index of graphene is obtained (figure 3) *UDSKHQHÂśV Gispersion relation shows that the density of states function has a logarithmic van Hove singularity corresponding to the M point of the Brillouin zone. The exciton binding energy is calculated as the difference between the resonant and the saddle point energies [4-7]. Afterwards, measurements are carried out with different doping levels. The influence of doping [8] on the extracted optical parameters is estimated.
References [1] K. S. Novoselov, A. K. Geim, S. V. Morozov, D. Jiang, Y. Zhang, S. V. Dubonos, I. V. Grigorieva and A. A. Firsov, Science, 306 (2004) 666-9. [2] V. G. Kravets, A. N. Grigorenko, R. R. Nair, P. Blake, S. Anissimova, K. S. Novoselov and A. K. Geim, Phys. Rev. B, 81 (2010) 155413. [3] J. W. Weber, V. E. Calado and M. C. M. van de Sanden, Appl. Phys. Lett. 97 (2010) 091904. [4] D-H. Chae, T. Utikal, S. Weisenburger, H. Giessen, K. V. Klitzing, M. Lippitz and J. Smet, Nano Lett. 11 (3) (2011) 1379-82. [5] K. F. Mak, J. Shan and T. F. Heinz, Phys Rev. Lett. 106 (2011) 046401. [6] A. Matkovic, U. Ralevic, G. Isic, M. M. Jakovljevic, B. Vasic, I. Milosevic, D Markovic and R Gajic, Phys. Scr. T146 (2012) (in print). [7] L. Yang, J. Deslippe, C-H. Park, M. L. Cohen and S. G. Louie, Phys. Rev. Lett. 103 (2009) 1086802. [8] L. Yang, Nano Lett. 11 (9) (2011) 3844±7. Figures (b) tan(<)
50 y [µm]
(a)
30 10 10
40
70
100
130
x [µm] Figure 1: (a) Single layer graphene sample used for extraction of the model parameters (presented in figure 3) (b) tan(< map of the sample measured at fixed incident light wavelength of 290nm and fixed incident o angle of 60 ; spectroscopic measurements (shown in figure 2) were carried out in the center of the flake (where tan(< has a maximal value).
Figure 2: (a) tan(<) and (b) cos(') of the graphene sample (dashed line) and the bare substrate (dot-dashed line) o o for two different incident angles of 60 and 50 . Filled lines represent fitted sample data.
Figure 3: (a) Complex dielectric susceptibility, (b) complex refractive index and (c) complex optical conductivity of graphene modeled by the Fano resonance with the best fit to measured data set of parameters.
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Complementary hot carrier transistor with vertical graphene base electrode for THz applications #
#
W. Mehr, J. Dabrowski, Ch. Scheytt, G. Lippert, Y.-H. Xie*, M.C. Lemme , S. Vaziri , G. Lupina IHP, Im Technologiepark 25, 15236 Frankfurt (Oder), Germany mehr@ihp-microelectronics.com * Department of Materials Science and Engineering, University of California at Los Angeles, USA #
KTH Royal Institute of Technology, Isafjordsgatan 22, 16440 Kista, Sweden
The ultimate thinness and low resistivity of graphene can be exploited to realize ultra-fast electronic devices. Here, we analyze a novel vertical concept of a graphene transistor called graphene base transistor (GBT). Figure 1 illustrates the difference between a graphene field-effect transistor (GFET) and a GBT [1]. In a GFET, the output current flows inside of the graphene sheet that forms a channel connecting the source to drain and that has its conductivity controlled by the gate electrode. In a GBT, the output current flows from the emitter to the collector perpendicularly to the graphene sheet that is contacted to the base electrode and that controls the amount of carriers ejected from the emitter. The carriers are injected from the emitter by Fowler-Nordheim tunneling through the emitter-base insulator (EBI) into the conduction band of the base-collector insulator (BCI) and further into the collector, traversing the graphene base. Due to monatomic thickness of graphene, the transport across the base is ballistic. Assuming that only electrons scattered within the base contribute to the base current I B, this may reduce IB even by two orders of magnitude compared to a similar design with a metal base. In addition, graphene is chemically inert, potentially reducing problems with interface reactions. A GBT has advantages over a GFET. In particular, it is designed to have a very high ION/IOFF ratio, a natural current saturation, and high power operation. For that reason, GBT is useable for analog radio frequency applications and as complementary transistors for ultra-high speed logic applications [2]. In addition, the BCI can be designed to sustain output voltages well above 10 V without compromising the transfer characteristics or the frequency response, so that a GBT can work also a high power transistor. Results of conservative simulations suggest that transition frequencies up to 2 THz can be achieved (Fig. 2). The simulations assume a simple one-band effective potential for which SchrĂśdinger equation with open boundary conditions is solved. The parameters correspond to n-GBT with Er2Ge3 emitter, Ge EBI, and a graded TiSiO/SiO2 BCI. Graphene is considered to be a tunneling barrier, as indicated by the results of ab initio calculation of self-consistent tunneling spectra in equilibrium. The quantum capacitance of graphene, CQ, is an important factor. Its role depends also on the tunneling properties of graphene, which at the moment can only be estimated. Assuming that the tunneling barrier is defined by the graphene EDQG VWUXFWXUH DW ÄŤ ZH H[SHFW that THz operation is possible when the EBI thickness is in the range of 3-5 nm and the EBI tunneling barrier height is about 0.2-0.4 eV.
References [1] W. Mehr et al., arXiv:1112.4520v1 [2] W. Mehr et al., accepted for publication in IEEE Electron Device Letters, (2012)
Figure 1
GBT
GFET
Emitter
Gate Base
EBI
Base
Source
Gate dielectric
Drain
BCI Collector
Silicon
Silicon Graphene
Schematic cross-section of a graphene base transistor (GBT) and a graphene field-effect transistor (GFET).
Figure 2
Simulated transition (cut-off) frequency fT of a high power GBT. Solid line: no quantum capacitance effects are accounted for (CQ Â&#x2019;). Broken line: with quantum capacitance effects (CQ = Č&#x203A;VQ). V = VEB - VQ defines the EBI electric field. VQ is the voltage drop on CQ and corresponds to the position of the Fermi level in graphene, measured with respect to the Dirac point, and is determined by the applied voltages and the EBI and BCI plate capacitances. For this particular design, VQ | 0.3 V at V | 1.3 V. Above 1.2 eV, quantum oscillations in fT begin.
Complementary hot carrier transistor with vertical graphene base electrode for THz applications #
#
W. Mehr, J. Dabrowski, Ch. Scheytt, G. Lippert, Y.-H. Xie*, M.C. Lemme , S. Vaziri , G. Lupina IHP, Im Technologiepark 25, 15236 Frankfurt (Oder), Germany mehr@ihp-microelectronics.com * Department of Materials Science and Engineering, University of California at Los Angeles, USA #
KTH Royal Institute of Technology, Isafjordsgatan 22, 16440 Kista, Sweden
The ultimate thinness and low resistivity of graphene can be exploited to realize ultra-fast electronic devices. Here, we analyze a novel vertical concept of a graphene transistor called graphene base transistor (GBT). Figure 1 illustrates the difference between a graphene field-effect transistor (GFET) and a GBT [1]. In a GFET, the output current flows inside of the graphene sheet that forms a channel connecting the source to drain and that has its conductivity controlled by the gate electrode. In a GBT, the output current flows from the emitter to the collector perpendicularly to the graphene sheet that is contacted to the base electrode and that controls the amount of carriers ejected from the emitter. The carriers are injected from the emitter by Fowler-Nordheim tunneling through the emitter-base insulator (EBI) into the conduction band of the base-collector insulator (BCI) and further into the collector, traversing the graphene base. Due to monatomic thickness of graphene, the transport across the base is ballistic. Assuming that only electrons scattered within the base contribute to the base current I B, this may reduce IB even by two orders of magnitude compared to a similar design with a metal base. In addition, graphene is chemically inert, potentially reducing problems with interface reactions. A GBT has advantages over a GFET. In particular, it is designed to have a very high ION/IOFF ratio, a natural current saturation, and high power operation. For that reason, GBT is useable for analog radio frequency applications and as complementary transistors for ultra-high speed logic applications [2]. In addition, the BCI can be designed to sustain output voltages well above 10 V without compromising the transfer characteristics or the frequency response, so that a GBT can work also a high power transistor. Results of conservative simulations suggest that transition frequencies up to 2 THz can be achieved (Fig. 2). The simulations assume a simple one-band effective potential for which SchrĂśdinger equation with open boundary conditions is solved. The parameters correspond to n-GBT with Er2Ge3 emitter, Ge EBI, and a graded TiSiO/SiO2 BCI. Graphene is considered to be a tunneling barrier, as indicated by the results of ab initio calculation of self-consistent tunneling spectra in equilibrium. The quantum capacitance of graphene, CQ, is an important factor. Its role depends also on the tunneling properties of graphene, which at the moment can only be estimated. Assuming that the tunneling barrier is defined by the graphene EDQG VWUXFWXUH DW ÄŤ ZH H[SHFW that THz operation is possible when the EBI thickness is in the range of 3-5 nm and the EBI tunneling barrier height is about 0.2-0.4 eV.
References [1] W. Mehr et al., arXiv:1112.4520v1 [2] W. Mehr et al., accepted for publication in IEEE Electron Device Letters, (2012)
Figure 1
GBT
GFET
Emitter
Gate Base
EBI
Base
Source
Gate dielectric
Drain
BCI Collector
Silicon
Silicon Graphene
Schematic cross-section of a graphene base transistor (GBT) and a graphene field-effect transistor (GFET).
Figure 2
Simulated transition (cut-off) frequency fT of a high power GBT. Solid line: no quantum capacitance effects are accounted for (CQ Â&#x2019;). Broken line: with quantum capacitance effects (CQ = Č&#x203A;VQ). V = VEB - VQ defines the EBI electric field. VQ is the voltage drop on CQ and corresponds to the position of the Fermi level in graphene, measured with respect to the Dirac point, and is determined by the applied voltages and the EBI and BCI plate capacitances. For this particular design, VQ | 0.3 V at V | 1.3 V. Above 1.2 eV, quantum oscillations in fT begin.
Atomistic control of the properties of nanographenes and design of devices: insights from simulations Francesco Mercuri, Matteo Baldoni, Daniele Selli, Antonio Sgamellotti CNR-ISMN, via P. Gobetti 101, 40129 Bologna, Italy francesco.mercuri@cnr.it The electronic properties of graphene can be fine-tuned through a careful control of the morphology at the nanoscale or by chemical functionalization, thus opening exciting perspectives for the development of novel devices for nanoelectronics. However, a comprehensive understanding of the key factors governing the relationships between morphology of nanographenes at the microscopic scale and potential functionalities is still in early stage of development.[1]
In this work, we explore the correlation between the atomistic structure and the properties of systems based on graphene by extending concepts borrowed from traditional organic chemistry corroborated by accurate theoretical calculations using density functional theory. Namely, we perform calculations on realistic models to rationalize the structural, electronic and transport properties of nanographenes and related systems.[2] The approach proposed allows to elucidate recent experimental observations on the effect of morphological details of graphene nanoflakes and nanoribbons on their properties beyond idealized models and provides viable synthetic routes for the design and synthesis of novel systems with tailored properties. Moreover, similar concepts are also applied to the design of nanostructured devices, based on functionalized nanographenes, able to perform basic operations through the application of external inputs.[3] In particular, we demonstrate the potential of our approach in the design of a reversible switch, based on functionalized graphene nanoribbons, exhibiting unprecedented ON/OFF ratios upon application of an external redox potential. The study presented points out the need for a precise control of the atomistic structure of systems based on graphene and, at the same time, suggests efficient routes for the design and optimization of novel materials, with controlled morphology and properties, which can be used in technological applications.
References [1] M. Baldoni, A. Sgamellotti, F. Mercuri, Chem. Phys. Lett, 464 (2008) 202. [2] F. Mercuri, M. Baldoni, A. Sgamellotti, Nanoscale, 4 (2012) 369. [3] D. Selli, M. Baldoni, A. Sgamellotti, F. Mercuri, Nanoscale, 4 (2012) 1350. Figures
Reduced (left) and oxidized (right) forms of a reversible switch based on a functionalized graphene nanoribbon and corresponding I(V) characteristics (bottom) showing the ON/OFF ratio.
Structure and interface of graphene films grown on SiC using propane-hydrogen-argon CVD 1
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A. Michon , S. Vézian , D. Lefebvre , A. Tiberj , J.-R. Huntzinger , J. Camassel , F. Cheynis , 3 3 4 4 5 5 1 F. Leroy , P. Müller , L. Largeau , O. Mauguin , T. Chassagne , M. Zielinski and M. Portail 1
2
CRHEA-CNRS, Valbonne, France L2C-CNRS/Université Montpellier II, France 3 CINaM-CNRS, Marseille, France 4 LPN-CNRS, Marcoussis, France 5 NOVASiC, Le Bourget du Lac, France am@crhea.cnrs.fr
Graphitization of annealed SiC, first studied in the mid 70's, has been widely explored last years as a way of producing graphene films on semi-insulating substrates for both fundamental studies and graphene-based applications. In this method, graphene grows from carbon atoms of the SiC substrate. More recently, graphene growth from external sources of carbon has been demonstrated under ultrahigh vacuum conditions [1,2] or argon atmosphere CVD conditions [3,4]. For all these methods, graphene/SiC interface and graphene structural properties depend mainly on the SiC surface orientation: on the Si-face, graphene/SiC interface is (6 3 6 3)-R30° reconstructed and graphene layers are bernally stacked, while on the C-face, no graphene/SiC interface reconstruction is observed and graphene layers are disoriented with respect to others (rotational disorder). In previous works, we have shown that propane-hydrogen CVD (i.e. using propane as the carbon precursor and hydrogen as the carrier gas) is an exception, as this method allows growing graphene films presenting either Bernal or disoriented stacking on the Si-face [5,6]. In this contribution, this original behavior is studied through samples grown using different hydrogen/argon mixtures for the carrier gas. We also discuss and compare our results with what can be obtained using SiC annealing to underline differences between propane-hydrogen-argon CVD and annealing methods. Samples are grown on 6H-SiC(0001) at 1450°C and 800 mbar with a propane flow of 5 sccm for 5 minutes. We use for the carrier gas different argon/hydrogen mixture with hydrogen ratio R H2 (RH2 = FH2 / (FH2 + FAr)) ranging from 38% to 100% (7 samples). Figure 1 presents low energy electron diffraction (LEED) patterns and atomic force microscopy (AFM) images of samples grown with RH2 = 38%, 53%, and 100%. For RH2 = 38%, AFM image shows SiC atomic steps (0.75 nm) or sometime, SiC
graphene atomic steps (0.75
0.35 nm), while LEED pattern presents graphene and
(6 3 6 3)-R30° interface reconstruction spots. When increasing the hydrogen ratio (RH2 = 53%), 6 3 spots become more shadow, while new graphene orientations (0°, 30 2°) and a ring appear. The appearance of rotational disorder on the LEED pattern is correlated to the appearance of few isolated wrinkles on the AFM view. Finally, for RH2 = 100% (pure hydrogen), the (6 3 6 3)-R30° interface vanishes totally and graphitic ring appears clearly on the LEED pattern. On the AFM view, wrinkles join to form a network on the surface. In our contribution, we will analyze this set of samples using X-ray photoemission spectroscopy (XPS), LEED, AFM, and Raman spectroscopy. LEED and AFM study will allow to discuss on the links between graphene/SiC interface and wrinkle formation, and Raman spectroscopy will be used to study the influence on graphene/SiC interface on the strain. Finally, we will compare our results to what can be observed using the annealing method on the Si or C-face in order to evidence the numerous differences between the samples grown using both techniques.
References [1] E. Moreau et al., Phys. Stat. Sol. a 207 (2009) 300-303. [2] A. Al-Temimy et al., Appl. Phys. Lett. 95 (2009) 231907. [3] J. Hwang et al., J. Crystal Growth 312 (2010) 3219-3224. [4] M. Portail et al., Graphene 2011 (Bilbao), poster communication. [5] A. Michon et al., Appl. Phys. Lett. 97 (2010) 171909. [6] A. Michon et al., Phys. Stat. Sol. c 9 (2012) 175.
Figure 1
AFM images and LEED patterns from samples grown by propane-hydrogen-argon CVD on 6H-SiC(0001) with different hydrogen/argon mixtures used as the carrier gas.
Surface Modification of Graphene Thorough Controlled Radical and Conventional Free Radical Polymerization 1,2
1
Somayeh Mohamadi , Naser Sharafi-Sanjani , and Holger SchĂśnherr
2
1- Polymer Group, School of Chemistry, University College of Science, University of Tehran, Tehran, Iran 2-Physical Chemistry I, Department of Chemistry and Biology, University of Siegen, Adolf-ReichweinStraĂ&#x;e 2, 57076 Siegen, Germany smohamadi.83 @gmail.com
Graphene and graphite have recently attracted considerable attention as versatile, environmentally friendly and readily available carbon materials. Pristine graphene comprises individual one-atom thick 2
two-dimensional layers of sp - bonded honeycomb carbon material, which is prepared by exfoliation of crystalline graphite. When crystalline graphite is exfoliated into individual graphene sheets, the specific 2
-1
surface can be as large as 2600 m g
so that extraordinary electronic, thermal and mechanical
properties are achieved [1]. Graphene sheets with a sufficiently large aspect ratio can be considered as a good candidate in the polymer nanocomposite field. Here the uniform dispersion of the graphene layers in the polymer matrices is crucial and (re)agglomeration must be avoided. Functionalization of these inorganic nanoparticles via organic oligomeric and polymeric chains represents a favorable way to promote the compatibility with polymeric media. In particular, surface initiated polymerization (SIP) from covalently immobilized initiators has been shown to be a powerful method to achieve a high degree of control over the functionality, density, and thickness of grafted materials. The surface functional groups typically introduced in the oxidative treatment, such as hydroxyl and carboxyl groups, afford a versatile handle in this context to further modify graphene [2-4]. In this study, the graphene surface was modified with poly(methyl methacrylate) (PMMA) chains through SIP with two different methods. In the first procedure, graphene was functionalized with vinyl groups by reaction with methacrylic anhydride, followed subsequently by in-situ polymerization with MMA monomers through free radical polymerization. In the second approach, bromoisobutyrate moieties were coupled as initiators to the surface of oxidized graphene and were exploited in Atom Transfer Radical Polymerization (ATRP). The functionalized graphene particles were incorporated into poly(vinylidene flouride)(PVDF) matrix by means of solution mixing. The characterization of the resulting polymer modified graphene and graphene-filled PVDF will be discussed in this contribution.
References [1]. S. Stankovich, D. A. Dikin, , R. D. Piner, K. A. Kohlhaas, A. Kleinhammes, Y. Jia, , Y. Wu, S. T. Nguyen, , R. S. Ruoff, Carbon ,45 (2007) 1558-1565. [2]. S. H. Lee, D. R. Dreyer, J. An, A. Velamakanni, R. D. Piner, S. Park, Y. Zhu, S. O. Kim, C. W. Bielawski, R. S. Ruoff, Macromol. Rapid Commun. 31 (2010) 281±288. [3]. G. Goncalves, P. A. A. P. Marques, A. Barros-Timmons, I. Bdkin, M. K. Singh, N. Emami, J.Gracio, J. Mater. Chem., 20 (2010) 9927±9934. [4]. M. Steenackers, A. M. Gigler, N. Zhang, F. Deubel, M. Seifert, L. H. Hess, C. H. Y. X. Lim, K. P. Loh, J. A. Garrido, R. Jordan, M. Stutzmann, I. D. Sharp, Ame. Chem. Soc. 133(2011)10490±10498.
Figures
Figure 1. Schematic illustration of graphene surface modification through (A) controlled radical and (B) conventional free radical polymerization yielding PMMA grafted from the surface.
Removal of oxidation debris from graphene oxide: influence on the formation of composites based on silver nanoparticles Ana C. M. Moraes, Andréia F. de Faria, Diego Stéfani T. Martinez, Amauri J. Paula, Oswaldo L. Alves Solid State Chemistry Laboratory (LQES), Chemistry Institute, State University of Campinas (UNICAMP), P.O. Box 6154, 13083-970, Campinas, São Paulo, Brazil ana.moraes@iqm.unicamp.br
Graphene oxide (GO) is derived from the exfoliation of graphite oxide and its structure contains reactive oxygen functional groups e.g. hydroxyl, epoxide, carboxyl, carbonyls, yielding stable dispersions that consist mostly of monolayers or few-layered stacks. GO is highly hydrophilic, facilitating the intercalation with inorganic and organic compounds. Therefore, the chemically reactive oxygen functionality, renders GO a more attractive material for a wide range of applications [1]. GO silver nanoparticle decoration has opened applications of this hybrid material in sensors, polymeric composites and new antimicrobial agents [2]. The oxidation of carbon nanomaterials is a common method employed for water dispersion, functionalization and purification. This approach can introduce oxygen groups functionalities on carbon nanomaterial surface, but in parallel, carbonaceous byproducts can be generated after the oxidation process, named oxidation debris – polycyclic aromatic sheets with oxidized edges also termed fulvic acids [3,4]. Meanwhile, the presence of oxidation debris in GO has been recently reported by Rourke et al. [5]. It has been proposed that oxidation debris is acting as a surfactant to stabilizing aqueous GO suspensions. However, it is lacking the literature information about the influence of oxidation debris absorbed on GO surface and their scientific and technological implications. In this endeavour, we have focused on the debris removal from GO and its effect on the colloidal stability of GO dispersions. We have also investigated the influence of debris removal on the formation of graphene oxide decorated with silver nanoparticles. Oxidation debris removal was performed by heating of GO suspension in 1 M NaOH at reflux, resulting in GO without debris (GOwD) and a supernatant liquid, oxidative debris. Graphene oxide silver nanocomposites were synthesized in situ with two different GO samples, giving: GO-Ag (composite in the presence of debris) and GOwD-Ag (composite without debris). Colloidal stability assays were taken by measuring the light absorbance of graphene oxide suspensions in deionized water after centrifuging step. For this, aliquots of the supernatant were drawn from the suspension. Figure 1 (a) shows colloidal stability assays. Both samples were submitted under centrifugation in various rotations per minute. It was found that the behavior of GO suspension is much more stable, moreover it suggests that debris acted as a stabilizer in the GO sheets suspensions, giving more stability in aqueous medium. Although, it is worth to say that GO dispersion is not totally dependent on the presence of debris. The removal of oxidative debris can affect the stability but not the ability of GO to form aqueous dispersions. Homogeneous colloidal dispersions of graphene oxide were produced in aqueous medium -1 (deionized water; 125 µg mL ). GO generated a light-brown suspension which was quite stable, no floating or precipitated particles were observed. The GOwD created a homogeneous black suspension which was quite stable also (Fig. 1b). The oxidation debris resulted in a yellow-brown solution dispersed in 1 M NaOH, as reported by Rourke et al. According to TEM analysis, the Fig. 2 (a) and (b), in both cases, there was the formation of graphene based on silver nanoparticles. From the size distribution histograms of silver nanoparticles, the sizes of the nanoparticles on the GO without debris and GO in the presence of debris are widely distributed from ı 2 to over 40 nm and ı 4 to over 90 nm, respectively. The Gauss fits indicate that the maximum distributions of silver nanoparticles sizes are 5 and 18 nm, respectively. These results demonstrate that the silver nanoparticles distribution is dependent on the presence of debris on the GO surface. Finally, the oxidation debris removal led to an increase in the average size of silver nanoparticles formed during GO decoration. References [1] D. R. Dreyer, S. Park, C. W. Bielawski, R.S. Ruoff, Chem. Soc. Rev., 39 (2010) 228-240.
[2] R. Pasricha, S. Gupta, A. K. Srivastava, Small, 20 (2009), 2253-2259. [3] D. StĂŠfani, A. J. Paula, B. G. Vaz, R. A. Silva, N. F. Andrade, G. Z. Justo, C. V. Ferreira, A. G. Souza Filho, M. N. Eberlin, O. L. Alves, J Hazard. Mater., 189 (2011) 391-396. [4] R. Verdejo, S. Lamoriniere, B. Cottam, A. Bismarck, M. Shaffer, Chem. Commun. (2007) 513-515. [5] J. P. Rourke, P. A. Pandley, J. J. Moore, M. Bates, I. A. Kinloch, R. J. Young, N. R. Wilson, 50 (2011) 3173-3177. Figures Figure 1
(b)
(a)
Figure 2
(a)
(b)
Figure caption Figure 1. (a) Colloidal stability assays of samples GO and GOwD in deionized water. (b) Picture of the GO and -1 GOwD suspensions (125 Âľg mL ), respectively. Figure 2. TEM micrographs (bright field mode) of samples (a) GO-Ag (composite in the presence of debris) and (b) GOwD-Ag (composite without debris). Respective histograms are inserted in each image. The histograms of Ag nanoparticles size distributions were obtained by counting more than 1000 nanoparticles for each example.
Optical biosensors based on graphene 1,2
1,3
Eden Morales-Narváez , Briza Pérez-López , Arben Merkoçi* 1
,1,4
Nanobioelectronics & Biosensors Group, Catalan Institute of Nanotechnology, CIN2 (ICN-CSIC), Barcelona, Spain. 2 Polytechnic University of Catalonia, ESAII department, Barcelona, Spain, 3 LEITAT Technological Center, Barcelona, Spain. 4 ICREA, Barcelona, Spain. *arben.merkoci@icn.cat
Since graphene bears innovative mechanical, electrical, thermal and optical properties, this twodimensional material is under active research [1±7]. In this regard, graphene displays advantageous characteristics to be used in biosensing platforms owing to the excellent capabilities for direct wiring with biomolecules, heterogeneous chemical and electronic structure, the possibility to be processed in solution and the availability to be tuned as insulator, semiconductor or semi-metal [3,6,8,9]. Moreover, after oxidation treatments, graphene can exhibit an interesting photoluminescence property in relation to resonance energy transfer donor/acceptor molecules exposed in a high planar surface and even can be proposed as a highly efficient quencher, which is opening the way to new biosensing strategies. We will discuss some exploitable properties of graphene in optical biosensig and our experimental results of the excellent capabilities of oxidized graphene as fluorescence quencher in order to be employed in biosensing applications. Graphene based optical biosensing platforms are versatile in configurations in addition of being highly sensitive, robust enough beside offering interesting multi-detection capacity in association to other nanomaterials (i.e. quantum dots). The preliminary results obtained so far seems to be with interest for future applications such as diagnostics (biomarkes detection) or safety and security applications (i.e. bacteria).
References
[1] [2] [3] [4] [5] [6] [7] [8] [9]
S. Park, R. S. Ruoff, Nature nanotechnology 2009, 4, 217-24. O. C. Compton, S. T. Nguyen, Small 2010, 6, 711-23. K. P. Loh, Q. Bao, G. Eda, M. Chhowalla, Nature chemistry 2010, 2, 1015-24. Y. Zhu, S. Murali, W. Cai, X. Li, J. W. Suk, J. R. Potts, R. S. Ruoff, Advanced materials 2010, 22, 3906-24. K. S. Novoselov, Angewandte Chemie (International ed. in English) 2011, 6986 - 7002. Y. Wang, Z. Li, J. Wang, J. Li, Y. Lin, Trends in biotechnology 2011, 29, 205-12. F. Schedin, a K. Geim, S. V. Morozov, E. W. Hill, P. Blake, M. I. Katsnelson, K. S. Novoselov, Nature materials 2007, 6, 652-5. M. Pumera, Materials Today 2011, 14, 308-315. A. Bagri, C. Mattevi, M. Acik, Y. J. Chabal, M. Chhowalla, V. B. Shenoy, Nature chemistry 2010, 2, 581-7.
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Development and Study of manufacturing method of few layers graphene dispersed solution for wet coating 1
1
Kazuo Muramatsu , Kouichi Sutani , Masahiro Toyoda
2
1
Incubation Alliance, Inc., 1-2-25-D307 Wdayamadori, Hyogo-ku, Kobe Hyogo, 652-0084, Japan Applied Chemistry, Faculty of Engineering, Oita University, 700 Dannoharu, Oita, 870-1192 Japan muramatsu@incu-alliance.co.jp, toyoda22@oita-u.ac.jp
2
In the portable terminals such as the smart phone, transparent conductive film is used as a component of touch panel. The rigid and transparent conductive film is used abundantly by sputtering ITO (Indium Thin Oxide) on PET film at present. However, its conductive film may be destroyed through flexible using. Therefore, research and development of transparent conductive material with flexible mechanical characteristic substitute the ITO was actively carried out. It has the excellent potential with material substitute the ITO because the graphene has the flexible mechanical characteristic with optical transparency and electrical conduction property. In recently, a large number of synthesis of graphene have been reported; method for copying the graphene synthesized in thermal CVD on the surface of the copper foil to the PET film [1,2] and method for reducing of substrate coated graphite oxide (graphene oxide) by using solution dispersed it [3,4]. It is possible that the graphene showing comparatively few layers selectively grows by the thermal CVD on the surface of the copper foil [5]. However, following problems have been indicated, 1) the graphene of scale leaf which depends on the polycrystal texture of the copper foil is formed, 2) The graphene is selectively and heterogeneously formed in the specific crystal orientation ( Cu 111 plane ) in the copper foil surface, 3) it is necessary to fix the graphene of the scale leaf in acrylic resin. In addition, there is a problem of dissolving the copper foil by the acid solution in order to separate the graphene from the o copper foil. And, high temperature processing at about 1000 C under inert atmosphere is necessary in the method for coating the graphite oxide (graphene oxide) to obtain the graphene with the high crystallinity. Therefore, application to the PET film is difficult, when its technique was applied. In this study and development, following items were examined to resolve those problems; 1) It is large production of the graphene showing few layers without catalyzing metal substrates such as the copper foil. 2) the graphene make dispersion without agglutination in the organic solvent. 3) the graphene is coated on PET film by using the wet coating process. CVD reaction to obtain the graphene was prepared though its calcinations of raw material derived from polyester resin heat-treated moderately which adjusted remaining hydrogen content under the high-pressure and isotropic gas pressure [6]. H2 and CH4 generated from raw material calcined have formed the graphene without catalyst. It was possible to form the few layers graphene (graphene flower) showing flower state on circumference of raw material calcined by using gas such as H2 and CH4 which was generated from raw material calcined. Fig. 1 (a) and (b) show interesting morphology of its graphene flower. Fig. 2 shows TEM observation of graphene flower. The heat-treatment time necessary for obtaining 500g graphene flower lump by small-scale experimental equipment are about 4 hours. Obtained it were atomized by using the rotary mixer in 2-propanol solution. Afterwards 0.8 mg/ml solution was obtained by sonication and following centrifugal separation treatment. Similar solutions dispersed graphene flower were also obtained in 2-methoxyethanol, PGMA (Propylene glycol monomethyl ether acetat) and NMP (N-methylpyrrolidone) solvent selected through Hansen solubility parameter test. 2-methoxyethanol and 2-propanol as organic solvent with lower boiling point and high volatility were selected, and then spray and dip coating by using its solution were carried out on the copper foil or PET film having preliminary heating to obtain the graphene through transferring to the substrate. Fig. 3 shows few layers graphene on PET film. From obtaining results, since the agglomeration for graphene is so large, evaporation rate of its solvent on the substrate is slow. In the case in which its dispersed solution was oversupplied, its configuration is changed in the roll, when the graphene independently exists. In the meanwhile, it agglutinates in the graphite stacking, when it exists in multiplicity. We found that the graphene layer is independently formed on the substrate by adjusting heat-treatment temperature of the substrate, amount of the dispersed solution and supply rate of the dispersed solution. In addition, we will be discussed and examined mass-productiveness of transparent conductive film by using roll-to roll method through wet coating process with optimization of the substrate heating system and accurate control of supply of dispersed solution.
References [1] S. Bae et al., Nature Nanotech. 5, (2010)574. [2] Y. Lee et al., Nano Lett. 10, (2010)490-493. [3] K. Ueno, J. Vac. Soc. Jpn. 53, (2010)73. [4] S. Park and R. Ruoff, Nature Nanotech, 4, (2009)217. [5] B. H. Hong, SKKU Advanced Institute of Nanotech., Korea, Graphene 2011. [6] K. Muramatsu and M. Toyoda, US PAT 13/321,944. Figures (a)
(b)
Fig. 1 Morphology of graphene flower(a) and extended one(b)
Fig. 2 TEM observation of graphene flower and SAED pattern
Fig. 4 Few layers graphene on PET film prepared through wet processing
Facile synthesis of high quality metal free reduced graphene nanosheets from expandable graphite oxide Nanjundan Ashok Kumar /LRQHO 'XERLV 6HUJH *DPEDUHOOL )ORUHQFH 'XFODLURLU DQG *pUDUG %LGDQĂ Laboratoire de Chimie Inorganique et Biologique, UMR-E CEA-UJF, Institute for Nanoscience and Cryogenics, Commissariat Ă l'ĂŠnergie atomique (CEA), 17 rue des martyrs, 38054, Grenoble, Cedex 09 FRANCE. Ă ,1$& ',5 &($-Grenoble E-mail: lionel.dubois@cea.fr A cost-effective, one-pot and environmentally benign process for reducing graphene oxide at room temperature is reported. High quality metal free and spectroscopically pure graphene nanosheets were obtained via metal induced reduction of expandable graphene oxide. Simultaneous exfoliation and reduction is achieved wherein, the reduction process is complete in a facile process. Magnetic measurements and EPR studies show that the prepared graphene is spectroscopically pure and no paramagnetic metallic impurities are detected. Due to its high purity, this graphene is particularly well suited to study the effect of magnetic or electronic doping and as a support for catalyst. References [1]. C. N. R. Rao, A. K. Sood, K. S. Subrahmanyam and A. Govindaraj, Angewandte Chemie International Edition, 2009, 48, 7752-7777. [2]. C. Soldano, A. Mahmood and E. Dujardin, Carbon, 2010, 48, 2127-2150. [3]. J. Wu, W. Pisula and K. Mullen, Chemical Reviews, 2007, 107, 718-747. [4]. Y. Sun, Q. Wu and G. Shi, Energy & Environmental Science, 2011, 4, 1113. [5]. Y. Wang, Z. Li, J. Wang, J. Li and Y. Lin, Trends in Biotechnology, 2011, 29, 205-212. [6]. D. A. C. Brownson, D. K. Kampouris and C. E. Banks, Journal of Power Sources, 2011, 196, 4873-4885. [7]. Z. Xu, H. Li, W. Li, G. Cao, Q. Zhang, K. Li, Q. Fu and J. Wang, Chemical Communications, 2011, 47, 1166-1168. [8]. S. Guo and S. Dong, Chemical Society Reviews, 2011, 40, 2644-2672. Figures
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Fluorographene with nanoswell surface relief obtained by hydrofluoric acid treatment N.A. Nebogatikova, I.V. Antonova, V.A. Volodin, V.Ya Prinz A.V. 5]DQRY ,QVWLWXWH RI 6HPLFRQGXFWRU 3K\VLFV /DYUHQWÂśHY Ave. 13, Novosibirsk, Russia antonova@isp.nsc.ru Fluorographene (FG) is one of the main derivatives of graphene attracting huge interest for last few years. It is a stoichiometric derivative of graphene with fluorine atom attached to each carbon atom [1]. o 12 FG is stable material (up to temperature 400 C) and a high-quality insulator (resistivity > 10 Ohm) with optical bandgap 3 eV [1, 2]. In the present report, we demonstrate a new, very simple approach for creation of FG. This process uses treatment of graphene or few-layer graphene in aqueous solution of hydrofluoric acid yielding fluorogrphene or few-layer FG with nanoswell relief on the surface. Fluorination of graphene or few-layer graphene (with thickness below 10 nm) by means of HF treatment resulted in dramatic changes of structural and electrical properties of the material. The interaction of graphene with HF observed as a function of HF treatment duration exhibited a two-stage behavior. At the first stage, Raman spectra of samples treated in HF:H2O solution demonstrated an increase in D-1 -1 peak intensity (1350 cm ) and a decrease in the intensity of G and 2D peaks (respectively ~1580 cm -1 and ~2700 cm ). A network relief with height 3-4 nm was revealed on the surface of such samples by atomic force microscopy (AFM). Grain boundary mapping is assumed to be responsible for the formation of this network. The resistivity of the graphene structures remained roughly unchanged at this stage of HF treatment. A surprising finding was that in HF-treated structures the current value in the Ids(Vg) characteristics measured in transistor configuration using the substrate as the gate electrode varied over 4Âą5 orders of magnitude with variation of gate voltage Vg. On the contrary, reference (not treated in HF:H2O) few-layer graphene samples demonstrated variation of drain-source current Ids within 30%. This effect was found to be due to an increase of carrier mobility. Thus, our films shortly treated in HF:H2O demonstrated a high potential in management with their conductivity. The second stage of the graphene-HF interaction proceeded over HF treatment times of 30 s for graphene proper and over times of 5 min for structures with thickness 3-5 nm. At this stage, graphenerelated peaks disappeared from Raman spectra. A step-OLNH LQFUHDVH LQ UHVLVWLYLW\ XS WR *Č? ZDV observed in such structures. This modification of few-layer graphene properties suggests the formation of FG during treatment of graphene in HF:H2O solution. A periodic nanoswell relief (step ~ 50 Âą 100 nm and height ~ 2Âą6 nm depending on treatment duration and sample thickness) was observed by AFM on the surface of the samples. Thermal stability of our FG samples was tested by giving them an additional o anneal. It is known that hydrogen desorption from graphene occurs at temperatures ~200 Âą 290 C [3]. o On the other hand, C-F bonds are stable up to ~400 C [1, 2]. We have performed an annealing of one o of the FG samples at temperature 300 C for 1 h. The properties of the annealed structure showed no changes after annealing. Repeated measurements performed over a one-year period proved time stability of created material. Fluorine ions present in HF/water solution were found to be necessary for the fluorination reactions. For comparison, several graphene samples were given treatment in HF vapor. The duration of the latter treatment was varied from 1 min to 17 hours. It was found that the resistivity value remained unchanged over 17 h of HF vapor treatment. We would like to mention here that such samples showed no changes in Raman spectra as well. The fluorination process could be made controllable using a preliminary treatment of graphene in isopropyl alcohol. The latter treatment was found to suppress fluorination of graphene in HF/water solution. Combination of the two treatments gives one a key for nanodesign of graphene-based devices. References [1] S.-H. Cheng, K. Zou, F. Okino, H.R. Gutierrez, A. Gupta, N. Shen, P.C. Eklund, J.O. Sofo, J. Zhu, J. Phys. Rev. B., 81 (2010) 205435. [2] R.R. Nair, W. Ren, R. Jalil, I. Riaz, V.G. Kravets, L. Britnell, P. Blake, F. Schedin, A.S. Mayorov, S. Yuan, M.I. Katsnelson, H.-M. Cheng, W. Strupinski, L.G. Bulusheva, A.V. Okotrub, I.V. Grigorieva, A.N. Grigorenko, K.S. Novoselov, A.K. Geim,, Small., 6 (2010) 2877. [3] S. Ryu, M.Y. Han, J. Maultzsch, T.F. Heinz, P. Kim, M.L. Steigerwald, L.E. Brus, Nano Letters, 8 (2008) 4597.
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b Fig. 1. (a) AFM images of the surface of samples treated in 5 % solution of HF in water during different times, 4 (a) and 9 min (b). (c) Resistivity of various few-layer graphene flakes versus the HF-treatment duration t. The few-layer graphene thickness is given in the figure sheet as a parameter. The curve labeled IPA+HF in (c) refers to the sample that was preliminarily (before HF-treatment) treated for 20 min in isopropyl alcohol (IPA).
Effect of grain boundary on the buckling of graphene nanoribbons Mehdi Neek-Amal, F. M. Peeters Mehdi.neekamal@gmail.com The buckling of graphene nano-ribbons containing a grain boundary are studied using atomistic simulations where free and supported boundary conditions are invoked. \textbf{We consider
the
buckling transition of two kinds of grain boundaries with special symmetry. When graphene contains a large angle grain boundary with $\theta=$21.8$^o$ the buckling strains are larger than those of perfect graphene when the ribbons with free (supported) boundary condition are subjected to compressive tension parallel (perpendicular) to the grain boundary. This is opposite for the results of $\theta=$32.2$^o$}. The shape of the deformations of the buckled graphene nanoribbons depends on the boundary conditions\textbf{, the presence of the particular used grain boundaries} and the direction of applied in-plane compressive tension. References: A. Hashimoto, K. Suenaga, A. Gloter, K. Urita, and S. Iijima, Nature (London) 430, 870~(2004). T. R. Albrecht, H. A. Mizes, J. Nogami, S.-i. Park, and C. F. Quate, Appl. Phys. Lett. 52, 362 (1988). X. Li, W. Cai, J. An, S. Kim, J. Nah, D. Yang, R. Piner, A. Velamakanni, I. Jung, E. Tutuc, S. K. Banerjee, L. Colombo, and R. S. Ruoff, Science 324, 1312~(2009). O. V. Yazyev and S. G. Louie, Phys. Rev. B 81, 195420 (2010). P. Y. Huang \emph{et al.} Nature 469, 389 (2011). O. V. Yazyev and S. G. Louie, Nature Materials, 9, 806 (2010). M. Neek-Amal and F. Peeters, Phys. Rev. B 82, 085432~(2010). M. Neek-Amal and F. M. Peeters. J. Phys.: Condens. Matter 23, 045002 (2011).
Low Temperature Graphene Growth Using Large Area Linear-Antenna Microwave Plasma Enhanced CVD System Miloš Nesládek1*, Andrew Taylor2, František Fendrych2, Otakar Frank3, Martin Kalbáč3 and Ladislav Kavan3 milos.nesladek@uhasselt.bel Graphene growth for large-area electronic and optical applications has recently attracted much interest. Several growth strategies including chemical exfoliation, thermal CVD, plasma CVD and others have been used. Recently, it has been suggested that graphene layers can be prepared by low pressure MW systems, such as slit-antenna delivery [1], at lower temperatures. However, the first results suggested the presence of amorphous carbon in the atomic monolayers. In our work we described using a novel concept of pulsed low-pressure linear antenna MW plasma delivery system, previously described in [2]. Advantage of using the pulsed mode and compared to CW [1] is a precise control about the substrate temperatures but also different plasma chemistry, allowing using pure CH4, CH4:H2 in addition to CH4:Ar:H2 gas mixtures for the growth of graphene on Cu foils at Ts 400 - 700°C. Graphemes layers are subsequently transferred to glass substrates. We use Raman spectroscopy to study transition from amorphous layers to graphene by optimising the growth conditions such as plasma parameters, gas composition and substrate temperature. For comparison we use thermal CVD to prepared graphene monolayers [3]. Raman spectroscopy using various laser excitations shows sharp graphene signatures at 1600 cm-1 and 2660 cm1, pointing to graphene layers, monolayers and bilayers. However the large D-peak intensity points towards the presence of disordered carbon at the boundary of domains, as demonstrated by TEM. The layers grown are highly transparent (i.e. 2.5 – 5%) but resistivity of the layers is higher than for the layers prepared by thermal CVD. Electrical transport characteristics of these layers are discussed.
Coaxial MW power divider (1:4)
“plasma line”
Substrate, up to 30cm in width
Rectangular waveguide
MW power generator
Figure 1: Schematic detail of LA- pulsed MW plasma installation
Rectangular waveguide
MW power generator
6000
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Figure 2: Graphene film lifted from Cu foil and transferred to glass substrate and its Raman spectra for different growth conditions. References [1] Kim J, Ishihara M, Koga Y, et al. App. Phys. Lett. 98, 091502, 2011 [2] Taylor A, Fendrych F, Fekete L, et al., Diam. Related Materials 20, p 613-615, 2011 [3 ] Kalbac M, Farhat H, Kong J, et al., NanoLetters, 11 , p 1957-1963, 2011
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Surface enhanced Raman scattering of graphene Zhenhua Ni,1 Yingying Wang,2 Zexiang Shen3 1 Department of Physics, Southeast University, Nanjing, China 211189 2 Department of Optoelectronic Science, Harbin Institute of Technology at Weihai, China 264209 3 Division of Physics and Applied Physics, School of Physical and Mathematical Sciences, Nanyang Technological University, 21 Nanyang Link, Singapore 637371, Singapore zhni@seu.edu.cn Graphene, the one monolayer thick flat graphite, has been attracting much interest since it was firstly reported in 2004. Graphene has many unique properties which make it an attractive material for fundamental study as well as for potential applications. Raman spectroscopy has been extensively used to study graphene, i.e. identify graphene layer numbers; probe electronic band structure; determine type of edges (zigzag or armchair); measure the concentration of electron and hole dopants. In addition, using graphene as a substrate for probing vibrational information from target molecules has been paid more attention recently. Here, we present our results on the study of using graphene as a surface enhanced Raman scattering (SERS) substrate.
Firstly, gold (Au) films with different thicknesses were deposited on single layer graphene (SLG) and used as SERS substrates for the characterization of rhodamine (R6G) molecules. We find that an Au film with a thickness of ~7 nm deposited on SLG is an ideal substrate for SERS, giving the strongest Raman signals for the molecules and the weakest photoluminescence (PL) background. While Au films effectively enhance both the Raman and PL signals of molecules, SLG effectively quenches the PL signals from the Au film and molecules. The former is due to the electromagnetic mechanism involved while the latter is due to the strong resonance energy transfer from Au to SLG. Hence, the combination of Au films and SLG can be widely used in the characterization of low concentration molecules with relatively weak Raman signals.[1]
Following, we report a simple method to recover the SERS activity of aged graphene. We found that, for graphene samples fabricated and exposed in ambient for a period (several month or more), i.e., aged graphene, the SERS activities are vanished and no vibrational information of absorbed molecule can be detected. The SERS activity of aged graphene is dramatically increased after vacuum annealing and comparable to those on fresh graphene. Atomic force microscopy measurements indicate that residues on aged graphene surface can efficiently be removed by vacuum annealing, which makes target molecule closely contact with graphene. We also find that the hole doping in graphene will facilitate charge transfer between graphene and molecule. These results confirm the strong Raman enhancement of target molecule absorbed on graphene is due to the charge transfer mechanism.[2]
Finally, we compare the SERS activity of graphene, graphene oxide(GO), and reduced graphene oxide (rGO), to explore the impacts of local chemical groups and global Ę&#x152;-conjugation network on the SERS effect of graphene. [3]
References 1. 2. 3.
Wang YY, Ni ZH, Hu HL, Hao YF, Wong CP, Yu T, Thong JTL, Shen ZX Gold on graphene as a substrate for SERS study Applied Physics Letters 97,163111 (2010) Wang YY, Ni ZH*, Li AZ, Zafar Z, Zhang Y, Ni ZH, Qu SL, Qiu T, Yu T, Shen ZX Surface enhanced Raman scattering of aged graphene: effects of annealing in vacuum Applied Physics Letters 99,233103(2011) Yang HP, Hu HL, Cong CX, Ni ZH, Shen ZX, Yu T Comparison of SERS on Graphene, Graphene oxide and Reduced Graphene oxide surface (Unpublished).
Figures
Figure 1. Raman spectra of R6G on SiO2 substrate, R6G on Au/SLG substrate, as well as R6G on Au substrate.
Figure 2. (a) Raman spectra of R6G adsorbed on aged SLG and annealed SLG samples. The Raman spectra of R6G adsorbed on fresh SLG is also included. (b) 2D bands of fresh SLG, aged SLG, and aged SLG annealed at different temperature.
Energy loss rates of hot Dirac fermions in epitaxial, exfoliated and CVD graphene under high magnetic fields. R.J. Nicholas*, A.M.R. Baker*, J.A. Alexander-Webber*, T. Altebaeumer*, D. McMullan*, Cheng-Te Lin# and Lain-Jong Li# * Department of Physics, University of Oxford, Clarendon Laboratory, Parks Rd., Oxford, OX1 3PU, U.K. # Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei, 10617, Taiwan r.nicholas1@physics.ox.ac.uk We have experimentally investigated energy loss rates for hot carriers in graphene produced by the three most common fabrication methods: micro-mechanical exfoliation, epitaxial growth from silicon carbide, and chemical vapour deposition onto copper. We have determined the energy loss rates both by studying the amplitude of Shubnikov ¹ de Haas oscillations, and the amplitude of the weak ¹ localisation peak at B=0 as a function of applied electric field. The electric field heats the carriers above the lattice temperature as the carriers are unable to lose energy to the lattice at a sufficient rate to reach thermal equilibrium causing the oscillations, and weak localisation peak, to be damped. Energy loss UDWHV IURP VXFK ³KRW FDUULHUV´ KDYH EHHQ LQYHVWLJDWHG H[Serimentally through magnetotransport measurements in a number of semiconductor heterostructures [1,2] and recently by our group in exfoliated graphene [3]. Lately there has been much theoretical interest in studying how Dirac fermions in graphene transfer energy to the lattice [4,5]. A thorough investigation of carrier energy loss rates in high magnetic fields comparing the different types of graphene samples, with their dramatically different carrier densities, could also lead to a better understanding of the high-current breakdown of the quantum Hall effect in graphene, and is of significant practical interest for nano-electronic devices since the energy loss rate per carrier is a key metric for high-performance transistor devices [6].
11
We present data taken across a range of carrier densities from 1x10
13
to 2x10
cm
-2
using samples
from the different production methods. Figure 1 shows the energy loss rates as a function of carrier 4
temperature and demonstrates that at low temperatures the data is well fitted by ÄŽ7 dependence, ZKHUH ÄŽ LV D PXOWLSOLFDWLYH FRQVWDQW IRU DOO JUDSKHQH IDEULFDWLRQ PHWKRGV )XUWKHU ZH VKRZ WKDW WKLV dependence is well modeled by deformation potential theory. Finally in figure 2 ZH VKRZ WKDW ÄŽ LV negatively correlated with increasing carrier density with a
dependence
References [1] D R Leadley, R J Nicholas et al., Semicond. Sci. Technol., 4 (1989) 879-884. [2] G. Stöger et al., Phys. Rev. B, 49 (1994) 10417. [3] A.M.R. Baker, J. A. Alexander-Webber, R J Nicholas et al. Phys. Rev. B. Awaiting Publication [4] Wang-Kong Tse and S. Das Sarma, Phys. Rev. B, 79 (2009) 235406. [5] S. S. Kubakaddi, Phys. Rev. B, 79 (2009) 075417. [6] Ashley M. DaSilva et al., Phys. Rev. Lett., 104 (2010) 236601. Figures
SiC 1.00E11 SiC 4.13E11 Exf 1.39E12 CVD 1.43E13 Q=42 CVD 1.43E13 WL CVD 1.62E13 Q=38 CVD 1.62E13 WL
1E-11
1E-17
Epitaxial Exfoliated CVD
T Coefficient
1E-12
1E-13
4
Energy Loss Rate per Carrier
1E-10
1E-14
1E-18 1E-15
1
10
100
Temp (K)
Figure 1: Energy loss rates per carrier as a function of carrier temperature for graphene grown on SiC, epitaxially on copper and exfoliated from natural 4 JUDSKLWH $Q Į7 dependence is observed from all samples measured.
1E11
1E12
1E13
Carrier Density
Figure 2 7KH YDOXH RI WKH PXOWLSOLFDWLYH FRQVWDQW Į as a function of carrier density for samples across the three graphene fabrication methods.
Tunneling Negative Differential Resistance in Flexible Silicone/Graphite Composites
Alain Nogaret, Sam Littlejohn, Simon Crampin Department of Physics University of Bath, Bath BA2 7AY, UK
Email: A.R.Nogaret@bath.ac.uk
Developing flexible electronic materials with the ability to amplify signals is a major challenge for bioelectronics. The need for both intelligent sensor arrays that stretch like a skin and implantable 1 control electronics is driving the search for soft conducting materials with active electronic properties. 2 Stretchable interconnects and flexible matrices have been obtained that exhibit pressure-sensing , 3 4 temperature-sensing and electroluminescent properties . The next step in developing an active nervous system calls for a flexible electronic material that produces a signal gain. The ability to tune the electrical conductivity of the graphene bilayer with an electric field provides a route for addressing this challenge through the generation of NDR. Here we demonstrate a wide NDR region in the current-voltage characteristics of silicone filled with 5 graphitic nanoparticles . At the peak, the conductor breaks up into domains of constant electric field separated by highly resistive domain boundaries. These boundaries are identified as individual graphite nanoparticles whose orientation in the electric field favours conduction across just two graphene layers â&#x20AC;&#x201C; in such nanoparticles, the tilt angle between the graphite planes and the electric field is 66°-78°. Increasing the electric field opens a partial energy gap at the Fermi level which causes the current carrying bilayer to undergo a semimetal-to-insulator transition. The nucleation of highly resistive domain boundaries fragments the composite into electric field domains whose size we measure to be ~0.3mm. This switches off the percolation paths through the composite and gives the observed NDR. This picture explains the dependence of the I-V curves on the concentration of graphitic nanoparticles, temperature, channel length, as well as the disappearance of the NDR when the graphitic nanoparticles are replaced with amorphous carbon nanoparticles. We obtain very good agreement with the experimental I-V curves and their dependence on graphite filling fraction by calculating the tunnelling current between two graphite nanoparticles through the silicone potential barrier.
References [1] J.A. Rogers, T. Someya, Y, Huang, Science 327, 1603 (2010) [2] T. Someya, T. Sekitani, S. Iba, Y. Kato, H. Kawaguchi, T. Sakurai, PNAS 101, 9966 (2004) [3] T. Someya, Y. Kato, T.Sekitani, S. Iba, Y. Noguchi, Y. Murase, H. Kawaguchi, PNAS 102, 12321 (2005) [4] H. Stirringhaus, N. Tessler, R.H. Friend, Science 290, 1741 (1998) [5] S. Littlejohn, A. Nogaret, S. Crampin, Adv. Mat. 23, 2815 (2011)
Figure 1: Current-voltage characteristic of silicone rubber filled with nanoparticles of Highly Oriented Pyrolytic Graphite (HOPG) 450nm in diameter. Nanoparticles whose graphite planes are tilted by 66°-78° from the direction of the electric field conduct across an embedded graphene bilayer (A). Higher electric field breaks the π-band of graphite (B). This semimetal-to-insulator transition is responsible for the NDR.
Figure 2: (a) Dependence of the NDR on the HOPG graphite filling fraction; (b) percolation plot
σ ∝ ( p − pc ) 4
(c) calculated tunneling current and its dependence on filling fraction; (d) composite made of pyrolytic carbon nanoparticles; (f) Joule heating; (g) The NDR vanishes when it is measured across smaller lengths of composite. The onset of the transition gives the size of the electric field domains ~0.3mm.
Synthesis of nanocrystalline graphene on Al2O3(0001) by molecular beam epitaxy M. H. Oliveira Jr., T. Schumann, M. Ramsteiner, R. Hey, L. Geelhaar, J. M. J. Lopes, and H. Riechert Paul-Drude-Institut für Festkörperelektronik, Hausvogteiplatz 5-7, 10117 Berlin, Germany oliveira@pdi-berlin.de The implementation of graphene in future nanoelectronics will depend, among many factors, on the capability of producing this material in large-scale and with high quality directly on substrates that are compatible with standard semiconductor technology. Additionally, growing mono- or few-layer graphene on semiconducting/insulating substrates
is highly desirable since it
enables its
electronic
characterization and device implementation without the need of a transfer process. In this context, molecular beam epitaxy (MBE) is particularly attractive since it may allow a precise layer-by-layer growth of graphene on such substrates that may result in high-quality material.
In the present work, we investigate the growth of graphene by MBE on 2-inch Al2O3(0001) wafers. The c-plane sapphire was chosen due to its hexagonal symmetry, which may facilitate graphene epitaxy since its in-plane lattice constant of 4.75 Å is about twice the graphene one (2.45 Å). Besides, the high thermal stability of sapphire allows growth experiments at high substrate temperatures. The carbon films were grown in a MBE system equipped with a solid source of carbon, which consists of a highly ordered pyrolytic graphite (HOPG) filament operating typically at temperatures of about 2400 ºC. Quadrupole mass spectrometry (QMS) analysis reveals that the source emits mostly atomic C, although CO-related species are also observed. The power applied to the source was kept constant in order to obtain similar carbon fluxes. The structural and electronic properties of films prepared with different growth times (between 15 and 480 min) and temperatures (from 800 to 1000 ºC) were studied. The thickness of the films, which cover the whole wafer surface, varied from around 1 to 14 nm depending on the growth time, as determined by atomic force microscopy after a lithography process.
The structural properties of the samples were investigated by Raman spectroscopy. The spectra show the characteristic peaks of a graphitic/graphene structure (see Fig. 1), namely the disorder/defect induced mode at ~1360 cm
í
(D peak) [1], the E2g mode at ~1600 cm
í
(G peak) [2], and a peak
-1
induced by a double-resonance electron-phonon process at ~2700 cm (2D peak) [3]. We observe that the sharpness of the D, G, and 2D peaks increase with the growth temperature, for samples prepared with the same growth time [Fig. 1 (a) and (b)]. Such variation is directly connected to an improvement in WKH GHJUHH RI FU\VWDOOLQLW\ LQ WKH ¿OP
For films prepared with growth times between 15 and 120 min, the analysis of the Raman peaks parameters, such as the relation between the full width at half maximum (FWHM) of the G peak and the intensity ratio between the D and G peaks [I(D)/I(G) , reveals that graphene domains are formed in the carbon film, however with high degree of disorder. On the other hand, for longer growth times [e.g. 480 min, Fig 1(c)] nanocrystalline graphene films with higher quality could be grown, as evidenced by the sharper and much more intense 2D peak. In this case, the observed D peak is likely related to the borders of the graphene domains. It is also worth to notice that the single peak shape of the 2D line [Fig. 1(c)] shows a lack of graphite stacking order between adjacent layers [4]. Thus, despite its thickness (~14nm), the film may behave electronically as a monolayer due to an electronic decoupling between each layer, similarly to what is observed for graphene grown on the C-face of SiC [5]. Finally,
to evaluate the electrical transport TXDOLW\ RI WKH ÂżOPV ZH FRQGXFWHG magneto transport measurements 2
using a van der Pauw geometry. A carrier mobility of 120 cm /Vs at room temperature and a strong ptype doping (~1x10
13
-2
cm ) were measured IRU WKH ÂżOPs deposited at 1000 ÂşC for 480 min. Further
experiments aiming at the achievement of graphene layers with better structural quality (i.e. larger domains) and thus better transport properties are currently under progress. References [1] A. C. Ferrari, Solid State Commun. 143, (2007) 47. [2] F. Tuinstra, and J. L. Koenig, J. Chem. Phys. 53, (1970) 1126. [3] A. C. Ferrari, J. C. Meyer, V. Scardaci, C. Casiraghi, M. Lazzeri, F. Mauri, S. Piscanec, D. Jiang, K. S. Novoselov, S. Roth, and A. K. Geim, Phys. Rev. Lett. 97, (2006) 187401. [4] M. A. Pimenta, G. Dresselhaus, M. S. Dresselhaus, L. G. Cançado, A. Jorio, and R. Saito, Phys. Chem. Chem. Phys. 9, (2007) 1276. [5] W. A. de Heer, C. Berger, M. Ruan, M. Sprinkle, X. Li, Y. Hu, B. Zhang, J. Hankinson, and E. Conrad, PNAS 108, (2011) 16900. Figure
Figure 1 - Raman spectra of MBE grown nanocrystalline graphene films prepared at 800 ÂşC (a) and 1000 ÂşC (b, c) for growth times of 15 min (a, b), and 480 min (c). It can be noticed that for increasing time and temperature the 2D peak intensity increases and the FWHM of the peaks decrease, showing the improvement in the structural quality of the grown layers.
Intraband electron focusing by a flat lens in bilayer graphene Laszlo Oroszlany, Csaba Peterfalvi, Jozsef Cserti, Colin Lambert Eötvös University Budapest Department of Physics of Complex Systems Eötvös University H-1117 Budapest Pázmány Péter sétány 1/A Budapest (Hungary)
We propose an implementation of a valley selective electronic Veselago lens, as a planar potential step in bilayer graphene. We demonstrate that low energy electrons radiating from a point source and being scattered by an appropriately oriented potential step can be focused again coherently within the same band. The phenomena is due to the negative refraction index which is a consequence of the anisotropy in the dispersion relation caused by the trigonal warping effect. We also consider an effective Hamiltonian in which the electron-electron interaction1, as well as external mechanical strain2 is taken into account, and we show how this affects the focusing phenomenon. Recent studies on the electron-phonon interaction in bilayer graphene3 suggest that the electrons' free path can be long enough even on room temperatures to enable the focusing.
Graphene nanoribbons as low-bandgap donor materials for organic photovoltaic: Quantumchemical aided design Silvio Osella, David Beljonne Chemistry of Novel Materials, University of Mons, Place du Parc 20, B-7000 Mons, Belgium Silvio.Osella@umons.ac.be Graphene nanoribbons (GNRs) are strips of graphene cut along a specific direction with a nanometer sized width (< 10 nm). Compared to graphene, GNRs feature peculiar electronic and optical properties, such as the opening of a finite bandgap, which make them suitable for various applications in nanoelectronics and nanophotonics [1-8]. It is well known that the opto-electronic properties of GNRs can be tuned by changing their shape (from armchair to zigzag), periphery, widths and length [9-10]. As such, they are widely investigated for various applications in nanoelectronics and nanophotonics, namely as transparent electrodes in organic photovoltaic (OPV) cells. We show here by means of (timedependent) density function theory calculations that GNRs with properly designed edge structures fulfill the requirements in terms of electronic level alignment with common acceptors (e.g., C 60) and solar light harvesting to be used as electron donors for OPV (considering the P3HT as refer donor), Figure 2. In addition, rearrangement of the electronic density at GNR-C60 interfaces strongly perturbs the energy diagram for electrons and holes favoring the splitting of the CT pairs into free charge carriers. Altogether, the electronic and optical properties of GNRs seem to be particularly well suited to ensure sunlight absorption and photoconvertion at interfaces with fullerenes in OPV devices.
References [1] M. Fujita, J. Phys. Soc. Jap. 65 (1996) 1920. [2] K. Nakada, Phys. Rev. B 54 (1996) 17954. [3] K. Wakabayashi, Phys. Rev. B 59 (1999) 8271. [4] L. Brey, Phys. Rev. B 73 (2006) 235411. [5] M. Ezawa, Phys. Rev. B 73 (2006) 045432. [6] H. Hsu, Phys. Rev. B 76 (2007) 045418. [7] D. Prezzi, Phys. Rev. B 84 (2011) 041401. [8] K. Gundra, Phys. Rev. B 83 (2011) 075413. [9] L. Yang, Nano Lett. 7 (2007) 3112. [10] K. Sasaki, Phys. Rev. B 84 (2011) 085458.
p-ANRn
m-ANRn
4-CNRn
6-CNRn
8-CNRn
Figure 1. Chemical structures of the graphene nanoribbons investigated here. The n index is varying from 1 (monomer) to 4 (tetramer). p-ANRn and m-ANRn have the same repeating unit but differ by their connectivity (dashed lines). The difference in the CNR n series is due to the increase of the width in the sequence 4-CNRn < 6-CNRn < 8-CNRn. For all structures, methyl group are used as side chains in the calculations.
Figure 2. One-electron energy diagram for GNRs, P3HT (donors) and C60 (acceptor).
Nature of Interaction of Graphene with Ag, Au, Pd Metals Michal Otyepka, Jaroslav Granatier, Petr Lazar, Pavel Hobza RCPTM, Dept. of Physical Chemistry, Faculty of Science, Palacky University Olomouc, tr. 17. Listopadu 12, Olomouc, Czech Republic Michal.Otyepka@upol.cz We studied the adsorption of Ag, Au, and Pd atoms on benzene, coronene and graphene using post Hartee-Fock wave-function theory (CCSD(T), MP2) and density functional theory (M06-2X, DFT-D3, PBE, vdW-DF) methods. The binding energies calculated by CCSD(T) method for benzene...M (M=Pd, Au, Ag) complexes are 19.7, 4.2, and 2.3 kcal/mol, respectively. The nature of binding of the three metals is different. Silver binds predominantly through dispersion interactions, the binding of palladium has a covalent character, and the binding of gold involves a subtle combination of charge transfer and dispersion interactions, as well as relativistic effects. These effects can be reproduced in plane-wave density functional theory calculations by including a fraction of the exact exchange and a nonempirical (vdW-DF) van der Waals correction (EE+vdW). The calculated EE+vdW energies agree well with the benchmark CCSD(T) energies for benzene...M complexes. The EE+vdW binding energies for the graphene...M (M=Pd, Au, Ag) complexes are 17.4, 5.6 and 4.3 kcal/mol, respectively. The interaction of larger metal clusters will also be discussed. References [1] Granatier J, Lazar P, Otyepka M, Hobza P J. Chem. Theory Comput., 7 (2011) 3743.
Formation of Graphene Nanoribbons in Solution 1
1
2
2
3
3
M. C. Paiva , E. Cunha , M. F. Proença , R. F. Araújo , F. Costa , A. J. Fernandes , M. A. Ferro
4
1
Instituto de Polímeros e Compósitos/I3N, Universidade do Minho, Campus de Azurém, 4800-058 Guimarães, Portugal 2
Departamento de Química, Universidade do Minho, Campus de Gualtar, 4710-057 Braga, Portugal
3
FSCOSD/I3N, Universidade de Aveiro, Campus Universitário de Santiago, 3810-193 Aveiro
4
CICECO, Complexo de Laboratórios Tecnológicos, Campus Universitário de Santiago, 3810-193 Aveiro, Portugal mcpaiva@dep.uminho.pt
Recently, the formation of graphene by exfoliation of carbon nanotubes (CNT) has shown increasing interest. This process originates graphene nanoribbons (GNR) that are expected to present excellent electrical properties, depending on their width and on their edge shape [1]. Several methods for the unzipping of graphene from CNT were proposed along the past few years [2-6]. These methods often present some limitation, such as low yield of GNR, or extensively oxidized GNR without electrical conductivity. Recently, the formation of GNR wDV REVHUYHG ³LQ VLWX´ by unzipping of carbon nanotubes under ultrahigh vacuum scanning tunneling microscopy (UHV STM) [7]. The CNT under observation were functionalized by the 1,3-dipolar cycloaddition reaction [8]. This particular functionalization route seems to be responsible for the unzipping of the CNT under STM imaging conditions. The present work demonstrates the formation of GNR in solution by unzipping of functionalized CNT, in different solvents. The GNR thus formed were analyzed by UV-vis and Raman spectroscopy, and by transmission electron spectroscopy. GNR bundles were deposited from an ethanol solution and observed by TEM, as depicted in Figure 1.
Figure 1. TEM of GNRs obtained by unzipping of CNT in ethanol.
References [1] K. Nakada, M. Fujita, G. Dresselhaus, M. Dresselhaus, Physical Review B (1996) 54, 24, 17954-61. [2] W. S. Jr Hummers, R. E. Offeman, , J. Am. Chem. Soc. (1958) 80, 1339. [3] D. V. Kosynkin, A. L. Higginbotham, A. Sinitskii, J. R. Lomeda, A. Dimiev, B. K. Price, J. M. Tour, Nature (2009) 458, 872-877. [4] Jiao, L. Zhang, X. Wang, G. Diankov, H. Dai, Nature (2009) 458, 877±880. [5] A. G. Cano-Márquez, et al., Ex-MWNTs: Nano Lett. (2009) 9, 1527±1533. [6] K. Kim, A. Sussman, A. Zettl, ACS Nano (2010) 4, 3, 1362-66. [7] M. C. Paiva, W. Xu, M. F. Proença, R. M. Novais, E. Lægsgaard, F. Besenbacher, Nano Letters (2010), 10, 1764±1768. [8] M. C. Paiva, F. Simon, R. M. Novais, T. Ferreira, M. F. Proença, W. Xu, F. Besenbacher, ACS Nano (2010) 4, 12, 7379-7386.
Figure 1. TEM of GNRs obtained by unzipping of CNT in ethanol.
Nitrogen-doped Graphene and its Iron-based composite as Efficient Electrocatalysts for Oxygen Reduction Reaction Md Khaled Parvez, Shubin Yang, Yenny Hernandez, Xinliang Feng, Klaus M체llen Max Planck Institute for Polymer Research, Ackermannweg 10, D55128 Mainz, Gremany parvez@mpip-mainz.mpg.de The expensive platinum based electrocatalyst for oxygen reduction reaction (ORR) has hindered the practical application of fuel cells [1]. Therefore, numerous efforts have been devoted to substitute Ptbased catalysts. Among them nitrogen doped carbon nanotubes, mesoporous graphitic arrays and graphene are among the most promising metal-free catalysts for replacing platinum [2-4]. In this work, we have developed a cost effective synthesis of nitrogen doped graphene (NG) by using cyanamide as nitrogen source and graphene oxide as precursor, which led to high and controllable nitrogen contents from 4.0% to 12.0% after thermal pyrolysis. The NG obtained by thermal treatment at 900 째C shows a -2 stable methanol cross-over effect, high current density (7.76 mA cm ) and durability (~ 87% after 10,000 cycles). Further, iron (Fe) nanoparticles could be incorporated into NG with the aid of Fe(III) chloride in the synthetic process. This allows one to examine the influence of non-noble metal on the electrocatalytic performance. Remarkably enough, we found that NG supported with 5 wt% Fe -2 nanoparticles displayed an excellent methanol cross over effect, high current density (10.83 mA cm ) and superior stability (~ 94%) in alkaline solution which outperformed the platinum and NG-based catalysts.
References [1] B.C.H. Steele, A. Heinzel, Nature, 414 (2001) 345. [2] K.P. Gong, F. Du, Z.H. Xia, M. Durstock, L.M. Dai, Science 323 (2009) 760. [3] R.L. Liu, D.Q. Wu, X. Feng, K. M체llen, Angew. Chem. Int. Ed. 49 (2010) 2565. [4] L. Qu, Y. Liu, J.-B. Baek, L. Dai, ACS Nano 4 (2010) 1321.
Figures
Figure caption: (a) Preparation scheme of nitrogen doped graphene, (b) HRTEM image of NG-900, (c) and (e) RRDE voltammograms and corresponding amperometric response for ORR in O 2 saturated -1 0.1M KOH at a scan rate 10mVs and; (d) and (e) electrochemical activity given as the kintetic-limiting current density (JK) at -0.5V for all NG and NG/Fex samples, respectively.
Graphene growth on Cu mono- and polycrystalline substrates 1
1,2
1,3
3
3
1
I.Pasternak , K.Grodecki , P.Dabrowski , I.Wlasny , Z.Klusek and W.Strupinski 1
Institute of Electronic Materials Technology, Wolczynska 133, 01-919 Warsaw, Poland 2 Faculty of Physics, University of Warsaw, Hoza 69, 00-681 Warsaw, Poland 3 Solid States Physics Department, University of Lodz, Pomorska 149/153, Lodz 90-236, Poland iwona.pasternak@itme.edu.pl
CVD is a crucial graphene growth technique, which is performed on the surface of transition metals. In particular, Cu is considered to be an excellent substrate for making high-quality graphene films with uniform thickness due to the low solubility of C in Cu [1]. The bonding of a single graphene layer to a metal surface depends sensitively on the metal surface itself, the quality of copper substrate and the grain size [2].
In this work, we collate data on the properties of graphene films grown on mono- and polycrystalline copper substrates by the CVD method. We compare commercially available graphene films on a Cu foil with graphene grown on different purity copper foils and a Cu(111) monocrystal substrate.
Graphene was deposited on a Cu (111) substrate with pretreatment surface and on a 100Âľm thick polycrystalline copper foil. The CVD process proceeded in two steps. First, to prepare the substrate 0
surface, copper substrates were annealed either in a H2/N2 or H2/Ar gas mixture at 1020 C. During the carbonization step, propane gas was used as a carbon precursor. The temperature was maintained at 0
1020 C. After deposition, copper substrates covering graphene films were cooled down to a room temperature under an Ar atmosphere.
Graphene grown on both mono- and polycrystalline substrates was investigated using a complementary characterization technique. Raman spectroscopy confirmed the formation of graphitic structures. Moreover, it provided information on the domain size, strain and stacking order of graphene films. The morphology of graphene on metal grains was analyzed by SEM. STM/STS techniques were used to show the topography of the graphene-substrate interface and confirm the uniformity of the graphene layer.
We report that proven quality graphene films on Cu mono- and polycrystalline substrates were obtained. We demonstrate the value of our graphene films by transferring graphene from Cu substrates to target substrates. References [1] X. Li, W. Cai, J. An, S. Kim, J. Nah, D. Yang, R. Piner, A. Velamakanni, I. Jung, E. Tutuc, S. K. Banerjee, L. Kolombo, R. S. Ruoff, Science, 324 (2009) 1312. [2] Gang Hee Han, Fethullah Gunes, Jung Jun Bae, Eun Sung Kim, Seung Jin Chae, Hyeon-Jin Shin, Jae-Young Choi, Didier Pribat, and Young Hee Lee, Nano Lett. 11 (2011) 4144.
Hydroxyl Functional Groups on Pristine, Defected Graphene, and Graphene Epoxide: insights from first principles calculations 1
Maria Peressi , Nahid Ghaderi
1,*
1
Department of Physics, University of Trieste, Strada Costiera 11- I-34151 Trieste, Italy and CNR-IOM Democritos National Simulation Center, Italy * presently at: Babol Noshirvani Univ. of Thechnology,Shariati Av., Babol, Mazandaran, Iran peressi@ts.infn.it Oxygen-containing functional groups can be present in considerable amount intentionally or unintentionally on graphene, and a complete reduction of graphene oxide is difficult to achieve. To address the origin of this behavior, we have performed pseudopotential density functional theory calculations to investigate in particular the adsorption of hydroxyl (OH) on perfect and defected graphene, individually and in presence of other coadsorbed functional groups. Structural, electronic, and magnetic properties and reactivity of the systems studied have been investigated, with the aim of understanding why complete deoxygenation of graphene oxide is not easy to achieve. We found that hydroxyl groups weakly adsorb on perfect graphene, with adsorption energy of 0.54 eV, and induces magnetization on graphene. They can diffuse with rather low barriers, less than 0.35 eV, and easily aggregate. Aggregation of hydroxyl groups give an energy gain of about 0.75 eV per each adsorbed OH in the case of pairs and even more in the case of triplets, quadruplets, and larger aggregates. Aggregation with coadsorbed epoxy groups is also energetically favored. Water formation from adsorbed hydroxyl groups can occur at the pristine surface with energy barriers of the order of 0.5 eV or slightly larger in the case of OH initially bonded to epoxy. The adsorption of hydroxyl on defected graphene is much stronger than that at pristine, with adsorption energy of 1.80 eV at Stone-Wales (SW) defects and 4.23 eV at single vacancies. However, single vacancies and SW defects play different roles. Hydroxyl adsorption at a single vacancy is dissociative, with a low barrier of about 0.2 eV, leading to the formation of stable ether groups with a strong magnetic character. At variance, SW defects could stabilize the hydroxyl groups on the graphene basal plane, with a stronger binding energy then pristine, and, even more important, higher barriers (more than 1 eV) for diffusion [Fig. 1], recombination of neighboring hydroxyl groups and water formation [Fig. 2]. This suggests that SW defects could be responsible for the stabilization of the hydroxyl groups which are also present at graphene oxide after reduction. References [1] N. Ghaderi and M. Peressi, J. Phys. Chem. C 114 (2010) 21625.
Figures
Fig. 1: Optimized structural models and energies in the diffusion process of an hydroxyl group individually adsorbed on a SW defect. Adsorption energies of OH in the initial and final states are reported, together with the diffusion barriers.
Fig. 2: Optimized structural models and energies in the process of water formation from hydroxyl aggregates on a SW defect.
High Conductance, Large Area, Single Layer Graphenes from Graphene Oxide 1
2
2
2
Søren Petersen , Yudong He , Jiang Lang , Wenping Hu and Bo W. Laursen
*1
1) Nano-Science Center, Institute of Chemistry, University of Copenhagen, 2100 Copenhagen, Denmark 2) Institute of Chemistry Chinese Academy of Science, 100190 Beijing, China bwl@nano.ku.dk The major route for low cost and large-scale production of graphene-like materials is chemical conversion via exfoliation of graphene oxide (GO). Unfortunately this method suffers from aggregation problems when the graphene is to be regenerated via reduction (deoxygenation). As a consequence its relatively high oxygen content has limited the quality of chemical converted graphene. Here we demonstrate a chemical conversion of single layer graphene oxide back into single layer highly reduced graphene. This is done by trapping reduced graphene oxide (RGO) sheets in two dimensions on a surface with the Langmuir-Blodgett technique before aggregation occurs [1]. The RGO is then further deoxygenated by multiple reduction steps including hydrazine treatment, acid treatment and annealing [2]. We have achieved single sheet conductivities up to 2.1x104 S/m. This is slightly lower than [3], but without the use of the extremely toxic pure hydrazine. The field effect has been examined by using the wafer as a gate and the RGO has been determined to be p-type. Coating of large areas with high quality single sheet graphene can be used for several purposes, e.g. preparation of transparent electrodes. The electrical and AFM characterization is supplemented with Raman analysis on individual single layered sheets to confirm the quality.
References [1] L. J. Cote et al., JACS, (2009), 131, 1043-1049. [2] Gao, W. et al., Nature Chem., (2009), 1, 403-408. [3] Tung, V.C. et al., Nature Nanotech., (2008), 4, 25-29. [4] Q. Tang et al., Adv. Mat., (2008), 20, 1511-1515.
Figures
Figure 1. LB-assembly of graphene oxide transferred to Si-wafer at different packing densities. On the left: diluted packing of GO resulting in isolated sheets which could be contacted. On the right: over packed GO resulting in a coherent film continues film consisting of primarily single sheets.
Figure 2. Electrical characterization of monolayered single graphene sheets. On the left: optical microscope image of a device fabricated from a single sheets of monolayered highly reduced graphene oxide in the middle with two gold electrode [4]. On the right: Plot of the conductivities measured for the reductions steps used as described in the text. The top and bottom bar represents the highest and lowest measured values respectively while the cross is the mean value.
Graphene Nanoribbon Heterojunctions via partial cyclodehydrogenation 1
1
1
1
1
1
2
C. A. Pignedoli , S. Blankenburg , J. Cai , P. Ruffieux , R. Jaafar , D. Passerone , X. Feng , 2 1 K. Müllen , R. Fasel 1
Empa, Swiss Federal Laboratories for Materials Science and Technology, nanotech@surfaces Laboratory, 8600 Dübendorf, Switzerland. 2 Max Planck Institute for Polymer Research, Ackermannweg 10, 55124 Mainz, Germany carlo.pignedoli@empa.ch
Graphene nanoribbons (GNRs) ± semiconducting quasi-one-dimensional graphene structures ± have great potential for the realization of novel electronic devices. Recently, graphene nanoribbon heterojunctions ± interfaces between nanoribbons with unequal band gaps ± have been realized with lithographic etching techniques and via chemical routes to exploit quantum transport phenomena. However, standard fabrication techniques are not suitable for ribbons narrower than ~5 nm and do not allow to control the width and edge structure of a specific device with atomic precision. A bottom-up approach based on surface-assisted cyclodehydrogenation reactions has recently emerged as a promising route to the synthesis of nanoribbons and nanographenes [1,2]. The key step of this bottom-up GNR fabrication method [1] is the thermally induced cyclodehydrogenation of linear polyphenylenes on Au(111) templates. The method, which does not need a Lewis acid or other catalyst than the supporting metal substrate, is highly selective and efficient, but nothing has been known about the details of this unique reaction. Here, we rationalize the mechanism responsible for the surfaceassisted cyclodehydrogenation of polyanthrylene oligomers into armchair graphene nanoribbons (AGNR) by means of a combined experimental and computational approach [3]. We identify a rather generic reaction mechanism that we expect to be relevant to other comparable surface-assisted synthesis processes. The identified, unanticipated reaction mechanism suggests that careful control of annealing temperature and duration might allow to fabricate partially reacted AGNR segments. In fact, controlled annealing produces partially cyclodehydrogenated graphene nanoribbons (Figure 1). These ribbons consist of segments exhibiting the 7-AGNR structure and of one-side-only dehydrogenated segments. The latter +
correspond to N=5 AGNRs with additional benzene rings ortho-fused to the naphthalene units (5 +
AGNR). These atomically precise 7-AGNR/5 -AGNR/7-AGNR nanostructures are a first realization of intra-ribbon heterostructures, with properties potentially very similar to the ones predicted by Prezzi and coworkers [4]. Finally, we also demonstrate the ability to selectively modify the nanoribbon heterojunctions via STM tip-induced cyclodehydrogenation, which suggests that the large scale IDEULFDWLRQ RI QDQRULEERQ KHWHURMXQFWLRQV E\ HOHFWURQ EHDP ³ZULWLQJ´ PLJKW EHFRPH WHFKQRORJLFDOO\ feasible. [1] J. Cai, P. Ruffieux, R. Jaafar, M. Bieri, M., et al. Nature 466 (2010) 470 [2] M. Treier, M., C.A. Pignedoli, T. Laino, R. Rieger, et al. Nat. Chem. 3 (2011) 61 [3] S. Blankenburg, J. Cai, P. Ruffieux, R. Jaafar, et al. ACS Nano (2012) DOI : 10.1021/nn203129a [4] D. Prezzi, D. Varsano, A. Ruini, E. Molinari Phys. Rev. B 4, (2011) 041401
Figure 1: Realization of GNR heterojunctions by partial cyclodehydrogenation of polyanthrylene oligomers. a, STM measurements and corresponding atomistic models demonstrating the synthesis of AGNRs starting from polyanthrylene chains assembled on a Au(111) substrate (left) and subsequent cyclodehydrogenation upon annealing at 670 K (right). b, Annealing at a reduced temperature of 600 K results in partial cyclodehydrogenation and produces intra-ribbon heterojunctions between fully reacted N=7 AGNRs and partially reacted polyanthrylene + segments (N=5 ). Image taken from [3]
Graphene: different fabrication technologies for solid state devices (1)
(1)
(1)
(1)
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T. Polichetti , L. Lancellotti , E. Massera , M. L. Miglietta , F. Ricciardella , S. Romano , O. (2) (1) (1) Tari , S. Gnanapragasam and G. Di Francia (3)
(3)
(3)
(3)
(3)
R. Giorgi , T. Dikonimos , N. Lisi , E. Salernitano , S. Gagliardi , M. Falconieri (1) (2)
(3)
ENEA-UTTP-MDB Portici ENEA, p.le E. Fermi, 1, Portici (Naples), I-80055, Italy
8QLYHUVLW\ RI 1DSOHV ³)HGHULFR ,,´ 'HSW (OHFWURQLF (QJLQHHULQJ 9LD &ODXGLR ¹ Naples, Italy (3)
ENEA-UTTMAT-SUP Casaccia ENEA, via Anguillarese 301, 00123 Roma, Italy Corresponding author: tiziana.polichetti@enea.it
The ideal technique for fabricating graphene, i.e. a method able to produce a real single monolayer on a large area, is still far from being developed; up to now the existing techniques [1-9,12] involve both advantages and drawbacks and imply the achievement of a tradeoff for the application it is intended to make. For example, the mechanical exfoliation has a limit in the dimension of flakes, usually in the range of the microns; this kind of graphene, however, posses the best electrical and mechanical properties, appropriate for fundamental research purposes. Herein we present the results related to the fabrication, characterization and applications performed XQGHU WKH WRSLF ³*UDSKHQH´ LQ WKH WZR ODERUDWRULHV RI (1($ &DVDFFLD DQG (1EA Portici, where we have diversified the fabrication technology of graphene, based on specific requirements of the application to which it was intended. At the Surface Technology laboratory of ENEA Casaccia the graphene growth by thermal CVD on metal substrates is being pursued: the effect of varying process parameters on the quality of the graphene film is being investigated, while aiming at obtaining large area, large single crystal domain, homogeneous and continuous films for the applications. The graphene film growth is performed in a hot wall CVD UHDFWRU FRQVLVWLQJ RI D TXDUW] WXEH HTXLSSHG ZLWK D ³KRPH-EXLOW´ V\VWHP IRU WKH IDVW LQVHUWLRQ DQG extraction of the growth samples as shown in Fig 1. The fast extraction of the sample from the hot zone rapid cooling of the samples, thus a better process control. A wet etching procedure for the transfer of the free floating graphene films from Cu to SiO 2/Si substrate is presently being adopted, without the use of a resist. Various gas precursor and vapour mixtures can be used, such as methane and ethanol vapour, diluted in argon and hydrogen. Acetonitrile was also utilized with the aim to deposit N-doped graphene films. The samples are routinely analysed by scanning electron microscopy, X-ray photoelectron spectroscopy 2 and Raman spectroscopy. Graphene films with area of the order of 9 cm , consisting of very few layer (1-2) regions with small inclusions of multilayered regions, are grown [9]. The testing of graphene as DSSC counter-electrode is presently under way. As for the ENEA Portici laboratory, the activity is concentrated mainly on the basic research on sensor devices and solar cells. Concerning the sensor application, the most of the scientific community is moving on the chemical exfoliation methods, since they are low cost, versatile and do not affect the sensing properties of the graphene. In our labs a simple approach to fabricate conductometric sensors based on chemical exfoliated natural graphite has been performed. The devices, tested upon sub-ppm concentrations of NO2 in controlled environments, have shown the ability to detect this toxic gas at room temperature (see Fig. 3), with an estimated detection limit as low as 40 ppb [10], consistent with the best performances observed in the few-layers devices [11]. In regards to the use of the material in fundamental studies, as said above, the technique that retains the original properties of graphene is undoubtedly the mechanical exfoliation. In our laboratories we have developed a method that relies on the highly oriented pyrolitic graphite exfoliation by means of a thermo-curable elastomer, polydimethylsiloxane (PDMS), that allows to achieve lateral dimensions of flakes of tens of microns [12]. Samples prepared by this technique were employed to realize MOS graphene-based structures and, through capacitance-voltage characterization, the graphene workfunction, which is dependent on the number of layers and is one of the most important physical parameters used in the solar cell simulations, was experimentally determined. The workfunction values were employed in a theoretical study on the performance of graphene-on-semiconductor Schottky barrier solar cells (SBSC), and the theoretical efficiency was then estimated as function of the graphene number of the layers [13]. The results allow to make predictions about the potential of graphene-based solar cells; in particular best performances have been achieved for a 6 layer graphene based SBSC with an efficiency value up to 6.86 in the case of graphene on p-type germanium (see Tab.1). References
[1] S. N. Yannopoulos, A. Siokou, N. K. Nasikas, V. Dracopoulos, F. Ravani, G. N. Papatheodorou, Adv. Funct. Mater, 22 (2012) 113. [2] P. J. Wessely, F. Wessely, E. Birinci, B. Riedinger, U. Schwalke, arXiv:1112.4320v1 [3] C. Vallés, J. D. Núñez, A M. Benito, W. K., Carbon , 50, (2012) 835 [4] W. Gannett, W. Regan, K. Watanabe, T. Taniguchi, M. F. Crommie A. Zettl, Appl. Phys. Lett., 98 (2011) 242105 [6] G. Lippert, J. Dabrowski, M. C. Lemme, C. M. Marcus, O. Seifarth, G. Lupina, Phys. Status Solidi B, 248 (2011), 2619 [7] C. T. Nottbohm, A. Turchanin, A. Beyer, R. Stosch, A. Golzhauser, arXiv:1105.5792v1 [8] Q. Yu, L. A. Jauregui, W. Wu, R. Colby, J. Tian, Z. Su, H. Cao, Z. Liu, D. Pandey, D. Wei, T. F. Chung, P. Peng, N. Guisinger, E. A. Stach, J. Bao, S.Pei, Y. P. Chen, Nature Materials 10 (2011) 443 [9] R. Giorgi, Th. Dikonimos, M. Falconieri, S. Gagliardi, N. Lisi,P. Morales, L. Pilloni and E. Salernitano Springer Series Carbon Nanostructures - Luca Ottaviano Vittorio Morandi Editors - GraphITA 2011 ISBN 978-3-642-20643-6 Springer Heidelberg New York Dordrecht London p.109 [10] T. Polichetti, E. Massera, M. L. Miglietta, I. Nasti, F. Ricciardella, S. Romano and G. Di Francia Springer Series Carbon Nanostructures - Luca Ottaviano Vittorio Morandi Editors - GraphITA 2011 ISBN 978-3-642-20643-6 Springer Heidelberg New York Dordrecht London, p.171 [11] F. Schedin, A. K. Geim, S. V. Morozov, E. W. Hill, P. Blake, M. I. Katsnelson and K. S. Novoselov, , Nat. Mater., 6 (2007) 652 [12] F. Ricciardella I. Nasti, T. Polichetti, M. L. Miglietta, E. Massera, S. Romano and G. Di Francia.: Springer Series Carbon Nanostructures - Luca Ottaviano Vittorio Morandi Editors - GraphITA 2011 ISBN 978-3-642-20643-6 Springer Heidelberg New York Dordrecht London, p. 187 [13] L. Lancellotti , T. Polichetti, F. Ricciardella, O. Tari, S. Gnanapragasam and G. Di Francia, paper to be submitted.
Figure 1 Scheme of the graphene growth apparatus and transfer
Figure 2. Tem micrograph of single layer graphene film grown in highly diluted methane in hydrogen
Material p-Si n-Si p-Ge n-Ge N
Figure 3: Normalized conductance response ((SS0)/(Smax - S0)) kinetics upon exposure to 350 ppb of NO2 in dry and wet carriers, with a flow of 500sccm at 22 °C. The device is DC biased at 1V. In the inset a device photograph of is reported.
0.11 0.01 1.08 0.43 3
Ș
0.232 0.98 0.07 0.7 1.37 3.99 0.8 2.87 4 5
1.58 1.3 6.86 5.7 6
Table 1: efficiencies calculated simulating the electric behavior of graphene based SBSC devices with different combinations of semiconductor and numberof-layers of graphene, N.
Magnetic and transport properties of graphene@MNPs hybrides Jana P. Vejpravova and Barbara Bittova Institute of Physics AS CR, v.v.i., Department of Functional Materials, Na Slovance 2, Prague 8, Czech Republic vejpravo@fzu.cz Martin Kalbac JH Institute of Physical Chemistry AS CR, v.v.i., Dolejskova 3, Prague 8, Czech Republic Jan Vlcek Department of Physics and Measurements, Institute of Chemical Technology, TechnickĂĄ 5, Prague 6, Czech Republic Jan Prokleska and Karel Carva Charels University Prague, Department of Condensed Matter Physics, Faculty of Mathematics and Physics, Ke Karlovu 5, Prague 2, Czech Republic Anton Repko Charles University Prague, Department of Inorganic Chemistry, Faculty of Science, Hlavova 2, Prague 2, Czech Republic We have investigated electronic and magnetic properties of the CVD-grown graphene monolayers under interaction with monodisperse magnetic nanoparticles (MNPs). The MNPs, mostly of iron oxide and cobalt ferrite, are used for many different purposes in medicine (MRI contrast agents, drug carriers, smart biosensors, and hyperthermia) and industry (cleaning of water facilities, recording media) [1]. Their magnetic properties, ranging from ordered ferrimagnetism to superparamagnetism can be effectively controlled by variation of their size [1], as is ensured by using smart fabrication procedures providing ensembles of nanoparticles with very narrow size distribution [2]. The huge magnetic response of an individual single-domain MNP is represented by its superspin, which is usually in order 4
of 10 Bohr magnetons. Hence tuning of the interaction of the particular electronic states of the graphene monolayers with those of the MNPs opens enormous possibilities to engineer new generation of the hybrid graphene-based nanostructure, as demonstrated recently for
few-layer graphene
composites with semiconducting NPs and MNPs [4]. The nanostructures, investigated in our work constitute of a CVD-grown graphene sheet(s) and CoFe2O4 nanoparticles with a size distribution below 0.5 nm, prepared by modification of the hydrothermal method, in the presence of oleic acid [2,3]. The three types of hybrids were fabricated on the SiO2(top layer)/Si substrate, as schematically shown in Figure 1. The MNPs were either dispersed directly on the graphene layer (1.), or on the substrate, and subsequently the graphene monolayer has been transferred over the fixed MNPs (2.), and finally a sandwich-like structure (3.) has been obtained by combining the two particular steps. Morphology of the samples, with focus on dispersion of the individual MNPs on the substrate (SiO2 or graphene) was investigated by high-resolution scanning electron microscopy (HR SEM) and atomic force microscopy (AFM). The graphene-MNPs interaction was inspected by Raman spectroscopy and magnetic force microscopy (MFM), and in addition magnetization (M), a.c. susceptibility ( ), electrical resistivity (R) and magnetoresistance (MR) were measured in the temperature range 2 Âą 400 K and magnetic fields up to 14 T (with orientation parallel and perpendicular to the substrate plane, respectively). Significant electronic and magnetic interactions between the nanoparticles and graphene were detected suggesting charge transfer between the deposited MNPs and the graphene. The results are supported by first principle calculations of the electronic structure and transport properties. The study thus demonstrates significant effects in tailoring the electronic structure of graphene@MNPs hybrids for future applications, e.g. as biosensors and various nanoelectronic biocompatible devices.
Figure 1. Schematic representation of the investigated nanostructures. The MNPs are depicted as the spheres with arrows (representing the superspins), while the graphene layers are illustrated as the solid black lines. The three possible scenarios of application of the external magnetic field (Bext = 0, Bext applied perpendicular or parallel to the substrate) are also demonstrated.
References [1] G. Schmidt, Nanoparticles: from theory to Application, WILEY-VCH, Verlag 2004. [2] X. Wang, J. Zhuang, Q. Peng,Y. Li, Nature 437 (2005) 121. [3] A. Repko, D. Niznansky, J.P. Vejpravova, J Nanoparticle Res. 13 (2011) 5021. [4] B. Das, B. Choudhury, A. Gomathi, A.K. Manna, S.K. Pati, C.N.R. Rao, Chem. Phys. Chem. 12 (2011) 937.
Chiral graphene nanoribbon inside carbon nanotube: ab initio study 1
Andrei M. Popov, Irina V. Lebedeva,
2,3
Andrey A. Knizhnik, 2,3 Potapkin
2,3
4
Andrei N. Khlobystov and Boris V.
1
Institute of Spectroscopy, Fizicheskaya Street 5, Troitsk, Moscow Region 142190, Russia National Research Centre "Kurchatov Institute", Kurchatov Square 1, Moscow 123182, Russia 3 Kintech Lab Ltd., Kurchatov Square 1, Moscow 123182, Russia. 4 School of Chemistry, University of Nottingham, University Park, Nottingham NG7 2RD, UK popov-isan@mail.ru
2
Though graphene is currently receiving significant attention because of the unique physical properties it exhibits, the absence of an electronic band gap in this material remains one of the main obstacles hindering its application in electronic devices. One of the most effective solutions for introducing an energy gap between the conduction and valance bands of graphene is to shape it into strips of fixed width Âą graphene nanoribbons (GNRs). Formation of GNRs with well-defined, atomically smooth edge is essential for controlling their electronic properties. However, as discovered very recently, this is possible only by assembly of these nanostructures inside carbon nanotubes [1]. We study structure and electronic properties of a sulfur-terminated zigzag GNR (S-ZGNR) synthesized recently inside a singlewalled carbon nanotube (SWNT) using calculations in the framework of dispersion-corrected density functional theory.
Two mechanisms that allow for the relatively wide GNR to be accommodated within the SWNT can be proposed on the basis of the experimental study [1]. The first mechanism is distortion of the SWNT cross-section into an elliptic shape and bending of the GNR (Fig. 1a) to fit into this cross-section. We show that the balance between the elastic and van der Waals forces between the GNR and nanotube wall leads to a non-trivial dependence of structure of the GNR@SWNT system on the nanotube diameter. The GNR bending is observed for nanotubes with the ratio of the nanotube diameter D0 to the GNR width w
1.4
D0 / w
in the ranges D0 / w
1.4 and D0 / w
1.6 (Fig. 2). In the SWNTs with
1.6 , the GNR is found to retain its flat structure. It is also revealed that the nanotubes are
deformed upon encapsulation of the GNR, in agreement with the experimental observations [1]. Strong deformation of the nanotube wall is observed for narrow nanotubes with D0 / w 1.5 .
The second mechanism that allows the GNR to be accommodated within the SWNT is transformation of the GNR to a helical conformation (Fig. 1b). We calculate the dependence of energy of the S-ZGNR on the helix angle assuming that atoms of the GNR inside the carbon nanotube lie on the cylindrical surface (Fig. 3a). The calculated dependence has two energy minima. It is seen from Fig. 3a that these minima arise from the van der Waals attraction of sulfur atoms at neighbouring edges of adjacent turns of the GNR.
The hybrid DFT calculations of band structure of the S-ZGNR consisting of 4 zigzag rows show that this GNR is metallic (Fig. 3b). The deformation of the S-ZGNR inside (9,9) Âą (14,14) carbon nanotubes is found to be insufficient to open the band gap. Therefore, we propose that S-ZGNRs are potentially interesting for the use as nanowires protected by the nanotube wall or inductance nanocoils.
References [1] A. Chuvilin, E. Bichoutskaia, M. C. Gimenez-Lopez, T. W. Chamberlain, G. A. Rance, N. Kuganathan, J. Biskupek, U. Kaiser, A. N. Khlobystov, Nature Materials,10 (2011) 687. Figures
Fig. 1. Schematic representation of two mechanisms of accommodation of a S-ZGNR in a SWNT: (a) distortion of the SWNT cross-section into an elliptic shape and bending of the GNR, and (b) transformation of the GNR to a helical conformation. (Left) side views, (right) on-end views. Sulfur and carbon atoms are coloured in yellow and gray, respectively.
Fig. 2. Optimized structures of the S-ZGNR in armchair SWNTs (end-on views). Sulfur atoms are coloured in yellow. Carbon atoms of the GNR and SWNTs are coloured in red and blue, respectively.
Fig. 3. (a) Energy E (in meV) of the S-ZGNR 74 Å x10.5 Å (240 C atoms and 66 S atoms) with the atoms lying on the cylindrical surface of radius 3.3 Å as a function of the helix angle (in degrees) calculated (Ɣ) with and (ż) o
0 corresponds to the GNR aligned along the nanotube axis. (b) without the dispersion correction. The case Calculated band structures of the S-ZGNR consisting of 4 zigzag rows in the cases when the GNR is isolated (solid lines) and encapsulated in the (10,10) SWNT (dashed lines). The energy is given in eV.
Reduced Graphene Oxide: fundamentals and applications. 1
1
1
1
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S. Prezioso , F. Perrozzi , M. Donarelli , F. Bisti , S. Santucci , L. Palladino 2 2 1 Treossi , V. Palermo , and L. Ottaviano
1, §
1
, M. Nardone , E.
1
Dipartimento di Fisica, Università dell'Aquila, Via Vetoio, 67100, L'Aquila, Italy 2 CNR-ISOF, Via Gobetti 101, 40129 Bologna, Italy § gc-LNGS INFN stefano.prezioso@aquila.infn.it
In this paper we report our recent studies on the fundamental physical/chemical properties of supported reduced Graphene Oxide (rGO) obtained either via standard thermal annealing or under extreme-UV (EUV) light exposure alongside with investigations on its possible technological applications. rGO has been studied by X-ray Photoelectron Spectroscopy (XPS), micro-Raman Spectroscopy (Č?56), and Optical Microscopy. rGO reduction degree has been calibrated on the basis of its color contrast (CC) 2
providing a handy tool to quantitatively determine the fraction of sp -hybridized carbon and the surface area ratio occupied by pure graphene puddles as GO is reduced [1]. 2
/DUJH DUHD V Č?P ) GO sheets have been prepared via a modified Hummers method [2] and deposited by spin coating on 72 nm thick Al2O3/Si(100) [3]. Thermal reduction has been performed in the range between room temperature and 670 °C (Fig. 1(a)). When going from pristine GO to 670 °C 2 annealed GO, the CC passes from 0.4 Âą 0.02 the one measured for graphene to 1, the fraction of sp hybridized carbon passes from 0.46 to 0.78, and the average size of pure graphene puddles passes from 6.0 nm to 8.0 nm at 400 - 500 °C (7.5 nm at 670 °C). EUV-assisted photoreduction has been performed with 46.9 nm coherent light produced by a table top capillary discharge plasma source almost unique in the world scenario. The energy of photons (26.4 eV) lies in the opportune range for GO photoreduction, i.e. beyond the minimum energy required for photoreduction (3.2 eV) [4] and below the energies that cause breaking of the in-SODQH FDUERQĂFDUERQ ERQGV LQ VLQJOH-layer exfoliated graphene 2
2
Ă HV) [5]. When exposed to 200 mJ/cm dose, GO exhibits a 6% increase of sp -hybridized FDUERQ DQG D GHFUHDVH RI &Ă2 ERQGV, corresponding Âą according to the CC-based calibration Âą to a 60 Âą 10 °C thermally induced reduction (Fig. 1(b)) [6]. Photoreduction performed under these conditions has proved to be 2 orders of magnitude more efficient than the one measured in the case of UV-assisted photopatterning [7]. The original choice of using EUV instead of UV light to photo-reduce supported GO is not only advantageous in terms of reduction efficiency but it also allows to introduce the concept of EUV photolithography (today limited to the silicon technology only) for the processing of graphene-based materials. 2
Here we demonstrate resistless sub-micrometer GO photo-patterning over large areas ( 10 mm ) [6], in line with the current request of nanometer level patterning on wafer-sized areas. Regular periodic patterns are obtained by interference lithography (Fig. 1(c)). The patterned features consist of GO stripes with modulated reduction degree. The darker stripes (as observed at the optical microscope) are WKRVH ZLWK WKH KLJKHU UHGXFWLRQ GHJUHH DV PHDVXUHG E\ Č?56 RQ SDWWHUQV ZLWK Č?P SHULRGLFLW\ consistently with the contrast enhancement that is observed as a function of the reduction degree [8]. This result is a relevant upgrade for the graphene-based technology that can take advantage, in this way, from the entire know-how of the EUV-based technology in view of an eco-sustainable all-carbon technology.
References [1] F. Perrozzi, S. Prezioso, M. Donarelli, F. Bisti, S. Santucci, M. Nardone, E. Treossi, V. Palermo, and L. Ottaviano, ³The use of optical contrast to estimate the degree of reduction of graphene oxide´ Submitted. [2] E. Treossi, M. Melucci, A. Liscio, M. Gazzano, P. Samorì, V. Palermo, J. Am. Chem. Soc., 131 (2009) 15576. [3] P. De Marco, M. Nardone, A. D. Vitto, M. Alessandri, S. Santucci, L. Ottaviano, Nanotechnology, 21 (2010) 255703. [4] V. A. Smirnov, A. A. Arbuzov, Yu. M. Shul'ga, S. A. Baskakov, V. M. Martynenko, V. E. Muradyan, and E. I. Kresova, High Energy Chem., 45 (2011) 57. [5] S. Y. Zhou, Ç. Ö. Girit, A. Scholl, C. J. Jozwiak, D. A. Siegel, P. Yu, J. T. Robinson, F. Wang, A. Zettl, and A. Lanzara, Phys. Rev. B, 80 (2009) 121409(R). [6] S. Prezioso, F. Perrozzi, M. Donarelli, F. Bisti, S. Santucci, L. Palladino, M. Nardone, E. Treossi, V. Palermo, and L. Ottaviano, Langmuir, 28 (2012) 5489. [7] Y. Matsumoto, M. Koinuma, S. Y. Kim, Y. Watanabe, T. Taniguchi, K. Hatakeyama, H. Tateishi, and S. Ida, ACS Appl. Mater. Interfaces, 2 (2010) 3461. [8] I. Jung, M. Pelton, R Piner, D. A. Dikin, S. Stankovich, S. Watcharotone, M. Hausner, and R. S. Ruoff, Nano Lett., 7 (2007) 3569. Figures
Figure 1: (a) optical images of GO reduced at different temperatures. (b) C 1s XPS spectra (left) and Raman spectra (right) of GO before and after EUV exposure. (c) Schematic view of tKH /OR\G¶V LQWHUIHURPHWHU XVHG WR pattern the GO flakes (left) and SEM images of GO patterns with ȝP DQG QP VL]HG IHDWXUHV ULJKW
Structure, Stability and Electronic Properties of Graphene Edges on Co(0001) 1,2
2
2
2
2
2
Deborah Prezzi , Daejin Eom , Kuang T. Rim , Hui Zhou , Michael Lefenfeld , Colin Nuckolls , 2 2 3 Tony F. Heinz , George W. Flynn , and Mark S. Hybertsen 1
CNR ² Nanoscience Institute, S3 Center ± via Campi 213/a, Modena, Italy 2 NSEC ² Columbia University ± New York, 10027 NY, USA 3 CFN ± Brookhaven Natl. Lab., Upton, 11973 NY, USA deborah.prezzi@unimore.it
The rapid development of epitaxial growth of graphene on transition metal (TM) surfaces has been recently exploited to develop nanostructures with well defined edges [1-4]. While armchair edges are preferentially observed for graphene nanostructures grown on weakly interacting surfaces [5] and/or at low temperature [6], straight zigzag edges tend to develop for epitaxial growth on TM substrates, irrespective of the precursor type [1-4]. Mounting evidence also suggests that the electronic properties of graphene are significantly affected. Overall, the factors that control edge orientation, roughness and ultimately functional properties remain poorly understood. Here we combine low-temperature scanning tunneling microscopy (STM) measurements and DFT calculations to study graphene edge stability and electronic properties, as resulting from the coupling with the substrate. Graphene edges display straight well-ordered structure with zigzag orientation. DFT calculations provide insights into their stability by comparing several edge morphologies with both armchair and zigzag orientation. Simulated images indicate that different edge structures can be clearly distinguished in topography at low bias. The calculated electronic properties for the low energy edge structures are consistent with the measured STS tunneling spectra, which show a prominent edgelocalized peak at low bias.
References
[1] D. Eom, D. Prezzi, K. T. Rim, H. Zhou, M. Lefenfeld, S. Xiao, C. Nuckolls, Mark S. Hybertsen, T. F. Heinz, and G. W. Flynn, Nano Letters 9 (2009) 2844. [2] J. Coraux et al., New J. Phys. 11 (2009) 22. [3] P. Lacovig et al., Phys. Rev. Lett. 103 (2009) 166101. [4] J. Lu et al., Nat. Nanotech. 6 (2011) 247. [5] G. M. Rutter et al., Phys. Rev. B 81 (2010) 245408. [6] J. Cai et al., Nature 466 (2010) 470.
Poster Modularity of CMOS-compatible synthesis of graphene by segregation methods 1, 12 1 1, Caroline Rabot *, Alexandru Delamoreanu , Aziz Zenasni , Patrice Gergaud 1
2
CEA, LETI, MINATEC Campus, 17 rue des Martyrs, 38054 GRENOBLE Cedex 9, France. UJF-Grenoble 1 / CNRS / CEA LTM, 17 rue des Martyrs, 38054 GRENOBLE Cedex 9, France. *caroline.rabot@cea.fr The outstanding properties of graphene are expected to provide leads to a variety of
technological challenges in microelectronics, alternative energies, health science, etc. One of the first expected commercial applications of graphene - where it could compete with actual materials due to its high transparency, conductivity, and flexibility (20% stretchable) - is the transparent electrode for photovoltaic devices, touch screens and light electroluminescent diodes. These applications require the development of graphene processes compatible with industrial implementation.
The present work emphasizes a simple and versatile route to grow graphene fully covering 200mm wafers in industrial-compatible equipments (Figure 1a). This route allows tuning the properties of graphene in term of topography, coverage, number of layers and defects using metals (Ni, Cu, Pt) and alloys. It is not limited to specific metals (even with very low carbon solubility) ; does not require any gaseous carbon source and can be performed at relatively low temperature (600-700째C). The whole process flow has been operated using standard semiconductor-compatible clean room facilities.
Using segregation behavior of carbon in metals [1, 2, 3], the solubility of carbon (from carbon-based source) in transition polycrystalline thin metal film (ex: nickel) leads to the formation of mono to few layers of graphene (Figure 1b). The resulting coverage is over 99% (fully interconnected) and highly uniform throughout all the 200mm wafer. Typical optical transmittance of ~80% at 550 nm and resistance of ~600-700 Ohm/sq have been obtained (Figure 2). This transmittance can be significantly improved with some additional treatments.
By tuning the materials chosen (Carbon-based materials and transition metal), different morphologies of graphene can be obtained (graphene on metals, graphene redeposited on the dielectric and suspended graphene) (Figure 1 c, d). All processes occur during the growth process itself on a metal thin film [4, 5]. Intermediate silicide phases are identified as a way to develop a large catalogue of graphene and offer new capabilities in the synthesis of large area of graphene at low temperature.
Poster References [1] M. Zheng, K. Takei, B. Hsia, H. Fang et al., Appl. Phys. Lett., 96 (2010) 063110. [2] Z. Sun, Z. Yan, J. Yao, E. Beitleret et al., Nature, 468 (2010) 549. [3] C. M. Orofeo, H. Ago, B. Hu, M. Tsuji, Nano Research, 4 (2011) 531. [4] A. Zenasni, A. Delamoreanu, C. Rabot. Submitted to Applied Physics Letters. [5] A. Zenasni, A. Delamoreanu, P. Gergaud, C. Rabot. Submitted to Nature Nanotechnologies. . Figures
Figure 1: (a) Graphene synthesized on 200mm wafer. (b) Raman spectrum. (c-d) Scanning Electron microscopy images of graphene partially and fully suspended between Pt-based islands.
Figure 2 Transmittance spectra of graphene transferred on glass substrate without UV-Ozone treatment (black curve), with 30min UV-Ozone treatment (blue) and 50min treatment (red).
Optical spin current injection in graphene Julien Rioux,1,* J. E. Sipe,2 and Guido Burkard1 Department of Physics, University of Konstanz, D-78457 Konstanz, Germany 2 Department of Physics and Institute for Optical Sciences, University of Toronto, Toronto, Canada * julien.rioux@uni-konstanz.de 1
Charge and spin current injection by optical methods is investigated in single-layer and bilayer graphene within the tight-binding model, including bias and interlayer coupling effects. Two-photon absorption in graphene presents a strong linear-circular dichroism that is frequency-independent as calculated from the Dirac Hamiltonian. This unusual nonlinear response is exploited in coherent two-color injection and control setups, resulting in a current response that is maximal for co-circularly polarized beams and vanishes for opposite-circularly polarized beams [1]. Further, the magnitude of the injected current is independent of the angle between polarization axes for linearlypolarized beams. This does no longer hold when we consider bilayer graphene. Interlayer coupling in bilayer graphene has a distinct qualitative effect on the polarization dependence of charge and spin current injection. This has recently been observed in the charge current injection into epitaxially-grown multilayer graphene [2]. In combination with interlayer coupling, which induces trigonal warping of the electronic bands, the bias voltage allows to control the warping at the Fermi surface. The resulting implications for the spin current injection are presented. The excitation scheme employs interference between two-photon absorption at ω and one-photon absorption at 2ω. The one- and two-photon transition amplitudes are given by ie ~ ~vcv (~k) · E(ω) , ~ω ~ ~ ~vmv (~k) · E(ω) 2e2 X ~vcm (~k) · E(ω) ~ Ω(2) , cv (ω, k) = 2 2 ~ ~ ~ ω m ωmc (k) + ωmv (k)
~ Ω(1) cv (ω, k) =
(1) (2)
~ where ~vmn (~k) indicate matrix elements of the velocity operator, E(ω) is the electric field, ~ωm (~k) is the energy of ~ ~ ~ ~ band m, and [ωmc (k)+ωmv (k)]/2 = ωm (k)−[ωv (k)+ω] is the usual energy denominator appearing in second-order perturbation theory. The reciprocal space, the linear energy-crystal momentum dispersion of graphene near K, and a diagram of the two-color excitation scheme are shown in Figure 1 a . The band dispersion of bilayer graphene and the breakdown of the transition amplitudes for two-photon absorption into contributions of different symmetry are shown in Figure 1 b . Current injection using linearly-polarized ω and 2ω light in an isotropic medium can be characterized by the ratio d = ηIxyyx /ηIxxxx , where ηI is the current injection tensor. For linearly-polarized ω and 2ω beams forming an angle θ between their polarization axes, different values of d lead to injected currents with different magnitudes but also with vastly dissimilar angular dependencies: h i ˙ J~ = 2ℑ [ηIxxxx ] Eω2 E2ω sin(∆ϕ) f (θ, d) ~eˆ2ω + g(θ, d) ~eˆ⊥ (3) 2ω , where f (θ, d) = cos2 θ + d sin2 θ and g(θ, d) = 12 (1 − d) sin 2θ. Thus, the current component that is parallel to ~eˆ2ω , the polarization of the 2ω field, has a nonseparable dependence on θ and d, whereas the perpendicular component ~˙ always follows sin 2θ. Polar plots of f (θ, d) and g(θ, d), the angular distributions of the orthogonal projections of J, are shown in Figure 1 c for various values of the parallel-perpendicular disparity parameter d. The dependence on d leads to different angular dependencies of the current in single- and bilayer graphene. References [1] J. Rioux et al., PRB 83, 195406 (2011) [2] D. Sun et al., Nano Lett. 10, 1293 (2010)
MECHANICAL STABILIZATION OF GRAPHENE AEROGELS BY VULCANIZATION WITH PURE SULPHUR 1
1
2
Valentina Romeo , Gianfranco Carotenuto , Giovanni Marletta 1
Institute of Composite and Biomedical Materials, National Research Council, P.leTecchio, 80 Napoli, 80125, ITALY; 2Department of Chemical Sciences, University of Catania, V.le A. Doria, 6, Catania, 95125, ITALY. E-mail: giancaro@unina.it
Graphene aerogels (see Figure 1) are very useful materials for those technological applications where high-surface development and good electrical conduction characteristics are required [1]. Such nanostructured materials can be prepared by drying a high concentrated graphite-nanoplatelets (GNP) colloidal suspension, which is obtained by treating expanded graphite with ultrasounds in a liquid medium (e.g., octane). However, the resulting mechanical properties are not enough for technological applications.
In order to give more strength to this material, it was vulcanized with pure sulphur (at 180°C).The carbon-sulphur chemical reaction can be advantageously used for the mechanical stabilization of the very fragile spongy graphite material [2]. The introduction of sulphur in the graphite structure is quite simple since the sulphur molucules (S8) are quite soluble into non-polar organic media and it can be dissolved in the GNP colloid before the drying process.
In particular, graphene is a very good substrate for chemical functionalization, since its reactivity is comparable with that of other polycyclic aromatic hydrocarbons. The presence of carbon-carbon double bonds (C=C) makes possible also radical addition reactions. Sulphur molecules (S8) decompose in the molten form, producing linear bi-radicals c-S8o•l-S8• (Ȝ-transition) [3], which are able to cross-link the graphene-based framework constituting the aerogel.
Such a process significantly improves the material mechanical stability (see Figure 2). To verify the presence of S in the vulcanized material and to establish its distribution in the sample composition of our materials, we carried out energy dispersive spectroscopic (EDS) mapping. EDS combined with scanning electron microscopy (SEM) images of the graphene-sulphur composite (after the vulcanization process) showed the presence of such sulphur phase segregated at the edges of graphene sheets. In addition, differential scanning calorimetry (DSC) was used as a really convenient approach to study this vulcanization process, in fact the complete cross-linking of the graphene structure was evidenced by the disappearance of the pure sulphur phase melting (see Figure 3). The amount of residual sulphur in the vulcanized material was evaluated by thermo-gravimetric analysis (TGA) and it corresponded to ca. 30% by weight. Further characterization approaches used to investigate the morphology and the structure of graphene aerogels have been: large angle X-ray powder diffraction, Fourier-transform spectroscopy (FT-IR), X-ray fluorescence spectroscopy. References [1] Worsley, M A, J. Phys. Chem. Lett, 2 (2011), 921-925. [2] Ji,X,Nat.Mater, 8(2009),500-506. [3] Eichinger, BE, Macromol. Symp, 171 (2001), 45-56.
Figures
Figure 1 - Graphene aerogel samples (left-side) and SEM-micrograph (right-side).
Figure 2 - Graphene aerogel samples before (A) and after (B) the vulcanization treatment III run
I run
0
Heat Flow (W/g)
Heat Flow (W/g)
0
-2
-1
-4 -2
-6 100
200
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300
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Figure 3 – DSC-thermogram of the reactive sulphur/GNPs mixture
240
Molecular graphene of shungite 1
Natalia Rozhkova and Elena Sheka
2
1
Institute of Geology Karelian Research Centre RAS, Petrozavodsk 185910, Russia 2 3HRSOHVÂś )ULHQGVKLS 8QLYHUVLW\ RI 5XVVLD 0RVFRZ 117198, Russia rozhkovat@krc.karelia.ru
Molecular graphene, consisting of size-restricted graphene sheets, has become a subject of intense theoretical and experimental studies and debates. It was supposed to be one of the most versatile systems in molecular science research that could be easily probed by various scanning probe techniques from mesoscopic down to atomic scales [1]. Such scenario has been realized in the current study of the nanocarbon, produced from natural shungite and consisting of nonplanar nanosize graphene sheets.
Shungite carbon (ShC) can be characterized either as a semiconductor or semimetal and soft matter. Its structural organization has been investigated to disclose the origin of this many-face behavior. It turns out that shungite can be quite easily dispersed. Consecutive processing of shungite powder in water using sonication, filtration and centrifuging results in the formation of a stable aqueous dispersion of 13
ShC nanoclusters [2, 3]. A number of techniques, among which there are MNR C , SANS and SAXS, Raman scattering and Auger spectroscopy, has been applied to perform a comparative study of ShC with fullerenes (C60 and other), graphite and onion-like carbon. The study has convincingly revealed graphene-like features of the matter. Estimated by dynamic light scattering, ShC aqueous dispersion, with the initial carbon concentration of ~0.1 mg/ml, consists of colloidal particles of 50-60 nm in average. High-resolution-TEM visualization has allowed for revealing the finest dispersions of the particles that includes nanosize carbonaceous globules of <6 nm in average, randomly distributed in water. The globules present clusters of curved graphene sheets with linear dimensions of <1 nm, molecular mass <300 m/z and dipole moment ~6.5 D. The condensation of the ShC aqueous dispersion results in the aggregation of the pristine particles followed with the formation of a 3-dimensional net [4]. An example of the globular cluster in the condensed phase is shown in the Figure.
Figure. HRTEM of ShC condensed aqueous dispersion, scale Âą 5 nm.
Additional evidence of the nanosize-cluster origin of shungite follows from the pore-size distribution that is characteristic for natural shungites as well as the condensate 3-D net. The distribution reveals the predominant pore size within 0.7-5 nm [5]. Small doping of the condensed powders, obtained from the stable aqueous dispersion, enhances nonlinear optical response and substantially influences the photorefractive parameters of the conjugated structures of polyimide matrices [6]. Stable toluene dispersions have revealed the quantum-dot behavior of the graphene globular clusters with respect to
the enhancement of spectral properties of the solutions themselves as well as additionally solved fullerene C60 [7].
Canopy- and basket-like shapes of the carbon skeleton of planar pristine graphene membranes subjected to one-side hydrogenation over their basal planes [8] are suggested to explain the origin of the curved graphene molecules of shungite. It was supposed that the shungite formation occurs in due course of the graphitization of the carbon deposits under particular hydrothermal conditions that, according to geological reports [9], are characterized by high temperature and high concentration of water vapors that include atomic hydrogen. Water does not adsorb on the graphite. In contrast, atomic hydrogen willingly makes this, but only from one side. Atomic-hydrogen-one-side adsorption on the top graphite layer causes the irreversible curving of its carbon skeleton that keeps the shape after the adsorbate is removed from the surface due to high temperature. The water, on one hand, promotes splitting this graphene layer one by one from the others and stabilizes the clusterization of individual, curved, one-atom-thick carbon sheets, on the other. A competitive character of the processes, related to graphitization, atomic hydrogen adsorption/desorption, complex water action, results in the finding a compromise, namely in the formation of globular clusters of curved graphenes of nanosize in average. The curved graphene fragments form globular clusters and stabilize them in dispersion due to their size and specific interaction with water. Water captured in between graphenes is responsible for stabilizing clusters in condensed form. The clusters, compressed into macroscopic blocks during the relevant geological age, were transformed into solid shungite deposits. References [1] A.H. Castro Neto, F. Guinea, N.M.R. Peres, K.S. Novoselov, A.K. Geim, Rev. Mod. Phys. 1, (2009) 109. [2] N.N. Rozhkova. Shungite nanocarbon. Petrozavodsk, KRC RAS (2011) 100 p. (in Russian) [3] 1 1 5R]KNRYD / ( *RUOHQNR * , (PHOÂś\DQRYD 0 9 .RURERY 9 9 /XQLQ ( Ă?VDZD 3XUH Appl. Chem. 11 (2009) 2093. [4] N.N. Rozhkova, A.V. Gribanov, M.A. Khodorkovskii, Diamond & Rel. Mat. 16, (2007) 2104. [5] N.N. 5R]KNRYD * , <HPHOÂś\DQRYD / ( *RUOHQNR $ 9 *ULEDQRY 9 9 /XQLQ, Glass Physics and Chemistry, 6 (2011) 613. [6] N.N. Kamanina, S.V. Serov, N.A. Shurpo, N.N. Rozhkova, Techn. Phys. Letts., 10 (2011) 949. [7] E.F. Sheka, B.S. Razbirin, N.N. Rozhkova, D.K. Nelson, A.N. Starukhin (submitted). [8] E.F. Sheka, N.A. Popova J. Mol. Mod. Doi: 10.1007/s00894-012-1356-9. [9] P.R. Buseck,, L.P. Galdobina, V.V. Kovalevski, N.N. Rozhkova, J.W. Valley, A.Z. Zaidenberg, Canadian Mineralogist, 6 (1997)1363.
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Graphene produced by electrochemical exfoliation of graphite: electroanalytical properties Virginia Ruiz, Aintzane Ochoa, Pedro M. Carrasco, Ibon Odriozola, Germán Cabañero New Materials Department, CIDETEC-IK4, Parque Tecnológico de San Sebastián, Pº Miramón 196 20009 Donostia-San Sebastián (Spain) vruiz@cidetec.es Among the different methods to produce graphene sheets by exfoliation of graphite, the electrochemical route has been very scarcely investigated. First reported by Liu and coworkers [1], the electrochemical exfoliation of graphite represents a simple one-step method to produce graphene sheets in ionic liquid/water mixtures where hydroxyl and oxygen radicals produced by anodic oxidation of water start the oxidation of the edge planes of graphite, facilitating intercalation of anions from the ionic liquid. More recent studies have shown that graphene sheets can also be obtained by electrochemical exfoliation of highly oriented pyrolitic graphite (HOPG) in aqueous electrolyte solutions containing H2SO4 albeit with high amount of defects [2].
Here we have explored the electrochemical exfoliation of graphite rods in electrolytic baths consisting of different ionic liquid/water mixtures, which has a clear influence in the extent of surface functionalization of the graphene sheets (EC-GS). The resulting graphene samples were characterized by X-ray diffraction, thermogravimetric analysis and electron microscopy (Figure 1). It was found that the amount of oxygen-containing functional groups increased linearly with the water content in the exfoliation bath. Conversely, increasing the ionic liquid content in the exfoliation media resulted in ionic liquidfunctionalized graphene sheets.
Thin films of the EC-GS were drop cast on glassy carbon (GC) electrodes and their electrochemical activity
for
several
redox
couples,
such
as
hydroquinone/o-quinone,
ferro/ferricyanide
or
dopamine/dopamine quinone, was compared to that of films of graphite and reduced graphene oxide featuring the same oxidation level. In all cases, EC-GS showed enhanced electrochemical activity surpassing all the other carbon films in electron transfer rates, including GC (Figure 2), demonstrating that graphene produced by electrochemical exfoliation of graphite is a good electrode material for applications in electroanalysis.
References [1] N. Liu, F. Luo, H.Wu, Y. Liu, C. Zhang, J. Chen, Advanced Functional Materials, 18 (2008) 1518. [2] C.-Y. Su, A.-Y. Lu, Y. Xu ,F.-R.Chen, A. N. Khlobystov, L.-J. Li, ACS Nano, 5 (2011) 2332. Figures
Figure 1. (a) FESEM image of EC-GS; (b) TEM image of rGO.
Figure 2. Cyclic voltammograms of bare GC electrode and modified with EC-GS, rGO and graphite in 5 mM hydroquinone (Britton Robinson pH 2) solution.
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Organic and Metallo-organic Doping of Graphene Alexander Samuels and J David Carey University of Surrey, ATI, Guildford, GU2 7XH, UK a.samuels@surrey.ac.uk Control of the carrier concentration is of utmosW LPSRUWDQFH IRU JUDSKHQH¶V DSSOLFDWLRQ DV DQ HOHFWURQLF material and in understanding of its properties. Using ab initio density functional theory calculations we have investigated the adsorption of organic (TTF, DDQ and F2-HCNQ) and metallo-organic molecules (cobaltocene and ferrocene) on graphene. We have discovered that the doping mechanism is not a simple one and that the open or closed shell natures of the molecules or their ionisation potentials do not always imply strong doping. Molecular doping is explained in terms of the density of states and hybridisation of the band structure of graphene with these adsorbed molecules along with the delocalisation of the HOMO and LUMO wavefunctions. Metallocenes such as cobaltocene and ferrocene are sandwich compounds in which a divalent metal ion is sandwiched between two cyclopentadienyl rings. Ferrocene has 18 valence electrons which is considered to be the most stable configuration for metallocene compounds, hence it possesses a relatively high ionisation potential of 6.3 eV. However, cobaltocene has 19 valence electrons and a very low ionisation potential of 3.8 eV, making it an ideal candidate as a dopant for graphene. We find that the doping effect from ferrocene is very weak, with a charge transfer of only ca. 0.03 e/molecule, suggesting doping by an inductive effect rather than a true doping mechanism while the n-type doping from cobaltocene is an order of magnitude higher at 0.31 e/molecule. The delocalisation of the HOMO wavefunction of cobaltocene is shown in Figure 1. Tetrathiafulvalene (TTF, H2C2S2C2)2) is a well known sulfur containing planar organic molecule which [1]
has shown promising results in the doping of carbon nanotubes . The adsorption of TTF results in ntype doping of 0.16 e/molecule and a relatively large binding energy of 0.72 eV, consistent with ʌ ± ʌ stacking between graphene and the TTF molecule. The adsorption of TTF also results in the opening of a modest bandgap in graphene (Figure 2). P-type doping of graphene by organic molecules is also possible using compounds such as DDQ (2,3-dichloro-5,6-dicyano-p-benzoquinone) and F2-HCNQ (3,6-difluoro-2,5,7,7,8,8-hexacyanoquinodimethane). Both of these molecules have a relatively high electron affinity of -4.86 eV and -4.7 eV respectively, comparable to the work function of graphene
[2]
(§ H9 , implying they should act as effective p-type dopants. Our calculations show DDQ results in a p-type doping of 0.32 e/molecule while F2-HCNQ accepts 0.77 e/molecule. The adsorption of F2-HCNQ also results in a 0.7 eV shift of the Fermi energy into the conduction band as shown in Figure 3.
References [1] Jing Lu et al. Phys. Rev. Lett. 93, 116804 (2004) [2] Filleter, T. et al. Appl. Phys. Lett. (2008), 93, 133117 Figures
Figure 1: Delocalisation of the HOMO of cobaltocene upon adsorption on a graphene layer.
Figure 2: Band structure of graphene with an adsorbed TTF molecule, TTF impurity band shown in red.
Figure 3: Density of states of graphene with an adsorbed F2-HCNQ molecule, DOS of graphene alone shown in red and graphene with an adsorbed F2-HCNQ shown in black.
Synthesis of conducting transparent few-layer graphene directly on glass at 450 °C 1
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Eric Vinod Sandana , Co Chang Seok Lee , Costel Sorin Cojocaru , Waleed Moujahid , BÊrengère 2,3 2 2 2,4 2, Lebental , Marc Chaigneau , Marc Châtelet , François Le Normand and Jean-Luc Maurice 1) Graphos, Ecole Polytechnique, 91128 Palaiseau Cedex, France 2) LPICM (Laboratoire de Physique des Interfaces et des Couches Minces) UMR 7647, CNRS-Ecole polytechnique, 91128 Palaiseau Cedex, France 3) UniversitÊ Paris-Est, Ifsttar, 58 boulevard Lefebvre, 75732 Paris Cedex 15, France 4) InESS (Institut d'Electronique du Solide et des Systèmes) UMR 7163, UniversitÊ de Strasbourg-CNRS, 23, rue du Loess, BP 20 CR, 67037 Strasbourg Cedex 2, France vinodsandana@aol.fr For the integration of graphene in microelectronics, it appears mandatory to be able to synthesize this material in a reproducible manner, and with a cost as low as possible. Early preparation routes, such as mechanical or chemical exfoliation of HOPG (highly ordered pyrolytic graphite) [1], or epitaxial growth by high temperature annealing of (0001) SiC [2] in ultra-high vacuum (UHV), fall short in terms of reproducibility or scalability.
In contrast, methods involving the catalytic crystallization of graphene on a metallic substrate have a potential of large-area fabrication and appear well adapted to meet the requirements for industrial applications. The carbon can be brought by (i) chemical vapor decomposition (CVD) [3] using gaseous (methane etc.) or liquid (ethanol etc.) sources [4, 5], possibly with laser-induced rapid growth of grapheme pattern [6], (ii) a solid-state source (organic layers, amorphous carbon, diamond etc.) [7-11] or (iii) it can be implanted in the metallic substrate [12]. In all these methods, graphene is obtained during a hightemperature stage or during the sample cooling immediately following it. It is often of variable thickness ³IHZ-OD\HU JUDSKHQH )/*´ +RZHYHU WKH VXEVWUDWH LQ WKHVH WHFKQLTXHV LV D conducting metal, such as nickel or copper; thus, after the graphene film is formed, further device fabrication requires its transfer on an insulating functional substrate; an operation that, in turn, introduces a variety of defects [13].
Post-growth transfer and high growth temperature are two major hurdles that research has to cross to get graphene out of research laboratories. Here, using a plasma-enhanced chemical vapour deposition process, we demonstrate the large-area formation of continuous transparent graphene layers at temperatures as low as 450 °C. Few-layer graphene grows at the interface between a pre-deposited 200-nm Ni catalytic film and an insulating glass substrate. After nickel etching, we are able to measure the optical transmittance of the layers without any transfer. We also measure their sheet resistance directly and after ink-jet printing of electrical contacts: it is locally as low as 500 Č?/sq. Finally the samples thus equipped appear to be efficient humidity sensors. References [1] Novoselov K. S., Geim A. K., Morozov S. V. et al. Science, 306 (2004), 666-69 [2] Berger C., Song Z. M., Li T. B., et al. J. Phys. Chem. B, 108 (2004), 19912-16 [3] Reina A., Jia X. T., Ho J., et al. Nano Lett., 9 (2009), 30-35 [4] Miyata Y., Kamon K., Ohashi K. et al. Appl.Phys. Lett., 96 (2012), 263105 [5]Guermoune A., Chari T., Popescu F. et al. Carbon, 49 (2011), 4204-10 [6] Park J. B., Xiong W., Gao Y. et al. Appl. Phys. Lett., 98 (2011), 123109 [7] Sun Z., Yan Z., Yao J., Nature, 468 (2010), 549-52 [8] Zheng M., Takei K., Hsia B. et al. Appl. Phys. Lett., 96 (2010), 063110 [9] GarcĂa J. M., He R., Jiang M. P. et al. Carbon, 49 (2011), 1006-12 [10] Hofrichter J., Szafranek B. N., Otto M. et al. Nano Lett., 10 (2009), 36-42 [11] RodrĂguez-Manzo J. A., Pham-Huu C. and Banhart F., ACS Nano, 5 (2011), 1529-34
[12] Baraton L., Cojocaru C. S., Pribat D. et al. Nanotechnology, 22 (2011), 085601 [13] Lin Y.-M., Valdes-Garcia A., Han S.-J. et al. Science, 332 (2011), 1294-97 Figures
(a)
Figure caption
(a) Optical transmittance in the 300 to 1000-nm range, for two plasma exposure times. The vertical dotted line indicates the 650-nm wavelength
(b) sheet resistance versus relative humidity for a 12-mn plasma exposure
Ecofriendly Reduction of Graphene Oxide Using Extremophile Bacteria 1,
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Y. Tanizawa Sreejith Raveendran , Neha Chauhan , R. Tero , Y. Yoshida , T. Maekawa , 2 1 D. Sakthi Kumar , and A. Sandhu 1
Electronics-Inspired Interdisciplinary Research Institute (EIIRIS) Department of Electrical and Electronic Information Engineering Toyohashi University of Technology, 1-1 Hibarigaoka, Tempaku, Toyohashi 441-8580 Japan 2
Bio-Nano Electronics Research Centre, Graduate School of Interdisciplinary New Science, Toyo University, 2100, Kujirai, Kawagoe, Saitama 350-8585, Japan Email: sandhu@eiiris.tut.ac.jp
Graphene shows tremendous potential for applications including high frequency transistors, fuels cells, biosensors, and transparent conducting electrodes. These wide range of applications necessitate a simple, inexpensive, and environmentally friendly method for mass production of graphene. Recent reports indicate the emergence of chemically derived graphene using toxic chemicals [1,2]. In an effort to develop an environmentally friendly process, here we describe the synthesis of graphene by biological reduction of graphene oxide using extremophilic bacteria. It has been already reported that heterotrophic bacteria Shewanella oneidensis can utilize graphene oxide (GO) as its terminal electron acceptor in its respiratory pathway [3, 4]. As an alternative to metal reducing bacteria we used moderately halophilic extremophile Halomonas species [5] for biologically reducing GO. The GO was produced by modified HummerÂśs method using natural graphite powder. A wide range of concentrations of GO flakes were added to Halomonas growth medium for both aerobic and anaerobic bacterial reduction of the GO flakes. This approach enabled the extraction of large areas of single to multi-layer graphene sheets.
The bacterially reduced GO was characterized using transmission electron
microscopy (TEM), atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy. XPS measurements clearly showed increases in the intensity of C-C bond after biological reduction. We also compared the probability of GO reduction in the presence as well as in the absence of oxygen, with results showing more effective GO reduction under anaerobic conditions rather than aerobic. The ability of Halomonas to reduce GO was further investigated using two different strains of H. eurihalina and H. maura. Subsequent analysis showed that both strains were effective in the reduction of GO under the same culture conditions, where H. eurihalina was more efficient at reducing GO over a given incubation time. A wide range of concentrations of GO were added to the culture medium to test the reducing power of Halomonas over a constant time period, with the result that the rate of reduction increased as the concentration of the GO added was decreased. We believe that this extremophile bacteria based process is suitable for large scale production of graphene for industrial applications. Our approach is an ecofriendly, cost effective, and efficient method for producing large volumes of high quality graphene. References [1] R. Ishikawa, M. Bando, A. Sandhu, Jpn. J. App. Phys. 49 (2010) 06GC02. [2] R. Ishikawa, M. Bando , Y. Morimoto , A. Sandhu, Nanoscale Research Letters 6 (2011)111. [3] E. C. Salas, Z. Sun, A. Luttge, J. M. Tour, ACS Nano, 4 (2010), 4852. [4] G. Wang, F. Qian, C. W. Saltikov, Y. Jiao, Y. Li, Nano Res. 4 (2011), 563. [5] I. Llamas, A.D. Moral, F. Martinez-Checa, Y. Arco, S. Arias, E. Quesada, Antonie van Leeuwenhoek 89 (2006) 395.
Figures
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A. AFM image showing Halomonas reduced graphene sheet with thickness curve B. TEM image showing thin layer graphene sheet.
Reflectance of pristine and N-doped epitaxial graphene from THz to mid-IR 1
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Cristiane N. Santos , FrĂŠdĂŠric Joucken , Domingos de Sousa Meneses , Patrick Echegut , Jessica 5 5 2 1 Campos-Delgado , Jean-Pierre Raskin , Robert Sporken , BenoĂŽt Hackens 1
IMCN/NAPS, UniversitĂŠ catholique de Louvain (UCL), 1348 Louvain-la-Neuve, Belgium 2 LPME, UniversitĂŠ de Namur (FUNDP), 5000 Namur, Belgium 3 CEMHTI-CNRS, 45071 OrlĂŠans cedex 2, France 4 8QLYHUVLWp GÂś2UOpDQV 45067 OrlĂŠans cedex 2, France 5 ICTM/ELEN, UniversitĂŠ catholique de Louvain (UCL), 1348 Louvain-la-Neuve, Belgium cristiane.nascimento@uclouvain.be
Graphene optical properties were first investigated with the motivation to maximize its visibility on various substrates [1], so that graphene flakes could easily be located and then patterned to fabricate electronic devices. But a few key experiments revealed that there was much more to expect from this research area, and, today, one can already state that graphene optical properties offer perspectives for various applications in the near future [2]. Recent experiments demonstrated that a gate voltage could tune graphene reflectance and transmittance, particularly in the infrared (IR) range [3]. This tunability was found to stem from strong changes in the Drude component of the optical conductivity, particularly important in the far-IR, and from gate-induced shift of interband transitions, governed by a peculiar threshold at 2EF in graphene (where EF is the Fermi energy). Here we investigate on the IR reflectivity spectra of graphene grown epitaxially on 6H-SiC. Contrary to IR transmission spectroscopy which is hampered over a large part of the IR range by the SiC reststrahlen band [4], IR reflectivity can give access to invaluable information over the full range from terahertz (THz) to mid-IR. Peculiar changes in the IR reflectivity spectra of the SiC substrate are observed when graphene is present. In the THz region, these changes are mostly related to the intraband electronic transitions in graphene, and the corresponding optical conductivity depends on the carrier concentration, carrier mobility, Fermi energy and layer number. These important parameters can be extracted by fitting the reflectivity data using a dielectric function model for the SiC, and the optical conductivity for graphene. We show that a consistent and simultaneous analysis of both intraband and interband transition contributions over this broad spectral range allow to obtain more precise information on the carrier properties and layer number in multilayer graphene. Moreover, we also present IR microscopy data in the mid-IR, which allow to investigate on samples homogeneity and to correlate with conventional IR spectroscopy data. Differences between pristine and N-doped graphene are also discussed.
References [1] P. Blake et al., Appl. Phys. Lett., 91 (2007) 063124; D. S. L.Abergel et al., Appl. Phys. Lett., 91 (2007) 063125; S. Roddaro et al., Nano Lett., 7 (2007) 2707. [2] F. Xia et al., Nature Nanotech., 4 (2009) 839; Q. Bao et al., Nature Photon., 5 (2011) 411. [3] F. Wang et al., Science, 320 (2008) 206; Z. Q. Li et al., Nature Phys., 4 (2008) 532. [4] J. M. Dawlaty et al., Appl. Phys. Lett., 93 (2008) 131905.
Graphene: In Our Food Stuffs since Mesolithic Age Manav Saxena, Sabyasachi Sarkar Department of Chemistry, Indian Institute of Technology Kanpur, Kanpur-208016, Uttar Pradesh, India msaxena@iitk.ac.in Fire is one of the five elements by which the universe has been created as per the mythologies of Hindu, Babylonian, Greece, Chinese, Japanese, Buddhist, and Tibetan [1]. Fire is an important discovery by human which revolutionized their life. The use of fire pushed up the evolution of human from Homo erectus to Homo sapiens during the Mesolithic age (200,000-40,000 years ago). Fire allowed human to manipulate their surrounding environment. Cooking gave a variety of food and Possibility of the fatal diseases from taking raw and uncooked meat was controlled.
Roasting is a method that uses dry heat whether an open flame or other heat sources. It is the first known method of cooking. Roasting technique is very popular among us in day-today life. This is the roasting that unknowingly giving us a material, named Graphene, of too much value, in free of cost. Recently in 2011, a low cost method to synthesize graphene was reported by G. Ruan and co-workers [2]. They had chosen much less expensive carbon sources, such as food, insects, and waste.
The author here will explain the presence of graphene in our daily food stuffs as shown in figure 1. The synthesis and presence of graphene and graphene type materials in food stuffs give us two very important future scopes: (a) new solutions for recycling of carbon from impure sources, like low-valued foods and negative-valued solid wastes, into high-valued graphene [2]; and (b) the knowledge that our food contains graphene, graphene type material will help us in in-vivo study from a different direction that might produce concrete results as regards to the toxicity of graphene and graphene type materials. References [1] en.wikipedia.org/wiki/classical_element. [2] G. Raun, Z. Sun, Z. Peng, J. M. Tour, ACS Nano, 5, (2011) 7601-7607.
Contribution (Oral) Figure 1
Figure caption: (A, B) FESEM image; (C, D) TEM image; (E) SAED confirms hexagonal lattice; (F) AFM image; and (G) Raman spectra (Black: experimental Raman spectra; Red: fitted spectra; Green: deconvoluted peaks) of graphene and graphene type materials present in our food.
Efficient graphene preparation by combined intercalation ± exfoliation steps 1
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Peter Schellenberg , César Bernardo , Hugo Gonçalves , Michael Belsley , José Alberto Martins , 1 3 Cacilda Moura , Tobias Stauber 1 2
Center of Physics, University of Minho, Campus de Gualtar, 4710-057 Braga, Portugal
Center of Chemistry, University of Minho, Campus de Gualtar, 4710-057 Braga, Portugal 3
Department of Condensed Matter Physics, University Autónoma of Madrid, Campus of Cantoblanco, E-28049Madrid, Spain peter.schellenberg@fisica.uminho.pt
Despite the impressive improvements in the quality of large CVD- synthesized graphene sheets, the highest quality graphene layers are still obtained by mechanical exfoliation of pristine graphitic crystals. However, the method is notoriously tedious and gives a low yield. Worse still, large graphene flakes are often damaged by the exfoliation process itself. In this poster, we wish to address these limitations and present a novel approach to prepare large graphene flakes with increased yield. To this end we combine the standard mechanical exfoliation with chemical pretreatment of the graphite crystals. In a first step, millimeter sized graphite crystals are treated with oleum (concentrated H2SO4 with free SO3 ). These compounds, in particular the free SO3 intercalate into the graphite layers. Furthermore, due to the acidic nature of the solution, interlayer bonds at the rims of the atomic sheets might be broken-up. Subsequent reaction with alcohols expands the interlayer distance and loosens the van der Waals bonds further. In additional reaction steps, further intercalators may be utilized. In the final step, the standard scotch tape method is applied to the expanded graphite crystals and the resulting flakes are immobilized on glass or on Si/PMMA layers, Fig.1. Potential graphene flakes on glass are identified with a reflection microscope using the droplet condensation technique [1], refractive index matching [2], or on Si:dielectric composite layers by interference enhancement. The final test of whether a given flake is actually a single layer graphene is done by Raman microscopy. The absence of significant amounts of oxidized graphene or other reaction products is also confirmed in this way.
References [1] Hugo Gonçalves, Michael Belsley, Cacilda Moura, Tobias Stauber and Peter Schellenberg Appl. Phys. Lett., 97 (2010) 231905. [2] Hugo Gonçalves, Peter Schellenberg Michael Belsley, Lúis Alves, Cacilda Moura and Tobias Stauber , Proc. SPIE, 8001, no 8001-133
Figure
Figure 1: Schematic presentation of the treatment procedure. First, H2SO4 / SO3 is intercalated into the graphite layers. Reaction with alcohol and posterior application of additional intercalation compounds expand the layers further. Eventually, scotch tape exfoliation is used to separate the layers.
The interaction of pyrene derivatives with graphene nanoplatelets Andrea SCHLIERF1, Emanuele TREOSSI1, Huafeng YANG2, Cinzia CASIRAGHI2, Vincenzo PALERMO1 1
ISOF Istituto Sintesi Organica e Fotoreattività, Bologna, Italy The Photon Science Institute, University of Manchester, UK
2
schlierf@isof.cnr.it Graphene has raised intensive research interest as a promising material for future carbon-base nanoelectronics, while upscalable low cost production of this remarkable material is still a bottleneck when heading towards graphene based applications. At the moment, chemical exfoliation of graphene is the most promising technique to obtain large quantities at low cost and easily processable. A wide amount of research works demonstrated, in the last years, that several different classes of molecules can exfoliate graphite flakes into soluble graphene sheets with different efficiencies in achieving soluble aggregates and obtaining mono- and multi-layer sheets.
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Fig. Macroscopic morphology of different pyrene derivatives on silicon (scale bar 100Pm) A class of molecules which seems very effective in exfoliation and stabilizing graphene sheets are small poly-aromatic hydrocarbons (pyrenes, perylenes, antracenes, etc.) functionalized with flexible or possibly polar sidechains. While graphene exfoliation is widely performed in academic and industrial labs, a clear understanding of the exfoliation mechanism at molecular level is still missing. Macroscopic models have been used to correlate exfoliation efficiency to empirical solubility parameters quantifying the dispersive, polar, and hydrogen bonding properties of the solvent, or more simply to the surface tension or the refractive index of the solvent. Using a quite simplistic description of this process, these molecules act as “molecular wedges”, with the polyaromatic part interacting with the graphene through S-S stacking, while the negatively charged part (the acid) favours sheet solubility in solvent, hindering re-aggregation. The true picture is likely much more complex, with a relevant interaction of both the aromatic and polar moieties with the graphene surface.
To better unravel the complex interaction between graphene and organic molecules, and possibly improve the exfoliation efficiency, we have performed a comparative study of the self-assembly with a whole series of pyrene derivatives, having an increasing number of functionalization and polar character. We studied different self-assembling behaviours of those model compounds both in solution and on different substrates in presence or not of graphene sheets, applying optical microscopy (OM), atomic force microscopy (AFM) and UV-Vis spectroscopy. References [1] [2] [3] [4] [5] [6]
J. Jang, Mater. Chem., 2011, 21, 3462–3466 X. Dong, Phys. Chem. Chem. Phys., 2010, 12, 2164–2169 X. Pan, J. Phys. Chem. C, 2012, 116, 4175–4181 X. An, Nano Lett. 2010, 10, 4295–4301 J. Coleman, Adv. Funct. Mater. 2009, 19, 3680–3695 R.S. Swathi, J. Chemical Physics, 2008, 129, 054703
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New promising pyrolytic graphite for micro-mechanical exfoliation of graphene 1-2
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O.V. Sinitsyna , E.A. Khestanova , A.A. Antonov , I.G. Grigorieva , I.V. Yaminsky 1
Lomonosov Moscow State University, Leninskie Gory, Moscow, Russia Advanced Technologies Center, Luzhnetskaya naberezhnaya 2/4 bld. 53, Moscow, Russia 3 Optigraph GmbH, Rudower Chaussee, 29 12489 Berlin, Germany o.sinitsyna@nanoscopy.ru
2
The highly oriented pyrolytic graphite (HOPG) was traditionally used as a monochromator in neutron and x-ray studies, for thermal management and other applications. Currently, HOPG is increasingly utilized in nanotechnology applications. HOPG is used as a substrate in scanning probe microscopy, since its surface contains large atomically smooth areas. Laboratory samples of graphene are produced by micro-mechanical cleavage of HOPG. In this work, a new pyrolytic graphite called HAPG [1], specially adjust for high resolution spectroscopy and nanotechnology applications, was investigated. The technology of the graphite production was optimized so that the graphite structure and the number of defects were most similar to natural graphite crystals. o
o
HAPG and HOPG samples with the mosaic spread of 0.4 and 0.8 were studied by atomic-force microscopy (AFM). The surface of graphite may contain a variety of defects after cleavage [2]. The most common defects are the cleavage steps and edge dislocations with the Burgers vector perpendicular to the basal plane of graphite. In the AFM images the edge dislocations are observed as the steps with a -1
FXUYHG VKDSH 7KH DYHUDJH OHQJWK RI WKH FOHDYDJH VWHSV SHU XQLW DUHD ZDV ȝP for HOPG. HAPG -1
KDV WKH DYHUDJH OHQJWK RI WKH VWHSV RI ȝP . The defects are almost absent in many AFM images 2
ZLWK WKH DUHD RI PRUH WKDQ ȝP (Figure 1). The edge dislocations and grain boundaries were observed less frequently on the surface of HAPG in comparison with HOPG. In general, the surface quality of HAPG is similar to the surface quality of mica, which is widely used in scanning probe microscopy. Graphene samples were obtained by the micro-mechanical cleavage of HOPG and HAPG. Silicon oxide, deposited on the polycrystalline silicon, was used as a substrate. The thickness of the silicon oxide was about 300 nm. An optical image of a graphene layer, produced by HAPG cleavage, is shown in Figure 2. The graphene samples were studied with the help of micro Raman spectroscopy. It was found that the number of graphene layers depended significantly on the graphene preparation procedure. It should be noted that the graphene layers, produced by HAPG cleavage, were thinner and contained less defects. This paper shows that HAPG is a very perspective material for nanotechnology applications. This research was supported by Rusnano (agreement MSU-06/1), the Ministry of education and science of Russia (state contract 16.512.11.2265), FP7 grant No 257511 (EU-RU.NET) and a program ³3DUWLFLSDQW RI MXQLRU UHVHDUFK DQG LQQRYDWLRQ FRPSHWLWLRQ´ References [1] Anklamm L., Pagels M., Legal H., Stiel H., Malzer W., Müller M., Beckhoff B., Grigorieva I., Antonov A., Kanngießer B., J.Appl.Cryst., 42 (2009) 572-579. [2] Sinitsyna O.V., Yaminsky I.V., Russian Chemical Reviews, 75(1) (2006) 23-30.
Figures
Figure 1. The surface of HAPG after cleavage. AFM image.
Figure 2. A graphene, prepared by HAPG cleavage. Optical image.
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In-situ Raman study on CVD-grown graphene microbridge under high current density
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Jangyup Son , Minkyung Choi , Sangjin Kim , Sangho Lee , Sukang Bae , Byunghee Hong *, 2 1 In-Sang Yang *, and Jongill Hong * 1
Materials Science and Engineering, Yonsei University, Seoul 120-749, Korea 2 Department of Physics, Ewha Womans University, Seoul 120-750, Korea 3 SKKU Advanced Institute of Nanotechnology, SungKyunKwan University, Suwon 440-746, Korea 4 Department of Chemistry, Seoul National University, Seoul 151-742, Korea hong.jongill@yonsei.ac.kr, yang@ewha.ac.kr, byunghee@snu.ac.kr In-situ Raman spectroscopy was performed on microbridges of single-layer graphene (SLG) and 8 2 double-layer graphene (DLG) under electrical current density up to ~10 A/cm at room temperature in air. The graphenes were prepared by chemical vapor deposition (CVD) in high vacuum and the microbridges (5 m Ă&#x2014; 80 m) were fabricated by electron beam lithography and etching with O2 plasma. The 2D phonon peak shifts to lower frequencies as the current density increases through the graphene microbridges, as well known by previous studies[1, 2]. The peak normally returns back to its original 7 2 values upon cooling, when the current density is lower than 10 A/cm . However, beyond the current 8 2 density of 0.6 Ă&#x2014; 10 A/cm , we find that the 2D and G peaks do not restore fully back to their initial values after switching off the current. The Raman peaks are found to be at higher frequencies than the initial values for both SLG and DLG microbridges. The up-shift of the 2D peaks, after switching off the electrical current, is believed to be due to p-doping of the graphene samples in air. Our findings suggest that the doping of graphene can be dependent on the current density.
References [1] I. Calizo et al., Nano Lett., 7 (2007) 2645. [2] I. Calizo et al., Appl. Phys. Lett., 91 (2007) 071913. Figures
FIG. 1. (a) A schematic diagram of the experimental setup for in-situ Raman measurements under electrical current flow. (b) Optical microscopy images and Raman spectra of single- and double-layer graphene microbridges with a 2 dimension of 5 x 80 m .
FIG. 2. Evolution of Raman spectra of graphene as a function of the applied current. Raman spectra of (a) SLG under current from 0 to 2600 A, and (b) DLG under current from 0 and 4000 A. The blue lines are the Raman spectra of the graphene at the initial state (zero current), the black lines under the electrical current and the red lines off the current. Note that the peaks of the red spectra are not fully restored to those of the blue spectra. The blue dotted lines are the guides for the G and 2D peaks of the graphene at the initial state.
FIG. 3. (a) The frequency shifts of the Raman G and 2D peaks ǝȌG, 2D) of SLG and DLG at various levels of applied current density on and off. (b) The down-shifts of Raman frequency correspond to Joule heating of the graphene microbridges under current. (c) The up-shifts of Raman frequency correspond to doping in air after the applied current is switched off.
Fabrication and Applications of Graphene in Loughborough University Mo Song, Jie Jin, Dongyu Cai, Yue Lin, Xiao Wang and Rehman Rafiq Department of Materials, Loughborough University, Loughborough LE11 3TU, United Kingdom m.song@lboro.ac.uk Graphene has recently been the subject of much interest internationally because it has unique electronic and optical properties, and therefore is being considered for applications in electronics, electrochromic devices and transparent conducting electrodes. The thermal conductivity of graphene is very high, and therefore potential as thermal management materials. It is also one of the strongest materials known and is currently being explored for its possibility as a reinforcement in polymeric matrices to make super strong composite materials. However, in order to realize the full potential of this material, there needs to be a cultural change so that routes from the test tube to the industrial plant are considered. In order to achieve this challenge, an integrated research approach following graphene from its production to processing and applications has being carried out in Loughborough University. The being carried out research includes the following parts.
1) Fabrication of graphene and functionalized graphene Graphene and functionalized graphene were fabricated from graphite based on top down routes for applications as energy storage, surface coatings, thermal management and composites. For realization of these applications, massive mass graphene and functionalized graphene are needed. Although lots of researches on graphene based electrodes have been done, up to now there are not viable methods to produce graphene based material for electrode with controlled quality in large scale and in massive quantity. New production methods for production of graphene in massive quantity must be developed. With the aid of dispersion agents and ultrasonication, a new physical method for fabrication of graphene and a new chemical method for production of functionalized graphene from graphite have been developed in our laboratory. Figures 1 and 2 show the TEM images of graphene and functionalized graphene produced based on the two methods, respectively.
2) Graphene-based hybrid materials for energy storage Recently, energy storage becomes a significant research domain in both civil and martial applications. Environmental friendly, portable, cheap, safe and high efficient storage is more preponderant than conventional ones. Graphene has emerged as an alternative energy storage material with superior properties, such as low weight, chemically, inert and low price. The surface area of graphene is about 2
2630 m /g, which is hugely favourable for energy storage applications. A graphene hybrid material has been developed as energy storage materials. It was found that the square resistance reduced to 10
-4
-1
Vsq when the percentage of graphene reached ca. 40 wt % in a graphene hybrid thin film [1]. The maximal specific capacitance was observed to be 114 F/g in a graphene/carbon nanotube (CNT) hybrid film. Figures 3 shows the specific capacitance of the hybrid graphene/CNT hybrid film. A theory [2] has been developed on geometrically enhanced extraordinary magnetoresistance in hybrids. According to the theory, it was noted such nanostructured graphene/CNT thin film [1] could be suitable for development of new structural materials with extraordinary magneto-resistance. The hybrid film as magnetic sensors is being develop.
3) Graphene/polymer composites for aerospace, automotive, construction and packing applications
High potential of functionalized graphene (FG) for reinforcing polymers has been recognized. The incorporation of graphene can toughen polymers such as nylon [3] and epoxy (Figure 4), reduce the permeation of water and gas (Figures 5 and 6) and increase the surface scratch resistance [4] significantly. In the communication, about research progress will be introduced in details References [1] D Cai, M Song and C Xu. Advanced Materials, 20, 1706 (2008) [2] F.V.Kusmartsev, M.B.Sobnack and Geim A. K., CMT, 16, 325 (2001) [3] R Rafig, D Cai, J Jin and M Song, Carbon, 48, 4309 (2010) [4] D Cai, C Yusof and M Song, Nanotechnology, 20, 085712 (2009)
Figure 1:TEM images of graphene made by the physical method
Figure 2: by the chemical method
2.6
Epoxy
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Figure 4: K1c with wt% FG
Permeaction of O2 (cc/m2/day)
Permeation of water (gm-ml/m2.day)
Figure 3: Capacitance with wt% graphene
27
0.2
Filler content (wt%)
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Figure 5: Permeation of water with wt% FG
Nylon
28 26 24 22 20 18
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Figure 6: Permeation of oxygen with wt% FG
Synthesis and characterization of graphene oxyfluoride f
a
b
cd
e
Giorgio SPeranza , Stefano Borini , Matteo Bruna , Barbara Massessi , Cristina Cassiago , Alfio d d f Battiato , Ettore Vittone and Luigi Crema a
Nokia Research Center, 21 JJ Thomson Av., CB3 0FA, Cambridge (UK) Center for Advanced Photonics and Electronics, 9 JJ Thompson Avenue, CB3 0FA, Cambridge(UK) c Thermodynamics Division, INRIM, Strada delle Cacce 91, 10135, Torino, Italy. d Physics Department, NIS excellence centre and CNISM, University of Torino, via P. Giuria 1, 10125, Torino, Italy. e Electromagnetics Division, INRIM, Strada delle Cacce 91, 10135, Torino, Italy. f FBK-IRST, Sommarive str. 18, 38123, Trento, Italy. speranza@fbk.eu b
Chemical functionalization of graphene is used to thinly modify its electrical and chemical properties. In particular, covalent bonding to halogen chemical elements leads to formation of a band gap useful for the realization of electronic devices such as sensors. In this work we describe an easy method to realize an oxyfluorination of graphene layers. It is based on the electrochemical intercalation of graphite, that could be used for adding various functional groups to the graphene lattice. The system was fully characterized in terms of chemical composition, structural and electrical properties. In particular Raman spectroscopy was used to discriminate between single and multi-layered systems. The electrical properties revealed hopping based conductivity and metallic like at low and high carrier concentration respectively. Moreover it was observed a colossal negative magneto-resistance which makes this novel material suitable for both fundamental research and graphene-based applications.
Figures
Figure 1, 2 and 3. XPS analysis of graphene oxyfluoride: C1s, O1s and F1s core lines showing the chemical bonds formed
New Electric Conductive Polymeric Nanocomposites Based on Graphene 1
1
1
1
2
Zdeno Spitalsky , Marketa Ilcikova , Jan Kratochvila , Igor Krupa , Manuel Pedro Graca , Luis Cadillon 2 Costa 1
Polymer Institute, Slovak academy of Sciences, Bratislava, Slovakia Physics Department and I3N, University of Aveiro, Aveiro, Portugal zdeno.spitalsky@savba.sk
2
The unusual mechanical and electronic properties of graphene make it a promising candidate for future devices. While these applications are a focus of further investigations, there are some areas where graphene can be used straightway. Graphene has attracted increasing attention for optoelectronic devices, super-capacitors, gas, pH, chemical, strain and bio-sensors, transparent films for liquid crystal devices, biodevices, DNA transistor, and nanocomposite applications. Proposed applications have enhanced properties comparing to carbon nanotube materials [1]. Here we report the results on polymeric nanocomposites based on the elastomeric matrix, namely styrene-isoprene-styrene copolymer and expanded graphite. As a source of graphitic nanofiller, expanded graphite having an average particle size of 50 microns was used. This dispersion was firstly prepared by sonication, followed by mixing with polymer solution and the thin films were casted onto a Teflon array. Thin films having a concentration of nanofiller up to 10 wt.%, with an average thickness around 400 microns, were prepared and tested. Dispergation of nanofiller within the polymeric matrix as well as the effect of exfoliation of expanded graphite into the individual graphene layers was investigated by scanning electron microscopy. The prepared nanocomposites shown the percolation threshold around 2 wt% of the filler. The increase in the storage modulus up to 80% in the glass state and up to 500% in the rubber state was observed by dynamic-mechanical analysis. Broadband dielectric spectroscopy in the range between 40 Hz up to 2.7 GHz was used to characterize the relaxation of polymer chains in the presence of graphene particles. Acknowledgement This work was supported by bilateral project SK-PT 0021-10, ORITUPOCO which is partly funded by the European Commission under contract no.FP7-ERG-213085, NMT-(5$1(7 ³$3*5$3+(/´ and by project VEGA 2/0119/12. References [1] Z. Spitalsky, M. Danko, J. Mosnacek, Current Organic Chemistry, 15 (2011) 1133-1155.
3D CVDGraphene Material Production Scale-up Process using Ni powders Karlheinz Strobl, Mathieu R. Monville, Subarna Banerjee, Shihsheng Chang CVD Equipment Corporation, 1860 Smithtown Ave, Ronkonkoma, NY 11779, USA kstrobl@cvdequipment.com 2
Graphene, a 2D sp -hybridized carbon sheet with one-atom thickness, has attracted increasing attention in recent years because of its unique structure and special properties. Recently several research publications have described the synthesis of 3D CVD graphene continuous networks
1,2
grown on Ni
foam substrates and their utilization for various novel products and application developments such as 3
4
5
high sensitive gas sensor , electrode or electrode additive for solar cells and ultra capacitors.
The commercially available Ni foam substrate which is used as a sacrificial template for CVD graphene 3
growth in these publications is typically 1 to 2 mm thick, has a density of ~0.3 g/cm and a surface area 2
2
3
in the order of 1 m /g. This results in the growth of around 0.3 m of graphene per cm of Ni foam used. However, higher density 3D graphene material is desired in applications (sensors, batteries, ultracapacitors, etc.) where more graphene material per mass/volume of active material is required to achieve better performance. In addition, one way to achieve higher yield per batch in a coating process is to use particles instead of flat substrates to exploit the increased surface area available for CVD. This path was already explored to achieve bulk growth of high-quality mono- to few-layer graphene sheets on nickel particles.
6
We improved upon these prior concepts to produce 10-100 higher batch quantity of 3D
CVD*UDSKHQH material by starting with a micron size filamentary Ni powder (Fig.1) and filling it into multiple, specially designed porous molds that allow the precursor gas to enter the Ni powder easily. This resulted in highly porous sintered Ni pellets (5mm tall, 16mm in diameter, Fig.2), uniformly covered with a graphene skin, as can be observed on the corresponding cross-sectional SEM images (Fig.3). Fig.4 shows the free standing 3D graphene network after Ni is etched away. This novel way to synthesize graphene on 3D interconnected Ni sintered powder allows to create D
macroscopic custom 3D structures of CVD3 *UDSKHQH which can be further customized by using other 3D shapes and sizes of molds and other types (spherical instead of filamentary) and sizes (nm to µm) of Ni powders. The CVD graphene batch yield can thus be 10-100 times improved (larger quantities and/or denser 3D CVD graphene can be achieved per oven volume) and its cost dramatically reduced. References [1] Chen Z., Ren W., Gao L., Liu B., Pei S., Cheng H-M., Nature Materials 10 (2011), 424-428. [2] Monville M.R., Strobl K., Stolyarov D., Polyakova E., MRS Fall (2011), Poster AA5.15. [3] Yavari F., Chen Z., Abhay V.T., Ren W., Cheng H-M., Koratkar N., Scientific Reports 1 (2011), #166. [4] Bi H., Huang F., Liang J., Tang Y., Lü X., Xie X., Jiang M., J. Mater. Chem. 21 (2011) 17366-17370. [5] Cao X., Shi Y., Lu G., Huang X., Yan Q., Zhang Q., Zhang H., Small 7 (2011), 3163-3168. [6] Chen Z., Ren W., Liu B., Gao L., Pei S., Wu Z-S, Zhao J., Cheng H-M., Carbon 48 (2010) 3543.
Figures
Fig.1: SEM image of the µm size filamentary Ni powder used as the starting material
Fig.2: Image of the resulting sintered Ni pellets D coated with CVD3 Graphene
D
Fig.3: SEM images of multilayer CVD3 *UDSKHQH covering the sintered Ni cellular material substrate made from Ni powder.
D
Fig.4: SEM image of a 3D multilayer CVD3 *UDSKHQH carbon network obtained after removal of the sintered Ni structure.
Graphene Epitaxy by Chemical Vapor Deposition on SiC W.Strupinski1, K.Grodecki1,2, R.Bozek2, A.Wysmolek2, R.Stepniewski2 and J.M.Baranowski1,2 1 2
Institute of Electronic Materials Technology, Wolczynska 133, 01-919 Warsaw, Poland Faculty of Physics, University of Warsaw, Hoza 69, 00-681 Warsaw, Poland E-mail: wlodek.strupinski@itme.edu.pl
Graphene deposited on a SiC has great potential for electronics applications, however, a major factor hindering the development of technology for the large-scale production of graphenebased nano-electronic devices is the lack of access to high-quality uniform graphene layers grown on large SiC substrates. In this paper, we report the CVD of epitaxial graphene (CVD-EG) on SiC substrates using propane gas as the carbon precursor. Graphene layers were grown using a commercially available horizontal CVD hot-wall reactor (Aixtron VP508), which is inductively heated with an rf generator. The epitaxial CVD of graphene relies critically on the creation of dynamic flow conditions in the reactor that simultaneously stop Si sublimation and enable the mass transport of propane to the SiC substrate. While protecting against Si sublimation, C deposition was enabled with one monolayer resolution by taking advantage of the high efficiency of kinetic processes at high T and low P. Additionally, the formation of FLG is possible on the Si-face SiC(0001) which, in comparison to max 2-3 ML of S-EG, creates greater research opportunities. The proposed method permits the growth rate of graphene on the C-face of SiC(000-1) to be considerably lowered enabling the growth of 1ML, which is extremely difficult in the case of S-EG. Our approach also enables precise growth rate control by adjusting the mass transport of the carbon precursor in a similar way to the method used in MOCVD/CVD, as well as the passivation of the SiC substrate by any substances prior to graphene growth. Moreover, one can tune the reactor conditions to grow both CVD-EG and S-EG in the same system. To provide information at the atomic scale, samples were characterized by scanning tunneling microscopy (STM), micro-Raman spectroscopy and transmission electron microscopy (TEM). The thickness of the graphene films were estimated by ellipsometry, the position of the V and S electronic energy bands were evaluated by angle-resolved photo-emission spectroscopy (ARPES). The transport parameters of the graphene samples were measured with the van der Pauw method at room temperature. The electron density in 1-2 ML graphene films was typically 8-10x1012 cm-2, with a macroscopically averaged electron mobility inferred from Hall voltage in the range 3000-3200 cm2/Vs, demonstrating the high electronic quality of the CVD-EG layers on the wafer scale. The micro Raman maps have been created with 3mm light spot using 530 points measured on 2,3 x 2,3 mm2 area in the center of the sample. Histograms reveal that CVD growth of graphene produces much less strained layers in comparison with S-EG. CVD epitaxial graphene has been also studied by LEED and LEEM methods. The approach proposed here offers numerous potential benefits including the application of well-developed commercial epi-systems for SiC epitaxy, a precise growth rate control by adjusting the mass transport of the carbon precursor in a similar way to the method used in MOCVD/CVD, doping of graphene, as well as the passivation of the SiC substrate by any substances prior to graphene growth.
&RQWUROOLQJ WKH &KLUDOLW\ RI *UDSKHQHÂśV HGJHV using Polarization Selective Laser Annealing 1
2
3
1
Milan Begliarbekov , Onejae Sul , , Ken-Ichi Sasaki , Eui-Hyeok Yang , Stephan Strauf
1
1
Stevens Institute of Technology, Castle Point on Hudson, Hoboken, NJ, U. S. A. Korea Advanced Institute of Science and Technology, Daehakno 291, Guseong-dong, Yuseong-gu, Daejeon, Republic of Korea 3 NTT Basic Research Laboratories, Nippon Telegraph and Telephone Corporation, Atsugi, Japan
2
ojsul@kaist.ac.kr
1
The electronic and optical properties of graphene strongly depend on the chirality of its edges . While zigzag edges are metallic, armchair-terminated edges are semiconducting and are thus desired for numerous photonic and electronic applications. However, conventional fabrication procedures favor the formation of the zigzag edge. Here we show that impure armchair edges may be purified post2
fabrication by using polarization-selective laser annealing . This technique was used to purify the edges of 30 nm wide graphene nanoribbon transistors. Transport measurements of optically annealed graphene nanoribbons (GNRs) reveal a 50% increase of the GNR energy gap after annealing, FRQVLVWHQW ZLWK DQ LQFUHDVHG SHUFHQWDJH RI DUPFKDLU VHJPHQWV )XUWKHUPRUH Č?-Raman spectroscopy reveals a greater armchair edge purity post annealing. These results suggest that edge chirality of graphene devices can be optically purified post fabrication, thereby enabling the realization of chiral graphene nanoribbons and heterostructures. References: 1. M. Begliarbekov, O. Sul, S. Kalliakos, E. H. Yang, S. Strauf, "Determination of Edge Purity in Bilayer Graphene Using micro-Raman Spectroscopy", Appl. Phys. Lett. 97, 031908 (2010). 2. M. Begliarbekov, K. Sasaki, O. Sul, EH Yang, and S. Strauf, "Optical control of edge chirality in graphene", Nano Letters 11, 4874 (2011).
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Reversible Formation and Hydrogenation of Deuterium-Intercalated Quasi-Free-Standing Graphene on 6H-SiC(0001) J.-M. Themlin1, F.C. Bocquet1, R. Bisson2, J.-M. Layet2, and T. Angot2 1
: IM2NP, UMR CNRS 7334, Aix-Marseille Univ., 2 : PIIM, UMR CNRS, Aix-Marseille Univ., Centre scientifique de St-Jérôme, Marseille, France jean-marc.themlin@im2np.fr
Using LEED and High Resolution Electron Energy Loss Spectroscopy (HREELS), we have studied two reversible hydrogenation processes of graphene/6H-SiC. We confirm that the high temperature hydrogenation around 700°C of the C-rich (6v3x6v3)R30° of 6H-SiC leads to a quasi free-standing monolayer graphene (QFMLG) through deuterium intercalation [1,2] under the carbonaceous buffer layer (SiC-BL). In contrast to the buffer layer, this QFMLG, stable under a moderate annealing, shows quasi-metallic properties. The decrease of the full-width at half-maximum of the elastic peak in HREELS shows that a subsequent room-temperature hydrogenation of the QFMLG restores an insulating phase with a bandgap dependent on the hydrogen coverage, while the carbon honeycomb structure remains intact.
The insulating character of the hydrogenated QFMLG allows to probe the vibrational properties of Si-H or Si-D species under the QFMLG. While no abstraction could be observed during hydrogenation (deuteration) of a D-intercalated (resp. H-intercalated) QFMLG at room temperature, the high temperature hydrogenation leads to a complete isotope substitution, i.e. SiC-D/QFMLG becomes SiCH/QFMLG [3].
The evolution of the HREELS elastic peak FWHM of SiC-D/QFMLG-H upon thermal annealing show marked changes which reveal the successive desorption of hydrogen chemisorbed on grapheme followed by the desorption of the intercalated deuterium, ending in a SiC-BL, and which can be used as a simple fingerprint to identify the nature of the hydrogenated system. These reversible hydrogenation processes appear promising for the practical realization of useful graphene-based devices.
References [1] C.Riedl et al., Phys.Rev.Lett.103 (2009) 246804 [2] S.Watcharinyanon et al.,Surf.Sci. 605 (2011) 1662 [3] F.C. Bocquet, R. Bisson, J.-M. Themlin, J.-M. Layet, and T. Angot, submitted to Phys.Rev.B (2012)
Figure 1 : HREEL spectra of SiC-BL (red), SiC-D/QFMLG (blue), and SiC-D/QFMLG:H (green) samples.
Figure 2 : HREEL spectra of deuterated H-intercalated SiC-H/QFMLG:D (black, and green â&#x20AC;&#x201C; x50), and hydrogenated D-intercalated SiC-D/QFMLG:H (red) samples.
Electron-beam-induced direct etching of Graphene 1,2
1,3
3,4
Cornelius Thiele , Alexandre Felten , Cinzia Casiraghi , Hilbert v. Löhneysen
2,5,6
, Ralph Krupke
1,2,7
1
Institut für Nanotechnologie, Karlsruhe Institute of Technology, 76021 Karlsruhe, Germany 2 DFG Center for Functional Nanostructures (CFN), 76028 Karlsruhe, Germany 3 Physics Department, Free University, 14195 Berlin, Germany 4 School of Chemistry and Photon Science Institute, University of Manchester, United Kingdom 5 Physikalisches Institut, Karlsruhe Institute of Technology, 76128 Karlsruhe, Germany 6 Institut für Festkörperphysik, Karlsruhe Institute of Technology, 76021 Karlsruhe, Germany 7 Institut für Materialwissenschaft, Technische Universität Darmstadt, 64287 Darmstadt, Germany Cornelius.Thiele@kit.edu Direct (maskless) lithography on graphene has so far been demonstrated using the high-energy electrons of a transmission electron microscope [1], helium ions of a scanning helium microscope [2] and recently also neon ions [3]. Scanning probe methods using an AFM or STM etch electrochemically [4], while the former methods rely on physical sputtering. Here we demonstrate direct lithography on single- and bilayer graphene sheets using a scanning electron microscope with a gas injection system. The injection of oxygen gas into the chamber during scanning leads to the formation of reactive species at the focal point of the primary beam. These species then locally etch graphene. The technique has been termed electron-beam-induced etching/oxidation (EBIO/EBIE) and has been shown to work on other carbon-based materials [5-7]. Voltage-contrast imaging techniques are used in conjunction with finite-element simulations of the electrostatics to the explain the observed secondary electron intensities and correlate them to the etch profile. References [1] M. D. Fischbein, M. Drndic, Appl. Phys. Lett, 93 (2008), 113107 [2] D. Bell et al., Nanotechnology 20 (2009), 455301 [3] D. Winston et al., Nano Letters 11 (2011), 4343 [4] Tapaszto et al., Nat. Nano. 3 (2008), 397 [5] Yuzvinsky et al., Appl. Phys. Lett. 86 (2005), 053109 [6] Spinney et al., Nanotechnology 21 (2009), 375301 [7] Thiele et al., Appl. Phys. Lett. 99 (2011), 173105
Excitations of bilayer graphene in the quantum Hall regime 1
2
3
&VDED 7Ä&#x2DC;NH , Vladimir I. Fal'ko , Judit SĂĄri 1
BME-07$ ([RWLF 4XDQWXP 3KDVHV ³/HQGXOHW´ 5HVHDUFK *URXS Budapest Univ. of Technology and Economics, Institute of Physics, H-1111 Budapest, Hungary tcsaba@phy.bme.hu 2
Department of Physics, Lancaster University, Lancaster, LA1 4YB, United Kingdom 3
Institute of Physics, University of PĂŠcs, H-7624 PĂŠcs, Hungary
We study the magnetoexcitons of the quantum Hall states, including those associated with spontaneous symmetry breaking, in bilayer graphene in a strong perpendicular magnetic and electric field. At zero filling factor [1], a perpendicular electric field may cause a transition between a spin-polarized quantum Hall ferromagnet and a layer polarized one. We identify a long wave length instability in both states, and argue that there is an intermediate range of the electric field where a gapless phase interpolates between the incompressible quantum Hall states. At generic integer filling factor [2], we analyze the subtle wave vector dependent many-body contributions of magnetoexcitons. These are finite even in the long wavelength limit, unlike in most conventional two-dimensional systems. We show that the mixing of different Landau level transitions significantly renormalizes these modes. Further, we argue that the mean-field theory of magnetoexcitons have limitations for the study of intra-Landau level excitations of zero-gap semiconductors. References [1] &VDED 7Ä&#x2DC;NH DQG 9ODGLPLU , )DO NR 3K\V 5HY % 83 (2011), 115455. > @ -XGLW 6iUL DQG &VDED 7Ä&#x2DC;NH PDQXVFULSW LQ SUHSDUDWLRQ
Experimental study of nucleation and growth mechanisms of graphene synthesized by Low Pressure Chemical Vapor Deposition on copper foil
1
2
1
2,3
Pierre Trinsoutrot , Caroline Rabot , Hugues Vergnes , Alexandru Delamoreanu , 2 1 Aziz Zenasni , Brigitte Caussat * 1
Université de Toulouse, Laboratoire de Génie Chimique, ENSIACET/INP Toulouse/ UMR CNRS 5503, 4 allée Émile Monso, BP 44362, 31432 Toulouse Cedex 4, France. 2 CEA, LETI, MINATEC Campus, 17 rue des Martyrs, 38054 GRENOBLE Cedex 9, France. 3 UJF-Grenoble 1 / CNRS / CEA LTM, 17 rue des Martyrs, 38054 GRENOBLE Cedex 9, France. * Brigitte.Caussat@ensiacet.fr During the past 40 years, the fields of micro-electronic, energy and communication devices have experienced an unbelievable evolution. To continue these progresses, the development of multifunctional materials presenting a broad range of properties such as high electronic and thermal conductivities, high transparency and good mechanical properties is needed. Graphene, a hexagonal arrangement of carbon atoms forming a one-atom thick planar sheet could match these demands. Several methods can be used for graphene synthesis, even though Chemical Vapor Deposition (CVD) on catalytic surfaces is foreseen to be the most compatible one with industrial -1 -1
requirements. Indeed, CVD graphene with an electronic conductivity of 7350 cm²V s , an electrical 1
resistance of 30 Ω/sq and a transparency of 90% has already been obtained. However, these values are still far from the theoretical ones announced by physicists, because graphene grows as randomly oriented domains in which scattering at the boundaries leads to lower physical properties. The CVD formation of graphene on Cu substrates has long been considered to be surface-mediated and selflimiting due to the very low carbon solubility in Cu, thus leading to single layers formation. However, numerous studies in 2011 have shown that this is true only in a small window of deposition conditions, 2
especially for methane partial pressure . As a consequence, the control of graphene thickness and crystalline uniformity on large surface areas still remains elusive and needs a better understanding of the mechanisms of graphene nucleation and growth. In this framework, the present study consists in synthesizing graphene on copper foils (25 µm thick, 99,999% Alfa Aesar) by CVD from methane diluted in hydrogen and argon at 0.5 Torr of total pressure. The operating temperature was fixed at 1000°C. Scanning electron microscopy (SEM), optical microscopy and Raman spectroscopy measurements were carried out to investigate the quality and extend of graphene sheets. First, the influence of the methane partial pressure PCH4 was investigated. During these experiments, as shown in Fig.1, for all tested PCH4, gradients in size, shape and density of graphene flakes were evidenced on the parts of the samples placed in the substrate holder slot. For the two lowest positions in the slot, graphene nucleation increases with the methane 2
partial pressure, as found by Wu et al. ; the preferential growth of multi lobes flakes observed at the lowest methane pressure is lost at higher values. The Raman spectra (not shown) indicated that lowering the CH4 partial pressure allows to decrease the defects in the graphene (D band), but the partial pressures tested were too high to obtain single layer graphene (I2D/IG ratios always lower than two). These observed gradients in size, shape and density of graphene flakes led to think to a mechanism of graphene nucleation and growth based on unsaturated species CxHy. These radicalar
species are probably formed by the catalytic cracking of methane and hydrogen on copper, as 3
proposed by Vlassiouk et al. To confirm our assumption, we set an enclosure in order to confine the Cu foil; this enclosure could act as a trap for the radicals present near the Cu surface. We performed several runs with two Cu foils (with and without confinement) in the isothermal zone of the reactor. The presence of the enclosure clearly led to less and smaller graphene flakes on the whole surface of the Cu foils. Moreover, Raman spectra (not shown) indicated that using the confinement allowed to decrease the ID/IG ratios, thus the number of defects in graphene, as obtained for lower methane partial pressures.
Figure 2: SEM image of multi-layers graphene flakes
For graphene flakes obtained on enclosed copper foils, and only in this case, the presence of additional layers is observed in the middle of the flakes, as shown in Fig.2. According to Vlassiouk et 3
al. , these additional top layers are formed at the beginning of the growth, when the nuclei perimeter is small. Therefore, the consumption of active surface-bound carbon is lower than the supplying, leading to multi-layers. When flakes grow, the amount of produced active carbon decreases due to a smaller area of open catalytic copper, and is mostly consumed by the first layers. In our case, without confinement of the copper foil, the amount of active surface-bound carbon formed is probably so high that complete multi-layers graphene flakes are formed.
References
ȏͳȐ Sukang Bae, Hyeongkeun Kim, Youngbin Lee, Xiangfan Xu, Jae-Sung Park, Yi Zheng, Jayakumar Balakrishnan, Tian Lei, Hye Ri Kim, Young Il Song, Young-Jin Kim, Kwang S. Kim, Barbaros Özyilmaz, Jong-Hyun Ahn, Byung Hee Hong, Sumio Iijima, Nature Nanotechnol, 5 (2010) 574-578.
ȏʹȐ Wei Wu, Qingkai Yu, Peng Peng, Zhihong Liu, Jiming Bao, Shin-Shem Pei, Nanotechnology, 23 (2012) 035603 (8pp). [3] Ivan Vlassiouk, Murari Regmi, Pasquale Fulvio, Sheng Dai, Panos Datskos, Gyula Eres, Sergei Smirnov, ACSNANO, 5(7) (2011) 6069-6076.
Strain analysis of CVD graphene by in situ Raman spectroscopy Gerald V. Troppenz, Marc A. Gluba, Jörg Rappich, Norbert H. Nickel Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Institut für Si-Photovoltaik, Kekuléstr. 5, 12489 Berlin, Germany gerald.troppenz@helmholtz-berlin.de High quality large scale graphene was grown on copper foil via chemical vapor deposition (CVD). Because of its unique optoelectric properties, graphene is suitable as a transparent contact to silicon based semiconducting devices. Since it can not be deposited on silicon directly, an adequate transfer process is required. The conductivity of the resulting layer depends strongly on strain, which can be characterized by Raman spectroscopy. It is shown that the application of transfer support materials, such as polymers and foils, causes tensile strain in graphene. In situ Raman spectroscopy is a powerful tool to investigate strain in graphene during the transfer from its growth substrate to other materials (e.g. SiO2 or Si). We employed this technique to monitor the strain of graphene at the polymer/graphene/copper interface via the 2D band vibrational frequency during removal of the substrate. Our results show that the strain relaxes due to the etch-back of the copper foil. As a reference we investigate the strain in graphene by mechanical stretching experiments.
These experiments aim to evaluate different transfer methods and -materials for graphene from copper to other substrates. Figures
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Graphite Oxide Reduction to Graphene Applying Ultrashort Laser Pulses 1
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Romualdas Trusovas , *HGLPLQDV 5DĂžLXNDLWLV , Jurgis Barkauskas 5HJLQD 0DĂĽHLNLHQĆĄ
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Center for Physical Sciences and Technology, Savanoriu 231, LT-02300 Vilnius, Lithuania romualdas.trusovas@ar.fi.lt 2 Faculty of Chemistry, Vilnius University, Naugarduko 24, LT-03225 Vilnius, Lithuania 3 Institute of Chemistry, Center for Physical Sciences and Technology, A. Gostauto 9, LT-01108 Vilnius, Lithuania Graphene can be produced by mechanical exfoliation, chemical exfoliation, chemical synthesis, pyrolysis, epitaxial growth, CVD and other methods [1]. There is still need in novel graphene production methods which are suitable for mass production and can avoid usage of harmful chemical materials. Chemical, thermal and light reduction methods are implied for graphite oxide (GO) reduction. Recently there were described several methods of producing graphene based on laser-induced GO reduction [25]. Samples prepared, using modified Hummers-Otieman method were used in our experiments. Congo red (CR) dye was used as an additive. The GO coatings were prepared on the polycarbonate membrane filters via slow filtration into alkaline media. The samples were treated using a picosecond laser (Atlantic, 10 ps, 100 kHz, Ekspla) with the scanner setup working at 1064 nm wavelength. During the tests, the average laser power and scanning speed was varied. Experiments were conducted in air, nitrogen and argon atmospheres. Laser treated samples were investigated with scanning electron microscope and Raman spectroscopy (Fig. 1 a,b). Morphology modifications appeared in laser irradiated areas. Raman spectra revealed formation of graphene by emerging of the 2D-line. Ratios of the Raman line intensities (ID/IG, I2D/IG) were found to be dependent on the CR concentration. Variation in the intensity ratios related to quality of the resulting graphene film indicates importance of the CR concentration on linkage of graphene sheets together and formation larger blocks with increase in stacking order. Simulation of transient temperature inside the GO film during and after pulsed laser irradiation was performed using COMSOL Multiphysics software. Simulation revealed that the 0
temperature above 1000 C, which is necessary to remove most carboxyl, hydroxyl and epoxy groups from GO [6] was achieved at the optimal laser beam scanning conditions. Irradiation of GO films with the picosecond laser created significant changes in material properties. We found that in our experiments the figure of merit of GO reduction to graphene was position and width of Raman lines as a function of a product of the pulse energy and the irradiation dose. Results of laser treatment depended on the Congo red dye concentration in GO. Optimal scanning irradiation dose Âą 50 J corresponds to the effective thermal reduction temperature of 1400 K.
References [1] Santanu Das, W. C. Graphene: Synthesis and Applications Nanomaterials and Their Applications(Crc Press Llc, 2011), 27. [2] Gao, W. et al. Nat Nano 6 (2011) 496 [3] Sokolov, D. A., Shepperd, K. R. & Orlando, T. M.s. The Journal of Physical Chemistry Letters 1 (2010) 2633 [4] Zhang, Y. et al. Nano Today 5 (2010) 15 [5] Zhou, Y. et al. Advanced Materials 22 (2010) 67 [6] Huh, S. H. Physics and Applications of Graphene - Experiments (InTech, 2011), 73. Figures 1.4
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A molecular route to 1 nm thick carbon nanomembranes (CNMs) and graphene for functional applications Andrey Turchanin and Armin Gölzhäuser University of Bielefeld, Universitätsstr. 25, 33615 Bielefeld, Germany turchanin@physik.uni-bielefeld.de Our group focuses its activities on the large-scale fabrication and applications of novel 2D carbon materials (graphene, carbon nanomembranes (CNMs) and their hybrid systems). We have developed an original and effective approach to fabricate CNMs and graphene from organic self-assembled monolayers (SAMs). This approach is based on the electron-radiation induced crosslinking of aromatic SAMs and their subsequent annealing. In this process, the SAM is converted into a graphene sheet with well-defined thickness and arbitrary dimensions. Sizes of the sheets are defined by the electron exposure that can be flexibly adjusted from a few nanometres to the macroscopic dimensions. Electric transport data and spectroscopy demonstrate that the conversion into graphene can gradually be adjusted via annealing temperature and it is accompanied by an insulator to metal transition. Thus by simple changing the annealing temperature such properties like electrical conductivity, electron mobility, ambipolar electric field effect as well as optical characteristics can be tuned. By the transfer procedure developed in our group, the fabricated CNM and graphene sheets can flexibly be placed onto arbitrary substrates, e.g., holey substrates where they from suspended membranes with extremely large lateral dimensions. The choice of molecular precursors can be used to tune the properties of CNMs and graphene, and to facilitate their direct preparation on semiconductor and insulator substrates. We are working on implementations of CNMs and graphene in field effect transistors, biosensors, transparent conductive coatings, nano-electro-mechanical systems (NEMS), separation technologies and high resolution electron microscopy (HRTEM).
References [1] A. Turchanin, D. Weber, M. Büenfeld, C. Kisielowski, M. Fistul, K. Efetov, R. Stosch, T. Weimann, J. Mayer, A. Gölzhäuser, ACS Nano 5 (2011) 3896-3904. [2] C.T. Nottbohm, A. Turchanin, A. Beyer, R. Stosch, A. Gölzhäuser, Small 7 (2011) 874-883. [3] D. Rhinow, M. Büenfeld, N.-E. Weber, A. Beyer, A. Gölzhäuser, W. Kühlbrandt, N. Hampp, A. Turchanin, Ultramicroscopy 111 (2011) 342-349. [4] Z. Zheng, C.T. Nottbohm, A. Turchanin, H. Muzik, A. Beyer, M. Heilemann, M. Sauer, A. Gölzhäuser, Angew. Chem. Int. Ed. 49 (2010) 8493-8497. [5] A. Beyer, A. Turchanin, C.T. Nottbohm, N. Mellech, M. Schnietz, A. Gölzhäuser, J. Vac. Sci. Technol. B 26 (2010) C6D6 [6] M. Schnietz, A. Turchanin, C.T. Nottbohm, A. Beyer, H. H. Solak, P. Hinze, T. Weimann, A. Gölzhäuser, Small 5 (2009) 2651-2655. [7] A. Turchanin, A. Beyer, C.T. Nottbohm, X. H. Zhang, R. Stosch, A. Sologubenko, J. Mayer, P. Hinze, T. Weimann, A. Gölzhäuser, Adv. Mater. 21 (2009) 1233-1237. [8] C. T. Nottbohm, A. Turchanin, A. Beyer, A. Gölzhäuser, J. Vac. Sci. Technol. B 27 (2009) 30593062. [9] A. Turchanin, D. Käfer, M. El-Desawy, C. Wöll, G. Witte, A. Gölzhäuser, Langmuir 25 (2009) 7342-7352.
Figures
Fig. 1 Schematic representation of the fabrication of carbon nanomembranes (CNM) and graphene from selfassembled monolayers (SAM). (a) An aromatic SAM (here biphenylthiol) is formed on the target surface. (c) The SAM is cross-linked via electron/photon irradiation to from a 1 nm thick CNM. (c-d) Annealing at high temperatures converts the CNM into graphene [1, 7, 8- 9].
Fig. 2. Engineering and lithography of individual CNMs, graphene sheets and their hybrid stacks. (a) Scheme of the fabrication of monolayer and multilayer structures from a1 nm thick CNMs. (1)-(2) Formation and e-beam crosslinking of a SAM; (3) transfer to an arbitrary surface; (4) assembly of multilayers by repeating this process. (b)(d) Line patterns fabricated on SiO2/Si substrates by transfer, photolithography and plasma etching. Carbonic nanolayers are blueshifted with respect to the substrate. (e) Stacks of 1 to 5 layers on a SiO2/Si substrate [2, 4, 67].
Fig. 3. Free-standing 1 nm thick CNMs. (a) A scanning electron microscope image of a CMN suspended on a metal grid. (b) An optical microscope image of a pattern of 30 nm thick Au dots on a suspended CNM. Such nanomembranes are of high potential for applications in NEMS and HRTEM. (c) Energy filtered TEM of tobacco mosaic virus (TMV) on a CNM; zero-loss image. Due to extreme thinnest of the CNM, the biological imaging is possible [3, 8].
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On the Interaction of Beryllium atoms with Graphene Nanostructures V.G. Chilkuri1 , T. Leininger1 , S. Evangelisti1 , A. Monari2 1
Laboratoire de Chimie et Physique Quantiques, University of Toulouse and CNRS, 118 R.te de Narbonne, Toulouse, F-31062, FRANCE 2
Equipe de Chimie et Biochimie Thoriques, University of Nancy and CNRS, B.P. 70239, Vandoeuvre les Nancy Cedex, F-54506, FRANCE vijay.gopal.c@gmail.com
Linear chains of Beryllium atoms have been shown to be local minima on the Potential Energy Surface of the isolated systems. These structures present, close to their equilibrium geometry, two edge orbitals. These orbitals are located at the two chain extremities, and give rise to two quasi-degenerated electronic states, a Singlet and a Triplet. Although these linear structures are locally stable as isolated chains, they could easily collapse into more compact, and more stable, clusters. One possibility to stabilize such structures could be via the interaction with a surface. In this context, Carbon surfaces (in particular, Graphene or Graphene fragments and Carbon Nanotubes) would be particularly appealing. As a preliminary step toward the study of Beryllium chains placed into Carbon Nanotubes, we performed a theoretical investigation of the interaction of Beryllium atoms with graphene nanostructures. In particular, rings composed of a small number of hexagonal cells (cyclic oligoacenes) have been considered in the present work. References [1] G.L. Bendazzoli, S. Evangelisti, and A. Monari, Theoret. Chem. Acc. 126, 257-263 (2010) [2] A. Monari, S. Evangelisti, T. Leininger, and G.L. Bendazzoli, Chem. Phys. Lett. 496, 306309 (2010). [3] S.-M. Choi and S.-H. Jhi, Phys. Rev. Lett. 101, 266105 (2008).
Figure 1: The Linear Be3 inside the nanotube (8,0)
3ODVPD SURGXFWLRQ DQG IXQFWLRQDOLVDWLRQ RI *UDSKHQH DQG *13ÂśV E\ SODVPD H[IROLDWLRQ Ian Walters Haydale Ltd ECM2 Port Talbot Wales SA132EZ ian@haydale.com
Graphene Graphene is known as the single-layer hexagonal form of carbon, corresponding to a single layer of the graphite structure but with properties exceedLQJ JUDSKLWHœV EHFDXVH RI WKH DEVHQFH RI neighboring layers. Graphene layers can be made to quite large sizes by careful mechanical ³H[IROLDWLRQ´ IURP JUDSKLWH UHGXFWLRQ RI H[IROLDWHG JUDSKHQH R[LGH RU E\ HSLWD[LDO JURZWK RQ VXEVWUDWHV of other materials. However the known methods are laborious and expensive. Haydale Plasma processing introduces a valuable method of obtaining a material containing a significant proportion of useful graphene flakes or particles by a more convenient, environmentally friendly and economical method. Plasma Haydale plasma treatment of particles of inorganic or mineral particulate material in which some or all of the particles comprise, consist of agglomerated, tangled or mutually cohering subsidiary or component particles or structures such as nanoparticles or atomic layers. Embodiments relate to carbon or carbon-containing materials, preferably in which the target component is graphene comprised in graphitic or stacked-graphene bodies. The Haydale plasma method is found to cause substantial and convenient disaggregation exfoliation and if required selective functionalisation that provides for enhanced dispersibility and covalent bonding within a desired matrix of the graphene flakes. The plasma treatment involves strategically positioning electrodes within a vacuum vessel. To control nano particle escape evacuation is provided via a suitable nano filter housing fabricated from materials that withstand plasma-processing conditions to avoid undesirable chemical or physical contamination of the nano product outputs. During the plasma treatment, the application of vacuum is combined and balanced with a feed of gas for plasma formation, so that the treatment atmosphere can be controlled and contaminated or spent treatment gas removed during the process. Ports are incorporated into the treatment vessel t for the injection of reagent or gas for chemical treatment.
Plasma Graphene Production
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Ultrafast Nonlinear Optical Responses of 2D MoS2 Nanosheets 1*
1
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Kangpeng Wang , Jintai Fan , Long Zhang , Mustafa Lotya , Danny Fox , Hongzhou Zhang , Werner 2 2 1* J. Blau , Jonathan N. Coleman , Jun Wang 1
Key Laboratory of Materials for High-Power Laser, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800, China 2 School of Physics and the Centre for Research on Adaptative Nanostructures and Nanodevices (CRANN), Trinity College Dublin, Dublin 2, Ireland wangkangpeng@siom.ac.cn jwang@siom.ac.cn
Research on graphene brings not only a new nanomaterial with excellent physical and chemical properties. Most importantly, the material revolutionized the traditional ideology on nanoscience and nanotechnology and opened up a door to a new two-dimensional (2D) nano-system [1]. Following the same vein on the graphene study, researchers have started the exploration of graphene analogue material comprising stacked atomic or molecular layers [2]. Strong covalent bonds in layer and weak van der Waals interaction between layers allow the graphene analogues forming robust 2D nanostructure. Layered molybdenum disulfide (MoS2) is one of the typical graphene analogues. Owing to the specific 2D confinement of electron motion and absence of interlayer perturbation, the monolayer MoS2 shows dramatic improvements in charge carrier mobility and photoluminescence quantum 2
4
efficiency by factors of 10 and 10 , respectively, in comparison with the bulk counterpart [3,4]. The remarkable physical properties of the layered semiconductor nanomaterial inject new opportunities in the field of photonics and optoelectronics. Whereas the electronic and luminescent properties of 2D MoS2 nanosheets have been generating much research interest, the ultrafast nonlinear optical (NLO) properties remain largely unexplored. We studied for the first time, to the best of our knowledge, the ultrafast NLO property of MoS2 nanosheets in liquidphase dispersions. Employing high-yield exfoliation of MoS2 in the liquid-phase, a series of dispersions with large populations of monolayer and few-layer MoS2 were prepared in N-methyl-pyrrolidone (NMP) [5]. As shown in Fig. 1(a) and (b), it is clearly seen that the micro-scale MoS2 bulk can be effectively exfoliated to 100-nm-scale monolayer and few-layer flakes. UV-Vis-NIR absorption and Raman spectroscopic studies in Fig. 1 (c) and (d) confirmed high quality of the MoS2 nanosheets. The ultrafast NLO properties were investigated using open-aperture Z-scan technique. All experiments were performed using 100 fs pulses at 800 nm from a mode-locked Ti:Sapphire laser and 6 ns pulses at 532 nm from a Q-switched Nd:YAG laser. Under the excitation of 800 nm fs pulses, the MoS 2 nanosheets exhibited strong saturable absorption (see Fig. 1(e)). Referring to the large bandgap difference between the MoS2 bulk (1.29eV, ~960nm) and the MoS2 monolayer (1.90eV, ~650nm), the saturable absorption at 800 nm implies a large number of few-layer MoS2 nanosheets exist in the NMP dispersions. The monolayer MoS2 would possess two-photon-absorption at 800 nm fs excitation. For the ns excitation at 532 nm, the MoS2 nanosheets showed intensity dependent nonlinear extinction behavior - saturable absorption at lower intensity region and induced nonlinear scattering at higher intensity region (see Fig. 1(f)). The variety of the ultrafast NLO responses verifies the 2D MoS2 a huge potential in the development of nanophotonic devices, such as, mode locker [6] and optical limiter [7]. Part of this work has been submitted for publication elsewhere. This work was supported in part by the 100-Talent Program of Chinese Academy of Sciences, the National Natural Science Foundation of China (NSFC, Grant No. 61178007), and Science and Technology Commission of Shanghai Municipality (STCSM Nano Project, Grant No. 11nm0502400).
References [1]. [2]. [3]. [4]. [5]. [6]. [7].
K. Geim, K. S. Novoselov, Nature Materials, 6, (2007) 183. R. Mas-Balleste, C. Gomez-Navarro, J. Gomez-Herrero, F. Zamora, Nanoscale, 3, (2011) 20. B. Radisavljevic, A. Radenovic, J. Brivio, V. Giacometti, A. Kis, Nature Nanotechnology, 6, (2011) 147. K. F. Mak, C. Lee, J. Hone, J. Shan, T. F. Heinz, Physical Review Letters, 105, (2010) 136805. J. N. Coleman, M. Lotya et al., Science, 331, (2011) 568. Z. Sun, T. Hasan, F. Torrisi, D. Popa, G. Privitera, F. Wang, F. Bonaccorso, D. M. Basko, A. C. Ferrari, ACS Nano, 4, (2010) 803. J. Wang, Y. Hernandez, M. Lotya, J. N. Coleman, W. J. Blau, Advanced Materials, 21, (2009) 2430.
Figures
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Fig. 1. Ultrafast nonlinear optical responses of 2D MoS2 nanosheets.
A Robust, Scalable Process for Automated Production of Highly Dispersed Graphene Oxide and its use in transparent conductive coatings Rune Wendelbo and Nils Berner Abalonyx AS, Forskningsveien 1, 0314 Oslo, Norway rw@abalonyx.no A robust, scalable process for the production of graphene oxide has been established based on a modification of the process of Hummers and Offeman [1]. The raw material graphite was chosen based on an extensive screening of different source materials, the process was optimized and automated and scaled to 100 g. Automation involved feeding of reagents, controlling temperature in each step, monitoring and logging of relevant data. In the pilot production phase, the synthesis was repeated 20 times, with excellent reproducibility, to produce a total of about 3 Kg of graphene oxide. A key test of quality is the suspension stability test. Our product forms a stable suspension in water over months, (Figure 1).
The product has been extensively characterized with TEM, XPS, SEM, Raman, XRD, TGA, BET and light scattering. The results indicate an average of about 2 layers per particle, with a substantial fraction of single layers. The raw graphite oxide has about one oxygen per carbon, but this ratio can be tuned to lower ratios by partial thermal or chemical reduction.
Chemically reduced graphene oxide (RGO) has been used to prepare transparent conduction coatings using a high throughput set-up for layer by layer deposition. Suspensions were prepared of RGO and magnesia nano-particles respectively, in 24-well microtiter plates, and a robotic arm was used to dip 24 glasslides forth and back between the two suspensions, washing water, a drop removal station, an oven and stations for on line measurement of conductivity and white light transmission (Fig. 2). Samples on glass slides are shown in Figure 3.
Figure 1. Graphene oxide and reduced graphene oxide. a) dispersions and dry powders. b) stable suspensions of GO. c) Electron diffractogram of single layer. d) freeze dried GO. e) thermally reduced GO 160,000 X and f) thermally reduced GO 80,000 X.
Figure 2. Robotic set-up for layer-by-layer deposition of particles and ions. a) robotic arm with well-plates and oven, b) hot samples withdrawn from oven, c) station for measurement of light transmission, d) drop removal station, e) dipping station detail, and f) station for online four-point measurement of conductivity.
Figure 3. Examples of transparent conducting cotings prepared. Grey samples contain reduced graphene oxide and magnesiaand greenish samples contain Cu-Zn-disulfide.
A further activity has now been started involving precipitation of metal oxides and phosphates on GO and RGO for the use in Li-ion battery electrode coatings.
References [1] Hummers, W.S. and Offeman, R.E., J. Am. Chem. Soc., 1958, 80 (6), pp 1339Âą1339. .
Development of Nano-Composites which include Plasma Functionalized Graphene Nanoplatelets J. Williams, S. Rahatekar, M. Williams ACCIS, Bristol University, Bristol, United Kingdom John.David.Williams@bristol.ac.uk
Commercially available nanomaterials such as Graphene Nanoplatelets and Carbon Nanotubes are chemically inert, and as such are difficult to disperse in various materials such as polymers. Plasma functionalization is a method for improving the compatibility of the nanomaterials with a matrix by adding surface functionalization such as amine, hydroxyl and carboxyl groups. The functionalization can be tailored to match the matrix material, improving the processing behavior and improving the physical properties of the composite material when compared to the untreated nano material. There are also significant rheological differences between nano-carbons of different morphologies, for example Carbon Nanotubes tend to dramatically increase the viscosity of the nanotube resin mix, whilst Graphene Nanoplatelets have a negligible change of the rheological properties of the original matrix.
The real graphene oxide revealed: stripping the oxidative debris from the graphene-like sheets 1
1
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Neil R. Wilson , Priyanka A. Pandey , Joseph J. Lea , Matthew Bates , Ian A. Kinloch , Robert J. 3 2 Young , and Jonathan P. Rourke 1
Department of Physics, University of Warwick, Coventry, CV4 7AL, UK Neil.Wilson@warwick.ac.uk 2 Department of Chemistry, University of Warwick, Coventry, CV4 7AL, UK 3 School of Materials, The University of Manchester, Grosvenor Street, Manchester, M1 7HS, UK Neil.Wilson@warwick.ac.uk Graphene oxide (GO) provides a potential route to large quantities of graphene, is cheap to make in bulk and easy to process. It is also a starting point for further functionalisation to create chemically modified graphenes (CMGs) e.g. for use in composites, light-harvesting, or as sensors. Understanding the chemical and physical structure of GO is a necessary step in its controllable functionalisation for CMGs and potential complete reduction back to graphene. We will provide compelling evidence that GO, as produced by the Hummers' method, is composed of functionalized graphene sheets decorated by strongly bound oxidative debris (also referred to as fulvic acids), which acts as a surfactant to stabilize aqueous GO suspensions. We will also show that the physical and chemical properties of the as-produced GO are strongly influenced by this oxidative debris 1
(OD). This OD-functionalised graphene complex appears to be indefinitely stable in water, but the removal of the OD can simply be effected with a base wash, whereupon the more highly functionalized debris dissolves fully into water, leaving a suspension of functionalized sheets. Careful independent weighing of the graphene oxide after washing, and the removed oxidative debris, shows that roughly a third of the mass of our as-produced graphene oxide can be ascribed to OD. The remaining graphenelike sheets are oxidized, but at a much lower level than current models for GO suggest. The importance of the OD is demonstrated by the change in properties of aGO after its removal: unlike as-produced GO, the resultant base-washed graphene oxide is not easily suspended in water and is conducting. Our results suggest that models for the structure of graphene oxide need revisiting. The oxidative debris non-covalently attached to as-produced GO has important implications for the synthesis and application of CMGs, particularly where direct covalent functionalisation of the graphene lattice is required. References [1]. Rourke, J.P. et al., Angew. Chem. Int. Ed., 50, 3173-3177 (2011) Figure HO2C
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Characterization of multilayer graphene obtained by SiC sublimation on C surface by far infrared magnetospectroscopy and Raman spectroscopy 1)
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1) ,QVWLWXWH RI ([SHULPHQWDO 3K\VLFV )DFXOW\ RI 3K\VLFV 8QLYHUVLW\ RI :DUVDZ +RĪD -681 Warsaw, Poland 2) Institute of Electronic Materials Technology, Wólczynska 133, 01-919 Warsaw, Poland 3) Laboratoire National des Champs MagnÊtiques Intenses, CNRS, BP 166, F-38042 Grenoble Cedex 09, France contact : Andrzej.Witowski@fuw.edu.pl Multilayer graphene obtained on C face of SiC substrates is composed of differently stacked layers [1]. It is not obvious that there are patches of differently stacked multilayer graphene (MLG) on the surface or those layers are stack one on another with diverse orientation. The far infrared magnetospectroscopy combined with micro-Raman (MR) spectroscopy is a perfect tool to study the problem. The observation of almost neutral graphene and highly doped graphene in FIR spectroscopy is well known [2]. Also MIR spectroscopy is helpful showing the existence of bilayers in such systems [3]. We investigated two sets of VDPSOHV ³JURZQ´ RQ + 6L& &-face substrate in the same conditions. The sets differ by the time of the sublimation process. The thickness of MLG was checked by means of optical transmission measurements allowing estimation of number of layers in MLG. The magnetospectroscopy measurements reveal not only the structure typical for free carriers in MLG and strong transition from ground VWDWH WR WKH ILUVW ODQGDX OHYHO ĺ RU - ĺ WUDQVLWLRQ ZLWK FODVVLFDO ¼% GHSHQGHQFH RI SRVLWLRQ HQHUJ\ EXW also weaker structures with linear dependence on B. Part of such lines can be explained in terms of transitions between Landau levels in bilayers. However the most intriguing one (see figure), the satellite of the main graphene line can be hardly explained within this model.
Figure 1. Relative magnetotransmission (T(B)/T(0)) for 4 different samples at 4T obtained in the 965 process. The splitting of the main graphene line at -1 about 620 cm is clearly visible.
In different samples the oscillator strength of the satellite line is almost the same in spite that the main line changes noticeable. The width is also almost two times smaller than the width of main line. We suggest that this is one of the spitted lines in trilayer system (ABA stacking) [4]. The second line is hidden in the JUDSKHQH RQH Äş RU - Äş WUDQVLWLRQ
The idea of observation of the trilayer is additionally supported by existence of week lines linearly dependent on magnetic field, suggesting the transitions between parabolic levels predicted by the theory [4]. The Raman experiments done on the same samples show that in the same spot exist differently oriented layers. Therefore one can conclude that sublimation take place making the dominant $$Âś RULHQWDWLRQ +RZHYHU in certain conditions, parts of the sample with trilayer system are obtained. [1] J Hass et al., J. Phys.: Condens. Matter 20 323202 (2008). [2] M. Orlita and M. Potemski, Semicond. Sci. Technol. 25, 063001 (2010). [3] M. Orlita et el., Phys. Rev. B 83, 125302 (2011). [4] Shengjun Yuan et el., Phys. Rev. B 84, 125455 (2011).
Scanning Tunneling Spectroscopy (STS) studies of Graphene-Au interactions a
I. Wlasny , P. Dabrowski
a, b
a
, Z. Klusek , J. Slawinska e
a, c
M. Wojtoniszak , E. Borowiak-Palen a
d
a
, I. Zasada , W. Kozlowski , e
Division of Physics and Technology of Nanometer Structures, Solid States Physics Department, University of Lodz, Pomorska 149/153, Lodz 90-236, Poland b
Institute of Electronic Materials Technology, Wolczynska 133, Warsaw 01-919, Poland
c
Theoretical Physics Department II, University of Lodz, Pomorska 149/153, Lodz 90-236, Poland
d
Solid States Physics Department, University of Lodz, Pomorska 149/153, Lodz 90-236, Poland e
Centre of Knowledge Based Nanomaterials and Technologies, Institute of Chemical and Environment Engineering, West Pomeranian University of Technology,ul. Pulaskiego 10, 70-322 Szczecin, Poland iwlasny@uni.lodz.pl
Graphene, an allotrope of carbon, has exciting potential for electronic applications. However, properties of graphene can be affected by the substrate it has been deposited on. It is crucial to understand the nanoscale electronic properties of graphene in terms of the electron density of states (LDOS) on different substrates. In case of some metallic substrates (Al, Cu, Ag, Pt and Au) the conical shape of dispersion relation is preserved [1-3]. However, presence of doping effect has been established position of Dirac point (ED) relative to Fermi level (EF) is shifted. It it especially vital in case of Au, which is widely used as contacts in graphene-based devices.
We present scanning tunneling microscopy and spectroscopy (STM/STS) investigations of graphene on Au substrate [4, 5]. Mono-, bi- and tri- graphene layers (MG, BG, TG) were deposited on 8nm Au / 0.5nm / 100 nm SiO2 / Si substrate. This setup allowed for both optical microscopy (OM) identification of graphene layers and measurements of uninterrupted electronic structure of graphene/Au system. Graphene has been identified with OM, Raman Spectroscopy (RS) with Renishaw InVia spectrometer and scanning electron microscopy (SEM) using Vega Tescan microscope. STM/STS experiments
were
conducted
using
VT-STM/AFM
microscope
integrated
with
the
XPS/UPS/AES/LEED/MULTIPROBE P system (Omicron GmbH) in room temperature. Experimental results has been confronted with theoretical predictions.
STS investigations of LDOS prove that graphene on Au substrate is doped with holes. For MG Dirac point is shifted to 0.25 Âą 0.45 eV above Fermi level (Fig. 1b). In case of BG doping is ranging from 0.22 to 0.30 eV. For TG doping value is about 0.10 Âą 0.15 eV. This shows that value of doping decreases with increasing number of graphene layers. CITS maps presented on Fig. 2b show heterogeneity of LDOS between MG, BG and TG regions. In case of MG, shift of Dirac point has been compared with theoretical values obtained using local density approximation and van der Waals density functionals.
This work was financially supported by Polish Ministry of Science and Higher Education in the frame of grant N N202 204737.
References [1] G. Giovannetti, P. A. Khomyakov, G. Brocks, V. M. Karpan, J.van den Brink, J.P. Kelly, Physical Review Letters 101 (2008) 026803. [2] P. A. Khomyakov, G. Giovannetti, P. C. Rusu, G. Brocks, J. van den Brink, P. J. Kelly, Physical Review B 79 (2009) 195425. [3] M. Vanin, J. J. Mortensen, A. K. Kelkkanen, J. M. Garcia-Lastra, K. Thygesen, W. Jacobsen, Physical Review B 81 (2010) 81408. [4] Z. Klusek, P. Dabrowski, P. Kowalczyk, W. Kozlowski, W. Olejniczak, P. Blake, M. Szybowicz, T. Runka, Applied Physics Letters 95 (2009) 113114. [5] J. Slawinska, I. Wlasny, P. Dabrowski, Z. Klusek, I. Zasada, arxiv:1201.5243 (2012) Figures
Fig. 1 a) 450 nm x 450 nm STM topography showing the details of MG and Au border line. b) dI/dV (E, line) map recorded on MG/Au border along the arrow in Fig. 1 a).
Fig. 2 a) 150 nm x 150 nm STM topography of MG/BG/TG region. Numbers denote number of graphene layers. Grey lines represent borders. b) dI/dV map of region presented in a) for 0.08 V
(b)
Novel method of graphite exfoliation towards synthesis of graphene Malgorzata Wojtoniszak, Ewa Mijowska Westpomeranian University of Technology in Szczecin, Pulaskiego 10, 70-322 Szczecin, Poland mwojtoniszak@zut.edu.pl Graphene is attracting an increasing research interest due to its amazing properties which may be applied in many fields, such as electronic devices (supercapacitors, lithium ion batteries), biomedicine (biosensors, drug delivery), and many others. However, there is still a huge need to find a method to synthesize graphene in a large scale. Among many methods, chemical exfoliation of graphite has been considered as the most promising way to large-scale production of graphene, where graphite is oxidized to graphene oxide and next reduced to graphene. The most common way to produce graphene oxide is Hummers method, where graphite is oxidized with KMnO4 and NaNO3 in concentrated H2SO4. In this study, we propose a novel method of graphite exfoliation. Here, graphite is treated with K2CrO4 in the mixture of perchloric and nitric acids to create graphene oxide, which is next reduced with glucose. Furthermore, the obtained materials was sorted according to its size and thickness via density gradient ultracentrifugation (DGU). The materials was characterized with transmission electron microscopy, FT-IR spectroscopy, Raman spectroscopy and XRD.
Gold decorated graphene flakes for large area transparent conductive films W. Wu; T. Hasan; F. Torrisi; M. Zelazny; G. Privitera; F. Bonaccorso; A.C. Ferrari Department of Engineering, University of Cambridge, Cambridge CB3 0FA, UK The scarcity, brittleness, processing limitations and fabrication costs of Indium Tin Oxide (ITO) are crucial limiting factors for the development of flexible displays and electronics requiring flexible, transparent conductors (TCs). Graphene liquid dispersions [1] offer an attractive proposition for roll to roll manufacturing of electronic devices requiring flexible TCEs [2]. We demonstrate an up-scalable gold-decorated graphene based TC. Gold decorated graphene flakes are prepared by refluxing liquidphase exfoliated graphene flakes with nitric acid to introduce carboxylic groups [3], followed by reduction of gold(III) chloride (AuCl3) driven by the addition of sodium hydroxide. The carboxylic groups improve the stability of the dispersion, as well as the wettability and processability of the graphene ink during coating. They also enable the efficient decoration of metal nanoparticles onto the graphene flakes. Reduced gold nanoparticles (AuNPs) homogeneously dispersed on the surface of the modified graphene sheets allow to improve the interfacial contact and achieve low sheet resistance. Large-area TC films (up to A4 size) are prepared with sheet resistancesa Â&#x; VT DQG ! WUDQVPLWWDQFH References [1] Y. Hernandez et al., Nat Nano.3 (2008), 563. [2] F. Bonaccorso et al., Nat Photon. 4 (2010), 611. [3] S. Niyogi et al., J. Am. Chem. Soc. 128 (2006), 7720.
Nitrogen Doped Graphene-Supported Fe3O4 Nanoparticles for Efficient Oxygen Reduction Reaction Zhong-Shuai Wu, Yi Sun, Long Chen, Khaled Parvez, Xinliang Feng,* and Klaus Müllen* Max-Planck-Institut für Polymerforschung, Ackermannweg 10, 55128 Mainz, Germany feng@mpip-mainz.mpg.de, muellen@mpip-mainz.mpg.de
One of the great challenges for oxygen reduction reaction (ORR) at the fuel cell cathodes is to explore innovative, alternative and high efficient non-precious metal catalysts with low cost, high activity and practical durability in the long-term.
[1-3]
The currently used platinum based catalysts are the best
electrode materials for ORR, which, however, suffer from slow reduction kinetics and high cost. As one of the reasonable alternatives for pure Pt catalysts, nonprecious metal based nitrogen-doped carbon catalysts, in particular, Fe-N/C and Co-N/C catalysts derived from transition-metal/nitrogen containing precursors/complexes, such as porphyrins, phthalocyanides, polypyrrole, and polyaniline, have been revealed to be viable alternative catalysts given their comparable catalytic activity toward ORR. Graphene is expected to be a good carbon support for uniformly anchoring the metal/ metal oxide nanoparticles for energy storage devices,
[4-7]
in particular, improving the ORR performance in fuel cell.
[8]
In this work, we reported a novel class of non-precious metal (Fe3O4)/N-doped graphene composite catalysts as high performance cathode catalysts for ORR in alkaline solution (Figure 1). An N-containing conducting polymer of polypyrrole (PPy) was chosen as nitrogen precursor to fabricate nitrogen-doped graphene due to its excellent catalytic activity and high durability. The resulting hybrid catalysts can greatly increase the active site density on the catalyst, therefore, the hybrid catalytsts exhibit high -
current density, low ring current, low HO2 yield, high electron transfer number (~4), and good durability. References [1]
R. L. Liu, D. Q. Wu, X. L. Feng, K. Mullen, Angew. Chem. Inter. Ed. 49 (2010) 2565-2569.
[2]
R. L. Liu, C. von Malotki, L. Arnold, N. Koshino, H. Higashimura, M. Baumgarten, K. Mullen, J.
Am. Chem. Soc. 133 (2011) 10372-10375. [3]
S. B. Yang, X. L. Feng, X. C. Wang, K. Mullen, Angew. Chem. Int. Ed. 50 (2011) 5339-5343.
[4]
Z. S. Wu, W. Ren, L. Wen, L. Gao, J. Zhao, Z. Chen, G. Zhou, F. Li, H. M. Cheng, ACS Nano 4
(2010) 3187±3194. [5]
Z. S. Wu, W. C. Ren, D. W. Wang, F. Li, B. L. Liu, H. M. Cheng, ACS Nano 4 (2010) 5835-5842.
[6]
Z. S. Wu, D. W. Wang, W. Ren, J. Zhao, G. Zhou, F. Li, H. M. Cheng, Adv. Funct. Mater. 20
(2010) 3595-3602. [7]
Z. S. Wu, W. C. Ren, L. Xu, F. Li, H. M. Cheng, ACS Nano 5 (2011) 5463-5471.
[8]
Y. Y. Liang, Y. G. Li, H. L. Wang, J. G. Zhou, J. Wang, T. Regier, H. J. Dai, Nature Mater. 10
(2011) 780-786.
Figure 1. a) XRD pattern and b) SEM image of Fe3O4-N-doped graphene. c) Cyclic voltammnetry curves of Fe3O4-N-doped graphene in nitrogen- and oxygen-staturated 0.1 M KOH aqueous electrolyte -1
solution. The scan rate is 100 mV s . d) linear sweep voltammetry of Fe3O4-N-doped graphene in an oxygen-saturated 0.1 M KOH at a scan rate of 10 mV s corresponding K-L plot of current density (J- YHUVXV ČŚ
-1/2
.
-1
and different rotation rates. Inset is the
Nanoscale Comparison of graphite exfoliation by supramolecular, chemical and electrochemical methods 1
1
1
Zhenyuan Xia , Emanuele Treossi , Vincenzo Palermo 1
Istituto per la Sintesi Organica e la FotoreattivitĂ -Consiglio Nazionale delle Ricerche (ISOF-CNR), via Gobetti 101, 40129 Bologna, Italy. xia@isof.cnr.it
Being a single-atom-thin carbon sheet, graphene has been intensively studied due to its impressive mechanical, thermal, optical and electronic properties. Meanwhile, Nature has provided us with large amounts of graphene sheets in high quality, stacked into graphite mineral; all we have to do is finding a way to exfoliate the single sheets in kilogram-scale yield and high quality. Graphite exfoliation can be simply achieved in different ways, such as chemical reduction of graphene oxide (GO) [1], exfoliation by extended sonication with organic solvent or surfactants [2], and electrochemistry approach [3]. While all these techniques yield graphite exfoliation, the exfoliation mechanism and the quality/size of the obtained graphene are greatly different. The exfoliation in organic solvents yields high quality sheets, but the mechanism of graphene dispersion in these solvents is still not clear. On the other side, the mechanism of chemical and electrochemical exfoliation involves gas production and subsequent large scale mechanical exfoliation, which have been previously studied [4]. In this work, we compare the nanoscale exfoliation process of graphite into graphene performed by sonication-assisted exfoliation in a widely used organic solvent (N,N-dimethylformamide, or DMF) with more disruptive exfoliation by oxidation (using a modified Hummers method) or by electrochemical oxidation. Differently from previous work, we focused our attention not only on the exfoliated sheets, but on the bulk material thatâ&#x20AC;&#x2122;s left behind after the exfoliation process. Watching at the similarities and differences in surface morphology etched by the exfoliation process, we shall have information on the mechanism behind it. For this, highly oriented pyrolytic graphite (HOPG) samples have been exfoliated using the aforementioned approaches, and then characterized by optical microscopy (OM), Atomic Force Microscopy (AFM) (in Fig.1), Scanning Electron Microscope (SEM), and X-rays Diffraction (XRD), monitoring the effects of different exfoliation techniques at the nanoscale.
References [1] J. M. Mativetsky, A. Liscio, E. Treossi, E. Orgiu, A. Zanelli, P. Samori, V. Palermo, J. Am. Chem. Soc., 133 (2011) 14320. [2] Y. Hernandez, V. Nicolosi, J. N. Coleman et al., Nature Nanotech., 3 (2008) 563. [3] C. Y. Su, A. Y. Lu, Y. P. Xu, F. R. Chen, A. Khlobystov, L. J. Li, ACS Nano., 5 (2011) 2332. [4] K. W. Hathcock, J. C. Brumfield, C. A. Goss, E. A. Irene, R. W. Murray, Anal. Chem., 67 (1995) 2201. Figures a) initial HOPG
c) chemical oxidation 10 s
b) sonication 10 h
d) electrochemical oxidation 2 V
Fig. 1. AFM images of a 15 Ă&#x2014; 15 Č?P region of basal planes HOPG prepared under different conditions: a) initial HOPG; b) HOPG after 10 hours of sonication in DMF;; c) HOPG after 10 second of chemical oxidation by a modified Hummers method; d) HOPG anode after 30 min of electrochemical oxidation by applying 2 V in 0.5 M H2SO4.
Chemical Potential of Inhomogeneous Single Layer of Graphene Y. E. Yaish, E. M. Hajaj, O. Shtempluk, A. Razin, and V. Kochetkov Department of Electrical Engineering, Technion, Haifa, Israel, 32000 yuvaly@ee.technion.ac.il We developed an alternative exfoliation method which enables us to receive high quality of graphene layers (single, double, etc... ) over insulating substrates. We have studied graphene performance on high k dielectrics, and obtained high gate geometrical capacitance, high tansconductance, and high mobilities. In such devices the energy assoFLDWHG ZLWK WKH TXDQWXP FDSDFLWDQFH EHFRPHV VLJQL多FDQW fraction of the total energy, and thus we could measure directly the chemical potential of single layer of graphene.
In the regime where inhomogeneous charge densities are found, electrons and holes
coexist, and the standard two carriers Hall Effect analysis fails to be adequate. Using direct measurements of the chemical potential as function of temperature we could extract the charge density of electrons and holes in the homogenous and inhomogeneous regime as well. It turns out that the temperature dependence of electron and hole densities is the dominant source for the temperature behavior of the graphene conductivity as well.
Reaction Mechanisms of Chemical Reduction of Graphene Oxide by Sulfur-Containing Compounds᧜A DFT Study Yan Su, Ji Jun Zhao* /DERUDWRU\ RI 0DWHULDOV 0RGL¿FDWLRQ E\ /DVHU (OHFWURQ DQG ,RQ %HDPV 6FKRRO RI 3K\VLFV DQG Optoelectronic Technology and College of Advanced Science and Technology, Dalian University of Technology, Dalian 116024, China. zhaojj@dlut.edu.cn Based on density functional calculations, the PHFKDQLVPV ZHUH LGHQWL¿HG as reducing epoxide and hydroxyl groups of GO with sulfur-containing compounds as a reducing agent. We also examined the reaction mechanisms of a series of GO structures with different coverage rates and OH:O ratios by sulfur-containing compounds. We provide atomic-level elucidation for the deoxygenation of GO, characterize the product structures, and suggest how to optimize the reaction conditions further.
Magnetic insulator proximity induced spin-polarization in graphene 1
1
2
3,4
H. X. Yang , D. Terrade , X. Waintal , S. Roche , M. Chshiev
1*
1
SPINTEC, CEA/CNRS/UJF-Grenoble, INAC, 17 rue des Martyrs, 38054 Grenoble, France 2 SPSMS-INAC-CEA, 17 rue des Martyrs, 38054 Grenoble, France 3 CIN2 (ICN-CSIC) and Universitat Autonoma de Barcelona, Catalan Institute of Nanotechnology, Campus de la UAB, ES-08193 Bellaterra (Barcelona), Spain 4 ICREA, Institucio Catalana de Recerca i Estudis Avancats, ES-08010 Barcelona, Spain mair.chshiev@cea.fr
Graphene is very attractive for spintronics [1] since long spin lifetimes are expected within this material due to its intrinsic weak spin-orbit coupling and hyperfine interaction [2]. However, inducing magnetism in graphene is still jeopardizing for its applications. One way to induce magnetic states in graphene is using magnetic substrates, e.g. transition metals Co and Ni [3]. The properties of these epitaxial films have been extensively studied, however they are grown on conducting substrates which limit graphene applications for electronic devices. Alternative possibility is to use magnetic insulating material EuO as a substrate [4]. Here we addressed this problem from first principles and report promising potential for producing high spin polarization and exchange splitting values. Our calculations were performed using Vienna Ab-Initio Simulation Package (VASP) which is based on density functional theory with generalized gradient approximation for exchange correlation and projector augmented wave based pseudopotentials [5]. All calculations have been performed to ensure the Hellman-Feynman forces acting on carbon atoms to be less than 10-3 eV/Å. Considering that Eu is a heavy element with atomic number of 63, and its outer shell (4f76s2) contains f electrons, GGA approach fails to describe strongly correlated localized 4f electrons of EuO giving the metallic ground state of EuO, while a clear band gap is observed in experiment [6]. Thus, we introduced Hubbard-U parameter to describe the strong intra-atomic interaction in a screened Hartree-Fock like manner, which produces correct ground state of EuO. Using the optimized structure of graphene on EuO, we calculated the local density of states for this system [Figure 1]. Due to the existence of EuO substrate, symmetry of carbon atoms in graphene lattices are broken into six folders as shown in Figure 1(a) with different colors. For clarity, EuO substrate atoms are shown with crosses. The calculated magnetic moment of surface Eu are found a little bit enhanced giving 7.0 ȝB compared to the bulk values of 6.9 ȝB. And sublayer oxygen atoms are found to be spin polarized also with magnetic moments of about -0.11 ȝB. Due to very strong spin polarization of EuO substrate, magnetic properties of graphene are strongly affected. As shown in Figure 1(b), the average spin polarization in graphene layer is found to be about 12%. This value is not large, but if we shift a little bit Fermi level, the spin polarization can be strongly enhanced even up to half-metallic state. Interestingly, around -1 eV, spin up and spin down densities are zero, but the energy ranges are different which could actually be related to Dirac point’s reshaping. Since these 18 carbon atoms are broken into 6 symmetry groups, their contributions to the total spin polarization are also different. For the purple one having largest magnetic moment in graphene, its spin polarization may reach up to 72%, while for the yellow one with smallest magnetic moment, its spin polarization gives just 9.6%. Also due to direct interaction between two sublattices of intrinsic graphene and interaction between graphene and EuO substrate, the spin polarization of some of carbon atoms becomes negative. Finally, the spin polarized electrons are mainly from pz orbital [Figure 1 (c)]. This work was supported by Chair of Excellence Program of the Nanosciences Foundation in Grenoble, France, by French National Research Agency (ANR) Projects NANOSIM_GRAPHENE, and by European Union funded STREP Project CONCEPT-GRAPHENE.
References [1] A. Fert et al, Mat. Sci. Eng. B, 84 (2001) 1; S. A. Wolf et al, Science, 294 (2001) 1488. [2] D. Huertas-Hernando et al, Eur. Phys. J. Special Topics, 148 (2007) 177. [3] A. Varykhalov et al, Phys. Rev. Lett., 101 (2008) 157601; O. Rader et al, Phys. Rev. Lett., 102 (2009) 057602. [4] H. Haugen et al, Phys. Rev. B, 77 (2008) 115406. [5] G. Kresse and J. Hafner, Phys. Rev. B, 47 (1993) 558; P. E. Blöchl, Phys. Rev. B 50 (1994) 17953; G. Kresse and J. Joubert, ibid. 59 (1999) 1758. [6] N. J. C. Ingle et al, Phys. Rev. B., 77 (2008) 121202; A Mauger et al, Phys. Reports, 141 (1986) 51.
Figures
Fig.1. (a) the six lattices of graphene on EuO represented with different colors, (b) total density of states for graphene layer and the average spin-polarization, (b)-(h) local density of states for each lattice of carbon atom in graphene, respectively.
High Power Light Emitting Diode with Graphene Transparent Electrode 1
1
1
3
3
Doo-Hyeb Youn , Hong-Kyw Choi , Young-Jun Yu , Seung-Hwan Kim , Seong-Ran Jeon , Sung-Yool 2 1 Choi , and Choon-Gi Choi 1
Creative Research Center for Graphene Electronics, Electronics and Telecommunications Research Institute, 138 Gajeongno, Yuseong-gu, Deajeon, 305-700, Korea 2 Department of Electrical Engineering, KAIST, 291 Daehakro, Yuseong-gu, Deajeon, 305-701, Korea 3 LED Fusion Research Cenetr, KOPTI, Wolchul-dong 971-35, Buk-gu, Gwangju, 500-779 Korea dhyoun@etri.re.kr
Summary High performance LEDs were fabricated using graphene transparent conducting electrode in ultraviolet (UV) and green region. In order to prevent the graphene delamination, two-layer lithographic patterning consisting of non-photosensitive low viscosity layer and common photoresist, was first demonstrated. Here we introduce the low-viscosity interfacial layer which prevents the strong interaction between photoresist and chemical groups on graphene. CVD (Chemical Vapor Deposition) graphene was doped by chemical treatment process using HNO3 solution. O2 plasma treatment process was introduced to increase the adhesion between Cr/Au electrode and p-GaN. Two layer lithograpjhic patterning and chemical doping of graphene resulted in enhancement of the current spreading over the p-GaN and the high power LED operation. Motivation Graphene, which is a high transparency from UV to near-infrared region, and the high thermal and electrical conductivity, graphene holds promise for use in LEDs as a transparent conductive electrode [1,2]. Indium tin oxide (ITO) has been conventionally used as the transparent conductive electrodes in solar cells and LEDs. However, ITO is an expensive material, unstable in chemical solutions and has low transparency in the UV region [3]. In addition, the high thermal conductivity of graphene is advantageous for lowering the operating temperature of high power LEDs. However, irrespective of these outstanding advantages, at cleaning process of residual photoresist after patterning, the strong dipole bonding between photoresist and graphene, cause graphene delamination due to the absence of interfacial bonds to the substrate. This graphene delamination blocks the current spreads over entire graphene sheet. Therefore it is essential to obtain reliable lithographic patterning without graphene delamination to produce large area current spreading over p-GaN. The large work function (̻ ) difference between p-GaN and graphene (respective ̻ values are 7.5 and 4.5 eV) results in a substantial Schottky barrier height at the interface. Thus, it is prerequisite to tune the electrical/electronic properties of graphene in order to attain a better electrical coupling between graphene and p-GaN. Chemical charge transfer doping has been shown to increase the ̻ of graphene as a result of modification of Fermi level [4-6]. Chemical doping can reduce the Rs and thus improve the conductivity. Results Graphene delamination was overcome by introducing the two-layer lithographic procedure. The sheet resistance of CVD-FLG was decreased from 700 ~ 1200 ȍ/sq to 90 ~ 150 ȍ/sq by immersing FLG into HNO3 solution. Current spreading over the p-GaN was increased from preventing graphene delamination. The adhesion between Cr/Au electrode and p-GaN was improved by O2 plasma ashing and it prevents the metal peeling off during LED operation. The Vf defined at an injection current of 20 mA is found to be 5.72 V for the LEDs with as-grown MLG electrodes. When a doped MLG electrode is applied to the LED, the Vf is reduced to a value of .4.59V and the current spreading is significantly improved. LED power was increased ~ 96 % from the adoption of graphene transparent electrode and ~ 14.3 % from the HNO3 chemical doping of graphene sheet, respectively. From above results, high power LED operation more than 60 mW in green is expected from the previous experimental conversion between non-integrating sphere and integrating sphere measurement. More details about the process optimization, power increase due to LED structure (electrode pattern design and chip shape), and analysis are to be discussed at the conference.
References [1] Wang, L. Zhi, and K. Mullen, Nano Lett. 8, 323 (2007). [2]. Wang, X. Chen, Y. Zhong, F. Zhu, and K. P. Loh, Appl. Phys. Lett. 95, 063302 (2009) [3]. I. Na, S. S. Kim, J. Jo, and D. Y. Kim, Adv. Mater. 20, 4061 (2008). [4]. K. K. Kim, A. Reina, Y. Shi, H. Park, L. J. Li, Y. H. Lee, and J. Kong, Nanotechnology 21, 285205 (2010). [5] Y. Shi, K. K. Kim, A. Reina, M. Hofmann, L.J. Li, and J. Kong, ACS Nano 4, 2689 (2010). [6] K. S. Kim, Y. Zhao, H. Jang, S. Y. Lee, J. M. Kim, K. S. Kim, J.-H. Ahn, P. Kim, J.-Y. Choi, and B. H. Hong, Nature 457, 706 (2009). Figures
Fig. 3
Fig. 4
Figure caption Fig. 1: Two-layer lithographic patterning Method. Fig. 2: Correlation between graphene delamination and current spreading. a) LED pattern after p-GaN etching without graphene electrode, b) LED pattern with graphene delamination, c) LED pattern without graphene delamination, d) current spreading limitation only at the electrode, e) current spreading limitation due to graphene delamination, f) current spreading over entire p-GaN. Fig. 3: Increase in current spreading due to chemical doping of graphene using HNO3 solution. Fig. 4: Increase in LED power due to adoption of grapheme transparent electrode and chemical doping of graphene.
Current Saturation in Few-layer MoS2 FET Lili Yu, Allen Hsu, Han Wang, Yumeng Shi, Jing Kong, Tomas Palacios Department of Electronic Engineering and Computer Science, Massachusetts Institute of Technology, Cambridge, MA 02139, USA e-mail: liliyu@mit.edu The discovery of graphene in 2004 has sparked great interest in 2-dimensional (2D) materials for their use in the next generation of electronic devices.[1] Although graphene exhibits some remarkable and really unique electrical properties that may help overcome some of the main limitations in analog electronics,[2][3] its lack of bandgap has limited its use for digital applications. On the other hand, molybdenum disulphide (MoS2), another two-dimensional material with a band gap of 1.8 eV in single layer and 1.2 eV in bulk, has recently been used in field effect transistors with excellent gate modulation and current pinch-off.[4][5][6] Monolayer MoS2 is composed of one layer of molybdenum atoms sandwiched between two layers of sulphur atoms for a total thickness of 0.65 Ă&#x2026;. Mobility around 300 2
7
cm /V.s and an on-off current ratio exceeding 10 have been experimentally demonstrated while a large gm P6 Č?P DQG excellent short channel behavior (drain induced barrier lowering ~10 mV/V and subthreshold swing ~60 mV/decade with gate length of 15nm) have been predicted. Its potential to reduce short channel effect in highly scaled devices thanks to its excellent electrostatic confinement, together with its high thermal stability, chemical inertness and mechanical properties makes MoS2 transistors excellent candidates for low power mixed-signal electronics. In this paper, we show MoS2 FETs with current saturation, for the first time. The saturation behaviour is extremely important for building both digital and analog circuits. This property is lacking in most graphene FETs due to its zero bandgap and is also not observed in the MoS2 FETs reported in the literature so far. Few-layer MoS2 flakes are first obtained from bulk crystals using adhesive-tape-based micromechanical exfoliation onto degenerately-doped Si substrates covered with 285nm thick SiO 2. The sample was then annealed at 300C for 6 hours in forming gas (20 sccm H 2 and 600 s ccm Ar) to flatten the flakes and remove the tape residues. The number of layers in the MoS2 flake is then confirmed by optical microscopy, Raman spectrum and Atomic Force Microscopy (AFM). The exfoliated MoS2 flakes we investigate always have 1 layer to 6 layers with thickness of 0.7 to 4nm. Figure 1(a) shows the optical micrograph of a typical few-layer MoS2 flake. The Raman spectrum (Inset of Figure 1 (a)) was -1
measured at room temperature using a 532 nm laser. Two peaks at 384 and 405 cm are attributed to the in-plane
1 E 2g
and out-of-plane A1g vibration of MoS2 respectively.[7] The AFM image (Figure 1(b))
demonstrates that the current sample has very clean and flat surface. This flake contains 5 layers of MoS2, with a total thickness of 3.5 nm, as shown in the step image in the inset of figure 1(b). The metal contacts are defined using electron beam (e-beam) lithography, followed by deposition of 3 nm titanium / 50 nm gold metal stacks using e-beam evaporation. Figure 1(c) shows an optical micrograph of two parallel FET devices fabricated on the flake in Figure 1(a). They have a gate length of 2 m and gate width of 3.5 m. DC characterization is performed using an Agilent 4155C Semiconductor Parameter Analyzer. All the fabricated few-layer MoS2 FETs exhibit clear n-type conduction and transistor behaviour (Figure 1(d)), which is consistent with the previous reports. [4][5] The transfer characteristics demonstrate the ability to modulate the resistance of the MoS2 channel by changing the back-gate voltage, demonstrating an 6
on/off current ratio of about 10 . The device also shows clear current saturation behavior in its output characteristics (Figure 1(e)). At the lower drain bias region, the current increases almost linearly with VDS. The device is in its linear (triode) region. At higher drain bias, the current changes little with the increase in VDS, showing that the gate voltage can independently change the current. The knee voltages for saturation are 0.1V, 0.2V, 0.5V and 0.6V for Vg=20V, 40V, 60V and 80V, respectively, which is
consistent with theoretical work. [6] 7KH RXWSXW FRQGXFWDQFH JG Â&#x2DC;,D Â&#x2DC;9DS) is 4.3, 2.8, 1.2
S/ m for
Vg=80, 60, 40V, respectively. Such saturation is observed in many of our devices. Figure 1(f) shows the output characteristics of a second MoS2 FET fabricated using photolithography. It has a channel length of 1.6
m, width of 24
m and thickness of 2.1 nm. At Vg=40V, the saturation current is 160
A,
corresponding to a current density of 6.7 A/ m. The output conductance is 6.1, 4.6, 3.0, 1.1 S/ m for Vg=40, 20, 0, -20V, respectively. In the lower bias region, the device shows some Schottky response and the current rise a little with the increase of drain and source voltage in the saturation region. It is believed that the Schottky behaviour may be due to photoresist residue in the contact region. In conclusion, we have fabricated few-layer MoS2 FETs and characterized their electronic performance. 6
These devices show on/off current ratios larger than 10 , current density as large as 6.7
A/ m. In
addition, current saturation has been observed for the first time in MoS2 FETs, which is extremely important for building both digital and analog circuits. Acknowledgements. This work has been partially funded by the ONR Young Investigator Program, monitored by Dr. Paul Maki. References [1] K. S. Novoselov et la., Science, 306 (2004) 666-669. [2] Y. Lin et la., Science, 327 (2010) 662. [3] Han Wang et la., IEEE Electron Device Letters, 30 (2009) 547-549. [4] RadisavljevicB et la., Nat Nano, 6 (2011) 147-150. [5] B. Radisavljevic et la., ACS Nano, 5 (2011) 9934-9938. [6] Y. Yoon et la, Nano lett. 11 (2011) 3768-3773. [7] J. Coleman et la., Science, 331 (2011) 568. Figures
Figure1. (a) Optical micrograph, Raman spectra (inset) and (b) AFM image of few-layer MoS2 flake. (c) Optical image of two parallel MoS2 field-effect transistors fabricated by electron-beam lithography. (d)Room temperature transfer characteristic for the FET in c). The inset shows the transfer characteristic in logarithmic scale to 6 demonstrate the on/off current ratio of >10 . (e) Output characteristics of the FET in (c) with back gate voltage from -20V to 80 V in steps of 20V. (f) Output characteristics of another FET fabricated by photolithography with back gate voltage from -40V to 40 V in steps of 20 V.
Interaction of Metals with suspended Graphene observed by Transmission Electron Microscopy 1
1
2
1
Recep Zan , Ursel Bangert , Quentin Ramasse , Konstantin S Novoselov 1
The University of Manchester, Manchester, M13 9PL, United Kingdom
2
SuperSTEM Laboratory, STFC Daresbury, WA4 4AD, United Kingdom recepzan@gmail.com
Graphene, the first two-dimensional material to be isolated, has become the focus of intense fundamental research due to its extraordinary properties, but even more so has spurred massive interest into studies regarding nanotechnology applications.[1, 2] An area of immense importance for the latter is the study of the metal-graphene interactions, because metals have to be used in every single application of graphene as functional material.[1, 3] The effects of metals on the transport, electronic,
magnetic
and
structural
properties
of
graphene
have
been
investigated
both
experimentally[3] and theoretically by means of Density Functional Theory[4] with arguably more emphasis on theoretical than on experimental studies. There is in particular a lack of high resolution transmission electron microscopy (TEM), thus limiting our understanding of this system as it is perhaps the only technique allowing direct imaging of the interactions between suspended graphene and metals. Here, various metal impurities have been introduced via evaporation onto graphene sheets obtained by CVD-growth. Au-, Cr-, Ti-, Ni-, Pd- and Aldeposited graphene sheets were then studied at atomic resolution in scanning TEM (STEM). Gold atoms and clusters are mainly observed on hydrocarbon contamination as previously reported.[5, 6] The cluster sizes vary and are not equally distributed on the graphene surface (fig 1a). As a result of surface treatments, in our case by exposing pristine graphene samples to a cold hydrogen plasma,[7] the cluster distributions and sizes are affected, although they remain on the hydrocarbon contamination.[7] Gold cluster distributions become more uniform in hydrogenated samples (fig. 1b and c) and cluster sizes become similar. Another way to study metal clusters on graphene is to anneal them either in a gas environment or in vacuum at elevated temperature to study their stability at high temperatures. As can be seen in fig. 1d, o
as the clean graphene areas widen during high vacuum annealing at 700 C, the gold clusters, which reside on the hydrocarbon contamination, are forced to move towards each other (fig. 1e). When the o
annealing temperature is increased to 950 C, we find that the gold clusters have agglomerated and almost melted, and as a result have flattened, while most of the contamination has disappeared (fig. 1f). Gold has never been observed to introduce any damage into graphene. This conclusion can be drawn with high certainty from STEM studies,[6] where a 60kV acceleration voltage has been used for imaging, which is known to be well below the displacement threshold for graphene.[8] In contrast, damaging of graphene has been observed in the presence of Al, Ti, Cr, Pd and Ni, as is predicted by theory.[9] Except in a few circumstances clusters of all these metals are also found to reside on hydrocarbon chains. However, observation during repeated STEM scans shows that smaller clusters and individual atoms are drawn out of their initial positions, i.e. from the middle of contamination patches, to the edge of the contamination. As soon as metals reach the border between hydrocarbon and clean graphene, they interact with the clean graphene surface. Initially point defects (vacancies) are created, and this process repeats itself as long as new metal atoms are supplied from nearby metal clusters to the emerging holes. High angle annular dark field (HAADF)-STEM imaging has been
employed to study individual ad-atoms on graphene. The etching process is shown for Al in the HAADF images in fig. 2. References [1] K. S. Novoselov, A. K. Geim, S. V. Morozov, D. Jiang, Y. Zhang, S. V. Dubonos, I. V. Grigorieva, and A. A. Firsov, Science 306 (2004) 666. [2] A. K. Geim and K. S. Novoselov, Nat Mater 6 (2007) 183. [3] K. Pi, K. M. McCreary, W. Bao, W. Han, Y. F. Chiang, Y. Li, S. W. Tsai, C. N. Lau, and R. K. Kawakami, Physical Review B 80 (2009) 075406. [4] K. T. Chan, J. B. Neaton, and M. L. Cohen, Physical Review B 77 (2008) 235430. [5] Y. Gan, L. Sun, and F. Banhart, Small 4 (2008) 587. [6] R. Zan, U. Bangert, Q. Ramasse, and K. S. Novoselov, Nano Letters 11 (2011) 1087. [7] R. Zan, U. Bangert, Q. Ramasse, and K. S. Novoselov, Small 7 (2011) 2868. [8] R. F. Egerton, F. Wang, and P. A. Crozier, Microscopy and Microanalysis 12 (2006) 65. [9] D. W. Boukhvalov and M. I. Katsnelson, Applied Physics Letters 95 (2009) 023109. Figures
Figure 1. Bright field (BF) image of 2Å gold evaporated a) on pristine, b) on 2-cycle hydrogenated, c) 4-cycle hydrogenated monolayer graphene. The scale (5nm) is chosen to be same in images a) to c) for accurate o o comparison. d) annealed at 700 C, e) showing magnified image of (d), f) as (d) but annealed at 950 C. The scale bar is 50nm in (d) and same in (e) and (f), 20nm.
Figure 2. HAADF images of graphene etching in the presence of an Al layer of 2Å nominal thickness a) before etching, b) after the start of hole formation, c) after hole enlargement in subsequent scans, d) after continued etching as result of sustained supply of Al atoms to the hole’s edge (some Al-atoms are arrowed in red in (b), (c) and (d)), e) after etching has almost stopped because Al atom supply has ceased; f) presents lower magnification overview of the Al distribution and hole evolution. The scale bar is same in images a) to e), 1nm.
Quantum spin transport in carbon chains with graphene-like contacts. Zeila Zanolli (1, 2), Giovanni Onida (3), and Jean-Christophe Charlier (1) 1. UniversitÊ catholique de Louvain, Institute of Condensed Matter and Nanosciences (IMCN) and ETSF, Chemin des Êtoiles 8, B-1348 Louvain-la-Neuve, Belgium 2. UniversitÊ de Liège, Institut de Physique, AllÊe du 6 Aout, 17, B-4000 Sart Tilman, Liege, Belgium 3. Universita' degli Studi di Milano, Dipartimento di Fisica and ETSF, via Celoria 16, I-20133 Milano, Italy. Contact email: zeilazanolli@gmail.com Linear carbon chains (or carbynes) have been recently synthesized via electronic irradiation of graphene inside a transmission electron microscope [1, 2, 3], showing experimental evidence of carbon chains terminated on graphitic fragments, as suggested in previous works about amorphous carbyne-rich pure carbon films produced via supersonic cluster beam deposition [4, 5]. The connections between carbon chains and graphitic nanofragments have the twofold effect of stabilizing the chain [6] and providing contacts for measurements, suggesting a possible use in nanoelectronics. In addition, the monatomic C chain can be considered as the smallest possible interconnect in all-carbon nanodevices [7]. 7R YDOLGDWH WKH ODWWHU DSSOLFDWLRQV ILUVW SULQFLSOHV DQG QRQ HTXLOLEULXP *UHHQœV IXQFWLRQV PHWKRGV DUH used to investigate the spin-polarized electronic transport properties of monatomic carbon chains covalently connected to graphene-like contacts as graphene nanoribbons (GNR). This study [8] reveals that a net spin polarization is always present on odd chains [Fig. 1], while even chains are not spin polarized. Besides, quantum electron conductance of the chain-GNR system is characterized by narrow resonant states resulting from the simultaneous presence of open conductance channels in the contact region and on the chain atoms. Such a behavior could be suggested as the physical mechanism underlying the observed on/off switching of graphene nanodevices [9]. Most interestingly, the magnetic and electronic properties of the complex chain-GNR structures can be tuned by tailoring graphene edges shape and chain parity to achieve any combination of spin polarization and electrical conductivity as semiconducting non spin-polarized, metallic spin-polarized, and even semiconducting spin-polarized systems, opening the way for the design of a new kind of spintronic nanodevices with tunable magnetic and conducting properties.
References [1] Meyer, J.C.; Girit, C.O.; Crommie, M.F.; Zettl, A. Nature 454 (2008) 319. [2] Jin, C.; Lan, H.; Peng, L.; Suenaga, K.; Iijima, S. Phys. Rev. Lett. 102 (2009) 205501. [3] Chuvilin, A.; Meyer, J.C.; Algara-Siller, G.; Keller, U. New Journal of Physics 11 (2009) 083019. [4] Ravagnan, L. et al. Phys. Rev. Lett. 89 (2002) 285506. [5] Ravagnan, L. et al. Phys. Rev. Lett. 98 (2007) 216103. [6] Ravagnan, L. et al. Phys. Rev. Lett. 102 (2009) 245502. [7] Avouris, Ph.; Chen. Z.; Perebeinos, V. Nat. Nanotechnology 2 (2007) 605. [8] Z. Zanolli, G. Onida, J.-C. Charlier, ACS nano, 4 (2010) 5174Âą5180. [9] Standley, B.; Bao, W.; Zhang, H.; Bruck, J.; Lau, C. N.; Bockrath, M. Nano Lett. 8 (2008) 3345.
Figures
Figure 1. Spin density (Un ± Up) on the C9 monatomic carbon chain between armchair graphene nanoribbon (AGNR) contacts. Blue and red correspond to positive and negative isodensities of Un ± Up. The lilac arrow indicates the spin-transport direction. [Z. Zanolli, et al., ACS Nano 4 (2010) 5174±5180].
Energy levels of quantum rings in bilayer graphene 1
2
1,2
M. Zarenia , J. M. Pereira Jr. , F. M. Peeters , and G. A. Farias
2
1
Department of Physics, University of Antwerp, Groenenborgerlaan 171, B-2020 Antwerpen, Belgium. 2 Departamento de Fisica, UniversidadH )HGHUDO GR &HDUÂśa, Fortaleza, Cear'a, 60455-760, Brazil. m.zarenia@gmail.com
We propose a novel system where electron and hole states are electrostatically confined into a quantum ring in bilayer graphene. This proposal is based on the fact that in bilayer graphene a gap in the electronic spectrum can be created and modified by means of a gate voltage [1]. Since in our proposed structure the bilayer graphene sheet is assumed to be defect-free and the confinement is brought about by an external bias the disadvantages of the edges do not arise in our system. In addition the ring parameters can be tuned by external fields. Our results display interesting new behaviors in the presence of a perpendicular magnetic field B, which have no analogue either in semiconductor-based or in lithography-based graphene quantum rings [2,3], such as an overlap between magnetically confined Landau levels (in which the carriers are mainly located in the center of the ring) and electrostatically confined states. In particular, the eigenvalues are not invariant under a B Äş -B transformation and, for a fixed total angular momentum index m, their field dependence is not parabolic, but displays two minima separated by a saddle point. The spectra also display several anti-crossings, which arise due to the overlap of gate-confined and magnetically-confined states. The existence of Aharonov-Bohm oscillations for both electrons and holes are still linked with flux quantization through the ring. This novel spectra obtained for a finite width quantum ring [4] can be understood by means of a toy model for an ideal zero width ring in which case analytical results can be obtained [5]. References [1] J. M. Pereira Jr., P. Vasilopoulos, and F. M. Peeters, Nano Lett. 7 (2007) 946. [2] S. Russo, J. B. Oostinga, D. Wehenkel, H. B. Heersche, S. S. Sobhani, L. M. K. Vandersypen, and A. F. Morpurgo, Phys. Rev. B 77 (2008) 085413. [3] A Bahamon, A. L. C. Pereira, and P. A. Schulz, Phys. Rev. B 79 (2009) 125414. [4] M. Zarenia, J. M. Pereira Jr., F. M. Peeters, and G. A. Farias, Nano Lett. 9 (2009) 4088. [5] M. Zarenia, A. Chaves, J. M. Pereira Jr., F. M. Peeters, and G. A. Farias, Phys. Rev. B 81 (2010) 045431. Figures
(a) Schematic depiction of a electrostatically confined quantum ring in bilayer graphene. Panels (b) and (c) show electron and hole states as function of external magnetic filed with gated voltage V=150 meV and ring radius R=50 nm respectively for the ring width W=20 nm and zero width. In panel (c) the energy levels are obtained analytically. The energy levels are shown for the quantum numbers |m|Â&#x201D;
TWO-DIMENSIONAL CHARGE RELAXATION IN GRAPHENE: GENERALIZED TELEGRAPH EQUATIONS AND PSEUDO-RELATIVISTIC INVARIANCE Gennady Zebrev National Research Nuclear University MEPHI, 115409, Kashirskoe sh., 31, Moscow, Russia gizebrev@mephi.ru Electrostatically doped graphene is not an intrinsic system and necessarily provided by the isolated conductive gate which controls electric neutrality and the Fermi energy position in graphene. The energy accompanying the current can be stored in associated magnetic field and as kinetic energy of flowing carriers. We will consider here the case v0 / L d Z c / L (*) that allows neglecting the magnetic inductance in comparison to kinetic inductance describing mechanical inertia. These inequalities enable us to set the light velocity equal to infinity neglecting in such way the photon retardation and magnetic inductance in comparison with plasmon retardation and kinetic inductance associated with mechanical motion. The non quasi-static operation is generally described by the electrodynamics and transport equations. Neglecting recombination and generation in the channel, the two first moments of the semi-classical Boltzmann transport equation are written as [1] ÂŞ P r, t Âş wj r , t j r, t 1 wU r, t
Â&#x2019;r ÂŤ Â&#x2019;r j r, t (1) 0, Âť, wt LK e Âź W wt ÂŹ where j r, t and U r, t are 2D current and charge densities, P r, t ] r , t eM r , t is local value of
electrochemical potential ( ] and M are chemical and electric potentials), V 0 is 2D conductivity, and kinetic inductance associated with mechanical carriersâ&#x20AC;&#x2122; motion is written for graphene as h 1 W W LK (2) V 0 eP0 nS 2e2 k F v0 For quasi-static case we have equations for incompressible fluid Â&#x2019;r j r, t 0 , j V 0Â&#x2019;r P r e . For
uniform conductivity the electrochemical potential distribution is obeyed to the Laplace equation Â&#x2019;r2 P r 0 . For long-range charge non-uniformities in graphene the ratio of the diffusion to the drift current components (assumed to be position-independent for a given electric biases) of the total current density j jDR jDIFF 1 N jDR may be obtained using the electric neutrality condition of gated graphene structure [2], which assumed to be instantaneous implying the condition (*) wVG wM ] Cox H a jDIFF § w] ¡ ¨ , N ¸ jDR Š ewM šVG e wVG w] M CQ H F where CQ
(3)
2H F S = 2 v02 is the graphene quantum capacitance, Cox is the gate oxide capacitance per unit
area, and the characteristic energy is introduced
Ha
S = 2 v02Cox
, (4) 2e 2 which is nothing but the full electrostatic energy stored in the capacitor with the area per one carrier in gapless graphene [3]. Quasi-static current as well as charge and potential distributions are obtained analytically in Ref.[2] in a such way. The Eqs.1 can be rewritten as follows w 2 U r, t 1 wU r, t
1 2 (5) Â&#x2019; P r, t . 2 wt eLK W wt This is nothing but a generalized form of the telegraph equations, which could be also derived from circuit consideration for a distributed RLC transmission line. Suppose a fluctuation GM is created and we ask how the system will go back to equilibrium. Notice that for long-range non-uniformities we have GU CQN GM , GP (1 N ) eGM e
GU GP
N 1 N
CQ
Cox CQ Cox CQ
wenS wVG
CCH
(6)
and â&#x20AC;&#x153;compressibilityâ&#x20AC;? is proportional to the channel capacitance. Taking this into account one can obtain a dissipative wave equation, which can be written in two equivalent forms vS2Â&#x2019; 2u r, t w t2u r, t W 1w t u r, t , DS Â&#x2019; 2 u r, t w t u r, t W w t2 u r, t , (7) where u r, t is any of functions GM , GU and GP , the characteristic velocity of the signal propagation vS and an effective diffusivity DS of signal are introduced as follows
vS2
CCH LK
v02 N 1 2N ,
1
D0 N 1 N ,
DS { vS2W
(8)
and the â&#x20AC;&#x153;signal diffusivityâ&#x20AC;? coincides with carrier diffusivity D0 v W 2 only for low doping when H a !! H F (notice the Einstein relation V 0 CQ D0 CCH DS ). For ZW 1 we have diffusion-like propagation 2 0
with loss of the signal w t u # DS Â&#x2019; 2u . In contrast, for ZW !! 1 we have the non-dissipative wave equation vS2 Â&#x2019; 2u x, t w t2u x, t
(9)
with the signal propagation speed dependent on the ratio of the Fermi energy and characteristic electrostatic attraction between graphene carriers and its gate images. Taking into account magnetic inductance we found vS2
v02 N 1 2 N H ox / 2 v02 c 2
that implies the light velocity in media as
propagation speed vS # c / H ox for very thick isolator layers ( N o 0 ). The wave equation is invariant under â&#x20AC;&#x153;pseudo-Lorentzâ&#x20AC;? transformations (where the signal velocity plays a role of the light velocity), which describe plasmon retardation, provide causality and conserve â&#x20AC;&#x153;pseudo-intervalâ&#x20AC;? between events s 2 r 2 vS2 t 2 , wavelet form, etc. The fundamental solution of Greenâ&#x20AC;&#x2122;s function for 2D wave equation has â&#x20AC;&#x153;relativistic-invariantâ&#x20AC;? form 1 T vS t r
(10) u r, t
2S vS vS2t 2 r 2 In contrast to 3D electrodynamics (where perturbation propagates as inflated sphere v G c 2t 2 r 2
according to the Huygensâ&#x20AC;&#x2122; principle) the fundamental solution of 2D (as well as 1D) wave equation for plasmons has no trailing edge of the signal. Interestingly that the substitution u r , t w r , t exp t / 2W
leads to a new equation [4]
v Â&#x2019; 2 S
2
w t2 w r , t 4W 2 w r , t , 1
(11)
which is nothing but the classic (2+1)D Klein-Gordon equation with the â&#x20AC;&#x153;lightâ&#x20AC;? velocity vS and imaginary mass m i / 2W which displays the slow decay of long-wavelength fluctuations (plasmons). Eq.7 allows also an exact explicit fundamental solution for one-dimensional geometry relevant for long transmission lines t § t 2 x2 / v2 ¡ 1 S ¸ u x, t
T t / vS | x | e 2W I 0 ¨ (12) ¨ ¸ 2vS 2W Š š where T(z) is the Heaviside unit step function and I0(z) is the modified Bessel function. This general solution exhibits explicitly a crossover between dissipative diffusion-like and lossless wave-like types of signal propagation (see Fig.1). References [1] S. Salahuddin, M. Lundstrom, â&#x20AC;&#x153;Transport effects on signal propagation in quantum wiresâ&#x20AC;?, IEEE Trans. ED-52, 1734, 2005. [2] G.I. Zebrev, â&#x20AC;&#x153;Graphene Field Effect Transistors: Diffusion-Drift Theoryâ&#x20AC;?, a chapter in â&#x20AC;&#x153;Physics and Applications of Graphene â&#x20AC;&#x201C; Theoryâ&#x20AC;?, Ed. by S. Mikhailov, Intech, 2011 [3] G.I. Zebrev et al. â&#x20AC;&#x153;Influence of interface traps and electron-hole puddles on quantum capacitance and conductivity in graphene field-effect transistorsâ&#x20AC;? arXiv: cond-mat 1011.5127 [4] A. Polyanin, â&#x20AC;&#x153;Handbook of Linear PD Equations for Scientist and Engineersâ&#x20AC;?, Chapman&Hall CRC, 2002
Fig.1a Crossover between wave-like t/W â&#x20AC;ŤŘ§â&#x20AC;Ź1 and diffusive-like (t/Wâ&#x20AC;ŤŘ¨â&#x20AC;Ź1)relaxation according to eq.12.
Fig.1b 1D perturbation spreading within the â&#x20AC;&#x153;pseudolightâ&#x20AC;? cone.
A Novel Way to Prepare Soluble Graphene through Organic Functionalization on Graphene 1,2
1
2
Xiaoyan Zhang , Wesley R. Browne , Bart J. van Wees and Ben L. Feringa
1
1
Stratingh Institute for Chemistry, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands 2 Physics of Nanodevices, Zernike Institute for Advanced Materials, University of Groningen, The Netherlands xiaoyan.zhang@rug.nl
Soluble functionalized graphene was prepared through organic functionalization on graphene, which is confirmed by several techniques. And the obtained graphene is useful in several applications. References (1) X. Y. Zhang, A. C. Coleman, N. Katsonis, W. R. Browne, B. J. van Wees, B. L. Feringa, Chem. Commun., 46 (2010), 7539. (2) X. Y. Zhang, L. L. Hou, A. Cnossen, A. C. Coleman, O. Ivashenko, P. Rudolf, B. J. van Wees, W. R. Browne, B. L. Feringa Chemistry-A European Journal, 17 (2011), 8957. (3) W. F. van Dorp, X. Y. Zhang, B. L. Feringa, J. B. Wagner, T. W. Hansen and J. Th. M. De Hosson, Nanotechnology, 22 (2011), 505303. (4) Y. Zhu, S. Murali, W. Cai, X. Li, J. Suk, J. Potts and R. Ruoff, Adv. Mater. 22 (2010), 3906.
Graphene nanomechanical piezoresistive sensor Shou-En Zhu,1 Victor E. Calado,2 Chao Zhang,2 G.C.A.M. Janssen1 1 Precision and Microsystems Engineering, Delft University of Technology, Delft, The Netherlands 2 Kavli Institute of Nanoscience, Delft University of Technology, Delft, The Netherlands shouen.zhu@tudelft.nl Graphene offers a wealth of possibilities for micro- and nano-mechanical applications. Our work on the piezoresistive effect of graphene synthesized by chemical vapor deposition and the graphene polymeric hybrid cantilever has already demonstrated a promising application [1]. The cantilever deflection was monitored by measuring the electrical resistance change of graphene with the mechanical stress. This method has a very high sensitivity and a high efficiency to miniaturize. However, the absence of a robust integration method for graphene films and the lack of a detailed understanding of the electromechanical behavior constitute the main obstacles for using this material. Here, we propose a scalable integration method by patterning graphene serpentine resistor onto more robust silicon nitride film (100 nm thickness) and cutting the cantilever shape (100-200 Pm length) by focused ion beam milling. We will demonstrate the robustness of the fabriaction method by measuring the extremely high Youngâ&#x20AC;&#x2122;s modulus, the piezoresistive effect, the negative thermal expansion coefficient, and the strong substrate adhesion force of graphene. By varying an applied vertical force or input current, we expect deflection and temperature dependent resistance of graphene to change, which can be utilized for ultra-sensitive metrological applications.
References [1] Shou-En Zhu et al. Graphene based bimorph microactuators, Nano Lett. 11 (2011) 977-981. Figures
Fig. 1 Tip displacement of the polymeric hybrid cantilever beam as a function of temperature. Each data point is the average value of five measurements, and the standard deviation is shown as error bars. The solid line shows the fit to the data obtained from finite element analysis. The top right figure shows the SEM images of the initial position of the cantilever beam, and bend up state upon applying electrical power (right bottom).
Fig. 2 Optical image and raman spectra of the large-area graphene layers transferred on silicon nitride membrane.
Visualization of electronic states along the boundaries of graphite nanoholes 1
2
3
Maxim Ziatdinov , Shintaro Fujii , Koichi Kusakabe , 2 1 2 Manabu Kiguchi , Takehiko Mori , and Toshiaki Enoki 1
Department of Organic and Polymeric Materials, Tokyo Institute of Technology, Ookayama, Meguro-ku, Tokyo 152-8552, Japan 2 Department of Chemistry, Tokyo Institute of Technology, 2-12-1, Ookayama, Meguro-ku, Tokyo 152-8551, Japan 3 Graduate School of Engineering Science, Osaka University, 1-3 Machikaneyama-cho, Toyonaka, Osaka 560-8531, Japan E-mail: ziatdinov.m.aa@titech.ac.jp Introduction. A crystalline matter with topologically non-trivial electronic band structure may support a special kind of electronic state on its boundary. This is so-called edge state (in 2D) or surface state (in 3D). In graphene nontrivial topology arises from pseudospinorial form RI ʌ-electron wavefunction which provides richer structure of edge boundary conditions. This leads, in particular, to a macroscopically degenerated manifold of single particle states (flat band) with nearly zero energy, localized along zigzag-shaped edges, while such states are absent at armchair-shaped edges of graphene [1-3]. Such enhancement of the density of states (DOS) at Fermi level of graphene can give rise to a number of interesting physical phenomena such as specific edge magnetism [4] and edge state superconductivity [5]. In the context of organic chemistry the appearance of edges state, a non-bonding ʌ-electron orbital, can be seen as geometrical frustration in ʌ-network and accounted for by simple counting rules such as Clar sextet rule or theorem on hexagonal graphs. In last decade several groups reported evidence from scanning tunneling spectroscopy (STS) measurements for presence of the edge state at respectively zigzag- and circular-shaped boundaries of graphite atomic terraces [2,3,6] and pits [7], and zigzag edges of unzipped carbon nanotubes [8]. However, a reproducible direct visualization of the edge state wavefunction along well-defined zigzag edges of graphene sheets has not been reported till present moment. Furthermore, with current trend in nanotechnology and nanoscience it is important to study the effect of finite size geometry on the electronic properties of graphene edges, as well as to investigate possible ways of controlling electronic and magnetic properties of nanographene via chemical modification of its edge. In the present work we used low-bias (30-100 mV) UHV scanning tunneling microscopy (STM) measurements and density functional theory (DFT) calculations to map an electronic wavefunction at the well-defined graphene edges of nanoholes with different types of hydrogen termination created by etching of graphite top layer with atomic hydrogen. Creation of nanoholes. The key point in achieving well-defined structure of graphene edges in nanoholes was to perform all preparations and measurement procedures strictly under the UHV conditions, avoiding contact with ambient environment. First, we covered graphite surface with atomic 0 vacancies and annealed at 600 C to remove all possible contaminations. Then, irradiated graphite 0 surface was exposed to atomic hydrogen. The temperature of sample during exposure was 900 C. Compared with previous reports on hydrogen etching of clean graphite basal plane [7], presence of atomic vacancies significantly eased creation of nanoholes, allowing us to use relatively low hydrogen pressures, and influenced the final shape of nanoholes boundaries. Although exact mechanism of nanoholes creation on defective graphite surface is not yet well understood, there are two possible explanations: i) Each atomic vacancy serves as a nucleation center for creation of nanohole; ii) Linearly attached hydrogen ³cuts´ the graphite surface along the lines connecting different vacancies. The nanoholes produced in the second way will have predominantly zigzag edges, which is consistent with our experimental observations. Two different types of zigzag edge in nahoholes. The length of studied edges typically ranges from 0.75 nm to 3 nm. :H LGHQWLILHG WZR W\SHV RI QDQRKROH¶V HGJHV SURSDJDWLQJ LQ WKH ]LJ]DJ direction of graphene lattice. The type 1 zigzag (zz1) edge is characterized by spheroidal, and not spherically symmetric, electronic charge distribution in the non-ERQGLQJ ʌ-orbital on edge carbon atoms. STS measurement at zz1 edge reveals the presence of an edge-state peak in the local density of states (LDOS) at +0.03 eV. This type of edge constitutes about 80% of all the observed zigzag edges in graphene nanoholes. The type 2 zigzag (zz2) edge constitutes around 20% of the observed edges and is characterized by honeycomb-like superperiodic pattern previously reported only for armchair edges. Furthermore, there are no features of non-bonding -state at this type of edge. Both features can be explained if we assume periodical arrangement of two mono-hydrogenated and one di-hydrogenated carbon atoms at the edge. The latter leads to elimination of every third pz orbital and introduces two additional carbon sites from different sublattice at the edge. The total sublattice imbalance is therefore zero and the situation is same to the case of armchair edge. We did, however, find deviations from ideal
periodicity of this structure observed as a mismatch between the honeycomb lattices at certain points along the zz2 edge. Effect of finite sized geometry on the electronic structure of zigzag edge. The important difference between graphene nanoholes and nanoribbons is the presence (absence) of edge corner 0 structure in the former (latter). When two zz1 edges are connected by 120 corner, our atomicallyresolved STM imaging mostly does not reveal the presence of edge localized state in the nanohole¶V corner. This is in good agreement with recent tight-binding (TB) model predictions [9]. However, the 0 similar absence of edge state in 60 corner of zz1 edges looks surprising and deviates from the results obtained within TB calculations scheme. To understand the origin of this discrepancy, we employed 0 DFT calculations to determine the LDOS in a relaxed structure of 60 corner, where two carbon atoms in the corner are not saturated by atomic hydrogen. The presence of two dangling bonds results in structural reconstruction in the corner and reduces the amplitude of the edge state wavefunction in the 0 60 corner. We have also found a circular-like superperiodic pattern in the vicinity of zz1 edge originating from non-uniform distribution of charge density along C-C bonds. Our DFT calculations suggest that 0 R30 superstructure. The latter such pattern can be due to the mixture of the edge state and originates from the intervalley scattering taking place at the ends of short zigzag edges in nanoholes. This is different from the case of infinitely long graphene nanoribbon model where intravalley scattering does not favor the formation of superstructure. We have not experimentally detected significant effects of finite size geometry on zz2 edge. However, the deviations from ideal periodicity mentioned above suggest a possible connection between edge length and thermodynamically stable arrangement of mono ± and di-hydrogenated carbon sites along the edge. Armchair edges of nanoholes. The electronic structure near the straight armchair edge of nanoholes represents wave-like superperiodic pattern, with bias-independent charge oscillations linearly decaying away from the edge. The appearance of the continuous wave-like superstructure indicates the presence of non-negligible charge density of ʌ-electron between carbon sites and therefore deviates from description of graphene honeycomb lattice within TB scheme. Previously, such patterns were considered to be specific to electronic scattering at armchair edges of monolayer graphene as a consequence of localization of the DOS along the C-C bonds [10]. However, our data obtained on multilayer graphene system suggests that this is a general property of graphene sheets regardless of the number of layers. In contrast, the electronic structure near disordered armchair edges can be seen as a combination of rhombic and honeycomb superperiodic patterns, in agreement with previous reports on edges of graphite nanopits [11]. The rhombic-honeycomb pattern can be therefore seen as a partial destruction of wave-like patterns at defective armchair edge. Conclusions. To summarize, we succeeded in detailed visualization of electronic states along the boundaries of graphene nanoholes with well-defined edges. The obtained knowledge will be useful in realization of future electronic devices based on periodical arrangement of nanoholes in graphene (quantum antidots), and may open way towards realization of magnetic nanographene which requires existence of highly hydrogenated zigzag edges [4]. References: [1] K. Nakada, M. Fujita, G. Dresselhaus, and M. S. Dresselhaus, Phys. Rev. B, 54 (1996) 17954. [2] T. Enoki, Y. Kobayashi and K. Fukui, Int. Rev. Phys. Chem., 26 (2007) 609. [3] T. Enoki, Phys. Scr. T, 146, (2012) 014008. [4] K. Kusakabe and M. Maruyama, Phys. Rev. B, 67 (2003) 092406. [5] K. Sasaki, et al., J. Phys. Soc. Jpn., 76 (2007) 033702. [6] Y. Niimi, et al., Phys. Rev. B, 73 (2006), 085421. [7] Z. Klusek, et al., Appl. Surf. Sci., 161 (2000) 508. [8] C. Tao, et al., Nature Phys., 7 (2011) 616. [9] Y. Shimomura, Y. Takane, and K. Wakabayashi, J. Phys. Soc. Jpn., 80 (2011) 054710. [10] H. Yang, et al., Nano Lett., 10 (2010) 943. [11] K. Sakai, et al., Phys. Rev. B, 81 (2010) 235417. Figures
Fig.1. STM image (U=100 mV) of one of the graphite nanoholes observed in the experiment. Two types of zigzag edge discussed in the text are marked by zz1 and zz2.
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