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Published by « Institut Nanosciences et Cryogénie »  INAC, CEA Grenoble 17, rue des Martyrs - F-38054 Grenoble Cedex 9 - France Chief Editor Engin Molva Editors Hélène Ulmer-Tuffigo, Jérôme Planès Scientific Committee Roberto Calemczuk, Pierre Dalmas de Réotier, Thierry Douki, Bruno Gayral, Engin Molva, Jérôme Planès, Jean-Paul Périn, Ricardo Sousa Layout Groupe Curious communication (Dialektic) Photography CEA, GIANT, Air Isère, V. Lassablière, A. Véron, D. Morel, P. Avavian, L. Godart, ESRF Contact jerome.planes@cea.fr Web site (with pdf release in french and english) inac.cea.fr ISSN 2106-3435

2

INAC celebrated its 40th birthday in 2011; at this occasion, a book was published in English and in French. This book can be downloaded or ordered on INAC website.


Contents Trends,

facts and figures

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 to 13

Fundamental

knowledge in condensed matter . . . . . . . . . . . . . . . . . . . . . . . . 14 to 17

Nanoscience

for electronics and photonics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 to 25

Nanoscience

for energy

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 to 30

Physics

and chemistry at the interface with biotechnologies . . . . . . . . . . . . . . . . . . . . . . . 31 to 33

Cryotechnologies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34 to 37

Organization

chart

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38

3


TRENDS,

FACTS AND FIGURES

Trends, facts and

INAC’s scientific positioning INAC’s research activities are organized in three major fields: nanoscience and condensed matter represent 75% of the programs of the institute, cryotechnology around 15% and physics and chemistry at the interface with biology almost 10%. Theses activities are supported by several technological facilities (nanocharacterization, nanofabrication, nanosimulation, cryogenic test facility) and large equipment (synchrotrons, neutron reactors, magnetic resonance).

WHAT DOES INAC DO? 3 MAJOR MISSIONS • Carry out fundamental research internationally acknowledged for excellence. • Do training through research. • Promote research results and emerging opportunities through patents and start-ups.

INAC HAS ALL THE QUALITIES OF CREATIVITY, DYNAMISM AND ADAPTABILITY, BASED ON A CLEAR AND PROACTIVE STRATEGY. 4


figures INAC today is the result of 40 years of scientific achievements During the spring 2011, INAC looked back on the major topics that have shaped its present state, on the women and men that have illustrated its vitality and dynamism. Three key events marked this anniversary year: five conferences, an anniversary day with numerous lectures on June 16th, and the publishing of a book* entitled FROM DRF TO INAC, 40 YEARS OF EXPLORATION INTO MATTER, written by journalists Anne Quantin-Pottecher and Béatrice Méténier, mainly based on interviews conducted during the fall 2010. * Available at inac.cea.fr

…FAST EVOLUTION… 1956-1971 Before DRF / landmarks

• 1956: CENG was born

1971-1990 DRF /

Department of Fundamental Research

• Building up to « the model »

1990-2000 DRFMC /

Department of Fundamental Research in Condensed Matter • The rise of « Nanoscience » > Condensed matter & Cryotechnologies

Louis Néel

Originator of CENG

2000-2012 DRFMC-INAC /

• the era of « excellence » > New funding schemes > Reinforced partnershipS and cross-division programmes > Shared FACILITIES

Daniel Dautreppe The 1st Director

The story continues… 5


Trends,

facts and figures

Organisation of INAC

Spin in Electronics Innovative concepts and applications in the field of spin electronics, up to demonstrators. Fundamental research in nanomagnetism, spindependent transport and innovative materials.

Physics of Materials and Microstructures

Improvement of fundamental knowledge in condensed matter, notably quantum properties of matter. Strongly correlated electron systems and frustrated systems; lowdimensional magnets; quantum transport in nanosystems.

50 peop le

le

76

peo p

Research in nanoscience and nanotechnologies, in the areas of nanomagnetism and spin electronics, nanophotonics and nanoelectronics. Nanostructure fabrication, atomistic simulation.

Statistical Physics, Magnetism and Superconductivity

12 4

pe o p le

450 people across 6 laboratories

peo p

le

l eop 65 p

e

68 people

Cryogenic engineering

Inorganic and Biological Chemistry Chemistry for nanoscience, energy and the environment, health and related biological issues‌ activities ranging from biochemistry to physical chemistry for a better understanding of fundamental mechanisms and to promote application development.

6

69

Structure and Properties of Molecular Architectures Interdisciplinary research in ÂŤ soft matter Âť. Physics and chemistry of molecular and macromolecular systems at the interface with biology. Dynamics of biological interactions, chemistry and physics for organic and hybrid electronics and photovoltaics, ion conducting polymers.

Cryogenics and cryotechnologies for major national and international fundamental research projects in various fields: space, inertial fusion, magnetic fusion. Projects are taken from the ground research phase up to final implementation. Study of turbulence and thermohydraulics.


Shared facilities used by INAC An outstanding environment, due to sharing of leading-edge instruments with our local partners, and thanks to the presence of the large European facilities ESRF and ILL.

PFNC

CRG beamlines INAC, Jülich

CRG beamlines INAC, CNRS

Upstream Technological Platform INAC, FMNT (CNRS, UJF, INP)

Cryotechnologies Cryogenic test facility 400W @ 1.8K INAC

Center for Nanocharacterization INAC, LETI, LITEN, INP

Simulation INAC, LETI, LITEN

New leading-edge equipment arrived in 2011 in the Center for Nanocharacterization (PFNC) Since September, the PFNC hosts the Ultimate Titan electron microscope, the most powerful microscope in the world, with a resolution of 0.5 ångström, and a Dynamic Nuclear Polarization setup at high magnetic fields, yielding a gain in sensitivity of 2 to 4 orders of magnitude as compared with conventional NMR, the third device of its kind in the world.

7


Trends,

facts and figures

INAC’s strategic positioning… A joint CEA-UJF research institute INAC’s strategy is based on three pillars 1 INAC is at the heart of academic research. 2 INAC in CEA plays a bridging role towards technological research. 3 INAC is also at the heart of a unique cryogenic pole in Grenoble. 1

2

At the heart of academic research, with University Joseph Fourier, CNRS, and Grenoble-INP.

INAC belongs to the Physical Science Division of CEA and is positioned upstream from all CEA technological programs: • energy, • technology for health and information, • very large research instruments, • defense and security.

3

1 2 3

A unique cryogenic pole.

INAC IS A MAJOR PARTNER OF THE DIFFERENT LOCAL STRUCTURES THAT ARE: GIANT, MINATEC, NANOSCIENCE FOUNDATION, UNIVERSITÉ DE GRENOBLE.

8

www.giant-grenoble.org

www.minatec.org

www.fondation-nanosciences.fr

www.grenoble-univ.fr

© ESRF


…in its local and national environment INAC TAKES AN ACTIVE PART IN THE NEW LABORATORIES OF EXCELLENCE

INAC takes part in 5 laboratories of excellence (LABEX) • 2 main structural LABEX, involving 75% of INAC’s staff: LANEF and ARCANE. • 3 national network LABEX, involving 8% of INAC’s staff: GANEX, SERENADE and PRIMES. INAC is also a partner in 4 instruments of excellence (EQUIPEX): CRG-F and ECOX related to CRG beamlines at ESRF, EQUIP@MESO dedicated to numerical simulation (partner of GENCI national network), and LASUP for high field magnets.

The laboratories of excellence were created by the French government through the « Plan d’Investissements d’Avenir » launched in 2010.

this scheme shows the distribution of the INAC staff among the different new laboratories of excellence.

LANEF / 260 people

« Laboratory of Alliances on Nanoscience and Energy for the future ». This LABEX brings together Giant’s skills in basic research. INAC represents 35% of LANEF.

serenade / 8 people

60%

2% 2% 4%

GANEX / 11 people « National network on GaN » for sharing knowledge and resources in gallium nitride.

ARCANE / 60 people

17% « Grenoble, a bio-driven chemistry ». This LABEX includes the chemistry labs located in east and west campuses of Grenoble, centered on the interface between chemistry and biology. INAC represents 25% of ARCANE.

« Towards the design of innovative, sustainable and safe nanomaterials » is aimed at eco-designing nanomaterials.

PRIMES / 17 people « Physics, Radiobiology, Medical Imaging and Simulation » for developping new methods and devices for medical imaging.

9


Trends,

facts and figures

Structuring the strategic programs In 2011 our research activities have been structured in thematic domains, supporting our business development. This structure aims at • emphasizing the internal coherence of our strategic programs, • sharing resources, • improving reporting, efficiency and visibility of our activities. The strategic activities of INAC out of which a business development is expected to grow have been gathered in five major domains.

MAJOR DOMAINS ALTERNATIVE ENERGIES

ADVANCED COMPONENTS

FACILITIES chemistry/biology interface advanced cryotechnologies

10

The activity manager and the business developer of INAC are in charge of: • conducting discussions with our partners, • reporting on the patent portfolios, • organizing brainstorming sessions in order to let disruptive approaches emerge, • helping for project submissions. ACTIVITiES Photovoltaics & LED Storage & Fuel cells Superconductors Quantum electronics Magnetics & Spintronics Advanced Optoelectronics Nanocharacterization Simulation Functionalization & Assembly Health technologies Space Cryotechnologies Cryotechnologies for Large Facilities and TGIR


ENERGY: an example of a fast and proactive development within INAC All INAC laboratories are concerned to varying degrees by the field of energy; this transverse thematic implies crossovers between the different laboratories of INAC, and increasing complementary partner­ ships with the Technological Research Division (DRT) of the CEA (LETI and LITEN).

Nano characterization

a common core of skills and knowledge for several thematic issues concerning energy

• PV • storage • Fuel cells • Catalysis • Thermoelectricity • Energy savings

THERMAL TRANSFER

Nano simulation

Nano fabrication

Photovoltaics Fuel cells

29

Storage Other energy thematics

INAC’s general strategy about ENERGY: three steps for launching a new activity.

21 16

1. Proactive redeployment of internal

2. Disruptive approaches relative

to state-of-the-art technologies by using INAC resources in physico-chemistry, as well as shared and large facilities.

3. New strategic partnerships

12

skills with an increased involvement from 42 full-time equivalent people working on energy in 2008 to 109 FTE in 2011 (including 40 permanent scientists).

with academic and industrial partners for joined project submissions (ANR, FP7…).

10

43

20 2008

2011

Evolution of the number of people working in the energy research area between 2008 and 2011, for the various energy thematics. Number of patents

30

energy PORTFOLIO

25 20 15 10 5 0

2002 2003 2004 2005 2006 2007 2008 2009 2010 2011

Evolution of INAC energy porfolio from 2002 to 2011.

11


Trends,

facts and figures

INAC, main figures Great succes in ERC calls (among the most prestigious grants in basic research) with 6 winners in 3 years (compared to 29 ERC grants awarded by CEA globally, and 14 ERC grants in Grenoble, during the same period). 2009 ERC « advanced grant »

2010 ERC « starting grant »

2011 ERC « starting grant »

Bernard Dieny

Dai Aoki

Max Hofheinz

Xavier Waintal

Silvano De Franceschi

Eva Monroy

Other prizes and awards in 2011 Evan Spafadora............... Graduate student research award from E-MRS Jean-Philippe Attané........ Junior member of Institut Universitaire de France Yoann Roupioz................. Masao Horiba Prize Mathieu Pierre................. PhD prize from the Nanoscience Foundation Thiery Livache.................. Oseo Emergence prize for PrestoDiag project Joël Cibert....................... Holweck prize and medal from French and English societies of physics Thuy Ung........................ L'Oréal / Unesco prize for women in science in Vietnam

Publications (a)

(c)

400

average if

300 200

4

3<IF<7,5

100 0

6

IF>7,5

2

0<IF<3

0

2006 2007 2008 2009 2010 2011

2006 2007 2008 2009 2010 2011

In 2011, INAC has published 325 articles in peer reviewed journals of various impact factors (IF).

The average impact factor (IF) lies between 3.5 and 4.5.

(b)

(d)

50

15%

40 10%

30 20

5%

10 0 United States

Germany

Spain Switzerland Japan

United Kingdom

Foreign partners for international co-publications are first the United States and Germany, then Spain, Switzerland, Japan and the United Kingdom.

12

0% 2005 2006 2007 2008 2009 2010

2011

The percentage of publications cosigned at least by two different units (“services”) of INAC has quite doubled in 6 years.


Training through research Increasing the number of PhD students at INAC is a good index of the attractiveness and vitality of the Institute. In 2011, 39 new students joined INAC to embark on doctoral thesis. 56% are French, 21% come from another European country, and 23% from the rest of the world. These 39 new theses are dispatched between several research areas as detailed in the following table:

In 2011, 27 theses were defended, as well as 7 HdR (post-doctoral degree).

New PhDs 50

Ongoing PhDs

120 100

40

80

30

Nanoscience for lnformation and health technologies................ 14 Physics and chemistry at the interface with biology..................... 6 Condensed matter................................................................... 6 Energy.................................................................................... 6 Simulation............................................................................... 3 Cryotechnologies..................................................................... 2 Instrument development .......................................................... 2

60

20

40

10 0

20 0

2003 2004 2005 2006 2007 2008 2009 2010 2011

Manpower Dispatching of the 474 people working at INAC as of December 31st, 2011 (permanent + temporary staff).

Dispatching of the permanent scientists.

13% 39%

CEA CNRS+UJF

12%

12%

CEA

48%

76%

CNRS

Temporary staff

UJF

Budget INAC has an annual budget close to 35 M€. External income represents a significantly increasing resource. External income comes from ANR (37%), from government, institutions, agencies… (27%), from Europe (26%), from industrial revenue for 8% (and 3% are others). In 2011, 27 projects have been accepted by ANR (amount to 5.3 M€) among 87 proposals, giving 32% success. In 2011, 5 projects have been accepted by FP7 (amount to 5.2 M€). Through the FP7 program, and at the end of 2011, INAC has obtained 25 projects which amount to 13.8 M€.

Income

13% 27%

8% 35%

60%

57%

CEA subsidy

Personnel

External income

Operation and equipment

Internal transfer

Internal transfer

Economical impact At the end of 2011, INAC had 144 patents; 22 of them have been filed in 2011. Since 2002, patents in spintronics represent 32% of the total amount of INAC’s filed patents. As a result of the strategic shift undergone in the research area of energy, the number of INAC’s patents has increased from 2 en 2002 to 21 en 2011 for inventions concerning PV, LEDs, electrochemical storage, fuel cells…

Expenses

Total energy 30

Total spintronics

25

Total others

20

New (any portfolio)

150 100

15 10

50

5 0

200

0 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011

Evolution of INAC’s patent portfolios

13


Fundamental

knowledge in condensed matter

-

Strongly correlated systems - Installation and commissioning of the new "top-loading" dilution cryostat with the ability to change the sample at very low temperature; first measurements of de Haas van Alphen oscillations in a pure sample of CeIn3. - Measurements of the phonon dispersion as a function of pressure in uranium validate the theory that describes the complex phase diagram of this element (p. 15). - Observation of magnetic resonance by vector polarization analysis in the new superconductor K0.8Fe2Se2 (Tc=29 K). - Study of the propagation vector of the antiferromagnetic superconductor CeRhIn5-xSnx by neutron diffraction near the quantum critical point. - Study of the ferromagnetic critical point in URhSi high quality single crystals synthesized in the laboratory: ferromagnetism vanishes beyond 3 GPa. - Larmor diffraction measurements of the temperature dependence of the multiferroic YMn2O5 lattice parameters. Characterization by spherical neutron polarization analysis of the excitations in the multiferroic compound HoMnO3, which are basically magnetic and free from lattice component. - Extending the theory of the spin nematic phase to 2D systems with competition between ferromagnetic and antiferromagnetic orders.

Mesoscopic physics, theory and simulation - Two international teams have experimentally validated a theory that predicted the increase the increase in supercurrent in thick Josephson junctions carrying triplet pairs (p. 15). - Measurement by low temperature tunneling spectroscopy of the density of states of graphene in contact with superconducting niobium electrodes: no superconductivity is induced. - Characterization by tunneling spectroscopy of the disappearance of coherence peaks in the local density of states at the disordered superconductor - insulator transition (p. 16). - Theory of Peltier cooling in SINIS junctions (S = superconductor, I = tunnel insulator, N = normal metal): effect of electron-electron and electron-phonon interactions. - Developping the numerical simulation code KNIT for the out-of-equilibrium quantum transport in high-frequency nanodevices. - Numerical simulation of structural and thermodynamic properties of metal clusters on a surface: towards the catalytic growth of carbon nanotubes (p. 17). - Ab initio atomistic study of new SiC cage-like clusters (stability, properties, synthesis routes...) with the BigDFT software. - Development of a new algorithm 30 times faster for the calculation of Green functions using the tight-binding model TB_Sim. Application to the study of nanowires used in transistors (typical diameter: 8 nm). - Fixing an artifact in the "broken symmetry" approximation in the calculation of the magnetic exchange interaction (p. 17).

Contact: pierre.dalmas-de-reotier@cea.fr

14 14


Fundamental

knowledge in condensed matter

Uranium unveiled at last Contact: Stéphane Raymond – SPSMS – stephane.raymond@cea.fr

Modeling strongly correlated systems is a very delicate task. For instance, the theory for the phonon spectrum of uranium has only been available since 2008. Our recent measurements have confirmed this theory and our results combined with new calculations provide an interpretation for the very original phase diagram of this metal. Among simple elements, uranium is certainly the most complex one. Not only is its crystal structure singular, its low tem­ perature behavior is unique as well. While metals are generally either supercon­ ducting or magnetic, uranium is neither: its conduction electrons are spontaneously distributed inhomogeneously and form what is called a charge density wave. This state is quite unstable though, because a modest pressure destroys this charge density wave and transforms uranium into a superconductor. Since superconductivity is the result of the coupling between lattice vibration modes, i.e. phonons, and

the electrons, we studied phonon disper­ sion. Our measurements at the ESRF from inelastic x-ray scattering as a function of pressure are in perfect agreement with the prediction of the ab initio calcula­ tions performed at DAM using the density functional theory. Then, confident in our model, we examined the electron-phonon coupling influence and we have shown that its variations as a function of pressure explain the crossover from charge density wave to superconductivity. These results will be the basis for future progress in our theoretical insight into strongly correlated systems. z

Further reading: Raymond S et al., Physical Review Letters 107 (2011) 136401

Single crystal of uranium (blue square) in the center of the diamond anvil pressure cell. The crystal was thinned to 10 µm at Los Alamos National Laboratory. Diameter of the pressure chamber: 250 µm.

The intensity of x-rays inelastically scattered by phonons varies with pressure according to the model prediction.

Long range proximity Contact: Manuel Houzet – SPSMS – manuel.houzet@cea.fr

The maximum supercurrent (I.e. the dissipationless current) that can flow through a Josephson junction made of two superconductors separated by ferromagnetic material, strongly decreases with the thickness of this material. We had theoretically predicted that the stacking of appropriate ferromagnetic layers would enable the transport of a supercurrent in relatively thick junctions. Three years later, our prediction has been confirmed by two experiments. A superconductor can transmit its properties to a normal metal through the exchange of quantum coherent electron pairs: this is called the proximity effect. When an electron pair in the singlet quan­ tum state is injected into a ferromagnetic metal, each of the electron spins is sub­ mitted to an exchange field of opposite directions. These fields rapidly diphase them, destroying the pair’s coherence. The proximity effect is therefore limited to a few nanometers. A much longer range proximity effect (a few tens of nanometers)

can be observed with multilayers having non-collinear magnetization: a singlet state electron pair in a layer is converted into an equal spin electron pair, called triplet state, in the next layer. The electrons then feel the same exchange field and the pair can survive. In 2007, in collaboration with a colleague from the University of Bordeaux, we had predicted that this long range proximity effect could be checked from the mea­ surement of a supercurrent through a

ferromagnetic trilayer connected to two superconducting electrodes. Using the very same geometry, the effect was experimentally verified in 2010. A group from Cambridge University used a trilayer of Ho/Co/Ho. Another one from Michigan State University considered a multilayer of PdNi/Co. The measurements of the charge current in these structures are consistent with the predictions of a spin-polarized current. We are currently studying whether it is possible to directly probe the pair spin state. z

Further reading: Robinson JWA et al., Science 329 (2010) 59; Khaire TS et al., Phys. Rev. Lett. 104 (2010) 137002; Houzet M and Buzdin AI, Phys. Rev. B 76 (2007) 060504

15


Fundamental

knowledge in condensed matter

An insulating superconductor Contact: Claude Chapelier – SPSMS – claude.chapelier@cea.fr

Were you aware that atomic disorder in a metal could result in a quantum transition to an insulating state? Then, what about it when the metal is also a superconductor? Spatially resolved tunneling spectroscopy reveals the spectral signature of this transition in this paradoxical context. Superconductivity relies on the formation of electron pairs, called Cooper pairs (see inset 1), which condense below Tc in a ground state described by a delocalized electronic wave function, i.e. it extends through the whole sample. The presence of moderate disorder does not funda­ mentally alter this mechanism, despite scattering effects arising from impurities and material defects. However, if the metal is strongly disordered, quantum inter­ ferences associated with the multiple reflections on the defects result in the localization of the electron wave functions. The metallic state is thus changed into an insulating state, a process called Anderson localization (see inset 2).

Superconductivity vs. localization Physicists very soon surmised that this quantum transition should affect the properties of superconducting metals. It is indeed difficult to imagine that the macroscopic spatial extension associated with superconductivity can coexist with the microscopic localization of the electrons. However, several theoretical studies pre­ dict that Cooper pairs are less prone to localization than single electrons. As a consequence, some strongly disordered

compounds could still exhibit supercon­ ductivity and shift to an insulating state only for a higher critical disorder, when Cooper pairs would also localize. Do these Cooper pairs exist for real or only in the wild imagination of some theore­ ticians? To settle this point, let's observe superconductivity … locally!

Tunneling spectroscopy to the rescue The differential conductance between the scanning tunneling electron microscope (STM) tip and the sample surface is pro­ portional to the local density of electronic states. This measurement therefore probes the energy gap in different locations on the surface. Usually, coherence peaks, are observed on each side of the gap. We have discovered though that in a strongly disordered superconductor close to the insulating state, amorphous indium oxide, the intensity of the coherence peaks strongly varies with the probed region on the nano­ meter scale, and that in numerous spectra obtained in the most disordered samples, the peaks were simply absent. The figure shows two thermal dependencies of the density of states for two different samples. In the less disordered sample, the majority of the spectra is similar to the left-hand

Hundred-year old electron pairs In 2011, we celebrated the centenary of the discovery of superconductivity, this dramatic property which enables the dissipation-less flow of electrons through some metals when they are cooled below a critical temperature Tc. The rigorously vanishing electrical resistivity was theoretically explained only in 1957 thanks to studies by Bardeen, Cooper and Schrieffer (BCS) who were awarded the Nobel Prize in physics for these results. These researchers realized that electrons need to form pairs in order to superconduct. The formation of pairs, called Cooper pairs, coincides with the disappearance of single electron states and with the opening of a gap in the density of states. This gap, of width 2 Δ, is characterized by coherence peaks at energies ± Δ.

side panel drawing. For the more disordered one, the right-hand side panel is observed almost everywhere. Based on systematic statistical studies, we have observed a proliferation of peakless spectra when disorder increases. The disappearance of the coherence peaks is thus correlated to the onset of an insulating state. z

Anderson localization

Thermal dependency of the density of electronic states measured by STM in regions of less than 1 nm2 in two disordered superconducting samples. The black line indicates the critical temperature Tc which decreases for an increasing disorder. For the region corresponding to the left-hand-side panel, coherence peaks are observed below Tc on each side of the gap, signaling the condensation of delocalized Cooper pairs. In the right-hand-side panel, these peaks are absent, indicating Cooper pair localization. Further reading: Sacepe B et al., Nature Physics 7 (2011) 239-244

16

Only a year after the theoretical interpretation of superconductivity and in another context, Anderson developed a model describing how an ordinary normal metal transforms into an insulator under the action of the lattice potential inhomogeneity. This inhomogeneity, which is related to atomic disorder, is responsible for an electron quantum confinement which results in an exponential decrease of the wave function spatial extension. In a three-dimensional space, this localization induces a metal-insulator transition for high enough disorders.


How do nanotubes grow? Contact: Steven Blundell – SPSMS – steven.blundell@cea.fr

Based on numerical simulations, we have reached a new step in our understanding of the mechanisms that drive the growth of carbon nanotubes. One of the most commonly used methods for synthesizing carbon nanotubes is chemical vapor deposition (CVD). The reaction involves a liquid catalyst placed on a substrate. This catalyst consists of metallic clusters (iron, cobalt, nickel…) whose size and shape, among other parameters, determine the properties of the nanotubes such as their diameter and quality. Since the details of the processes involved in the growth of nanotubes are not well understood at present, it is helpful to model the cluster properties by com­ puter simulation. The interactions between the few tens of atoms forming the cluster, as well as their interaction with the substrate and the carbon atoms, are the input parameters

for the calculation. Using the molecular dynamics method, and by modeling the interaction between atoms of the cluster with a classical potential with parameters valid for bulk materials, we had in 2009 determined the shape of the clusters and their melting temperature as a function of the strength of the interaction with the substrate, as shown in the figure obtained for a cluster of Co55. However, for clusters with so few atoms, quantum effects that were not accoun­ ted for in the above-mentioned work are important. We have recently modeled the interactions between the atoms by an ab initio quantum calculation, starting with sodium clusters as the first simple application. We find notable differences

with the classical model regarding the shape and melting points of the clusters. We are now going to consider the metals which are used to grow nanotubes. z

Further reading: Blundell SA et al., Physical Review B 84 (2011) 075430

The dialog of spins revisited Contact: Jean-Marie Mouesca – SCIB – jean-marie.mouesca@cea.fr

In a molecule bearing several unpaired local spins, one of the pertinent parameters that govern its magnetic properties is the magnetic exchange interaction constant J that describes the way spins “talk to each other” through the atoms connecting them (bridges). The RM laboratory, by revisiting one of the theoretical chemistry methods dedicated to the computation of J, has identified a major flaw in this approach, the correction of which opens the way to an improvement of the calculation of J. When a molecule has several unpaired spins (for example two Cu2+ ions corres­ ponding each to a local spin ½), these spins can establish a “dialog”, and a scale of magnetic levels appears (in the pre­ vious example, a triplet ↑↑ and a singu­ let ↑↓ – ↓↑ states). The relative energies of these levels are directly related to the magnetic exchange interaction constant J, the value of which depends on the nature of the spin carriers (metal, radical) as well as on the geometry and chemistry of the diamagnetic bridges that connect them

(OH¯ ions for instance). From a practical point of view, these interactions are responsible for the magnetic behavior of the molecule: ferro- or antiferromagnetic. A widespread method for the calcula­ tion of J is based on what is called the “broken symmetry” state (BS, ↑↓). Yet, when compared to experiment, this calculation overestimates J typically by a factor of 2. In order to identify the source of the problem, an analytical expression of J was derived by taking into account

the physical mechanisms involved in the BS state, and was parameterized by the Density Functional Theory (DFT). We have shown that most of the error came from the BS procedure itself that artificially breaks the symmetry of the bridge orbitals ΦC, which were no longer diamagnetic (a process never reported until now). After correction of this unwanted effect, the J values become comparable to those obtained by other more sophisticated methods at the same physical level. z

Further reading: Onofrio N. and Mouesca J.-M., Inorganic Chemistry 50 (2011) 5577-5586

17


Nanoscience

for electronics and photonics

Nanomagnetism, spintronics, nanoelectronics - Demonstration of the SPICE software for noise modeling of hybrid CMOS/magnetic tunnel junctions electric circuits. - Fabrication in the Actemium deposition machine of a magnetic tunnel junction with a perpendicular magnetization exhibiting 70% magnetoresistance. - Development of a method of sample preparation of multilayer discrete magnetic media for the observation of local magnetic fields by electron holography (p. 19). - Proof of concept of MRAM writing by spin transfer (STT) and thermal assistance in perpendicular magnetized junctions. - New method of writing magnetic memory cells by a polarized current which does not cross the multilayer (p. 20). - Measurement of the magnon magnetoresistance in a planar nanowire (p. 21). - Observation of the spin Hall effect in platinum detected in a non-local spin valve geometry, and detection of dimensionality effects. - First observation of the effect of an electric field on a GeMn layer; correlation between the magnetic anisotropy and the crystalline or amorphous structure of GeMn nano-columns (p.22). - Observation of a spin injection signal at 300 K in a germanium channel. - Fabrication and characterization of transistors based on self-organized SiGe quantum dots showing a very high on-off current ratio (> 105) up to 200 K. - Electron pump developed using single electron transistors on SOI in CMOS technology: its high quality makes it appropriate for use in amp metrology (p. 22). - Detecting the charge granularity in a silicon transistor gate and the charge state change of a single dopant through a radio frequency single-electron transistor.

Nanophotonics - Study of an optical nanosystem consisting of twin nanocavities on an SOI integrated chip with sub-wavelength coupling. - Tuning of the growth geometry of InGaN/GaN heterostructures by acting on the substrate polarity (p. 23). - Fabrication of GaN nanowire based photodetectors with a very high UV vs. vis. contrast and fast switching (p. 24). - Controlling the polarization of the spontaneous emission of quantum dots in photonic wires with elliptical cross-section. - Single photon emission at 300 K by a II-VI quantum dot analyzed by photon correlation (p. 24). - Development of a photonic switch with switching time less than a picosecond (p. 25). Continuous operation at room temperature of a dual-mode microlaser, precursor of a THz source. - Development of a GaN quantum dot based chemical sensor working in a hostile environment.

Contacts: ricardo.sousa@cea.fr bruno.gayral@cea.fr

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Nanoscience

for electronics and photonics

Imaging the magnetic field of multilayer dots Contact: Éric Gautier – SPINTEC – eric.gautier@cea.fr

At Spintec, in collaboration with the Laboratory of Advanced Microscopy of SP2M, we have developed a sample preparation technique for electron holography imaging. With this technique, we can draw a map of the magnetic field in a sample consisting of magnetic bilayers developed for multilevel magnetic storage.

Fig. 1: Stages in the manufacture of the sample and observation by holography.

The sample is a pre-patterned substrate, i.e. a matrix of pads prepared on a silicon substrate by conventional microelec­ tronic techniques, and on which magnetic materials for storage of information are deposited. In our approach, each pad contains two pieces of information: the average value of the magnetic field above the pad and the field gradient between the two edges of the pad. Thus, we obtain four distinct magnetic configu­ rations (2 bits). The goal here is to measure by elec­ tron holography (inset) the spatial distri­ bution of the magnetic field around these pads, like the one that is captured by a read-head. The sample must be very thin, about 100 nm typically. The pads are 180 × 80 nm spaced by 80 nm. It is the­ refore necessary to reduce the sample to a thin slice consisting of a single row of dots. These slices are obtained by FIB (inset).

The sample is first thinned from the rear face down to a thickness of about 5 microns. At this thickness and with a voltage of 30 kV, it is then possible to observe the dot array by electronic trans­ parency. The substrate can then be milled by a beam of gallium from the rear face. This avoids having to use protective layers that could affect the observation of the magnetic field around the dots. Once the sample is reduced to a single row, it is glued onto a sample holder and observed by electron holography perpendicular to the plane of the strip (Figs. 1 and 2.) The hologram that is obtained is then used to calculate the spatial distribution of the magnetic field within the sample. The measurements obtained are fully consistent with theoretical predictions (Fig. 3). z

Electron holography Electron holography is an imaging method that uses a transmission electron microscope and is based on the phase difference between two halves of the electron beam. One passes through the sample to form the wave image, while the other part forms the reference wave. The interference pattern between these two waves gives the hologram, i.e. the spatial phase shift induced by the magnetic field of the sample. It is then possible to calculate through integration the spatial distribution of the magnetic field.

FIB (Focused Ion Beam) An FIB is an instrument that resembles a scanning electron microscope that uses a focused ion beam (typically gallium) instead of an electron beam. The ions etch away the target atoms, therefore the FIB is a tool for micro­fabrication and not observation. It is used for preparing samples for transmission electron microscopy which requires very thin samples. The equipment acquired jointly by the PTA, the PFNC and the CMTC, also includes an electron column for in situ imaging.

Fig. 2: Sample preparation: slices of (a) a line of dots, (b) six lines and (c) ten lines. (d) Sample slice glued to holder.

Fig. 3: Interference images (a) measured and (b) obtained by simulation. The magnetic configuration of the dots is presented in (c), which also shows the distribution of the magnetic field around the dots.

Further reading: Moritz J et al., IEEE Magnetics Letters 2 (2011) 4500104

19


Nanoscience

for electronics and photonics

Better memories at lower power Contact: Gilles Gaudin – SPINTEC – gilles.gaudin@cea.fr

Spintec researchers, in collaboration with the Catalan Institute of Nanotechnology and the Autonomous University of Barcelona have developed a new technique to write information in a magnetic memory cell in a more stable manner and at a lower energy cost. This technique opens up very promising concepts in terms of performance memories and offers new features to develop logic functions using magnetic memory cells. MRAMs (Magnetic Random Access Memories) are a new type of memory that combines speed, low power, high-density non-volatile information (persistent even in the absence of power) and immunity to ionizing radiation. A set of properties that no other type of memory possesses. MRAMs consist of magnetic tunnel junc­ tions (MTJ), stacks of two ferromagnetic layers (FM) and a tunnel barrier oxide. Their resistance varies with the relative orientation of the magnetization of FM layers. To write information in a cell, the magnetization direction of these layers must be changed with a magnetic field, or a spin-polarized current must be injected perpendicular to the layer plane (STT writing). Each method has its drawbacks: difficulty of increasing the integration density for the first, loss of long-term relia­ bility due to the passage of the polarized current for the second. In these structures, a compromise must always be struck between stability and power consumption for writing: an FM layer that is "hard" (with higher anisotropy) retains information better because it is less sensitive to thermal fluctuations but it requires more energy to be written.

magnetic field that can flip the magne­ tization of the Co layer. Our device mea­ sures 200 x 200 nm, and the current pulse duration is less than 10 ns with an amplitude of the order of 2 mA. These values should improve significantly in the future with technological advances. What did we win? By being able to flip the magnetization of the storage layer of an MTJ with a write current in-plane, the current flow through the tunnel barrier is avoided (in STT writing) thus avoiding deterioration. Reading is done as usual by measuring the resistance of the MTJ, thus the paths of reading and writing are separate. This makes it possible to use higher resistance values that improve readability and avoid the risk of accidental writing.

Writing with an in-plane current Our new method is to reverse the magne­ tization of a magnetic nanostructure with perpendicular magnetization by applying a current in the plane of the layers. How? The nanostructure - cobalt in our sample is inserted between two different electrodes of platinum-and aluminum oxide (Fig. 1). The asymmetry of the interfaces creates an electric field which, combined with the injected current, generates an effective

Fig. 1: The magnetization of the Co layer is perpendicular to the layer plane, and according to its direction, it can encode a 1 or a logic 0. The asymmetry of interface between the electrodes of Pt and AlOx of the device creates an electric field E parallel to the magnetization direction. Then, by applying a current I in the plane of layers, an effective magnetic field Heff parallel to Ux is created due to the Rashba effect, but also a torque directed along the same axis. Applying a weak magnetic field Happ parallel to the current direction simultaneously with the injection current then results in the bipolar reversal of the magnetization.

Further reading: Miron IM et al., Nature 476 (2011) 189-193

20

Fig. 2: Demonstrator of a programmable switch using two magnets (in blue) for generating a static magnetic field. The Pt electrode is shown in gray, AlOx in yellow and the Co dot is at the center of the stack

Balancing stability and low power This method has other advantages. First, this property seems to be stronger in hard layers compared with soft layers. This means that unlike existing structures, high stability would come with low power consumption for writing! Second, it is possible to control the direction of the reversal due to the presence of a weak constant magnetic field applied parallel to the current direction. Thus, the same current can lead to a parallel or antiparal­ lel state depending on the direction of the field. This makes it possible for example to change the functionality of a circuit or part of it by changing the direction of the static field locally (Fig. 2). Finally, the separation of reading and writing current paths allows for three-terminal devices (ex. transistors), where a control terminal determines the electrical behavior between the other two. This additional degree of freedom in particular, makes it possible to associate an intrinsic memory function and a logical function in the same device. Although the physical origin of this phenomenon is still poorly unders­ tood and will require developing a solid theoretical framework, the actual performance and expected improvements suggest that it will contribute significantly to expand the boundaries of micro­ electronics. z


Wall detection with mmr Contact: Van Dai Nguyen – SP2M – van-dai.nguyen@cea.fr

Magnetic imaging techniques can be used to observe magnetic walls. We have shown that this can also be achieved by measuring electrical resistance. This is done based on the collective vibrations of the magnetic moments (magnons). We are able to position a domain wall in a lithographed nanowire with a precision of around one percent.

Fig. 1: Magnetoresistance of a FePt nanowire. The “bow-tie” effect mimics the magnetization hysteresis.

In a ferromagnetic metal, magnetic moments (i.e. spins) are not rigorously aligned if the external magnetic field is not very strong or if the temperature is high enough. Still, their directions and their motions are not random and follow a collective behavior described as “ma­ gnons” (see inset). In the same way as phonons, magnons interact with the conduction electrons and thus have an influence on the resistance. This magne­ toresistance effect (see inset) is called MMR, magnon magnetoresistance. MMR has been known for a long time, in parti­cular its linearity and symmetry as a function of magnetic field, if it is strong enough. We have studied what happens under weak applied magnetic field in “magnetically structured” materials such as nanowires that contain magnetic domains separated by a wall. We study a nanowire made of FePt, a ferromagnetic material with perpendicular magnetization that has two stable ma­ gnetization states (up/down or positive/ negative). Let us follow the green arrows on Fig. 1. With a strong positive field, the magnetization is up and the spin lattice is stiffened with respect to the thermal vibrations: the magnon population is weak and so is its contribution to the resistance.

We diminish the field intensity, the ma­ gnon population increases, increasing the resistance. This stays true until the magne­ tization reverses for a certain negative value of the field. A jump in the resistance is then observed, followed by a continuous decrease as the field becomes more and more negative. The magnon population diminishes again due to the alignment of the magnetic moments with the field. By considering in more detail the portion of the curve described by the blue arrows, a resistance plateau is observed during the reversal. This intermediate state corresponds to the pinning of a magnetic domain in the wire. One part of the wire is magnetized in one direction and the other in the opposite direction. The fraction of the wire with a given magnetization can be measured as explained in Fig. 2. We have also detected the magnetization reversal in permalloy nanowires by MMR curves similar to the one shown in Fig. 1. Permalloy is however a soft magnetic material, but shape anisotropy is sufficient to force a magnetization aligned along the nanowire main axis, in one direction or the other, thus defining two magnetization states. z

Fig. 2: Determination of the location of the wall responsible for the plateau observed in Fig.1. By stopping at this plateau and subsequently further decreasing the applied magnetic field, a diminution of the magnetoresistance is observed. The slope is directly related to the fraction of the wire length having a positive magnetization, so that the position of the wall in the wire can be determined: 24%. This position is confirmed by visualizing the wall by magnetic force microscopy.

Further reading: Nguyen VD et al., Physical Review Letters 107 (2011) 136605

Magnons When a ferromagnetic material at zero temperature is saturated, the magnetization points in the same direction in the entire material. At the microscopic level, the magnetic moment of each atom points in this direction. If the temperature increases or if the applied field decreases, the magnetic moments can deviate slightly from their equilibrium position and start to precess around it. Due to the strong coupling between neighboring moments (responsible for ferromagnetism) this precession is coherent. For an atom chain carrying a magnetic moment, the trajectory of the moments carried by the atoms can be described as a wave called spin wave. Based on the wave-particle duality principle, we can then associate a quasi-particle called magnon with this excitation. All the tools from quantum mechanics can then be used to describe the interaction between these magnons and the electrons of an electric current flowing through the ferromagnetic material.

Magnetoresistance Magnetoresistance (MR) is the variation of electrical resistance as a function of applied magnetic field. Depending on the orientation of the electric current, of the magnetic field and on the measured voltage, several MR values are defined. The longitudinal MR is the classical MR where the current and the field are aligned and the voltage drop is measured on the same line. Anisotropic MR and Hall MR stand for in-plane and out-of-plane perpendicular field orientation. Let us now take a stack consisting of two different ferromagnetic layers separated by a non-magnetic metallic layer. Let us flow some current and measure the voltage through this stack. By applying a magnetic field that reverses the magnetization in the softest material without modifying the hardest one, two configurations – parallel and antiparallel – of the magnetization are obtained. The resistance of the first one is much lower than that of the second one. One speaks of giant magneto­ resistance (GMR). This is the effect discovered by Albert Fert for which he received the Nobel prize. Its amplitude can reach around ten percent. If the non-magnetic layer is replaced by a tunnel barrier, one speaks of tunnel magnetoresistance (TMR). Its amplitude is even larger than in the case of GMR. This effect is currently used in the read heads of hard disks.

21


Nanoscience

for electronics and photonics

Morphology induced magnetization Contact: Abhinav Jain – SP2M – abhinav.jain@cea.fr

Ferromagnetic semiconductors are very attractive for spintronics: the spin polarization of carriers is strong and the magnetic properties can be controlled with an electric field. So far, the manganese-germanium alloy (GeMn) grown in thin films is the best candidate because of its Curie temperature of 400 K and its compatibility with silicon technology. We have successfully established a correlation between the magnetic anisotropy and the structural quality of the layers. GeMn thin films are deposited by mole­ cular beam epitaxy at low temperature (Te). These layers contain Mn-rich zones (40% Mn) that form nanocolumns. At Te=100 °C, these nanocolumns are well crystallized and in epitaxy with the Ge substrate. At Te=150 °C, they partially relax and become amorphous. An unders­ tanding of magnetic anisotropy is impor­ tant as it determines the behavior of the layer when it is subject to a magnetic field.

[001] surface of a Ge0,9Mn0,1 thin film for Te= 100 °C (left) and for Te= 150 °C (right). The Mn-rich columns are seen standing. The insets show that they are crystalline on the left and amorphous on the right.

Measuring the magnetic anisotropy is difficult as Mn, which carries the magnetic moment, is also present in low quantities in the Ge matrix. By coupling SQUID magnetometry and electronic paramagnetic resonance, we have shown that the structure of the nanocolumns determines

the material anisotropy. When they are amorphous, the columnar aspect is pre­ dominant and the easy magnetization axis is perpendicular to the thin film. On the contrary, when they are crystalline, the easy axis is in-plane. z

Further reading: Jain A et al., Applied Physics Letters 97 (2010) 202502

Electron pumps Contact: Xavier Jehl – SPSMS – xavier.jehl@cea.fr

We have designed and fabricated the first electron pump using silicon on insulator (SOI) in a standard microelectronics technology. These devices could be used as electrical current standards. The definition of the ampere in the in­ ternational system of units is based on the force associated with an electrical current flowing in two parallel conductors assumed to be of infinite length. A new definition of this unit in terms of a funda­ mental constant, namely the elementary charge, is forthcoming.

an appropriate sequence of voltages at frequency f, a single electron goes through the device at each period. The intensity of the direct current flowing through the device is thus I = e f. Frequency being a quantity that can be determined with high precision, the electron pumps are of great interest for metrology.

An electron pump is a device with a series of islands which are sufficiently decoupled from each other so that their charge, i.e. their number of individual electrons, can be controlled. Through the polarization of these Coulomb islands using control gates, single-electron transfers can be achieved. If, in addition these gates are driven with

Current devices which are made of aluminum need to be operated at a tem­ perature near 50 mK. Moreover, they are very fragile and limited to operation up to 50 MHz. In collaboration with Leti, we have designed an electron pump from single electron transistors made on SOI in CMOS technology. For the first time,

Electron micrograph of our electron pump showing the gates (red), the spacers or barriers (green) and the nanowire (blue) as well as the source and drain.

the detection stage has been integrated into the device which, in addition, is very weakly sensitive to nearby electric charges. The measured current is proportional to the frequency up to 400 MHz. The device’s characteristics are going to be tested at metrological level. z

Further reading: M. Pierre et al, Proceedings of the Conference on Precision Electromagnetic Measurements (2010) 755-756

22


Growth of GaN: wire or pyramid? Contact: Joël Eymery – SP2M – joel.eymery@cea.fr

The CEA-CNRS-UJF joint “nanophysics and semiconductors” group studies the vapor phase growth of GaN-based nanostructures, including InGaN heterostructures. By understanding the growth mechanisms, we can obtain original structures such as wires or pyramids. Their practical interest is demonstrated in emitters and photodetectors.

Fig. 1: Radial and longitudinal heterostructures in nanowires.

The use of gallium nitride (GaN) compound semiconductors has recently become very popular in visible optoelectronics to obtain blue and white light sources. Current technologies for solid-state lighting or LCD screen backlighting are based on planar growth of InGaN/GaN quantum wells. The indium composition and the thicknesses are controlled so as to choose the emitted color. Blue light is obtained quite easily, while green and yellow emitters are more difficult to master. The use of GaN wires brings several important advantages. The light extrac­ tion efficiency is increased due to a larger free surface area. Their structural qua­ lity and hence their optical properties are much less affected by a lattice parameter mismatch with the substrate than equiva­ lent planar layers. Longitudinal or radial heterostructures can be grown (Fig.1) that take advantage of the elastic relaxation properties at free surfaces in nanowires. Finally, the integration of wire-like objects is of interest both for electrical devices by simplifying the contacting and the definition of the doped zones and for optoelectronics devices for which an adaptation of the wire diameter provides efficient waveguiding of the light along the wire.

Our laboratory has had a precursor role in France in the last few years by developing the growth of nitride semiconductor-based wires by metal-organic vapor phase epitaxy. This research has led to some surprising results. In the simplest case of GaN homoepitaxy, it is possible to choose the growth morphology either as wires (Fig. 2 a) or as pyramids (Fig. 2 b) by changing the polarity of the substrate (see inset), i.e. by flipping it upsidedown! Both these morphologies are used to grow heterostructures. For wires, (Fig. 3 a,b) the lateral surface covered by InGaN/GaN quantum wells is controlled by the growth conditions. In this case,

Fig. 2: Influence of the substrate polarity on the crystal morphologies. For the same growth conditions through a thin Si3N4 layer deposited on a GaN substrate, the result is either (a) -c oriented wires or (b) +c oriented pyramids (see the inset for the definition of the axes).

Polarity Two views of the wurtzite structure of GaN, with opposite polarities. The +c axis that links the cation to the anion is a polar axis, meaning that one of its ends cannot be linked to the opposite end by a symmetry transformation of the crystal. This property results from the absence of inversion symmetry along the growth axis.

the growth is on non-polar surfaces (the lateral facets of the wire) so that contrarily to growth along the c-axis of the crystal (in that case the wire’s long axis), there is no local electric field to induce a spontaneous spatial separation of electrons and holes. For pyramids, whose section is controlled by the Si3N4 mask and a selective growth, quantum dots can be grown at the tip (Fig. 3 c,d). The regularity of these objects determines precisely their emission properties. Heterostructures based on dispersed horizontal wires have already been used in collaboration with Institut d’Électronique Fondamentale (Orsay) to make photo­ detectors and emitters. Vertical radial heterostructures on Si substrates covered by a thin polar AlN buffer are currently being investigated in collaboration with Institut LETI. Directly contacted through the substrate, their integration in devices is facilitated. z

Fig. 3: Multiple InGaN quantum wells (around 16% indium) on GaN wires. The wires are oriented along -c (see inset) and are observed (a) from the side in scanning electron microscopy and (b) in cross-section in transmission electron microscopy. (c-d) Views from the top of +c oriented pyramids (see inset) with InGaN quantum dots grown at their apex.

Further reading: Bugallo ADL et al., Applied Physics Letters 98 (2011) 233107; Chen XJ et al., Journal of Crystal Growth 322 (2011) 15-22; Koester R et al., Nano Letters 11 (2011) 4839-4845

23


Nanoscience

for electronics and photonics

A single nanowire photodetector Contact: Fernando González-Posada – SP2M – fernando.gonzalez-posada@cea.fr

Semiconductor nanowires have become the building blocks for the fabrication of a new generation of photonic components. In this context, the CEA-CNRS joint group NPSC fabricates high crystalline quality GaN nanowires. They show a remarkable sensitivity in the ultraviolet as we have demonstrated by measuring the photocurrent on a single nanowire. Nanowire conductivity increases drastically under ultraviolet excitation (wavelengths shorter than 360 nm) polarized along the nanowire main axis. In contrast, no response is measured under visible (blue, green or red) excitation. This blindness in the visible spectrum is a particularity of nitride semiconductor based photo­ detectors. When the GaN is grown as a thin film, the contrast between the ultraviolet and the visible absorptions is

Photocurrent measurement principle and intensity variation as a function of the angle between the nanowire axis and the linear polarization of the incident light. The figure of 8 is characteristic of the light coupling with the nanowire. In the background: micrograph of MBE-grown GaN nanowires.

around 1,000. Now with GaN nanowires, this contrast reaches 1,000,000, due to their exceptional crystalline quality. Another advantage of the nanowire photodetectors is their response time in the millisecond range, while a photo­ current can persist during minutes or even hours in the case of thin films.

We now will study heterostructured nanowires in order to increase the photoconduction gain. Once the single nanowire response has been optimized, we will fabricate organized matrices of nanowires, the ultimate goal being the fabrication of high resolution imaging devices that mimic the human retina. z

Further reading: González-Posada F et al., Nano Letters, 12 (2012) 172

Photon correlations probe spectral diffusion Contact: Kuntheak Kheng – SP2M – kuntheak.kheng@cea.fr

Semiconductor quantum dots have potential applications, in particular in the field of quantum cryptography. The precise determination of the emission properties of these emitters is thus an important stake. The CNRSCEA joint group NPSC has developed a new experimental technique that can be used to measure a fundamental property of these emitters based on the correlation of photons stemming from the same transition. A single quantum dot can be considered to be an artificial atom that emits photons at a well defined wavelength upon the radiative relaxation of an excited electron. This nanometer-scale box is not isolated but is embedded in a matrix in which other charges fluctuate, for instance by trapping/detrapping on crystal defects. This fluctuating environment leads to fast random spectral jumps (called spectral

diffusion) of the narrow emission line and results in an apparent broadening of the transition. We present a new experimental method for measuring the characteristic times of spectral diffusion of the photolumines­ cence based on the correlation of photons emitted by the same quantum dot transi­ tion. We performed both autocorrelation measurements on half of the emission

peak and cross-correlation measurements between the high energy and low energy halves of the emission peak. This allowed us to measure its characteristic times τD and τCX. This measurement is done with a time resolution of 90 ps, four orders of magnitude better than previously reported results. z

Further reading: Sallen G et al., Physical Review B 84 (2011) 041405; Sallen G et al., Nature Photonics 4 (2010) 696-699

24


An ultrafast optical transistor Contact: Jean-Michel Gérard – SP2M – jean-michel.gerard@cea.fr

In the same way that the invention of the transistor revolutionized electronics, the development of ultrafast all-optical switches is a major stake for very high rate information processing, for instance for very high rate telecommunications. In collaboration with the University of Twente, the NPSC team has developed a photon switch with a response time below one picosecond, i.e. ten times faster than previous devices. An optical switch is for instance a resonant system that lets light through only at a well defined wavelength λ0 that can be modified so as to switch from an "off-state" to an "onstate". In our case, we use a semiconductor cavity with a transmission resonance in the infra-red at λ0 = 1.28 µm. Under a “pump” optical beam, λ0 varies as electron-hole pairs are created in the active material (GaAs) and modify its optical index. The “on” switch is ultra-fast, but not the transi­ tion back to “off” because it is dependent upon the recombination of the carriers. To improve the device, we have used the Kerr effect: a material under strong optical

illumination undergoes an instantaneous modification of its index of refraction. In this case, the pump wavelength (2.4 µm) is such that no carriers are photocreated by a oneor two-photon absorption. We have shown the relevance of this approach by studying the transmission of the cavity with a probe beam. When the pump beam is shone onto the sample, the micro­ cavity resonance is shifted due to the Kerr effect and the probe beam is blocked. In our experiment (Fig.), we vary the delay between pump and probe pulses; a shift of the ca­ vity resonance λ0 during the pulse length is observed. The double commutation time

Reflectivity as a function of the wave-number for various delays between the probe and pump pulses.

(on-off-on) is below one picosecond. This switch could thus be used to modulate an optical telecom beam at record high rates, above 1 THz. Our study now aims at miniaturizing the device so as to reduce the commutation energy. z

Further reading: Ctistis G et al., Applied Physics Letters 98 (2011) 161114

25


Nanoscience

for energy

Low carbon energies - Demonstrating the feasibility of in situ and in operando confocal Raman microscopy study of water management in fuel cells. Increased spatial and temporal resolution in cell measurements by use of X-rays at ESRF (p. 23). - Ex situ validation of the concept of polyaromatic membranes with improved stability for PEMFC. - Structure and transport characterization of Nafion thin film in the catalyst layer of a fuel cell (p. 24). - Simulation by molecular dynamics of ionomer membranes for PEMFC: equilibrium configurations, nanostructure of the matrix, dynamical properties. - Designing cells for the study of the solid electrolyte interface (SEI) of Li-ion battery electrodes by scanning electrochemical microcopy without water and oxygen. - Demonstration of the influence of the silicon nanowire diameter and of the n or p doping level on the capacity of the supercapacitor electrochemical double layer in organic electrolytes and ionic liquids. - MBE synthesis of GaAs / AlGaAs heterostructured nanowires and early studies of the alloy Cu2ZnSn(S1-xSex)4 free of rare or toxic elements. - Setup of the ultra-high resolution transmission electron microscope FEI Titan Ultimate; identification of lithiated phases in a battery electrode using precession. - 3D reconstruction by coherent scattering of synchrotron radiation of nanowires containing a strained layer. - Characterization of spin states in electroactive conjugated polymers by pulsed EPR (p. 24). - Correlating specific Si surface chemical functions with high-resolution solid-state NMR data in functionalized silicon nanoparticles. - First tests of CO2 reactivity with a family of heavy metal complexes in low oxidation state. - Coupled pulsed ESR / electrical detection setup for the study of materials for quantum bits and for photovoltaics (p. 25). - New polymers based on fluorenone and benzothiadiazole exceeding 1.8% and 2.1% yield in bulk heterojunction organic solar cells with C70 fullerene. - Deposition of multi-scale electrodes on donor-acceptor heterojunction thin film, and in situ electrical characterization under selective irradiation by QPlus AFM nanostencil (p. 26). - First transport measurements by time of flight in CdSe/P3HT hybrid materials for photovoltaics; Monte Carlo simulation of transport properties. - Selective grafting of CuInS2 nanoparticles on ZnO nanowires and first development of Extremely Thin Absorber type solar cells. - Identification of the quenching mechanism of the fluorescence of semiconductor nanocrystals occurring during "click" chemistry functionalization.

Contacts: bruno.gayral@cea.fr roberto.calemczuk@cea.fr thierry.douki@cea.fr

26


Nanoscience

for energy

An x-ray study of fuel cells: down-to-water management Contact: Gérard Gebel – SPrAM – gerard.gebel@cea.fr

A fuel cell generates electricity, heat and water. Water distribution strongly impacts the performance of the battery: the how and why deserves attention! Researchers at Inac and Liten had already put an operating fuel cell in a neutron beam. Now they have placed the system in an X-ray beam at ESRF. The spatial resolution improves from the centimeter to the millimeter, the temporal resolution from 10 minutes to 15 seconds. X-rays thus provide observations of the water at the heart of the battery with very fine accuracy and in transient regime.

Fig. 1: Fuel cell designed for X-ray scattering experiments. In the center, the 6 recesses specially made for the beam. Typical size of the cell is 10 cm.

Water is a reaction product of the proton exchange membrane fuel cell (PEMFC): it is formed at the cathode. It is also the vector of proton transport: without water, the electrolyte membrane does not conduct. Water is however inhomoge­ neously distributed (see inset). This uneven distribution causes serious performance and degradation problems. Knowledge of local operating conditions by non-intrusive measurement methods is essential for understanding the degradation mechanisms and for optimizing PEMFC operation.

Fig. 2: Small-angle X-ray scattering spectra. When the current density increases, the peak grows and shifts to smaller angles, indicating the swelling of the membrane, much more marked under the rib than in the channel.

For several years, Liten and Inac have worked together to determine the water concentration profiles in the PEMFC membranes. The analysis of the early work done by neutron scattering revealed non-uniform concentration profiles in the thickness of the membrane. The origin of these profiles is the competition between the electro-osmosis process, i.e. the process that drives a flow of water molecules from the anode to the cathode by the protons, and the backscattering process, i.e.the diffusion of water from the cathode to the anode under the effect of a concentration gradient.

Water and gases: where do they go? The anode and the cathode of a PEMFC are both composed of a monopolar plate in which a gas circulates. It is distributed in a serpentine channel, while the ribs press the stack consisting of a gas diffusion layer (porous carbon cloth), an electrode where the chemical reaction takes place (carbon and platinum catalyst) and finally the proton exchange membrane, impermeable to gases. The heterogeneity of the water concentration in a cell occurs as well between the two electrodes - the cathode is always wetter than the anode - as between gas inlet and outlet. The water is extracted by gravity and is recycled in particular for humidifying gases.

More accurate and faster In-operando characterization of a fuel cell has recently been adapted to small angle X-ray scattering (Fig. 1). Why X-rays? To significantly improve the spatial and temporal resolution. Spatially, the area of analysis is reduced from 1 cm² with neutrons to 1 mm² with X-rays. Therefore, a much finer local analysis is possible. For instance, it is possible to compare the amount of water under a rib and in front of a channel of the monopolar plate. As for the temporal aspect, with X-rays, measurementsevery 15 s are possible. This is a very important asset for the study of transient phenomena: when you switch on the battery for example, or when studying the kinetics of hydration. The experiment performed at ESRF is the only non-invasive technique delivering this kind of information with such a resolution for an operating fuel cell. Two important results have already been obtained: - At the cathode, the amount of water in the membrane increases between the

Fig. 3: Microfluidic simulations predicting the amount of water in the membrane: it is higher under the rib than in the channel. On both sides of the membrane are the electrodes and gas diffusion layers.

inlet and the outlet, although water is generated continuously and everywhere in the membrane. - The amount of water is always higher in front of a rib than in front of a channel (Fig. 2). These results are in good agreement with fluid modeling of water transfer through the various parts of the fuel cell (Fig. 3). Further analysis of the results will specify the local water concentration profiles in the membrane. Later on, these studies will be carried out for thin membranes and operating temperatures below 0 °C and above 100 °C. z

27


Nanoscience

for energy

Like membranes, like electrodes Contact: Sandrine Lyonnard – SPrAM – sandrine.lyonnard@cea.fr

Some of the secrets of fuel cell electrodes have been uncovered thanks to large facilities: microstructure and transport properties have recently been studied by neutron scattering in the in the context of a joint Nissan and CEA work project. Surprisingly, Nafion® as a thin electrolyte layer collecting protons in the electrode has similar properties to those of Nafion® in its thick membrane form. In each electrode of a fuel cell, the catalytic layer (CL) is a multi-function compound of carbon grains containing nanoparticles of Pt and impregnated with a solution of Nafion®. The porous carbon collects electrons and lets gases (O2 and H2) diffuse as they arrive from the diffusion layer (GDL). Pt catalyzes the electrochemical reactions. Nafion® is the electrolyte that collects protons and disseminates them to (or from) the electrode/membrane interface. It takes the form of a 5 to 10 nm-thick film cove­ ring the carbon grains. We know very little about its properties. Yet everything happens

precisely there: optimizing fuel cells currently involves optimizing membranes but also electrodes, and understanding the mass transfer. Can we characterize the microstructure and proton transport in Nafion® inside the electrode? The answer is yes! We have been able to perform this specific study by neutron scattering at the ILL and the LLB in Saclay. The structural organization, the

swelling properties in water, and the proton dynamics within a 5 nm-thick film of Nafion® are very close to those of the membranes, yet 10 ,000 times thicker! This study provides the first quantitative set of data on the mole­ cular and nanometer scale, which will supply useful input for the current models of fuel cell design, still based on assumptions and not on measurements. z

Pulsed esr sorts spins Contact: Vincent Maurel – SCIB – vincent.maurel@cea.fr

How much do spins in a complex and disordered paramagnetic material contribute to macroscopic magnetization? Through the study of doped polyaniline samples, the SCIB/RM laboratory jointly with the SPrAM/LEMOH and the Polytechnic University of Warsaw have shown that pulsed Electron Spin Resonance (ESR) is an efficient method to “sort” and identify the different spin states. How to quantify the magnetic properties of a sample? With a magnetometer, one sums without distinction all the contributions. ESR spectroscopy with continuous wave is better suited for paramagnetic systems, but does not permit to easily identify the spin states present in a complex material. In pulsed ESR, it is possible to measure the contributions of the different spin states – the famous S of quantum mechanics – in a univocal way. We applied this method to new materials of interest for molecular magnets: the doped polyanilines. These polymers, mostly studied for their electrical properties, have recently

been found to be interesting for their ma­ gnetic properties also. Our colleagues from Warsaw have synthesized specific polymers for several purposes: i) to reach a high level of doping. More than 70% of the monomers carry an unpaired electron and thus an electronic spin. ii) to induce ferromagnetic coupling between the spins created by the doping. The figure shows the spectrum corres­ ponding to a branched polymer exhibiting several different spin states: non-coupled spins (S=1/2), but also higher spin states showing that a noticeable fraction of

Further reading: Gosk J et al., Journal of Applied Physics 109 (2011) 074911

28

electrons are coupled by pairs (S=1), by 3 (S=3/2) and by 4 (S=2). The second spectrum, corresponding to a linear polymer is very different: all of the electrons are coupled but only two by two (S=1). It is thus possible to better understand the magnetic interactions that occur in the polymer and to propose novel structures in order to visualize higher spin states. z


Impulsive and sensitive esr spectroscopy Contact: Serge Gambarelli – SCIB – serge.gambarelli@cea.fr

Pulsed ESR (electron spin resonance) is a method of magnetic resonance where magnetic moments associated with electronic spins are detected and analyzed. In the SCIB/LRM group, we use a technique of electrical or optical detection of the ESR signal that makes it possible to increase the sensitivity of an ESR spectrometer by 6 orders of magnitude. The principle of ESR is simple: the studied sample is placed in a static magnetic field that orientates the electronic spins. These are then manipulated and detected by pulses of very short and very intense hyperfrequency waves (a few GHz). Because ESR detects magnetic moments associated with electronic spins, it is possible to study all the compounds bearing unpaired electrons (free radicals, metal ions in specific oxidation states, triplet states, ferromagnetic systems …). In contrast, ESR is completely blind to the diamagnetic systems that constitute the vast majority of the matter that sur­ rounds us. At first glance, it may be seen as a severe limitation. Actually, this is a major advantage because a variety of small paramagnetic sub-systems are created from diamagnetic systems in response to perturbations (ionizing radiations, photons, oxidation, ageing processes...). ESR can thus study just the active parts in the middle of a sea of diamagnetic systems: it seems like the microscopic version of the search for a needle (paramagnetic) in a haystack (diamagnetic).

Let’s take the example of a photovoltaic device: light interacts with the compo­ nents (pigments, doping, silicon …) that are initially diamagnetic, and creates excitons, triplet states, pairs of radicals … all paramagnetic. We study these pheno­ mena on a model system, for example a photoactive compound like C60, directly irradiated in an ESR spectrometer, usually with a pulsed laser. Using specific detection methods, it is possible to ana­ lyze the paramagnetic species present in the sample as a function of time with a resolution of a few nanoseconds. The result is bidimensional maps (Fig. 1). In our laboratory, over the years we have developed a pulsed ESR coupled to a laser. More recently, we have purchased an optic device allowing us to generate laser pulses of a duration of a few nanoseconds on a continuous range of wavelengths (0.4 – 2 µm). It is thus possible to study physical phenomena involved in these systems as a function of the wavelength. Do we have a totally efficient spectros­ copic method? Almost! In order to accurately study individual systems

Fig. 1: Example of a pulsed ESR study on a simple system: C60 diluted in an organic matrix. Just after the laser pulse (t=0), a detectable species rapidly appears, characterized by the general shape of the signal and the resonance magnetic field. Specific pulsed-ESR experiments (not shown) attribute to each species an electronic spin S=1 and a chemical structure made only of carbons with no hydrogen: this is the triplet state of C60. The signal of this species rapidly decays as a function of time because of relaxation phenomena.

(for instance in nanostructures), the sensitivity of the approach is sometimes too low. How could we increase it? By optical detection or detection of electrical current. In this case, the electromagnetic wave of the ESP spectrometer is used to manipulate the electronic spins of the photoinduced species in order to modify their yield of recombination or dissociation. This results in a change of the fluorescence or photocurrent inten­ sity directly and very efficiently detectable by the pODMR or pEDMR methods (Optically or Electrically Detected Magnetic Resonance, Fig. 2). They combine a sensitivity increase of a factor of 106 compared with classical ESR with all the possibilities of pulsed ESR spectroscopy. This apparatus is under development in our laboratory. It will be unique in France and comparable with the best systems designed in Germany and in the United States. It will be used in particular for our research on new technologies for energy and on quantum calculation. z

Fig. 2: In a classical spectrometer (left), the manipulation of spins and the detection are done with microwaves (in red). In an EDMR system (middle), the manipulation of spins is still done with microwaves but the detection relies on the measurement of a photocurrent. Last, in the case of an ODMR apparatus, the detection is done through a monochromator and a photomultiplier which collect the photons emitted by the sample.

29


Nanoscience

for energy

Drawing through the mask, it’s on the tip Contact: Benjamin Grévin – SPrAM – benjamin.grevin@cea.fr

An ambitious instrumental project has just succeeded in the LEMOH laboratory, providing a way for developing mesoscopic devices within an atomic force microscope (AFM), by evaporation through a mask, in combined mode with imaging. This achievement paves the way for the fabrication and in situ study of new devices of interest in physics, nanoelectronics, photovoltaics...

Fig. 1: The evaporator/fork/mask system.

The concept originated in 2005 from the need for measurements of local electronic properties in organic materials for the future: conjugated polymers, blends, hybrid systems. At that time, a few labs had developed nanostencils, but the coupling with the AFM required highly technical know-how. Routine service was thus almost impossible, especially because the most classical detection method was used: the deflection of a laser beam focused on the AFM tip. However, our lab masters the technique of the tuning fork, which is a resonator and a self-sensor for the AFM. We therefore built our project on this basis and mounted a chip with the stencil and the AFM tip on one branch of the fork (a configuration called QPlus.)

Theoretical and practical approach We thus launched two major tasks: i) fabrication of the system evaporator / fork / mask; ii) integration into the AFM. The device consists of (Fig. 1): - an evaporator and a collimating diaphragm, - a piezoelectric scanner tube driving the fork, drilled in its center to let out evaporated molecules or atoms, - the tuning fork,

- a silicon-based chip with a structured Si3N4 membrane, part of which contains the stencil, - an integrated AFM tip, - an X-Y closed loop nano-positioning table for wide field imaging. The masks were etched by Leti. The modeling of QPlus vibrational modes in every configuration was performed by Iramis. The mechanical developments were done entirely by Lemoh (Fig. 2). All had to meet the requirements of AFM imaging (wide field, high resolution, accuracy in (re)positioning), of static and dynamic design of electrodes (Fig. 3).

Fig. 2: The nanostencil integrated in the AFM housing.

We have experimentally demonstrated that, despite the addition of a massive chip on the fork, high-resolution imaging (about 10 nm) is possible on a wide range of samples, including soft materials. Spec­ troscopic measurements show that the AFM control processes are fully efficient. The AFM tips made by focused ion beam (FIB) appear well suited, both for topogra­ phic imaging and spectroscopy. The po­ sitioning of the probe is reproducible to a few tens of nanometers after a trip of several tens of microns. The simultaneous evaporation of patterns in the micrometer (several hundred µm²) and nanometer range is possible, either in static, dynamic or sequential mode. To date, patterns that are 165 nm-wide

Fig. 3: Examples of performance in simple mode (pattern imaging with a separate probe) and combined mode (imaging with the tip integrated in the nanostencil).

have been obtained in dynamic mode. Gold, silver or C60 were deposited under high vacuum at a speed of 0.1 to 1 nm per minute.

New implementation project Although nanoelectronics (organic transistors) and optoelectronics were originally the main objectives, new needs have emerged, for example for organic photovoltaics. A new project (Fig. 4) involves model planar junc­ tions between thin-film organic single crystals (hole conducting materials) and C60 (electron conductor). The QPlus nanostencil is used for the measurement of both cell performance under illumi­ nation and intrinsic properties (mobi­ lity) of the components. We thus have a flexible way of fabricating and studying multiple samples without using lithography at any step. z

Fig. 4: Planar junction for organic photovoltaics. The organic single crystal (blue) is deposited on the ITO and glass transparent anode. The C60 (purple) and the aluminum cathode (yellow) are evaporated through the stencil. Several cells of this type (varying thicknesses and sizes) have been produced and characterized on a unique single crystal.

Further reading: Grévin B et al., Review of Scientific Instruments 82 (2011) 063706; Hayton J et al., Review of Scientific Instruments 81 (2010) 093707

30 30


Physics and chemistry at the interface with biotechnologies

- First "nanoswimmer" type magnetic particles for biotechnologies. - Development of two approaches for cell sorting and release, one based on enzymatic cleavage, the other on an original desorption process by plasmon heating. - Innovative design of artificial tongue from combinatorial sugar biochip systems. - Aqueous solubilization of InP/ZnS fluorescent nanocrystals and multi-functionalization by rare earth complexes (MRI contrast agent and fluorescence) and by cell penetrating peptides (p.â&#x20AC;Ż32). - Development of turnkey data analysis bioinformatics tools for biochip test users. - Development of new original DNA structures (hairpin oligonucleotides) as sensors and modulators of DNA repair systems. - Demonstration of the genotoxicity of isolated TiO2 nanoparticles on single cells (p.â&#x20AC;Ż33). - Market survey for copper chelators developed in the laboratory and potentially interesting in the treatment of diseases involving copper deregulation (e.g. Wilson's disease). - Dynamic Nuclear Polarization (DNP) setup on a new NMR spectrometer leading to increased sensitivity and reduced acquisition time (factor of 10 to 1,000).

Contacts: thierry.douki@cea.fr roberto.calemczuk@cea.fr

31


Physics

and chemistry at the interface with biotechnologies

Shine on you crazy nanocrystal Contact: Peter Reiss – SPrAM – peter.reiss@cea.fr

In collaboration with Leti, we have successfully transferred fluorescent semiconductor nano­crystals into aqueous medium without degrading their colloidal stability nor their fluorescence efficiency. This technique applies to many families of nanocrystals and is particularly attractive for non-toxic ones, used in biological applications. With our colleagues from SCIB and Grenoble Institute of Neuroscience, we have developed imaging agents that penetrate the cells, are fluorescent, and increase the contrast in MRI.

Fig. 1: Left, the nanocrystals covered by their hydrophobic synthesis ligands are concentrated in the organic phase (chloroform). On the right, capped by their new hydrophilic ligands, they find themselves in water. In order to obtain high colloidal stability, it was necessary to select a basic pH (~ 9.2) that promotes the deprotonation of the thiol function -SH: the link between the nanocrystal and the thiolate -S¯ function is more stable. The fluorescence of the nanocrystals is obtained under UV illumination.

Fluorescent semiconductor nanocrystals (or QDs for "quantum dots") offer high performance today: their quantum yield is almost 100%, the emission line is extremely narrow (i.e. their color is very well defined), and they show excellent photostability. The first bright QDs were based on cadmium, a toxic metal, but nowadays more "biocompatible" systems such as InP/ZnS core/shell nanocrystals are synthesized, which are the subject of this work. All QD features sound very attractive for biotechnologies, except for the (big) detail that the synthesis occurs in organic media. To transfer QDs into aqueous media, the environment of the living, we must replace the hydrophobic surface ligands of the synthesis by

hydrophilic ligands. Simple? No! Known protocols lead to two major flaws: i) the solutions are not stable, ii) 90% of the fluorescence is lost.

Colloidal stability For a good use of QDs it is essential to keep them in colloidal suspension and avoid aggregation. This is the role of the ligands covering their surface. The thiol function -SH is conventionally used, particularly for anchoring on metals such as gold. But in the case of QDs with a ZnS shell, bonding with the thiolate function -S¯ is much stronger. Therefore, the ligand has to be deprotonated, which is done by precisely controlling the pH.

Fig. 2: An undesirable side effect of the phase transfer in basic pH is the formation of a dithiol bond between two cysteine ligands. The S-S bridge is a trap for photogenerated positive charges in the nanocrystal, which are no longer available for fluorescence. An efficient remedy is the addition of the reducing agent TCEP. In addition to its reducing action, it may also serve as a ligand for passivating the nanocrystal. With this treatment, more than 90% of the initial fluorescence in chloroform is retained in water.

For the cysteine ligand, the optimum value is pH 9.2. With this protocol, we increased the colloidal stability of the QDs from one day to several months (Fig. 1).

Luminescence The second problem that we solved is the loss of luminescence during the exchange with cysteine. This problem, already encountered in previous works that ope­ rated the ligand exchange in acid medium, is accentuated in basic medium. We found out the main mechanism and how to avoid as detailled in (Fig. 2). Even better is to use TCEP directly in the phase transfer step, which virtually eliminates the problem. The protocol developed for InP/ZnS is equally valid for CdSe QDs, with 0, 1 or 2 shells, for nano-rods, etc.

Probes for imaging Cysteine is a small molecule, so that the size of the nanocrystal with its ligands is less than 10 nm, an advantage allowing the natural excretion of the QDs. We made multimodal probes by grafting other func­ tions, such as rare earth chelates. Among them, gadolinium is an excellent contrast agent for MRI. Our optical/magnetic probes significantly improve the perfor­ mance of commercial products: higher contrast due to their high relaxivity and retention times extended to several hours (Fig. 3a, b). We also managed to co-graft a peptide, maurocalcine (MCa), essential for cell penetration (Fig. 3c). All this in one step!. z

Fig. 3: a) MRI image of a rat brain 4 hours after injection of the contrast agent Gd-MCa QDs. b) With the commercial contrast agent Dotarem ®, there is no signal after 4 hours. c) False-color view of the presence of contrast agents within Chinese hamster ovary cells. Penetration of the cell does not happen without the presence of MCa peptide, grafted on the fluorescent nanocrystal (green). The cell walls (red) are colored with rhodamine.

Further reading: Stasiuk GJ et al., Acs Nano 5 (2011) 8193-8201; Tamang S et al., Acs Nano 5 (2011) 9392-9402

32


Cells under TiO2 cream Contact: Marie Carrière – SCIB – marie.carriere@cea.fr

The impact of nanoparticles on health is a burning question for the general public. Teams from the SCIB and IRAMIS/SPAM have studied the damage caused to cultured human cells by nanoparticles of titanium dioxide, used in sunscreens and in paints. The toxicity of nanoparticles is not related to the chemical nature of the materials they are made of, which are very classical, but rather to their extreme state of division that gives them a potentially novel surface reactivity. The determination of the intrinsic toxic properties of nanomaterials has a strong implication: they must stay in this size range for the entire duration of the experiment. Using ultrasounds to disperse nanoparticles of controlled size and shape, we have limited their aggregation and created more reproducible experimental conditions than those of a number of published works.

A nanoparticle is toxic only if it enters the cell. To study this parameter, we incubated cells in the presence of TiO2 nanoparticles. We observed by electron microscopy and analyzed by synchrotron-based techniques that nanoparticles may go through the cell membrane and even reach the nucleus. Is there then a risk of inducing DNA damage? Yes, free radicals are produced in the pre­ sence of nanoparticles and induce strand breaks and oxidized bases. The presence of nanoparticles also strongly decreases the DNA repair capacities, which makes the phenomenon worse. Thus, when they are isolated, and if the cells themselves are isolated, TiO2 nanoparticles have a

A cell that has internalized TiO2 nanoparticles. These accumulate in vesicles of the cytoplasm and sometimes reach the nucleus.

genotoxic potential. It now remains to be evaluated how the natural barriers of the organism (skin, mucous membranes) in an environment conducive to the aggregation of particles (salts, proteins …) decrease the direct contact with cells and limit the toxicity observed in these model experiments. z

Further reading: Jugan ML et al., Journal of Biomedical Nanotechnology 7 (2011) 22-23

33


Cryotechnologies

Magnetic or inertial fusion and big lasers - Operability of the JT60-SA tokamak cryogenic system designed for the absorption of power peaks up to twice the average power demonstrated in HELIOS loop (p. 35). - Technical specifications recommended by the SBT for the JT60-SA tokamak cryogenics reviewed by a European panel. - SBT involved in cryogenics of large power lasers in the framework of the European Extreme Light Infrastructure (ELI) coordinated by the Institute of Physics in Prague, on the basis of SBT expertise in cryogenic hydrodynamics and pellet injectors.

Space and cryocoolers - Development phase of the engineering model of the cooler for the Spica/Safari mission, for which SBT is in charge of the thermal architecture and of the 50 mK refrigeration system design (p. 36). - Favorable position in a European proposal for the cryogenic chain of Athena and for the Bliss project for which the SBT delivered a cooler at the Jet Propulsion Laboratory (NASA). - Development of a new type of thermometer, contactless and insensitive to electromagnetic interference, using the luminescence decay of semiconductor nanocrystals (p. 36). - Development of a miniature pulse tube operating at 100 Hz with a cooling power of 0.25 W at 120 K. - The Large Pulse Tube Cooler "LPTC", for which a license was granted to Air Liquide, has been selected for Meteosat Third Generation and for military observation satellites CSO. - Development of pulsated heat pipes (PHP), their priming system, and upstream research investigations on these PHP (p. 37).

Contact: jean-paul.perin@cea.fr

34 34


Cryotechnologies

Helios smoothes hot flashes Contact: Christine Hoa – SBT – christine.hoa@cea.fr

JT-60SA is the future tokamak of the Japan Atomic Energy Agency (JAEA). Superconductive magnets will be used to confine the plasma. The fusion reactor is jointly constructed through the Broader Approach Agreement involving Europe and Japan and it shall be in operation in 2016. CEA has the responsibility of the cryogenic system procurement. One of the difficulties of the system is the high variable heat loads that the system shall absorb during the cycling plasma operation. In order to validate the technical solution, a dedicated experimental cryogenic loop has been designed, built and tested with success at CEA Grenoble. down mock up of the cryo-distribution of the central solenoid superconducting magnets. The HELIOS loop is driven by forced convection with Supercritical helium at 4.4 K under 5 bar (secondary loop, see schematic). The experimental investiga­ tions have brought a better understanding of the transient phenomenon induced by the heat pulses. This large installation can reproduce conditions of pressure, tempe­ rature and transport times, similar to those expected in the cooling circuits of the central solenoid superconducting magnets of JT-60SA. Fig. 1: HELIOS loop in the multi-test cryostat at SBT.

The thermal loads from the DeuteriumDeuterium nuclear reactions, the current losses in the coils, and the eddy currents in the structures, induce high variable heat loads. The cryogenic system shall absorb the transient loads encountered in large tokamaks and maintain the supercon­ ducting magnets at 4.4 K. In JT-60SA, the peak loads could reach up to twice the average refrigeration power over one cycle. Different methods are under study to protect the system from an overcharge during a pulse load, to optimize the refrigeration capacity and to reduce the investment and operation costs. The aim of the research work is to ensure safe operation with a refrigeration capacity close to the average power over one cycle.

The first experimental campaigns were dedicated to commissioning the test loop and calibrating the instrumentation. The first pulse scenarios were tested under different configurations. The thermal buffer operation has been validated (Fig. 2): the peak loads representative of the tokamak operation have been repro­ duced and smoothed before they arrive in the refrigerator, by means of a saturated helium bath ( thermal reservoir, see sche­ matic). The stored energy in the reservoir induces an evaporation of helium and a mass flow to the refrigerator. The latter is regulated (see schematic) and remains within the available refrigerator capacity limits. For the Japanese reactor, the peak

Operation of the HELIOS loop This loop is a mock up of the cooling circuits of the magnets. It is a secondary loop that exchanges heat with the primary loop of the refrigerator though 2 heat exchangers, immersed in a thermal reservoir at 4.3 K. The flow is driven by a cold centrifugal circulator with a mass flow of 32 g/s. It takes about 10 minutes for one helium particle to travel the complete loop. In order to smooth the thermal loads, the amplitude shall be reduced and the phase shifted so the arrival of the loads to the refrigerator is delayed. The high pulsed loads are applied on the secondary loop during the first 170 seconds of the 30 minute cycle, and then they are stored in the loop and in the thermal helium reservoir. The energy is released progressively to the refrigerator during the remaining time of the cycle, before another cycle starts. Instrumentation can characterize the thermo-hydraulic evolution (temperature, pressure, mass flow) both of the loop and of the thermal reservoir.

load of about 12 kW can be absorbed by the system with a refrigeration capacity of 6 KW (average power over one cycle) and with a thermal reservoir of about 7 m3. Investigations have started to explore other modes of operation and to find proper smoothing methods with dedicated controls. z Fig. 2: The thermal reservoir stores the energy deposited at the beginning of the cycle and releases it progressively to the refrigerator.

Smoothing methods applied upstream the refrigerator, at the cryo-distribution level are being explored and validated with a versatile test bench built at the laboratory: HELIOS (HElium Loop for hIgh lOads Smoothing, Fig. 1) is a 1/20 scaled Further reading: Hoa C et al., Cryogenics in press (2012) DOI: 10.116/j.cryogenics.2012.02.004

35


Cryotechnologies

Safari cryocooler on its way Contact: Lionel Duband – SBT – lionel.duband@cea.fr

Gaze at the Eagle Nebula, close to the centre of our Galaxy. Star forming regions can be spotted in this picture (centre and left portion). According to astrophysicists, hundreds of stars are born here. This picture was taken in April 2010 by two of the instruments on board the Herschel satellite whose detectors are cooled by cryocoolers provided by CEA-SBT.

The follow-up, SPICA (Space Infrared Telescope for Cosmo­ logy and Astrophysics), is currently under definition. SPICA is a Japanese led mission, that features a European core instrument SAFARI (SpicA FAr infraRed Instrument).

The follow-up, SPICA (Space Infrared Telescope for Cosmology and Astrophysics), is currently under definition. SPICA is a Japanese led mission, that features a Euro­ pean core instrument SAFARI (SpicA FAr infraRed Instrument). Among other objectives, SAFARI will focus on the star forming mechanisms. This instrument is expected to be at least one order of magnitude more sensitive than Herschel. This can only be achieved by developing detectors cooled down to 50 mK. A cooler developed at CEA-SBT, combining a sorption stage and an adiabatic demagnetization stage, will provide this temperature. The cooler shown in the picture was developed under a research program funded by the European Space Agency. For SAFARI, CEA-SBT is in charge of the coo­ lers and will deliver engineering, qualification, flight and spare models for a launch expected around 2020. Our development plan has been validated by CNES and funding for the first phase has been approved. Further reading: Duband L et al., Cryogenics 52 (2012) 145-151; Luchier N et al., Cryogenics 52 (2012) 152-157

A nanocrystal-based thermometer Contact: Daniel Communal – SBT – daniel.communal@cea.fr

In collaboration with SPrAM, SBT has developed a temperature sensor based on fluorescent nanocrystals and optical fiber for temperature measurements in the range 4-40 K. This type of sensor is based on the fluorescence decay of nanocrystals. The decay time, i.e. the time it takes for optically excited electrons to return to their ground state, depends strongly on temperature between 4 and 40 K. The SPrAM laboratory produces CdSe nanocrystals surrounded by a double shell of CdS and ZnS, whose fluorescence yield is the highest in the world. They are also very photostable. Their decay time was Further reading: Patent FR2959308 (2011)

36

measured in the laboratory in the range 7-40 K for different excitation frequencies (see Fig.) In practice, the nanocrystals are mixed with PMMA (plexiglas) and deposited in layers onto the tip of the optical fiber. With this kind of device remote measurements are possible, even without contact, and are insensitive to the magnetic field (at least up to 7 Tesla) or to electromagnetic interference. A patent was filed. z

Fluorescence decay time of CdSe/CdS/ZnS nanocrystals as a function of temperature. Inset: Orange fluorescence of nanocrystals excited by a blue diode.


Cryogenic pulsating heat pipe Contact: Philippe Gully – SBT – philippe.gully@cea.fr

The distribution of cold power in cryogenic systems is becoming increasingly important, in particular because of the replacement of cryogen baths by mechanical cryocoolers which are local cold sources. Two-phase systems (heat pipes) are good candidates to address this new problem. The SBT has designed, manufactured and tested a cryogenic pulsating heat pipe (PHP) that uses helium as a working fluid. For distances greater than several meters, metallic thermal straps made of copper, aluminum, etc. have mediocre conduction over mass ratios. Thermal links that use a two-phase liquid vapor fluid circulation are attractive. Among them, the PHP has an efficient thermal performance. The laboratory prototype is a simple small and sealed pipe partially filled with liquid helium. The tube is bent into several turns (5 for this prototype, see Fig.) between the condenser (the cold point) and the eva­ porator, where the object to be cooled is

located. The tube diameter must be chosen so that a series of liquid slugs and vapor bubbles can form inside the tube. The heat transfer between helium, the condenser and the evaporator leads to self-sustained oscillations between the two ends of the PHP. These oscillations provide for an efficient heat transfer with a complex mechanism. Good thermal

performance is achieved with a 0.4 K temperature difference at 4.2 K and for 145 mW transferred heat power. Our prototype is 10 cm long, but could be longer. The cooling down time of a PHP alone as presented in the diagram takes 9 hours, which is still very long. But SBT already have the solution: stay tuned… z

Heat pipe priming Contact: Philippe Gully – SBT – philippe.gully@cea.fr

Two-phase thermal links can transfer high power over long distances with an attractive efficiency/mass ratio. At cryogenic temperatures, a priming system is needed to reach the operating temperature. The SBT has designed, manufactured, and successfully tested a new priming system to equip a pulsating heat pipe. Two-phase thermal links use the circulation of liquid and vapor at thermal equilibrium. They are capable of trans­ ferring high thermal power over several meters. The capillary forces, the gravity forces or oscillations generated by evaporation and condensation processes are used for the fluid circulation. The fluid (hydrocarbon, nitrogen, helium) is chosen according to the working temperature. At cryogenic temperatures, these ther­ mal links must be primed or pre-cooled before operation. The priming system designed by SBT features two reservoirs with liquid and vapor at saturation and

thermally anchored to the cold source. They are connected together with a capillary tube via the object to be cooled. One reservoir is heated, the pressure increases and pushes the liquid outside of it towards the object to be cooled where it evapo­ rates. Then the vapor is condensed in the other reservoir. The alternate heating of

the two reservoirs quickly drains heat from the object towards the cold source. This priming system has been successfully used and tested for the pre-cooling of a cryogenic pulsating heat pipe. z

Further reading: F. Bonnet et al., to appear in AIP Conf. Proc. 1435 (2012); Patent # 11 53726 (France, May 2, 2011)

37


Organization chart as of June 2012

Head of Institute: Engin Molva Deputy Head: Armelle Mesnard Strategic evaluation, International relations: Hélène Ulmer-Tuffigo Patents, Business development: Pascal Besesty Education and training: Isabelle Schuster Program manager of “Chemistry for nanoscience”: Gérard Bidan Safety engineer: Laurent Miquet Communication and Scientific information: Jérôme Planès

SP2M ( 124 people ) Physics of Materials and Microstructures Jean-Michel Gérard

SPrAM ( 68 people ) Structure and Properties of Molecular Architectures Jean-Pierre Travers

Semiconductors, Spintronics, Photonics, Nanoelectronics, Nanomagnetism, Energy, Clusters, TEM, Simulation, Near field microscopy, ESRF

Polymers, Nanocrystals, Biochips, Molecular electronics, Photovoltaics, Fuel cells, STM, ESRF Joint research unit CEA/CNRS/UJF

Joint research unit CEA/UJF

SPSMS ( 76 people ) Statistical Physics, Magnetism and Superconductivity Jean-Pascal Brison Superconductivity & magnetism, Strongly correlated electronic systems, Mesoscopic physics, Superconductor compounds, ILL

SCIB ( 65 people ) Inorganic and Biological Chemistry Pascale Maldivi DNA damage, Coordination chemistry, Contrast agents, Heavy metals scavengers, Molecular materials for nanoelectronics, NMR, ESR Joint research unit CEA/UJF

Joint research unit CEA/UJF

SBT ( 69 people ) Low Temperatures Alain Girard

SPINTEC ( 50 people ) Spintronics and Technology of Components Jean-Pierre Nozières

Cryotechnologies for national & international programs or organizations, LMJ, CERN, ITER, Space applications

MRAM magnetic memories, Ultra High Density recording, Reprogrammable magnetic logic, RF components

Joint research unit CEA/UJF

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Joint research unit CEA/CNRS/UJF associated wih G-INP


N° 12 - July 2012

SBT

I SCIB I SPINTEC I SPRAM I SPSMS I SP2M

Highlights

BULLETIN inac.cea.fr

2011

Joint units with

For six years now, Inac has been committing an original work to an artist for the Bulletin’s cover. Photographer and video maker Jing Wang is the author of the 2012 edition. She lives and woks in Grenoble. www.jingwang-art.com

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la recherche, ressource fondamentale research - a fundamental resource Direction des Sciences de la Matière - Grenoble


Bulletin INAC 2012