On the Impact of Swift Electrons on Mass Transport Phenomena at the Nanoscale
Daniel Knez
(1,2,3)
(4)
(4)
(4)
(4)
, Martin Schnedlitz , Maximilian Lasserus , Alexander Schiffmann , Wolfgang Ernst , Ferdinand Hofer
(2,3)
, Gerald Kothleitner
(2,3)
1. Istituto Officina dei Materiali (IOM), Italian National Research Council, Trieste, Italy 2. Institute of Electron Microscopy and Nanoanalysis, Graz University of Technology, Graz, Austria 3. Graz Centre for Electron Microscopy, Graz, Austria 4. Institute of Experimental Physics, Graz University of Technology, Graz, Austria
Introduction Ÿ Ÿ Ÿ Ÿ
In situ heating experiments in the TEM can give valueable information about diffusion processes at surfaces and interfaces. Processes induced by heat are, however, often difficult to separate from electron beam induced effects on an atomic scale. In metallic systems: beam induced dynamics only due to elastic scattering (radiolysis and beam heating effects can be neglected) [1] We compare high-resolution STEM experiments with molecular dynamics simulations.
Results
Methods 4
Synthesis of metallic clusters in superfluid He:
Beam induced phase segregation in NiAu clusters: Experiment: Ÿ In situ heating above the miscibility gap leads to alloying Ÿ Temperature: 600 °C, electron energy: 300 keV Image 1
+ High purity structures (UHV, no surfactants) [2] + Clusters can be deposited on any surface + With variable morphologies
7
n = 5.6 x 10 e
Image 23
Image 49
2 nm
-
9
n = 1.4 x 10 e
Image 88
Au
-
9
-
n = 3.0 x 10 e
Image 150
Image 205
Elastic scattering: Theory of momentum transfer m0
e
Ni
9
n = 5.4 x 10 e
-
9
n = 9.2 x 10 e
10
-
-
n = 1.3 x 10 e
Segregation kinetics: Ÿ Dose needed for visible segregation as function of temperature Ÿ Exponential relationship Ÿ knock-on displacements are followed by diffusive relaxation processes
M
y
q Simulation of beam induced atom dynamics [3]: Ÿ Based on Molecular Dynamics (MD) Ÿ Elastic scattering events generated randomly from
differential cross sections given by NIST [4]
MD Simulation: Calculation of displacement cross sections sD Ÿ sD largely dependent on local atomic configuration Ÿ sD for Au atoms about 10 times larger compared to Ni
Ni Equilibrate structure at given temperature (Molecular Dynamics or Metropolis Monte Carlo) Evolve structure, calculate trajectories and velocities of each atom (Molecular Dynamics)
z x
z
y x
z
z
y
Au
y
x
y
x
Select one atom randomly and generate a scattering event
Continue with Molecular Dynamics to evolve structure
-> Au displacements create vacancies that are filled by Ni atoms -> Difference in atomic radius and sD leads to segregation
References/ Literature No
displaced?
Yes
[1] Egerton, R. Mechanisms of radiation damage in beam-sensitive specimens, for TEM accelerating voltages between 10 and 300 kV. Microsc. Res. Tech. 75, 1550–556 (2012). [2] Volk, A. et al. The impact of doping rates on the morphologies of silver and gold nanowires grown in helium nanodroplets. Phys Chem Chem Phys 18, 1451–459 (2016). [3] Knez, D. et al. Modelling electron beam induced dynamics in metallic clusters. Ultramicroscopy 176, 105-111 (2017) [4] Jablonski, A. et al. NIST Electron Elastic-Scattering Cross-Section Database. Version 3.2, National Institute of Standards and Technology, Gaithersburg, MD, (2010).
Contact
Acknowledgements We acknowledge funding from the European Union's Horizon 2020 research and innovation programme under grant agreement No. 823717-ESTEEM3.
knez@iom.cnr.it www.iom.cnr.it