With the availability of aberration-correctors, scanning transmission electron microscopy (STEM) has become one of the most versatile tools for the characterization of nanostructured materials. In combination with electron energy loss spectrometers (EELS) and energy dispersive X-ray (EDX) detectors in principle chemical information can be extracted at atomic resolution. Due to the high current densities occurring in a highly focused electron beam, however, STEM has to be classified as destructive method in many cases.^[1] This is especially true for small metallic clusters with a high percentage of low-coordinated atoms, which are interesting in particular for heterogeneous catalysis.

In this work electron beam induced dynamics was studied via transient STEM HAADF image sequences for different nanostructured material systems like bimetallic alloys and clusters. All clusters were synthesized by means of the superfluid helium droplet method, which guarantees their high purity.^[2] Furthermore, we present a methodology for the simulation of elastic electron damage processes using an algorithm based on molecular dynamics and Monte Carlo techniques.^[3] Figure 1a shows simulation results for a bimetallic AuAg-cluster which exhibits selective Ag-sputtering and a rapid change of morphology during electron radiation. In Figure 1b HAADF image time series of an Au-CrO_x core-shell nanowire can be seen. Due to electron beam enhanced diffusion of Au atoms in CrO_x, significant morphology changes of the Au core can be observed after a few seconds of electron beam exposure, even at 60 keV electron energy.

Deeper understanding of electron beam induced dynamics can not only help to develop strategies to prevent sample damage, which is important especially for analytical and quantitative STEM analysis. Electron induced motion of single atoms on the surface or inside crystalline materials may also be useful to estimate surface energies, analyse defect generation and investigate diffusion processes.^[4,5] Figure 1c shows the electron beam mediated diffusion of Au atoms on and inside an Al-matrix, for instance.

Moreover, electron beam induced chemical reactions can be used to tailor nanostructures with properties not obtainable otherwise. We show how Ni clusters transform into single crystalline, hollow and toroidal NiO clusters, consisting of less than 3000 Ni atoms, during electron exposure (see Figure 1d and e). The transformation is mediated by adsorbed water and driven by a nanoscale Kirkendall effect. We studied the oxidation process via HAADF time-lapse series and EELS analysis.

References

[1]    R. Egerton, Ultramicroscopy 2013, 127, 100.

[2]    P. Thaler, A. Volk, D. Knez et al., The Journal of chemical physics 2015, 143, 134201.

[3]    A. Santana, A. Zobelli, J. Kotakoski et al., Phys. Rev. B 2013, 87.

[4]    A. Surrey, D. Pohl, L. Schultz, B. Rellinghaus, Nano letters 2012, 12, 6071.

[5]    W. Xu, Y. Zhang, G. Cheng et al., Nature communications 2013, 4, 2288.

Acknowledgements

Our research is supported by the European Union within the 7^th Framework Programme (FP7/2007-2013) under Grant Agreement no. 312483 (ESTEEM2) as well as by the Austrian Research Promotion Agency (FFG). 

Figures:

(a) Simulation of the radiation dynamics of a supported AgAu cluster (electron energy E=300 keV); Selective Ag sputtering and a change of the shape are visible (time indication for a beam current of I=100 pA). Experiments: (b) HAADF time lapse shows the evolution of an elongated Au-CrOx core-shell particle (on amorphous carbon) during electron irradiation (E=60 keV, I~50 pA). (c) Motion of single Au atoms on/inside an Al matrix (E=300 keV, I=100 pA). (d) HAADF time lapse series (5 out of 140 images) of a Ni cluster (on amorphous carbon) at different states during beam induced oxidation. (e) illustration of the morphological change in (d).

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