Electron Beam Induced Oxidation of Nickel Nanoclusters
D. Knez
Institute for Electron Microscopy and Nanoanalysis, Graz University of Technology
09:40 - 10:00 Monday 28 September 2015 Hörsaal I, Alte Technik Electron beam induced oxidation and rearrangement of metallic nanostructures, driven by the nanoscale Kirkendall effect, was reported recently by several authors (1–3). This effect can be used as a new route for the modification of such structures with the high spatial resolution of a focused electron beam. However, we report here for the first time the creation of hollow NiO nanoclusters consisting of less than 3000 nickel atoms with electron beam induced oxidation in a scanning transmission electron microscope (STEM).
Nickel clusters were synthesized by using superfluid helium nanodroplets (composed of ~106 helium atoms) at 0.37 K under ultra-high vacuum (UHV) conditions (4). This approach provides exceptional advantages over conventional methods in terms of purity and flexibility concerning morphologies and materials (5). The clusters are directly deposited on TEM-grids and can be characterized in-situ via time-of-flight mass spectrometry. The high purity and small size of these clusters give us an ideal playground for the study of electron beam driven dynamics of metallic clusters using both experimental and computational approaches.
By using a probe corrected, monochromated FEI Titan3 60-300 (equipped with a Super-X detector (EDX) and a Gatan Quantum energy filter) we performed analytical high resolution STEM in order to oxidize and analyse the clusters at the same time.
We draw conclusions about elemental composition and morphology before, while and after oxidation and give new insights into the oxidation mechanisms of nickel clusters on different substrates, which differ in the amount of initially adsorbed oxygen. We also observed a strong dependency on the electron primary energy; at 300 kV the process runs much faster than at 60 kV.
The transient dynamics of the oxidation was documented by time series of high angle annular dark field (HAADF) images and electron energy loss spectra (EELS). Figure 1a shows the temporal evolution of a cluster on a 3 nm amorphous carbon film during electron beam exposure. The graph illustrates the lateral enlargement of the cluster in an image series of 142 images recorded over a time span of 447 s. With EELS quantification we show that the average Ni L/O K-edge intensity ratio does not change over the whole oxidation process. This leads us to the conclusion that the amount of oxygen needed for the oxidation of the cluster is adsorbed close to the cluster in the first place, which is also supported by EELS elemental maps acquired on a Graphene substrate at 60 kV (Figure 1b and 1c). No native oxide layer can be seen and oxygen is only adsorbed to the cluster surface. Accordingly, we propose that water adjacent to the cluster and dissociated by electron radiolysis, acts as oxygen source for the electron induced oxidation.
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Institute of Solid State Physics
