Quantitative Scanning Transmission Electron Microscopy and Application to Gold Nanocubes

Dan Thuy Duc Nguyen
Many nanotechnology applications involve nano-sized metallic particles, the size, shape and composition of which play a key role in device functionality. Recent developments in aberration-correcting lenses have enabled scanning transmission electron microscopy (STEM) to become an immensely effective tool in studying nanoparticles, allowing for images to be obtained at the resolution of individual atomic columns. However, at this resolution understanding of the spreading and scattering of the
more » ... ctron probe as it travels through the specimen becomes of the utmost importance for quantitative analysis. This thesis presents a theoretical investigation on how the electron scattering dynamics inside a specimen affects quantitative STEM analysis, as well as an experimental application of quantitative STEM to gold nanoparticles, to obtain insight into the particle morphology and composition. Numerical simulation is used to examine the consequences of effective source distribution and probe geometry on the spatial origin of incoherent STEM signals. Three different source distribution models, each with different distribution 'tails', are applied to the STEM probe on a GaAs <001> crystal case study. The shape of the effective source distribution is found to have a strong influence not only on the STEM image contrast, but also on the spatial origin of the detected electron intensities. In particular, the length of the effective source distribution's 'tails' is found to have a non-trivial influence on the degree to which the electron probe scatters onto adjacent atomic columns. Source distributions with longer 'tails' will amplify signal contributions originating from atomic sites in neighbouring columns, potentially leading to incorrect attribution of STEM signal to a given atomic column if the actual effective source distribution is significantly different to the commonly assumed Gaussian distribution. Effects of the probe geometry on the spatial origin of STEM signals are tested through variations in [...]
doi:10.4225/03/58c61c5145ced fatcat:jsgeus5embgpfaosljrkwlkyk4