Quantitative Scanning Transmission Electron Microscopy and Application to Gold Nanocubes

2017-03-06T01:26:52Z (GMT) by 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 electron 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 the convergence angle and defocus of the incident probe. Specimen tilting and more exotic probes, such as hollow cone and vortex probes, are also explored. Due to the complex nature of electron scattering dynamics, each probe type results in a different specimen volume from which the STEM signal originates, with no one probe being unilaterally better than any other. With each probe type having its own advantages and drawbacks, the probe most suitable for a given experiment depends on the specific material and information desired. Therefore, a 'menu of probes' is presented to assist in the choice of probe for STEM analysis.
   Through the use of quantitative STEM, a detailed experimental study on the shape and composition of copper-assisted gold nanocubes finds that these particles are divided into two main morphological categories: cubes and bars. Each have similar characteristics, with {001} facets and very rounded edges and corners. EDX mapping results show copper to be present on the surfaces of the cuboid particles. An approach to quantifying the copper on the surfaces utilising 'atom counting' is presented as a proof of concept. While further experimentation is needed to determine the exact atomic structure, the results suggest copper forms a layer around gold cuboids, which directs the nanocube's growth and formation. This work builds the platform for further studies to be performed to gain additional insight into the role of Cu2+ ions in Au nanoparticle growth and shape formation.