Quantitative Scanning Transmission Electron Microscopy and Application to Gold Nanocubes
Creators: name: Nguyen, Dan Thuy Duc
Publisher: name: Figshare
Year: 2017
Subjects: Uncategorized

description: 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. <br>
   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 &lt;001&gt; 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. <br>
   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. <br>
   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 Cu<sup>2+</sup> ions
in Au nanoparticle growth and shape formation.