Complete Report - University of New South Wales

Generating a description of the sample in a suitable form for the multislice algorithm
can be the most diffi cult part of the simulation problem. The scattering potential of
many crystalline materials, oriented along a high-symmetry direction with respect to
the electron beam, can be described as a repetitive sequence of a small number of
identical layers and is a comparatively straightforward problem. The problem is more
diffi cult for nanoparticles, as the scattering potential is no longer strictly periodic in the x-
y plane and the number of atoms in each slice varies along the z-, or beam direction. The
particle also needs to be embedded in a suffi ciently large supercell to avoid the effects
of wraparound errors. Similar problems are encountered when simulating images of
defects and crystal surfaces.
To examine the image simulation process, a large number of images of particles of
various sizes, shapes and orientations have been performed and a library of images is
being generated. This will form the basis of an image-matching system which will be able
to identify particular crystal structures found in experimentally acquired images. As an
example, a series of images along high-symmetry orientations has been simulated for a
32Å wide truncated cubeoctahedral Si nanoparticle containing 1251 atoms. The atomic
coordinates are based on a bulk Si lattice which has been truncated along 8 sets of {111}
planes and 6 sets of {100} planes. For the (110) and (111) orientations, the particle
is rotated about the z-axis, followed by a tilt around an axis perpendicular to the z-axis.
Coordinate data is generated using Mathematica [4.5.11] and centred in a 100Å x
100Å x 60Å supercell, hardsphere
images are rendered
using the VMD package
[4.5.12] and the simulated
images are generated
using the Kirkland Multislice
code [4.5.10] at electron
beam energy of 200kV
and Cs=1mm for defocus
settings of 613 (Scherzer),
900 and 1200Å. The effects
of beam divergence, beam tilt
and defocus spread are not
included. A contrast reversal
occurs between 613Å and
900Å and the particle
appears to be at least 1
fringe larger at 1200Å than
at 613Å. The apparent
particle size is approximately
10% larger than the actual
size.
Figure 4.5.16: The highsymmetry
orientations of
the truncated cuboctahedral
Si nanoparticle. The fi rst
panel shows a hard-sphere
structural image in real space
of a particle viewed along
the (100), (110) and (111)
directions. The second panel
shows the corresponding
simulated HRTEM images.
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