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Title: Simulation and Modeling of the Powder Diffraction
Pattern from Nanoparticles: Studying the Influence of Surface Strain
Summary:
Nanostructured materials are currently at the forefront of nearly
every emerging industry, as they offer promising solutions to problems ranging
from those facing energy technologies, to those concerning the structural
integrity of materials. With all of these future applications, it is crucial
that methods are developed which can offer accurate, and statistically reliable
characterization of these materials in a reasonable amount of time. X-ray
diffraction is one such method which is already widely available, and can offer
further insight into the atomic structure, as well as, microstructure of
nanomaterials.
This thesis work then focuses on investigating how different
structural features of nanoparticles influence the line profiles of the x-ray
powder diffraction pattern. Due to their extremely small size, the contribution
from crystallite size broadening becomes the dominating feature in an observed
diffraction peak. Therefore, the theory of size broadening is critically
reviewed concerning the considerations necessary when the crystallite size
approaches a few nanometers. Furthermore, the analysis of synthesized shape
controlled platinum nanoparticles was carried out using a developed line
profile analysis routine, based on the Debye function analysis (DFA) approach,
to determine the distribution of particle size and shape in the sample.
The Debye function simulates the powder diffraction pattern from
atomistic models. This allows for the coupling of this technique with
atomisitic simulations, like molecular dynamics (MD), to gain further
understanding of the diffraction pattern from nanoparticles. Techniques were
developed to study how lattice dynamics, and the resulting thermal diffuse
scattering, are affected by the small crystallite domains. Also, the features
in the peak profiles from simulated surface relaxation of free-standing
nanoparticles were studied, and used to test the existing models found in the
diffraction literature. In both cases the different results from Al and Cu
particles were discussed to compare the features from an elastically isotropic
and anisotropic material. This study then improves the understanding of
diffraction from small crystallites, and showcases the level of insight which
is achievable through the coupling of simulation and diffraction pattern
analysis.
