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Title: Computational electron microscopy of small gold nanoclusters
Author: Aveyard, Richard
ISNI:       0000 0004 5348 861X
Awarding Body: University of York
Current Institution: University of York
Date of Award: 2014
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Scanning transmission electron microscopy is amongst the most valuable techniques for nanoscale structural characterization. It is capable of providing atomic resolution images with lesser sample damage than is typically incurred by other techniques of comparable resolution. Additionally, recent studies have found that the intensity in these images can be used to deduce the three dimensional structures of samples. The atomic resolution, three-dimensional characterization of gold nanoclusters is particularly desirable, as it is expected to provide significant insights into their surprising catalytic activity. Unfortunately, the image formation process in scanning transmission electron microscopy is not straightforward, with many microscope and sample parameters affecting image intensities. Recently, there has been a concerted effort in the electron microscopy community to achieve more quantitative analyses of images to maximise the information which can be extracted from them. This is typically achieved through the comparison of experimental and simulated images of model structures. To apply such methods to nanoscale structures, the simulations should account for the large inhomogeneities expected in these structures. In particular, both the static structural disordering induced by strain, and the dynamic disordering caused by thermal motion, should be included. These effects are frequently overlooked in reports in the literature, principally because there is currently no means by which they can be accurately measured. In the work presented here, molecular dynamics simulations are used to predict the structural relaxations and thermal motion in small gold nanoclusters, in order to produce more rigorous electron microscope simulations than any previously reported. This method is equally applicable to any system for which accurate molecular dynamics simulations can be performed. Images produced using this new method are compared with those produced using more conventional techniques and found to be sufficiently different to confirm the value of this approach. The results of the comparisons also prompt a systematic study into the effect of structural disorder on image intensities. It is found that electron channelling effects play a large role in image formation and cause a non-trivial relationship between thermal motion and image intensities. The results of this work show that the interrelated effects of the many factors affecting image formation in scanning transmission electron microscopy preclude parametrizations, so that the physical interpretation of images is expected to continue to rely upon rigorous computational simulations.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available