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Electron energy-loss spectroscopy: DFT modelling and application to experiment

By Che Royce Seabourne

Abstract

The all-electron density functional theory (DFT) code Wien2k has an established track record of modelling energy-loss near-edge structure (ELNES). The pseudopotential DFT code CASTEP can reproduce results found using Wien2k. A methodology was developed for DFT code parameter selection,\ud based on converging parameters to the ELNES prediction. Various aluminium systems were studied; aluminium, aluminium nitride and aluminium oxide. Uniquely for aluminium metal, a ground state calculation provided strong\ud agreement with experiment, as the core-hole is well screened. It was quantitatively demonstrated that the core-hole causes ionisation edge peaks to shift towards the Fermi level, and increases the intensity of those peaks -\ud effects found to be larger for the cationic species. Group 4 and 5 transition metal carbides were modelled using CASTEP. Systems with vacancies were considered; TiC0.79, TiC0.58N0.30, TiC0.45N0.43, TiC0.19N0.65 and TiN0.82. By\ud comparison with experimental data, structures for these systems were proposed. CASTEP was used to model oxygen K edges in various systems. For bulk MgO, acceptable experimental agreement was found using a ground state\ud calculation. This was rationalised by observing that the introduction of a corehole had relatively little effect on the p orbital DOS prediction for oxygen. For the interface of Fe (001) / MgO (001), it was demonstrated by careful\ud comparison of theory and experiment that some degree of oxidation was present. Nanoscale analysis of multilayered CrAlYN/CrN coatings was performed. Experimentally observed ELNES was reproduced using ground state Wien2k calculations. Combined experimental and theoretical analysis\ud indicated that the nominal CrN layers were close to stoichiometric CrN, the Cr/N ratio being 1.05 ± 0.1. For the CrAlYN layers, the theoretical system showing the best agreement was Cr0.5Al0.5N. This thesis has established methodologies for utilising DFT codes, illustrating how links between experimental and theoretical ELNES can be used in the nanoscale characterisation of technologically important materials

Publisher: School of Chemical and Process Engineering (Leeds)
Year: 2010
OAI identifier: oai:etheses.whiterose.ac.uk:1447

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