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THE INTERPRETATION OF ELECTRON ENERGY-LOSS SPECTROSCOPY IN COMPLEX SYSTEMS: A DFT BASED STUDY

Nichol, Robert M
http://rave.ohiolink.edu/etdc/view?acc_num=osu1431145968

Abstract Details

2015, Master of Science, Ohio State University, Materials Science and Engineering.
Electron energy-loss spectroscopy (EELS) is capable of probing the electronic structure of materials at the nanometer scale; however, accurate models for the interpretation of energy-loss spectra is lacking, particularly for complex material systems. The focus of this work is on the modeling and interpretation of EELS for two complex systems, bone mineral and the battery cathode material lithium iron phosphate (LFP). For the first case study, DFT was employed to model the spectra of HA and carbonated HA (CHA) using a density of states interpretation. It was found that the twin peak appearance in both HA and CHA may be due to a distribution in the magnitude of core-hole screening on either crystallographically inequivalent sites, meaning the magnitude of screening is site-specific, or across equivalent sites following a general statistical distribution. It was also found that the low-energy shoulder peak that is present in the oxygen K-edge spectra of CHA, as well as the overall broadening seen in the twin peak appearance relative to HA is due to additional states added by the substitutional carbonate groups. The second case study deals with the battery cathode material, lithium iron phosphate (LFP). While considerable work has gone into the predictive modeling capabilities of DFT fore core-loss spectra, the same is not true for the valence loss region, which contains collective excitations effects as well as single-excitation effects. Even less research has focused on the modeling of VEEL spectra for complex highly correlated systems, such as LFP and FP. DFT+U formalism was used to model the complex dielectric function of LFP and delithiated LFP (FP) from which the valence electron energy-loss function could be extracted. The Hubbard correction term, Ueff, was calculated from linear response theory (LRT) found in literature and employed and the corresponding models were compared with models found in literature employing other values. The only major discrepancy between experiment and our models was found in the magnitude of the band gap, which is a known shortcoming of DFT calculations of insulators and semiconductors. However, DFT+U models for FP predicted a semi-metallic material with no measurable band gap at all, contrary to models and experimental results found in literature. In all the models generated for FP, there were spurious peaks inside the band gap that were not witnessed in experiment. Differences in fine structure between the models for FP using different values of Ueff were nearly negligible, showing that a simple energy-independent shift in the Hubbard bands was not enough to reproduce experimental observations in FP. Comparisons were made between literature models for LFP and FP which used scissor operators to shift the unoccupied DOS up in energy relative to the occupied states, GW approximations for the calculation of quasi-particle energies, and the artificial introduction of antiferromagnetic ordering for spin-polarized calculations. It was found that the differences between literature models and our models were likely due to incorrect excitation energies, something that can be approximated more accurately using GW approximations, or introduced artificially with similar results by use of a scissor operator. However, the influence antiferromagnetic ordering may play in the VEEL spectra for these materials must be investigated before this can be confirmed. DFT Models with and without spin ordering that employ post DFT GW approximations as well as those that employ scissor operators should be studied to determine what if any contribution to spectra this artificial ordering may generate.
David McComb, Dr. (Advisor)
Wolfgang Windl, Dr. (Committee Member)
147 p.
Biochemistry; Biomedical Research; Chemistry; Materials Science; Physical Chemistry; Physics
EELS; electron energy-loss spectroscopy; DFT; VASP; valence electron energy-loss; loss function; hydroxyapatite; bone mineral; lithium iron phosphate; screening; core-hole; core hol

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Citations

  • Nichol, R. M. (2015). THE INTERPRETATION OF ELECTRON ENERGY-LOSS SPECTROSCOPY IN COMPLEX SYSTEMS: A DFT BASED STUDY [Master's thesis, Ohio State University]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=osu1431145968

    APA Style (7th edition)

  • Nichol, Robert. THE INTERPRETATION OF ELECTRON ENERGY-LOSS SPECTROSCOPY IN COMPLEX SYSTEMS: A DFT BASED STUDY. 2015. Ohio State University, Master's thesis. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=osu1431145968.

    MLA Style (8th edition)

  • Nichol, Robert. "THE INTERPRETATION OF ELECTRON ENERGY-LOSS SPECTROSCOPY IN COMPLEX SYSTEMS: A DFT BASED STUDY." Master's thesis, Ohio State University, 2015. http://rave.ohiolink.edu/etdc/view?acc_num=osu1431145968

    Chicago Manual of Style (17th edition)

osu1431145968
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© 2015, some rights reserved.Creative Commons License THE INTERPRETATION OF ELECTRON ENERGY-LOSS SPECTROSCOPY IN COMPLEX SYSTEMS: A DFT BASED STUDY by Robert M Nichol is licensed under a Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License. Based on a work at etd.ohiolink.edu.
This open access ETD is published by The Ohio State University and OhioLINK.