Graduation Semester and Year

2021

Language

English

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Physics and Applied Physics

Department

Physics

First Advisor

Muhammad N Huda

Second Advisor

Jonathan Asaadi

Abstract

Strongly correlated materials such as d and f-electron systems manifest a broad range of interesting phenomena such as metal-insulator transition, high temperature superconductivity, topologically insulating behavior etc. Due to the highly localized nature of d and f-electrons, they are tightly bound about the associated atoms within the crystal lattice and hence strongly interact with intra-site electrons and the corresponding ionic cores. Such a strong localization of electrons results in the creation of polaron quasi-particles in strongly correlated system like BiVO4, a promising photoanode material for photoelectrochemical (PEC) water splitting for H2 fuel generation using solar irradiation. Using first principles density functional theory (DFT), we have theoretically characterized the polaron formation in BiVO4 and have studied the transport behavior of polarons within BiVO4 crystal. We have also studied the aliovalent doping mechanism of polaron formation suppression in BiVO4. In conjunction with the d-electron system like BiVO4, we have also studied solid solutions with rare-earth chalcogenides, Ca(La1-xCex)2S4 (0 ≤ x ≤ 1). The rear-earth chalcogenides are strongly correlated 4f-electron systems. Using post DFT methods (DFT+U theory), we have theoretically determined the optical transition mechanism of experimentally observed colors for Ca(La1-xCex)2S4 (0≥x≥1). This Ph.D. research is divided into four different projects: 1) The 1st project demonstrated the electronic band structure engineering of d-electron system, BiVO4. In this project, the highly localized V 3d bands within the conduction band of BiVO4 were replaced by less localized Nb 4d-orbitals via isovalent substitutional doping. We have tested the effect of hydrostatic pressure on the microscopic structure of BiVO4 as well as the stability of Nb-doped BiVO4 metastable excited states, and how it affects the transport of free carriers by means of effective mass calculations. 2) The 2nd project is about theoretical characterization of polaron formations in BiVO4, and the calculation of polaronic electronic structure. We have explored how polaron adversely affects the transport of carriers and limits the available photovoltage of BiVO4 by pinning the quasi Fermi-level splitting. 3) In the 3rd project, we have demonstrated a mechanism to suppress polaron formations in BiVO4 crystal lattice. I have applied the aliovalent doping mechanism to suppress polaron formation and showed that this mechanism could serve as a powerful tool to suppress polaron formation in other materials. 4) The 4th and the last research project was to investigate the role of f-electrons toward structural, opto-electronic, and magnetic properties of strongly correlated 4f systems- Ca(La1-xCex)2S¬4 solid solution. In this project, we have successfully explained the optical transition mechanism responsible for experimentally observed color as well insulating to semiconducting to metallic phase transition based on electronic structure calculations. Overall, this dissertation will help to understand some aspects of the electronic structure and transport properties of strongly correlated d and f-electron materials. The dissertation not only shows the applicability and limitations of the DFT in understanding the strongly correlated systems, but also will facilitate the search for the suitable and efficient materials for solar energy conversions.

Keywords

Strongly correlated materials, Polaron, BiVO4, Solar energy converison, DFT

Disciplines

Physical Sciences and Mathematics | Physics

Comments

Degree granted by The University of Texas at Arlington

Included in

Physics Commons

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