Electronic Theses and Dissertations

Date

2021

Document Type

Dissertation

Degree Name

Doctor of Philosophy

Department

Engineering

Committee Chair

Xiao Shen

Committee Member

Xiao Shen

Committee Member

Thang Hoang

Committee Member

Ranganathan Gopalakrishnan

Abstract

First-principles calculations are performed to investigate the electronic, optical, mechanical, and phase transition properties of two-dimensional silicon telluride (Si2Te3) nanostructures. We aim to build a comprehensive understanding of Si2Te3, which will be useful for its potential applications in optoelectronics, chemical sensors, photovoltaic cell, et al. The foundation of calculations is based on the density functional theory (DFT) implemented in the Vienna Ab initio Simulation Package (VASP) and Quantum ESPRESSO package. After the structural optimizations, the post-processing calculations are carried out taking the lowest energy structure (structure with all the silicon dimers oriented horizontally along y-direction). The optoelectronic properties of Si2Te3 are analyzed under the GW approximation and the Bethe-Salpeter equation (BSE) on the top of the DFT method, which show there exists an in-plane optical anisotropy. Also, BSE calculations reduce the quasiparticle bandgap by 0.3 eV in bulk and 0.6 eV in the monolayer, which indicates the strong excitonic effect. Further, the analysis of Raman intensity shows that the horizontally misaligned silicon dimers do not have a noticeable impact on the Raman spectrum. However, the vertically orientated dimers result in an additional peak around 127 cm-1 in the Raman spectrum, which is close to an experimentally observed side-peak at 130 cm-1. In addition, the monolayer Si2Te3 is studied under uniaxial tensile strain, which shows that Si2Te3 can sustain a tensile strain up to 38%, making it one of the most flexible 2D materials. The uniaxial strain causes the monolayer Si2Te3 to undergo an indirect-direct-indirect-direct band gap transition. It is also found that the orientation of silicon dimers within the cell can be controlled by applying strain, which is particularly important for controlling the structural variability for potential applications. Furthermore, the study of Si2Te3 subject to external hydrostatic pressure shows there is a structural phase change at 7.4 GPa pressure at which the material goes to semiconductor-metal transition, which is close to the experimental result. The continuous blue shifting of major Raman peaks with the pressure and finally their disappearance from the spectrum confirms this transition. Two possible high pressure-low energy metallic phases of Si2Te3, M1, and M2, are predicted using the genetic algorithm combined with the DFT calculations. Further, the external pressure causes the semiconducting Si2Te3 to undergo indirect-direct-indirect gap transition.

Comments

Data is provided by the student.

Library Comment

Dissertation or thesis originally submitted to ProQuest

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