Date on Master's Thesis/Doctoral Dissertation

8-2021

Document Type

Doctoral Dissertation

Degree Name

Ph. D.

Department

Physics and Astronomy

Degree Program

Physics, PhD

Committee Chair

Sumanasekera, Gamini

Committee Co-Chair (if applicable)

Freelon, Byron

Committee Member

Yu, Ming

Committee Member

Ramezanipour, Farshid

Author's Keywords

2D materials; semiconductor; strain engineering; thermoelectricity; quantum materials; x-ray/neutron diffraction

Abstract

Electron correlation effects in quantum materials are very strong. It is critical to investigate the structure of quantum materials to better understand and manipulate their physical properties. Quantum effects are prominent at the atomic microscopic length scale, which can not be examined by average long range structural measurements using traditional diffraction methods. Instead, pair distribution function (PDF) analysis, a local structure probe, can effectively unveil the mystery of local structure, which is more sensitive to local behavior than bulk average features. The first section of my dissertation will concentrate on the local structural study of the Iron oxy-chalcogenides, {\cSSe}, which are layered materials formed by stacking layered units of La$_{2}$O$_{2}$ and Fe$_{2}$O$M_{2}$ ($M$ = S, Se). Local crystal structure was studied using the PDF technique, which involves Fourier transforming the measured total scattering intensity to obtain a real space representation of inter-atomic correlations. This technique was used to study local, short range structural correlations that deviate from the average structure. PDF analysis for $M$ = S, Se shows short-scale structural distortions in a typical range of 1-2 nm, indicating nematic fluctuations. However, neutron powder diffraction (NPD) provides clear evidence that the average, long-range structure remains tetragonal throughout the high and low temperature regimes. A comparable result was obtained for Fe$_{1.1}$Te. These findings highlight the ubiquity of nematic fluctuations in iron-based superconductors and related materials. The second part of my research is focused on measuring the transport and vibrational properties of black phosphorus and related materials. Phosphorene, a novel two-dimensional (2D) material, is gaining researchers' attention due to its exceptional properties, including a unique layer structure, a widely tunable band gap, strong in-plane anisotropy, and high carrier mobility. Strain in 2D materials can tune the material properties. The effect of tensile strain on the Raman spectra of black phosphorus (BP) was studied by using a simple custom strain device revealed clear red shifting of all three phonon modes, A$^{1}_{g}$, B$_{2g}$ and A$^{2}_{g}$. We anticipate that our method of in-situ Raman spectroscopy could be an effective tool that can allow observation of strain effects directly, which is critical for future flexible electronic devices. Even though, black phosphorus has several unique properties, there are some limitations in its application in devices due to its environmental instability. Several passivation techniques have been employed, but just covering the surface may not be the long term solution. Doping engineering is found to be an efficient strategy in order to enhance the properties of BP, and its promising performance in electronic devices. We have synthesized a series of As$_{x}$P$_{1-x}$ (x = 0, 0.2, 0.5, 0.83, 1) alloys which show similar properties to black phosphorus and are more stable in comparison to BP. Temperature dependent transport properties of As$_{x}$P$_{1-x}$ alloys show that small arsenic doping greatly increases the thermoelectric power of black phosphorus. Thermoelectric properties of these materials provide an environmentally friendly solution for direct and reversible conversion between heat and electricity. They have potential applications in a wide range of fields, including transportation, industry, and power generators/solid-state refrigerators, and may also provide solutions for sustainable energy sources.

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