Graduate Thesis Defense - 2021 Spring
STRUCTURAL, VIBRATIONAL AND TRANSPORT PROPERTIES OF NOVEL COMPLEX SYSTEMS
Speaker: Bhupendra Man Shing Karki, University of Lousiville (PhD Thesis Defense)
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 cannot 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, La2O2Fe2OM2 (M = S, Se), which are layered materials formed by stacking layered units of La2O2 and Fe2OM2 (M = S, Se). Local crystal structure was studied using the pair distribution function 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. Our results for M = S, Se show 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 Fe1.1Te. This finding highlights 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. The effect of tensile strain on the Raman spectra of black phosphorus (BP) by using a simple custom strain device revealed clear red shifting of all three phonon modes, A1g, B2g and A2g. In a comparative study, we found that the effect of strain on the Raman shifting is larger for BP than that for MoTe2, presumably due to the smaller Young’s modulus of BP. 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. In another study, temperature dependent transport properties of AsxP1-x (x = 0, 0.2, 0.5, 0.83, 1) alloys show that small arsenic doping greatly increases the thermoelectric power of black phosphorus. This alloy’s thermoelectric properties 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.
Ga-based III-V Semiconductor Photoanodes for Solar Fuels and Novel Techniques to Investigate Their Photocorrosion
Speaker: Sahar Pishgar, University of Louisville (PhD Thesis Defense)
Abstract: Solar energy is one of the most abundant renewable energy sources. However, the diurnal variation of the sun as well as seasonal and weather effects, limits the widespread global implementation of solar energy. Thus, cost-effective energy storage is critical to overcome the intermittent nature of solar energy available on the earth. Photoelectrolysis of water to oxygen and hydrogen fuel is a promising large-scale solution to store solar energy in a dense and portable form. Photoelectrochemical water-splitting research strives to develop a semiconductor photoelectrode with both high efficiency and long-term stability. III–V semiconductors are among the most promising materials for high efficiency solar fuels applications. However, they suffer from severe instability in acidic and alkaline electrolyte and fundamental understanding of their corrosion mechanism is of significant importance for the solar fuels community. This dissertation is focused on study of gallium based III-V semiconductors for water splitting systems. Corrosion of n-GaP, a promising III–V material for tandem top subcells, was investigated in strongly acidic electrolyte using an in-situ UV-Vis spectroscopy technique to interpret the corrosion process in conjunction with SEM and XPS characterization. Further, photocorrosion of n-GaAs, one of the most well-developed III-V semiconductors was studied. Three type of Ir, OER co-catalyst, were tested to explore their affect on photocorrosion of n-GaAs photoanodes. Synthesis of ternary III-V alloys enable us to tune the band gap and band edge positions of III-V semiconductors according to the requirements of desired PEC system. Herein, optical and electrical properties of a novel III-V ternary alloy GaSbxP(1-x), synthesized by halide vapor phase epitaxy is also investigated with various characterization techniques such as diffuse reflectance spectroscopy, X-ray diffraction spectroscopy, and Hall effect measurement.