Browsing by Author "Joshi, Jaydeep"
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Item Investigation of the Optical Properties in Low-Dimensional Materials and Heterostructures(2021) Joshi, Jaydeep; Joshi, Jaydeep; Vora, Patrick MThe introduction of layered two-dimensional (2D) materials that offer unconventional pathways to harness and engineer many-body quantum effects has provided new possibilities to develop next-gen optoelectronic and computational applications. In the last decade, the rediscovery of a new class of 2D materials called the transition metal dichalcogenides (TMDs) have proved to be key candidates for advanced technology needs, made possible through materials synthesis, growth, and ease of exfoliation. Additionally, TMDs can be easily integrated in stacks, also called heterostructures, tailoring them for emergent phenomena at interfaces between materials with varying structural and electronic properties. Particularly fascinating is their innate polymorphic nature, which offers energetically-inexpensive alternatives to achieve phase-transitions in materials that exhibit multiple crystallographic and electronic characteristics. In this dissertation, we employ fundamental spectroscopy techniques to investigate intriguing properties and quantum phenomena in 2D TMDs. This involves studying atypical materials that exhibit novel structural and electronic phase transitions, understanding the role of defects in the optical properties of synthetically grown materials, and proximity effects coupled to interfaces shared between two materials with distinct band-structure, work functions, and electron-correlated physics. These results elucidate the importance of quantum correlations in complex low-dimensional materials and heterostructures and highlight some of the challenges that continue to be barriers for the 2D community.Item The structural phases and vibrational properties of Mo1−xWxTe2 alloys(2D Materials, 2017-09) Oliver, Sean M; Beams, Ryan; Krylyuk, Sergiy; Kalish, Irina; Singh, Arunima K; Bruma, Alina; Tavazza, Francesca; Joshi, Jaydeep; Stone, Iris R; Stranick, Stephan J; Davydov, Albert V; Vora, Patrick MThe structural polymorphism in transition metal dichalcogenides (TMDs) provides exciting opportunities for developing advanced electronics. For example, MoTe2 crystallizes in the 2H semiconducting phase at ambient temperature and pressure, but transitions into the 1T' semimetallic phase at high temperatures. Alloying MoTe2 with WTe2 reduces the energy barrier between these two phases, while also allowing access to the T d Weyl semimetal phase. The $\text{M}{{\text{o}}_{1-\text{x}}}$ WxTe2 alloy system is therefore promising for developing phase change memory technology. However, achieving this goal necessitates a detailed understanding of the phase composition in the MoTe2-WTe2 system. We combine polarization-resolved Raman spectroscopy with x-ray diffraction (XRD) and scanning transmission electron microscopy (STEM) to study bulk $\text{M}{{\text{o}}_{1-\text{x}}}$ WxTe2 alloys over the full compositional range x from 0 to 1. We identify Raman and XRD signatures characteristic of the 2H, 1T', and T d structural phases that agree with density-functional theory (DFT) calculations, and use them to identify phase fields in the MoTe2–WTe2 system, including single-phase 2H, 1T', and T d regions, as well as a two-phase 1T' + T d region. Disorder arising from compositional fluctuations in $\text{M}{{\text{o}}_{1-\text{x}}}$ WxTe2 alloys breaks inversion and translational symmetry, leading to the activation of an infrared 1T'-MoTe2 mode and the enhancement of a double-resonance Raman process in $\text{2H-M}{{\text{o}}_{1-\text{x}}}$ WxTe2 alloys. Compositional fluctuations limit the phonon correlation length, which we estimate by fitting the observed asymmetric Raman lineshapes with a phonon confinement model. These observations reveal the important role of disorder in $\text{M}{{\text{o}}_{1-\text{x}}}$ WxTe2 alloys, clarify the structural phase boundaries, and provide a foundation for future explorations of phase transitions and electronic phenomena in this system.