Raman Study of gated devices in two-dimensional materials and pressure effects

Abstract

This thesis presents Raman study of 2D materials of contemporary interests: 2H-MoTe_2, type-II Weyl semimetal (T_d-MoTe_2), structurally anisotropic semiconductors (1T'-ReX_2 (X=S, Se)) and Mott insulator (FePS_3) under different conditions such of, high-pressure (up to 19 GPa), low-temperature (80 K), and electron/hole doping at carrier concentration of ∼ 10^13/cm^2. In-situ Raman measurements were carried out on electrochemically top-gated field-effect transistor (FET) devices with few-layer 2H-MoTe_2 as a channel, to probe the asymmetry of phonon coupling between electrons and holes. We also showed that the doped holes are localized in the top two layers of the nanocrystal. In addition, temperature-dependent Raman measurements were performed in few-layer 1T'-MoTe_2 to look for the Raman signatures of semiconductor to Weyl semimetallic transition. Phonon renormalization with electron doping in isostructural bilayer ReS_2 and trilayer ReSe_2 were done to highlight their different EPC. We showed that bilayer ReS_2 exhibits doping tunable Fano resonance as a result of its high EPC. In the last part of the thesis, high-pressure Raman experiments on bulk FePS_3 were carried out to gain insight into the Mott insulator to metal transition.