Raman spectroscopy in MoS2-type transition-metal dichalcogenides

Abstract

This thesis presents a multiple energy excitation resonant Raman study on mono- and few-layered MoS2. Our aim is to obtain the understanding of their exciton and phonon dynamics and the origin of the second-order features, as well as their potential to the understanding other semiconducting transition metal dichalcogenide materials. In our investigation of the Raman excitation profile of the two most pronounced features of the Raman scattering spectrum of MoS2 (E1 2g and A1g bands). We demonstrate that the A1g feature is enhanced when the excitation laser is in resonance with the A, B and C excitons of MoS2, while the E1 2g feature is shown to be enhanced only when the excitation laser matches the C exciton energy. We analyze the exciton-phonon interaction and show that the excitons orbital and the phonons are the responsible for the enhancement behavior of both modes. In the study of the second-order modes, Raman spectroscopy in conjunction with density functional theory calculations unveil the double-resonance Raman scattering process in monolayer and bulk MoS2. Our findings show that the frequency of some Raman features shifts when changing the excitation energy and, first-principle simulations confirm that such bands arise from distinct acoustic phonons, connecting different valley states. The double-resonance Raman process is affected by the indirect-to-direct bandgap transition, and a detailed comparison of results in monolayer and bulk allow the assignment of each Raman feature to specific phonons near the M or K points of the Brillouin zone. These findings highlight the underlying physics of intervalley scattering of electrons by acoustic phonons in semiconducting two dimensional transition metal, which is essential for valley depolarization in MoS2.