Excitons in monolayer transition metal dichalcogenides

Abstract

Excitons are quasiparticles formed due to electrostatic attraction between the electrons and the holes in a semiconductor. This Coulomb attraction is very strong in the mono- layers of Transition Metal Dichalcogenides (TMDs) mainly because of strong quantum confinement, reduced dielectric screening, and high effective mass of electrons and holes in these material systems. A 2D hydrogen atom is a simple model to describe confined excitons in these monolayer films. A more formal way to describe excitons in thin semi- conductors is through the Bethe-Salpeter formalism which describes these excitons as a superposition of the electronic states in momentum space. In order to understand exci- tons further, we explore the following excitonic features in this thesis: Probing intrinsic exciton linewidth: Monolayer TMDs are highly luminescent materials despite being sub-nanometer thick. This is due to the ultrashort radiative life- time of the strongly bound bright excitons hosted by these materials. The intrinsically short radiative lifetime results in a large broadening in the exciton band with a magnitude that is about two orders greater than the spread of the light cone itself. The situation calls for a need to revisit the conventional light cone picture. We present a modified light cone concept which places the light line as the generalized lower bound for allowed radia- tive recombination. A self-consistent methodology, which becomes crucial upon inclusion of large radiative broadening in the exciton band, is proposed to segregate the radiative and the nonradiative components of the homogeneous exciton linewidth. We estimate a fundamental radiative linewidth of 1:54 0:17 meV, owing purely to finite radiative lifetime in the absence of nonradiative dephasing processes. As a direct consequence of the large radiative limit, we nd a surprisingly large ( 0:27 meV) linewidth broadening due to zero-point energy of acoustic phonons. This obscures the precise experimental determination of the intrinsic radiative linewidth and