Photoluminescence from Defects in Two-Dimensional Transition Metal Dichalcogenides

Abstract

Atomic defects in monolayer transition metal dichalcogenides (TMDs) such as chalcogen vacancies significantly affect their properties. In this thesis, I provide a reproducible and facile strategy to rationally induce chalcogen vacancies in monolayer MoS2 by annealing at 600 °C in an argon/hydrogen (95 vol.%/5 vol.%) atmosphere. At sulfur vacancy densities of ~1.8×1014 cm−2, I observe defect-mediated photoluminescence (PL) at ~1.72 eV (referred to as LXD) at room temperature. The LXD emission is attributed to excitons trapped at defect-induced in-gap states and is typically observed only at low temperatures (≤77 K). Time-resolved PL measurements reveal that the lifetime of defect-mediated LXD emission is longer than band edge excitons, both at room and low temperatures (~2.44 ns at 8 K). My results provide insight into how excitonic and defect-mediated PL emission in MoS2 are influenced by sulfur vacancies at room and low temperatures. I also show that the LXD peak can be suppressed by annealing the defective MoS2 in sulfur or selenium vapor, which indicates that it is possible to passivate the vacancies. At annealing temperatures ≥600 °C in selenium vapor, I observed the filling of sulfur vacancies by selenium. I extended the defect generation strategy to monolayer MoS2 on various dielectric substrates to investigate the influence of dielectric substrates on defect-mediated emission. The LXD emission after annealing was observed on SiO2 and hBN, but not on HfO2. When the MoS2 is first annealed on SiO2 and then transferred onto HfO2, the LXD emission is present on untreated HfO2, but not on annealed HfO2. These results suggest that surface states in dielectric substrates can influence the defect-mediated emission.