Manish Chhowalla1
University of Cambridge1
Atomic defects in monolayer transition metal dichalcogenides (TMDs) such as chalcogen vacancies significantly affect their properties. In this work, we provide a reproducible and facile strategy to rationally induce chalcogen vacancies in monolayer MoS<sub>2</sub> by annealing at 600 °C in argon/hydrogen (95%/5%) atmosphere. Synchrotron X-ray photoemission spectroscopy shows that a Mo 3<i>d</i><sub>5/2</sub> core peak at 230.1 eV emerges in the annealed MoS<sub>2</sub> associated with non-stoichiometric MoS<sub>x</sub> (0<x<2), and Raman spectroscopy shows an enhancement of the ~380 cm<sup>-1</sup> peak that is attributed to sulfur vacancies. At sulfur vacancy densities of ~2.4×10<sup>14</sup> cm<sup>−2</sup>, we observe a defect peak at ~1.72 eV (referred to as LX<sub>D</sub>) at room temperature in the photoluminescence (PL) spectra. The LX<sub>D</sub> peak 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 LX<sub>D</sub> emission is longer than band edge excitons, both at room and low temperatures (~2.77 ns at 8 K). The LX<sub>D</sub> peak can be suppressed by annealing the defective MoS<sub>2</sub> in sulfur vapor, which indicates that it is possible to passivate the vacancies. Our results provide insights into how excitonic and defect-mediated PL emission in MoS<sub>2</sub> are influenced by sulfur vacancies at room and low temperatures.