LAUSR

We investigate the excitonic peak associated with defects and disorder in low-temperature photoluminescence of monolayer transition metal dichalcogenides (TMDCs). To uncover the intrinsic origin of defect-related (D) excitons, we study their dependence on gate voltage, excitation power, and temperature in a prototypical TMDC monolayer MoS2. Our results suggest that D excitons are neutral excitons bound to ionized donor levels, likely related to sulphur vacancies, with a density of 7×1011 cm-2. To study the extrinsic contribution to D excitons, we controllably deposit oxygen molecules in-situ onto the surface of MoS2 kept at cryogenic temperature. We find that in addition to trivial p-doping of 3×1012 cm-2, oxygen affects the D excitons, likely by functionalizing the defect sites. Combined, our results uncover the origin of D excitons, suggest an approach to track the functionalization of TMDCs, to benchmark device quality, and pave the way towards exciton engineering in hybrid organic-inorganic TMDC devices.