Oxygen Passivation Induced Tunability of Trion and Excitons in Molybdenum Disulphide*

Molybdenum disulphide (MoS₂) plays a central role in the burgeoning field of two-dimensional (2D) materials. Particularly, it has extraordinary optical properties which relies on strong electron-hole interactions. However, experimental reports as well as fundamental understanding of the optical properties isolating the roles of both defects and extrinsic environmental factors are still lacking for MoS₂. We have performed in situ spectroscopic ellipsometry in a wide spectral range and under ultra-high vacuum (UHV) conditions to explore the complex dielectric function of MoS₂. Combining high temperature UHV annealing and oxygen exposure, we show tunability of the exciton and trion features of the optical spectra. We attribute these tunability to desorption and chemisorption of oxygen upon UHV annealing and oxygen exposure, respectively. Ab initio density functional theory calculations suggest that oxygen chemisorption passivates gap states originating from sulphur vacancies, which influences the recovery of the exciton and trion features. Overall, this study highlights the importance of adsorbed oxygen as well as defects in the interpretation of experimental results from MoS₂.