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Using wide spectral range in situ spectroscopic ellipsometry with systematicultra high vacuum annealing and in situ exposure to oxygen, we report thecomplex dielectric function of MoS$_2$ isolating the environmental effects andrevealing the crucial role of unpassivated and passivated sulphur vacancies.The spectral weights of the A ($1.92$ eV) and B ($2.02$ eV) exciton peaks inthe dielectric function reduce significantly upon annealing, accompanied byspectral weight transfer in a broad energy range. Interestingly, the originalspectral weights are recovered upon controlled oxygen exposure. This tunabilityof the excitonic effects is likely due to passivation and reemergence of thegap states in the bandstructure during oxygen adsorption and desorption,respectively, as indicated by ab initio density functional theory calculationresults. This work unravels and emphasizes the important role of adsorbedoxygen in the optical spectra and many-body interactions of MoS$_2$.

Oxygen passivation mediated tunability of trion and excitons in MoS$_2$