Spatially Selective Enhancement of Photoluminescence in MoS₂ by Photo-mediated Adsorption and Defect Passivation

Monolayers of transition metal dichalcogenides (TMD) are promising components for optoelectronic devices due to their direct band gap and atomically thin nature. Their photoluminescence (PL) is strongly dependent on mid-gap defects which serve as non-radiative recombination sites for excitons. We demonstrate up to a 200x increase in PL intensity by exposing MoS₂ synthesized by chemical vapor deposition (CVD) to laser light in ambient. This spatially resolved passivation treatment is air and vacuum stable, which indicates strong bonding of moieties from ambient. A wavelength dependent study confirms that this PL brightening is concomitant with exciton generation in the MoS₂; laser light below the optical band gap of MoS₂ fails to brighten the TMD. We highlight the photo-sensitive nature of the process by successfully brightening with a broadband white light source (< 10 nW/mm²). We decouple changes in absorption from defect passivation by examining the degree of circularly polarized PL. This measurement, which is independent of exciton generation, confirms that the laser brightening reduces non-radiative recombination sites in the MoS₂. We propose that H₂O molecules passivate sulfur vacancies in the MoS₂ but requires photo-generated excitons to overcome the adsorption barrier.