Reversible Photoluminescence Tuning by Defect Passivation via Laser Irradiation on Aged Monolayer MoS.

Atomically thin (1L)-MoS emerged as a direct band gap semiconductor with potential optical applications. The photoluminescence (PL) of 1L-MoS degrades due to aging-related defect formation. The passivation of these defects leads to substantial improvement in optical properties. Here, we report the enhancement of PL on aged 1L-MoS by laser treatment. Using photoluminescence and Raman spectroscopy in a gas-controlled environment, we show that the enhancement is associated with efficient adsorption of oxygen on existing sulfur vacancies preceded by removal of adsorbates from the sample's surface. Oxygen adsorption depletes negative charges, resulting in suppression of trions and improved neutral exciton recombination. The result is a 6- to 8-fold increase in PL emission. The laser treatment in this work does not cause any measurable damage to the sample as verified by Raman spectroscopy, which is important for practical applications. Surprisingly, the observed PL enhancement is reversible by both vacuum and ultrafast femtosecond excitation. While the former approach allows switching a designed micropattern on the sample ON and OFF, the latter provides a controllable mean for accurate PL tuning, which is highly desirable for optoelectronic and gas sensing applications.