Photoluminescence evolution in WS2 via optical irradiation and substrate interactions

Transition metal dichalcogenides, such as monolayer MoS2 and WS2, can be easily transferred onto or integrated with various substrates, and are therefore promising on-chip light sources. Although are direct-bandgap materials with more efficient optical gain compared to indirect band-gap materials such as silicon, they are more sensitive to the surrounding environment and substrate materials than bulk direct-bandgap semiconductors. Herein, the photoluminescence intensity of WS2 transferred onto silicon nitride is demonstrated to be tunable and substantially enhanced by violet laser excitation, owing to irradiation-induced chemical doping with atmospheric moisture and oxygen, which lead to changes in the densities of neutral excitons and trions; this process is reversible in vacuum conditions. However, with continuous excitation, WS2 with trapped molecules is more easily damaged by laser irradiation than bulk semiconductors. Upon coating with Al2O3, the stability of WS2 under laser irradiation is enhanced. We show that the isolation of WS2 from air can extend the lifetime of transition metal dichalcogenides on chip-communication devices. The results of this study indicate that further optimization is required to realize practical on-chip applications of these monolayer materials as light sources.