Tunable Exciton Modulation and Efficient Charge Transfer in MoS/Graphene van der Waals Heterostructures.

Monolayer transition metal dichalcogenides (TMDs) are direct gap semiconductors where the optical properties are dominated by strongly interacting electron-hole quasi-particles. Understanding the interactions among these quasi-particles is crucial for advancing optoelectronic applications. Here, we examine the electrical tunability of light emission from the A and B excitons in monolayer MoS and MoS/graphene heterostructures and unravel the competition between the A exciton to trion formation and charge transfer processes. Our results show significant gate-tunable quenching of the photoluminescence intensity from A excitons with notable differences due to charge transfer in the heterostructure. Furthermore, we observe a distinct superlinear correlation between the A exciton photoluminescence intensity and high doping levels in MoS, which continues until the density of photoexcited excitons exceeds and saturates the free carrier density. This phenomenon ceases to occur in MoS/graphene, where MoS remains almost undoped across all values of the applied external voltage. In contrast, the B exciton photoluminescence is unaffected by doping in MoS, while it decreases analogously to that of the A excitons in the MoS/graphene heterostructure, indicating the relevance of gate-tunable charge transfer from hot electrons before any internal recombination.