Tunable Exciton Modulation and Efficient Charge Transfer in MoS₂/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 MoS₂ and MoS₂/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 MoS₂, which continues until the density of photoexcited excitons exceeds and saturates the free carrier density. This phenomenon ceases to occur in MoS₂/graphene, where MoS₂ remains almost undoped across all values of the applied external voltage. In contrast, the B exciton photoluminescence is unaffected by doping in MoS₂, while it decreases analogously to that of the A excitons in the MoS₂/graphene heterostructure, indicating the relevance of gate-tunable charge transfer from hot electrons before any internal recombination.