Revealing the Fundamental Limit of Gate-Controlled Ultrafast Charge Transfer in Graphene-MoS2 Heterostructures

When graphene forms heterostructures with transition metal dichalcogenides (TMDCs), the photons with energy below the TMDCs' bandgap can be harvested by graphene and injected into TMDCs through ultrafast charge transfer. Controlling and understanding this ultrafast charge transfer are crucial for developing advanced photonic and optoelectronic devices. Here, we use ultrafast terahertz and transient absorption spectroscopy to demonstrate the significant potential of a gate-controlled method in controlling the ultrafast charge transfer efficiency in graphene-MoS2 heterostructures and reveal the fundamental limitation of the method. Our results show that the number of hot electrons transferred from graphene to MoS2 can be modulated several fold by gate bias, achieved by altering the Fermi distribution of hot electrons in graphene. There is an upper limit to the gate-controlled method in the aforementioned modulation, and we reveal that the underlying mechanism of this limitation is that, at high gate bias, the chemical potential of graphene surpasses the band edge of MoS2, leading to an increased energy barrier for charge transfer. A photothermionic emission model incorporating the gate-controlled limit can well reproduce the experimental findings. Our study demonstrates the role and fundamental limitation of the gate-controlled method in regulating ultrafast charge transfer in graphene-MoS2 heterostructures, providing insights for the development of related photodetectors, solar cells, and optoelectronic devices.