Revealing the\nFundamental Limit of Gate-Controlled\nUltrafast Charge Transfer in Graphene–MoS<sub>2</sub> Heterostructures

When graphene forms heterostructures with transition\nmetal dichalcogenides\n(TMDCs), the photons with energy below the TMDCs’ bandgap can\nbe harvested by graphene and injected into TMDCs through ultrafast\ncharge transfer. Controlling and understanding this ultrafast charge\ntransfer are crucial for developing advanced photonic and optoelectronic\ndevices. Here, we use ultrafast terahertz and transient absorption\nspectroscopy to demonstrate the significant potential of a gate-controlled\nmethod in controlling the ultrafast charge transfer efficiency in\ngraphene–MoS<sub>2</sub> heterostructures and reveal the fundamental\nlimitation of the method. Our results show that the number of hot\nelectrons transferred from graphene to MoS<sub>2</sub> can be modulated\nseveral fold by gate bias, achieved by altering the Fermi distribution\nof hot electrons in graphene. There is an upper limit to the gate-controlled\nmethod in the aforementioned modulation, and we reveal that the underlying\nmechanism of this limitation is that, at high gate bias, the chemical\npotential of graphene surpasses the band edge of MoS<sub>2</sub>,\nleading to an increased energy barrier for charge transfer. A photothermionic\nemission model incorporating the gate-controlled limit can well reproduce\nthe experimental findings. Our study demonstrates the role and fundamental\nlimitation of the gate-controlled method in regulating ultrafast charge\ntransfer in graphene–MoS<sub>2</sub> heterostructures, providing\ninsights for the development of related photodetectors, solar cells,\nand optoelectronic devices.