Charge Transfer in Graphene-MoS2 Vertical Heterostructures Tuned by Stacking Order and Substrate-Introduced Electric Field

Vertical van der Waals heterostructures composed of graphene (Gr) and transition metal dichalcogenides (TMDs) have created a fascinating platform for exploring optical and electronic properties in the two-dimensional limit. Numerous studies have focused on Gr/TMDs heterostructures to elucidate the underlying mechanisms of charge-energy transfer, quasiparticle formation, and relaxation following optical excitation. Nevertheless, a comprehensive understanding of interfacial charge separation and subsequent dynamics in graphene-based heterostructures remains elusive. Here, we have investigated the carrier dynamics of Gr-MoS2 heterostructures (including Gr/MoS2 and MoS2/Gr stacking sequences) grown on a fused silica substrate under varying photoexcitation energies by comprehensive ultrafast means, including time-resolved terahertz (THz) spectroscopy, THz emission spectroscopy, and transient absorption spectroscopy. Our findings highlight the impact of the substrate electric field on the efficiency of modulating the interfacial charge transfer (CT). Specifically, the optical excitation in Gr/MoS2 generates thermal electron injection from the graphene layer into the MoS2 layer with photon energy well below A-exciton of MoS2, whereas the interfacial CT in the MoS2/Gr is blocked by the electric field of the substrate. In turn, photoexcitation of the A exciton above leads to hole transfer from MoS2 to graphene, which occurs for both Gr-MoS2 heterostructures with opposite stacking orders, resulting in the opposite orientations of the interfacial photocurrent, as directly demonstrated by the out-of-phase THz emission. Moreover, we demonstrate that the recombination time of interfacial exciton is approximately ∼18 ps, whereas the defect-assisted interfacial recombination occurs on a time scale of ∼ns. This study provides valuable insights into the interplay between interfacial CT, substrate effects, and defect engineering in Gr-TMDs heterostructures, thereby facilitating the development of next-generation optoelectronic devices.