Vertically stacking van der Waals (vdW) layers allows forming bond-free heterostructures, opening up exciting possibilities for optoelectronic applications due to their unique optical and electronic properties. To fully harness these properties, it is crucial to understand and control the interfacial charge transfer and recombination dynamics across vdW heterostructures. This poster presents our recent efforts in understanding and controlling interfacial charge carrier dynamics in graphene-WS₂ vdW heterostructure, combining terahertz and transient absorption spectroscopies. For charge transfer, our study unveils the critical role of optical excitations in dictating the CT pathway: we show that while the photo-thermionic emission dominates the sub-A-exciton excitation regime, the direct hole transfer from WS₂ to the valence band of graphene prevails for above-A-exciton excitation. More importantly, we reported the critical role of defects in governing the interfacial CT and recombination in graphene-TMD interfaces: these defects at interfaces can efficiently “capture” (within ~ 1 ps) and “store” (over ~ 1 ns) the transferred charge carriers across the heterojunction, leading to effective long-lived photogating effect in graphene. Finally, we demonstrate full control over the photogating field direction and efficiency by electrically tuning the defect occupancy. Our results show that electron occupancy of the two in-gap states, presumably originating from sulfur vacancies, can account for the observed rich interfacial charge transfer dynamics and electrically tunable photogating fields.

These comprehensive findings shed light on the critical processes of interfacial charge transfer and recombination in graphene-WS₂ heterostructures, which are pivotal for optimizing optoelectronic devices, particularly in the field of photodetection.