▎ 摘　　要

The van der Waals heterojunction involving graphene (Gr) with transition metal dichalcogenides (TMDs) is regarded as a promising structure for its outstanding performance in optoelectronic response. The electron-hole thermalization has been deemed to be the main reason for the subband gap excitation charge transfer from Gr to TMDs. However, the role of the intricate interlayer interaction of Gr and TMDs still requires intensive investigation. Here, we have investigated the photocarrier dynamics in a five-layer PtSe2/Gr heterojunction by using time-resolved optical pump and terahertz probe spectroscopy. Interestingly, after photoexcitation, electron transfer from PtSe2 to Gr has been demonstrated successfully in the PtSe2/Gr/substrate heterojunction; by contrast, no observable charge transfer occurs in the Gr/PtSe2/substrate heterostructure. The prominent difference for the different stacking sequences between Gr and PtSe2 can be ascribed to the effective electric field introduced by the fused silica substrate. A physical picture accounting for the effective electric field introduced by the substrate has been proposed to interpret the charge transfer process in the TMD/Gr heterostructure. More importantly, the electric field induced by the substrate plays a dominant role in controlling the charge transfer pathway in the TMDs/Gr heterojunction. This study not only sheds light on the substrate engineering of the van der Waals heterojunction but also provides new insight into the layer interaction dynamics in the Gr/TMD heterojunction.