  Vertical van der Waals (vdW) heterostructures consisting of graphene (Gr) and
transition metal dichalcogenides (TMDs) have created a fascinating platform for
exploring optical and electronic properties in the two-dimensional limit.
Previous study has revealed the ultrafast formation of interfacial excitons and
the exciton dynamics in the Gr/MoS2 heterostructure. However, a fully
understanding of interfacial charge separation and the subsequent dynamics in
graphene-based heterostructures remains elusive. Here, we investigate the
carrier dynamics of Gr-MoS2 (including Gr/MoS2 and MoS2/Gr stacking sequences)
heterostructures under different photoexcitation energies and stacking
sequences by comprehensive ultrafast means, including time-resolved terahertz
spectroscopy (TRTS), terahertz emission spectroscopy (TES) and transient
absorption spectroscopy (TAS). We demonstrate that the Gr/MoS2 heterostructure
generates hot electron injection from graphene into the MoS2 layer with
photoexcitation of sub-A-exciton of MoS2, while the interfacial charge
separation in the MoS2/Gr could be partially blocked by the electric field of
substrate. Charge transfer (CT) occurs in same directions for the Gr-MoS2
heterostructures with opposite stacking order, resulting in the opposite
orientations of the interfacial photocurrent, as directly demonstrated by the
terahertz (THz) emission. Moreover, we demonstrate that the recombination time
of interfacial charges after CT is on a timescale of 18 ps to 1 ns, depending
on the density of defect states in MoS2 layer. This work provides a
comprehensive and unambiguous picture of the interfacial charge dynamics of
graphene-based heterostructures, which is essential for developing Gr/TMDs
based optoelectronic devices.
