Directly Observe Charge Injection of Graphene in the Graphene/WSe2 heterostructure by Optical-pump THz-probe Spectroscopy.

Charge transfer across the interface and interlayer coupling in graphene van der Waals heterostructure, which is constructed by graphene and semiconducting transition metal dichalcogenides (TMDCs), is critical for their electronic and optoelectronic applications. Photo-induced charge injection from TMDC to graphene has been studied in several heterostructure photodetector. However, the response time significantly varies among different reports, ranging from microseconds to milliseconds. In this work, using graphene/WSe2 heterostructure as an example, we directly observe the carriers density change of graphene by time-resolved optical-pump terahertz (THz)-probe spectroscopy and show ultrafast picosecond photoresponse of graphene. In the absence of photoexcitation, THz time-domain spectroscopic measurements show that WSe2 can transfer holes to graphene and pull down the Fermi level of graphene. After excitation by the ultrafast laser pulse, the transient THz response shows a rapid (∼0.35 ps) increase in the graphene conductivity mainly due to the holes injection from the WSe2 into graphene. Unlike previous reports on band bend as the guidance mechanism for charge transfer, our results show that the relevant mechanism is band offset across the atomically sharp interface.