Probing effect of electric field on photocarrier transfer in graphene-WS van der Waals heterostructures.

We report spatially and temporally resolved measurements of photocarrier transfer process in van der Waals heterostructures. Graphene-WS hetero-bilayers were fabricated by manually stacking monolayers of graphene and WS obtained by mechanical exfoliation. Photocarriers were excited in WS by an ultrafast laser pulse. Their transfer to graphene was monitored by measuring the differential reflection signal as a function of both time and space. Surprisingly, we found that the width of the photocarrier profile in graphene decreases with time. This counter-intuitive phenomenon suggests that the Coulomb field of the holes transferred from WS to graphene can effectively drag the electrons to speed up their transfer. This effect illustrates that an externally applied electric field can be used to control the ambipolar photocarrier transfer in van der Waals heterostructures.