Time-Resolved Observation of Hole Tunneling in van der Waals Multilayer Heterostructures.

We reported a time-resolved study of quantum-mechanical tunneling of holes between two MoSe monolayers that are separated by a monolayer WS energy barrier. Four-layer heterostructures of MoSe/WS/MoSe/graphene, as well as control samples, were fabricated by mechanical exfoliation and dry transfer techniques. To time-resolve the hole tunneling process, an ultrashort laser pulse was used to excite electrons and holes in both MoSe layers. By utilization of the graphene layer to eliminate carriers in the third MoSe layer, the first MoSe layer is selectively populated with the holes, which then tunnel to the third MoSe layer. By monitoring decay of the hole population with an ultrashort probe pulse, we measure a hole tunneling time of about 20 ps, which is found to slightly increase with the injected carrier density. Besides the fundamental interests of real-time observation of the quantum-mechanical tunneling effect across a nanometer barrier, these results provide quantitative understanding on tunneling mechanisms of charge transfer in van der Waals heterostructures, which is useful for designing sophisticated van der Waals multilayer heterostructures.