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 MoSe₂ monolayers that are separated by a monolayer WS₂ energy barrier. Four-layer heterostructures of MoSe₂/WS₂/MoSe₂/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 MoSe₂ layers. By utilization of the graphene layer to eliminate carriers in the third MoSe₂ layer, the first MoSe₂ layer is selectively populated with the holes, which then tunnel to the third MoSe₂ 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.