Spatiotemporal Observation of Quasi-Ballistic Transport of Electrons in Graphene

We report spatiotemporal observations of room-temperature quasi-ballistic electron transport in graphene, which is achieved by utilizing a four-layer van der Waals heterostructure to generate free charge carriers. The heterostructure is formed by sandwiching a MoS₂ and MoSe₂ heterobilayer between two graphene monolayers. Transient absorption measurements reveal that the electrons and holes separated by the type-II interface between MoS₂ and MoSe₂ can transfer to the two graphene layers, respectively. Transient absorption microscopy measurements, with high spatial and temporal resolution, reveal that while the holes in one graphene layer undergo a classical diffusion process with a large diffusion coefficient of 65 cm² s^-1 and a charge mobility of 5000 cm² V^-1 s^-1, the electrons in the other graphene layer exhibit a quasi-ballistic transport feature, with a ballistic transport time of 20 ps and a speed of 22 km s^-1, respectively. The different in-plane transport properties confirm that electrons and holes move independently of each other as charge carriers. The optical generation of ballistic charge carriers suggests potential applications for such van der Waals heterostructures as optoelectronic materials.