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Since 2012, heterostructures formed by two-dimensional materials have drawn considerable attention due to the potentials of combining their novel properties. In developing these materials, one key issue is to understand and control charge transfer process. In this presentation, latest progress on experimental studies of ultrafast electron transport in van der Waals heterostructures, including both vertical and lateral heterostructures, will be discussed. First, previous studies by several groups on electron transfer in van der Waals vertical hetero-bilayers, such as MoS2/MoSe2 and WS2/graphene, will be reviewed. Secondly, latest results on van der Waals bilayers will be presented, including the demonstration of a structure with type-I band alignment and study of electron transfer in homo-bilayers. Next, new experiments on electron transport in van der Waals multilayers, such as 3 layers and 4 layers, will be presented. The coherent nature of transport and the systematic control of the carrier dynamics by thickness will be demonstrated. Attempts of using band-alignment engineering to control flow of carriers will be discussed. As an example, unipolar optical doping of graphene with significantly extended photocarrier lifetimes were achieved. Finally, latest ultrafast measurements of charge transfer in lateral junctions will be discussed, including a heterojunction formed by connecting monolayers of MoS2 and MoSe2 and a homojunction by varying the bandgap of a region of a MoSe2 monolayer by a dielectric top-layer. In both cases, transient absorption measurements with high temporal and spatial resolution revealed efficient photocarrier transfer across the lateral junctions.
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