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In-plane heterojuctions formed from two monolayer semiconductors represent the finest control of electrons in condensed matter and have attracted significant interest. Various device studies have shown the effectiveness of such structures to control electronic processes, illustrating their potentials for electronic and optoelectronic applications. However, information about the physical mechanisms of charge carrier transfer across the junctions is still rare, mainly due to the lack of adequate experimental techniques. Here we show that transient absorption measurements with high spatial and temporal resolution can be used to directly monitor such transfer processes. We studied MoS2–MoSe2in-plane heterostructures fabricated by chemical vapor deposition and lithographic patterning followed by laser-generated vapor sulfurization. Transient absorption measurements in reflection geometry revealed evidence of exciton transfer from MoS2to MoSe2. By comparing the experimental data with a simulation, we extracted an exciton transfer velocity of 104m s–1. These results provide valuable information for understanding and controlling in-plane carrier transfer in two-dimensional lateral heterostructures for their electronic and optoelectronic applications.

We observe subpicosecond charge separation and formation of indirect excitons a van der Waals heterostructure formed by molybdenum disulfide and molybdenum diselenide monolayers. The sample is fabricated by manually stacking monolayer MoS2and MoSe2flakes prepared by mechanical exfoliation. Photoluminescence measurements confirm the formation of an effective heterojunction. In the transient absorption measurements, an ultrafast laser pulse resonantly injects excitons in the MoSe2layer of the heterostructure. Differential reflection of a probe pulse tuned to the MoS2exciton resonance is immediately observed following the pump excitation. This proves ultrafast transfer of electrons from MoSe2to MoS2layers, despite the strong Coulomb attraction from the holes in the resonantly excited excitons. Conversely, when excitons are selectively injected in MoS2, holes transfer to MoSe2on an ultrafast time scale, too, as observed by measuring the differential reflection of a probe tuned to the MoSe2resonance. The ultrafast charge transfer process is followed by the formation of spatially indirect excitons with electrons and holes residing in different layers. The lifetime of these indirect excitons are found to be longer than that of the direct excitons in individual MoS2and MoSe2monolayers.