Due to its high carrier mobility, broadband absorption, and fast response time, graphene is attractive for optoelectronics and photodetection applications. However, the extraction of photoelectrons in conventional metal-graphene junction devices is limited by their small junction area, where the typical photoresponsivity is lower than 0.01 AW-1. On the other hand, the atomically thin layer of molybdenum disulfide (MoS2) is a two-dimensional (2d) nanomaterial with a direct and finite band gap, offering the possibility of acting as a 2d light absorber. The optoelectronic properties of the heterostructure of these two films is therefore of great interest. The growth of large-area graphene using chemical vapour deposition (CVD) has become mature nowadays. However, the growth of large-area MoS2 monolayer is still challenging. In this work, we show that a large-area and continuous MoS2 monolayer is achievable using a CVD method. Both graphene and MoS2 layers are transferable onto desired substrates, making possible immediate and large-scale optoelectronic applications. We demonstrate that a phototransistor based on the graphene/MoS2 heterostructure is able to provide a high photoresponsivity greater than 107 A/W while maintaining its ultrathin and planar structure. Our experiments show that the electron-hole pairs are produced in the MoS2 layer after light absorption and subsequently separated across the layers. Contradictory to the expectation based on the conventional built-in electric field model for metal-semiconductor contacts, photoelectrons are injected into the graphene layer rather than trapped in MoS2 due to the alignment of the graphene Fermi level with the conduction band of MoS2. The band alignment is sensitive to the presence of a perpendicular electric field arising from, for example, Coulomb impurities or an applied gate voltage, resulting in a tuneable photoresponsivity.
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