  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.
