Ultrahigh-Gain Photodetectors Based on Atomically Thin Graphene-MoS₂ Heterostructures

Due to its high carrier mobility, broadband absorption, and fast response time, the semi-metallic graphene is attractive for optoelectronics. Another two-dimensional semiconducting material molybdenum disulfide (MoS₂) is also known as light- sensitive. Here we show that a large-area and continuous MoS₂ monolayer is achievable using a CVD method and graphene is transferable onto MoS₂. We demonstrate that a photodetector based on the graphene/MoS₂ heterostructure is able to provide a high photogain greater than 10⁸. 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 presence of a perpendicular effective electric field caused by the combination of the built-in electric field, the applied electrostatic field, and charged impurities or adsorbates, resulting in a tuneable photoresponsivity.