We report on the gas-sensing characteristics of a van der Waals heterojunction consisting of graphene and a MoS2 flake. To extract the response actually originating from the heterojunction area, the other gas-sensitive parts were passivated by gas barrier layers. The graphene/MoS2 heterojunction device demonstrated a significant change in resistance, by a factor of greater than 103, upon exposure to 1 ppm NO2 under a reverse-bias condition, which was revealed to be a direct reflection of the modulation of the Schottky barrier height at the graphene/MoS2 interface. The magnitude of the response demonstrated strong dependences on the bias and back-gate voltages. The response further increased with increasing reverse bias. Conversely, it dramatically decreased when measured at a large forward bias or a large positive back-gate voltage. These behaviors were analyzed using a metal-semiconductor-metal diode model consisting of graphene/MoS2 and counter Ti/MoS2 Schottky diodes.
Keywords: MoS2; Schottky barrier; gas sensor; graphene; heterojunction; metal−semiconductor−metal diode; thermionic emission.