Transition metal dichalcogenides (TMDCs) have recently been the focus of extensive research activity owing to their fascinating physical properties. As a new member of TMDCs, Mo doped ReSe 2 (Mo:ReSe 2) is an octahedral structure semiconductor being optically biaxial and highly anisotropic, different from most of hexagonal layered TMDCs with optically uniaxial and relatively high crystal symmetry. We investigated the effects of physisorption of gas molecule on the few-layer Mo:ReSe 2 nanosheet based photodetectors. We compared the photoresponse of the as-exfoliated device with annealed device both in air or ammonia (NH 3) environment. After annealing at sub-decomposition temperatures, the Mo:ReSe 2 photodetectors show a better photoresponsivity (∼55.5 A/W) and higher EQE (10893%) in NH 3 than in air. By theoretical investigation, we conclude that the physisorption of NH 3 molecule on Mo:ReSe 2 monolayer can cause the charge transfer between NH 3 molecule and Mo:ReSe 2 monolayer, increasing the n-type carrier density of Mo:ReSe 2 monolayer. The prompt photoswitching, high photoresponsivity and different sensitivity to surrounding environment from the few-layer anisotropic Mo:ReSe 2 can be used to design multifunctional optoelectronic and sensing devices.
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