No abstract

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 ReSe2 (Mo:ReSe2) 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:ReSe2 nanosheet based photodetectors. We compared the photoresponse of the as-exfoliated device with annealed device both in air or ammonia (NH3) environment. After annealing at sub-decomposition temperatures, the Mo:ReSe2 photodetectors show a better photoresponsivity (~55.5 A/W) and higher EQE (10893%) in NH3 than in air. By theoretical investigation, we conclude that the physisorption of NH3 molecule on Mo:ReSe2 monolayer can cause the charge transfer between NH3 molecule and Mo:ReSe2 monolayer, increasing the n-type carrier density of Mo:ReSe2 monolayer. The prompt photoswitching, high photoresponsivity and different sensitivity to surrounding environment from the few-layer anisotropic Mo:ReSe2 can be used to design multifunctional optoelectronic and sensing devices.

Vertically stacked Van der Waals heterojunctions of atomically thin transition metal dichalcogenides (TMDs) offer new physical properties and new strategies for designing novel device functionalities that are vastly different from homostructured TMDs. The Raman intensity is strongest and frequency difference is largest in monolayer WSe2 compared with that in few‐layers, which is opposite to MoS2 and WS2. In the WSe2/MoS2 bilayer heterostructures, inefficient charge transfer quenches light emission of monolayer WSe2 but strengthens those of MoS2 monolayer. Interestingly, rectification and ambipolar effects emerge due to tunneling‐assisted interlayer recombination and dual conducting channels of p‐WSe2 and n‐MoS2 in the heterojunctions system. Gate‐induced holes tunneling also leads to a novel “anti‐bipolar” behavior with a sharp current peak. Under light illumination, charge transfer competes with the holes tunneling between the WSe2 and MoS2 layers, which can greatly influence the electrical transport leading to the disappeared rectifying and “anti‐bipolar” properties.