Layer-dependent electrical and optoelectronic responses of ReSe₂ nanosheet transistors

The ability to control the appropriate layer thickness of transition metal dichalcogenides (TMDs) affords the opportunity to engineer many properties for a variety of applications in possible technological fields. Here we demonstrate that band-gap and mobility of ReSe₂ nanosheet, a new member of the TMDs, increase when the layer number decreases, thus influencing the performances of ReSe₂ transistors with different layers. A single-layer ReSe₂ transistor shows much higher device mobility of 9.78 cm² V^-1 s^-1 than few-layer transistors (0.10 cm² V^-1 s^-1). Moreover, a single-layer device shows high sensitivity to red light (633 nm) and has a light-improved mobility of 14.1 cm² V^-1 s^-1. Molecular physisorption is used as "gating" to modulate the carrier density of our single-layer transistors, resulting in a high photoresponsivity (R _λ) of 95 A W^-1 and external quantum efficiency (EQE) of 18645% in O₂ environment. This work highlights the fact that the properties of ReSe₂ can be tuned in terms of the number of layers and gas molecule gating, and single-layer ReSe₂ with appropriate band-gap is a promising material for future functional device applications.