Engineering the photoresponse of liquid-exfoliated 2D materials by size selection and controlled mixing for an ultrasensitive and ultraresponsive photodetector

The performances of percolating thin-film photodetectors fabricated from liquid-exfoliated 2D materials such as MoS2 and WS2 are inferior compared to their single-crystalline counterparts due to the low-mobility arising from edge defects and inter-nanosheet resistance. Moreover, the mechanism controlling the photoresponse time is not well understood. In this study, we demonstrated better control of the photoresponse time via the size selection of the liquid-exfoliated nanosheets via cascade centrifugation. We discovered that a faster photoresponse was observed when using a large average nanosheet size, while a much slower response was observed when using small nanosheets, giving rise to the linear component of the photoresponse in addition to the exponential component. Furthermore, we discovered that mixing a controlled proportion of WS2 and graphene nanosheets exactly at the percolation threshold results in a significantly increased photocurrent, up to 40 times higher compared to the pure WS2 photodetector, due to good interfacial charge transfer between WS2 and graphene, reaching a responsivity of 1.15 A W-1 under blue light. Above the percolation threshold, the photocurrent vanishes due to increased dark current noise. This strategy of nanosheet size-selection and controlled mixing with graphene opens up the possibility for 2D material photodetectors fabricated via liquid-exfoliation to be more competitive with their single-crystalline counterparts. We believe that this approach can be applied to other emerging 2D materials, such as phosphorene, MXene, and graphitic carbon nitride.