Ultrahigh-Gain and Fast Photodetectors Built on Atomically Thin Bilayer Tungsten Disulfide Grown by Chemical Vapor Deposition.

The low responsivity observed in photodetectors based on monolayer transition-metal dichalcogenides has encouraged the pursuit of approaches that can efficiently enhance the external quantum efficiency, which relies predominantly on the light absorption, the lifetime of the excess carriers, and the charge collection efficiency. Here, we demonstrate that phototransistors fabricated on large-area bilayer tungsten disulfide (WS2) grown by chemical vapor deposition exhibit remarkable performance with photoresponsivity, photogain, and detectivity of up to ∼3 × 103 A/W, 1.4 × 104, and ∼5 × 1012 Jones, respectively. These figures of merit of bilayer WS2 provide a significant advantage over monolayer WS2 due to the greatly improved carrier mobility and significantly reduced contact resistance. The photoresponsivity of bilayer WS2 phototransistor can be further improved to up to 1 × 104 A/W upon biasing a gate voltage of 60 V, without evident reduction in detectivity. Moreover, the bilayer WS2 phototransistor exhibits a high response speed of less than 100 μs, large bandwidth of 4 kHz, high cycling reliability of over 105 cycles, and spatially homogeneous photoresponse. These outstanding figures of merit make WS2 bilayer a highly promising candidate for the design of high-performance optoelectronics in the visible regime.