  Two-dimensional transition metal dichalcogenides are emerging with tremendous
potential in many optoelectronic applications due to their strong light-matter
interactions. To fully explore their potential in photoconductive detectors,
high responsivity and weak signal detection are required. Here, we present high
responsivity phototransistors based on few-layer rhenium disulfide (ReS2).
Depending on the back gate voltage, source drain bias and incident optical
light intensity, the maximum attainable photoresponsivity can reach as high as
88,600 A W-1, which is a record value compared to other two-dimensional
materials with similar device structures and two orders of magnitude higher
than that of monolayer MoS2. Such high photoresponsivity is attributed to the
increased light absorption as well as the gain enhancement due to the existence
of trap states in the few-layer ReS2 flakes. It further enables the detection
of weak signals, as successfully demonstrated with weak light sources including
a lighter and limited fluorescent lighting. Our studies underscore ReS2 as a
promising material for future sensitive optoelectronic applications.
