Photocurrent Generation in a Metallic Transition Metal Dichalcogenide

Light induced current in two-dimensional (2D) layered materials emerges from mechanisms such as photothermoelectric effect, photovoltaic effect or nonlocal hot carrier transport. Semiconducting layered transition metal dichalcogenides have been studied extensively in recent years as the generation of current by light is a crucial process in optoelectronic and photovoltaic devices. However, photocurrent generation is unexpected in metallic 2D layered materials unless a photothermal mechanism is prevalent. Typically, high thermal conductivity and low absorption of the visible spectrum prevent photothermal current generation in metals. Here, we report photoresponse from two-terminal devices of mechanically exfoliated metallic 3R-NbS$_2$ thin crystals using scanning photocurrent microscopy (SPCM) both at zero and finite bias. SPCM measurements reveal that the photocurrent predominantly emerges from metal/NbS$_2$ junctions of the two-terminal device at zero bias. At finite biases, along with the photocurrent generated at metal/NbS$_2$ junctions, now a negative photoresponse from all over the NbS$_2$ crystal is evident. Among our results, we realized that the observed photocurrent can be explained by the local heating caused by the laser excitation. These findings show that NbS$_2$ is among a few metallic materials in which photocurrent generation is possible.