Photocurrent Generation in a Metallic Transition Metal Dichalcogenide

TL;DR

This study demonstrates that metallic 3R-NbS₂ layered crystals can generate photocurrent through local heating effects, challenging the common understanding that metals do not produce photocurrent due to high thermal conductivity and low absorption.

Contribution

It provides the first evidence of photocurrent generation in metallic layered transition metal dichalcogenides, specifically NbS₂, using scanning photocurrent microscopy.

Findings

Photocurrent mainly originates from metal/NbS₂ junctions at zero bias.

Finite bias induces a negative photoresponse across the NbS₂ crystal.

Photocurrent can be explained by local heating caused by laser excitation.

Abstract

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.