Photocurrent generation in a metallic transition-metal dichalcogenide

Photocurrent generation is unexpected in metallic 2D layered materials unless a photothermal mechanism is prevalent. Yet, typical 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-$\mathrm{Nb}{\mathrm{S}}_2$ thin crystals using scanning photocurrent microscopy (SPCM) both at zero and finite bias. SPCM measurements reveal that the photocurrent predominantly emerges from metal/$\mathrm{Nb}{\mathrm{S}}_2$ junctions of the two-terminal device at zero bias. At finite biases, along with the photocurrent generated at metal/$\mathrm{Nb}{\mathrm{S}}_2$ junctions, now a negative photoresponse from all over the $\mathrm{Nb}{\mathrm{S}}_2$ 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 $\mathrm{Nb}{\mathrm{S}}_2$ is among a few metallic materials in which photocurrent generation is possible.

Figure 1

(a) Crystal structure of $3R-\mathrm{Nb}{\mathrm{S}}_2$ is depicted from the side and (b) from the top. (c) Measurement configuration is depicted on a scanning electron microscope micrograph of a two-terminal device. Laser beam raster scans over the sample with a diffraction-limited spot. Throughout the paper, left contact is grounded, and the bias is applied through the right contact using a current preamplifier.

Figure 2

(a) Optical microscope image of TC-1 is shown. SPCM measurement is taken from the region boxed with a dashed rectangle. Scale bar is 10 µm. (b) Reflection map and (c) the corresponding photocurrent map of TC-1 under zero bias shows photocurrent emerging around the contacts. Black dashed lines indicate the outlines of the contacts. (d) $R-T$ graph shows the metallic characteristic of the sample. $\partial R/\partial T$ is given in the inset from 200 to 295 K as a reference. (e) Upper panel shows $V_{\mathrm{PC}}$ vs $P$ measured from the metal/$\mathrm{Nb}{\mathrm{S}}_2$ junctions [positions indicated by yellow dashed circles in (c)], red points from the left contact, and blue points from the right contact. Lower panel shows the calculated $\mathrm{\Delta }{T}_C/P$ from the $V_T$ vs $P$ data.

Figure 3

(a) Photocurrent map of TC-1 under 50-mV and (b) −50-mV bias is given. Photocurrent is generated all over the crystal between the contacts when the bias is applied. Dashed gray line represents the outline of the metal contacts. (c) Line trace along the center of the crystal shows the photocurrent (upper panel) under −50-, −20-, 0-, 20-, and 50-mV biases and respective photoconductances (lower panel). It is clear that the photoconductance $G_{\mathrm{PC}}$ is the same for all different biases.

Figure 4

(a) Optical microscope image of bottom contact device (BC-1) is shown. Scale bar is 10 µm. Photocurrent maps (b) under zero bias, (c) 50-mV, and (d) −50-mV bias show that there is no observable photoconductance change in the center of the crystal. This can be explained by the fact that the contact resistance of BC-1 is so high that $V_{\mathrm{eff}}$ on the crystal is small and hence local resistance change caused by photothermal heating becomes insignificant. Dashed gray line represents the outline of the metal contacts.