Photocurrent generation in a metallic transition-metal dichalcogenide

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-NbS2 thin crystals using scanning photocurrent microscopy (SPCM) both at zero and finite bias. SPCM measurements reveal that the photocurrent predominantly emerges from metal/NbS2 junctions of the two-terminal device at zero bias. At finite biases, along with the photocurrent generated at metal/NbS2 junctions, now a negative photoresponse from all over the NbS2 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 NbS2 is among a few metallic materials in which photocurrent generation is possible. TextPhotocurrent generation in semiconducting 2D layered materials is dominantly due to photothermal [1], photovoltaic effects [2,3] as well as excitation of nonlocal hot carriers [4][5][6]. In metals, these mechanisms typically do not result in photocurrent generation except in a few cases. Photothermal effects are generally not significant in metals because of typical high thermal conductivity and low absorption of the optical excitation. Moreover, optically excited electrons in metals would not have a measurable contribution to the large number of intrinsic electrons near the Fermi level when a bias is applied. In a few cases such as metallic carbon nanotubes [7], graphene [4,8,9] and gold nanoparticle networks [10], photocurrent generation has been reported. Light induced current generation in metallic 2D layered transition metal dichalcogenide (TMDC) is unprecedented, thus, it is not clear what mechanism will be prevalent. Here, we investigate photoresponse of mechanically exfoliated thin Niobium Disulfide (NbS2) crystals using scanning photocurrent microscopy (SPCM) as an exemplary metallic TMDC.NbS2 can be found in layered form both in hexagonal (2H) and rhombohedral (3R) polytypes. While both polytypes are metallic [11], only 2H-NbS2 is superconducting. It is reported in the literature [12][13][14][15]] that 3R-NbS2 shows a conductivity maximum, ranging from 20K to 50K, whose origin is still under debate [16,17]. The conductivity maximum may indicate charge density wave correlations [18] as well as weak localization [19], strong Coulomb correlations [20] or the Kondo effect [21]. Sketches of the 3R structure are depicted in Fig. 1(a) and 1(b).

Thermal Conductivity Measurements via the Bolometric Effect

“…Placing the crystal on the gold contacts with minimal contact resistance is crucial. This has been demonstrated in an earlier study by our group [2]. When the contact resistance dominates the overall resistance of the device, the photoresponse becomes very localised at the contacts and the bolometric response diminishes.…”

Thermal conductivity measurements in nanosheets via bolometric effect

“…When no bias is applied, photoresponse results due to the electromotive force generated by the Seebeck effect at the metal-TaS2 junctions. When the bias is applied, we observe photoresponse from all over the nanoflake due to the local resistance change upon laser beam heating 29 . As proposed in the previous paragraph, there is an enhancement of the absolute value of the photoresponse when the laser scans the region above the hole.…”

“…Moreover, since the residual resistance for the metals at very low temperatures have very weak temperature dependence, our method would fail at such regimes as well. Another problem associated with the bolometric measurement method we introduce would be the large contact resistance [32]. When the contact resistance dominates the total resistance of the device, bolometric response is significantly reduced.…”

“…However, these assumptions have no or minimal effect on the thermal distribution over the suspended part of the crystal for the aforementioned reasons and as reported for the Raman based thermal conductivity technique 10 . 29 . However, as the crystal geometries here are typically complicated and the local current density also depends on the temperature, we used a commercially available finite element method (FEM) package (COMSOL Multiphysics) to solve for 𝑅 𝐻 .…”

Enhancement in the transient visible light anisotropic photoresponse of cobalt‐incorporated tungsten diselenide ternary alloy

“…Mehmood et al have reported that light-induced local temperature rise causes a reduction in the conductivity and hence the photocurrent. 30 However, since the decrement has been observed solely for white light, it points towards a wavelength-induced photocurrent polarity. 31 Similar temperature and wavelength induced reduced photocurrents and negative photoconductance has been reported in works by Ratha et al 32 Wang et al 33 attributed the decrement in current due to the bolometric effect where the scattering between charge carriers and phonons reduces the mobility of the former; this is likely to be the reason for the sudden decrement in photocurrent in the present work.…”