The main challenge encountered by most 2D materials for their use in non-volatile memory technology is their low R_on/off ratio.

Vertical integration of two dimensional (2D) layered materials is indispensable in making van der Waals (vdWs) heterostructures for promising electronic and optoelectronic devices. Herein, we report excellent electrical and photoelectrical measurements where the current ON & OFF ratio of FET is increased by decreasing the temperature in the graphene/ReSe₂/graphene heterojunction. We investigated the photoresponsivity in broad spectral range (UV-Vis-NIR) and achieved high photoresponsivity of 1.5 × 10⁷ A W⁻¹ and external quantum efficiency of ∼64% at λ = 220 nm. Further, the photovoltaic effect was examined, which significantly modulated the short circuit current (I_sc) from 4.2 × 10⁻⁸ A to 2.6 × 10⁻⁷ A and open-circuit voltage (V_oc) from 0.21 V to 0.44 V at different wavelengths (1064, 840, 514 and 220 nm), attributed to the photo-generation and recombination rate of the carriers. Moreover, photoresponsivity was observed near 1.2 × 10⁶, 8.6 × 10⁶ and 1.5 × 10⁷ A W⁻¹ by applying different gate biases (0, 20 and 40 V), respectively. Further, we have explored the photocurrent and photoresponsivity at different intensities of incident light (200, 260, 400, 620 and 850 μW cm⁻²). In addition, we calculated the rise and decay response times of photodetectors at different wavelengths and power densities, which depend upon the trap sites in the energy band of ReSe₂. These devices opened up new ways to improve the performance of photodetectors from the UV to the NIR region.

2D transition metal dichalcogenides are promising in various electronics and optoelectronics applications and have gained popularity owing to their carrier transport and strong light–matter interactions. To fully realize their potential in field-effect transistors (FETs) and photodetectors, high mobility and high responsivity are imperative. Here, we demonstrate the highest mobility of ~166 cm ² V ⁻¹ s ⁻¹ at 200 K for single-layer rhenium diselenide (ReSe ₂ ) FETs encapsulated between h-BN flakes at V _g = 47 V. The high mobility is attributed to low-resistance contacts of scandium/gold (Sc/Au), with a low Schottky barrier height and reduced charge scattering platform of h-BN. Further, we elucidated the Schottky-barrier-height dependent high photoresponsivity (~3.2 × 10 ⁶ A W ⁻¹ ) of few-layer ReSe ₂ (FL-ReSe ₂ ) at 532 nm-wavelength laser light on an h-BN substrate with Sc/Au contacts. Moreover, broadband light detection of undoped and Co-doped few-layer (FL) ReSe ₂ was performed under different laser wavelengths (400–1100 nm). After the deposition of Co nanoparticles, the photocurrent of FL-ReSe ₂ increased due to n-doping, as confirmed by the transfer curves of the FL-ReSe ₂ -based undoped and co-doped FETs. Further, the work function decreased from 4.856 to 4.791 eV in FL-ReSe ₂ , as measured by Kelvin probe force microscopy. No light signal was observed at 1100 nm for the undoped ReSe ₂ (1050 nm < λ _cut-off < 1100 nm); however, after doping with Co nanoparticles, the cut-off wavelength exceeded to ( λ _cut-off > 1100 nm), due to the additional trap states generated in the energy band gap of ReSe ₂ after Co doping. Further, the transient response of ReSe ₂ and Co + ReSe ₂ FETs was estimated so that the rise and decay times are decreased from 1.9 s & 2.7 s to 1.1 s & 1.8 s, respectively. ReSe ₂ is therefore a promising semiconducting material for electrical and optoelectrical applications.