Transition metal dichalcogenides (TMDCs) have shown tremendous potential for application in the field of optoelectronics owing to their extraordinary characteristics. The $\rm{WS_{2}/V_{0.25}W_{0.75}Se_{2}}$ van der Waals heterostructure was fabricated by layer transfer technique and its $I – V$ characteristic was measured at room temperature. The fabricated pn-junction heterostructure shows obvious current rectification with a rectification ratio of $\thicksim{39}$ at $\pm{1}$ V. The heterostructure was analysed in the dark and under polychromatic illumination. The noticeable rise in reverse current is observed at higher intensity of illumination. The photocurrent and photoresponsivity are found to be enhanced as intensity and bias voltage are increased. The higher value of the ideality factor of $\thicksim{2}$ is attributed to the inhomogeneity of the heterojunction.

Two-dimensional transition metal chalcogenides like GeSe, GeS and SnSe are widely used in a variety of electrical and optoelectrical applications. Alloying becomes the most important tool to alter the structural, optical and electrical properties of the material. Here, efforts have been applied to grow the crystals of GeS$_x$Se$_{1−x}$ ($x = 0, 0.5, 1$) using iodine (I$_2$) as a transporting agent by the chemical vapour transport technique. The elemental confirmation of grown crystals was done by the energy-dispersive analysis of X-rays. The lattice parameters were obtained from powder X-ray diffraction patterns of all the grown compounds. For the optical study of grown compounds, UV–Vis spectroscopy was performed in the wavelength range of 700–1450 nm. The optical absorption process was studied in detail using the direct and the indirect transitions from two- and three-dimensional models. Moreover, band-gap modification by incorporating sulphur in different concentrations is studied. It is found that the band gap increases with increasing sulphur content in germanium selenide crystals.