The transition-metal dichalcogenides (TMDC) such as MX₂ (M=W or Mo and X= S, Se) show a wide variety of interesting anisotropic optical and electrical properties. They display a whole spectrum of electronic properties covering semiconductors, metals, insulators and superconductors. Most TMDCs form hexagonal layered structures. Structurally, these compounds can be regarded as strongly bonded two dimensional X – M – X layers or sandwiches which are loosely coupled to one another by relatively weak Van der Waals type forces. This structure makes these compounds extremely anisotropic in character and leads to unusual structural properties. The band gaps of many of these materials are around 1 eV and it has been suggested that the band gaps are indirect. These TMDCs have been used for many years as solid state lubricants, photovoltaic/photocatalytic solar energy converters, Schottky and liquid junction solar cells as catalysts in many industrial applications and in secondary batteries etc.  Looking to such diverse applications of TMDCs, the molybdenum diselenide, tungsten diselenide and mixed compounds of molybdenum and tungsten diselenides belonging to this family have been grown by direct vapour transport (DVT) technique using a two zone horizontal furnace. Optical absorption spectra of these compounds have been measured in the range 200–2000 nm at room temperature near the fundamental absorption edge using light parallel to c – axis incident normally on the basal plane. Results have been analyzed on the basis of two dimensional (2D) and three dimensional (3D) models. Both direct and indirect transitions are involved in the absorption process. Absorption near the fundamental edge was found to be due to indirect and direct allowed transitions on the basis of 3D model and indirect allowed transition on the basis of 2D model. The optical energy gaps corresponding to both transitions have also been determined. Some feeble disorder in the crystals is conceived to be present.

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1. Growth and Structural Studies of Compounds of TMDC Single Crystals
2. Electrical, Optical and Surface Microtopographic studies of Mo_0.25W_0.75Se₂ Single Crystals grown by DVT Technique
3. Growth and Surface Studies of Mo_XW_1-xSe₂ Crystals