Recently, there has been a rise of interest in layered crystals, particularly transition metal dichalcogenides show signs of layered structures. The group VI Transition Metal Dichalcogenide compounds have been found to be highly stable against the environment. Several TMDCs are semiconducting compounds and widely used as a PEC solar cells, catalysts and also as a good lubricants. The compounds of TMDCs consists sandwiched layers i.e. X – M – X in which M atom layer is enclosed within two X chalcogenes layers. The interlayer binding forces are of van der Waals nature resulting in large interlayer distances and easy cleavability of these crystals. The crystals are anisotropic and leads to unusual structural properties. The Molybdenum diselenide, tungsten diselenide and mixed compounds of molybdenum and tungsten diselenides are diamagnetic indirect semiconductors belonging to this family.

Besides some technical complications the vapour transport technique has been proved to be very good technique to grow good quality single crystals. Single crystals of MoSe₂, WSe₂ and mixed system of these two compounds have been grown by direct vapour transport (DVT) technique using a two zone furnace from stoichiometric mixture of molybdenum (99.95% pure), tungsten (99.99% pure) and selenium (99.9 % pure). The source zone and growth zone temperatures were 1075⁰C and 1055⁰C respectively. The grown crystals were found to be gray and shiny irregular platelets. The average crystal dimensions were 3-10mm² in cross sectional area and few micrometers in thickness. The surface features of as grown crystals have been studied with the help of Optical Microscopy. The crystals grown by DVT technique possess 2H-hexagonal structure having /mmc space group and also contain only a little proportion of 3R polytype form. The optical micrograph shows the presence of screw dislocation on the surface of the grown crystals, which is clearly an indication that the growth of present materials in the form single crystals takes place via screw dislocation mechanism. Also the simultaneous presence of 2H and 3R polytypes is clearly seen in the optical micrograph, which confirms the results of XRD. On the other hand, the size and properties of grown crystals has been found to depend on the growth conditions also. 

1. FULLTEXT: Growth and Surface Studies of Mo_{X}W_{1-x}Se_{2} Crystals, Microsoft Office Document, 0MB

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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. Optical Absorption in  Mo_xW_1-XSe₂ (x=0, 0.25, 1) Single Crystals