Transition-metal dichalcogenides (MX₂) have attracted wide research interest due to their unusual structural properties. Many of these compounds crystallize in sheets of hexagonally coordinated atoms stacked in the sequence X-M-X, X-M-X.  Within each of the layers the atoms are bonded covalently while the interlayer forces are of Van der Waal’s type. The resulting strong anisotropy has made these materials an interesting system for the study of electronic and ionic transport, intercalation, superconductivity and many other physical properties. The single crystals of Mo_0.25W_0.75Se₂ belonging to this family have been grown by direct vapour transport (DVT) technique and the Hall Effect and thermoelectric power measurements have been carried out. The temperature dependence of Seeback coefficient (S) shows semiconducting behaviour of as-grown crystals. The value of S is found to be negative over the entire range of temperatures, revealing n-type nature of the crystals. This is also confirmed by Hall effect measurements.  Optical absorption studies of the grown crystals have been used to evaluate the energies of the direct band gaps and optical constants. The detailed surface microtopographic studies of these crystals by means of optical microscopy have shown that the screw dislocation mechanism is predominant during crystal growth. The scanning electron micrographs show the presence of hexagonal crystallites on the surface of as grown crystals. Such crystallites form island on the as-grown surface where other units come and sit. However, the size and properties of grown crystals as well as the surface morphology has been found to depend on the temperature profile, heating and cooling rate, duration of growth process etc. Atomic force microscopy studies show that the arrangement of atoms is found to be uniform with variation from 3 nm to 6 nm. The overall surface of Mo_0.25W_0.75Se₂ crystal investigated in the present case is found to be homogeneous.

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1. Growth and Structural Studies of Compounds of TMDC Single Crystals
2. Optical Absorption in  Mo_xW_1-XSe₂ (x=0, 0.25, 1) Single Crystals
3. Growth and Surface Studies of Mo_XW_1-xSe₂ Crystals