High pressure studies of as grown WX2-x single crystals

The structural optical and transport properties of tungsten metal dichalogenides having layered structure have been extensively studied in the last two decades. These materials shows highly anisotropic behaviour and have been receiving considerable interest for a variety of applications. Several of these layered semiconductors have attracted attention as a new class of solar cell material. We present here the results of simultaneous resistivity and thermoelectric power (TEP) measurements upto 7 GPa on single crystals of WS2, WS1.9, WSe2 and WSe1.9 grown using Direct Vapour Transport (DVT) technique. The observations clearly shows WS2 and WS1.9 are more resistive compared to other two crystals. In all samples an exponential fall of resistivity on increases in pressure upto 2.1 GPa but after 2.2 GPa the resistivity decreases substantially with increases pressure. The TEP of WSe2 increases steadily and reaches maximum at 0.65 GPa, while for WSe1.9 TEP increases upto pressure 0.5 GPa. In both the cases after attaining the maximum TEP, then decreases monotonically with increase in pressure. TEP of WS2 and WS1.9 increase upto pressure 1.1 GPa, beyond 1.1 GPa pressure in both the cases TEP decreases steadily with further increase in pressure. In all the samples, the sign of TEP is positive indicating that all of them are p-type and remain p-type with increase in pressure. The variation of thermoelectric power factor with pressure has been thoroughly studied. An analysis of the data point out that perfectly stoichiometric crystals of WSe2 work as superior thermoelectric materials. The results have been presented and implications have been discussed. (author)