Strain induced valley degeneracy: a route to the enhancement of thermoelectric properties of monolayer WS2

Two-dimensional transition metal dichalcogenides show great potential as promising thermoelectric materials due to their lower dimensionality, the unique density of states and quantum confinement of carriers. Here the effects of mechanical strain on the thermoelectric performances of monolayer WS2 have been investigated using density functional theory associated with semiclassical Boltzmann transport theory. The variation of the Seebeck coefficient and band gap with applied strain has followed the same type of trend. For n-type material the relaxation time scaled power factor (S-2 sigma/tau) increases by the application of compressive strain whereas for p-type material it increases with the application of tensile strain due to valley degeneracy. A 77% increase in the power factor has been observed for the n-type material by the application of uniaxial compressive strain. A decrease in lattice thermal conductivity with the increase in temperature causes an almost 40% increase in ZT product under applied uniaxial compressive strain. From the study, it is observed that uniaxial compressive strain is more effective among all types of strain to enhance the thermoelectric performance of monolayer WS2. Such strain induced enhancement of thermoelectric properties in monolayer WS2 could open a new window for the fabrication of high-quality thermoelectric devices.