Spin-orbit and strain effect on power factor in monolayer MoS2

Biaxial strain dependence of electronic structures and thermoelectric properties of monolayer MoS2, including compressive and tensile strain, are investigated by using local-density approximation (LDA) plus spin-orbit coupling (SOC). Both LDA and LDA + SOC results show that MoS2 is a direct gap semiconductor with optimized lattice constants. It is found that SOC has important effect on power factor. In n-type doping, the power factor with SOC is larger than that without SOC. However, in p-type doping, the power factor using LDA + SOC becomes very small compared to one using LDA. Both compressive and tensile strain can induce direct-indirect gap transition, which produce remarkable influence on power factor. Calculated results show that strain can induce significantly enhanced power factor in n-type doping by compressive strain and in p-type doping by tensile strain at the critical strain of direct-indirect gap transition. These can be explained by strain-induced accidental degeneracies, which leads to improved Seebeck coefficient. Calculated results show that n-type doping can provide better power factor than p-type doping. These results make us believe that thermoelectric properties of monolayer MoS2 can be improved in n-type doping by compressive strain. (C) 2016 Elsevier B.V. All rights reserved.