Biaxial strain tuned electronic structures and power factor in Janus transition metal dichalchogenide monolayers

Tuning the physical properties of transition metal dichalcogenide (TMD) monolayers by strain engineering is an area that has been widely studied, and recently a Janus TMD monolayer MoSSe has been synthesized. In this work, we systematically study the biaxial strain dependence of the electronic structures and transport properties of Janus TMD MXY (M = Mo or W, X/Y = S, Se, or Te) monolayer by using generalized gradient approximation (GGA) and spin-orbit coupling (SOC). It is found that SOC has a noteworthy detrimental influence on power factor in p-type MoSSe, WSSe, n-type WSTe, p-type MoSeTe and WSeTe, and has a negligible influence on n-type MoSSe, MoSTe, p-type WSTe and n-type MoSeTe. This can be understood by considering SOC effects on their valence and conduction bands. For all six monolayers, the energy band gap firstly increases, and then decreases, when strain changes from a compressive one to a tensile one. It is found that strain can tune the strength of band convergence of both valence and conduction bands by changing the numbers and relative position of valence band extrema (VBE) or conduction band extrema (CBE), which can produce very important effects on their electronic transport properties. By applying appropriate compressive or tensile strain, both n- or p-type Seebeck coefficient can be enhanced by strain-induced band convergence, and then the power factor can be improved. Our works further enrich studies on the strain dependence of electronic structures and the transport properties of new-style TMD monolayers, and will motivate further experimental works.