Strain-mediated stability and electronic properties of WS2, Janus WSSe and WSe2 monolayers

Monolayers of transition metal dichalcogenides (TMDs) have been proposed as the next generation electronic materials for nanoscale devices. We evaluated the thermodynamic stability of WS2, Janus WSSe and WSe2 monolayers under biaxial tensile and compressive strain using density functional approach. The phonon band structures of unstrained WS2, WSSe and WSe2 monolayers confirm their thermodynamic stability. The stability of these monolayers is investigated under strain and phonon softening is observed for acoustic and optical mode upto 8% tensile strain. The bond length reduction under compressive strain resulted in out of plane deformation, leading the instability of monolayers under compressive strain. The bond length W-S (Se) and bond angle W-S(Se)-W exhibit significant contribution in coupling strength between transition metal d-orbitals and chalcogen p-orbitals that mediated the electronic properties. The spin orbit coupling showed strong effect on splitting of the valence band for unstrained mono-layers. The spin splitting in valence band increases with increasing the tensile strain and decreases with increasing the compressive strain. The effective masses and mobility values are 0.57m(e), 0.54m(e), 0.47m(e); and 0.059, 0.062, 0.072 m(2)V(-1)s(-1) for unstrained WS2, WSSe and WSe2 monolayers, respectively. The tungsten d(x2-y2) orbitals are mainly contributing to the conduction and valence band electronic states in compressive strained monolayers. In contrast under tensile strain tungsten d(z2) orbitals contribute to conduction and valence band electronic states, causing the direct to indirect band gap transition in these monolayers. We observed that the impact of tensile strain is more sensitive as compared to that of compressive strain.