Band valley flattening and exciton appearance/disappearance under isotropic strain in monolayer WS 2

Mechanism of exciton appearance and disappearance under isotropic strain in monolayer WS 2 is investigated using the non-local van der Waals density functionals theory with taking into account the spin–orbit coupling (SOC). The essential effects such as vertical shift (the shift in the binding energy scale) of the d and p partial orbitals of the W and S atoms, respectively, and the flattening of band valleys caused by strain are explored. The exciton appearance and disappearance in the isotropic strained WS 2 are discussed. Thanks to the spin splitting by SOC effect, light and dark excitons are visually shown. Under strain, the flattening of the band valleys may lead to the disappearance of excitons, causing the direct-to-indirect transition of the band gap. Furthermore, the vertical shift of electron orbitals at band edges can result in a notable reduction in the band gap. These findings can open a possible path to manipulate excitons using strain scheme.