Analysis of localized excitons in strained monolayer WSe by first principles calculations.

Although there is growing interest in enhancement of single-photon emission (SPE) in two-dimensional transition metal dichalcogenides, particularly when introducing strain by the generation of wrinkles in monolayer WSe, understanding the effects of the wrinkle type on the response has been lacking. In this work, by investigating the electronic and optical properties of monolayer WSe with wrinkles by first principles calculations, we gain insight into the tunability of the response, where the band gap is found to be modulated by the wrinkle type. Our detailed analyses of the local electronic structures show that the strain distribution from regions with different local strain magnitudes and types affect the band alignments. We demonstrate that introducing a wrinkle in the monolayer results in a red-shift, including the bright A exciton and the lowest energy dark exciton, and the dependence on strain is consistent with available measurements. The energy difference between the A exciton and the lowest mid-gap energy for a single Se vacancy in the wrinkled WSe monolayer as a function of strain is consistent with a suggestion on the origin of SPE in monolayer WSe. Our results will encourage engineering of wrinkle types for enhanced SPE at specific wavelengths, which could potentially originate from hybridization of the localized strained dark exciton and a mid-gap point-defect exciton.