Effect of Strain on the Fine Structure of Exciton States in Atomically Thin Transition Metal Dichalcogenides

In two-dimensional (2D) transition metal dichalcogenides, the sequence and splitting energy between spin-allowed (bright) and spin-forbidden (dark) excitons controls the optical and transport properties. In this paper, we discuss the effect of strain at both compression and tension on the band structure and fine spectrum of exciton states in MoS 2 nanostructures. Using a combination of micro-Raman and time-resolved micro-photoluminescence, we found that the exciton spectrum in unstrained layers in complete agreement with the theoretical predictions. In the A-exciton series, the bright state is the lowest in the monolayer, while in the bilayer the exciton states are spin-degenerate due to the even number of layers. However, strain can lift the degeneracy and increase the splitting value in the monolayer by several times. On folds subjected to local tension, the splitting decreases down to the reversed sequence of dark and bright excitons. With both types of strain, the band structure tends to transform towards the indirect type.