Emergence of non-uniform strain induced exciton species in homo- and heterobilayer transition metal dichalcogenides

Abstract: Full control of excitons in 2D materials is an important step to exploit them for applications. Straintronics is one method that can be used to effectively control the movement of excitons. Unfortunately, the effects of non-uniform strain in 2D materials are not yet well understood theoretically, although these strain fields can be present in experiments in the form of wrinkles, bubbles, and folds, or even explicitly applied to 2D materials through pre-patterned surfaces. The effects of these non-uniform strain fields on multilayers are even less studied due to the sheer size of these systems. In the present investigation, we study wrinkles that form in homo- and heterobilayers of 2D transition metal dichalcogenides using density functional theory. We show that the non-uniform strain leads to the formation of interlayer excitons in homobilayers of $ \mathrm WSe_2 $ and to exciton localization in heterobilayers of $ \mathrm WSe_2$-$ \mathrm MoSe_2$. Our results also reveal that the spin angular momentum is changed due to the mixing of in- and out-of-plane states which can explain the brightening of the formerly dark excitonic states under strain. Our results will pave the way towards a full understanding of the strain-control of excitons in 2D materials.