Emergence of Nonuniform Strain-Induced Exciton Species in Bilayer Transition Metal Dichalcogenides

Full control over excitons in 2D materials is an important step toward their exploitation for applications. Strain modulation is one method that can be used to effectively control the movement of the excitons. Unfortunately, the effects of nonuniform strain in 2D materials are not yet well understood theoretically. However, these strain fields can be present in experiments in the form of wrinkles, bubbles, and folds, or even explicitly applied to 2D materials through prepatterned surfaces. The effects of these nonuniform strain fields on multilayers are even less studied because of 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 nonuniform strain could lead to the formation of spatially localized, momentum-direct, bright interlayer excitons IX^KK in homobilayers of transition-metal dichalcogenides such as WSe₂ and WS₂ and to exciton localization in transition-metal dichalcogenide heterobilayers. 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 provide insights into a better understanding of the strain control of excitons in 2D materials.