From Dark to Bright: First-Principles Prediction of Ground State Excitons and Trions in Janus Transition Metal Dichalcogenide Bilayers

We investigate bilayer Janus transition metal dichalcogenides using ab initio many-body screened configuration interaction calculations and find that MoSSe–WSSe and WSSe–WSSe, with Se–S interfaces, exhibit spin-allowed interlayer exciton and trion ground states, in contrast to the spin-forbidden ground states of conventional transition metal dichalcogenides bilayers. This behavior arises from intrinsic structural asymmetry and interface-induced polarization, which rearrange the band structure. We further show that besides the intrinsic asymmetry also the applied external strain enables tuning of the ground state exciton brightness, allowing for detailed control of their optical properties.