Controlling exciton-exciton annihilation in WSe2 bilayers via interlayer twist

The twist angle between two van der Waals coupled monolayers has emerged as a new and powerful degree of freedom for engineering physical properties of semiconductor homo- and hetero-bilayers. While the interlayer twist has shown prominent effect on electronic and optical properties of transition metal dichalcogenide (TMD) bilayers, it remains unclear how it could be used to manipulate the exciton dynamics, especially exciton-exciton annihilation (EEA) process which is the dominant energy loss channel in TMDs under moderate to high exciton density due to strong Coulomb interaction. Herein, we show that the twist angle in TMD bilayers can act as an effective knob to control the EEA process. Specifically, EEA rate constant increases from 1° twisted WSe2 bilayers (0.026 cm2/s) by more than twice to 32° twisted bilayers (0.053 cm2/s) and then drops again in 60° twisted bilayers (0.019 cm2/s). This twist-angle dependence can be attributed to the energy difference between indirect and direct excitons arising from the interlayer interaction. Our work opens up the possibility of artificially managing the exciton dynamics in TMD materials for optoelectronic applications via interlayer twist angle.