Tuning the valley polarization in WS₂ monolayers via control of active defect sites induced by photochlorination

Transition metal dichalcogenides (TMDs) arise as an exciting class of atomically thin, two-dimensional (2D) materials for electronics, optics, and optoelectronics. Apart from light-based applications, these materials are good candidates for quantum information processing based on valley degrees of freedom i.e., valleytronics. In this work [1], we demonstrate spin-valley polarization tunability by more than 40% in 1L-WS₂ on hBN via photochlorination. The PL intensity of the neutral exciton is significantly enhanced after several laser pulses in a chlorine environment and the circular polarization (P_circ) decreased systematically. Polarization PL spectroscopy was performed in the temperature range from 4K to 300K. The decrease of P_circ after the photochlorination treatment is attributed to the significant reduction of the active defect sites in the 1L-WS₂ and consequently to the increase of the non-radiative exciton lifetime. Ultrafast time-resolved transient absorption spectroscopy measurements support our findings. These results indicate a useful approach to controll the density of the active defect sites as well as the degree of the emitted circular polarization in all 2D-TMDs. [1] I. Demeridou et al, Applied Physics Letters 118, 123103 (2021).