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

The sufficient control of the carrier density of a single layer WS2 (1L-WS2) has been realized by the pulsed laser irradiation doping technique. Chlorine atoms are incorporated on the surface of the atomically thin lattice in a precursor gas atmosphere. In this work, we demonstrate spin-valley polarization tunability by more than 40% in 1L-WS2 on hBN via photochlorination. Polarization photoluminescence spectroscopy was performed in the temperature range from 4 K to 300 K. The decrease in circular polarization after the photochlorination treatment is attributed to the significant reduction of the active defect sites in 1L-WS2 and, consequently, to the increase in the non-radiative exciton lifetime. Ultrafast time-resolved transient absorption spectroscopy measurements support our findings. The above results indicate a useful approach of controlling the density of the active defect sites and the valley polarized light emission in doped monolayer crystal lattices.