Valley Polarization in Superacid-Treated Monolayer MoS₂

The interplay between excitons and point defects in semiconductors plays a critical role in optical and optoelectronic properties, especially within a reduced dimensional system. Recently, it has been shown that the superacid treatment on a monolayer sulfur-based transition metal dichalcogenide (TMDC) provided efficient defect repairing/passivation, leading to an enhanced photoluminescence (PL) quantum yield and exciton radiative lifetime by more than two orders of magnitude. Meanwhile, the TMDC monolayer possesses valley-contrast properties and consequently valleytronics applications in which one key parameter, valley polarization, is in principle inversely proportional to the exciton lifetime, suggesting the degraded valley polarization after the superacid treatment. The behavior of the valley properties in the superacid-treated sulfur-based TMDC monolayer remains largely unexplored. Here, we perform detailed steady-state circularly polarized PL characterizations on superacid-treated MoS2 monolayers. We show the ultrahigh stability of the acid molecule’s decoration in the high vacuum and low temperature environments. While one counterintuitive finding is the presence of defect-bound emissions at low temperature after the treatment, the temperature-dependent behavior of the valley polarization does not show any significant variation before and after the treatment. An exciton-decaying model involved with deep trapped states is utilized, accompanied by the theoretically re-built valley polarization expression, to comprehend experimental findings. Our results suggest that the superacid-treated sulfur-based TMDCs with superior PL performance and unaffected valley properties are an excellent candidate for applications of highly efficient valleytronic devices.