Valley Polarization in Superacid-Treated Monolayer MoS₂

Point defects play a critical role in numerous phenomena such as transport, optical, and optoelectrical properties. With a lower dimensionality, due to reduced dielectric screening the interplay between charge carriers, excitons, and point defects becomes stronger in the two-dimensional materials, such as monolayer transition metal dichalcogenides (TMDCs).
TMDCs have shown the fascinating spin-valley-coupled physics, which is strongly related to the lifetime of photoexcited exciton and the polarization decay time. Recently the superacid treatments have been demonstrated to increase the life time of excitons in monolayer MoS₂. The valley properties of treated MoS₂ monolayers are largely unexplored. With the variable temperature steady state PL and polarization-resolved PL spectroscopy, we show the robust absorption of superacid molecules and unexpectedly enhanced defect-bound emission under high vacuum under low temperature. The unaffected valley polarization of monolayer MoS₂ emphasizes the exciton trapping effect by the shallow defect levels. Our finding paves the way for the understanding of point defeat engineering for monolayer TMDCs.