Investigation of carrier migration from WS2 monolayer to substrate by photoluminescence

Carriers in atomically thin two-dimensional (2D) semiconductors like WS2 monolayers are prone to interact with their surroundings, which may have a significant impact on both the electric and optical properties of 2D semiconductors. In this work, abnormal dependence of photoluminescence (PL) intensity on excitation power densities is observed in mechanically exfoliated WS2 monolayers on SiO2/Si substrates, which is manifested as a sudden intensity drop of exciton and trion emissions at certain power density followed by much slowed intensity increase. As-exfoliated and wetted WS2 monolayers exhibit similar abnormal variation of PL intensity but different power densities that trigger the intensity drop. The sudden drop of PL intensity is ascribed to the modification to the contact status of the monolayer with the substrate, caused by the interplay of the trapped air or water with the monolayer and the substrate upon laser irradiation, which therefore alters the carries transport rate from the monolayer to the substrate. A rate equation model has been developed to delicately deal with the carrier recombination and transport processes in a steady state, and the variation dependence of exciton, trion and free charge populations on the cross-plane transport rate is demonstrated, which consistently explains the abnormal PL intensity dependence on the excitation power density. The important role of carrier transport in the PL performance has been verified in this work, and further understanding of the carrier interactions in atomical thin 2D semiconductors has been achieved.