Stokes spectroscopy applied to monolayer MoS₂ photoluminescence at room temperature; different helicities observed for A-exciton and A-trion

Strong electron-electron and/or electron-hole Coulomb interactions play important roles in optical transitions in monolayer semiconducting transition metal dichalcogenides (TMDCs). These optical transitions reflect the behaviors of excitons and/or trions. Here, we present our investigation of room temperature Stokes spectroscopy applied to the photoluminescence emitted by a large-area MoS₂ monolayer grown by CVD technique standing on a SiO₂ substrate. Stokes spectroscopy allows for simultaneous measurement of any helicity, which is advantageous compared to the traditional helicity-resolved photoluminescence measurement. Our results show that the PL emission presents significant amount of circularly polarized light when non-resonantly excited with linearly polarized light at 532 nm. Analyzing the Stokes spectra leads to a deconvolution in which the A-exciton and A-trion peaks have opposite helicities. While the exact valley polarization dynamics and selection rule mechanisms that leads to the optical properties still need to be clarified, this study opens the door for a better understanding of the room temperature exciton and trion physics on monolayer MoS₂, an ideal candidate for industrial atomically thin opto-electronic devices such as solar cells and photodetectors.