monolayers*

Transition metal dichalcogenide (TMD) monolayers (1L) in the 2H-phase are 2D semiconductors with two valleys in their band structure that can be selectively populated using circularly polarized light. The choice of the substrate is an essential factor for the optoelectronic properties and for achieving a high degree of valley polarization at room temperature (RT). In this work, we investigate the RT valley polarization of 1L-WS₂ on different substrates. A polarization degree of 27% is measured from neutral excitons in 1L-WS₂/graphite at RT, under resonant excitation. Using photochlorination doping, we modulate the polarization of the neutral exciton emission from 27% to 38% for this system. We show that the valley polarization strongly depends on the interplay between doping and the choice of the supporting layer of TMDs. Time-resolved photoluminescence measurements, corroborated by a rate equation model accounting for the bright exciton population in the presence of a dark exciton reservoir, support our findings. These results suggest a pathway towards engineering valley polarization and exciton lifetimes in TMDs, by controlling the carrier density and/or the dielectric environment at ambient conditions.