Exciton and trion photoluminescence evolution in 2D MoS₂ treated with controlled thermal annealing

MoS₂, isolated into the monolayer phase, is the subject of intense research works due to its rich physics and unique excitonic properties. In this study, we investigate the response of A-type excitons (one neutral exciton and one trion) of mechanically exfoliated MoS₂ monolayers, via post exfoliation thermal annealing, in the 200–300 °C annealing temperature range, by means of room temperature photoluminescence (PL). Thermal annealing, in this temperature range, introduces point defects (sulfur vacancies). This induces a variation of doping, via oxygen adsorption on vacancies once exposed to air, and allows tuning the intensity of the PL yield. The PL yield increase is quantified for each post-annealing temperature, reaching a gain of 4 times to 300 °C annealed sample. The PL signal of the 200 °C annealed sample is dominated by the trion, while the PL signal of the 300 °C annealed one is dominated by the neutral exciton, and the PL spectral weight of excitons is perfectly tunable in this temperature range. We also demonstrate that the thermal annealing, in the 200–300 °C annealing temperature range, causes a slight variation of doping with values comparable to those of the as-exfoliated MoS₂.