Impeding Exciton-Exciton Annihilation in Monolayer WS₂ by Laser Irradiation

Monolayer (1L) transition metal dichalcogenides (TMDs) are two-dimensional direct-bandgap semiconductors with promising applications of quantum light emitters. Recent studies have shown that intrinsically low quantum yields (QYs) of 1L-TMDs can be greatly improved by chemical treatments. However, nonradiative exciton-exciton annihilation (EEA) appears to significantly limit light emission of 1L-TMDs at a nominal density of photoexcited excitons due to strong Coulomb interaction. Here we show that the EEA rate constant (γ) can be reduced by laser irradiation treatment in mechanically exfoliated monolayer tungsten disulfide (1L-WS₂), causing significantly improved light emission at the saturating optical pumping level. Time-resolved photoluminescence (PL) measurements showed that γ reduced from 0.66 ± 0.15 cm²/s to 0.20 ± 0.05 cm²/s simply using our laser irradiation. The laser-irradiated region exhibited lower PL response at low excitation levels, however at the high excitation level displayed 3× higher PL intensity and QY than the region without laser treatment. The shorter PL lifetime and lower PL response at low excitation levels suggested that laser irradiation increased the density of sulfur vacancies of 1L-WS₂, but we attribute these induced defects, adsorbed by oxygen in air, to the origin for reduced EEA by hindering exciton diffusion. Our laser irradiation was likewise effective for reducing EEA and increasing PL of chemically treated 1L-WS₂ with a high QY, exhibiting the general applicability of our method. Our results suggest that exciton-exciton interaction in 1L-TMDs may be conveniently controlled by the laser treatment, which may lead to unsaturated exciton emission at high excitation levels.