Pressure-Dependent Strong Photoluminescence of Excitons Bound to Defects in WS2 Quantum Dots

There are various efforts to tailor the excitonic properties in monolayer transition metal dichalcogenides (TMDs) for exploring their potential applications in optoelectronic devices. However, the low quantum yields (QYs), despite their direct bandgap nature, have limited the application in much fields. Encouragingly, excitons combined with defects endow WS2 quantum dots (QDs) with certain desirable properties through strain engineering. A strong exciton photoluminescence (PL) of WS2 QDs even up to approximate to 20 GPa by PL measurements is reported. Their PL reveals that a distinct defect-induced peak D is located below the neutral exciton peak A. This peak D originates from defect-bound excitons and intensifies with increasing pressure as more electrons transfer from WS2 QDs to O-2. In addition, a transition from direct to indirect bandgap above 4.5 GPa is revealed by both experimental measurements and theoretical calculations. The evolution of electronic structure is related to lattice structural distortion. The results provide a new direction for modulating the optical properties of TMDs QDs through utilizing defects-excitons interactions. The pressure-tuned emission of excitons combined with strong PL from defects sites of WS2 QDs may have promising applications in optoelectronic devices.