Competitive Effects of Oxidation and Quantum Confinement on Modulation of the Photophysical Properties of Metallic-Phase Tungsten Dichalcogenide Quantum Dots.

Metallic-phase transition metal dichalcogenide quantum dots (TMDs-QDs) have been reported in recent years. However, a dominant mechanism for modulating their intrinsic exciton behaviors has not been determined yet as their size is close to the Bohr radius. Herein, we demonstrate that the oxidation effect prevails over quantum confinement on metallic-phase tungsten dichalcogenide QDs (WX-QDs; X = S, Se) when the QD size becomes larger than the exciton Bohr radius. WX-QDs with a diameter of ~12 nm show an obvious change in their photophysical properties when the pH of the solution changes from 2 to 11 compared to changing the size from ~3 nm. Meanwhile, we found that quantum confinement is the dominant function for the optical spectroscopic results in the WX-QDs with a size of ~3 nm. This is because the oxidation of the larger WX-QDs induces sub-energy states, thus enabling excitons to migrate into the lower defect energy states, whereas in WX-QDs with a size comparable to the exciton Bohr radius, protonation enhances the quantum confinement.