Ultrasmall and Monolayered Tungsten Dichalcogenide Quantum Dots with Giant Spin–Valley Coupling and Purple Luminescence

Monolayered tungsten dichalcogenide quantum dots (WS 2 QDs) have various potential applications due to their large spin-valley coupling and excellent photoluminescence (PL) properties. What is expected is that with the decrease in lateral size of QDs, the stronger quantum confinement effect will dramatically strengthen the spin-valley coupling and widen the band gap. However, ultrasmall monolayered WS 2 QDs prepared by ion intercalation unavoidably undergo the problem of structural defects, which will create defect levels and significantly change their properties. In this study, we report that by annealing defective monolayered WS 2 QDs in sulfur vapor, pristine monolayered WS 2 QDs with an ultrasmall lateral size of ca. 1.8-3.8 nm can be obtained. The results show that the ultrasmall monolayered WS 2 QDs exhibit a giant spin-valley coupling of ca. 821 meV. Moreover, the pristine ultrasmall monolayered WS 2 QDs show purple PL centered at 416 nm, and the defect PL peaks in defective WS 2 QDs can be effectively removed by annealing. All of these results afford the ultrasmall monolayered QDs various applications such as in optoelectronics, spintronics, valleytronics, and so on.