Inhibiting the Surface Oxidation of Low-Cadmium-Content ZnS:(Cd,Se) Quantum Dots for Enhancing Application Reliability

Quantum dots (QDs) are advancing display and innovating lighting but still encountering unsolved stability problems mainly caused by material oxidation. In this work, the effect of Se composition (0.8 at. % vs 4.3 at. %) and thick ZnS shell (shell thickness ∼ 5 nm) on the oxidation of low-Cd content ZnS:(Cd,Se) QDs (Cd, <20 at. %) were investigated. Surface analysis and thermodynamic calculation were employed to study the QD oxidation. The results show that the oxidation products were correlated to (Zn,Cd)SO4, (Zn,Cd)(OH)2, and SeO2 phases, which lead to QD surface defects/amorphization inducing ligand detachment and particle agglomeration, indicating that the shell should be a much more dominant role in the QD reliability. It was also found that a heavier Se doping and thicker ZnS overcoating indeed prevented the oxidation of Cd, Zn, and S so the low-Cd QD stability could be improved. By considering the application criteria, optimized green QDs with diameter of ∼12 nm and chemical composition of Zn0.443S0.463:Cd0.057Se0.037 are suggested for QD-based lighting and displays. A QD color enhancement film based on low-Cd QDs exhibited superior stability compared with a commercial product, demonstrating their potential in practical applications.