Hydrothermal synthesis of water-soluble CdSe quantum dots with enhanced photoluminescence through precursor optimization and ZnS-related surface modification

Water-soluble CdSe quantum dots (QDs) were synthesized by a hydrothermal method using N-acetyl-L-cysteine (NAC) as a ligand. The effects of precursor pH, [Se]/[Cd], and [NAC]/[Cd] molar ratios on the optical properties of CdSe QDs were examined. Under optimized conditions, CdSe QDs with relatively strong photoluminescence (PL) were obtained. X-ray diffraction and PL measurements suggested the formation of CdS shell-like structures at [Se]/[Cd] ≤ 0.1 or [NAC]/[Cd] ≥ 9, accompanied by an increase in PL quantum yield (PLQY) to 20%. Further improvement was achieved by ZnS-related surface passivation through stepwise precursor addition and microwave-assisted heating. The resulting CdSe/ZnS QDs showed enhanced PL intensity, prolonged PL lifetimes, and a maximum PLQY of 70%, which is among the highest values reported for water-soluble CdSe-based QDs directly synthesized in aqueous solution without post-synthetic ligand exchange. These results show that control of precursor composition and ZnS-related surface passivation is essential for obtaining highly luminescent water-soluble CdSe QDs.