Dual-ligand capping modulates defect-related emission in AgInS₂-based ternary quantum dots

This study systematically investigates the effect of single- and dual-ligand surface capping on the spectroscopic properties and defect-related emission of two classes of quantum dots (QDs). ZnS, AgInS₂, and AgInS₂/ZnS QDs were synthesised and stabilised using two surface-capping strategies: first by 3-mercaptopropionic acid (MPA), then by a dual-ligand system combining MPA and N-acetyl-L-cysteine (NAC). UV-Vis absorption and fluorescence spectroscopy showed that the optical response of MPA-capped ZnS QDs was principally governed by quantum confinement. In contrast, the emission of AgInS₂ and AgInS₂/ZnS ternary quantum dots (TQDs) was dominated by defect-related electronic states. Comparative analysis of the capping strategies demonstrated that dual-ligand capping reduced the optical bandgap estimated from Tauc plots and notably enhanced fluorescence emission, with the most pronounced effect observed for AgInS₂-based TQDs. For AgInS₂ TQDs, the absolute photoluminescence quantum yield (PLQY) increased from 0.32% with MPA capping to 1.22% with MPA:NAC capping, providing quantitative evidence of enhanced photoluminescence efficiency. The results obtained here highlight that functionalising AgInS₂-based TQDs with an MPA:NAC dual-ligand system modulates defect-related emission, enhancing the fluorescence response and hence expanding their potential for fluorescence-based applications.