Ligand-engineered ZnS quantum dots synthesized from substituted thioureas: scalable production, polymer grafting, and emissive film fabrication

Abstract

Zinc sulfide (ZnS) quantum dots (QDs) were synthesized using a series of substituted thioureas as single-source sulfur precursors in a high-temperature 1-octadecene medium. The hot-injection method offered excellent reproducibility and enabled straightforward scale-up to multigram quantities without compromising particle size or optical characteristics. The as-prepared ZnS QDs exhibited a high organic content (∼46 wt.%), originating from in situ-generated surface ligands, which was quantitatively determined through acid digestion. This surface composition provided a versatile platform for subsequent ligand exchange. Functional ligands, including 2-mercaptopropionic acid (2-MPA), bis[2-(methacryloyloxy)ethyl] phosphate (BMEP), and 10-(phosphonooxy)decyl methacrylate (PODM), were successfully introduced, yielding hydrophilic, hydrophobic, and polymer-reactive ZnS QDs. Structural analysis (XRD, STEM, EDS, FTIR, XPS) confirmed the formation of cubic ZnS QDs with uniform particle sizes (6-8 nm) and verified the incorporation of the new ligands without altering the ZnS core. Optical measurements revealed size-dependent absorption and emission properties across the thiourea series, as well as pronounced ligand-dependent modulation of photoluminescence intensity and decay kinetics. Finally, pristine and functionalized QDs were incorporated into PMMA, PVK, PEG, and methacrylate-based copolymers to form uniform emissive thin films, with AFM demonstrating smooth surface morphology for most systems. These results establish substituted thioureas as effective precursors for scalable ZnS QD synthesis and highlight ligand engineering as a powerful tool for tuning surface chemistry and enabling direct polymer integration.