Lead sulfide colloidal quantum dots (PbS CQDs) have become a research focus in infrared optoelectronic materials due to their significant quantum size effect, tunable absorption covering the near-infrared region, ease of synthesis, and good compatibility. The ability to precisely tune the optical absorption of PbS CQDs lies at the heart of high-performance device fabrication. To elucidate the influence of anionic precursor reactivity on the optical absorption of PbS CQDs, the hot-injection synthesis was optimized using lead oxide as lead source and oleic acid as ligand. Three sulfur sources of distinct reactivity, together with different coordinating co-solvents, were employed as sulfur precursors to systematically investigate their effects on the nucleation, growth, crystal structure, and optical absorption properties of PbS CQDs. Three combinations of sulfur precursors adopted in this work are bis(trimethylsilyl) sulfide paired with trioctylphosphine, thioacetamide with oleylamine, and elemental sulfur with oleylamine. For each precursor system, the reaction temperature, growth time, and molar ratios of oleic acid ligands were systematically varied to investigate their influence on the optical absorption of PbS CQDs. Comprehensive characterization confirmed that all PbS CQDs obtained in this work possessed a pure cubic rock salt structure. Notably, the quantum dots synthesized with bis-trimethylsilyl sulfide and thioacetamide as sulfur sources showed a quasi-spherical morphology. The particle sizes of the quantum dots obtained from the bis-trimethylsilyl and thioacetamide systems were comparable, while those synthesized with elemental sulfur displayed a distinct cubic morphology. The quantum dots prepared with elemental sulfur exhibited larger particle size. The PbS CQDs synthesized with thioacetamide as sulfur source exhibited the narrowest particle size distribution (coefficient of variation 8.5%), the highest monodispersity, and the best colloidal and ambient stability. Furthermore, precise tuning of the reaction temperature, molar ratio of oleic acid ligands relative to lead and growth time enabled accurate control over the particle size and optical absorption properties of the resulting quantum dots. The optimized PbS CQDs combined a high crystallinity of 81.74% with excellent optical absorption properties, thus providing a promising material platform for infrared optoelectronic device applications.

Key words: PbS, colloidal quantum dots, hot-injection, optical absorption property, size control