Prenucleation Clusters Assisting Formation of CdS Quantum Dots in Aqueous Environments

The nucleation mechanism of colloidal semiconductor quantum dots (QDs) in the aqueous phase remains unknown. Using CdS as a model system, we show that prenucleation clusters (PNCs) play a key role in the formation of QDs, which supports the multistep nonclassical nucleation model. For the reaction of CdCl₂, thioacetamide, and 3-mercaptopropionic (MPA), a prenucleation cluster forms at room temperature via the chemical self-assembly of the precursors. PNCs transform to magic-size clusters (MSCs) in the dispersion with amine and thus are also defined as precursor compounds (PCs) of MSCs. PNCs fragment to monomers (Mos) at elevated temperature or at room temperature after dilution. The supersaturation of Mos leads to the nucleation and growth of QDs, which is influenced by the pH value. Our findings provide a new synthesis route to QDs in aqueous phase and brings an in-depth understanding of the nucleation mechanism.