Development of CdSe magic-size clusters displaying optical absorption singlets from one prenucleation-stage sample in dispersion under mild conditions

Cadmium selenide (CdSe) is a model system that has been widely used to study the synthesis of semiconductor magic-size clusters (MSCs) and quantum dots (QDs), together with their formation pathways, optical properties, and quantum confinement effects. Here, we show the occurrence of MSC-330, MSC-360, MSC-390, and MSC-415 from a prenucleation-stage sample of CdSe in dispersion at temperatures much lower than the sample preparation temperature. The number represents the lowest energy peak in nanometers of optical absorption. We develop a model based on prenucleation clusters (PNCs) to explain and rationalize our experimental data, and propose that the MSCs are a group of isomers emerging from their corresponding precursor compounds (PCs). The PNC−MSC-415 isomerization is driven thermodynamically, with the loss in enthalpy ( $\mathrm{\Delta }H$ < 0) and entropy ( $\mathrm{\Delta }S$ < 0). With that, we introduce a PNC-based approach to control the MSC development in dispersion, and provide compelling evidence for the existence of the PNC and of two categories of MSCs displaying either absorption singlets or doublets.