General Evolution of CdSe Magic-Size Clusters First with Optical Absorption Singlet Peaks and Later with a Doublet Peak.

Cadmium selenide (CdSe) has been a model system in the study of colloidal semiconductor magic-size clusters (MSCs), which can appear in reactions to photoluminescent quantum dots (QDs). A sample of CdSe MSCs can display two peaks of optical absorption around 325-340 nm and 348-361 nm, while their assignment to two singlet peaks or a doublet peak remains under debate. Here, we show for the first time that at an early stage of a reaction, the two peaks are two singlet peaks corresponding to MSC-330 and MSC-360, respectively. At a later stage, the peaks at 335 and 350 nm are a doublet peak corresponding to a different type of MSC, dMSC-335/350. The evolution of the various CdSe MSCs in reactions was directly monitored by optical absorption spectroscopy at room temperature. Our findings narrow the knowledge gap of the general formation of MSCs in reactions, first with optical absorption singlet peaks and later with a doublet peak.