Little is known about the synthesis of colloidal ternary semiconductor magic-size clusters (MSCs) and quantum dots (QDs) in an aqueous environment. We report here the first synthesis of aqueous-phase CdSeS MSC-380 (displaying sharp optical absorption peaking at ~ 380 nm) at room temperature and QDs at elevated temperatures. The reaction contains CdCl₂·2.5H₂O, 3-mercaptopropionic acid (MPA, HS−(CH₂)₂−COOH), selenourea (SeU, NH₂−C(Se)−NH₂), and thioacetamide (TAA, CH₃−C(S)−NH₂). Prior to the nucleation and growth (N/G) of QDs, there are clusters formed at 25 °C. The prenucleation-stage clusters are the precursor compound of CdSeS MSC-380 (PC-380). The PC is relatively transparent in optical absorption; in the presence of a primary amine butylamine (BTA, CH₃−(CH₂)₃−NH₂), the PC transforms to absorbing CdSeS MSC-380. At 80 °C, the PC decreases and the N/G of CdSeS QDs appears. The present study paves the way to the aqueous-phase synthesis of ternary CdSeS MSCs and QDs, providing an in-depth understanding of the cluster formation in the prenucleation stage of CdSeS QDs.

We report, for the first time, the synthesis of CdS magic-size clusters (MSCs) which exhibit a single sharp absorption peaking at ~ 361 nm, along with sharp band edge photoemission at ~ 377 nm and broad trap emission peaking at ~ 490 nm. These MSCs are produced in a single-ensemble form without the contamination of conventional quantum dots (QDs) and/or other-bandgap clusters. They are denoted as MSC-361. We present the details of several controlled syntheses done in oleylamine (OLA), using Cd(NO₃)₂ or Cd(OAc)₂ as a Cd source and thioacetamide (TAA) or elementary sulfur (S) as a S source. A high synthetic reproducibility of the reaction of Cd(NO₃)₂ and TAA to single-ensemble MSC-361 is achieved, the product of which is not contaminated by other bandgap clusters and/or QDs. In some cases, the reaction product exhibits an additional absorption peak at ~ 322 nm. We demonstrate that the two peaks, at 361 and 322 nm, do not evolve synchronously. Therefore, the 322 nm peak is not a higher order electronic transition of MSC-361, but due to the presence of another ensemble, namely MSC-322. The present study suggests that there is an outstanding need for the development of a physical model to narrow the knowledge gap regarding the electronic structure in these colloidal semiconductor CdS MSCs.