Four types of colloidal semiconductor CdTe magic-size clusters (MSCs), each of which is in a single-ensemble form, have been obtained at room temperature from a single induction period (IP) sample in dispersion. The induction period is the prenucleation stage that occurs prior to nucleation and growth of colloidal quantum dots (QDs). Three types display sharp optical absorption peaking at either 371, 417, or 448 nm, and the fourth type exhibits a sharp absorption doublet with peaks at 350 and 371 nm. These MSCs are respectively denoted as sMSC-371, sMSC-417, sMSC-448, and dMSC-371. We show that the evolution of the various MSCs is affected by the nature of their dispersions. We hypothesize that the evolution of MSCs involves their precursor compounds (PCs), which are transparent in optical absorption. The present study explores new avenues for the exclusive synthesis of four types of CdTe MSCs (with each in a single-ensemble form) and provides an improved understanding for their formation.

There are two types of colloidal semiconductor nanocrystals (NCs) that exhibit band gap absorption that is relatively sharp compared to conventional quantum dots (QDs). One type displays an absorption doublet, while the other displays an absorption singlet. Here, we report the evolution of the two types of NCs at room temperature from a single CdTe sample extracted during the induction period (IP) prior to nucleation and growth of conventional QDs. The resulting NCs exhibit band gap absorption peaking at ∼371 nm and are magic-size clusters (MSCs), labeled as dMSC-371 and sMSC-371 for the doublet and singlet cases, respectively. We demonstrate that dMSC-371 (with another peak at ∼350 nm) evolves when the sample is incubated. When the sample is dispersed without incubation into a toluene and octylamine mixture, dMSC-371 or sMSC-371 grows depending on the amine amount. We propose that dMSC-371 and sMSC-371 are a pair of polymorphs (with identical CdTe core compositions). The present study brings insight into the formation relationship between dMSCs and sMSCs.

Conspectus A knowledge of colloidal semiconductor magic-size clusters (MSCs) is essential for understanding how fundamental properties evolve during transformations from individual molecules to semiconductor quantum dots (QDs). Compared to QDs, MSCs display much narrower optical absorption bands; the higher cluster stability gives rise to a narrower size distribution. During the production of binary QDs such as II–VI metal (M) chalcogenide (E) ones, binary ME MSCs observed were interpreted as side products and/or the nuclei of QDs. Prior to the current development of our two-step approach followed by our two-pathway model, it had been extremely challenging to synthesize MSCs as a unique product without the nucleation and growth of QDs. With the two-step approach, we have demonstrated that MSCs can be readily engineered as a sole product at room temperature from a prenucleation stage sample, also called an induction period (IP) sample. It is important that we were able to discover that the evolution of the MSCs follows first-order reaction kinetics behavior. Accordingly, we proposed that a new type of compound, termed as “precursor compounds” (PCs) of MSCs, was produced in an IP sample. Such PCs are optically transparent at the absorption peak positions of their MSC counterparts as well as to longer wavelengths. It is thought that quasi isomerization of a single PC results in the development of one MSC. In this Account, we provide an overview of our latest advances regarding the transformations among binary CdE MSCs as well as from binary CdTe to ternary CdTeSe MSCs. Optical absorption spectroscopy has been employed to study these transformations, all of which display well-defined isosbestic points. We have proposed that these MSC to MSC transformations occur via their corresponding PCs, also called immediate PCs. It is reasonable that the as-synthesized PC (in an IP sample) and the immediate PC (in an incubated and/or diluted sample) probably have different configurations. A transformation between two PCs may involve an intermolecular reaction, with either first-order reaction kinetics or a more complicated time profile. A transformation between one immediate PC and its counterpart MSC may contain an intramolecular reaction. The present Account, which addresses the PC-enabled MSC transformations with isosbestic points probed by optical absorption spectroscopy, calls for more experimental and theoretical attention to understand these magic species and their transformation processes more precisely.

A fundamental understanding of formation pathways is critical to the controlled synthesis of colloidal semiconductor nanocrystals. As ultrasmall‐size quantum dots (QDs) sometimes emerge in reactions along with magic‐size clusters (MSCs), distinguishing their individual pathway of evolution is important, but has proven difficult. To decouple the evolution of QDs and MSCs, an unconventional, selective approach has been developed, along with a two‐pathway model that provides a fundamental understanding of production selectivity. For on‐demand production of either ultrasmall QDs or MSCs, the key enabler is in how to allow a reaction to proceed in the time prior to nucleation and growth of QDs. In this prenucleation stage, an intermediate compound forms, which is the precursor compound (PC) to the MSC. Here, the two‐pathway model and the manipulation of such PCs to synthesize either ultrasmall QDs or binary and ternary MSCs are highlighted. The two‐pathway model will assist the development of nucleation theory as well as provide a basis for a mechanism‐enabled design and predictive synthesis of functional nanomaterials.

Little is known about the pathway of room‐temperature formation of ternary CdTeSe magic‐size clusters (MSCs) obtained by mixing binary CdTe and CdSe induction period samples containing binary precursor compounds (PCs) of MSCs, monomers (Ms), and fragments (Fs). Also, unestablished are dispersion effects that occur when as‐mixed samples (without incubation) are placed in toluene (Tol) and octylamine (OTA) mixtures. The resulting ternary MSCs, exhibiting a sharp optical absorption peak at 399 nm, are labelled CdTeSe MSC‐399, and their PCs are referred to as CdTeSe PC‐399. When the amount of OTA is relatively small, single‐ensemble MSC‐399 evolved without either binary CdTe or CdSe MSCs. When the OTA amount is relatively large, CdTe MSC‐371 appeared initially and then disappeared, while single‐ensemble MSC‐399 developed more deliberately. The larger the OTA amount, the more slowly these changes proceeded. The substitution reaction of CdTe PC + CdSe M/F↔CdTeSe PC‐399 + CdTe M/F is proposed to be rate‐determining for the MSC‐399 formation in a Tol and OTA mixture. This study provides further understanding of the transformation pathway between MSCs.