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.

The formation pathway of colloidal semiconductor ZnSe magic-size clusters (MSCs) in a reaction that display an optical absorption doublet remains poorly understood. The reaction of Zn(OAc)₂/OLA (made from zinc acetate and oleylamine) and tri-n-octylphosphine selenide (SeTOP) in OLA in the presence of diphenylphosphine (HPPh₂) is studied, in which dMSC-345 displays a doublet peaking at 328/345 nm. We suggest that the development is from the clusters that form in the initial prenucleation stage of the reaction. The clusters are the precursor compound (PC-299) of MSC-299 (displaying an absorption singlet peaking at 299 nm). PC-299 transforms to PC-345 at a later stage. The presence of alcohol (such as methanol or ethylene glycol) promotes another pathway, which is the PC-299 to PC-320 transformation. PC-320 transforms to dMSC-320 (with a doublet at 305/320 nm), followed by dMSC-345 via PC-345. The present study provides additional evidence that clusters (PC-299) form and transform (such as to dMSC-345 via PC-345) in the prenucleation stage of ZnSe quantum dots (QDs).

Little is known about how to precisely promote the selective production of either colloidal semiconductor metal chalcogenide (ME), magic-size clusters (MSCs), or quantum dots (QDs). Recently, a two-pathway model has been proposed to comprehend their evolution; here, we reveal for the first time that the size of precursors plays a decisive role in the selected evolution pathway of MSCs and QDs. With the reaction of cadmium myristate (Cd(MA)₂) and tri-n-octylphosphine selenide (SeTOP) in 1-octadecene (ODE) as a model system, the size of Cd precursors was manipulated by the steric hindrance of carboxylic acid (RCOOH) additive. Without RCOOH, the reaction produced both CdSe MSCs and QDs (from 100 to 240 °C). With RCOOH, the reaction produced MSCs or QDs when R was small (such as CH₃−) or large (such as C₆H₅−), respectively. According to the two-pathway model, the selective evolution is attributed to the promotion and suppression of the self-assembly of Cd and Se precursors, respectively. We propose that the addition of carboxylic acid may occur ligand exchange with Cd(MA)₂, causing the different sizes of Cd precursor. The results suggest that the size of Cd precursors regulates the self-assemble behavior of the precursors, which dictates the directed evolution of either MSCs or QDs. The present findings bring insights into the two-pathway model, as the size of M and E precursors determine the evolution pathways of MSCs or QDs, the understanding of which is of great fundamental significance toward mechanism-enabled design and predictive synthesis of functional nanomaterials.

The formation pathway of aqueous-phase colloidal semiconductor magic-size clusters (MSCs) remains unrevealed. In the present work, we demonstrate, for the first time, a precursor compound (PC)-enabled formation pathway of aqueous-phase CdSe MSCs exhibiting a sharp absorption peaking at about 420 nm (MSC-420). The CdSe MSC-420 is synthesized with CdCl₂ and selenourea as the respective Cd and Se sources, and with 3-mercaptopropionic acid or L-cysteine as a ligand. Absorption featureless CdSe PCs form first in the aqueous reaction batches, which transform to MSC-420 in the presence of primary amines. The coordination between primary amine and Cd²⁺ on PCs may be responsible to the PC-to-MSC transformation. Upon increasing the reactant concentrations or decreasing the CdCl₂-ligand feed molar ratios, the Cd precursor self-assembles into large aggregates, which may encapsulate the resulting CdSe PCs and inhibit their transformation to MSC-420. The present study sheds essential light on the syntheses and formation mechanisms of nanocrystals.

Precursor compounds (PCs) link quantum dots (QDs) and magic-sized clusters (MSCs), which is pivotal in the conversion between QDs and MSCs. Here, for the first time, we report the transformation, synthesis, and composition of a type of ZnSe PCs. ZnSe PCs can be directly transformed to two different MSCs with the assistance of octylamine and acetic acid at room temperature. The two types of MSCs exhibit sharp absorption peaks at 299 and 328 nm which are denoted as MSC-299 and MSC-328. In the preparation of ZnSe PCs, diphenylphosphine (DPP) as an additive plays a key role which not only inhibits the thermal decomposition of Zn precursor, but also acts as a reducing agent to reduce the by-products produced in the reaction. The composition was explored by X-ray photoelectron spectroscopy, energy dispersive spectrometer, matrix-assisted laser desorption/ionization time-of-flight mass spectra with ZnSe PC powder appeared as white powder after purifying by toluene (Tol) and methanol (MeOH). The results indicate that the molar ratio of Zn/Se is 2:1 with a molecular of ∼ 3, 350 Da. Therefore, we propose that the molecular formula of ZnSe PCs is Zn₃₂Se₁₆. In addition, at the molecular level, the covalent bond of Zn–Se is formed in ZnSe PCs. This study offers a deeper understanding of the transformation from PCs to MSCs and for the first time proposes the composition of PCs. Meanwhile, this research provides us with a new understanding of the role of DPP in the synthesis of colloidal semiconductor nanoparticles.

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.