Evolution of Two Types of ZnTe Magic-Size Clusters Displaying Sharp Doublets in Optical Absorption

Alloyed Zinc Chalcogenide Magic-Sized Nanoclusters and Their Transformation to Alloyed Quantum Dots

Alloying provides the opportunity to widen the physical and chemical properties of quantum dots (QDs); however, the precise controlled composition of alloyed QDs is still a challenge. In this work, a few quaternary alloyed zinc chalcogenide magic-sized nanoclusters (MSCs) were synthesized using the active chalcogen precursors of tri(dimethylamine)phosphine chalcogen, such as Zn21S4Se3Te4 (MSCs-348), Zn14S4Se4Te7 (MSCs-350), Zn15S1Se4Te6 (MSCs-349), and Zn17S2Se2Te7 (MSCs-355) MSCs. The composition of alloyed zinc chalcogenide MSCs was tuned with the different amounts of added chalcogen precursors. Finally, the produced alloyed zinc chalcogenide MSCs can be used as precursors to synthesize alloyed zinc chalcogenide QDs, and the composition of zinc chalcogenide QDs can be adjusted with different alloyed MSCs. This work provides methods to alloy MSCs with controlled composition, providing efficient precursors for alloyed QDs.

Lower-Temperature Nucleation and Growth of Colloidal CdTe Quantum Dots Enabled by Prenucleation Clusters with Cd-Te Bond Conservation

The reason why heating is required remains elusive for the traditional synthesis of colloidal semiconductor quantum dots (QDs) of II-VI metal chalcogenide (ME). Using CdTe as a model system, we show that the formation of Cd-Te covalent bonds with individual Cd- and Te-containing compounds can be decoupled from the nucleation and growth of CdTe QDs. Prepared at an elevated temperature, a prenucleation-stage sample contains clusters that are the precursor compound (PC) of magic-size clusters (MSCs); the Cd-Te bond formation occurs at temperatures higher than 120 °C in the reaction. Afterward, the PC-to-QD transformation appears via monomers at lower temperatures in dispersion. Our findings suggest that the number of Cd-Te bonds broken in the PC reactant is similar to that of Cd-Te bonds formed in the QD product. For the traditional synthesis of ME QDs, heating is responsible for the M-E bond formation rather than for nucleation.

CdTe magic-size cluster synthesis via a cation exchange method and conversion mechanism

The quasi-metallic nature of Te is not conducive to telluride formation and crystallization, which makes the synthesis of CdTe magic-size clusters (MSCs) in a single-ensemble form still challenging. CdTe MSCs are usually synthesized by direct synthesis, a method that must avoid the formation of quantum dots by selecting suitable active precursors and precisely controlling the reaction temperature. In addition, the organic Cd compounds and superhydrogenated precursors used are air-sensitive. Herein, CdTe MSC-448 in a single-ensemble form was synthesized for the first time via a cation exchange method using ZnTe MSC-389 as a template and Cd2+ as an exchange ion. In situ absorption spectroscopy characterization combined with the two-pathway model proposed by Yu's group reveals that the conversion of ZnTe MSC-389 into CdTe MSC-448 is assisted by their corresponding precursor compounds (PCs). After the addition of Cd precursors to ZnTe MSC-389 solution, ZnTe MSC-389 is transformed into ZnTe PC-389, which then undergoes a rapid cation exchange reaction with Cd2+ to yield CdTe PC-448, and CdTe PC-448 is finally converted into CdTe MSC-448. CdTe MSCs in single-ensemble form were obtained by cation exchange in air at room temperature, avoiding the formation of quantum dots (QDs) at high temperatures in the direct synthesis method conducted without the use of toxic and expensive active precursors, which provides a new route to the synthesis of CdTe MSCs.

Room-temperature formation of alloy ZnxCd13-xSe13 magic-size clusters via cation exchange in diamine solution

Magic-size clusters (MSCs) are molecular materials with unique properties at the border between molecules and solids, providing important insights into the nanocrystal formation process. However, the synthesis of multicomponent alloy MSCs in a single-ensemble form remains challenging due to their tiny size and difficult doping control. Herein, for the first time, we successfully synthesized alloy Zn_xCd_13-xSe₁₃ MSCs (x = 1-12) with a unique sharp absorption peak at 352 nm by cation exchange between Cd²⁺ ions and pre-synthesized (ZnSe)₁₃ MSCs in a diamine solution at room temperature. The experimental results show that the use of diamines is crucial to the formation of stable Zn_xCd_13-xSe₁₃ MSCs, which may be attributed to two amine groups that can coordinate to the surface of MSCs simultaneously. Limited by the robust interaction between diamine ligands and MSCs, the partial cation exchange results in the formation of alloy Zn_xCd_13-xSe₁₃ MSCs. In contrast, complete cation exchange occurs in a monoamine solution, giving (CdSe)₃₄ MSCs. Besides, a lower reaction temperature and a higher concentration of diamine favor the formation of Zn_xCd_13-xSe₁₃ MSCs. Our study provides an important basis for further understanding of the transformation of MSCs and a new approach to the controllable synthesis of alloyed MSCs.

A Two-Pathway Model for the Evolution of Colloidal Compound Semiconductor Quantum Dots and Magic-Size Clusters

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.

Size matters: Steric hindrance of precursor molecules controlling the evolution of CdSe magic-size clusters and quantum dots

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.

Electronic Supplementary Material

Supplementary material (additional optical absorption spectra, TEM, NMR, FT-IR, and XRD) is available in the online version of this article at 10.1007/s12274-022-4421-4.