Evolution of CdTe Magic-Size Clusters with Single Absorption Doublet Assisted by Adding Small Molecules during Prenucleation

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.

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.

Effect of One-Coordinated Atoms on the Electronic and Optical Properties of ZnSe Clusters

To understand the influence of one-coordinated Zn and Se atoms on the structures, electronic, and optical properties of ZnSe clusters, we investigate the Zn₃₇Se₂₀ clusters employing first-principles theoretical calculations. The Zn₃₇Se₂₀ cluster, constructed from the InP nanocrystal structure, possesses a Zn₂₁Se₂₀ core and 16 one-coordinated surface atoms. The effect of one-coordinated atoms is studied by adding or removing one-coordinated atoms of the Zn₃₇Se₂₀ cluster. The calculations show that the modifications of one-coordinated atoms change slightly the coordination states and bond lengths of the atoms on the cluster surface. The clusters with the same core structure and different amounts of one-coordinated atoms have similar optical spectra, suggesting the importance of the cluster core structure in their optical properties.

Transformation Pathway from CdSe Magic-Size Clusters with Absorption Doublets at 373/393 nm to Clusters at 434/460 nm

Divergent interpretations have appeared in the literature regarding the structural nature and evolutionary behavior for photoluminescent CdSe nanospecies with sharp doublets in optical absorption. We report a comprehensive description of the transformation pathway from one CdSe nanospecies displaying an absorption doublet at 373/393 nm to another species with a doublet at 433/460 nm. These two nanospecies are zero-dimensional (0D) magic-size clusters (MSCs) with 3D quantum confinement, and are labeled dMSC-393 and dMSC-460, respectively. Synchrotron-based small-angle X-ray scattering (SAXS) returns a radius of gyration of 0.92 nm for dMSC-393 and 1.14 nm for dMSC-460, and indicates that both types are disc shaped with the exponent of the SAXS form factor equal to 2.1. The MSCs develop from their unique counterpart precursor compounds (PCs), which are labeled PC-393 and PC-460, respectively. For the dMSC-393 to dMSC-460 transformation, the proposed PC-enabled pathway is comprised of three key steps, dMSC-393 to PC-393 (Step 1), PC-393 to PC-460 (Step 2 involving monomer addition), and PC-460 to dMSC-460 (Step 3). The present study provides a framework for understanding the PC-based evolution of MSCs and how PCs enable transformations between MSCs.

Evolution of Photoluminescent CdS Magic-Size Clusters Assisted by Adding Small Molecules with Carboxylic Group

We report our investigation on the formation of photoluminescent CdS magic-size clusters (MSCs), which exhibit absorption peaking at 373 nm, along with sharp band edge emission at ∼385 nm. Denoted as MSC-373, the MSCs were synthesized from the reaction of cadmium oleate (Cd(OA)₂) and S powder in 1-octadecene at room temperature, together with the addition of acetic acid (HOAc) or acetate salts (M(OAc)₂, M = Zn and Mn) during the prenucleation stage (120 °C). The morphology of as-synthesized MSC-373 was dot-like, which could be altered to flake-like morphology after purification. We found the formation of MSC-373 was related to the ligand exchange, resulting from the addition of small molecules with carboxylic group. The addition of HOAc not only promotes the formation of CdS MSC-373 but suppresses the formation of MSC-311 and nucleation and growth of quantum dots (QDs). When the amount of HOAc addition was increased, another photoluminescent CdS MSCs, namely, MSC-406, evolved. This study provides an overall understanding of the CdS MSC-373 and introduces a new approach to synthesize photoluminescent CdS MSCs.

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

The two-pathway model proposed for colloidal semiconductor metal chalcogenide (ME) quantum dots (QDs) and magic-size clusters (MSCs) is demonstrated for ZnTe. Two new types of ZnTe MSCs have been found, which exhibit sharp optical absorption doublets peaking at 356/389 and 389/420 nm. Labeled dMSC-389 and dMSC-420, respectively, they were produced from reaction mixtures in 1-octadecene (ODE) of zinc oleate (Zn(OA)₂), tri-n-octylphosphine telluride (TeTOP), diphenylphosphine (HPPh₂), and acetic acid (HOAc, CH₃COOH). The collective use of HOAc and HPPh₂ enabled the exclusive production of dMSC-389 and dMSC-420 from reaction mixtures that had high Zn-to-Te feed molar ratios and a high Te feed concentration of 60 mmol/kg. The present findings demonstrate the utility of HOAc in synthesizing MSCs displaying a sharp absorption doublet, as well as of a secondary phosphine that decreases the temperature at which the M-E covalent bonds form.

Reversible Transformations at Room Temperature among Three Types of CdTe Magic-Size Clusters

We report the first observation of the reversible transformations that occur among three types of CdTe magic-size clusters (MSCs) in dispersion at room temperature and discuss our understanding of the transformation pathway. The reversible transformations were achieved with CdTe prenucleation stage samples, which were prepared with reactions of cadmium oleate [Cd(OA)₂] and tri-n-octylphosphine telluride in 1-octadecene and were then dispersed in mixtures of toluene and a primary amine at room temperature. Three types of OA-passivated CdTe MSCs evolved, exhibiting sharp optical absorption singlets peaking at 371, 417, and 448 nm. The MSCs and their immediate precursor compounds (PCs; with no sharp optical absorption) are labeled by the MSC absorption peak wavelengths. The transformation between MSC-371 and MSC-417 has a distinct isosbestic point at ∼385 nm and that between MSC-417 and MSC-448 at ∼430 nm. Our findings suggest that these PC-enabled reversible transformations occur through a process of quasi-isomerization, transforming between PCs and their counterpart MSCs, combined with substitution reactions that cause transformation between the two involved PCs.

Transformations Among Colloidal Semiconductor Magic-Size Clusters

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.

Interplay between Perovskite Magic-Sized Clusters and Amino Lead Halide Molecular Clusters

Recent progress has been made on the synthesis and characterization of metal halide perovskite magic-sized clusters (PMSCs) with ABX ₃ composition (A = CH₃NH₃ ⁺ or Cs⁺, B = Pb²⁺, and X = Cl^-, Br^-, or I^-). However, their mechanism of growth and structure is still not well understood. In our effort to understand their structure and growth, we discovered that a new species can be formed without the CH₃NH₃ ⁺ component, which we name as molecular clusters (MCs). Specifically, CH₃NH₃PbBr₃ PMSCs, with a characteristic absorption peak at 424 nm, are synthesized using PbBr₂ and CH₃NH₃Br as precursors and butylamine (BTYA) and valeric acid (VA) as ligands, while MCs, with an absorption peak at 402 nm, are synthesized using solely PbBr₂ and BTYA, without CH₃NH₃Br. Interestingly, PMSCs are converted spontaneously overtime into MCs. An isosbestic point in their electronic absorption spectra indicates a direct interplay between the PMSCs and MCs. Therefore, we suggest that the MCs are precursors to the PMSCs. From spectroscopic and extended X-ray absorption fine structure (EXAFS) results, we propose some tentative structural models for the MCs. The discovery of the MCs is critical to understanding the growth of PMSCs as well as larger perovskite quantum dots (PQDs) or nanocrystals (PNCs).