Thermally‐Induced Isomerization of Prenucleation Clusters During the Prenucleation Stage of CdTe Quantum Dots

Abstract

The evolution of prenucleation clusters in the prenucleation stage of colloidal semiconductor quantum dots (QDs) has remained unexplored. With CdTe as a model system, we show that substances form and isomerize prior to the nucleation and growth of QDs. Called precursor compounds (PCs), the prenucleation clusters are relatively optically transparent and can transform to absorbing magic‐size clusters (MSCs). When a prenucleation‐stage sample at 25, 45, or 80 °C is dispersed in a mixture of cyclohexane (CH) and octylamine (OTA) at room temperature, either MSC‐371, MSC‐417, or MSC‐448 evolves with absorption peaking at 371, 417, or 448 nm, respectively. We propose that PC‐371 forms at 25 °C, and isomerizes to PC‐417 at 45 °C and to PC‐448 at 80 °C. The PCs and MSCs are quasi isomers. Relatively large and small amounts of OTA favor PC‐371 and PC‐448 in dispersion, respectively. The present findings suggest the existence of PC‐to‐PC isomerization in the QD prenucleation stage.

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

“…A fundamental understanding of the initial formation of the prenucleation clusters, PC-371, and the transformation to dPC-485 in the reaction can be found elsewhere (Figure S1C). , As shown in Figure S1–4, a 135 °C/60 min sample contained dMSC-485 and QD-580 (a). For the mixture, TEM suggested an average size of ∼3.4 nm (b), X-ray diffraction (XRD) showed a structure that is more zinc blende-like rather than wurtzite-like (c), and energy dispersive X-ray spectroscopy (EDX) and scanning electron microscopy (SEM) showed the 1.1Cd to 1.0Te number ratio (d).…”

“…20−27,30−36,38−41 Meanwhile, the PC-to-PC transformations in reactions have been elaborated elsewhere (Figure S1C). 24,25 The prenucleation stage of CdTe QDs, for example, can be as long as 20 min at a constant temperature of 135 °C for a reaction in oleylamine (OLA, CH 3 (CH 2 ) 7 −CH�CH− (CH 2 ) 8 −NH 2 ) of Cd(OAc) 2 /OLA (made from cadmium acetate and OLA) and TeTOP. 20−23 When a prenucleationstage sample (which was preheated at 135 °C for ∼20 min) was dispersed in a mixture of toluene (Tol) and a primary amine octylamine (CH 3 (CH 2 ) 7 −NH 2 , OTA) at room temperature, CdTe MSC-371 (displaying sharp optical absorption singlet peaking at ∼371 nm) was seen (Part a of Figure S1B).…”

“…Recent advances based on the heating-up, noninjection-based approach have experimentally demonstrated that clusters form in the prenucleation stage of CdTe, − CdSe, − CdS, − ZnTe, , ZnSe, ,− and ZnS QDs at elevated temperatures ( T 1 ). , The heating-up approach can readily stop reactions in the prenucleation stage which exists prior to the N/G of ME QDs, while the injection-based approach cannot. The prenucleation stage is also termed as the induction period (IP) .…”

“…The PC-to-MSC transformation can occur in reactions at elevated temperatures (Figure S1A) ,,,,− ,, or in dispersion at lower temperatures (Figure S1B). − ,− ,− Meanwhile, the PC-to-PC transformations in reactions have been elaborated elsewhere (Figure S1C). , …”

Anion Exchange in Semiconductor Magic-Size Clusters

“…In this work, a complete anion exchange process in CdSe, CdS, and CdTe MSCs was achieved for the first time under mild conditions, and the intrinsic transformation mechanism was systematically investigated. We proposed that covalent inorganic complexes (CICs) played a critical role as intermediates in the intermolecular transition of template-free MSCs (nonstoichiometric composition), which was distinct from the intramolecular anion exchange process found in templated MSCs (stoichiometric composition). , An in situ absorption spectroscopy study showed that the conversion pathway from CdE 1 -MSCs to CdE 2 -MSCs (Scheme ) involved three key steps, namely the disassembly of CdE 1 -MSCs into CdE 1 -CICs (step 1, eq ), the anion exchange reaction that transformed CdE 1 -CICs into CdE 2 -CICs (step 2, eq ), and the assembly of CdE 2 -CICs into CdE 2 -MSCs (step 3, eq ). Steps 1 and 2 were relatively fast, and step 3 was a rate-determining step, following the behavior of first-order reaction kinetics with a rate constant ( k 1 ) of 0.01 min –1 ( t 1/2 of 69.3 min), which was five times faster than the rate constant of the anion exchange reaction in templated MSCs.…”