Transformation Pathways in Colloidal CdTeSe Magic-Size Clusters

Room-Temperature Formation of CdTeSe Magic-Size Clusters from Oleate-Capped CdTe Precursor Compounds via CdSe Monomer Substitution

We report the first room-temperature synthesis of ternary CdTeSe magic-size clusters (MSCs) that have mainly the surface ligand oleate (OA). The MSCs display sharp optical absorption peaking at ∼399 nm and are thus referred to as MSC-399. They are made from prenucleation-stage samples of binary CdTe and CdSe, which are prepared by two reactions in 1-octadecene (ODE) of cadmium oleate (Cd(OA)2) and tri-n-octylphosphine chalcogenide (ETOP, E = Te and Se) at 25 °C for 120 min and 80 °C for 15 min, respectively. When the two binary samples are mixed at room temperature and dispersed in a mixture of toluene (Tol) and octylamine (OTA), the CdTeSe MSC-399 develops. Also, when the CdSe sample is added to CdTe MSC-371 in a dispersion, the transformation from CdTe MSC-371 to CdTeSe MSC-399 is seen. We propose that the MSCs develop from their precursor compounds (PCs) that are relatively transparent in optical absorption, such as CdTeSe MSC-399 from CdTeSe PC-399 and CdTe MSC-371 from CdTe PC-371. The formation of CdTeSe PC-399 undergoes monomer substitution and not anion exchange, which is the reaction of CdTe PC-371 and the CdSe monomer to produce CdTeSe PC-399 and the CdTe monomer. Our study provides evidence of monomer substitution for the transformation from binary CdTe to ternary CdTeSe PCs.

Pathway of Room-Temperature Formation of CdSeS Magic-Size Clusters from Mixtures of CdSe and CdS Samples

The synthetic application of prenucleation-stage samples of colloidal semiconductor quantum dots (QDs) is in its infancy. It is shown that when two prenucleation-stage samples of binary CdSe and CdS are mixed, ternary CdSeS magic-size clusters (MSCs) grow at room temperature in dispersion. As the amount of the CdS sample increases, the optical absorption of the CdSeS MSCs blueshifts from ≈380 to ≈360 nm. It is proposed that the cluster in the CdSe sample reacts with the CdS monomer from the CdS sample. The monomer substitution reaction of CdSe by CdS can proceed continuously; thus, CdSeS MSCs with tunable compositions are obtained. The present study provides compelling evidence that clusters formed in the prenucleation stage of QDs. The clusters are precursor compounds (PCs) of MSCs, transforming at room temperature with the thermoneutrality principle of isodesmic reactions. The nucleation and growth of QDs follows a multi-step non-classical instead of one-step classical nucleation model.

Surface-Ligand Tuned Reversible Transformations in Aqueous Environments Between CdSe Magic-Size Clusters and Their Precursor Compounds

That magic-size clusters (MSCs) have their counterpart precursor compounds (PCs) has not been generally accepted by expertise circles. Here, experimental evidence to support this new concept is presented. With aqueous-phase CdSe MSCs as a model system, it is shown that when the MSCs are dispersed in water containing a certain amount of L-cysteine (Cys), the MSCs disappear slowly. Upon the addition of CdCl2 , the MSCs recover. It is proposed that after dispersing, the MSCs transform to their quasi-isomeric, non-absorbing PCs upon Cys addition. In the presence of CdCl2 , the PCs transform back to the MSCs due to Cys elimination. The surface ligand Cys of the MSCs plays a significant role in the reversible transformations. The present study provides compelling evidence that absorbing MSCs have their non-absorbing PCs. The study findings suggest that the transformation between two MSCs that display absorption spectral shifts in a stepwise pattern is assisted by their PCs.

Precursor Compound-Assisted Formation of CdS Magic-Size Clusters in Aqueous Solutions

Investigations of the formation pathway of semiconductor magic-size clusters (MSCs) in aqueous solutions are quite limited. Here, we present our understanding about a precursor compound (PC)-assisted formation pathway of aqueous-phase CdS MSCs exhibiting a characteristic absorption peak at about 360 nm (MSC-360). The reaction uses CdCl2 as the Cd source and thioglycolic acid (TGA) as both the S source and ligand in alkaline aqueous solutions. The mixture remains absorption featureless upon incubation at room temperature but with MSC-360 absorption observed upon adding butylamine. The longer the incubation period of the aqueous solution, the more MSC-360 forms after adding butylamine. We propose that Cd-TGA complexes form first, in which the TGA moieties then decompose partially to form PC of MSC-360 (PC-360) that cannot be observed in the optical absorption spectrum. The resulting PC-360 transforms to MSC-360 via quasi-isomerization in the presence of butylamine. The present study provides an in-depth understanding about the formation of aqueous-phase MSCs.

Direct and Indirect Pathways of CdTeSe Magic-Size Cluster Isomerization Induced by Surface Ligands at Room Temperature

The field of isomerization reactions for colloidal semiconductor magic-size clusters (MSCs) remains largely unexplored. Here, we show that MSCs isomerize via two fundamental pathways that are regulated by the acidity and amount of an incoming ligand, with CdTeSe as the model system. When MSC-399 isomerizes to MSC-422 at room temperature, the peak red-shift from 399 to 422 nm is continuous (pathway 1) and/or stepwise (pathway 2) as monitored in situ and in real time by optical absorption spectroscopy. We propose that pathway 1 is direct, with intracluster configuration changes and a relatively large energy barrier. Pathway 2 is indirect, assisted by the MSC precursor compounds (PCs), from MSC-399 to PC-399 to PC-422 to MSC-422. Pathway 1 is activated when PC-422 to MSC-422 is suppressed. Our findings unambiguously suggest that when a change occurs directly on a nanospecies, its absorption peak continuously shifts. The present study provides an in-depth understanding of the transformative behavior of MSCs via ligand-induced isomerization upon external chemical stimuli.

Transformations of Magic-Size Clusters via Precursor Compound Cation Exchange at Room Temperature

The transformation of colloidal semiconductor magic-size clusters (MSCs) from zinc to cadmium chalcogenide (ZnE to CdE) at low temperatures has received scant attention. Here, we report the first room-temperature evolution of CdE MSCs from ZnE samples and our interpretation of the transformation pathway. We show that when prenucleation stage samples of ZnE are mixed with cadmium oleate (Cd(OA)₂), CdE MSCs evolve; without this mixing, ZnE MSCs develop. When ZnE MSCs and Cd(OA)₂ are mixed, CdE MSCs also form. We propose that Cd(OA)₂ reacts with the precursor compounds (PCs) of the ZnE MSCs but not directly with the ZnE MSCs. The cation exchange reaction transforms the ZnE PCs into CdE PCs, from which CdE MSCs develop. Our findings suggest that in reactions that lead to the production of binary ME quantum dots, the E precursor dominates the formation of binary ME PCs (M = Zn or Cd) to have similar stoichiometry. The present study provides a much more profound view of the formation and transformation mechanisms of the ME PCs.

A Real-Time In Situ Demonstration of Direct and Indirect Transformation Pathways in CdTe Magic-Size Clusters at Room Temperature

The transformations of colloidal semiconductor magic-size clusters (MSCs) are expected to occur with only discrete, step-wise redshifts in optical absorption. Here, we challenge this assumption presenting a novel, conceptually different transformation, for which the redshift is continuous. In the room-temperature transformation from CdTe MSC-448 to MSC-488 (designated by the peak wavelengths in nanometer), the redshift of absorption monitored in situ displays distinctly continuous and/or step-wise behavior. Based on conclusive evidence provided by real-time experiments, the former transformation is apparently direct and intra-cluster with a relatively large energy barrier. The latter transformation is indirect and assisted by MSC precursor compounds (PCs). The former transformation follows the latter often, being predominant at a relatively high temperature. The present findings encourage a reconsideration of the absorption redshift reported previously for transformations of binary II-VI MSCs, together with the pathway associated without the increase of cluster mass.