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

Abstract

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 intracluster 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.

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

“…This important result is not addressed in the previous work on the chemical transformation of MSCs from CdSe to Cu 2 Se and from InP to Cd 3 P 2 . , When the CE reaction occurs on ZnE MSCs, it is our view that a continuous redshift pattern of optical absorption would be observed during the MSC transformation from ZnE to CdE. The uninterrupted pattern is similar to that reported for the intracluster isomerization from CdTe MSC-448 to MSC-488. , After the addition of Cd­(OA) 2 to a dispersion containing ZnSe MSC-299 at RT, the CE reaction (Step 2) that transfers the ZnSe PC to the CdSe PC takes place immediately. Step 2 facilitates Step 1 via which ZnSe MSC-299 transforms to its counterpart ZnSe PC-299.…”

“…In addition to the present CE reaction, we have demonstrated that the binary ME PC can experience another four chemical reactions. They are isomerization to binary ME MSCs, − ,− ,,, substitution reactions followed by isomerization to binary or ternary MSCs (Figure S1B), ,− ,, addition reactions followed by isomerization to photoluminescent MSCs (Figure S1B), and fragmentation to QDs. , For the reaction of M and E precursors to ME QDs, the E precursor dominates the self-assembly of the M and E precursors that results in the formation of the ME PC with similar stoichiometry. The present finding of the E-dominated, magic self-assembly principle, together with the recent discovery of these binary and ternary MSCs and the successful synthesis of small-size QDs with enhanced yield, demonstrates that we are able to push further “chemical self-assembly” …”

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

“…Very recently, an in situ, real-time examination by optical absorption spectroscopy reveals a strikingly different pathway for the room-temperature evolution of CdTe MSC-488 from CdTe MSC-448. , An unexpected continuous red-shift pattern is observed, in addition to the stepwise one described above. However, the study begins with prenucleation stage samples of CdTe QDs, also called induction period (IP) samples that contain no QDs but the PC for CdTe MSC-371, together with CdTe monomers and fragments (Mo/Fr).…”

“…The discrete pattern of the spectrum change usually displays a characteristic isosbestic point located between the two absorption peaks of MSC-1 and MSC-2, without intermediate absorptions. MSCs have their precursor compounds (PC), ,− and the isomerization pathway is modeled as going through their PCs. ,,, That is from MSC-1 to PC-1, then to PC-2, and finally to MSC-2. Being relatively optically transparent, the PCs do not absorb at the peak position of their counterpart MSCs or to longer wavelengths.…”

Unraveling the Transformation Pathways in Semiconductor Clusters by Studying the Formation of Spectroscopically Pure (CdS)13 Magic-Size Clusters

“…Following the initial observation of (CdSe) 33 and (CdSe) 34 in 2004, intense research activity has focused on the study of synthetic pathways and related formation mechanisms. − This field has expanded dramatically since (CdSe) 13 MSCs were synthesized and isolated in solid form by Buhro and co-workers. , In recent years, several stoichiometric MSCs have been isolated in discrete, single-size purity. , For example, Hyeon and co-workers recently reported the crystal structure of a semiconductor Cd 14 Se 13 cluster . It is worth noting that although five members of stoichiometric cadmium-based II–VI MSCs have already been prepared and isolated, ,,− the synthesis of pure (CdS) 13 MSCs remains a challenge.…”