Manipulating Reaction Intermediates to Aqueous‐Phase ZnSe Magic‐Size Clusters and Quantum Dots at Room Temperature

Abstract

It is not resolved which model describes better the aqueous-phase nucleation and growth of semiconductor quantum dots (QDs), the classical one-step one or the nonclassical multi-step one. Here, we design a room-temperature reaction to trap reaction intermediates in the prenucleation stage of ZnSe QDs (as a model system). We show that the trapped intermediate can transform to magic-size clusters (MSCs) via intra-molecular reorganization and can fragment to enable the growth of QDs. The MSCs exhibit a sharp optical absorption peaking at 299 nm, labelled MSC-299. The intermediate, the precursor compound (PC-299) of MSC-299, is optically transparent at 299 nm and to longer wavelengths. This intermediate forms in various Zn and Se reaction systems. The present study provides unambiguous evidence that the nonclassical and classical pathways are both necessary to explain the nucleation and growth of aqueous-phase QDs, with the former pathway favored more by high reaction concentrations.

Anion Exchange in Semiconductor Magic-Size Clusters

“…The stoichiometry of the MSCs was further determined by laser desorption ionization time-of-flight mass spectrometry (LDI-TOF-MS) and ICP-OES measurements. The (CdSe) 34 MSCs showed a spectrum containing a prominent ion centered at m / z 6506.5 (black line in Figure d) and the Cd/Se ratio of 1:1 obtained from the ICP-OES measurement, similar to the previous report. , However, the CdSe-415 exhibited a nonstoichiometric composition, evidenced by a prominent base peak at m / z 4920.5 (red line in Figure d), and a higher Cd/Se ratio of up to 1.9:1 in the ICP-OES analysis. ,, The difference in crystallization was further studied by X-ray diffraction (XRD) (Figure S8c). For stoichiometric (CdSe) 34 MSCs (red trace), relatively sharp peaks were observed along the (100), (002), and (103) planes, which were comparable to the previously proposed wurtzite (WZ) structure of ultrasmall nanocrystals (NCs). , In comparison, the XRD peaks of nonstoichiometric CdSe-415 MSCs (black trace) matched well with the phase of zinc blende (ZB) structure.…”

“…The CdSe-415 and CdTe-448 MSCs were prepared by mixing Cd­(OAc) 2 ·2H 2 O and TOP-E (E = Se or Te) in primary amines at high temperature and then incubating in toluene and octylamine (Tol/Oct) at room temperature (RT). , Unlike the templated MSCs spontaneously generated from the lamellar metal–amine bilayer mesophases (such as (CdS) 13 , (CdSe) 13 , and (CdTe) 13 ), the CdSe-415 and CdTe-448 MSCs did not form with the templates and therefore had better colloidal stability in nonpolar solvents and were expected to be suitable precursors for QD synthesis (Figure S1). − …”