Individual Pathways in the Formation of Magic-Size Clusters and Conventional Quantum Dots

Abstract

The formation relationship between colloidal magic-size clusters (MSCs) and conventional quantum dots (QDs) has not been well established. Here, we report our systematic study on their formation pathways, using cadmium sulfide (CdS) as a model system. Two Cd precursors were prepared from CdO with branched 2-methyloctadecanoic acid (CHCH(CH)-COOH) and linear oleic acid (CHCH-COOH), reacting with elemental S powder in 1-octadecene (ODE). We show that the presence of MSC-311 (exhibiting a sharp absorption peaking at 311 nm) is regulated by the growth of conventional QDs. We demonstrate that MSC-311 cannot directly convert into conventional QDs but to its immediate precursor (IP-311), which is transparent in optical absorption (>310 nm). We propose that there are two individual pathways for the formation of MSCs and conventional QDs, linked by an intrinsic pathway from MSCs to IPs to fragments to QDs. The present study introduces new avenues to precisely control their formation.

One-Step Approach to Single-Ensemble CdS Magic-Size Clusters with Enhanced Production Yields

“…By a side note, the reactivity of Cd(OA) 2 is higher than that of Cd(MA) 2 , and the PC produced from a Cd(OA) 2 + S reaction batch at 180 C (with a growth period of 15 min) seemed to be more than that from a Cd(MA) 2 + S reaction batch at 200 C (with the same growth period of 15 min). 1 In conclusion, we have demonstrated that it is possible to tune the pathways proposed in the two-step model for the prenucleation stage of colloidal semiconductor ME QDs. 1 Meanwhile, the PC to MSC-311 transformation is also facilitated under such temperatures without a second-step incubation at a lower temperature, which is required for the two-step approach reported.…”

“…1 In conclusion, we have demonstrated that it is possible to tune the pathways proposed in the two-step model for the prenucleation stage of colloidal semiconductor ME QDs. 1 Meanwhile, the PC to MSC-311 transformation is also facilitated under such temperatures without a second-step incubation at a lower temperature, which is required for the two-step approach reported. 5 The combined use of a secondary phosphine and an methyl carboxylic acid results in a significant overlap in the temperature range required for the PC formation (Step 1) as well as for the PC to MSC transformation (Step 2), making a one-step approach to single-ensemble MSC production possible.…”

“…This may be reasonably attributed to the fact that 2methyloctadecanoic acid has a stronger coordination effect towards Cd than oleic acid does. 1 Thus, methyl carboxylate capped MSC-311 ( Figure 4b) displayed a higher thermal stability than oleate capped MSC-311 ( Figure 4a). By a side note, the reactivity of Cd(OA) 2 is higher than that of Cd(MA) 2 , and the PC produced from a Cd(OA) 2 + S reaction batch at 180 C (with a growth period of 15 min) seemed to be more than that from a Cd(MA) 2 + S reaction batch at 200 C (with the same growth period of 15 min).…”

“…Recently, a two-pathway model was proposed for the induction period that occurs prior to nucleation and growth of colloidal semiconductor II-VI metal (M) chalcogenide (E) quantum dots (QDs). 1 Starting from M and E precursors, one pathway involves the formation of monomers and fragments. This pathway follows the conventional LaMer model of the classical nucleation theory (CNT), which goes forward to nucleation and growth of QDs.…”

“…The present study provides additional insight into the two-pathway model proposed recently. 1 The collective effort on the synthesis and formation pathway has been moving the field of colloidal semiconductor clusters and nanocrystals one-step forward to transform from an empirical art to science, similar to the advance of organic chemistry. [26][27][28][29][30][31][32][33][34][35][36][37] ASSOCIATED CONTENT…”