Formation and Transformation of CdS Clusters during the Prenucleation Stage and in a Dilute Dispersion at Room Temperature.

The formation and transformation of colloidal semiconductor clusters remain poorly understood. With CdS as a model system, we show that, in the reaction of cadmium myristate (Cd(MA)2) and S powder in 1-octadecene (ODE), clusters form in the prenucleation stage of quantum dots (QDs). Called precursor compounds (PCs), the clusters can transform to magic-size clusters (MSCs) in reaction at a relatively high temperature (MSC-322 displaying optical absorption peaking at 322 nm) or in a dispersion at room temperature (MSC-360). When the reaction temperature is increased, PC-360 forms at 140 °C, while PC-322 and MSC-322 form at 180 °C. In a dispersion of cyclohexane and octylamine, MSC-322 transforms to MSC-360 via MSC-345. The MSC-345 to MSC-360 transformation displays continuous and discontinuous shifts in the optical absorption. The PCs and MSCs are a group of isomers. The present findings bring insight into the cluster formation and isomerization in the prenucleation stage of QDs and in a dispersion.

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.