Rapid Stepwise Growth of Water-Dispersive CdS Quantum Dots in Ethylene Glycol

Water-dispersive CdS quantum dots (QDs) have been rapidly synthesized in a polyol solvent of ethylene glycol at 160 degrees C without using nonpolar organic solvents, and their stepwise growth mechanisms have been investigated thoroughly. The growth rate of CdS QDs in the second step of >= 70 s is prominently slower than that in the first step of <= 70 s. The dominant growth mechanism of CdS QDs switches from the Ostwald ripening (OR) growth in the first step to the oriented attachment (OA) growth in the second step. The fast decay time of 40 ns and the slow decay time of 300 ns in photoluminescence decay profiles are due to the decay times of excitons in internal defects and OA-induced defects, respectively. In the first step, internal defects are eliminated by the OR growth to reduce photoluminescence, and the center of the photoluminescence of the slow decay component shifts from 550 to 600 nm. In the second step, the OA growth occurs to enhance the number of OA-induced defects, increasing photoluminescence at 600 nm.