Control over the Size Effect in the Spectroscopic Properties of ZnxCd1 – xS Colloidal Quantum Dots

Abstract

Approaches are developed for controlling the size effect in the absorption and photoluminescence properties of colloidal CdS and Zn_0.5Cd_0.5S quantum dots (QDs) in a gelatin matrix and thioglycolic acid. We analyze spectroscopic manifestations of dark Ostwald ripening and size-selective photoetching and demonstrate that the illumination of molten colloidal CdS and Zn_0.5Cd_0.5S QD solutions at a wavelength lying at the optical absorption edge in the presence of hydrogen peroxide as a catalyst for photoetching leads to a blue shift of the optical absorption and photoluminescence spectra of the QDs. Analysis of TEM images of QD samples leads us to conclude that photoetching reduces the average QD size as a consequence of the photodissolution of the CdS and Zn_0.5Cd_0.5S compounds. We demonstrate that, by contrast, heat treatment of the synthesized CdS and Zn_0.5Cd_0.5S QD sols at a temperature between 75 and 90°C leads to an increase in nanocrystal size through recrystallization, which shows up as a redshift of the optical absorption and photoluminescence spectra of the corresponding samples.