Silica-coated CdTe Quantum Dots of Unchanged Size with Intense Photoluminescence at Various Wavelengths

Abstract

A new method for controlling the photoluminescence (PL) wavelength of silica-coated quantum dots (QDs) without changing the sizes of the core QDs is reported. When silica-coated green-emitting CdTe QDs were refluxed in aqueous solution containing Cd2+ and a sulfur-containing surfactant (thioglycolic acid (TGA)), the PL peak wavelength shifted to red with a significant increase in PL efficiency. This spectral shift was attributed to the reduction of the quantum confinement effect by the formation of small CdS-like clusters very close to the QDs in the silica shells. When the silica-coated QDs were refluxed in a solution of higher pH, the extent of this red shift increased. The larger red shift of the PL with pH is attributed to the hydrolysis speed of TGA increasing with pH. There is faster generation of S2− and as a consequence the growth rate of CdS-like clusters is enhanced. When a different sulfur-containing surfactant (thioglycerol (TG)) was used for reflux, the PL spectral shift and enhancement of PL efficiency became very small. This was because TG did not hydrolyze significantly enough to release S2−. These results demonstrate that the reported reflux procedure is a promising method for tuning the PL color of QDs through selection of the solution pH and the surfactant during reflux.