Photostability of the Oleic Acid-Encapsulated Water-Soluble Cd_xSe_yZn_1-xS_1-y Gradient Core-Shell Quantum Dots

Composite systems where quantum dots (QDs) are combined with other nanomaterials (e.g., gold nanorods) in aqueous solutions have attracted broad attention - both for their potential in applications and for studies of fundamental processes. However, high-quality QDs are typically prepared in organic solvents, and the transfer of QDs to an aqueous phase is needed to create the desired QD composites. Photostability of the transferred QDs - both the steady-state and photo-induced dynamic properties - is essential for studying the processes in the composites and for their applications. We present a detailed study of the photostability of aqueous Cd_xSe_yZn_1-xS_1-y gradient core-shell QDs obtained by various approaches using linker exchange and surfactant encapsulation. Beside the steady-state photoluminescence (PL) emission stability, we also study changes in the PL decay. From the variety of the studied samples, the water-soluble QDs encapsulated by a double layer of oleic acid show superior properties, that is, stable PL emission and PL decay under continuous light or pulsed-laser light irradiation. We demonstrate that the double-layer encapsulation of QDs can be used to create QDs-metal nanoparticle composites.