Mercaptopropionic acid capped CdS@ZnS nanocomposites: Interface structure and related optical properties

CdS@ZnS core-shell nanoparticles stabilized by mercaptopropionic acid were prepared by a two-step aqueous synthesis: a hydrothermal route at 180 °C and different heating times to prepare the CdS core and the successive ion layer adsorption and reaction method to grow ZnS shell layers. The structure of the composites formed was established from X-ray diffraction and high-resolution transmission electron microscopy data complemented by UV-vis, photoluminescence, energy-disperse X-ray and IR spectra. At the core-shell interface evidence of formation of a solid solution Cd _1-xZn _xS was obtained. The interface formed between the ZnS layers and the capping molecule (MPA) appears to be amorphous. The optical properties of the resulting nanostructures were evaluated from UV-vis and photoluminescence spectra. The recorded spectral information supports the nature of the core-shell structure and of the interface regions. Photoluminescence peak showed a blue shift of about 20 nm relative to the emission peak for the precursor CdS core and an increase of 3 times in its intensity. Such a blue shift was ascribed to the C _1-xZn _xS solid solution formation at the core-shell interface. The CdS@ZnS obtained composite remains stable for weeks within the aqueous solution. The formation of fluorescent nanostructures stable in aqueous media is highly desirable in many potential applications, including biological labels.