Optical Properties of ZnS:Co+Cr Nanocrystals Synthesized by a Low Temperature Hydrothermal Process

Water-soluble ZnS:Co+Cr nanocrystals (NCs) were synthesized by a low temperature hydrothermal process using 3-mercaptopropionic acid (MPA) as capping agent. X-ray diffraction, laser particle size analyzer, transmission electron microscope (TEM), UV-Vis spectrophotometer, fluorescence spectrophotometer, XPS were used to characterized crystalline structure, morphology, particle size and optical properties of the samples. It is found that the ZnS:Co+Cr NCs are monodisperse and show zinc blende structure with an average particle size of about 9.3 nm. The absorption edge of ZnS:Co+Cr NCs is observed at 320 nm from the UV-Vis absorption spectra, furthermore the characteristic absorption peak at 728 nm of Co is observed. The photoluminescence characteristics indicate that ZnS:Co+Cr NCs shows the maximum PL intensity value under the condition: Cr-doping concentration of 0.75at% and the hydrothermal reaction temperature of 160°C. As confirmed by X-ray photoelectron spectroscope(XPS) , the Cr dopants are partialiy oxidized into Cr.