Synthesis of CdTe, CdSe and CdTe/CdSe core/shell QDs from wet chemical colloidal method

In the present study, CdTe, CdSe, and CdTe/CdSe core/shell quantum dots (QDs) have been synthesized with 1-octadecane from colloidal method. The effect of reaction temperature on the properties of CdTe QDs was investigated. The CdTe/CdSe core/shell QDs were synthesized by coating number layers of CdSe onto CdTe core at 190 degrees C. The resultant QDs were characterized with a range of characterization techniques to study the optical, structural, compositional and morphological properties. A systematic red shift was observed in the optical spectra's of CdTe QDs synthesized for reaction temperatures in the range 130-250 degrees C. The size of CdTe QDs was estimated in the range 2.5-4 nm from the position of absorption peak. The effect of reaction temperature on the interplanar distance 'd', crystallite size and elemental composition of the CdTe QDs is studied. The emission spectra of CdTe/CdSe core/shell QDs attributed at higher wavelength as compared to pristine CdTe demonstrates the indirect excitation in CdTe core. The XRD pattern revealed the cubic crystal structure of CdTe and CdSe QDs with reflections (111), (220) and (311). The synthesis of monodispersed spherical QDs with narrow size distribution are confirmed by TEM images. Three diffused ring observed in SAD patterns of CdTe, CdSe and CdTe/CdSe core/shell are corresponds to (111), (220) and (311) reflection of cubic crystal structure. The lattice spacing obtained by SAD patterns and HRTEM images are in good agreement with standard 'd' spacing. The compositional analysis studied by EDS revealed the stoichiometric growth of QDs. The effect of shells of various layers on the Raman spectra of CdTe/CdSe core/shell QDs is studied. The shift reveled in longitudinal optical (LO) peaks for the CdTe/CdSe core/shell QDs is associated to the lattice mismatch between CdTe and CdSe QDs.