Optical characterization of CdTe nanoparticles embedded in a nanoparticulate SnO2 matrix

Cadmium telluride (CdTe) nanoparticles were grown in the interior of a SnO_(2) transparent matrix by means of the r.f. sputtering technique. X-ray diffraction (XRD) patterns reveal that the CdTe quantum dots (QD) grow in the hexagonal wurtzite (W) phase. Diffraction data show that SnO_(2) is constituted by nanoparticles also, with size of the same order of the W-CdTe QD. The broad bands of the XRD patterns and the Scherrer formula allowed, by assuming a spherical shape, the W-CdTe QD size calculation, which have average diameters in the range 4.8 - 14.0 nm. These data were confirmed by electron microscopy images. Optical absorbance gives information to calculate the energy of the two lowest excitonic states (band gap). The Raman spectra show several broad bands in the range 100 - 200 cm^(−1) , on which deconvolution allows separate five modes in the 100 - 200 cm^(−1) interval. The bands correspond to CdTe and tetragonal Te. The transversal optic (TO) and longitudinal optic (LO) modes of CdTe at the Γ point of the first Brillouin zone, for phonons in nanoparticles, follow the expected behavior if the radius of crystal decreases, taking into account that the selection rules for momentum conservation are relaxed.