Growth and characterisation of cadmium sulphide nanocrystals embedded in silicon dioxide films

Abstract

In this communication we report on the growth and the optical characterisation of CdS quantum dots in the diameter range of 4–10 nm embedded in silicon dioxide glass films grown by magnetron rf-sputtering technique with post-deposition annealing. Optical transmission measurements display a marked blue shift of the absorption band edge due to the quantum confinement of the electrons and holes in the CdS nanocrystals. The mean diameter of the nanocrystals was estimated from absorption spectra using a standard theoretical model. A study of the effect of the nanocrystal size on the Raman spectra has been made from an analysis of the first order LO mode lineshape. The increase in the linewidth observed with decreasing crystal size is in agreement with the results of a calculation based on the phonon confinement model. The asymmetric lineshape is well reproduced by the model, but they fail to reproduce the large low-frequency tail. The origin of this tail is discussed considering several possible sources proposed in the literature and is under study. Room temperature infrared reflectance measurements have also been obtained showing a diffuse band at about 230–300 cm-1, which closely corresponds to CdS optical polar phonons observed in the bulk crystals. However, we are unable to fit the observed spectra by assuming just this contribution. Instead, by including an additional contribution due to CdO we are able to well simulate the experimental spectra using a model based on the Maxwell–Garnet theory.