Modification of the crystalline structure of ZnO nanoparticles embedded within a SiO₂ matrix due to thermal stress effects

Self-assembled nanocrystals of ZnO were embedded within a SiO₂ matrix by a sequential deposit using reactive R.F. sputtering. The ZnO nanoparticles (NP's) were obtained by depositing a very thin layer (∼20 nm) of Zn on the bottom of the valleys of a first SiO₂ rough surface and then covered by another SiO₂ layer at 500 °C. The stress produced, due to the cooling process, by the SiO₂ matrix on the ZnO NP's generates an unusual crystalline phase of ZnO. The crystal structure was determined by means of X ray diffraction patterns. The ZnO + SiO₂ composite shows a transmittance higher than 80 % for wavelengths > 450 nm. Optical absorption allows to reveal the character and value of the optical band gap. Vibrational modes of the material were determined by Raman spectroscopy.