Optical and structural properties of sol-gel $SiO2$ layers containing cobalt

$SiO2$ coatings containing cobalt were prepared using the sol-gel method and the Si to Co nominal atomic ratio in the coatings was varied from 1.3 to 7.1. The structure and optical properties of the coatings, heat-treated in air at 300 and $500 °C,$ were characterized using optical, x-ray diffraction, and Auger depth profile measurements. The optical transmission data in the UV-visible range, in samples with low cobalt concentrations, show only the absorption bands associated with the tetragonal $Co2+,$ regardless of the heat treatment temperature. Coatings with a larger amount of Co treated at $300 °C$ show absorption bands associated with both tetragonal and octahedral $Co2+.$ When these coatings are heat treated at $500 °C,$ most of the cobalt migrates to the free surface of the coatings, where it is oxidized by the atmospheric oxygen and forms a top layer of $Co3O4.$ The thickness of the cobalt oxide layer depends on the heat treatment temperature and on the Co concentration. To describe the absorption bands of both tetragonal and octahedral Co, the Lorentz oscillator model was used to represent the complex effective dielectric function of the $SiO2$ cobalt doped layers. The frequency dependence of the optical constants, in the UV-visible range for the top cobalt oxide layer, was obtained with a Jellison–Modine model.