Characterization of Sol–Gel Materials by Raman and Brillouin Spectroscopies

The use of Raman and Brillouin spectroscopies in sol–gel-derived materials is reviewed. It covers a quite vast domain of investigation, from the basic glass science to the characterization of materials produced for many different applications. The theory of inelastic light scattering is briefly presented and the basic physical mechanisms involved in Raman and Brillouin spectroscopies are discussed. The instrumentation for measurements by visible, ultraviolet, and X-ray excitation is described. The vibrational dynamics in low-density aerogel is discussed in terms of a random fractal model, where the low-frequency acoustic vibrations are distorted phonon-like extended propagating modes, whereas highfrequency modes can be spatially localized. As a function of the vibrational frequency, the phonon wavelength and mean free path depend on the size of the porosity. The dependence of the sound velocity and attenuation on the densities is measured by Brillouin spectroscopy. Attenuation in low-density sol–gel-derived solids is nearly temperature independent and is due to structural disorder instead than to anharmonicity, as it is in crystals and compact glasses. It is shown that Raman and Brillouin spectroscopy can be used to follow the different steps of densifications from the wet-gel to the compact glass during thermal annealing. Waveguided spectroscopies for dip- and spin-coated films are described. Low-frequency Raman spectra from the localized acoustic vibration in nanocrystals are presented in sol–gel-derived glass ceramics.