Effect of experimental conditions on the structure and properties of nano-porous silica films

The evolution of structure and properties of nano-porous silica films was investigated under different experimental conditions. A new method to adjust refractive index of the films rapidly and continuously is reported. The thin films were prepared with the base,acid one-step and base/acid two-step catalytic sol-gel processes and the dip-coating method. The structure and properties of the films were analyzed by atom force microscopy,scanning electron microscopy, X-ray photo electron spectroscopy, Fourier transform infrared, ellipsometry, separately. From the base one-step, the base/acid two-step,and the acid one-step catalytic process,silica particles in the films become small, the porosity decreases,and the refractive index increases continuously from 1.18 to 1.41.XPS analysis has shown Si2p peak shifts to a higher binding energy and near that of the silica bulk. The frequency shift of omega (4)(transverse optical mode) of infrared absorption to a lower wave number indicates a decrease in the average Si-O-Si bridging angle. These results suggest that the effect of the above experimental conditions on properties of nano-porous silica films is attributed to the change of the silica structure of the films.