Sol-gel synthesis of xTiO2(100 − x)SiO2 nanocomposite thin films: Structure, optical and antireflection properties

Sol-gel xTiO2(100 − x)SiO2 nanocomposite thin films with x = 0, 25, 50, 75 and 100 mol % were dip-coated on glass and silicon substrates. The influence of the composition on structure, morphology and optical properties was studied by X-ray diffraction, Raman spectroscopy, Fourier transform infrared spectroscopy, scanning electronic microscopy, monochromatic ellipsometry at λ = 632.8 nm and ultraviolet-visible absorbance spectroscopy. The optical properties were discussed on the basis of the microstructure. After annealing at 500 °C, results showed high pure materials, homogenous crack-free surfaces with good adherence and high optical qualities. The 100% TiO2 crystallizes in the anatase phase and exhibits nanograins of 6 to 10 nm in size. However, all the other compositions are amorphous with comparable grains around 4 nm in size. The averaged transmittance decreases with increasing the TiO2 content but remains higher than 90%. Whatever the composition, the coating thickness increases linearly as the withdrawal speed increases from 10 to 54 mm/minute. The relationship between refractive index and composition was analyzed by fitting the experimental data to different theoretical models for the refractive index of mixed films. Results showed that depending on the withdrawal speed, the data can properly fit either the Drude or the linear models. Thus, the film thickness can be adjusted by the control of the withdrawal speed. However, the control of the composition allows easily the tuning of the refractive index from 1.48 to 2.18 at λ = 632.8 nm to achieve optimum anti-reflection characteristics. Different anti-reflection designs of both single and double layers were experimentally examined. Gains (assigned to the reduction of reflection losses) up to 54 and 63% were predicted with 75% TiO2 single-coating and SiO2/TiO2 double-coatings respectively.