Optical characterisation of anatase: a comparative study of the bulk crystal and the polycrystalline thin film

The optical parameters of crystalline anatase and polycrystalline anatase films prepared by reactive magnetron sputtering were examined by spectroscopic ellipsometry and optical transmittance spectroscopy, respectively. Some films were doped with aluminium or chromium. In general, the values of the optical bandgap for the films are consistently blue-shifted as compared with the crystal. The blue shift is also found for some films doped with Al and Cr. The width of the absorption bands is considerably narrower in the films than in the crystal. These effects are interpreted as resulting from the spatial confinement of electrons and holes in the TiO2 microcrystallites. The average crystallite size determined from the shift of the optical gap was found to be in the range ∼5–8 nm, in excellent agreement with the data obtained from small-angle X-ray diffractometry.