Optical properties of sol-gel processed TiO_2 thin films up to the vacuum ultraviolet energy region

摘要

We have investigated structural and optical properties of TiO_2 thin films prepared by a conventional sol-gel method. X-ray diffraction and transmission electron microscopy demonstrate that the TiO_2 films are polycrystallized and consist of anatase phase. The absorption spectra above the band-edge of the TiO_2 thin films exhibit the shoulder structures around 3.9 and 4.3 eV and the broad bands around 4.8 and 6.2 eV. Several absorption bands revealed by decomposing the absorption spectra can be assigned to the transitions fromoxygen 2p to titanium3d states in TiO_2. Excitation spectra for a visible luminescence peaking at 2.23 eV exhibit a gradual increase with increasing energy from8.0 eV, whose energy corresponds to the beginning of the transitions from oxygen 2p to titanium 4s and 4p states. The result gives the characteristics of the relaxation processes from titanium 4s and 4p to the relaxed excited states.