Optical properties of very thin (<100nm) sol-gel TiO2 films

Very thin TiO2 films (less-than 100 nm) were prepared by a sol-gel dip-coating process on glass substrates. The films were characterized with optical transmission measurements and atomic force microscopy. The optical properties of these films were analyzed within the Forouhi-Bloomer model, [A. Forouhi, I. Bloomer, Phys. Rev., B34 (1986) 7018], describing the optical dispersion in disordered dielectrics. Theoretical spectra were generated and fitted, using standard regression analysis techniques, to the experimental ones having as adjustable parameters, the five Forouhi-Bloomer physical model constants and the film thickness. It was found that the model describes satisfactorily the optical properties of very thin sol-gel TiO2 films within the range of 200-2500 nm. The energy difference between bonding-antibonding states was found to be approximately 4.3 eV, while the energy gap values (as defined by the physical model) obtained, ranged between 2.8 and 3.2 eV. The energy gap values using Tauc's formula have also been estimated and found to be approximately 3.6 eV, slightly higher than other corresponding values found in the literature. (C) 1998 Elsevier Science S.A. All rights reserved.