F. Gracia

Determination of texture by infrared spectroscopy in titanium oxide-anatase thin films

A theoretical model to determine the effective dielectric tensor of heterogeneous materials composed by anisotropic microcrystallites has been introduced to explain the infrared spectral features of textured thin films of uniaxial materials as the function of a textural parameter. This theoretical treatment is able to satisfactorily reproduce the experimental absorbance spectra of TiO2–anatase thin films chosen as a model system. Comparison of texture data obtained from infrared spectroscopy and x-ray diffraction are in good agreement which support the validity of the proposed model.

Optical and crystallisation behaviour of TiO2 and V/TiO2 thin films prepared by plasma and ion beam assisted methods

This paper reports the synthesis of TiO2 and V/TiO2 thin films, prepared by plasma enhanced and ion beam induced chemical vapour deposition procedures. Thin films with different contents of vanadium from a few to some tenth percents of this element have been prepared. Vanadium oxide thin films were also prepared for comparison. Morphological, structural and chemical characterisation of the samples was carried out by means of several techniques including scanning electron microscopy, X-ray diffraction, Fourier-transform infrared spectroscopy and X-ray photoelectron spectroscopy. Optical properties of these samples were studied by ellipsometry and ultraviolet–visible absorption spectroscopy. It has been shown that the absorption threshold of V/TiO2 thin films shifts continuously towards the visible by increasing the V content. On the basis of this behaviour it is proposed the use of these thin films as optical filters in the UV–vis region of the spectrum. In the ‘as prepared’ samples vanadium is homogeneously distributed within an amorphous Ti–O structure. After annealing in air at T>673 K, both segregation of vanadium as vanadium oxide and crystallisation of the TiO2 into the rutile structure take place. It is realised that infrared spectroscopy can be a complementary technique of XRD to determine the crystallisation behaviour of the samples. The observed structural modifications are accompanied by a change in the light absorption properties of the films that can now be interpreted as due to the superposition of the absorption spectra of TiO2 and vanadium oxide. In the ion beam assisted films, only slight changes in their refraction index (n) are found as a function of the vanadium content and/or the annealing treatments. The changes were greater for the less compact films obtained by plasma deposition, indicating that the main parameter controlling the value of n is the density of the films.

Phase mixing in Fe/TiO2 thin films prepared by ion beam-induced chemical vapour deposition: optical and structural properties

Abstract Transparent Fe/TiO2 thin films with different Fe/Ti atomic ratios (0.07, 0.18 and 0.73) have been prepared at room temperature by ion beam-induced chemical vapour deposition (IBICVD). These thin films can be of interest as optical filters and for photo-catalytic applications. Ion beam bombardment of the growing thin films made the original samples amorphous, presenting complete mixing of the iron and titanium oxide phases. The original samples were annealed in air (T=473–873 K) and then structurally characterised by X-ray diffraction (XRD) and Fourier-transform infrared spectroscopy (FT-IR). It is shown that FT-IR spectra can be used, on a fingerprint basis, to ascertain the structure of TiO2 thin films. Anatase (pure TiO2 sample), mixtures of anatase and rutile (sample with a Fe/Ti ratio of 0.07), or rutile phases (samples with Fe/Ti ratios higher than 0.18) were obtained by annealing at T>673 K. Iron-containing samples present a shift towards the visible in the absorption edge of their UV-vis absorption spectra. The magnitude of this shift increases with the amount of iron in the films and with the annealing temperature. Both effects, rutilisation and the increase in the magnitude of the shift, are attributed to the presence of iron in the films and discussed in terms of the solubility/segregation of the Fe2O3 and TiO2 phases.

Angle dependence of the O K edge absorption spectra of TiO2 thin films with preferential texture

O K and Ti L2,3 edge X-ray absorption spectra have been recorded for TiO2 thin films (∼100 nm thickness) prepared by ion beam induced CVD at room temperature. The as prepared films were amorphous but they crystallize into the anatase structure of TiO2 after annealing at T>650 K. In the amorphous films, the magnitude of the crystal field, determined from the evaluation of the spectra, was smaller than in the well crystallized anatase samples. In these latter samples the O K edge spectra show a strong dependence on the angle of incidence of the radiation with respect to the sample surface. This dependence rises some concerns about the use of the O K edge spectra of oxide materials for a universal “fingerprint” characterization of thin films. A preliminary theoretical evaluation of the evolution of the t2g/eg intensity ratio with the relative orientation of the electric field of the radiation and the surface normal of the samples is carried out. For this evaluation, the texture of the thin film (i.e. the preferential growth of certain crystallographic planes parallel to the surface) and the existence of surface modifications of the environment around the Ti are taken into account.