Diana Mardare

On the structural properties and optical transmittance of TiO2 r.f. sputtered thin films

Abstract Pure and doped TiO 2 thin films were obtained by r.f. sputtering method. The samples were deposited onto glass and glass covered with indium tin oxide (ITO) substrates. Phase and surface morphology were investigated using X-ray diffraction (XRD) and scanning electron microscopy (SEM). The structure of TiO 2 thin films is influenced by the substrate used and also by doping with Ce, Nb and Fe impurities. Consequently, the transmittance will also be modified.

Optical dispersion analysis of TiO2 thin films based on variable-angle spectroscopic ellipsometry measurements

Pure and doped TiO2 thin films, deposited by reactive r.f. sputtering onto glass substrates, have been investigated by spectroscopic ellipsometry in the wavelength range between 360 and 830 nm. Doping with Ce and Nb induces structural changes in TiO2 and modifies its optical constants. While undoped TiO2 films crystallize in a mixed rutile/anatase phase at a substrate temperature of 250°C, the doped films exhibit the anatase phase only. Using a polynomial and a single oscillator model dispersion function the spectral dependency of the refractive index and the extinction coefficient, as well as the optical band-gap have been determined. It turned out that even a moderate surface roughness in the range of 10 nm significantly influences the absolute value of the refractive index. Therefore, the surface roughness has been measured by atomic force microscopy and has been taken into account in the ellipsometric model.

On the structure, morphology and electrical conductivities of titanium oxide thin films

Abstract Titanium oxide thin films were prepared by a d.c. sputtering technique onto glass substrates. The morphology of the films was analysed by atomic force microscopy and their structure by X-ray diffraction. The structure and phase composition of the films depend on the deposition conditions. The small values for roughness obtained from AFM, ranging from 3 to 9 nm, show relatively smooth surfaces. Temperature dependences of the electrical conductivities were studied in a wide range, 13–560 K. The values of activation energies of electrical conduction, calculated from the temperature dependences of the electrical conductivity, varied between 0.13 and 0.39 eV, for temperature range 310–468 K. The current–voltage characteristics are ohmic for values of applied voltage lower than 0.5 V. For higher values, the mechanism of electrical conduction is determined by space-charge-limited currents.

Polaron transport in TiO2 thin films

Undoped and Fe-doped TiO2 thin films were obtained by rf-sputtering technique onto heated glass substrates (250 °C) covered with indium tin oxide. The temperature dependence of the electrical conductivity was investigated in the temperature range 13–320 K, and it shows that the conduction mechanism in the studied samples is described by small-polaron hopping (SPH) at temperatures higher than half of the Debye temperature (θD). It was found that the magnitude of the SPH coupling increases by Fe doping in TiO2 thin films. With decreasing temperature, the conduction behavior transited from SPH conduction to variable-range hopping (VRH) conduction. In the intermediate temperature domain (200 K<T<θD/2), the VRH conduction was found to be dominant, while a temperature-independent conductivity behavior was observed in the lower temperature range (T<200 K).

On the optical constants of TiO2 thin films. Ellipsometric studies

Abstract TiO 2 thin films were obtained on unheated glass substrates by a DC reactive magnetron sputtering method. The as-deposited films exhibit an amorphous structure as observed from X-ray diffraction (XRD) patterns. The structure changes to a mixed one of 70% anatase and 30% rutile after heat treatment in air in the temperature range 293–673 K. Using ellipsometric measurements, and a computer to solve the corresponding equations, a modeling technique was used to find the optical constants of the studied thin films. A sensitivity analysis was performed.

Synthesis and hydrophilic properties of Mo doped TiO2 thin films

Amorphous undoped TiO2 and Mo-doped TiO2 thin films were obtained by spray pyrolysis. By heat treatment, they became polycrystalline consisting in anatase, or mixed anatase/rutile phases, if deposited on glass or silicon, respectively. Mo enters in the TiO2 matrix as Mo5+, determining a slight red shift of the absorption edge. Based on Fourier transmission infrared analysis, performed on the studied films, before and after irradiation, providing information on the relationship between hydrophilicity and the amount of the adsorbed hydroxyl groups, we certify once again that the anatase phase (present in proportion of 100% in the films deposited on glass) is superior concerning the hydrophilic properties. Mo enhances wettability for the films deposited on silicon, compared to the undoped one, even if determines a rise in the rutile weight percentage, as a combined effect with the substrate nature. The decomposition of the oleic acid confirms that titania films with higher hydrophilic performances are also g...

Polaronic transport in TiO2 thin films with increasing Nb content

The temperature dependence of the charge transport in TiO2 films was investigated to establish the correlation between the Nb content and electrical properties. It was identified that temperature-dependent conductivity of the films is dominated by a phonon-assisted small polaron hopping model in the non-adiabatic regime. Applying the polaron hopping models of Mott, Schnakenberg and Emin to describe the observed behavior, temperature-dependent conductivity data of the films were analyzed. A detailed analysis in terms of small polaron hopping parameters in the investigated temperature regime was used to correlate electrical properties with the percentage of Nb.

On the electrical properties of some new stable disubstituted ylides in thin films

The synthesis of six new 3-(4-halogenophenyl)-pyridaziniu-(4-chloroanilido)-(4-R-benzoyl)-metylides is described and the temperature dependence of the electrical conductivity and Seebeck coefficient for respective compounds is investigated. The measurements have been performed using thin films deposited from solution. The studied compounds have semiconducting properties. Some correlations between these properties and the chemical structures of the compounds are established. The values of some characteristic parameters of investigated compounds have been determined.

Gas sensing materials based on TiO2 thin films

TiO2 thin films were prepared by spray pyrolysis using a solution of titanium tetrachloride and ethyl alcohol. The deposition was performed onto different substrates (silicon, quartz, glass) maintained at the same temperature, 270°C. After annealing, a predominant rutile structure is obtained for films deposited onto silicon and quartz substrates, as revealed by x-ray diffraction patterns. The TiO2 films were exposed to different gases, at different temperatures, in order to evaluate their gas sensitivity. The optimum operating temperatures, showing the highest gas sensitivity, were determined for some gases (acetone, ethanol, methane, and liquefied petroleum gas).

Platinum role in hydrophilicity enhancement of Cr-doped TiO2 thin films

Abstract In this paper, we have investigated the hydrophilic properties of the titania films doped with increasing chromium percentages (from 2.1 at.% till 4.0 at.%). Cr-doping induces an increase in the rutile weight %, a more compact structure, and a significant red shift of the TiO2 absorption edge, the last property being very important in the self cleaning applications. For the chosen Cr concentrations, the films did not show promising hydrophilic properties. To improve them, we have applied a novel surface modification method, reported in literature mainly for powders, namely, surface metallisation. We have observed that, by depositing Pt islands on the film with the highest Cr content, its hydrophilic properties improve for a certain metal coverage area. The explanation was based on FT-IR and X-ray photoelectron spectroscopy analysis, performed on the UV irradiated and non-irradiated films, which gives information on the relationship between hydrophilicity and the amount of the adsorbed hydroxyl groups.