Parametrization models of optical constants, namely Tauc–Lorentz (TL), Forouhi–Bloomer (FB) and modified FB models, were applied to the interband absorption of amorphous carbon films. The optical constants were determined by means of transmittance and reflectance measurements in the visible range. The studied films were prepared by rf sputtering and characterized for their chemical properties. The analytical models were also applied to other optical data published in the literature pertaining to films produced by various deposition techniques. The different approaches used to determine important physical parameters of the interband transition yielded different results. A figure-of-merit was introduced to check the applicability of the models and the results showed that FB modified for an energy dependence of the dipole matrix element adequately represents the interband transition in the amorphous carbons. Further, the modified FB model shows a relative superiority over the TL ones for concerning the determination of the band gap energy, as it is the only one to be validated by an independent, though indirect, gap measurement by x-ray photoelectron spectroscopy. Finally, the application of the modified FB model allowed us to establish some important correlations between film structure and optical absorption properties.

The influence of hydrogen addition to an Ar plasma on the structural properties of TiO2−x films produced by RF sputtering of a TiO2 target at room temperature was studied. The structural properties of the films were characterized by x-ray photoelectron spectroscopy while the surface morphology was analysed using scanning electron microscopy (SEM). The valence band analysis showed the crystal field splitting of d states into doubly and triply degenerate states. H2 addition to the Ar plasma created additional d-state splitting due to distortions in the TiO2 structure by the Jahn–Teller mechanism. The occurrence of the Jahn–Teller split is well-correlated with oxygen vacancies in the TiO2−x films. Water adsorption at the TiO2−x surface and film hydroxylation were also addressed. The as-grown films were amorphous and SEM analysis showed a columnar structure for all the films but with a lower packing density of the columns after H2 introduction in the Ar plasma.

No abstract

The key aspects related to surface states present on an anodically formed TiO2 nanotube array are here discussed. Impedance measurements performed on similar layers, in solutions at different pH, evidenced quite different behaviors and conducting properties. The comparison between the surface state capacitances at three different pH values showed that surface states have to be correlated with hydroxyl groups at the surface of TiO2. They are characterized by a broad distribution in energy in the band gap. Under polarization in alkaline solution, charge transfer then mainly occurs through the surface states. The situation is quite different in acidic solutions in which a low density of surface states was measured and in which proton insertion and negative charge storage in the tubular layer is responsible for the high doping level of the tube walls and band bending in a narrow space charge zone in the walls. In neutral solutions at pH close to the zero-charge pH, only the bottom of the tubes is active, and a localized state attributed to the adsorption of undissociated water molecules is observed.

No abstract