The electronic structure of nitrogen-containing diamondlike films prepared by sputtering was determined by photoelectron spectroscopy. The N 1s core-level spectra are constituted by two peaks at 400.5 and 398.2 eV associated with substitutional N sp 2 in aromatic rings and N bonded to C sp 3 , respectively. On increasing N, the top of the valence band suffers profound changes. The new features are identified by a comparison of the experimental spectra with theoretically calculated density of states of nitrogen-containing graphite and C 3 N 4 structures. ͓S0163-1829͑98͒04004-1͔

A series of randomly nitrogen-substituted carbon clusters in graphitelike structures, containing up to 96 carbon atoms, is theoretically investigated through semiemipirical pseudopotential techniques. The evolution of conformation and electronic structure is obtained as a function of nitrogen content. Results from semiempirical geometry optimizations reveal that the clusters are planar for nitrogen concentrations up to ͓N͔/͓C͔ϳ20%. Above this concentration, buckling develops in the clusters. One of the characteristics of these corrugated clusters is the presence of carbon dangling bonds. Chemical stabilization imposes that these structures evolve to either a three-dimensional, fully covalent carbon nitride network, or to molecular forms. Among the welldefined molecular structures that could develop in amorphous carbon nitride, we found nanotubules and a molecular cage of elemental compositions CN and C 3 N 4 , respectively.

TiO 2 films prepared by sol-gel route are active photocatalysts for the oxidation of organics in photoelectrochemical cells. The as-grown films for photocatalysis applications and those exposed to Ar + or H 2 + +Ar + ion bombardment are characterized by different spectroscopic methods, such as X-ray diffraction (XRD), atomic force microscopy (AFM), UV-vis transmittance, photothermal deflection spectroscopy (PDS) and X-ray photoelectron spectroscopy (XPS), as well as by conductance. This material has defects associated with oxygen vacancies produced during the sample preparation which support nondissociative adsorption of O 2 when films are exposed to air. Charge transfer from reduced Ti species to adsorbed dioxygen leads to Ti-O 2surface complexes that are partially removed by heating at 200 °C, and fully removed after 30 min ion bombardment. By comparison with the relatively well-understood structural defects of bombarded TiO 2 we arise to a quite complete structural model of the as grown material which corresponds to an amorphous semiconductor possessing relative low disorder and density of states as compared with a pure amorphous material. These TiO 2 films are modeled as low size crystalline domain embedded in an amorphous matrix whose electronic structure exhibit exponential band tails and a narrow band close to the conduction band. The latter is fully or partially occupied depending on the presence of adsorbed electron scavengers such as dioxygen.

Structural properties of hydrogenated carbon-nitride films produced by ion-beam-assisted evaporation of the molecular precursor C 4 N 6 H 4Influence of ion-beam energy and substrate temperature on the synthesis of carbon nitride thin films by nitrogenion-assisted pulsed laser deposition

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