J.P. Espinós

The state of the oxygen at the surface of polycrystalline cobalt oxide

Abstract XPS and factor analysis (FA) have been applied to characterize the surface state of three polycrystalline cobalt oxide samples with different crystallographic bulk structure (CO 3 O 4 and CoO) and surface characteristics. The study of the thermal behaviour of the O1s and Co2p spectra and of their changes as an effect of Ar + bombardment and exposure to O 2 have permitted verification of the existence of three components at the O1s spectra with binding energies at 529.6 (O I ), 531.1 (O II ) and 532.1 eV (O III ) and three components at the Co2p level. The shape of these components is similar to the Co2p spectra of Co 3 O 4 (Co III ), CoO (Co II ) and Co 0 (Co I ) compounds. In the three samples component O II yields component O III by heating at 473 K T III and O I decreases to a minimum at 923 K. Simultaneously, component Co III , the most abundant in the original samples, yields component Co II by outgassing at T > 473 K. Co II is the only one detected at 923 K. After Ar + sputtering of the samples heated at that temperature, components Co III and Co I were generated. Co I disappears, while the intensity of components O II and Co III increases when the samples are exposed to O 2 . The most intense O I species is attributed to oxygen atoms in sites with a well defined coordination. Species O II and O III are attributed to low coordinated oxygen atoms at special sites or domains of the surface where the covalence of the CoO bond is higher. Differences in the relative abundance of the three oxygen components, either during outgassing or after Ar + sputtering, are dependent on the texture of the three samples expressed in terms of their respective surface areas.

XPS study of oxidation processes of CeOx defective layers

Abstract A CeO 2 thin film has been subjected to Ar + bombardment at 298 K to induce the reduction of its outmost layers by preferential removal of oxygen. An XPS study of the altered layer at normal and grazing angle has been carried out. The Factor Analysis (FA) of the XPS spectra of this Ar + reduced film shows that it has a stoichiometry close to Ce 2 O 3 , being Ce 3+ the dominant species at both collection angles. Simultaneously, the O1s spectra depict a lateral peak whose relative intensity is higher for those spectra recorded at grazing angle. Exposure to successive doses of O 2 at 298 K of the reduced layers produces the increase of the O/Ce ratio and a progressive reoxidation of Ce 3+ into Ce 4+ as determined by FA of the Ce3d spectra. Simultaneously, the lateral component at the O1s peak also decreases, thus discarding that it is due to surface contamination by –OH or similar species, as previously suggested in the literature. After exposure to a high pressure of oxygen (ca. 1 Torr), the XPS spectrum obtained at a normal collection angle shows an almost complete oxidation of the film to CeO 2 . However, in the spectrum at grazing angle, Ce 3+ species and the lateral component of oxygen are still detected. The lateral O1s component is tentatively attributed to oxygen ions with unusual coordinations in a defective CeO x ( x 3+ ions might be due to fully coordinated species. Enrichment of the surface of the defective cerium oxide with these oxygen species seems to be a result of the same structural rearrangements that favour the observed stabilization of Ce 3+ species at the surface.

Compositional changes induced by 3.5 keV Ar+ ion bombardment in Ni-Ti oxide systems: A comparative study

Abstract Compositional changes induced by 3.5 keV Ar + sputtering in TiO 2 , NiO, NiTiO 3 and a (TiO 2 + NiO) mixture have been quantitatively studied by XPS. Although all the samples show important changes in their stoichiometry, the extent of the decomposition depends on the compound. The stability of Ti 4+ appears to be enhanced by the presence of Nisu2+ cations which, on the other hand, are more easily reduced to Ni 0 than in pure NiO. To explain these results a redox solid state reaction between the intermediate phases formed during sputtering is proposed, which tends to preserve the most stable phases.

Elastic and orbital effects on thickness-dependent properties of manganite thin films

Mn-nuclear magnetic resonance data and x-ray photoemission spectroscopy, signal that the depression of themagnetic properties of the more strained 001 LCMO films is not caused by an elastic deformation of theperovskite lattice but rather due to the electronic and chemical phase separation caused by the substrate-induced strain. On the contrary, the thickness dependence of the magnetic properties of the less strained 110 LCMO films are simply described by the elastic deformation of the manganite lattice. We will argue thatthe different behavior of 001 and 110 LCMO films is a consequence of the dissimilar electronic structure ofthese interfaces.DOI: 10.1103/PhysRevB.76.224415 PACS number s : 75.47.Lx, 75.70. i

Control of the stoichiometry in the deposition of cobalt oxides on SiO2

Both CoO and Co3O4 overlayers have been deposited on SiO2 by evaporation from metallic Co and subsequent oxidation with oxygen and a plasma of oxygen. The combined use of ion scattering spectroscopy and XPS shows that both oxides grow in the form of small particles on the surface of SiO2. Ion scattering spectroscopy also shows that the surface of cobalt oxide exposed to a plasma of oxygen is enriched in oxygen ions with respect to the surface of the cobalt oxide formed by exposure to oxygen. The Co 2p spectra corresponding to the deposits obtained by oxidation with O2 are characteristic of CoO, while those corresponding to the deposits obtained after oxidation with a plasma are typical of Co3O4. Moreover, the OCo/Co ratios determined by XPS and factor analysis indicate the formation of CoO stoichiometry in the former case and Co3O4 stoichiometry in the latter. It has also been observed that no shift in either binding energy or modified Auger parameter α′ appears as a function of coverage. This absence of shifts is interpreted as a consequence of the type of screening mechanism that dominates the relaxation of the photoholes in these oxides.

XRD, XPS and 119Sn NMR study of tin sulfides obtained by using chemical vapor transport methods

Application of the chemical vapor transport method to the Sn–S system allowed three different phases (viz. SnS, SnS2 and Sn2S3) to be synthesized. No evidence of the formation of other, previously reported mixed valence compounds such as Sn3S4 or Sn4S5 was found, whichever the Sn:S atomic ratio and temperature gradient used. Except for SnS2, which was always obtained as a pure phase as a result of starting from the required stoichiometry, a mixed phase was invariably obtained. The XPS spectrum for Sn2S3 was only slightly different from those for SnS and SnS2 in spite of the presence of Sn(II) and Sn(IV) in the former compound, which hindered the identification of mixed valence compounds in the Sn–S system by the use of this spectroscopic technique. By contrast, the chemical shifts, anisotropy and skew parameters for Sn2S3 as obtained by 119Sn NMR were markedly different from those for SnS and SnS2, and reflected the severe distortion of Sn(IV) and, especially, Sn(II) in the former compound relative to the latter two. The time scale for this resonance technique is shorter than the lifetime of the valence states, which allows one to unambiguously distinguish the two oxidation states of Sn.

Oxidation and diffusion processes in nickel-titanium oxide systems

Abstract The oxidation of nickel in Ti/Ni (titanium evaporated on Ni), Ni/TiO2 (nickel evaporated on TiO2) and Ni/TiOx (nickel evaporated on TiO2 previously submitted to Ar+ bombardment) systems has been studied by XPS and ISS. The deposition of Ti on Ni, as followed by ISS occurs in a two-dimensional way up to approximately half a monolayer, when it starts to agglomerate in three-dimensional islands. XPS has shown that while for clean nickel oxidation up to 80% of the topmost layers occurs after exposure to 1800 L of O2 at 573 K, only partial oxidation (i.e. ∽ 31%) takes place when nickel is covered with half a monolayer of Ti. No oxidation occurs for a coverage of 2.5 monolayers (i.e. full coverage). These results show that besides a physical protection by coating, oxidation of nickel can be prevented by a modification of its oxidation pathways due to TiO2 deposits. Oxidation of Ni evaporated on TiO2 and TiOx (x ≈ 1.5) was followed by XPS. In Ni/TiOx, some diffusion of Ni into the defective oxide occurs at 298 K during deposition of the metal, while small differences in the degree of oxidation of nickel with respect to Ni/TiO2 can be ascribed to a Ni2+ + Tin + (n 773 K enhances the diffusion of Ni into the TiOx defective layer as indicated by the loss of the Ni peak in the ISS spectrum and a drastic decrease in the Ni/Ti XPS peak ratio. Under these conditions, exposure to O2 (1800 L) at 298 and 573 K only produces a further decrease in the Ni/Ti peak ratio while TiOx is oxidized at the surface. These results suggest that diffusion of Ni0 in defective TiOx can be a very important process during the oxidation of Ni-Ti systems.

XPS study of the surface carbonation/ hydroxylation state of metal oxides

Abstract The carbonation and hydroxylation state of a series of basic oxides (La 2 O 3 , RhLaO 3 and YBa 2 Cu 3 O 7 ) has been examined by XPS. While hydroxyl groups are characterized by an O(1s) peak at ∼531.4 eV, when carbonate species are present C(1s) and O(1s) peaks at 289.4 and 531.4 eV are recorded, giving an atomic O/C ratio near 3. Heating of samples in UHV up to 973 K leads to removal of most of the carbonate and hydroxyl species, carbonate being much more stable than hydroxyls remaining partially even at the highest temperatures.

Preparation of transparent and conductive Al-doped ZnO thin films by ECR plasma enhanced CVD

The objective of this work is to optimise the deposition conditions of aluminium-doped ZnO by ECR plasma enhanced CVD to produce transparent and conductive thin films. Films were deposited by the reaction of Zn(C 2 H 5 ) 2 and Al(CH 3 ) 3 with pure oxygen plasma or mixtures of oxygen and hydrogen plasmas, obtained by an ECR downstream plasma source operated at low pressure (0.1-1 Pa). Control of the resistivity and UV-Vis transparency of the films was achieved by changing the aluminium dose in the film, the preparation temperature and the composition of the plasma (O 2 :H 2 ratio). Optical constants were determined by transmittance data in the UV-Vis region, all samples presenting very good optical characteristics (96-92% transmittance in visible range). On the other hand, resistivities, as obtained at room temperature by a four-point probe, ranged from > 10 +7 to 6×10 -3 Ω-cm, depending on the preparation conditions. Thus, the lowest resistivities were obtained for samples prepared with a plasma of H 2 +O 2 (2.0:1.5). Composition, microstructure, cristallinity and thickness of the films were characterised by XRF, XPS, RBS and XRD.

Influence of the chemical and electronic structure on the electrical behavior of zirconium oxynitride films

This work is devoted to the investigation of decorative zirconium oxynitride, ZrOxNy, films prepared by dc reactive magnetron sputtering, using a 17:3 nitrogen-to-oxygen-ratio gas mixture. The color of the films changed from metallic-like, very bright yellow pale, and golden yellow, for low gas mixture flows [from 0 to about 9SCCM (SCCM denotes cubic centimeter per minute at STP)] to red brownish for intermediate gas flows (values up to 12SCCM). Associated to this color change there is a significant decrease of brightness. With further increase of the reactive gas flow, the color of the samples changed from red brownish to dark blue (samples prepared with 13 and 14SCCM). The films deposited with gas flows above 14SCCM showed only apparent colorations due to interference effects. This change in optical behavior from opaque to transparent (characteristic of a transition from metallic to insulating-type materials), promoted by the change in gas flow values, revealed that significant changes were occurring in...