Etude par spectroscopie de photoelectrons des interactions metal—support dans des catalyseurs a l'iridium depose sur des oxydes metalliques

Several iridium supported catalyst were studied by means of x-ray photoelectron spectroscopy. These catalysts contain 5 wt. % iridium impregnated from aqueous solution of hexachloroiridic acid on δ-alumina, silica, zinc oxide, titanium oxide and Ketjen silica alumina and are used in catalytic oxidation of olefins.Photoelectron spectra of oxides, impregnated oxides and hydrogen reduced catalysts have been recorded. Chemical shifts observed on impregnated oxides indicate that alumina and titanium oxide make possible the reduction of some of the hexachloroiridates ions into an iridium(III) chloro complex. This reaction is not detected with zinc oxide and silica.For the reduced catalysts, the iridium 4f doublet is shifted to higher binding energies with respect to that of unsupported iridium and the iridium lines have an anomalous breadth. We have postulated that an electronic interaction occurs between the support acting as an electron acceptor and the metal acting as an electron donor. The chemical shifts depend on the nature of the support, increasing ZnO < SiO₂ < TiO₂ < Al₂O₃, and may be correlated with the Fermi level in the metallic oxide. With silica-alumina and progressively de-aluminated silica-alumina, a simultaneous variation of the iridium chemical shift and catalytic activity is observed.     

Comparative photoemission study of the electronic properties of L-CVD SnO2 thin films

In this work we present the results of comparative XPS and PYS studies of electronic properties of the space charge layer of the L-CVD SnO₂ thin films after air exposure and subsequent UHV annealing at 400 °C, with a special emphasis on the interface Fermi level position.From the centre of gravity of binding energy of the main XPS Sn 3d_5/2 line the interface Fermi level position E_F − E_v in the band gap has been determined. It was in a good correlation with the value estimated from the offset of valence band region of the XPS spectrum, as well as from the photoemission yield spectroscopy (PYS) measurements. Moreover, from the valence band region of the XPS spectrum and PYS spectrum two different types of filled electronic band gap states of the L-CVD SnO₂ thin films have been derived, located at 6 and 3 eV with respect to the Fermi level.     

Effects of hydrogen treatment on ohmic contacts to p-type GaN films

This study investigated the effects of hydrogen (H₂) treatment on metal contacts to Mg-doped p-GaN films by Hall-effect measurement, current-voltage (I-V) analyzer and X-ray photoemission spectra (XPS). The interfacial oxide layer on the p-GaN surface was found to be the main reason for causing the nonlinear I-V behavior of the untreated p-GaN films. The increased nitrogen vacancy (V_N) density due to increased GaN decomposition rate at high-temperature hydrogen treatment is believed to form high density surface states on the surface of p-GaN films. Compared to untreated p-GaN films, the surface Fermi level determined by the Ga 2p core-level peak on 1000 °C H₂-treated p-GaN films lies about ∼2.1 eV closer to the conduction band edge (i.e., the surface inverted to n-type behavior). The reduction in barrier height due to the high surface state density pinned the surface Fermi level close to the conduction band edge, and allowed the electrons to easily flow over the barrier from the metal into the p-GaN films. Thus, a good ohmic contact was achieved on the p-GaN films by the surface inversion method.     

Improved electrical properties of silicon-incorporated anodic niobium oxide formed on porous Nb-Si substrate

In the present study, porous Nb-Si alloy films with isolated nano-column morphology have been successfully developed by oblique angle magnetron sputtering on to aluminum substrate with concave cell structure. The deposited films are amorphous with the 15 at% silicon supersaturated into niobium. The porous Nb-15 at% Si films, as well as niobium films with similar morphology, are anodized at several voltages up to 50 V in 0.1 mol dm⁻³ ammonium pentaborate electrolyte. Due to the presence of sufficient gaps between neighboring columns, the gaps are not filled with anodic oxide, despite the large Pilling-Bedworth ratio (for instance, 2.6 for Nb/Nb₂O₅) and hence, a linear correlation between the reciprocal of capacitance and formation voltage is obtained for the Nb-15 at% Si. From the comparison with the anodic films formed on porous niobium films, it has been found that silicon addition improves the thermal stability of anodic niobium oxide; the change in capacitance and increase in leakage current become small for the Nb-Si. The findings indicate the potential of oblique angle deposition to tailor porous non-equilibrium niobium alloy films for high performance niobium-base capacitor.     

Electrosynthesis of TiO2 oxide film on ITO substrate and electrochemical comparative study of the oxide with its hydrated gel

Thin TiO₂ films obtained by cathodic electrosynthesis from an acidic aqueous bath containing TiOSO₄, H₂O₂ and KNO₃ on conductive glass indium tin oxide have been physically and electrochemically characterised. Secondary ion mass spectroscopy profile of the crystallised gel after heat treatment at 400 °C shows the presence of TiO₂ with traces of TiO and oxygen. X-ray patterns confirm the presence of anatase nanocrystallites for the annealed film and an amorphous structure for the non-annealed gel. Scattering electron microscopy surface micrographies reveal an opened porous nanostructure of the deposits. Cyclic voltammetry and impedance spectroscopic measurements reveal the different behaviour of the films obtained before and after the annealing, showing an important electrical activity of the non-annealed films. The dependence of capacitance values with potential in the anodic domain of depletion is obviously remarked from impedance plots for both gel and crystal forms of the film, which confirmed the fact that films obtained in this way have n-type properties. The potential of flat band equals −0.6 V/Ag/AgCl in pH range of 6.5 has been estimated according to Mott–Shottky curves for the crystallised oxide; meanwhile, the Mott–Shottky curve for the hydrated gel was nonlinear.     

Electronic structure of the conduction band of the interface region of ultrathin films of substituted perylenedicarboximides and the germanium oxide surface

The results of the investigation of the electronic structure of the conduction band and the interfacial potential barrier during the formation of interfaces of dioctyl-substituted perylenedicarboximide (PTCDI-C₈) and diphenyl-substituted perylenedicarboximide (PTCDI-Ph) ultrathin films with the oxidized germanium surface have been presented. The experimental results have been obtained using the very low energy electron diffraction (VLEED) technique in the total current spectroscopy (TCS) mode at energies in the range from 5 to 20 eV above the Fermi level E_F. The positions of the maxima of the fine structure of total current spectra (FSTCS) of the PTCDI-C₈ and PTCDI-Ph films differ significantly in the energy range from 9 to 20 eV above the Fermi level E_F, which can be associated with the difference between the substituents of the chosen molecules, dioctyl- and diphenyl-, respectively. At the same time, the positions of the lowenergy maxima in the FSTCS spectra at an energy 6–7 eV above the Fermi level E_F for the PTCDI-C₈ and PTCDI-Ph films almost coincide with each other. It has been suggested that these maxima are attributed to the electronic states of the perylene core of the molecules under investigation. The process of the formation of interfacial potential barriers of the PTCDI-C₈ and PTCDI-Ph films with the oxidized germanium surface has been analyzed. It has been found that the work functions of the surface, E_vac–E_F, differ little from 4.6 ± 0.1 eV over the entire range of organic coating thicknesses from 0 to 6 nm.     

The interaction of oxygen molecules with iron films studied with MIES,UPS and XPS

X-ray Photoelectron Spectroscopy (XPS), Metastable Induced Electron Spectroscopy (MIES) and Ultraviolet Photoelectron Spectroscopy (UPS) were applied to study the interaction of oxygen molecules with iron films. Supplementarily, iron oxide was investigated for comparison.With XPS from the Fe 2p_3/2 range contributions of metallic Fe as well as Fe²⁺ and Fe³⁺ can be distinguished. During the interaction with oxygen an oxide film is formed on the iron surface. Nevertheless, XPS still shows metallic contributions even for a surface which is saturated with more than 10⁴ L. The oxide film hinders the dissociation of further impinging oxygen molecules.The interaction of He* atoms with iron oxide surfaces during MIES is dominated by Auger Neutralization. This surprising result follows from the high work function and the fact that intrinsic defects result in a Fermi level pinning to the conduction band.     

紫外波段高透过率铜铟掺杂SnO2薄膜的研究

采用反应蒸发的方法,在玻璃、Corning7059玻璃及石英玻璃衬底上制备了SnO₂:(Cu,In)透明导电薄膜,对薄膜的各种元素的含量做了分析, 给出了各种元素在膜中的分布情况;测量了薄膜的透过率,结果显示个别样品对紫外线有较高透过率,退火过程对透过率有影响.测量了电阻率与温度的关系,同时解释了样品的阻-温特性.对材料的光学带隙与吸收系数的关系做了讨论,给出了用透过率曲线确定光学带隙的简易方法.讨论了扩展态迁移率与迁移率边和费米能级之间的关系.结果显示,用铜、铟掺杂的氧化锡透明导电 关键词： 金属氧化物薄膜 电阻率 光电特性     

Deposition of sputtered iridium oxide—Influence of oxygen flow in the reactor on the film properties

Thin films of iridium oxide have been deposited by reactive magnetron sputtering. The influence of oxygen partial pressure in the sputtering plasma on the composition, surface structure and morphology of the films has been studied by XRD, SEM and AFM analysis. An optimal combination of sputtering parameters yields stable microporous amorphous films with highly extended fractal surface. The electrochemical properties of these films have been investigated in view of their application as catalysts for water splitting, using the electrochemical techniques of cyclovoltammetry, electrochemical impedance spectroscopy, and steady state polarization. The SIROFs have shown an excellent electrochemical reversibility and a high catalytic activity toward the oxygen evolution reaction in 0.5 M H₂SO₄. A current density of 150 mA cm⁻² at potential of 1.7 V (versus Ag/AgCl) has been obtained at catalyst load of only 100 μg cm⁻². These results combined with the established long-term mechanical stability of the sputtered iridium oxide films (SIROFs) proved the advantages of the reactive magnetron sputtering as simple and reliable method for preparation of catalysts with precisely controlled composition, loading, and surface characteristics.     

Oxygen dependence of the fermi-level and electron-phonon coupling constant in YBa2Cu3Ox films

We present a detailed investigation of the dynamics of laser-excited carriers in YBa₂Cu₃O_x thin films with various oxygen contents x. We observe a Fermi level shift due to oxygen reduction and determine the energy gap between the Fermi level and the upper Hubbard band. The electron-phonon coupling constant is determined as a function of transition temperature.     

The electronic structure of 1,2-PCB10H11 molecular films: a precursor to a novel semiconductor

The band gaps and electronic structure of undoped films of molecular icosahedra of closo-1-phospha-2-carbadodecaborane (1,2-PCB₁₀H₁₁) are reported. 1,2-PCB₁₀H₁₁ adsorbs on Au and Ag substrates to generate molecular thin films with the Fermi level (chemical potential) placed closer to the lowest unoccupied molecular orbital than has been observed with closo-1,2-dicarbadodecaborane (1,2-C₂B₁₀H₁₂, orthocarborane) adsorbed on Co, Cu or Ag. Both 1,2-PCB₁₀H₁₁ and 1,2-C₂B₁₀H₁₂ molecular films exhibit an unoccupied molecular defect state above the Fermi level. The vibrational modes, observed in infra-red absorption, are close to the values expected for the isolated 1,2-PCB₁₀H₁₁ molecule. Consistent with the placement of the Fermi level in the molecular films, fabrication of heterojunction diodes from partially dehydrogenated 1,2-PCB₁₀H₁₁ indicates that the resultant PCB₁₀H_x semiconductor film is more n-type than the corresponding boron carbide semiconductor formed from 1,2-C₂B₁₀H₁₂, orthocarborane. PACS 68.43.-h; 73.20.Hb; 31.15.Ct; 79.60.-i     

Evolution of electronic states in thin Ca(001) flms in strong electrostatic fields

The transformation of electronic states in Ca(001) films in strong electrostatic fields is studied using electron density functional theory. It is shown that an excess film charge of either sign pins the Fermi level (with respect to the conduction band edge) in a wide range of fields. For positively charged films, the change in the density of states at the Fermi level is small but the energy derivative of the density of states changes sign with increasing excess charge of the film. For negatively charged Ca(001) films, the change in the density of states at the Fermi level plays the main role in stabilizing the width of the occupied part of the conduction band; this should be manifested in the electronic thermodynamic and transport properties of negatively charged Ca(001) films with quantum confinement.     

Surface effects on the magnetic properties of Co2FeAl(0 0 1): An ab initio study

Electronic structures of the Co₂FeAl(0 0 1) surface are studied theoretically via first-principles calculations based on density functional theory. It is found that the minority spin band gap at the Fermi level in bulk Co₂FeAl disappears at the surface due to space localization of the states. However, beneath the surface, the density of states of individual atoms shows a trend of minority spin gap opening at the Fermi level, which indicates that the electronic structures become close to that of bulk Co₂FeAl. The termination of FeAl is more favorable for spin polarization of Co₂FeAl films than that of Co. Accordingly, we present a composite tri-layer model to illustrate the fading of the half-metallic property in Co₂FeAl films against the ideal character in bulk materials.     

Electronic structure and magnetism of FeGe2

The electronic band structure of FeGe₂ has been calculated using the self-consistent full potential non-orthogonal local orbital minimum basis scheme based on the density functional theory. In the band structure of FeSn₂, Fe 3d and Sn 5p states play important roles near the Fermi level. Our calculations show that large enhancement of the static susceptibility over its non-interacting value is found due to a peak in the density of states at the Fermi level.     

Effects of H2 ambient annealing in fully 0 0 2-textured ZnO:Ga thin films grown on glass substrates using RF magnetron co-sputter deposition

Gallium doped zinc oxide (ZnO:Ga) thin films were grown on glass substrates using RF magnetron co-sputtering, followed by H₂ ambient annealing at 623 K to explore a possibility of steady and low-cost process for fabricating transparent electrodes. While it was observed that the ZnO:Ga thin films were densely packed c-axis oriented self-textured structures, in the as-deposited state, the films contained Ga₂O₃ and ZnGa₂O₄ which had adverse effect on the electrical properties. On the other hand, post-annealing in H₂ ambient improved the electrical properties significantly via reduction of Ga₂O₃ and ZnGa₂O₄ to release elemental Ga which subsequently acted as substitutional dopant increasing the carrier concentration by two orders of magnitude. Transmittance of the ZnO:Ga thin films were all over 90% that of glass while the optical band gap varied in accordance with the carrier concentrations due to changes in Fermi level. Experimental observation in this study suggests that transparent conductive oxide (TCO) films based on Ga doped ZnO with good electrical and optical properties can be realized via simple low-cost process.     

Surface segregation in Nb-doped BaTiO3 films

We have used in situ photoemission spectroscopy to investigate Niobium doping in polycristalline BaTiO₃. The valence band maximum position progressively shifts from 2.5 eV for undoped to 2.84 eV for Nb-doped films. Ceramics and single crystal have been investigated for comparison with thin films. Nb-doped BaTiO₃ ceramics and Nb-doped SrTiO₃ single crystal show higher Fermi level position indicating that our doped films are less conducting regarding their bulk parents. This was confirmed by impedance spectroscopy under variable temperature. Large amount of niobium is clearly observable at surface but the amount of dopant is drastically reduced below the near-surface region, as evidenced by depth profile. Therefore, we provide evidence of surface segregation which would explain the contrasted resistivity values reported in literature for such donor-doped films.     

Gd-doping of HfO2

An increase in the density of states between the oxygen 2p bands and the Fermi level is seen with increasing Gd concentrations. In addition, for the Gd-doped HfO₂ films, the Gd 4f photoexcitation peak at 5.5 eV below the valence band maximum was identified using resonant photoemission. Electrical measurements show pronounced rectification properties for lightly-doped Gd:HfO₂ films on p-Si and for heavily-doped Gd:HfO₂ films on n-Si, suggesting a crossover from n-type to p-type behavior with increasing doping level. In addition, there is an increase in the reverse bias current with neutron irradiation.     

Electrophysical characteristics of epitaxial cadmium telluride films

Equilibrium electrophysical characteristics of epitaxial CdTe films grown by the thermal screen method and the quasiclosed volume method are investigated. It is established that the films in both cases are semiconductors with inhomogeneous potential relief of the band to whose formation the main contribution is from barriers on the boundaries of the growth patterns. It is shown that jump conductivity with a variable jump length due to inhomogeneity of the potential relief of the bands and the high states density at the Fermi level is observed in CdTe films synthesized by the quasiclosed volume method for relatively high temperatures.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 3, pp. 72–76, March, 1990.     

W掺杂ZnO透明导电薄膜的理论及实验研究

本文从理论与实验两方面入手, 对高价态差金属W掺杂ZnO (WZO) 薄膜材料的特性进行分析讨论. 采用基于密度泛函理论的平面波赝势方法对WZO材料特性进行理论分析, 计算结果表明: W以替位形式掺入ZnO六角纤锌矿晶格结构中, 由于W-O键的键长较长引起晶格常数增加, 产生晶格畸变; 掺杂后费米能级进入导带, 其附近的导电电子主要由W 5d, O 2p及Zn 3d电子轨道提供, 材料表现出n型半导体的特性; 同时能带简并效应使其光学带隙展宽. 为进一步验证该理论分析结果的适用性, 本文采用脉冲直流磁控溅射技术进行了本征ZnO及WZO薄膜的实验研究, 结果表明: W掺入未改变ZnO的生长方式, 但引起薄膜的晶格常数增加, 电阻率由本征ZnO的1.35× 10^-2 Ω·cm减小到1.55× 10^-3 Ω·cm, 光学带隙由3.27 eV展宽到3.48 eV. 制备的WZO薄膜在400-1100 nm的平均透过率大于83%. 实验结果对理论计算结果进行了验证, 表明WZO薄膜作为透明导电薄膜的应用潜力.     

Luminescence properties of anodic aluminum oxide films with organic luminophores embedded into pores

Luminescence properties of porous anodic aluminum oxide films formed in a 0.6 M solution of citric acid and luminescence of paraterphenyl, perylene, coumarin 7, and rhodamine 6G dyes adsorbed by the films are investigated. The nature of emitting centers in anodic aluminum oxide is revealed. Intense photoluminescence of all tested dyes embedded into pores of anodic aluminum oxide has been found. A redshift of fluorescence spectra of dyes adsorbed by the matrix and emergence of an additional longwave band have been detected. Data obtained can be used in developing new thin-film luminescent coatings for future applications in optoelectronics and molecular electronics. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 64, No 4, pp. 483–488, July–August, 1997.