XPS Valence Band Spectra Research Articles

AU4 PAN-based Carbon Fiber by Core Level and Valence Band XPS

Hercules AU4 PAN-based carbon fiber surface was analyzed with both core level and valence band XPS. Compared to Du Pont and Amoco's pitch-based carbon fibers with different modulus, the AU4 fiber had more oxidized components on the surface than the pitch-based carbon fibers. [See Y. Xie and P. M. A. Sherwood, Appl. Spectrosc. 43, 1153 (1989); Chem. Mater. 1, 417 (1989); 2, 293 (1990); Appl. Spectrosc. 44, 797 (1990); Chem. Mater. 3, 164 (1991); Appl. Spectrosc. 44, 1621 (1990); 45, 1158 (1991); Y. Xie, T. Wang, O. Franklin, and P. M. A. Sherwood, ibid. 46, 645 (1992).] The AU4 PAN-based fiber had a much less graphitic structure in both the surface and the bulk than the pitch-based fibers as evidenced by both XPS and XRD. It also had a small amount of nitrogen in its structure, but no nitrogen was found on the pitch-based carbon fiber surfaces. Our previously reported work [Y. Xie and P. M. A. Sherwood, Chem. Mater. 1, 427 (1989); 2, 293 (1990); Appl. Spectrosc. 44, 797 (1990); Chem. Mater. 3, 164 (1991); Appl. Spectrosc. 44, 1621 (1990); 45, 1158 (1991); Y. Xie, T. Wang, O. Franklin, and P. M. A. Sherwood, ibid. 46, 645 (1992)], showed that the XPS valence band spectra were more sensitive to chemical environment on the carbon fiber surface than core level spectra and could be well interpreted by X–α calculations with model compounds. In this work, the valence band spectrum showed that there were nitrogen and at least two different types of oxygen species on the AU4 fiber surface.

AS4 PAN-based Carbon Fiber by Core Level and Valence Band XPS

Hercules AS4-6k PAN-based carbon fiber surface was analyzed with both core level and valence band XPS. Compared to Du Pont and Amoco's pitch-based carbon fibers with different modulus and Hercules’ AU4-12k PAN-based fiber, the AS4 fiber had much more oxidized components on the surface [Y. Xie and P. M. Sherwood, Appl. Spectrosc. 43, 1153 (1989); Chem. Mater. 1, 427 (1989); 2, 293 (1990); Appl. Spectrosc. 44, 797 (1990); Chem. Mater. 3, 164 (1991); Appl. Spectrosc. 44, 1621 (1990); 45, 1158 (1991); Y. Xie, T. Wang, O. Franklin, and P. M. Sherwood, ibid. 46, 645 (1992)]. The AS4 PAN-based fiber had a much less graphitic structure in both the surface and the bulk than the pitch-based fibers as evidenced by both XPS and XRD. It also had a higher nitrogen content on its surface than the AU4-12k PAN-based fiber, but no nitrogen was found on any of the pitch-based carbon fibers measured in our laboratory. Our previously reported work [Y. Xie and P. M. Sherwood, Chem. Mater. 1, 427 (1989); 2, 293 (1990); Appl. Spectrosc. 44, 797 (1990); Chem. Mater. 3, 164 (1991); Appl. Spectrosc. 44, 1621 (1990); 45, 1158 (1991); Y. Xie, T. Wang, O. Franklin, and P. M. A. Sherwood, ibid. 46, 645 (1992)] showed that XPS valence band spectra were more sensitive to chemical environment on the carbon fiber surface than core level spectra, and could be well interpreted by X–α calculations with model compounds. In this work, the valence band spectrum showed that there were nitrogen and at least two different types of oxygen species on the AS4 fiber surface.

The Eu/Pd(111) interface: spectroscopic and structural studies

Growth of Eu ultra thin films on a Pd(111) single crystal, kept at room temperature, has been studied by spectroscopic (XPS, UPS, AES) and structural (RHEED) methods. Eu atoms of the first two deposited monolayers are divalent and form a p(2 × 2) arrangement on the Pd(111) surface. During the completion of the third Eu monolayer, this ordered structure smears out in connection with the appearance of trivalent Eu atoms at the interface. The fine structure modifications of Pd and valence band XPS spectra indicate the 4d band filling, via a charge transfer mechanism, of the Pd atoms involved in the interface. For intake Eu coverages a diffusion process leads to formation of disordered divalent EuPd alloys.After annealing the sample at 820 K, RHEED diffraction patterns reappear (p(2 × 2)) and, simultaneously, an increase in the Eu mean valence is observed. XPS results show that segregation of divalent Eu occurs at the surface of the new ordered bulk-trivalent EuPd compound which forms at the interface. This behaviour occurs whatever the thickness of the Eu deposit is.

X-ray photoelectron spectroscopy analysis of photostimulated chemical vapor deposition hydrogenated amorphous silicon/amorphous aluminum oxide

We have fabricated multilayer structures consisting of hydrogenated amorphous silicon (a-Si:H) and amorphous aluminum oxide (a-AlxO1−x) by photostimulated chemical vapor deposition (photo-CVD) using ArF excimer laser. X-ray photoelectron spectroscopy (XPS) analysis of the multilayer samples demonstrated that the a-Si:H/a-AlxO1−x heterostructure grown by the photo-CVD had an ideally sharp interface and no mixing layer. The XPS valence-band spectrum for a-AlxO1−x deposited on a-Si:H also determined the valence-band offset at about 4 eV in the a-Si:H/a-AlxO1−x superlattices.

XPS Spectra and Optical Reflectivity of Yttrium Stabilised Zirconia

AbstractX-ray photoelectron spectroscopy measurements and synchrotron radiation reflectivity and measurements in the photon energy range between 5eV and 30 eV are reported for yttrium stabilised zirconia single crystals. From the present data some behaviour of the valence band and conduction band electronic structure are elucidated. Two features present in the valence band XPS spectra, not explained on the basis of one-clectron theory, are tentatively assigned to simultaneous two electron excitation.

Changes in Surface Structure of Acrylate Resin Substrate by Plasma Exposure.

Surface structures of the acrylate resin substrates with and without plasma exposure were analyzed by SEM, XPS, and static SIMS in order to examine the influence by plasma exposure prior to sputtered film deposition. In general, both the morphology and composition of a resin surface are changed by plasma exposure. However, it was found that the difference in the surface roughness was hardly discernible at the initial stage of the exposure. On the other hand, surface chemical structures such as functional groups are markedly modified. Furthermore, analysis of XPS valence band spectrum based on the molecular orbital calculation has been successfully applied to distinguish the position of cleaved band in the molecule. The results obtained from XPS and static SIMS analyses are described in detail.

The electronic structure of the [CuO5]8? unit: A photoemission study of Y2BaCuO5

A photoemission study of the ‘green phase’ Y2BaCuO5, including core level spectra of the elements, CuL3VV Auger, and XPS and UPS valence band spectra has been performed. The crystal structure of Y2BaCuO5 contains discrete [CuO5]8− units with divalent copper. The surface status of the Y2BaCuO5 samples is discussed with respect to the problems encountered in photoemission studies of the YBaCuO superconductors. The photoemission data are analysed within a simple cluster configuration interaction model by considering in addition previously reported optical spectra. Y2BaCuO5 is a charge transfer insulator with the charge transfer energy Δ≈0.5 eV being much smaller than thed−d Coulomb interaction energyU≈6eV. Cluster model parameters and valence band shapes of Y2BaCuO5 are compared with those of Nd2CuO4 the structure of which contains divalent copper within an extended copper-oxygen network. More detailed cluster and impurity models for cuprates described in literature are discussed in view of the present results.

X-ray photoelectron spectroscopy study of chemically etched Tl 2Ba 2CaCu 2O 8+ x thin film surfaces

X-ray photoelectron spectroscopy (XPS) is used to characterize the surfaces of epitaxial c-axis oriented Tl 2Ba 2CaCu 2 O 8+ x (TBCCO) thin films which have been chemically etched in a solution of Br 2 in absolute ethanol. Comparison of the XPS core level spectra presented here with spectra in the literature obtained from single crystals cleaved in vacuum or polycrystalline pellets fractured or scraped in vacuum shows that the chemically etched thin films exhibit lower levels of nonsuperconducting surface species. Cation disorder is found to be restricted to the Ba sites. XPS valence band spectra show a clear Fermi edge, indicating that the room temperature conductivity near the surface is metallic. To the authors' knowledge, this is the first report of a clear Fermi edge detected on TBCCO. Exposure of the chemically etched films to air results in a significant surface degradation even for exposure times as short as 1 min.

Electronic structure investigation at a zirconia-nickel interface

X-ray photoelectron spectroscopy (XPS) is used to study electronic structure effects at a zirconia-nickel interface. A method is described to fabricate ZrO 2 coatings on metallic substrates under ultrahigh vacuum conditions with a sufficiently low coverage to allow examination of the interfacial region. Charging effects which relate to the size of oxide particles deposited on the metal (and not to accumulated charge on the oxide) have been identified for small ZrO 2 particles on nickel, palladium and NiO substrates equivalent to a coating approximately 6A˚thick. The charging effects show up as a broadening of the XPS Zr 3d core-level spectrum. By examining the XPS Ni 2p 3/2 and XPS valence band spectra it is found that an oxidized nickel surface layer is reduced during ZrO 2 deposition.

Study of chemically-etched Bi 2Sr 2−xCa 1+xCu 2O 8+δ surfaces

X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM) are used to characterize the surfaces of polycrystalline Bi 2Sr 2− x Ca 1+ x Cu 2O 8+δ thin films which have been chemically etched in a solution of Br 2 in absolute ethanol. XPS measurements from individual grains (∼1 mm 2) are obtained, and the spectra presented here are comparable to spectra in the literature obtained from single crystals cleaved in vacuum. XPS valence band spectra show a clear Fermi edge, indicating that the room temperature conductivity near the surface is metallic. This is the first report of a detectable Fermi edge from a high temperature superconductor surface which has been treated in vacuum.

Expanded lattice ruthenium pyrochlore oxide catalysts II. Catalyst surface investigations by electron microscopy, X-ray photoelectron spectroscopy, and temperature-programmed reduction and oxidation

Five expanded lattice ruthenium pyrochlore oxide powders with the general formula A 2+ x Ru 2− x O 7− y ( A = Pb: x = 0.06, 0.15, 0.62; y = 0.5 and A = Bi: x = 0.39, 0.86; 0 < y ≤ 0.5) were investigated using high-resolution electron microscopy (HREM), X-ray photoelectron spectroscopy (XPS), and temperature-programmed reduction/oxidation (TPR/TPO) to ascertain factors that contribute to their low-temperature catalytic oxidation activity toward 1,2-diols and alcohols in aqueous alkaline solution. HREM data find small crystallites that vary from about 25 to 200 Å in diameter. Image analysis techniques applied to one 100-Å crystallite of Pb 2.62Ru 1.38O 6.5 reveal a 5% range of lattice spacings about the exterior portions of the particle. XPS data were collected in both the valence-band and core-level regions. XPS valence-band spectra for samples with higher levels ( x > 0.15) of Ru-site substitution by the A-site atoms display a less intense band near the Fermi energy level indicating a reduced Ru 4 d character compared to the more stoichiometric analogues. Core level XPS bands contain contributions from two different valence states for each of the Pb, Bi, and Ru atoms in the A 2+ x Ru 2− x O 7− y series. XPS-derived compositions show a higher A/Ru ratio for Pb 2.62Ru 1.38O 6.5 than the bulk whereas the other four oxides have A/Ru ratios that are similar in the surface (XPS) and bulk (XRD) compositions. TPR using H 2 of the A 2+ xRu 2− xO 7− y oxides reveals remarkable reactivity below 200°C that is in line with loosely bound oxygen in these oxides. A sample of Pb 2.62Ru 1.38O 6.5 with a lead-rich surface shows markedly less reactivity toward H 2 below 100°C than the other oxides. No distinct differences are seen in TPR data for all five expanded lattice ruthenium pyrochlore oxides between the surface and bulk oxygen. Reduced oxides reoxidize reversibly with oxygen after up to 20 mol% reduction (based on oxygen content) but at rates that are slow compared to H 2 reduction. The lack of a correlation between the TPR/TPO data and the liquid-phase catalytic activities toward 1,2-diols and alcohols is explained through differences in the active site preferences by the various substrates.

XPS Study on the Electronic Structure of Bi2Sr2Ca1-xNdxCu2Oy

XPS core level and valence band spectra of sintered Bi2Sr2Ca1-xNdxCu2Oy which manifests a superconducting to semiconducting phase change around x=0.5 have been measured. The fraction of Cu1+ to the total Cu ions evaluated from Cu2p spectra amounts to 7% in Bi2Sr2CaCu2Oy, but decreases to 0% with increasing Nd content. From Ca2p, Sr3d, Nd3d, Bi4f and O1s spectra, it is inferred that Ca2+ and Sr2+ are mixed and distributed over both Ca and SrO layers, whereas Nd3+ and Sr2+ are rather separate. From the valence band spectra, a finite density of states at EF is deduced for Ca-rich oxides, but it diminishes in Bi2Sr2NdCu2Oy. The analysis of ionic configurations based on the mixed valencies of copper and oxygen ions gives a consistent account of the above spectra and the superconductor to semiconductor transition.

X-Ray Photoelectron-Spectroscopic Studies of Carbon Fiber Surfaces. Part XII: The Effect of Microwave Plasma Treatment on Pitch-Based Carbon Fiber Surfaces

X-ray photoelectron spectroscopy has been used to monitor the surface chemical changes on DuPont high-modulus pitch-based carbon fiber surfaces brought about by microwave air-plasma treatments for different periods of time. The air-plasma treatment increased the fiber surface functionalities, but extended treatment caused fiber damage. X-ray diffraction studies showed that the bulk structure was not affected by the air-plasma treatment. XPS valence band spectra proved a more sensitive probe of the surface chemistry of the carbon fibers than XPS core studies. Air-plasma treatment of fibers promises to be an effective method of fiber treatment.

Vacancy-induced modifications to the local densities of states of VNx

The valence states of vanadium nitride have been studied by X-ray emission spectroscopy (XES) and X-ray photoelectron spectroscopy (XPS) as a function of the nitrogen vacancy concentration. Vacancy induced states are observed in the V K emission band (and in the XPS valence band spectrum) ~2eV below the Fermi energy suggesting that the V p states are strongly affected by the presence of nitrogen vacancies. This is confirmed by simulations of XES and XPS spectra from Korringa-Kohn-Rostoker coherent potential approximation densities of states.

XPS studies on the electronic structures of sputter-deposited and hydrogenated Ti-Pd alloy films

XPS Ti2p, Pd3d and valence band spectra of metastable crystalline and amorphous Ti 1−xPd x alloy films are measured. Effects of hydrogen charging are also examined. The spectral characteristics are related to the local densities of states at Ti and Pd sites at the Fermi level and to preferential TiH chemical bonding.

Spectroscopic study of potential magneto-optic storage layers

We present an experimental investigation of magneto-optic (MO) thin film materials of interest in optical storage. XPS valence band spectra of ultrahigh vacuum deposited rare earth — Fe50Co50 alloys are correlated to magneto-optic and reflectivity spectra measured ex-situ in the photon energy range 0.5 ≦E ph ≦ 5.5 eV. It is found that 4f n →4f n−15d 1 optical transitions contribute to room temperature Kerr spectra in Pr-FeCo and Nd-FeCo, but only in the UV spectral region (λ≦300 nm). Eu is found to enter Eu-FeCo intermetallic alloys with a 4f 7 ground state at −2 eV binding energy. However, no substantial contribution to the Kerr effect from excitations of this electronic state has been observed n either at room temperaturen or at 100 K. We present a general argument that 4f excitations of RE ions in typical intermetallic alloys can never be expected to contribute more than some tenths of a degree toθ k in contrast to the well known huge rotations in RE-chalcogenides. For Yb-FeCo we have observed in XPS a mixed valence behavior of Yb showing a discontinuous transition from divalent Yb2+ in the pure metal to mixed valent Yb2.55+ in amorphous Yb-FeCo films. Nof→d contributions to the room temperature MO spectra have been detected. Strong optical enhancement effects are presented for metallic bilayer and multilayer structures of the type MO-layer/reflector. The total thickness of an optimized structure corresponds to about two times the penetration depth of light. Enhancement occurs predominantly at photon energies below the plasma frequency of the reflector material and is caused by low values of the optical constantsn andk of the reflector. In storage materials like Tb-FeCo the read-out performance can be improved by about a factor of 2 with a bilayer of 30 nm Tb-FeCo on Cu. Quantitative modelling of the optical and magneto-optic properties of these films is discussed.

Photoemission and electronic structure of FeOOH: Distinguishing between oxide and oxyhydroxide.

Valence-band and core x-ray-photoemission spectroscopy has been used to examine the electronic structure of FeOOH and ${\mathrm{Fe}}_{2}$${\mathrm{O}}_{3}$. The known small difference in the core-level Fe 2p spectra of the two compounds is discussed. X\ensuremath{\alpha} calculations of an ${\mathrm{FeO}}_{6}$${\mathrm{H}}_{3}^{3\mathrm{\ensuremath{-}}}$ cluster are shown to give good agreement with the two-peak FeOOH valence-band XPS spectrum, and it is clear that FeOOH and ${\mathrm{Fe}}_{2}$${\mathrm{O}}_{3}$ can be clearly distinguished in the valence-band region. X\ensuremath{\alpha} calculations of an ${\mathrm{FeO}}_{6}^{9\mathrm{\ensuremath{-}}}$ cluster are found to predict the three-peak structure of the ${\mathrm{Fe}}_{2}$${\mathrm{O}}_{3}$ valence band. The O 2s region of FeOOH is found to be more sensitive to chemical environment than the O 1s region, and the former region is well explained by the X\ensuremath{\alpha} calculation. Little decomposition of FeOOH is found during data collection.

X‐RAY PHOTOELECTRON VALENCE BAND STUDIES ON FIREFLY D‐(‐)‐LUCIFERIN

Abstract— The valence band spectrum of firefly D‐(‐)‐luciferin (Ln) was measured for the first time by x‐ray photoelectron spectroscopy (XPS). Since the conversion of Ln to decarboxydehydroluciferin (dcrL) by x‐ray irradiation occurred even within ∼30 min, the effect of the irradiation on the valence band spectrum of Ln was investigated quantitatively. We found that the XPS valence band spectrum of Ln during irradiation for 3 h corresponds to the state density of Ln calculated by the LCAO‐ASMO‐SCF‐CI method for π‐electrons. Moreover, the weak coupling model for π‐electron transfer between Ln and oxygen was proposed for the primary electronic process in Ln chemiluminescence, assuming that the calculated π‐electron state densities of Ln in acidic and basic solutions reflect the characteristics of their XPS valence band spectra.

Electronic structure of superconducting MoN thin films produced by ion implantation

The electronic structure of Mo thin films implanted with nitrogen ions was investigated by X-ray photoelectron spectroscopy (XPS). With increasing nitrogen dose the binding energies of the Mo3p, Mo3 d 5 2 and N1s levels shifted in a significant manner. This indicated that an electronic charge transfer occurred from Mo atoms to N atoms. The shifts in binding energy of the core levels reached a maximum value of ΔE = ± 0.5 eV for a critical dose of 5 × 10 17 ions/cm 2. The observed changes were due to the formation of a B1-type MoN with the maximum superconducting transition temperature of ∼ 7 K in the present MoN thin films. The XPS valence-band spectrum consisted primarily of three peaks which were centered around 16, 7 and 2 eV, respectively, below the Fermi level. The energy positions of these three peaks were in good agreement with theoretical results. However, the relative intensity of the three peaks was found to differ considerably from those calculated for an ideal B1-MoN compound. This suggested that electronic structure of the implanted MoN compound differed somewhat from that of an ideal B1-MoN.

The electronic structure of Co silicides and Co germanides

Polycrystalline Co silicides and Co germanides of various stoichiometric compositions have been examined by ECSA experiments. From core level shifts and XPS valence band spectra it has been concluded that the bonding is similar in the silicides and germanides and can be expressed in terms of hybridisation, and that the chemical bonding is stronger in the silicides than in the germanides. Furthermore, the authors report on samples of Si and Ge which have been implanted with Co at an energy of 80 keV. The valence band spectra showed that in these samples the bonding of the Co atoms is similar to CoSi2 and CoGe2, respectively. Experimentally determined depth profiles of these samples are comparable with Monte Carlo calculations.