Core Photoelectron Research Articles

Synthesis and X-ray study of ammonium polyvanadomolybdate xerogels

, and Mo powder byusing solŒgel technology. Synthesized gels were investigated by X-ray photoelectron spectroscopy methods. The shape andposition of the complex core photoelectron lines V 2p, O 1s, C 1s, Mo 3p, Mo 3d, and N 1s were analysed for the gels withdifferent x values. The valences of the V and Mo ions, which have an important role in the electrical conductivity of thecompounds, was studied. It has been found that the molybdenum is in the stable Mo

Monitoring in situ catalytically active states of Ru catalysts for different methanol oxidation pathways

One of the prerequisites for the detailed understanding of heterogeneous catalysis is the identification of the dynamic response of the catalyst surface under variable reaction conditions. The present study of methanol oxidation on different model Ru pre-catalysts, performed approaching the realistic catalytic reaction conditions, provides direct evidence of the significant effect of reactants' chemical potentials and temperature on the catalyst surface composition and the corresponding catalytic activity and selectivity. The experiments were carried out for three regimes of oxygen potentials in the 10(-1) mbar pressure range, combining in situ analysis of the catalyst surface by synchrotron-based photoelectron core level spectroscopy with simultaneous monitoring of the products released in the gas phase by mass spectroscopy. Metallic Ru with adsorbed oxygen and transient 'surface oxide', RuO(x), with varying x have been identified as the catalytically active states under specific reaction conditions, favouring partial or full oxidation pathways. It has been shown that the composition of catalytically active steady states, exhibiting different activity and selectivity, evolves under the reaction conditions, independent of the crystallographic orientation and the initial pre-catalyst chemical state, metallic Ru or RuO(2).

Reactions of sulfur dioxide on calcium carbonate single crystal and particle surfaces at the adsorbed water carbonate interface

Sulfur dioxide reactions with calcium carbonate interfaces at 296 K in the presence and absence of adsorbed water result in the formation of adsorbed sulfite and sulfate. The extent of reaction is significantly enhanced, approximately five- to ten-fold for particulate and single crystal CaCO(3) (calcite), respectively, in the presence of adsorbed water between 30 and 85% RH. Atomic force microscopy following the reaction shows that adsorbed water facilitates surface reactivity by enhancing the mobility of surface ions, giving rise to the formation of nanometer sized product crystallites approximately 1 nm in height. Simultaneous with the formation of these crystallites is pitting and etching of the underlying substrate, which occurs preferentially in the vicinity of monoatomic surface steps. In the absence of water, there is little pitting and no evidence for the formation of crystallites. X-Ray photoelectron core and valence band spectra confirm the presence of two sulfur adsorbed species, SO and SO, with nearly equal amounts of SO and SO in the absence of adsorbed water and approximately five times more SO relative to SO in the presence of adsorbed water. From these data, it is proposed that the nanometer-sized crystallites are composed primarily of CaSO(3).

Intrinsic and extrinsic excitations in deep core photoelectron spectra of solid Ge

AbstractIn the quantitative analysis of buried interfaces or the exploration of local electronic structures around component atoms in bulk solids, spectroscopy of transitions in the deep atomic core has an increasingly significant role to play. A key issue in understanding the deep core photoelectron spectra is the identification and separation of the contributions that are attributable to different excitations leading to loss of energy of the emitted photoelectrons. In this work, Ge 1s and 2s photoelectron spectra are analyzed using various models to interpret the measured spectral shapes. A detailed comparison of the results derived from the different approaches is presented. Copyright © 2006 John Wiley & Sons, Ltd.

Theoretical study on the circular dichroism in core and valence photoelectron angular distributions of camphor enantiomers

In the present work the photoelectron circular dichroism of camphor has been theoretically studied using B-spline and continuum multiple scattering-Xalpha methods, and comparisons are made with available experimental data. In general, rather large dichroism effects have been found for both valence and core (O 1s, C 1s) photoionizations. The agreement between the two calculations reported here and previous experimental measurements for core C 1s data is essentially quantitative. For valence ionization satisfactory agreement between theory and experiment has been obtained and the discrepancies have been attributed to both exchange-correlation potential limitations and the absence of response effects in the adopted formalism. The calculations predict, moreover, important features in the cross-section profiles, which have been discussed in terms of dipole-prepared continuum orbitals.

The influence of lysine on InP(001) surface ordering and nanowire growth

We report on high resolution photoelectron core level spectroscopy (HRCLS)and scanning tunnelling microscopy (STM) measurements of the decompositionof lysine adsorbed on InP(001) substrates. We find that components fromthe lysine can be present on the InP surface even after annealing to600 °C and desorption of the native surface oxide. We further observe that while a crystalline surfacephase can be observed on the epi-ready surface after annealing, the lysine treated surface stillappears rough. We conclude that lysine residues inhibit the formation of a flat crystallinestructure on the InP(001) surface. These results are discussed in terms of the lysine promotion of[001] nanowire growth.

Oxygen defects and Fermi level location in metal-hafnium oxide-silicon structures

We describe an in situ method for measuring the band bending of Si substrates in complex metal-oxide-semiconductor systems using femtosecond pump-probe photoelectron spectroscopy. Following deposition of metal layers (Pt, Re, or Re oxide) on the high-k dielectric HfO2, measurement of the band bending in the underlying Si provides a direct determination of the location of the Fermi level within the Si band gap at the Si-dielectric interface. Changes in the Fermi level with post-deposition anneals and oxygen exposure were correlated with valence and core photoelectron spectroscopy as well as capacitance-voltage measurements. These studies illuminate the roles that gate metal work function, modified by metal induced gap states and defects within the oxide, such as oxygen vacancies, play in defining the location of the Fermi level in metal-oxide-semiconductor structures.

Shakedown in core photoelectron spectra from aligned laser-excited Na atoms

In an experiment combining a state-of-the-art visible laser system with the vacuum ultraviolet beamline I411 at the storage ring MAX II, shakedown satellite lines in the 2p photoelectron emission of excited Na atoms have been observed. A close investigation of these lines shows a strong dependency of the line intensities from the linear polarization angle of the laser light. The shakedown electrons are preferably emitted into the direction of the laser electric field vector.

Hard X-ray photoelectron spectroscopy: sensitivity to depth, chemistry and orbital character

Photoelectron spectroscopy is growing in importance as a tool for characterizing not only the surface but also the bulk of solids. Photon fluxes of modern synchrotron radiation sources compensate the cross-section lowering of photoelectrons with kinetic energy in the hard X-ray range, the only electrons able to escape from bulk regions. We present examples of photoelectron emission experiments where we measured core and valence photoelectrons up to ∼ 5 keV kinetic energy. We studied the evolution of photoelectron emission spectra collected at increasing incident photon energy: core-level spectra of Samarium and its compounds indicate increased bulk-sensitivity, valence band spectra reveal significant cross-section changes of electrons with different orbital character. The chemical and depth sensitivity given by the energy dependence of the attenuation length has been quantitatively assessed in the case of layered GaAs/AlAs/GaAs. These are examples of the wide scientific importance that hard X-ray photoelectron spectroscopy is acquiring in many aspects of the study of solids.

A theoretical study of the role of the hydrogen bond on core ionization of the water dimer

Motivated by the interest in using X-ray spectra for probing of hydrogen bonded networks we developed a quantum model for simulations of the electronic-vibrational profile of the X-ray core photoelectron spectrum of the water dimer. It is found that the potential surfaces of the donor and acceptor O1s core-ionized states of this system display a qualitative difference. Large gradients of the potential in the core ionized state along some intermolecular coordinates combined with small vibrational frequencies breaks down completely the harmonic approximation. The band profiles are therefore treated using a quasi-continuum approximation. The weak hydrogen bonding and the drastic change of water dimer potential under core ionization is responsible for the anomalously strong vibrational broadening: 0.4 eV for the acceptor band and 0.6 eV for the donor band. The core ionization of the donor oxygen is accompanied by proton transfer which should be observable in X-ray fluorescence or Auger spectra.

Studies of Carbon Nanotubes and Fluorinated Nanotubes by X-ray and Ultraviolet Photoelectron Spectroscopy

Carbon single-walled nanotubes (SWNTs) and fluorinated SWNTs (F-SWNTs) were investigated by core and valence band X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS). The core spectra were interpreted by chemical shift calculations based upon CNDO calculations, and the valence band spectra were calculated using ab initio molecular orbital calculations. Both calculations used various molecules representing different structural models for SWNTs and F-SWNTs. Core XPS studies of SWNTs showed carbon and small amounts of chemisorbed oxygen, while F-SWNTs showed a C 1s region split into four principal components with peaks shifted by 1.25, 4.00, and 7.20 eV from the principal carbon peak. The valence band showed characteristic features that could be interpreted by calculated spectra. The model for SWNTs consisted of a carbon framework with terminating hydrogen atoms. This study indicates that it is possible to see significant differences in the photoelectron spectra for different nanotube structures, especially in the case of fluorinated nanotubes. The fluorinated nanotubes of this study give the best agreement with the calculated spectra for an armchair arrangement nanotube.

Disorder induced core photoelectron linewidth broadening in AgPd alloys

High resolution core level photoelectron spectra for a series of Ag x Pd 1− x alloys have been measured and reveal a large “disorder broadening” effect. A comparison of measurements of this effect for three alloy systems, Ag x Pd 1− x , Cu x Pd 1− x and Cu x Zn 1− x , with results of ab initio calculations is presented. Poor agreement is obtained and possible reasons for this observation are discussed.

Acid−Base Characterization of Aluminum Oxide Surfaces with XPS

The localized acid−base properties of different, aluminum oxide thin layer surfaces have been evaluated with X-ray photoelectron spectroscopy (XPS). Five types of oxide layers were studied, which were produced by oxidizing aluminum in a vacuum, with an alkaline and acidic pretreatment, and in boiling water. The photoelectron core level binding energies, as measured with XPS, are evaluated for this purpose, while taking into consideration the initial and final state effects. For the structurally comparable oxides, the shifts in the O 1s binding energies are determined by their initial state chemistry. The values of the O 1s binding energy can be directly related to the surface-averaged charge on the O anions. For the Al cations, a correlation between the photoelectron core level binding energy shift and changes in the initial state chemistry was observed, but the Al 2p binding energy shifts were found to be partially due to changes in extra-atomic relaxation. The measured Al 2p binding energies and the binding energies of the resolved OH and O components in the O 1s peak showed that the studied aluminum oxides have OH sites with the same Brönsted/Lewis acid−base properties, O sites with the same Lewis base properties, and Al sites with very similar Lewis acid properties. The pseudoboehmite oxide, obtained by boiling aluminum in water, exhibits more basic O, OH, and Al sites. This oxide deviates structurally from the other oxides studied, resulting in a lower extra-atomic relaxation and Madelung potential contribution to the binding energies.

Room-temperature photoluminescence and electroluminescence properties of sputter-grown gallium nitride doped with europium

We have studied the room-temperature photoluminescence (PL) and electroluminescence (EL) properties of europium (Eu)-doped gallium nitride (GaN) thin films grown by radio frequency planar magnetron cosputter deposition. X-ray photoelectron core level spectra collected for Eu 4d revealed that the Eu ions in the GaN host were mainly in the trivalent state. A series of PL peaks were observed in the region of 530–700 nm, with the most intense peak at 614 nm. They were assigned to the radiative transitions between the 4f6 energy levels of the Eu3+ ion, specifically DJ5→7FJ′ (J=0,1;J′=1,2,3,4). The maximum PL intensity was obtained at a Eu concentration of ∼1.8 at. %. Electroluminescent devices were fabricated with an Al electrode, Eu-doped GaN light-emitting layer, Al2O3–TiO2 dielectric layer over an indium-tin-oxide electrode on a Corning 7059 glass substrate. The EL emission spectra from the fabricated devices were almost identical to the PL spectra. Higher electron energies (higher applied voltages) were needed to excite Eu3+ ground-state electrons into the higher-lying D15 state, which is consistent with the EL excitation mechanism being direct impact by hot electrons.

Interfacial interactions of polymer coatings with oxide-free phosphate films on metal surfaces

A method for producing thin oxide-free phosphate films using an easier process than those previously reported is described. The process is carried out under ambient conditions using aqueous phosphoric acid. The chemistry of the phosphated surface prepared by the process and the interaction of this surface with a thin film of polyvinyl alcohol (PVA) are examined with the purpose of evaluating the potential of this film to serve as an adhesion promoting layer. The surface chemical changes are monitored by core and valence band x-ray photoelectron spectroscopy and ultraviolet photoelectron spectroscopy, with the resultant spectra compared to those from band structure calculations. Valance band photoemission interpreted by band structure calculations can be used to study the interfaces involved, and proves to be a very effective tool for conclusively identifying the surface species present. The process for forming oxide-free phosphate films forms the same surface chemistry as previously reported for other processes. PVA is found to strongly interact with the oxide-free phosphate film on aluminum metal illustrating the potential of such films as adhesion promoters.

Angle-resolved Auger-photoelectron coincidence spectroscopy (AR-APECS) of the Ge(100) surface

We have measured the angular distribution of Ge ${L}_{3}{M}_{45}{M}_{45}$ Auger electrons in coincidence with Ge ${2p}_{3/2}$ core photoelectrons along the (001) azimuth of the Ge(100) surface. Intensity modulations arising from diffraction effects are suppressed in the coincidence Auger angular distribution and, when specific emission angles of the photoelectrons are considered, new features appear. We attribute the former effect to enhanced surface specificity of the coincidence technique and the latter to sensitivity of the coincidence measurement to alignment of the core hole state.

Oxidation state of chromium associated with cell surfaces of Shewanella oneidensis during chromate reduction

Employing electron energy loss spectroscopy (EELS) and X-ray photoelectron spectroscopy (XPS), we demonstrate that in both aerobic and anaerobic culture Shewanella oneidensis cells are capable of chromate reduction. No Cr(VI) or Cr(V) species were identified at the cell surfaces in Cr 2p 3/2 core photoelectron spectra. More chromium was associated with cell surfaces recovered from anaerobic medium than aerobic. Multiplet-splitting models derived for Cr(III) and Cr(IV) were employed to determine contributions from each ion to Cr 2p 3/2 photopeaks collected from the various cell treatments. Whilst in all cases Cr(III) was the major ion associated with cell surfaces, a significant contribution was identified due to Cr(IV) in anaerobically grown cells. The Cr(IV) contribution was far less when cells were grown aerobically. Moreover, when anaerobically grown cells were exposed to oxygen very little re-oxidation of Cr-precipitates occurred, the precipitates were again identified as a mixture of Cr(III) and Cr(IV). A positive relationship was observed between amounts of chromium and phosphorous associated with cell surfaces resulting from the various treatments, suggesting the precipitates included Cr(III)-phosphate. The fact that Cr(IV) remained associated with precipitates following re-oxidation suggests that under anaerobic conditions the intermediate ion is afforded sufficient stability to be incorporated within the precipitate matrix and thus conferred a degree of protection from oxidation.

Photoemission study of Zr- and Hf-silicates for use as high-κ oxides: Role of second nearest neighbors and interface charge

We show a systematic trend of x-ray photoelectron binding energy shifts for Zr- and Hf-silicates, which are related to the composition of the films. The binding energy for the core photoelectrons can shift by up to 2 eV, depending on the relative electronegativities of the second nearest-neighbor elements. Understanding these shifts helps determine the underlying local electronic and chemical nature of the silicate network. Furthermore, we explain how charge at the dielectric-semiconductor interface can lead to shifts in the measured Si 2p peak binding energy by as much as 1 eV. The direction and magnitude of the binding energy shift can be used to determine the sign and density of the charge at the interface.

Metal/AlQ 3 interface structures

X-ray photoelectron core level spectroscopy has been used to probe the buried metal (Au, Ag, Mg, Mg:Ag alloy)/AlQ3 interface structures from working organic light-emitting diodes. It is found that there is no chemical reaction between (Au,Ag)/AlQ3 interfaces, while there is a significant reaction/diffusion at (Mg,Mg:Ag)/AlQ3 interfaces. The reaction is AlQ3 oxidation-reduction reaction in nature, and is well explained by thermodynamic consideration of equivalent type of reaction. The reaction involves formation of MgO, metallic Al and fragmented hydroxyquinolines, gaseous N, and then followed by metallic Mg diffusion to the interface and metallic Al diffusion into the metal cathodes.

Quadrupole effects in core and valence photoelectron emission from crystalline germanium measured via a spatially modulated x-ray interference field

Near a crystal x-ray Bragg reflection, the incident and reflected x-ray beams that travel with opposite wave vectors create an x-ray standing-wave (XSW) interference field. The quadrupole (and higher order nondipole) contributions to the photoelectron emission matrix element differ for these two beams due to their different wave vectors. By monitoring the angle-resolved photoelectron yield as a function of photon energy near the (11-1) Bragg back-reflection condition of crystalline Ge, we measure the contribution of nondipole effects to Ge $3p,$ Ge $3d,$ and Ge valence-band $(4s$ and $4p)$ XSW photoelectron emission. Significant changes due to nondipole emission are measured in both the apparent amplitude and phase of the Ge structure factor relative to the true Ge atomic distribution, and compared to theory.