Valence Band Photoemission Spectra Research Articles

The Redistribution of Electrons in La<sub>1-<i>x</i></sub>Sr<em><sub>x</sub></em>CoO<sub>3</sub> Induced by Distortion of CoO<sub>6</sub> Octahedron

The La1-xSrxCoO3(x=0.0, 0.1, 0.2 and 0.3) samples were prepared by solid-state reaction method. The X-ray diffraction patterns show that the samples are ofR-3cspacegroup and the relative intensity of (024) plane diffraction peak increases with the increase of Sr concentration. The x-ray absorption fine structure data of Co ions can be fitted with 2 and 3 different scattering paths for the samples withx=0, 0.1 and forx=0.2 and 0.3, respectively. A new peak arises in x-ray absorption spectra of O ions. These results suggest a relationship between the variations of electron densities and local structure distortion. Both the valence band photoemission and x-ray absorption spectra indicate the existence of possible phase separation scenario that comes from the different types of Co-O bonds which coexist in one sample.

Polarization Sensitive Surface Band Structure of DopedBaTiO3(001)

We present a spatial and wave-vector resolved study of the electronic structure of micron sized ferroelectric domains at the surface of a BaTiO(3)(001) single crystal. The n-type doping of the BaTiO(3) is controlled by in situ vacuum and oxygen annealing, providing experimental evidence of a surface paraelectric-ferroelectric transition below a critical doping level. Real space imaging of photoemission threshold, core level and valence band spectra show contrast due to domain polarization. Reciprocal space imaging of the electronic structure using linearly polarized light provides unambiguous evidence for the presence of both in- and out-of-plane polarization with two- and fourfold symmetry, respectively. The results agree well with first principles calculations.

Optical spectroscopy and electronic structure of the face-centered icosahedral quasicrystals Zn-Mg-R(R=Y, Ho, Er)

Results of optical spectroscopy studies of the face-centered icosahedral (fci) single-grain Zn-Mg-Y, Zn-Mg-Ho, and Zn-Mg-Er quasicrystals (QCs) are presented. The dielectric function of the QCs was measured in the 0.01--6 eV spectral range by IR-UV spectroscopic ellipsometry and far infrared reflection spectroscopy techniques. A theoretical scheme of optical conductivity calculations is extended to account for the Fermi level positions within and below a pseudogap. The model of the QC electron energy spectrum, based on a band structure hypothesis, is suggested, which treats the electronic subsystem as a nearly free electron gas affected by intersections of the Fermi surface with several families of Bragg planes. The experimental optical spectra are reproduced in detail by theoretical calculations carried out within the framework of the model. The parameters of the electron energy spectrum deduced from an analysis of optical data are close to those previously determined in an analysis of fci Zn-Mg-$R$ valence band photoemission spectra.

Sulfur passivation of surface electrons in highly Mg-doped InN

Electron accumulation with a sheet density greater than 1013 cm−2 usually occurs at InN surfaces. Here, the effects of treatment with ammonium sulfide ((NH4)2Sx) on the surface electronic properties of highly Mg-doped InN (>4×1018 cm−3) have been investigated with high resolution x-ray photoemission spectroscopy. The valence band photoemission spectra show that the surface Fermi level decreases by approximately 0.08 eV with (NH4)2Sx treatment, resulting in a decrease of the downward band bending and up to a 70% reduction in the surface electron sheet density.

Role of oxygen vacancies on the structure and density of states of iron-doped zirconia

In this paper we study, both with theoretical and experimental approach, the effect of iron doping in zirconia. Combining density functional theory (DFT) simulations with the experimental characterization of thin films, we show that iron is in the Fe3+ oxidation state and accordingly that the films are rich in oxygen vacancies (VO). VO favor the formation of the tetragonal phase in doped zirconia (ZrO2:Fe) and affect the density of state at the Fermi level as well as the local magnetization of Fe atoms. We also show that the Fe(2p) and Fe(3p) energy levels can be used as a marker for the presence of vacancies in the doped system. In particular the computed position of the Fe(3p) peak is strongly sensitive to the VO to Fe atoms ratio. A comparison of the theoretical and experimental Fe(3p) peak position suggests that in our films this ratio is close to 0.5. Besides the interest in the material by itself, ZrO2:Fe constitutes a test case for the application of DFT on transition metals embedded in oxides. In ZrO2:Fe the inclusion of the Hubbard U correction significantly changes the electronic properties of the system. However the inclusion of this correction, at least for the value U = 3.3 eV chosen in the present work, worsen the agreement with the measured photo-emission valence band spectra.

Interpretation of valence band photoemission spectra at organic-metal interfaces

Adsorption of organic molecules on well-oriented single crystal coinage metal surfaces fundamentally affects the energy distribution curve of ultra-violet photoelectron spectroscopy spectra. New features not present in the spectrum of the pristine metal can be assigned as "interface states" having some degree of molecule-substrate hybridization. Here it is shown that interface states having molecular orbital character can easily be identified at low binding energy as isolated features above the featureless substrate sp-plateau. On the other hand much care must be taken in assigning adsorbate-induced features when these lie within the d-band spectral region of the substrate. In fact, features often interpreted as characteristic of the molecule-substrate interaction may actually arise from substrate photoelectrons scattered by the adsorbates. This phenomenon is illustrated through a series of examples of noble-metal single-crystal surfaces covered by monolayers of large pi-conjugated organic molecules.

Honeycomb Cupric Oxide and Hematite Nanotubes Synergism Photoelectro-reduction CO2

Vertically aligned Fe2O3 nanotubes (Fe2O3 NTs) arrays were prepared on the iron foil by potentiostatic anodization method. The anodization experiments were performed under 30 V for 2 h (20 degrees C). The ferrous foil was working electrode and titanium foil was counter electrode. The electrolytes were consisted of 0.25 wt% NH4F in an aqueous solution (Volume of water : glycol=3 : 97). After sonicating for 5 min, the samples were put under oxygen atmosphere with the flow rate of 60 sccm (standard cubic centimeters/min) and heated to 500 degrees C with the rate of 3 degrees C.min(-1) and maintained at 500 degrees C for 2 h. Then cooled to room temperature with the rate of 3 degrees C.min(-1). Honeycomb structure cupric oxide (CuO HCs) were decorated on the surface of Fe2O3 NTs by the pulse electrochemical deposition method. This was done by CHI660D potentiostat in 1 mol.L-1 CuSO4 solution (40 degrees C) in a three-electrode configuration. The Fe2O3 NTs electrode worked as counter electrode, Hg/Hg2Cl2 in saturated KCl solution as reference electrode, and platinum wire as working electrode, respectively. A 10 ms cathodic pulse with a current density of -70 mA.cm(-2) was employed. After that followed by a 1 ms anodic pulse and its density was +70 mA.cm(-2) to prevent damage of the Fe2O3 NTs structure. Finally was a 1000 ms rest time. The electrodeposition time was 36 s, with a forward duty cycle of 0.99% (considering cathodic, anodic and rest components). Then the samples were annealed under oxygen atmosphere with the flow rate of 60 sccm then heated to 300 degrees C with the rate of 3 degrees C.min(-1) and maintained at 300 degrees C for 2 h. Then cooled to room temperature with the rate of 3 degrees C.min(-1). Then the CuO HCs/Fe2O3 NTs catalysis was obtained. The SEM images showed that CuO HCs distributed uniformly on the surface of Fe2O3 NTs that arranged in order were volcano-like. Measured by UV-vis DRS (ultraviolet-visible diffuse reflection spectrum) and valence band photoemission spectrum, its band energy has been narrowed from 2.03 eV to 1.84 eV after CuO decorating. It also has admirable photocatalytic and electrocatalytic reduction performance. The main product of photoelectrocatalytic CO2 reduction identified by chromatography was methanol and its concentration reached 3.77 mmol/L after 6 h. The study has some guidance and reference significance for photoelectrocatalytic CO2 reduction.

Electronic structure of buried Co-Cu interface studied with photoemission spectroscopy

Depth profiling type of measurement has been performed on the Co(100 Å)/Cu(50 Å) bilayer thin film. Valence band photoemission spectra were recorded at 50 eV photon energy as a function of sputtering time. The motivation of the present work is to understand the electronic structure of the buried Co/Cu interface and the nature of intermixing in the Co and Cu layers. X-ray reflectivity and transmission electron microscopy corroborate with the photoemission results and shows a very broad intermixed Co-Cu interface. The valence band of intermixed Co/Cu interface shows the Co and Cu 3d states which are considerably shifted towards higher and lower binding energy, respectively, as compared to the bulk elemental Co and Cu 3d states. The experimental observations are explained with the help of calculations based on projected augmented wave pseudopotential method using density functional theory. The origin and the shift of feature in the valence band of the Co-Cu interface are mainly due to the formation of two different Co and Co-Cu mixed nanoclusters.

Electronic and magnetic properties of CeCoGa

We report the magnetic and electronic transport properties of CeCoGa, using magnetic susceptibility, magnetization, specific heat and electrical resistivity investigations. We also present X-ray photoemission valence band spectra (VB-XPS) as well as ab-initio band structure calculations for the ferromagnetic and antiferromagnetic ordering. The dc and ac magnetic susceptibility data reveal a magnetic phase transition at TN = 4.3 K. For T < TN magnetization M vs. magnetic field B isoterms have two metamagnetic transitions. Above TN susceptibility shows evidence of spin-glass like behavior. The magnetic entropy is strongly reduced at TN due to the on-site Kondo coupling. For CeCoGa the Kondo temperature is ∼20 K, much larger than the temperature of the magnetic ordering. Our calculations show that within bare local spin density approximation (LSDA) the ground state of CeCoGa is weakly magnetic, however, the 4f correlations when they are included within LSDA + U approach lead to stable antiferromagnetic ground state. Calculations for moderate U parameter indicate that magnetic ordering is present not only at the Ce sublattice, but also for the Co sites, moreover small induced magnetic moment is also present at the Ga sites. All sublattices exhibit antiferromagnetic ordering with different values of local magnetic moments what explains the ferrimagnetic behavior of CeCoGa. The magnetization M vs. magnetic field B shows at T = 2 K two metamagnetic features at B = 0.1 T and B = 5 T, which well correlates with the calculated magnetic moments of Co and Ga. The first transition can be attributed to spin–flop within the magnetic moments induced in Ga sublattice, whereas the anomaly at B = 5 T is related to the spin–flop within the Co sublattice.

The electronic and chemical structure of the a-B3CO0.5:Hy-to-metal interface from photoemission spectroscopy: implications for Schottky barrier heights

The electronic and chemical structure of the metal-to-semiconductor interface was studied by photoemission spectroscopy for evaporated Cr, Ti, Al and Cu overlayers on sputter-cleaned as-deposited and thermally treated thin films of amorphous hydrogenated boron carbide (a-BxC:Hy) grown by plasma-enhanced chemical vapor deposition. The films were found to contain ∼10% oxygen in the bulk and to have approximate bulk stoichiometries of a-B3CO0.5:Hy. Measured work functions of 4.7/4.5 eV and valence band maxima to Fermi level energy gaps of 0.80/0.66 eV for the films (as-deposited/thermally treated) led to predicted Schottky barrier heights of 1.0/0.7 eV for Cr, 1.2/0.9 eV for Ti, 1.2/0.9 eV for Al, and 0.9/0.6 eV for Cu. The Cr interface was found to contain a thick partial metal oxide layer, dominated by the wide-bandgap semiconductor Cr2O3, expected to lead to an increased Schottky barrier at the junction and the formation of a space-charge region in the a-B3CO0.5:H y layer. Analysis of the Ti interface revealed a thick layer of metal oxide, comprising metallic TiO and Ti 2O 3, expected to decrease the barrier height. A thinner, insulating Al2O3 layer was observed at the Al-to-a-B3CO0.5:Hy interface, expected to lead to tunnel junction behavior. Finally, no metal oxides or other new chemical species were evident at the Cu-to-a-B3CO0.5:Hy interface in either the core level or valence band photoemission spectra, wherein characteristic metallic Cu features were observed at very thin overlayer coverages. These results highlight the importance of thin-film bulk oxygen content on the metal-to-semiconductor junction character as well as the use of Cu as a potential Ohmic contact material for amorphous hydrogenated boron carbide semiconductor devices such as high-efficiency direct-conversion solid-state neutron detectors.

X-ray Photoelectron Spectroscopic Characterization of Ag Nanoparticles Embedded Bioglasses

This article reports on synthesis and X-ray photoemission spectroscopy (XPS) results regarding the development of silver nanoparticles (AgNPs) in CaO-SiO2–B2O3 bioactive glasses with three different Ag2O contents (0.7, 1, 3%). The analysis of the core level and valence band photoemission spectra is discussed in terms of AgNPs shape, size, and distribution at the sample surface for all investigated samples. A comparison with a theoretical valence band photoemission spectrum (VB-XPS) is presented to check the correlation between clusters size and electronic structure. Our data provide complete experimental evidence of the correlation between AgNPs size and electronic structure that has been predicted theoretically.

Potassium-intercalated H2Pc films: Alkali-induced electronic and geometrical modifications

X-ray spectroscopy studies of potassium intercalated metal-free phthalocyanine multilayers adsorbed on Al(110) have been undertaken. Photoelectron spectroscopy measurements show the presence of several charge states of the molecules upon K intercalation, due to a charge transfer from the alkali. In addition, the comparison of valence band photoemission spectra with the density functional theory calculations of the density of states of the H(2)Pc(-) anion indicates a filling of the formerly lowest unoccupied molecular orbital by charge transfer from the alkali. This is further confirmed by x-ray absorption spectroscopy (XAS) studies, which show a decreased density of unoccupied states. XAS measurements in different experimental geometries reveal that the molecules in the pristine film are standing upright on the surface or are only slightly tilted away from the surface normal but upon K intercalation, the molecular orientation is changed in that the tilt angle of the molecules increases.

Orbital degeneracy and Peierls instability in the triangular-lattice superconductor Ir1−xPtxTe2

We have studied electronic structure of triangular lattice Ir$_{1-x}$Pt$_x$Te$_2$ superconductor using photoemission spectroscopy and model calculations. Ir $4f$ core-level photoemission spectra show that Ir $5d$ $t_{2g}$ charge modulation established in the low temperature phase of IrTe$_2$ is suppressed by Pt doping. This observation indicates that the suppression of charge modulation is related to the emergence of superconductivity. Valence-band photoemission spectra of IrTe$_2$ suggest that the Ir $5d$ charge modulation is accompanied by Ir $5d$ orbital reconstruction. Based on the photoemission results and model calculations, we argue that the orbitally-induced Peierls effect governs the charge and orbital instability in the Ir$_{1-x}$Pt$_x$Te$_2$.

Iron Resonant Photoemission Spectroscopy on Anodized Hematite Points to Electron Hole Doping during Anodization

Anodization of α-Fe(2)O(3) (hematite) electrodes in alkaline electrolyte under constant potential conditions the electrode surface in a way that an additional current wave occurs in the cyclic voltammogram. The energy position of this current wave is closely below the potential of the anodization treatment. Continued cycling or exchanging of the electrolyte causes depletion of this new feature. The O 1s and Fe 2p core-level X-ray photoelectron spectra (XPS) and near-edge X-ray absorption fine structure (NEXAFS) spectra of such conditioned hematite exhibit a chemical shift towards higher binding energies, in line with the general perception that anodization generates oxide species with dielectric properties. The valence band XPS and particularly the iron resonant valence band photoemission spectra, however, are shifted towards the opposite direction, that is, towards the Fermi energy, suggesting that hole doping on hematite has taken place during anodization. Quantitative analysis of the Fe 2p resonant valence band photoemission spectra shows that the spectra obtained at the Fe 2p absorption threshold are shifted by virtually the same energy as the anodization potential towards the Fermi energy. The tentative interpretation of this observation is that anodization forms a surface film on the hematite that is specific to the anodization potential.

On the absence of resonance in the valence band photoemission at L2 resonance edge and the delayed onset of the normal Auger-decay

No clear resonance is present in the resonant valence band photoemission spectrum of a Mn6 (S=4) single molecule magnet grafted on Au (111) for photon energy at the L2 resonance edge. This is due to the L2-hole’s dominant decay by the L2–L3V Coster–Kronig (CK) decay. The excess energy of the photon at which the Mn resonant Raman scattering regime transits to the normal Mn Auger-decay regime in MnO depends on the Mn Auger-decay channels. This is because of the localization and delocalization of two holes in the Auger final states. The Mn resonant Auger-electron spectroscopy spectra of MnO are analyzed.

Valence band dependence on thermal treatment of gold doped glasses and glass ceramics

This paper reports x-ray photoemission spectroscopy results regarding the nucleation and growing process of gold nanoparticles (AuNPs) in CaO-SiO2-P2O5 bioactive glasses heat treated at different temperatures. The analysis of the valence band (VB) spectra is discussed in terms of the AuNPs’ shape, size, and distribution at the sample surface at different temperatures. Moreover, the theoretical VB photoemission spectra of the investigated samples are presented, and their agreement with experimentally obtained spectra is discussed in detail.

Valence Electron Photoemission Spectrum of Semiconductors:Ab InitioDescription of Multiple Satellites

The experimental valence band photoemission spectrum of semiconductors exhibits multiple satellites that cannot be described by the GW approximation for the self-energy in the framework of many-body perturbation theory. Taking silicon as a prototypical example, we compare experimental high energy photoemission spectra with GW calculations and analyze the origin of the GW failure. We then propose an approximation to the functional differential equation that determines the exact one-body Green's function, whose solution has an exponential form. This yields a calculated spectrum, including cross sections, secondary electrons, and an estimate for extrinsic and interference effects, in excellent agreement with experiment. Our result can be recast as a dynamical vertex correction beyond GW, giving hints for further developments.

Hydrogen spillover monitored by resonant photoemission spectroscopy

We have employed resonant photoemission spectroscopy to investigate the phenomenon of hydrogen spillover on well-defined model Pt/ceria catalysts. On Pt/CeO2(111)/Cu(111), hydrogen spillover and reverse spillover give rise to reversible changes of the oxidation state of the surface cerium ions. These changes are monitored with highest sensitivity via resonant enhancement of Ce3+- and Ce4+-related features in the valence band photoemission spectra. The temperature regions are precisely determined in which specific processes such hydrogen spillover, Ce3+ transport, and reverse spillover are activated.

Surface and Bulk Oxygen Vacancy Defect States near the Fermi Level in 125 nm WO3−δ/TiO2(110) Films: A Resonant Valence Band Photoemission Spectroscopy Study

An approximately 125 nm thick pulsed laser deposited blue, nonstoichiometric WO3−δ film grows on TiO2 (110) in the [220] direction. Oxidative treatment at 400 °C turns the film color from blue to yellow and improves the film quality considerably, as shown by improvement of the Kiessig oscillations in the X-ray reflectometry curves. Detailed analysis of resonant valence band photoemission spectra of the as-deposited nonstoichiometric blue film and oxidized yellow film suggests that a transition near the Fermi energy originates from the nonstoichiometry, i.e., oxygen deficiency, and insofar poses electronic defect states that partially can be eliminated by heat treatment in oxygen. The defects of the as-deposited blue film seem to be located throughout the film, except for the top surface due to exposure to oxygen in ambient air. Thermal after-treatment under oxygen heals the defects in the bulk, whereas residual defect states appear to remain near the film–substrate interface. Potential strain at the substrate–film interface due to lattice mismatch may be one origin for the remanence of the defect states in the bulk.

Nature of the Binding of a c(2×2)-CO Overlayer on Ag(001) and Surface Mediated Intermolecular Coupling

We present a first-principles study of the nature of the binding of a c(2×2)-CO overlayer on Ag(001) and of the origin of CO-CO interactions upon adsorption. Electronic structural changes induced by molecular adsorption provide an interpretation for earlier X-ray photoemission valence band spectra of CO/Ag(001). Our results establish that CO chemisorbs on clean Ag(001) and follows the Blyholder model of donation and back-donation between CO and metal orbitals. We analyze the origin of the dispersion of the C-O stretch mode and attest that it is caused by the metal-CO coupling. Specifically, the coupling of CO to Ag, although the weakest of those between it and transition and other noble metals, greatly enhances the intermolecular force constants. We also find that the response of the charge density around CO is much stronger and of longer range when the molecule stretches than when it rigidly vibrates against the surface. This difference explains why the C-O stretch mode disperses while the Ag-CO stretch mode does not.