Photoelectron Valence-band Spectra Research Articles

CO dissociation on K-modified Cu(1 1 2) and Cu(1 1 7)

Using valence band photoelectron spectroscopy it is demonstrated that CO dissociates on the potassium-modified stepped copper surfaces, Cu(112) and Cu(117). The dissociation process is conditioned by both the presence of steps and an alkali metal coverage above a certain threshold value. Carbonate is formed at 375 K via the process CO+CO→CO2+C and reaction with oxygen. Further heating to 600 K results in decomposition of the carbonate. The threshold alkali metal coverage for dissociation of CO on stepped copper surfaces depends on the step density. It is found that the height of the Fermi edge in the valence band photoelectron spectra is correlated with the CO uptake for a given potassium coverage and the perturbation and dissociation of the CO molecule with increasing temperature.

Preparation and characterization of lithium doped indium sesqui-oxide

Doping of lithium into indium sesqui-oxide (In 2O 3) brought about an increase in the lattice constant of cubic cell from 10.116 to 10.163 Å and a decrease in the optical absorption energy from 2.72 to 2.68 eV with increasing the nominal molar ratio x (=[Li]/[In 2O 3])<0.5. Enlargement of the lattice constant obeying the Vegard’s law suggests that the doped Li atoms occupied interstitial sites of In 2O 3 crystal. Tight-binding (TB) band-structure calculations with two Li atoms at 8 a sites and those at 16 c sites of the crystal showed decrements in energy splitting between the valence and conduction bands by 0.2 and 0.1 eV, respectively, and supported the observed decrements of the optical absorption energy in In 2O 3:Li x . First-principles molecular-orbital (MO) calculations with clusters for Li atoms at 8 a sites showed similar result to the TB calculation. Changes in the X-ray photoelectron valence band spectra of the In 2O 3:Li x ( x=0, 0.2, and 0.5) agreed with those in the density of states expected from the TB and MO calculations. Non-linear current–voltage characteristics were due to the surface states and surface band bending observed in the valence band spectra.

Electrochromism in lithiated nickel oxide films deposited by rf sputtering

Lithiated nickel oxide films were deposited by rf sputtering from an LiNiO 2 target. Different samples were obtained by changing the deposition atmosphere or rf power during deposition, all other parameters remaining constant. The electrochemical/electrochromic performance during lithium extraction/insertion was investigated using aprotic electrolytes, over the whole stability range of the electrolyte. The results showed that all films, independent of the deposition conditions, were electrochemically active. Films deposited under pure Ar atmosphere and high rf power or under Ar+O 2 atmosphere and low power did not present significant electrochromic activity, and the valence band photoelectron spectrum did not show the presence of the Ni 3d line for the as-grown samples. In contrast, samples deposited under Ar+O 2 atmosphere and high power, or pure Ar atmosphere and low power showed a 70% transmittance change, and a stable voltammetric profile after some cycles. For these samples, the valence band photoelectron spectrum clearly presented the Ni 3d line. The electron population at the Ni 3d levels in the as-grown state seems to be responsible for the electrochromic ability.

Electronic structure of ScN determined using optical spectroscopy, photoemission, andab initiocalculations

Experimental and ab initio computational methods are employed to conclusively show that ScN is a semiconductor rather than a semimetal; i.e., there is a gap between the N $2p$ and the Sc $3d$ bands. Previous experimental investigators reported, in agreement with band structure calculations showing a band overlap of 0.2 eV, that ScN is a semimetal while others concluded that it is a semiconductor with a band gap larger than 2 eV. We have grown high quality, single crystalline ScN layers on MgO(001) and on TiN(001) buffer layers on MgO(001) by ultrahigh vacuum reactive magnetron sputter deposition. ScN optical properties were determined by transmission, reflection, and spectroscopic ellipsometry while in-situ x-ray and ultraviolet valence band photoelectron spectroscopy were used to determine the density of states (DOS) below the Fermi level. The measured DOS exhibits peaks at 3.8 and 5.2 eV stemming from the N $2p$ bands and at 15.3 eV due to the N $2s$ bands. The imaginary part of the measured dielectric function ${\ensuremath{\varepsilon}}_{2}$ consists of two primary features due to direct X- and \ensuremath{\Gamma}-point transitions at photon energies of 2.7 and 3.8 eV, respectively. For comparison, the ScN band structure was calculated using an ab initio Kohn--Sham approach which treats the exchange interactions exactly within density-functional theory. Calculated DOS and the complex dielectric function are in good agreement with our ScN valence-band photoelectron spectra and measured optical properties, respectively. We conclude, combining experimental and computational results, that ScN is a semiconductor with an indirect $\ensuremath{\Gamma}--X$ bandgap of $1.3\ifmmode\pm\else\textpm\fi{}0.3\mathrm{eV}$ and a direct X-point gap of $2.4\ifmmode\pm\else\textpm\fi{}0.3\mathrm{eV}.$

XPS-studies of the electronic structure of Fe–X (X=Al, Si, P, Ge, Sn) systems

For the stoichiometric compounds FeSi, FeGe, FeSn, FeAl, and FeP the X-ray photoelectron spectra of valence bands are obtained and the calculations of their electronic structure are carried out by the full-potential linear muffin–tin–orbital method (FP-LMTO) and by the tight-binding linear muffin–tin–orbital atomic sphere-approximation method (TB-LMTO-ASA). A satisfactory agreement of the X-ray photoelectron spectra and the calculated density of states curves is obtained. The comparison of the experimental spectra with the calculated curves shows that if the second component (X) of an alloy belongs to the same (IVth) group of the periodic table, the p–d hybridization is maximum in the alloy FeSi and decreased with the increase in the atomic radius (atomic number) of X(Ge, Sn). If X belongs to the same (IIIrd) row of the periodic table, then the p–d hybridization is maximum in FeAl and decreased with the increase in the number of p electrons of X(Si, P).

Electron structure of thin fullerene films deposited by various methods

The valence-band photoelectron spectra and the photoelectron energy loss spectra behind the C1s peak in the X-ray photoelectron spectra were studied in thin fullerene films deposited by various methods. The fine structure in the density of states near the valence band bottom observed for the fullerene films deposited by the method of pulsed supersonic molecular beam (SMB) with a helium carrier may be indicative of the presence of a long-range order. The spectra of π-plasmon losses in the films obtained by methods of thermal and SMB deposition exhibit a significant difference that can be related to a closer packing of C60 molecules in the latter case.

X-RAY PHOTOELECTRON SPECTRA IN TiC-NbC SOLID SOLUTIONS

The inner-level and valence-band X-ray photoelectron spectra of TiC–NbC solid solutions have been studied. The TiLα, NbLβ2,15, and CKα X-ray emission spectra and the valence-band photoelectron spectra were fitted to a single energy scale relative to the Fermi level, and the results of the fitting are analyzed. It is shown that formation of the electronic structure of these mixed carbides is strongly affected by the variation of the lattice parameters and also by the mutual effects of the Ti and Nb atoms on charge redistribution between them.

Valence States of Lead and Bismuth Atoms in the High-Temperature Superconductor BaPb1-xBixO3

X-ray photoelectron spectra of valence bands and core levels of BaPb0.8Bi0.2O3, PbO, PbO2, BaPbO3, BaBiO3, NaBiO3, and Bi2O3 were studied. Comparison of the electron binding energies of the Pb 4f7/2 or Bi 4f7/2 core levels for all the oxides studied showed that the high-temperature oxide superconductor BaPb0.8Bi0.2O3 contains simultaneously two different valence forms of lead atoms (PbIV and PbII) and two different valence forms of bismuth atoms (BiV and BiIII). Parameters of the X-ray photoelectron spectra of the valence bands do not contradict the conclusion on heterovalent states of lead and bismuth atoms in BaPb0.8Bi0.2O3.

Electronic properties of the diamond films with nitrogen impurities: An x-ray absorption and photoemission spectroscopy study

X-ray absorption near-edge structure (XANES) measurements have been performed for nitrogen (N) containing diamond films with three different N concentrations at the C K-edge using the sample drain current mode. The C K-edge XANES spectra of these diamond films resemble that of the pure diamond regardless of the N concentration, which suggests that the overall bonding configuration of the C atom is unaltered. N impurities are found to reduce the intensities of both the sp2- and sp3-bond derived resonance features in the XANES spectra. The valence-band photoelectron spectra indicate that N atoms cause the broadening of the valence band σ- and π-bond features and the enhancement and reduction of the σ- and π-bond features, respectively.

Soft-x-ray fluorescence study of the quasi-one-dimensional Heisenberg antiferromagnet tetraphenylverdazyl

Soft-x-ray fluorescence measurements have been performed on a single crystal of the organic antiferromagnet 2,4,6-triphenylverdazyl. Resonant and nonresonant C $K\ensuremath{\alpha}$ and N $K\ensuremath{\alpha}$ $(2\stackrel{\ensuremath{\rightarrow}}{p}1s$ transition) x-ray emission spectra (XES) were measured and compared with x-ray photoelectron valence band spectra and deMon density-functional theory calculations. It is shown that intramolecular interactions are much stronger than intermolecular ones and give the main contribution to the formation of C $2p$ density of states. We present evidence of a delocalization of unpaired N $2p$ electrons over the verdazyl ring. The excitation energy dependence of C $K\ensuremath{\alpha}$ and N $K\ensuremath{\alpha}$ XES observed below the C $1s$ and N $1s$ thresholds, respectively, is discussed in terms of symmetry selective resonant inelastic x-ray scattering.

Surface crystallization of ion-implantation damaged Si 95Ge 5 on Si(100)

Si(100) was implanted with 80 keV Ge ions to a Si 95Ge 5 composition. The implanted layer (120 nm thick) was amorphous as judged by XTEM, but the valence band photoelectron spectra from this system were clearly different from earlier data on amorphous Si. Thermal regrowth of the damaged layer was found to proceed, as expected, from the damaged–undamaged interface. However, even after extended annealing at temperatures well above that required for interfacial regrowth, the surface region remained disordered (non-single crystal). The results are interpreted in terms of parallel recrystallization of the implantation-damaged layer at the interface as well as at the surface. Due to lack of a template, the latter process does not result in a single crystal.

XPS study on pristine and intercalated tantalum carbosulfide

Core level and valence band spectra of pristine and Fe, Co, Cu, or Ni intercalated tantalum carbosulfide (Ta 2S 2C) have been studied by X-ray photoelectron spectroscopy. The aim was to compare pristine tantalum carbosulfide (a layered compound, stacking sequence: S–Ta–C–Ta–S) to tantalum carbides (TaC and Ta 2C) and tantalum sulfide (TaS 2) and compare intercalated with pristine carbosulfides. The chemical shift of carbon layers in the carbosulfides was comparable to a graphitic carbon monolayer on (111) faces of TaC. Therefore, the carbon in carbosulfides does not have a carbidic character, it is a single carbon layer with graphitic character. From an electronic point of view, the carbosulfides should no longer be called complex carbides. Sulfur and tantalum in the pristine compound showed shifts comparable to tantalum sulfide. X-ray photoelectron valence band spectra can be only interpreted as fingerprints. However, intercalated carbosulfides with guests at the octahedral sites between two adjacent sulfur layers (Fe, Co, and Ni) show valence band spectra distinguishable from Cu intercalated compounds (Cu is located in tetragonal sites between sulfur and tantalum layers). Distinguishable from all valence band spectra of intercalated materials is the valence band spectrum of the pristine compound.

Core and valence-level photoemission study of theInP(001)−(2×1)Ssurface: Surface structure and electronic states

The ex situ sulfur treatment of InP(001) by $({\mathrm{NH}}_{4}{)}_{2}\mathrm{S}$ and subsequent annealing in situ finally results in a sulfur-induced $(2\ifmmode\times\else\texttimes\fi{}1)$ superstructure as observed by low-energy electron diffraction. We have investigated the development of the In $4d,$ P $2p,$ and S $2p$ core levels with increasing temperature by using soft x-ray photoelectron spectroscopy. The measured core level shifts are compared with the corresponding data obtained on the $(2\ifmmode\times\else\texttimes\fi{}4)$ reconstruction of the clean InP(001) surface. On the basis of the core level data some models of the $(2\ifmmode\times\else\texttimes\fi{}1)\mathrm{S}$ structure are discussed. Valence-band photoelectron spectra of the $(2\ifmmode\times\else\texttimes\fi{}1)\mathrm{S}$ and the $(2\ifmmode\times\else\texttimes\fi{}4)$ surfaces have been measured in dependence on photon energy at normal emission and derived ${E(k}_{\ensuremath{\perp}})$ dispersions are analyzed. Although significantly different surface features are observed on both surfaces, there are similarities, which may be interpreted in terms of contributions from similar structural elements, possibly surface dimers.

A study of the FeX (X=Si, Ge) system electronic structure by X-ray photoelectron spectroscopy

For the two stoichiometric compounds FeSi and FeGe, the X-ray photoelectron spectra of valence bands are obtained and the calculations of their electronic structure are carried out by the first principles full-potential linear muffin-tin-orbital method. For FeSi, which is nonmagnetic, a good agreement of the calculated density of states and the X-ray photoelectron spectrum (Al Kα excitation) is obtained in both the peak positions and the width of energy gap. For FeGe, which is antiferromagnetic, neither the nonspin-polarized nor the antiferromagnetic calculations shows good agreement with the experiment, and the former clearly suggests instability of the paramagnetic state. In both compounds the calculation reveals a remarkable degree of p– d-hybridization and covalency, which are evaluated quantitatively. In FeGe a paradoxical result is obtained, that the covalency of the FeFe bond is higher than the covalency of the bond FeGe.

Combined study of KNbO 3 and KTaO 3 by different techniques of photoelectron and X-ray emission spectroscopy

The new results of the experimental study of KNbO 3 and KTaO 3 by means of X-ray photoelectron spectroscopy, soft X-ray emission and X-ray fluorescence spectroscopy are presented. In particular, Nb M 4,5 spectra ( 5p,4f→ 3d 5/2,3/2 transition) have been measured over a range of near-threshold excitation energies 206.5–240.6 eV, and Ta N 3 spectra ( 5d,6s→ 4p 3/2 transition) over 399.4–420.4 eV at the Beamline 8.0 of the Lawrence Berkeley National Laboratory's Advance Light Source. These spectra were found to be strongly dependent on the excitation energy. Moreover, the O Kα X-ray emission spectra in both compounds as well as the valence-band photoelectron spectra are brought to the binding energy scale and discussed in combination. The trends in the spectra are explained on the basis of first-principles band-structure calculations, with the dipole transition matrix elements taken into account.

Soft X-ray fluorescence measurements of polyimide films

Fluorescent X-ray emission spectroscopy has been used for the first time to study the electronic structure of polyimide films (PMDA-ODA) prepared on Si-substrates by spin-coating and ionized cluster beam deposition (ICBD) methods. The observed C Kα and O Kα X-ray emission spectra that probe the C 2p and O 2p partial density of states in the valence band, respectively, were compared with X-ray photoelectron valence band spectra. Theoretical simulations of X-ray energies and intensities for polyimide were performed with molecular orbital (MO) calculations of the model monomer, containing 41 atoms, utilizing a semi-empirical Hydrogenic Atoms in Molecules, V. 3 (HAM/3) method. Calculated Al Kα photoelectron spectra were obtained using Gaussian lineshape functions of an approximate linewidth (0.10 Ik for the model molecule): Ik=I′k−WD, as indicated for analysis of valence X-ray photoelectron spectra for 63 polymers from some of our previous work. Theoretical C Kα and O Kα X-ray emission and valence X-ray photoelectron spectra show good agreement with the experimental results, and were used to identify certain spectral features.

In situ observation of a high-temperature Si(001) surface during SiH 2Cl 2 exposure by photoelectron spectroscopy

In situ observation by photoelectron spectroscopy using synchrotron radiation was performed to investigate the species and coverage of surface adsorbates while exposing the high-temperature Si(001) surface to SiH 2Cl 2 atmosphere. Photoelectron spectra of valence bands made it possible to evaluate the clean surface area with dimer dangling bonds (Si db). Curve-fitting analysis of Si 2p spectra showed that the chlorine adsorption species is only monochloride (Si-Cl) in the whole temperature region from room temperature ~700 °c examined. From comparison between the surface coverages of Si-db and Si-Cl in connection with the 2 × 1 structure of the SiH 2Cl 2-adsorbed Si(001) surface as observed by reflection high energy electron diffraction, we assigned the hydrogen adsorption state to be dominantly monohydride (Si-H). The temperature dependencies of the surface coverages obtained for Si-Cl, Si-H and Si-db are discussed using a surface reaction model of SiH 2Cl 2 adsorption.

Electronic structure of crystalline FeSi and FeGe

X-Ray photoelectron valence band spectra were recorded for stoichiometric compounds FeSi and FeGe; their electronic structure was calculated by the ab initio full-potential linear muffintin orbital method. For nonmagnetic FeSi, good agreement was obtained between the calculated densities of states and the X-ray photoelectron spectrum in both peak positions and forbidden gap. For FeGe, which is an antiferromagnet, the nonmagnetic calculation yields worse agreement with experiment and explicitly indicates that the paramagnetic phase is unstable. In both compounds, the calculation gives a high degree of d—p hybridization and covalence, which is estimated quantitatively. In FeGe, the degree of covalence of the Fe-Fe bond is higher than that of the Fe-Ge bond.

Electornic structure of emeraldine and pernigraniline base: A joint theoretical and experimental study

The electronic structure of the emeraldine and pernigraniline base forms of polyaniline is revisited with a combined experimental and theoretical approach. On the one hand, high-resolution valence band photoelectron spectra are presented. High-quality electronic structural information is obtained due to the use of chemically pure and well-defined materials, which could be studied for the first time with a high-resolution photoelectron spectroscopy facility. On the other hand, the electronic band structures and the density of valance states spectra associated with these polymers are calculated by means of the pseudopotential valence effective Hamiltonian (VEH) technique. The excellent agreement observed for the first time between the experimental and theoretical results obtained on these polymers clearly validates the description of the emeraldine and pernigraniline valence band electronic structure within the VEH formalism and allows us to evaluate the ionization potential of both polymers.

Analysis of photoelectron spectra of high-temperature superconductors

We present the calculated valence-band photoelectron spectra for the superconducting Bi 2Sr 2CuO 6, Bi 2Sr 2CaCu 2O 8 and YBa 2Cu 3O 7 thin films at photon energies of 21.2–70 eV. Angle-integrated photoelectron spectra were calculated using the single-scatterer final-state approximation and results of a local-density linearized augmented-plane-wave band-structure calculation for thin films. Film geometry immediately takes into account surface contribution.