Manifestation of auger processes in C1s-satellite spectra of single-walled carbon nanotubes

Using the equipment of the Russian-German beamline of the synchrotron radiation at the BESSY II electron storage ring, satellite spectra accompanying the C1s core lines in the cases of single-walled carbon nanotubes and highly ordered pyrolytic graphite have been measured with a high energy resolution. The Auger spectra corresponding to shaking of the valence system of carbon by the core vacancy have been found and investigated. The Auger spectra of the studied single-walled carbon nanotubes and highly ordered pyrolytic graphite are caused by annihilation of the excited π* electron with a hole in the π subband. It has been established that the electron states in the conduction band have 3π* (gT, K, M) symmetry; i.e., they correspond to flat 3π* subband, which is localized by 12–13 eV above the Fermi level. It has been revealed that the general regularities of the distribution of electron states in the valence system insignificantly change during its shake-up by the excited core.     

Electronic structure of PbI2 from photoelectron spectra and the exciton problem

The valence band density of states for PbI₂ is determined from X-ray and u.v. induced photoelectron spectra. It is shown that the band derived from Pb 6s states is at 8 eV binding energy and not at the top of the valence bands as suggested by band structure and charge density calculations. A rigid shift in the predominantly iodine 5p derived bands to lower binding energy brings the band structure calculations into essential agreement with experiment. Pb 5d core level binding energies determined here are used to derive core level exciton energies of 0.7 eV from published reflectivity spectra.     

Electron spectroscopic studies of the adsorption of methanol on zinc oxide powder

X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), and Auger electron spectroscopy (AES) have been used to study the adsorption of methanol on ZnO powder. The He(II) spectrum of electron-beam-eleaned ZnO was similar to that reported by Rubloff .¹ for the ZnO (11̄ 0 0) surface. The spectrum of methanol condensed at ?150°C matched that of gas-phase methanol. At low methanol coverages, difference spectra showed no increase in the methanol O(2p) lone pair binding energy reported to be indicative of chemisorption. A residual carbon and oxygen surface component on the ZnO powder may have prevented chemisorption. XPS measurements were found to be more useful in observing this layer than was AES. A decrease in the ZnO valence band intensity near the valence band maximum indicated that a strong interaction with the substrate had occurred.In this system the valence bands of the adsorbate and substrate overlap, and the true position of the methanol O(2p) lone pair orbital is obscured by the effects of the substrate band modification. Also, for insulators or semiconductors, a change in the measured position of the valence band maximum relative to the Fermi level may occur upon adsorption. Since the Zn(3d) core level should not be affected, it was used as an energy reference for the present work. The problems of energy referencing in UPS studies of adsorption on semiconductors are discussed.     

Valence band of molybdenum by photoelectron spectroscopy

Experimental photoelectron spectra of a clean polycrystalline Mo surface excited by monochromatized Al K _α X-rays are presented. The spectra are compared with valence bands obtained by UPS and by band structure calculations within the 5 eV region below the Mo Fermi level. All results mentioned above display peaks at 0.3, 1.7, 2.8 and 4 eV belowE _F. The energy distribution of the valence band does not vary with photon energy and electron emission angle for the four different polycrystalline Mo surfaces compared. It is concluded that the four peaks representing the Mo valence band are predominantly of bulk origin.     

N23-core losses and autoionization emissions of clean and oxygen exposed zirconium

The electron energy loss spectra associated with N₂₃-excitation and the low energy N₂₃VV Auger emission have been studied for both the clean and oxygen exposed zirconium. In the high energy side of the N₂₃VV Auger spectrum, autoionization emission of electrons of the valence band due to the decay of 4p electrons excited to states ≈9eV above the Fermi level has been identified. The excitation process can be also observed in the electron energy loss spectra. This is the first time that an autoionization feature is observed in a electron excited Auger spectrum of a 4d transition metal.     

A study of Al/Si(111)-cleaved interface by photoemission,Auger electron yield,and Auger electron spectroscopies

Photoemission, Auger electron yield, and Auger electron spectra are observed for Al/Si(111)-cleaved interfaces. The Al 3p derived state which becomes metallic at monolayer coverage is introduced near the top of the valence band. The Fermi level is stabilized in this metallic state. The present result favors the on-top geometry of Al atoms on a Si(111)-cleaved surface.     

X-ray Photoelectron Spectroscopy of Sb2S3 Crystals

The paper presents the X-ray photoelectron spectra (XPS) of the valence band and core levels of semiconductor ferroelectric Sb₂S₃ single crystals, which show weak phase transitions and anomalies of various physical properties. The XPS were measured with monochromatized Al K _α radiation in the energy range 0-1450 eV and the temperature range 160-450 K. The valence band is located 0.8-7.5 eV below the Fermi level. Experimental results of the valence band and core levels are compared with the results of theoretical ab initio calculations of the molecular model of Sb₂S₃ crystal. The chemical shifts in Sb₂S₃ crystal for the Sb and S states are obtained. Results revealed that the small structural rearrangements at the phase transition T _c1 = 300 K shift the Fermi level and all electronic spectrum. Also, temperature dependence of a spontaneous polarisation shifts the electronic spectra of the valence band and core levels. Specific temperature-dependent excitations in Sb 3d core levels are also revealed.     

The effect of transition probability on the shape of X-ray photoelectron spectra of diamond and silicon

X-ray photoelectron spectra of valence bands in diamond and silicon have been calculated. It is shown that the probability of electron excitation from s-states is higher than that from p-states. The density of the electron states in the valence band of these crystals differs markedly from the energy distribution of photoelectrons.     

Photoemission studies of single crystals of titanium carbide

The electron distribution in the valence band from single crystals of titanium carbide has been studied by photoelectron spectroscopy with photon energies h?ω = 16.8, 21.2, 40.8 and 1486.6 eV. The most conspicious feature of the electron distribution curves for TiC is a hybridization between the titanium 3d and carbon 2p states at ca. 3–4-eV binding energy, and a single carbon 2s band at ca. 10 eV. By taking into account the strong symmetry and energy dependence of the photoionization crosssections, as well as the surface sensitivity, we have identified strong emission from a carbon 2p band at ? 2.9-eV energy. Our results are compared with several recent energy band structure calculations and other experimental data. Results from pure titanium, which have been used for reference purposes, are also presented.The valence band from single crystals of titanium carbide have been studied by means of photoelectron spectroscopy, with photon energies ranging from 16.8 to 1486.6 eV.By taking into account effects such as the symmetry and energy dependence of the photoionization cross-sections and surface sensitivity, we have found the valence band of titanium carbide to consist of two peaks. The upper part of the valence band at 3–4 eV below the Fermi level consists of a hybridization between Ti 3d and C 2p states. The C 2p states observed in our spectra were mainly excited from a band about 2.9 eV below the Fermi level. The APW^5–9, MAPW¹⁰ and EPM¹¹ band structure calculations predict a flat band of p-character between the symmetry points X′₄ and K₃, most likely responsible for the majority of C 2p excitations observed. The C 2s states, on the other hand, form a single band centered around ?10.4 eV.The results obtained are consistent with several recent energy band structure calculations^{5–11, 13} that predict a combined bonding of covalent, ionic and metallic nature.     

The YbNi interface studied with photoemission spectroscopy

The development of the ytterbium valence band region was followed with Synchrotron radiation induced Photoemission Spectroscopy (SPS) by interdiffusion of Yb into a Ni (110) single crystal in order to identify the valence conditions of Yb in the bulk and on the surface. During this process, also the width of the Ni L₃VV Auger transition was investigated with X-ray induced Photoemission Spectroscopy (XPS), reflecting the electron donation of Yb to the Ni valence band. By comparison between theory and experiment, strong multiplet splittings were found to take place in the 4d and 5p core level spectra of Yb due to the promotion of one 4f electron to the valence band by reaction with Ni. The 5p level is demonstrated to resonate strongly at hν=181 as a consequence of the 4d–4f giant resonance.     

Surface electronic structure of the organic superconductor κ-(ET)2Cu(NCS)2 studied via photoemission microscopy

The surface electronic structure of cleaved single crystals of the organic superconductor κ-(ET)₂Cu(NCS)₂ has been studied using photoemission microscopy. Two types of cleaved surfaces were observed, displaying different valence band photoemission spectra and different spectral behavior near the Fermi level, E_F. In particular, spectra from one surface type display relatively broad spectral features in the valence band and finite spectral intensity at E_F, while spectra from the other surface type show well-defined valence band emission features and zero photoemission intensity at E_F. We propose that the spectral differences are due to a very short electron mean free path in this material, and our results are used to explain the differences between previously published photoemission spectra from this superconductor. We also report the results of an investigation of the electronic structure of defects in this material.     

Screening effects in the Ti 2p core level spectra of Ti-based ternary nitrides

The electronic structure of Ti-based ternary nitrides, Ti-X-N, with X = Si, Al or Cr, has been investigated by X-ray photoelectron spectroscopy and ultraviolet photoelectron spectroscopy. Striking screening effects associated with the decrease of the occupied states at the Fermi level have been observed in the Ti 2p core level spectra of the Ti-Si-N and Ti-Al-N systems with decreasing the Ti content, due to the reduction of the screening ability of conduction electrons. As a consequence of this effect, a strong increase of the ratio between the satellite or “poorly-screened” peak and the main line or “well-screened” peak in the Ti 2p spectra is observed. This behaviour strongly supports the explanation proposed by Porte et al. for the origin of the Ti 2p satellite structure in nearly stoichiometric TiN_x films. Likewise, shifts of the Fermi edge have been observed in the Ti-Si-N and Ti-Al-N systems with decreasing the Ti content. These shifts could be associated with the formation of a nanocomposite structure in which metallic clusters are embedded in an insulator matrix, or just to reflect a metal-to-insulator transition in those ternary systems. Although, the core level and valence band spectra do not allow to univocally decide on this point. The comparative study of the evolution of Ti 2p spectra and of the occupied states at the Fermi level in Ti-Si-N and Ti-Al-N systems suggests that the disappearance of the well-screened peak upon decreasing the Ti content is related not only with the decrease of the occupied states at the Fermi level, but with the formation of isolated metallic clusters embedded in an insulating matrix. The formation of small isolated TiN clusters could change the degree of localization of screening orbitals, therefore promoting the satellite at the expense of the main line in the Ti 2p core level spectra.     

YNi4Cu: XPS measurements and electronic structure calculation

The valence band and the core levels of the YNi₄Cu compound are studied using the X-ray photoemission spectroscopy. The valence band is compared with the theoretical calculation by the spin-polarized Tight Binding Linear Muffin Tin Orbital method. The dominance of the Ni 3d and Cu 3d states down to 5 eV below the Fermi level is found. The modification of the bands’ widths and positions can be well explained by the d–d repulsion model. The Ni 2p and Y 3d peaks resemble the results for pure metals.     

pd Interference effect in extreme ultraviolet photoelectron spectra of valence bands of cuprous halides

To interpret the spectra of the valence bands showing strong pd mixing, the interference between the copper and halogen sublattices in the photo-excitation process is taken into account. The ratio of integrated intensity of the upper band spectrum to that of the lower one can be explained well in terms of the pd mixing rates and the interference effect.     

Comparison of site-specific valence band densities of states determined from Auger spectra and XPS-determined valence band spectra in GeS (001) and GeSe (001)

Auger lineshapes of the Ge M₁M_4,5V and M₃M_4,5V and Se _M1M_4,5V transitions in GeS (001) and GeSe (001) are measured and compared to XPS valence band spectra. Distortions in both types of spectra due to inelastic scattering, analyzer and source broadening, and core level lifetime broadening are removed by deconvolution techniques. The valence band consists of three main peaks at ?2 eV, ?8 eV, and ?13 eV. There is excellent agreement of peak positions in AES and XPS spectra. The Auger lineshapes can be interpreted in terms of site-specific densities of states. They indicate that the states at ～?8 eV and at ～?13 eV are associated with the cation and anion sites respectively. The bonding p-like states at the top of the valence band have both cation and anion character. The Auger lineshapes indicate that the states closest to the valence band maximum are preferentially associated with Ge.     

A study by XPS and XRS of the participation in chemical bonding of the 3d electrons of copper,zinc and gallium

The participation of 3d electrons in chemical bonds and their part in the formation of valence bands was studied by X-ray photoelectron- and X-ray-spectroscopy for Cu, Zn and Ga phosphides, sulphides and oxides. Incomplete screening of (n + 1)s,p electrons through the nd shell leads to non-systematic changes of orbital energies and radii of the valence electrons in the first, second and third Group elements. It is reflected in the electronic structure of the respective compounds. A qualitative interpretation of XPS and XRS data for Cu, Zn, Ga phosphides is given. Possible reasons for phosphorus s band splitting for zinc and copper phosphides are considered. The interaction of 3d metal states and 3s, p third Period element states considerably affects the valence band of compounds, and this interaction should be taken into account when considering electronic structures of Cu, Zn and Ga compounds.     

Theory of Auger XVV spectra of solids: Many body effects in incompletely filled bands

The availability of empty electron states above the Fermi level and the presence of strong intra-atomic correlations may be expected to lead to new features in the Auger XVV spectra of conduction bands. Here, the qualitative aspects of the problem are studied within a simplified model. The case of a low equilibrium hole density n_h in the band is considered in detail and the various propagators are calculated in a low density approximation. It is found that correlation and shake up effects can be approximately factored out and the singularity exponent relevant to the problem is linear in n_h. In the single particle density of states we find a peak in the high binding energy side, which appears to be consistent with the experimental XPS spectra of Fe, Co and Ni. The Auger XVV spectra are obtained in the same approximation. As in the case of closed bands (n_h = 0) two-hole resonances appear in the spectra provided that intra-atomic correlations are strong enough (quasiatomic case), but the condition for their occurrence is found to involve the total band width, rather than the width of the occupied portion of the band only. Moreover, even in the quasiatomic case, the resonances have a width that is related to the energy difference between the top of the band and the Fermi level.     

Electronic structure of carbyne studied by X-ray photoelectron spectroscopy and X-ray emission spectroscopy

The electronic structure of amorphous carbyne has been investigated by X-ray photoelectron spectroscopy and X-ray emission spectroscopy. Carbyne band structure has been calculated semiempirically and the experimental data have been interpreted on the basis of the calculation results. The valence band width was found to be about 20 eV which is the same as that in all other condensed carbon structures. The fine satellite structure near the 1s line of carbon has been studied. It is shown that the energy bands in carbyne are arranged in a mirror-like way relative to the Fermi level. The real carbyne structure is susceptible to conformations which affect primarily the π-subband structure.     

X-ray photoemission studies of the electronic structure of single-crystalline CdO(100)

The electronic structure of single-crystalline CdO is investigated using X-ray photoemission spectroscopy. The surface is prepared by in situ Ar⁺ ion bombardment and annealing (IBA). The core level spectra before and after the IBA are presented, and reveal a dramatic reduction in the surface contamination. The semi-core Cd 4d level and valence band region following the IBA are also shown. The surface Fermi level is pinned 1.3±0.10 eV above the valence band maximum.     

Effect of preheating temperatures on microstructure and optical properties of Na-doped ZnO thin films by sol–gel process

The chemical composition, crystalline structure, surface morphology and photoluminescence spectra of Na-doped ZnO thin films with different heat treatment process were investigated by X-ray photoelectron spectroscopy, X-ray diffraction, atomic force microscopy and a fluorescence spectrometer. The results show that preferred orientation, residual stress, average crystal size and surface morphology of the thin films are strongly determined by the preheating temperature. The effects of preheating temperature on microstructure and surface morphology have been discussed in detail. The photoluminescence spectra show that there are strong violet & UV emission, blue emission and green emission bands. The violet & UV emission is ascribed to the electron transition from the localized level below the conduction band to the valence band. The blue emission is attributed to the electron transition from the shallow donor level of oxygen vacancies to the valence band, and the electron transition from the shallow donor level of interstitial zinc to the valence band. The green emission is assigned to the electron transition from the level of ionized oxygen vacancies to the valence band.