The probing depth in photoemission and auger-electron spectroscopy

Electron scattering lengths, or escape depths, have been compiled from the literature for twenty different materials in the energy range 0–3000 eV. The energy dependence of the escape depth and its implications for interpretation of photoemission and Auger-electron data are discussed.     

Direct photoemission spectroscopy and electronic properties of in situ grown,strained high-Tc and related oxide films

We have optimized laser ablation thin film deposition and transfer procedures within synchrotron vault, specifically to perform angle-integrated and angle-resolved photoemission spectroscopy (ARPES) on high-T_c and related films without cleaving the samples. However, the chain-containing phases like YBCO-123 easily loose surface oxygen and do not exhibit stable Fermi edge, hence are not suitable for ARPES studies. Direct in situ ARPES studies on strained LSCO-214 films show striking strain effects on the electronic structure. The Fermi surface (FS) of LSCO evolves with doping, yet changes even more significantly with strain. The strain changes the FS topology from hole-like to electron-like, and causes band dispersion along k_x and the Fermi level crossing before the Brillouin zone boundary, in sharp contrast to the ‘usual’ flat band remaining ≈30 meV below E_F measured on unstrained samples. The associated reduction of the density of states does not diminish the superconductivity; T_c is enhanced in all our strained samples. Implications for the evolving high-T_c theory and studies of nano-engineered film heterostructures are briefly discussed.     

Bulk and surface plasmon excitation in X-ray photoemission

A quantitative theory of bulk and surface plasmon excitation by X-ray photoelectrons in thin metallic films is presented. The shape and strength of inelastic peaks in energy spectra are calculated. Their dependence on the point of primary excitation, energy and direction of the outgoing electron is discussed in the case of X-ray photoemission from aluminium.     

X-ray photoelectron spectroscopy analysis of electronic states in the oxide layer on an ultradisperse copper surface

X-ray photoelectron spectroscopy (XPS) is used to study the electronic states of copper oxide covering a thin (2.0 ± 0.5 nm) layer of ultradisperse copper. A reduction in the thickness of the CuO layer is found to cause extremal behavior in the XPS spectrum parameters as opposed to their monontonic variations for CuO nanoparticles. This behavior is explained by the competition of two factors which affect the system under study: the dimensions of the substrate material and the electrostatic field in it.     

X-ray absorption and photoemission spectroscopy of 3C- and 4H-SiC

We have studied the electronic properties of 3C- and 4H-SiC with X-ray absorption (XAS). Particular emphasis is placed on the conduction bands because they exhibit larger differences between the various SiC polytypes than valence bands. XAS spectra at the Si2p and C1s edges provide projections onto Si3d, 4s and C2p conduction band states. We explain the observed differences in the Si L_2,3 XAS data to arise from transition into dispersive bands which occur at the M and K point of the hexagonal Brillouin zone. The XAS data are sensitive to a difference in the dispersion of the two lowest conduction bands. For 3C-SiC the dispersion is larger than for 4H-SiC in agreement with theory. We compare the XAS data at the Si L edge with CFS and CIS spectra and find that the SiLVV Auger is dominant.     

Buildup of the InSe/M interface (M Pd, Au) studied by X-ray photoemission and X-ray absorption spectroscopy

The electronic properties of InSe/M (M  Pd, Au) interfaces have been studied by X-ray photoemission measurements. For the InSe/Pd interface, it has been found that Pd atoms diffuse into the InSe lattice at early stages of Pd coverage, acting as acceptor centers. As the Pd coverage increases, a Pd-InSe reaction determines the electronic behaviour of the interface. However, for Pd coverages higher than 1 ML, the barrier formation tends to be controlled by an emerging bulklike Pd overlayer. Despite the atomic structure of this system is far from that expected for an ideal Schottky one, the final electronic barrier value is close to that expected for an abrupt InSe/Pd Schottky interface. On the contrary, the InSe/Au system appeared to behave as a quasi-ideal abrupt Schottky interface. Annealing processes performed at temperatures higher than 600 K alter this scheme, as revealed by X-ray absorption spectroscopy measurements, enhancing diffusion of Au atoms into InSe. In any case, the electronic barrier results to be determined by the Au overlayer formed.     

Coherent peaks and minimal probing depth in photoemission spectroscopy of Mott-Hubbard systems

We have measured hard x-ray photoemission spectra of pure vanadium sesquioxide (V(2)O(3)) across its metal-insulator transition. We show that, in the metallic phase, a clear correlation exists between the shakedown satellites observed in the vanadium 2p and 3p core-level spectra and the coherent peak measured at the Fermi level. Comparing experimental results and dynamical mean-field theory calculations, we estimate the Hubbard energy U in V(2)O(3) (4.20+/-0.05 eV). From our bulk-sensitive photoemission spectra we infer the existence of a critical probing depth for investigating electronic properties in strongly correlated solids.     

Electronic surface states investigated by means of photoemission spectroscopy

Ultraviolet photoemission spectroscopy (UPS) as an investigative method for surface states is presented. The measured energy distribution curve (EDC) furnishes information on the electronic density of states distribution in the valence band, nearest to the core levels and surface states. The measured angle resolved EDC's provide the possibility to determine the energy-momentum E(k) dependence of electrons in the bulk of the crystal and on the surface. Application of the synchrotron-storage ring system as a source of ultraviolet radiation in the energy range from 10 to 300 eV opens new avenues to investigate structure of electronic states. It features the following possibilities: (1) To distinguish the contribution of the surface and bulk states to the obtained EDC by measuring the change of the EDC when varying the exciting energy hv around the minimum of the escape depth (E = 80 eV) and outside of this region. (2) To discriminate in the valence band the contribution of the d-electrons from that of the s-p electrons due to a different change of the photoemission cross section of the d and s-p electrons with a change of hv. (3) To recognize the localized and delocalized contribution to the density of states in the valence band and to determine E(k) for these electrons by measurements of the angle resolved EDC. (4) To obtain information on the initial and final state distribution using the constant initial states (CIS) and/or constant final states (CFS) techniques. The potential of the photoemission technique will be illustrated by the results of the electronic structure investigation of some metals and semiconductors.     

The electronic structure of KMnO4 investigated by photoemission and electron-energy-loss spectroscopy

From photoemission and electron-energy-loss data the following picture of KMnO₄, with Mn^VII (with a formal charge state Mn⁷⁺ (3d ⁰)) tetrahedrally surrounded by four O^2–-ions, is deduced: strong covalent bonding between Mn^VII and O^2– leads to a considerable occupation of the Mn-3 d shell. The ground state of the (MnO₄)^–1 molecule is an orbital and spin singlet as seen by the absence of any multiplet splitting in the Mn core levels. The valence band shows a four peak structure extending form 4 eV to 8 eV below the Fermi energy. The first peak at 4.2 eV has mainly O-2p character. The remaining peaks are of strongly mixed Mn-3d/O-2p character due to the covalent bonding. This mixing decreases with increasing binding energy. The electron energy loss data show a variety of structures between 2 eV and 10 eV independent of the primary electron energy which defines them as dipole allowed charge-transfer transitions. An additional excitation at 1.8 eV decreases quickly in intensity with increasing electron energy which classifies it as a dipole or spin forbidden transition in the compound. This energy is close to the value of 1.6 eV reported for the activation energy observed in electrical transport data. The results are compared to quantum chemical molecular orbital calculations of the (MnO₄)^–1 molecule.Physics Department, Allahabad University Allahabad 211002, India     

Structural,electronic, and optical properties of bulk Cu2Se

By using first-principles calculations within the density functional theory and the many-body perturbation theory, we investigate the structural, electronic, and optical properties of bulk Cu₂Se with a recently discovered low-temperature layered configuration. We demonstrate that the effects of the van der Waals forces significantly modify the interlayer binding and distance in the layered Cu₂Se, while the band gap is invariant. Our density functional theory and post-processing GW calculations reveal that for the layered structure, GW correction remedies the serious band-gap underestimation of the density functional theory from 0.12 eV to 0.99 eV. By solving the Bethe–Salpeter equation, we find that the optical gap of the layered Cu₂Se is 0.86 eV, which is in close agreement with previous experimental observations. In addition, we show that the high-temperature fluorite structure has no band gap, even after GW correction, explaining that the band gap controversy among the theories stems from different structural models. This work may serve as an important guide in designing and evaluating photovoltaic devices using Cu₂Se-based materials.     

X-ray photoemission spectroscopy and secondary-ion mass spectroscopy applied to the compositional study of pre-colonial pottery from Pantanal,Brazil

X-ray photoemission spectroscopy (XPS) and time-of-flight (TOF) secondary-ion mass spectroscopy (SIMS) have been applied to investigate potsherd samples from Pantanal, Brazil. One of the potsherds presented burnt bone as an additive, which was characterized by XPS as carbonate hydroxyapatite. For shell-tempered ceramics the phase present in shells after firing was identified by X-ray diffraction. TOF-SIMS was used to study the distribution of quartz as temper in one of the sherds. XPS was also applied to the characterization of the finishing external layer of the ceramic vessels. In this case, based on the Fe 2p spectra of the sherd's interior and outermost layers, it was possible to prove that their difference in coloration is due to black heart formation. Hierarchical clustering analysis and principal component analysis of the XPS elemental data enabled the potsherds to be classified with respect to their clay composition.     

Effects of surface roughness on surface analysis via soft and ultrasoft X-ray fluorescence spectroscopy

A model for calculating surface roughness effect on the intensity of emitted X-rays in the soft and ultrasoft X-ray region is presented and discussed. The results of calculations based on this model are compared to the results obtained experimentally using Al gratings as models for rough surfaces. It is shown that the correspondence between calculated and experimental intensities for Al–K is sufficient to corroborate the use of this model as a means of estimating maximum allowable surface roughness without a significant loss in intensity. It is also shown that this maximum allowable roughness estimate can be calculated with a knowledge of only the sample density and wavelength of the X-ray emission.     

X-ray standing wave analysis with synchrotron radiation applied for surface and bulk systems

The advantageous use of synchrotron radiation for X-ray standing wave measurements is demonstrated. For bulk-like systems As implanted in Si was analysed with two reflection orders (220) and (440) which shows that lattice relaxation around the As atom can be measured. The case of Br chemisorbed on Si(111) illustrated the accomplished decrease in measuring time and increase in precision.     

Valence fluctuations and oxygen dimerization in high-temperature superconductors from X-ray photoemission spectroscopy

The high-temperature superconducting Cu oxides — which are intrinsically Mott insulators — are strongly correlated metals because of valence fluctuations involving Cud ⁹–d ¹⁰ configurations. Temperature-dependent X-ray photoemission spectroscopy of theO 1s and Cu 2p levels in the range 10–300 K provides evidence for O-dimerization, with corresponding increase of the Cud ¹⁰O 2p ⁵ well-screened XPS-final state belowT _c . This supports the suggestion of dynamical mixed-valence and double-exchange antiferromagnetic interaction as an important mechanism for the origin of highT _c superconductivity.     

X-ray photoelectron spectroscopy for surface film analysis in corrosion research

Surface films on metals and alloys often protect them from reaction with the environment, and hence a knowledge of their protective properties and composition could be invaluable for predicting their corrosion behaviour. XPS (x-ray photoelectron spectroscopy) could provide a quantitative analysis of the chemical composition, the nature of valence states and elemental distribution within the surface films. The present paper reviews the potential of this technique in corrosion studies. A brief review of the work done on the passivation of iron and iron-chromium alloys and on the inhibition studies on copper base alloys has been given. A few examples of investigations carried out at authors’ laboratory are also included. An attempt has been made to establish a correlation between the compositions of the films formed and corrosion behaviour of carbon steel in 10.5 pH lithium hydroxide solution and of Cu-Ni alloys and sacrificial Al-Zn-Sn alloys in synthetic sea-water.     

Optical spectroscopy of electronic surface states

In this paper a number of optical spectroscopic methods for investigating surface electronic structure are discussed, including reflectance techniques, ellipsometry, surface photoconductivity and surface photovoltage spectroscopy. In addition to electron scattering techniques and UV-photoemission, optical spectroscopic methods have contributed much in recent times to the understanding of electronic surface states on solids. A discussion and comparison is given of the nature and significance of information obtained by these methods and exemplary experimental results are presented to illustrate the contribution of the optical techniques to the present knowledge about surface states. The relation between information obtained from optical measurements and electron spectroscopy is considered.     

Band alignment at the In2S3/Cu2ZnSnS4 heterojunction interface investigated by X-ray photoemission spectroscopy

The band alignment at the In₂S₃/Cu₂ZnSnS₄ heterojunction interface is investigated by X-ray photoemission spectroscopy. In₂S₃ is thermally evaporated onto the contamination-free polycrystalline Cu₂ZnSnS₄ surface prepared by magnetron sputtering. The valence band offset is measured to be 0.46 ± 0.1 eV, which matches well with the valance band offset value 0.49 eV calculated using “transitivity” method. The conduction band offset is determined to be 0.82 ± 0.1 eV, indicating a ‘type I’ band alignment at the heterojunction interface.     

Resonant photoemission spectroscopy study of electronic structures of LiMn2O4

The valence-band resonant photoemission spectra (RPES) of LiMn₂O₄ have been measured throughout the Mn3p absorption edge. Based on the RPES data, the contribution of Mn3d states to the valence band of LiMn₂O₄ has been described and, consequently, the detailed hybridization between O2p and Mn3d states in the valence-band was determined.     

Evidence of SiO at the Si-oxide interface by surface soft X-ray absorption near edge spectroscopy

Using synchrotron radiation a new surface sensitive spectroscopy has been applied to determine the local structure of the first surface oxide layer formed on the Si(111) surface. The Surface Soft X-ray Absorption (SSXA) spectra have been measured. From the analysis of the X-ray Absorption Near Edge Structures (XANES) we have extracted structural information. We have first determined that bulk amorphous SiO has a characteristic microsopic structure, which cannot be described by the random alloy or microcrystalline (Si + SiO₂) mixture models. The oxide layer formed on the Si(111) surface by ground-state molecular excitation in ultra high vacuum at temperatures (～700°C) approaching the oxide dissociation point has this unique SiO local structure. Such SiO layer not formed at room temperature is expected to be present in the SiSiO₂ interface grown at high temperature. An electronic transition to empty states at the SiSiO₂ interface has been observed.