S. Hüfner

Photoelectron spectroscopy : principles and applications

1. Introduction and Basic Principles.- 2. Core Levels and Final States.- 3. Charge-Excitation Final States: Satellites.- 4. Continuous Satellites and Plasmon Satellites: XPS Photoemission in Nearly Free Electron Systems.- 5. Valence Orbitals in Simple Molecules and Insulating Solids.- 6. Photoemission of Valence Electrons from Metallic Solids in the One-Electron Approximation.- 7. Band Structure and Angular-Resolved Photoelectron Spectra.- 8. Surface States, Surface Effects.- 9. Inverse Photoelectron Spectroscopy.- 10. Spin-Polarized Photoelectron Spectroscopy.- 11. Photoelectron Diffraction.- A.1 Table of Binding Energies.- A.2 Surface and Bulk Brillouin Zones of the Three Low-Index Faces of a Face-Centered Cubic (fcc) Crystal Face.- A.3 Compilation of Work Functions.- References.

The Cu valence in the highT c superconductors and in monovalent, divalent and trivalent copper oxides determined from XPS core level spectroscopy

A number of theoretical models have been developed to explain the unexpected high temperature superconductivity in La-Me-Cu-oxides (Me=Ba, Sr) and Y-Ba-Cu-oxides. Some of these models invoke charge fluctuations on the copper ions in the superconductors between a 2+ and a 3+ state. In order to test these possibilities we have measured the Cu−2p3/2-core level spectra of NaCuO2 in which the copper ion is in a 3+ state and compared it with the core line position in pure CuO and to superconducting oxides. The data strongly suggest, that there is, if any very little Cu3+ in the superconducting compounds present. However, we notice, that in comparison to trivalent and monovalent copper oxides the Cu−2p3/2 line in CuO and the superconducting oxides is unexpectedly broad. The cause of this large linewidths remains so far unexplained.

Electronic structure of 3d-transition-metal oxides : on-site Coulomb repulsion versus covalency

We have performed photoemission and inverse photoemission experiments on a series of 3d-transition-metal oxides with formal ionic configuration from to . The photoemission core-level spectra are analysed in terms of a simple cluster model leading to estimates for the charge-transfer energy , the Coulomb correlation energy , and the hybridization strength V. It is found that the ratio of the correlation energy to the hybridization energy significantly decreases from the late to the early transition metal oxides. This trend is attributed mostly to the increasing number of empty d states in the early transition metals which enhances the effective metal-ligand hybridization. We also compare the experimental valence band spectra with densities of states (DOS) from band-structure calculations. The rather good agreement between the theoretical DOS and the measured single-particle excitation spectra of the early 3d-transition-metal oxides as opposed to the failure of the one-electron description for most of the late transition metal oxides supports the results of the cluster model analysis.

The electronic structure of PdO found by photoemission (UPS and XPS) and inverse photoemission (BIS)

The electronic structure of the 4d transition-metal oxide PdO is investigated by photoemission (UPS and XPS), inverse photoemission (BIS; ), and electron energy loss spectroscopy in reflection geometry (REELS; primary energy, ). The valence band spectra are compared to recent theoretical ab initio band-structure calculations. Good agreement between theory and experiment is found in the occupied part of the band structure down to 8 eV below as well as in the unoccupied part up to 6 eV above . This confirms the common view that the electronic structure of the 4d transition-metal oxides, e.g. PdO, can be explained in terms of a single-electron picture. Nevertheless correlation effects among the Pd 4d electrons are clearly visible in the spectra, as e.g. satellites of the Pd core level spectra. In order to explain the origin of these satellites we performed simple cluster model calculations and as a result we can explain one satellite in a screening picture by means of a charge transfer process. In addition radiation damage effects in PdO during the electron bombardment in the BIS experiments are reported. This is explained by the formation of the Pd -like states connected with oxygen loss due to the electron bombardment.

Supramolecular Assemblies Formed on an Epitaxial Graphene Superstructure

The seminal work of Novoselov et al. has stimulated great interest in the controllable growth of epitaxial graphene monolayers. While initial research was focussed on the use of SiC wafers, the promise of transition metals as substrates has also been demonstrated and both approaches are scalable to large-area production. 12] The growth of graphene on transition metals such as Ru, Rh and Ir leads to a moir!-like superstructure, 10,12,13] similar to that observed for BN monolayers. Here we show that such a superstructure can be used to control the organization of extended supramolecular nanostructures. The formation of two-dimensional supramolecular arrays has received increasing attention over recent years primarily due to potential applications in nanostructure fabrication as well as fundamental interest in self-assembly processes. Such studies can be highly dependent on the nature of the substrate used, and the interplay between surface and adsorbed supramolecular structure is a topic of significant conjecture. Until now metallic surfaces or highly oriented pyrolytic graphite (HOPG) have typically been the surfaces of choice for such studies. Our results demonstrate that graphene is compatible with, and can strongly influence molecular selfassembly. We have studied the adsorption of perylene tetracarboxylic diimide (PTCDI) and related derivatives on a graphene monolayer grown on a Rh(111) heteroepitaxial thin film (Figure 1). In particular, we show that a near-commensur-

Strong hybridization in vanadium oxides: evidence from photoemission and absorption spectroscopy

We present x-ray photoemission spectra of the vanadium oxides , and , and their analysis in terms of a simple cluster model based on the Anderson impurity Hamiltonian. The electronic structure of these materials is characterized by a strong V 3d-O 2p hybridization energy which exceeds the energy scales related to on-site Coulomb correlation and metal-ligand charge transfer. This result is at variance with the usual Mott-Hubbard picture, but agrees with recent studies of other early 3d transition metal compounds. The V 3d ground-state occupations obtained by the cluster-model analysis are considerably higher than the values derived from the formal valencies. Covalency also affects the exchange splitting observed in the V 3s core-hole spectra. X-ray absorption measurements and resonant photoemission spectroscopy at the V 2p-3d threshold provide further evidence for a strong V 3d-O 2p coupling.

The hole concentration on oxygen sites in the high Tc superconductor Y1-Ba2-Cu3-O7−x

From XPS core level spectroscopy the average copper charge on the Cu sites in the high temperature superconductor Y1Ba2Cu3O7−x is determined as function of the oxygen vacancy concentrationx. Analysis of these data leads to the suggestion that there are holes on the oxygen sites in the basal plane of the crystal structure. The probability for holes on these oxygen ions is rather constant for 0≦x≦0.3 with a value of 0.64 and decreases to zero forx=0.5. The dependence of the superconducting transition temperature on the hole concentration is discussed. An energy level diagram for Cu2+ and Cu3+ in YBa2Cu3O7−x is constructed.

Photoemission and inverse photoemission spectroscopy of NiO

Abstract Photoemission (UPS) and inverse photoemission (BIS) spectra of NiO grown as a thin film on metallic Ni are reported. The data are discussed with respect to the electronic structure of the occupied and empty states in NiO.

Core-level binding energy shifts in dilute alloys

Abstract Core level binding energy shifts for the element A of a large number of dilute alloys AB have been measured, where the concentration of the dilute component A is 10% or less. The experimental shifts are analysed in terms of alloy heat of formation data, for which we have used Miedemas semi-empirical scheme. Good overall agreement is obtained between the experimental data and the calculated values, encouraging the use of electron spectroscopy for evaluating solution energies of metallic systems.

Photoemission on the highT c superconductors Y−Ba−Cu−O

XPS and UPS photoemission experiments on the highTc superconductors (Tc≈90 K) with nominal composition YBa2Cu3O9-y (y≈2) show the following:a)The density of electronic states at the Fermi energy is very small, much smaller than in pure Cu.b)The Cu 2p spectra show only a Cu2+ contribution.c)The Ba core levels show a structure with two components of nearly equal magnitude, which leads to the suggestion that these compounds have large O2− vacancies coordinated to Ba2+ sites.d)Annealing at 400°C under UHV conditions leads possibly to a partial reduction of Cu2+ to lower Cu valence states and to a small increase of the O2− vacancy component of the Ba2+ line.