Core-hole Screening Research Articles

Understanding mixed valent materials: Effects of dynamical core-hole screening in high-pressure x-ray spectroscopy

Changes in the electronic structure of Yb, a material whose valence is modified under pressure, are observed with remarkable detail in x-ray absorption and emission data measured between ambient conditions and $20\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$. These changes are reproduced by a theory that essentially does not rely on experimental parameters, and includes dynamical core-hole screening. From the combined experimental and theoretical data we can firmly establish on a quantitative level how the valency of an intermediate valence material is modified by pressure. In metallic Yb it increases from 2 to $2.55\ifmmode\pm\else\textpm\fi{}0.05$ between 0 and $20\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$.

Surface states, surface potentials, and segregation at surfaces of tin-dopedIn2O3

Surfaces of ${\mathrm{In}}_{2}{\mathrm{O}}_{3}$ and tin-doped ${\mathrm{In}}_{2}{\mathrm{O}}_{3}$ (ITO) were investigated using photoelectron spectroscopy. Parts of the measurements were carried out directly after thin film preparation by magnetron sputtering without breaking vacuum. In addition samples were measured during exposure to oxidizing and reducing gases at pressures of up to $100\phantom{\rule{0.3em}{0ex}}\mathrm{Pa}$ using synchrotron radiation from the BESSY II storage ring. Reproducible changes of binding energies with temperature and atmosphere are observed, which are attributed to changes of the surface Fermi level position. We present evidence that the Fermi edge emission observed at ITO surfaces is due to metallic surface states rather than to filled conduction band states. The observed variation of the Fermi level position at the ITO surface with experimental conditions is accompanied by a large apparent variation of the core level to valence band maximum binding energy difference as a result of core-hole screening by the free carriers in the surface states. In addition segregation of Sn to the surface is driven by the surface potential gradient. At elevated temperatures the surface Sn concentration reproducibly changes with exposure to different environments and shows a correlation with the Fermi level position.

A study of many-body phenomena in metal nanoclusters (Au, Cu) close to their transition to the nonmetallic state

The results of a study of many-body phenomena in gold and copper nanoclusters are presented. The measured conductivity as a function of nanocluster height h was found to have a minimum at h ≈ 0.6 nm. Conductivity was local in character at nanocluster sizes l ≤ l c ≈ 2.5 nm. Changes in core hole screening and an anomalous increase in the Anderson singularity index α in gold and copper nanoclusters could be caused by changes in permittivity from metallic (ɛ → ∞) to nonmetallic (ɛ ∝ l 2). The many-body phenomenon characteristics observed in the X-ray photoelectron and tunnel spectra of gold and copper nanoclusters as the size of the nanoclusters changed led us to suggest changes in the band structure of the nanoclusters and, therefore, their possible transition from the metallic to nonmetallic state.

Dynamical core-hole screening in the x-ray absorption spectra of graphite,C60, and carbon nanotubes: A first-principles electronic structure study

We have implemented the effect of dynamical core-hole screening into a first principles electronic structure computational scheme. The method has been applied to the x-ray absorption spectrum of the carbon $K$-edge for graphite, fullerene ${\mathrm{C}}_{60}$ molecules, and single-wall carbon nanotubes. We demonstrate that a theory that includes dynamic core-hole screening improves drastically the description of the experimental data in comparison with both initial- and final-state calculations. Most of the experimentally observed spectral features are well described by our theory, both regarding the position, shape, and relative intensity of the peaks.

Momentum selectivity and anisotropy effects in the nitrogenK-edge resonant inelastic x-ray scattering from GaN

High-resolution soft x-ray emission and absorption spectra near the N $K$-edge of wurtzite GaN are presented. The experimental data are interpreted in terms of full-potential electronic structure calculations. The absorption spectra, compared with calculations including core hole screening, indicate partial core hole screening in the absorption process. The resonantly excited x-ray emission spectra show pronounced dispersion of spectral structures, which is attributed to effects of momentum conservation in the resonant inelastic x-ray scattering (RIXS) process. In development of GaN based optoelectronics, momentum selectivity in RIXS can be utilized to control development of band structure in GaN nanostructures.

Ab InitioTheory of Dynamical Core-Hole Screening in Graphite from X-Ray Absorption Spectra

We have implemented the effect of dynamical core-hole screening, as given by Mahan, Nozières, and De Dominicis, in a first-principles based method and applied the theory to the x-ray absorption (XA) spectrum of graphite. It turns out that two of the conspicuous peaks of graphite are well described, both regarding the position, shape, and relative intensity, whereas one peak is absent in the theory. Only by incorporation of both excitonic and delocalized processes can a full account of the experimental spectrum be obtained theoretically, and we interpret the XA spectrum in graphite to be the result of a well screened and a poor screened process, much in the same way as is done for core level x-ray photoelectron spectroscopy.

Modeling core-hole screening in core-excitation spectroscopies

We consider screening of the core-hole potential experienced by the ejected electron in core-excitation processes. This potential affects near-edge structure strongly, but it appears difficult to consistently obtain reliable screened core-hole potentials without substantial computation. Screening effects have both atomic and solid-state (or molecular/cluster) contributions. We emphasize aspects that must be true of “correct” screening models, in an attempt to circumvent the need to carry out full-scale randomphase approximation (RPA) or self-consistent-field calculations of the screening. We present and discuss three spectra computed with RPA and model screened potentials, suggesting ideas to be considered in future work developing screening models.

Experimental and theoretical surface core-level shifts of aluminum (100) and (111)

The surface core-level shifts of Al(111) and Al(100) have been measured using high-resolution core-level photoemission spectroscopy and calculated using density functional theory (DFT). For Al(100), the 2p core-level shift of the first (second) layer was determined to be –75 meV (+20 meV) from experiment and –71 meV (+20 meV) from the DFT calculations. For Al(111), the corresponding values are –27 meV (0 meV) from experiment and –14 meV (–) from the DFT calculations. Core-level splittings caused by the low-symmetry crystal fields at the (111) and (100) surfaces have also been studied. These splittings turn out to be much smaller than previously reported provided proper care is taken of the influence of the core hole screening and of core–valence exchange beyond the DFT level. Finally, the experimental Al 2p line shape was found to contain structure caused by a sharp no-phonon line and a broad and weak phonon replica. (Less)

Structural study of adsorption of isonicotinic acid and related molecules on rutile TiO 2(1 1 0) II: XPS

The adsorption of monolayers of the pyridine-carboxylic acid monomers (isonicotinic acid, nicotinic acid, and picolinic acid) on rutile TiO 2(1 1 0) has been studied by means of X-ray photoemission spectroscopy. An investigation of the O 1s spectra shows that the molecular carboxylic groups are deprotonated and, hence, that the molecules bind to the surface in a bidentate mode. Moreover, the binding energy of those core levels that are related to the pyridine ring atoms shift as a function of molecule relative to the substrate O 1s and Ti 3p levels, while the position of the core levels related to emission from the carboxylic group are constant relative to the substrate levels. The molecule-dependent shifts are attributed to local intermolecular interactions that determine the proximity of adjacent molecular rings and thus the core-hole screening response of the neighbouring molecules. We propose a simple molecular arrangement for each case which satisfies the known constraints.

Soft x-ray photoelectron spectroscopy of (HfO2)x(SiO2)1−x high-k gate-dielectric structures

Soft x-ray photoelectron spectroscopy has been used to study several (HfO2)x(SiO2)1−x film compositions. The relationships between composition and Si 2p and Hf 4f core level binding energies were investigated using nominally thick films. Both the Si 2p [Si4+] and Hf 4f features shift to lower binding energy by approximately 1.3 eV as the composition is varied from SiO2 to HfO2. The shift to lower binding energy is consistent with both an electron transfer model of the chemical environment and final-state core-hole screening resulting from differences in material polarizability. In addition, the Gaussian widths of the core levels narrow with increasing HfO2 content. Calculations of phonon broadening indicate that this trend is due instead to inhomogeneous disorder. The dielectric/Si interface was investigated with ultrathin (∼10–20 Å) films. There was virtually no difference in binding energies or Gaussian width among the various compositions, indicating a preferential composition of approximately x=0.5 at the interface. Interface suboxides were also investigated and indicate a quality entropy-driven oxide/silicon interface.

Photoemission spectroscopy of the evolution of In-terminatedInP(100)−(2×4)as a function of temperature: Surface- and cluster-related In4dlines

The evolution of clean, In-terminated $\mathrm{InP}(100)\ensuremath{-}(2\ifmmode\times\else\texttimes\fi{}4)$ surfaces is investigated by synchrotron-radiation-excited photoemission spectroscopy as a function of annealing temperature. As-prepared $\mathrm{InP}(100)\ensuremath{-}(2\ifmmode\times\else\texttimes\fi{}4)$ surfaces are found to be free of metallic indium, and the $4d$ core level shows two clear surface components. A third, indium-cluster-related component appears after annealing above $360\ifmmode\pm\else\textpm\fi{}10\ifmmode^\circ\else\textdegree\fi{}\mathrm{C},$ due to phosphorous desorption, and is accompanied by a corresponding reduction in intensity in the In-P surface component. Further annealing leads to a decrease in binding energy of the indium-cluster-related peak due to increased metallicity and hence core-hole screening in the clusters. The increasingly metallic nature of the indium clusters is also revealed by the appearance and growth of a Fermi edge in valence-band spectra.

Core-hole screening response in two-dimensional cuprates: A high-resolution x-ray photoemission study

We have studied the core level photoemission spectra of the two-dimensional cuprates ${\mathrm{Sr}}_{2}{\mathrm{CuO}}_{2}{\mathrm{Cl}}_{2},$ ${\mathrm{Sr}}_{2}{\mathrm{CuO}}_{2}{\mathrm{Br}}_{2},$ ${\mathrm{Ca}}_{2}{\mathrm{CuO}}_{2}{\mathrm{Cl}}_{2},$ ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{CaCu}}_{2}{\mathrm{O}}_{8+\ensuremath{\delta}},$ and ${\mathrm{Nd}}_{2}{\mathrm{CuO}}_{4},$ with particular focus on the screening response to core-hole creation in the $\mathrm{Cu}{\ensuremath{-}2p}_{3/2}$ level. The influence of the apex positions on the shape of the so-called main line is investigated, and found to be weak. Additionally, an Anderson impurity model was used to fit the shape of the main lines, obtaining good agreement with the data from ${\mathrm{Nd}}_{2}{\mathrm{CuO}}_{4}.$ For the other compounds, while the energy spread of the two screening channels (local and nonlocal) is well reproduced, the theory underestimates the width of the nonlocally screened feature. The shapes of the main lines are discussed in detail.

Importance of interatomic hole screening in core-level spectroscopy of transition metal oxides: Mn3shole states in MnO

Ab initio theoretical results are reported to determine the role of interatomic screening of the metal core hole in Mn 3s x-ray photoelectron spectra (XPS) of MnO. We present information that finally resolves the controversy about the relative importance of intra-atomic and interatomic effects to the XPS spectra of ionic manganese compounds. We focus on the transitions to high-spin 3s core hole states, using a nonorthogonal configuration interaction method. This method allows for a balanced treatment of configurations that involve different degrees of screening of the core hole. The differences between MnO and NiO are analyzed. Although at first glance the two XPS spectra seem to have similar features, the 3s hole final states have completely different characteristics. In MnO, interatomic screening of the core hole is found to play a minor role. This is in contrast with NiO, where the inclusion of interatomic screening of the metal core hole has been shown to be crucial for a proper explanation of the Ni 3s XPS. The main reason for the difference is an essentially atomic effect, namely the smaller electron affinity of Mn as compared to Ni. We have assigned the weak feature observed in the Mn 3s XPS spectrum of MnO at around 10 eV to be a charge-transfer-type high spin satellite.

Chemical configuration of nitrogen in ultrathin Si oxynitride on Si(100)

Chemical bonding structures of nitrogen within very thin oxynitride films on Si(100) have been investigated by high-resolution photoemission spectroscopy using synchrotron radiation. The oxynitride films nitrided by the rapid thermal process with NO and N 2 O are systematically compared. Two distinct N 1s components are resolved for both films with a binding-energy difference of ∼0.61 eV, which are assigned to the N atoms at the interfaces and those in the SiO 2 matrix. Both components are unambiguously attributed to represent a N-Si 3 like chemical configuration with three nearest-neighbor Si atoms. The energy shift of 0.61 eV between these components is thought be due to the second-nearest-neighbor effect combined with core-hole screening. The difference between N 2 O- and NO-nitrided films and the interface suboxide species identified by Si 2p core levels are discussed.

Ab initiocalculation ofKLVAuger spectra in Si

We present an ab initio Green's-function formalism for predicting $\mathrm{CCV}$ Auger spectra for solids. The formalism takes into account core-hole screening, final-state interaction effects, and angular momentum dependence. It is applied to calculate the ${\mathrm{KL}}_{1}V$ and ${\mathrm{KL}}_{2,3}V$ Auger spectra for bulk silicon. We achieve fair agreement with high-resolution Auger experiments recently performed on silicon and gain a more quantitative understanding of the role of the final-state hole-hole interaction in Auger processes.

MULTISITE EFFECTS ON THE CORE-LEVEL PHOTOEMISSION IN Sr2RuO4

Multi-Ru-site effects on the photoemission from the Ru 3d core level in Sr 2 RuO 4 are discussed on the basis of numerical diagonalization calculations for large-size cluster models. It is shown that the electrons on the Ru 4d ions neighboring the core-hole site participate actively in the core-hole screening, which indicates the importance of so-called "nonlocal screening" in Sr 2 RuO 4. However, since it occurs via the Ru 4d(xy)– O 2pπ bonding, the photoemission intensity which corresponds to the nonlocal screening is not large, compared with the Cu 2p photoemission in high-Tc-related cuprates. From the spectral analysis, the Coulomb interaction strength between Ru 4d electrons is estimated to be comparable with the width of Ru dε bands. In other words, Sr 2 RuO 4 belongs to a category of strongly correlated electron systems.

Growth and electronic structure of CuFPc on Si( [formula omitted

The stability of hexadecafluoro copper phthalocyanine ultrathin films after the deposition onto clean Si(1 0 0)2×1 substrate has been studied by means of X-ray and ultraviolet photoemission spectroscopies. In particular the nature of a strong metallic Cu 2p component in the monolayer regime film has been investigated. These data have been compared with the results acquired on a thick film deposited on SiO 2 substrate before and after an in situ thermal annealing. The valence band features indicate that the bulk-like molecular electronic structure is already present even at monolayer stage. Instead, when the annealed film presents the metallic Cu 2p structure, the molecular structure is completely destroyed. This indicates that the metallic component in the Cu 2p spectrum is not due to the loss of the Cu atom during the deposition but can be attributed to the screening of the Cu 2p core hole by the high density of electrons at the Si(1 0 0)2×1 surface.

Core-hole screening effects in the initial state of Auger emission across the transition metal series

Supercell method is used to study the relaxation and screening effects on the initial state of the Auger transition in metals. Our consideration is based on the assumption that when a core-hole exists long enough before the Auger transition occurs, the occupied valence states relax to screen the core-hole which results in a redistribution of the valence electrons, in particular within the atom that contains the core-hole. In order to make the interaction between the core-holes sites at different atoms negligible, the real metal is simulated by supercells repeated periodically. In each supercell one atom is considered to have a core-hole and many others not to have one. The electronic states concerned by the Auger transition are calculated by the self-consistent full-potential linearized augmented plane wave (FLAPW) method. Different responses of the local valence band on the site of the core-hole have been shown depending on whether the d-bands are partially or completely filled. According to the final state rule, the screening to the two holes in the local valence band after the Auger transition has also been considered, as examples, for Ni and Cu metals. The result shows that, with the existence of two holes in them, the states of the local valence band of Cu relax to atomic-like impurity states, while the local valence band of Ni changes to a much narrow band at the bottom of the original band. As examples, L 3VV and M 1VV Auger spectral profiles of Cu have been calculated in reasonably good agreement with the experiment.

Mn 3d derived contribution to the valence band of MBE grown cubic MnTe

Photoemission spectroscopy was applied to determine the valence band density-of-states in zinc blende MnTe grown by MBE on a CdTe(100) substrate. The Mn 3d derived contribution to the electronic structure was revealed by resonant photoemission experiments. Our experimental results are compared to the available configuration-interaction cluster models. An important role of the core hole screening (through charge transfer from ligand orbitals) on the whole Mn 3d derived photoemission spectrum (including the satellite structure observed below the valence band) was clearly confirmed.

Core-level shifts of low coordination atoms at the W(320) stepped surface

We study the surface core-level shifts (SCLS's) at the W(320) surface by ab initio calculations, including both initial-state and final-state effects. This stepped surface features (110) terraces with five atom rows, separated by single atomic steps. We find that the final-state effect is important for the proper prediction of the SCLS at the W(320) surface. Unlike the W(110) surface where the final-state effect increases the SCLS to lower binding energy, the reverse is true in the W(320) surface, indicating inefficient core-hole screening at the stepped surface. Our results provide a theoretical basis for the interpretation of core-level photoemission spectra from the W(320) surface.