Surface termination of BaTiO3 (001) single crystals: A combined electron spectroscopic and theoretical study

The surface termination of oxide surfaces is of crucial importance for the growth of a second material like metals or other oxides. In this study we have investigated the surface of a BaTiO₃ (001) single crystal during sample preparation by various electron spectroscopic methods. It is shown by X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS) and metastable induced electron spectroscopy (MIES) that during sputtering a Ba rich overlayer is formed, in which the Ba²⁺ ions are under coordinated. Below this layer, an oxygen deficient BaTiO₃ layer is found. During annealing, we observe the reformation of the crystalline structure. UP and MIE spectra provide clear evidence of a BaO terminated surface. X-ray photoelectron diffraction studies support this result, comparing recorded polar angle scans with calculated intensity modulations using multiple scattering cluster models.     

X-ray photoelectron diffraction study of Cu(1 1 1): Multiple scattering investigation

Multiple scattering theory based on a cluster model is used to simulate full-hemispherical X-ray photoelectron diffraction measurements in order to verify how state of the art multiple scattering simulations are able to reproduce the experiment. This approach is applied to the Cu(1 1 1) surface for two different photoelectron kinetic energies. Differences and similarities between single and multiple scattering are discussed in comparison with experimental results. We find that the present approach gives very good results despite some limitations.     

Photoelectron diffraction at low energies: the applicability of single scattering calculations

The energy dependence of the 4f core level photoemission intensity from Ta(100) has been modeled at low electron kinetic energies using the single scattering cluster (SSC) approximation. The results show a strikingly similar shape to that of a full multiple scattering calculation, particular at electron kinetic energies less than 75 eV. A comparison with the experimentally measured surface/bulk core level intensity oscillations indicates that the first interlayer spacing of the Ta(100) surface is contracted by 11 ± 3%, consistent with previous determinations. These results suggest that SSC calculations can model low energy photoelectron diffraction data from surface shifted core levels with sufficient accuracy to enable a quantitative structural determination of clean, unreconstructed surfaces.     

GaN表面极性的光电子衍射研究

利用x射线光电子衍射的极角扫描模式,采集了GaN（0001）表面由(1010)和(1120)晶面产生的光电子衍射实验曲线,并运用光电子衍射的前向聚焦效应确定了GaN(0001)表面是Ga在最外层的极性面.利用与能量有关的光电子衍射即角分辨光电发射精细结构谱技术并结合多重散射团簇模型计算对GaN(0001)表面的极化性质进行了研究,进一步证实了GaN表面是Ga在最外层的极性面. 关键词： GaN 表面极性 光电子衍射     

Defect states at the TiO2(110) surface probed by resonant photoelectron diffraction

The charge distribution of the defect states at the reduced TiO(2)(110) surface is studied via a new method, the resonant photoelectron diffraction. The diffraction pattern from the defect state, excited at the Ti-2p-3d resonance, is analyzed in the forward scattering approach and on the basis of multiple scattering calculations. The defect charge is found to be shared by several surface and subsurface Ti sites with the dominant contribution on a specific subsurface site in agreement with density functional theory calculations.     

FDMX: extended X‐ray absorption fine structure calculations using the finite difference method

A new theoretical approach and computational package, FDMX, for general calculations of X‐ray absorption fine structure (XAFS) over an extended energy range within a full‐potential model is presented. The final‐state photoelectron wavefunction is calculated over an energy‐dependent spatial mesh, allowing for a complete representation of all scattering paths. The electronic potentials and corresponding wavefunctions are subject to constraints based on physicality and self‐consistency, allowing for accurate absorption cross sections in the near‐edge region, while higher‐energy results are enabled by the implementation of effective Debye–Waller damping and new implementations of second‐order lifetime broadening. These include inelastic photoelectron scattering and, for the first time, plasmon excitation coupling. This is the first full‐potential package available that can calculate accurate XAFS spectra across a complete energy range within a single framework and without fitted parameters. Example spectra are provided for elemental Sn, rutile TiO₂ and the FeO₆ octahedron.     

Structure of the Rh2O3(0001) surface

We have studied the (0001) surface termination of Rh₂O₃ on a Rh(111) single crystal using a combination of high resolution core level spectroscopy, low energy electron diffraction, scanning tunneling microscopy and density functional theory. By exposing the Rh(111) to atomic oxygen we are able to grow Rh₂O₃ layers exposing the (0001) surface. The experiments support the theoretical predictions stating that the surface is terminated with an O–Rh–O trilayer yielding a RhO₂ termination instead of a bulk Rh₂O₃ termination. The structural details as found by the DFT calculations are presented and reasons for the previously observed strong differences in catalytic activity between the structurally similar RhO₂ surface oxide, and the Rh₂O₃(0001) surface are discussed.     

Characterization of 1T-TiSe2 surface by means of STM and XPD experiments and model calculations

Scanning tunneling microscopy (STM) and X-ray photoelectron diffraction (XPD) are applied to study the surface of layered dichalcogenide 1T-TiSe₂. XPD pattern simulation for the 1T-TiSe₂ surface is performed in the approach of electron multiple scattering within the EDAC code: considered are models of structural defects in the 1T-TiSe₂ lattice, relaxation contraction (expansion) of surface layers and van der Waals gap, and deviation of the 1T-TiSe₂ surface geometry from the basal plane (001). The atomic structure of 1T-TiSe₂ surface layers is reconstructed from the XPD pattern on Se(LMM) and Ti2p core level using the photoelectron holography scattering pattern extraction algorithm with maximum entropy method (SPEA-MEM). The results of the 3D reconstruction are in agreement with the XPD pattern simulation data. In both cases, the TiSe₂ surface corresponds to 1T polytype; an increase is observed in the parameter a₀ and in the van der Waals gap between two surface slabs. It is assumed that similar structural distortions of the 1T-TiSe₂ lattice lead to the formation of an energy gap between the valence band and the conduction band of titanium diselenide, which was observed earlier by photoemission spectroscopy and follows from the theoretical calculations.     

X-ray photoelectron diffraction study of ultrathin PbTiO3 films

Full hemispherical X-ray photoelectron diffraction (XPD) experiments have been performed to investigate at the atomic level ultrathin epitaxial c-axis oriented PbTiO₃ (PTO) films grown on Nb-doped SrTiO₃ substrates. Comparison between experiment and theory allows us to identify a preferential ferroelectric polarization state in a 60 ? -thick PTO film. Multiple scattering theory based on a cluster-model [ Phys. Rev. B , 075404 (2001)] is used to simulate the experiments.     

A photoelectron spectroscopy study of ultra-thin epitaxial alumina layers grown on Cu(111) surface

Thin epitaxial alumina layers were grown on the Cu(111) surface using simultaneous aluminum deposition and oxygen exposure. Different substrate temperatures during the deposition resulted in layers with different thicknesses, growth rates, crystallinity and epitaxy. Low energy electron diffraction measurements confirmed the epitaxial growth for substrate temperatures above 870 K. The Al 2p doublet was studied by means of photoelectron spectroscopy in order to determine the alumina termination at the metal-oxide interface. A strong dependence on the preparation temperature was found and both aluminum and oxygen terminated interfaces were created.     

Structural defects on a 1T-TiSe2 surface: Experiment and model calculation of photoelectron diffraction

A 1T-TiSe₂ surface is studied by means of scanning tunneling microscopy (SMT) and X-ray photoelectron diffraction (XPD). The diffraction patterns were modeled in the multiple electron scattering approximation using the EDAC code. Variants of the point and structural defects on the 1T-TiSe₂ surface are considered. The experimental and theoretical XPD patterns are compared on the basis of a convergence analysis of their R factors.     

A multiple-scattering approach to X-ray photoelectron diffraction

We present an alternative method for multiple scattering calculations of X-ray photoelectron diffraction from periodic surfaces. The technique uses a time-reversed RHEED wave function as the final state in a one-step model of the photoemission process. We apply the method to the Ni(001)c(2×2)S system and find an excellent agreement with the experimental data for vertical height of the S overlayer of 1.35±0.05 Å above the top Ni layer. Our calculations display a much better agreement with the experimental data than has been previously reported.     

Unintentional F doping of SrTiO3(001) etched in HF acid-structure and electronic properties

We show that the HF acid etch commonly used to prepare SrTiO₃(001) for heteroepitaxial growth of complex oxides results in a non-negligible level of F doping within the terminal surface layer of TiO₂. Using a combination of x-ray photoelectron spectroscopy and scanned angle x-ray photoelectron diffraction, we determine that on average ~ 13% of the O anions in the surface layer are replaced by F, but that F does not occupy O sites in deeper layers. Despite this perturbation to the surface, the Fermi level remains unpinned, and the surface-state density, which determines the amount of band bending, is driven by factors other than F doping. The presence of F at the STO surface is expected to result in lower electron mobilities at complex oxide heterojunctions involving STO substrates because of impurity scattering. Unintentional F doping can be substantially reduced by replacing the HF-etch step with a boil in deionized water, which in conjunction with an oxygen tube furnace anneal, leaves the surface flat and TiO₂ terminated.     

Ionic potential analysis of RHEED rocking curves from fluoride structures

Rocking curves of reflection high energy electron diffraction from MnF₂ and CaF₂(1 1 1) surfaces have been calculated with ionic scattering potentials. The potentials were derived from tabulated X-ray scattering factors and expressed in the Doyle-Turner representation. The Coulomb potential of the ions was introduced into the calculations of diffracted intensity using dynamical diffraction theory. Comparison of the calculated and measured rocking curves for CaF₂(1 1 1) surface confirmed that it is bulk terminated; the correction to volume average potential was found to be 1.1 eV (much less than when using atomic potentials). Analysis of the rocking curves from ultra-thin MnF₂ layers on CaF₂(1 1 1) indicated that MnF₂ inherits the cubic lattice of fluorides up to a thickness of three molecular layers.     

A comparative multi-property analysis of existing models for the He–N2 potential energy surface

The ability of an improved version of a recent three-dimensional ab initio potential energy surface for the He–N₂ interaction [Phys. Rev. A 66, 042703 (2002)], determined from high-level coupled-cluster calculations (including full singles and doubles, perturbative triples, and Brueckner orbitals), to predict scattering cross-sections and various bulk gas mixture properties is tested. The full three-dimensional potential energy surface has been employed for the calculation of vibrational relaxation rates, and a two-dimensional version (at a fixed N₂ bond length of 2.0743 a ₀) has been used for the calculation of molecular beam scattering cross-sections using quantum close-coupling methods and for the calculation of bulk gas phenomena using classical trajectory methods. The results obtained from the two-dimensional version of the present potential energy surface are compared with those obtained from three other recent accurate two-dimensional representations of the He–N₂ interaction.     

Interband transitions and electronic Raman continuum in systems with strong inelastic scattering: Applications to YBa2Cu3O7-δ

We consider a model for the electronic Raman continuum which takes into account strong inelastic scattering and interband transitions. Calculations are based on four-vertex Raman scattering diagrams (Kawabata formalism) within the RPA for Coulomb interaction and the ladder diagram Bethe-Salpeter equation for the vertex. We apply this method to an analysis of the nature of the electronic Raman continuum in the normal state of the high-T _c superconductor YBa₂Cu₃O₇. In numerical calculations we take into account all the self-energy effects and make simulations for vertex corrections assuming that inelastic scattering is due to electron-phonon interaction. Theab-plane polarized continuum contains a large contribution from interband processes and does not depend strongly on temperature and inelastic scattering strength. The in-plane anisotropy is determined by interband transitions rather than by anisotropy of the Fermi surface. The ZZ continuum contains only weak contribution from interband transitions. It can be crudely described within a single band model with inelastic scattering and is strongly dependent on the relaxation rates of inelastic scattering. The nature of the oxygen-deficiency dependence of the Raman spectra is also commented upon.     

Photoelectron elastic scattering effects in XPS

The photoelectron elastic scattering effects in XPS are reviewed. Using the transport theory approach and Monte-Carlo calculations as well experimental data, the influence of elastic scattering is demonstrated for the escape probability of the photoelectrons from solids, the intensities from bulk and surface systems, the angular intensity dependence including dipole and quadrupole transitions, attenuation lengths from various definitions, mean escape depths, the path-length distribution of photoelectrons in solids, the overlayer thickness determination, in-depth profiling and photoelectron diffractions patterns from single crystals.     

Spin polarization of electrons elastically scattered from europium and bismuth atoms

We present calculations of differential, integrated elastic, total, momentum transfer cross-sections and spin-polarization parameters S, T and U for scattering of electrons from Eu and Bi atoms in the energy range 2.0 to 500.0 eV using semi-relativistic approach. The target-projectile interaction is represented both by real and complex parameter-free optical potentials in the solution of Dirac equation for the scattered electrons. The results for the differential cross-sections and spin-polarization parameters have been compared with the available calculations and experimental results. Received 17 February 2000 and Received in final form 15 June 2000     

Energy scan photoelectron diffraction: An integrated method for adsorbate structure determinations

In order to determine the structure of adsorbates on surfaces an integrated multi-step approach has been developed for the analysis of energy scan photoelectron diffraction data. The first step of the method gives an approximate three-dimensional image of the dominant nearest neighbour scatterers surrounding an emitter atom. In a second step this approximate structure is further refined using multiple scattering cluster calculations. Finally, a very recently developed procedure is employed in which the dynamics/displacements of the emitter atom are examined utilising the Maximum Entropy Method. The application of this integrated method is illustrated with examples of molecular adsorbates.     

Angular parameters in X-ray diffraction theory

In this article, we provide a detailed mathematical analysis of the Helmholtz equation, in the context of dynamical wide-angle X-ray scattering by deformed crystals. We show that the dynamical theory of diffraction may be written without introducing a sophisticated dispersion theory, by using the following simple condition for wavevectors: |K_h|=|K₀|=k. This choice gives simple way to describe the reciprocal space mapping phenomena in a dynamical approach and simulate the diffraction curves for strained crystals over many orders of reflection. Several theoretical approaches to obtaining the angular distribution of scattered intensity are discussed.