Electronic structure of a cobalt monolayer on Cu(100)

Angular resolved photoemission spectra using synchrotron radiation have been measured for different amounts of cobalt evaporated on Cu(100). At room temperature cobalt grows layer-bylayer forming well-ordered layers in registry with the substrate, as judged by AES, LEED and UPS measurements. The energy position and linewidth of the Cu peaks remain unchanged when cobalt is deposited onto the surface, suggesting a rather weak interaction between the d-bands of Co and Cu. The two-dimensional band structure of the monolayer of cobalt has been determined. We have obtained a value for the magnetic exchange splitting of ΔE_exch = 0.80 ± 0.15 eV, which is nearly identical to the bulk value. A shift in the energy positions of the critical points for the monolayer versus bulk of cobalt is interpreted in terms of a narrower 2D density of states in the monolayer as compared to the bulk. A resonant valence-band two-electron satellite has been found. The correlation energy and screening effects of the two d-holes are very similar to the corresponding bulk values, while the decreased intensity of the satellite at resonance compared to the one for Co(0001) suggests that there are more d-states relative to s-states in the monolayer than in a bulk cobalt single crystal, in agreement with recent models of the valence band electronic structure at surfaces.     

Bulk and surface electron states in ruthenium

Using ab initio pseudopotentials and a mixed basis, we have examined the electronic structure of ruthenium. We have concentrated on the bulk and surface energy bands with specific emphasis on the close packed (0 0 0 1) surface. We present energy band spectra for the surface and bulk bands, and compare these bands to experiment and to other calculations. We find good agreement between our bulk bands and recent photoemission measurements. With respect to our surface bands, the results are in good accord with other calculations both for ruthenium and for “analogous” transition metal surfaces.     

First-principles study of electronic structure and ferromagnetism in Ti-doped ZnO

We perform first-principles spin polarized calculations of the electronic structure of Ti-doped in ZnO. Ferromagnetism in Ti-doped ZnO is identified, which is in agreement with recent experimental and calculated results. A net magnetic moment of 0.715μ_B is found per Ti. At a Ti concentration of 12.5%, total energy calculations show that the ferromagnetic state is 68 meV lower than the antiferromagnetic state. The electronic states near Fermi energy are dominated by strong hybridization between O 2p and Ti 3d, which is just the origin of impurity band in Ti-doped ZnO and also implies that the Ti-O bond is quite covalent instead of purely ionic. Since there is no magnetic element in this compound, Ti-doped ZnO appears to be an unambiguous dilute magnetic semiconductor.     

Surface electronic structure of SnO2(110)

We report theoretical investigations on the surface electronic structure of the (110)-face of SnO₂, a semiconductor of rutile bulk structure. Starting with a tight binding Hamiltonian for the bulk, we determine the surface electronic structure using the scattering theoretic method. As results we obtain the surface bound states, the surface resonances and the wave-vector resolved surface layer densities of states. The dominant features are two backbond states in the stomach gap of the main valence band and two Sn-s derived states in the lower conduction band region. In the upper valence band region, only weak resonances occur, like in other materials with relatively strong ionicity.     

Photoelectron spectroscopy of the valence electronic structure of polymers

Photoelectron spectroscopy has been highly developed as an investigative tool of the bulk and surface chemical and electronic structure of condensed matter during the past 2 decades. The use of soft X-ray sources led to the development of electroq spectroscopy for chemical applications (ESCA),^1,2 which is also known to the physicst as X-ray photoelectron spectroscopy (XPS). These developments were followed by the use of ultraviolet sources for the study of gases³ and the bulk and surface electronic structure of solids.^4,5 Recently, the use of the continuous spectral distribution of synchrotron radiation has had a major impact upon the study of solid surfaces^6,7 and has made the largely historical terminology of XPS (or ESCA) and ultraviolet photoelectron spectroscopy (UPS) somewhat less meaningful. Angle-resolved UPS and XPS (ARUPS and ARXPS) can provide additional information about electronic band structure, surface states, and adsorbates, but are typically applied to the study of the surfaces of ideal single crystals, where crystallographic high-symmetry directions as well as the polarization of the light source can be used to high advantage.⁷ Since polymers are almost never single crystals, the angle-resolved issue will not be addressed here.     

Photoemission study of electronic structure evolution across the metal-insulator transition of heavily B-doped diamond

We studied the electronic structure evolution of heavily B-doped diamond films across the metal-insulator transition (MIT) using ultraviolet photoemission spectroscopy (UPS). From high-temperature UPS, through which electronic states near the Fermi level (E_F) up to ∼5k_BT can be observed (k_B is the Boltzmann constant and T the temperature), we observed the carrier concentration dependence of spectral shapes near E_F. Using another carrier concentration dependent UPS, we found that the change in energy position of sp-band of the diamond valence band, which corresponds to the shift of E_F, can be explained by the degenerate semiconductor model, indicating that the diamond valence band is responsible for the metallic states for samples with concentrations above MIT. We discuss a possible electronic structure evolution across MIT.     

Surface studies by modulation spectroscopy

Surface barrier and electronic surface states are important parameters for characterizing semiconductor surfaces. Qualitative information on the surface electric field can be deduced from electroreflectance studies. Energetic positions of surface states have been determined by spectroscopic methods using effect modulation techniques. Besides the field effects of surface conductivity and absorption, which are limited to low densities of surface states, new information on surface states was gained by investigating the spectral dependence of surface photoconductivity. Also surface phonons were detected in “spectral oscillations” of photoconductivity. The measurement of surface photovoltage at photon energies where the bulk is transparent promises a new tool for surface state research in the future. To demonstrate these modulation techniques examples are given for Ge, Si, ZnO, and CdS surfaces.     

Tight—binding calculation of the electronic states of bulk—terminated GaAs（311）A and B surfaces

We have carried out theoretical investigations on the electronic structure of GaAs(311)A and GaAs(311)B surfaces. The bulk electronic structure of GaAs has been described by the second-neighbour tight-binding formalism and the surface electronic structure was evaluated via an analytic Green function method. First, we present the surface band structure together with the projected bulk band of both Ga-terminated and As-terminated for GaAs(311)A and GaAs(311)B surfaces, respectively. In each case, the number of surface states is determined, and the localized surface features and orbital properties of these surface states along Γ-Y-S-X-Γ high symmetry lines of the surface Brillouin zone are discussed. For the Ga-terminated GaAs(311)A (1×1) surface, we have tested two possible structure models, i.e. "the bridge site" and "the hollow site" models. In comparison with the angle-resolved photoelectron spectroscopy studied recently, the results have shown that the surface electronic states of the hollow site model are in good agreement with the experiments, whereas those of the bridge site model are not. So we have concluded that the hollow site model is favourable for the Ga-terminated GaAs(311) (1×1) surface and the bridge site model should be excluded.     

A theoretical study of the electronic structure of Pd(111) clean surface

A detailed investigation of the electronic structure of Pd(l 11) clean surface is presented in terms of a LCAO band model. Results including surface bands and local densities of states are given and a comparison with recently performed photoemission experiments is presented. For hω = 21.2 eV we find that the angle resolved distribution curves can be explained in terms of initial state density and k_∥ conserving transitions. For lower photon energies a mixture of bulk transition calculations and surface density of states seems more appropriate. Comparison with previous theoretical work is also presented.     

Disulphide and metal sulphide formation on the reconstructed (0 0 1) surface of chalcopyrite: A DFT study

The bulk and (0 0 1) surface of chalcopyrite have been investigated using Plane Waves Density Functional calculations. The band structure and the optimized lattice parameters are in good agreement with previously published results. The relaxed S-terminated (0 0 1) surface led to the formation of disulphide, S₂²⁻, upon the reduction of Fe(III) to Fe(II). The relaxed M-terminated (0 0 1) surface led to the formation of metal sulphides and metal-metal bonds. The calculated Fe-Fe, Fe-Cu and Cu-Cu bond lengths are close to the typical bond distances found in the metal. Löwdin population analysis, density of states and electron localization function have been used to understand the electronic structure of the chalcopyrite surfaces. Implications to the chalcopyrite surface reactivity are briefly discussed.     

Valence surface electronic states on Ge(001)

The optical, electrical, and chemical properties of semiconductor surfaces are largely determined by their electronic states close to the Fermi level (E{F}). We use scanning tunneling microscopy and density functional theory to clarify the fundamental nature of the ground state Ge(001) electronic structure near E{F}, and resolve previously contradictory photoemission and tunneling spectroscopy data. The highest energy occupied surface states were found to be exclusively back bond states, in contrast to the Si(001) surface, where dangling bond states also lie at the top of the valence band.     

Electronic structure of Si(111) surfaces

We report on new angle-resolved photoemission studies of Si(111) 2 × 1 and 7 × 7 surfaces. The emission from the 2 × 1 surface shows much structure. For normal emission the energy positions are insensitive to the photon energy in the range 19–27 eV. The emission has been interpreted as a probe of the surface density of states, SDOS, including both surface states, resonances and bulk-like states. The SDOS was also calculated as a function of parallel momentum k_∥ for a model of the Si(111) 2 × 1 surface obtained from energy minimization considerations. We identify emission from the dangling bond band, which has a positive dispersion of 0.6 eV, and also emission from surface resonances which have some character of the compressed and stretched back bonds. There are also other predicted surface resonances that correspond to experimental peaks which have not been identified in previous work. Except for the dangling bond band, the surface resonances are limited in k_∥ space, so that it is not possible to follow these resonance bands over all angles. Maximum intensity for the normal emission from the dangling bond is obtained at 23 eV, while the emission from the lowest s-like states monotonically increases towards 30 eV photon energy. When annealing the cleaved 2 × 1 surface to the 7 × 7 reconstructed surface, the spectra broaden significantly. The intensity of the dangling bond decreases and we see a very small metallic edge.     

Energy loss mechanisms in low energy electron scattering from W(100) and their use as a surface sensitive spectroscopy

The energy loss spectrum of low energy (0 < E_p < 200 eV) electrons scattered from W(100) has been experimentally investigated, and mechanisms giving rise to the fine structure analyzed using a dielectric response formalism. The dielectric medium is characterized by available optical data and energy band calculations for tungsten. All of the structure for loss energies, w, less than 18 eV is attributed to intra- and interband transitions involving the bulk valence and conduction bands. The surface and bulk plasmon excitations are observed at w = 21 eV and w = 25.5 eV respectively which is in reasonable agreement with the optical data. A very narrow peak in the density of conduction d-band states apparently functions strongly in well defined excitations involving the $\text{5p}\text{3}\text{2}$ and 4f tungsten orbitais and the 2s and 2p orbitais of adsorbed oxygen. These conduction band states form a “window” with which to measure the electronic orbital structure of both the substrate and adsorbate during adsorption and reaction. We demonstrate this for the room temperature adsorption of oxygen on W(100) in which we observe the sequential filling of two electronically inequivalent binding states. The stability of the “d-band window” during thermally activated reaction, and the likelihood of its existence in other transition metals makes this an attractive surface sensitive spectroscopy.     

The reaction of carbon and water as catalyzed by a nickel surface

Many of the individual steps which make up the reaction of carbon and water to produce CO and H₂ were studied on a nickel foil surface using temperature-programmed reaction spectroscopy (TPRS), Auger electron spectroscopy (AES), and ultraviolet photoelectron spectroscopy (UPS). Surface graphite and carbide, two metastable surface carbon forms, were prepared by dehydrogeneration of C₂H₂ and served as reactant carbon. UPS of the graphite monolayer in contact with the metal yielded a valence electronic structure that could be interpreted in terms of the bulk band structure of graphite. The fully carbided Ni surface was active for H₂O dissociation with an estimated activation energy ≤ 5 kcal/mol. The reaction of graphitic carbon in contact with the nickel surface and adsorbed oxygen occurs directly without isolated prior breaking of carbon-carbon bonds. The estimated activation energy for the direct reaction was 44 kcal/mol. A different catalytic reaction cycle involving carbon-carbon bond breaking followed by oxidation of the carbide is energetically more demanding. The activation energy for direct carbon-carbon bond breaking was estimated to be between 65 and 70 kcal/mol. Following this demanding step, the reaction between carbidic carbon and oxygen proceeded with estimated activation energy of 31 kcal/mol.     

Strong Coulomb correlation effects in ZnO

The electronic structure, band parameters, and optical spectra of wurtzite-type ZnO were studied by first-principles calculations within different approximations of the density functional theory. The local-density approximation underestimates the band gap, the energy levels of the Zn-3d states, the band dispersion, the crystal-field splitting, the spin-orbit interaction, and location of peaks in the optical spectra. The generalized-gradient approximation slightly corrects the discrepancies with the experimental findings and it shows good agreement for the optical spectra with experimental data at energies 10-20 eV for E⊥c. Studies within the local-density approximation with the multiorbital mean-field Hubbard potential show that strong Coulomb correlations are in operation. From effective mass calculations it is found that holes are much heavier and more anisotropic than the conduction-band electrons in ZnO.     

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.     

Theoretical study of the electronic structure of the Si3N4(0 0 0 1) surface

Density functional theory has been applied to a study of the electronic structure of the ideally-terminated, relaxed and H-saturated (0 0 0 1) surfaces of β-Si₃N₄ and to that of the bulk material. For the bulk, the lattice constants and atom positions and the valence band density of states are all in good agreement with experimental results. A band gap of 6.7 eV is found which is in fair accord with the experimental value of 5.1-5.3 eV for H-free Si₃N₄. Using a two-dimensionally-periodic slab model, a π-bonding interaction is found between threefold-coordinated Si and twofold-coordinated N atoms in the surface plane leading to π and π* surface-state bands in the gap. A surface-state band derived from s-orbitals is also found in the gap between the upper and lower parts of the valence band. Relaxation results in displacements of surface and first-underlayer atoms and to a stronger π-bonding interaction which increases the π-π* gap. The relaxed surface shows no occupied surface states above the valence band maximum, in agreement with recent photoemission data for a thin Si₃N₄ film. The π* band, however, remains well below the conduction band minimum (but well above the Fermi level). Adsorbing H at all dangling-bond sites on the ideally-terminated surface and then relaxing the surface and first underlayer leads to smaller, but still finite, displacements in comparison to the clean relaxed surface. This surface is more stable, by about 3.67 eV per H, than the clean relaxed surface.     

Temperature and concentration dependences of the electronic etructure of copper oxides in the generalized tight binding method

The electronic structure of p-type doped HTSC cuprates is calculated by explicitly taking into account strong electron correlations. The smooth evolution of the electronic structure from undoped antiferro-magnetic to optimally and heavily doped paramagnetic compositions is traced. For a low doping level, in-gap impurity-type states are obtained, at which the Fermi level is pinned in the low-doping region. These states are separated by a pseudogap from the valence band. The Fermi surfaces calculated for the paramagnetic phase for various concentrations of holes are in good agreement with the results of ARPES experiments and indicate a gradual change in the Fermi surface from the hole type to the electron type.     

On the preparation and electronic properties of clean W(1 1 0) surfaces

We have studied the influence of oxygen pressure during the cyclic annealing used for the cleaning of W(1 1 0) surfaces. For this purpose the surface morphology and electronic properties are measured by means of scanning tunneling microscopy (STM) and spectroscopy (STS), respectively. It is found that the surfaces with impurity atom densities as low as 2 × 10⁻³ can be obtained by gradually reducing the oxygen pressure between subsequent annealing cycles down to about 2 × 10⁻⁸ mbar in the final cycle. Only on the clean surface a bias-dependent spatial modulation of the local density of states (LDOS) is observed at step edges and around impurity sites by STS. In addition, we find a pronounced peak in the occupied states. In combination with density functional theory calculations these features can be traced back to a dispersive p_z-d_xz-type surface resonance band and the lower band edge of a surface state, respectively.