The study of the energy band structures of TiC,VC, Ti4C3 and V4C3 by the LMTO-ASA method

The energy band structures of TiC, VC, Ti₄C₃ and V₄C₃ have been studied by the linear muffin-tin-orbital method (LMTO). The influence of vacancies on the density of states (DOS) in the 2s C-band is a uniform reduction and a narrowing of this DOS. The splitting of the peak in the low-energy part of the 2p C, 3d Me-band (Me = Ti, V) is observed, while in the neighbourhood of the Fermi energy two peaks of DOS appear which consist of 3d Me- and vacancy states. Under the influence of vacancies the electronic charge in the carbon atomic sphere is shifted partly into the vacancy sphere. For Ti₄C₃, the increase of the Fermi energy and DOS at this energy is observed, while in the case of V₄C₃ these values decrease. In the presence of vacancies a contraction of crystal lattice is observed while the bulk moduli of TiC and VC diminish which is most pronounced for VC. The results are compared with previous data obtained by the LCAO coherent potential and APW calculations and with the experimental data concerning the DOS at the Fermi energy.     

Initial growth at the F16CoPc/Ag(111) interface

An extended, combined (STM/STS)–(UPS/XPS) study was carried out towards a comprehensive understanding of the mechanism responsible for the F₁₆CoPc/Ag(111) interface formation. The evolution of the morphology and the electronic properties at the organic/metal interface is investigated for the early-stage growth of the ultrathin molecular film. Template-guided molecular structures are formed via a strong molecule–substrate interaction which leads to the formation of a new adsorption-induced interface state close to the Fermi energy (E_F). With increasing the thickness the molecular coupling to the metal surface states becomes less important while the more dominant molecule–molecule interaction governs the second layer formation. The quenching of the interface state upon increasing the molecular thickness, together with the changes observed in the Co 2p and F 1s core levels, is explained based on a charge transfer at the interface and a corresponding charge redistribution within the molecular ligand. A detailed “picture” of the energy level alignment close to E_F is achieved by correlating the high resolution UPS and highly localized STS data.     

Vacancy induced changes in the electronic structure of titanium carbide—I. Band structure and density of states

Results of a self-consistent “Augmented Plane Wave” (APW) band-structure calculation are presented for substoichiometric titanium carbide with 25% vacancies on the carbon sublattice sites (TiC_{0 75}) assuming a model structure with ordered vacancies Comparison with an earlier APW study on stoichiometric TiC reveals that the carbon vacancies induce two pronounced peaks in the density of states (DOS), 0.4 eV below and 0.8 eV above the Fermi energy E_γ, thus forming electronic states in a region where the DOS for stoichiometric TiC exhibits a minimum So-called “vacancy states” with an important amount of charge on the vacancy site are found to be derived from Ti 3d states extending into the vacancy muffin tin sphere An angular momentum decomposition with respect to the center of the vacancy muffin tin sphere shows that the s character predominates for the occupied and the p character for the unoccupied “vacancy states” The theoretical findings explain features near E_γ, observed in recently published X-ray emission spectra Furthermore, we find a slight increase of electronic charge in the carbon muffin tin spheres as compared with stoichiometnc TiC.     

Electron states on the (111) surface of copper

A calculation of electronic states at the (111)-surface of Cu in the energy region around E_F is presented. The calculation is based on the empirical pseudopotential method. A proper position of an abrupt potential barrier ensures overall charge neutrality. A surface state is found at 0.13 eV below E_F, being localized midway between atomic layers. The effective mass of the associate bands is 0.39m_e. Our results are in good agreement with directional photoemission data.     

Surface enhanced covalency and Madelung potentials in Nb doped SrTiO3 (100), (110) and (111) single crystals

The influence of surface enhanced covalency on the Madelung potential is experimentally investigated using angle-resolved photoemission for (100), (110) and (111) SrTiO₃ surfaces after annealing in UHV at 630 °C. Deconvolution of the core level spectra (O 1s, Sr 3d and Ti 2p) distinguishes bulk and surface components, which are interpreted in terms of surface enhanced covalency (SEC). By comparing the experimentally measured binding energies with theoretical calculations developed in the framework of the Localized-Hole Point-Ion Model, we quantitatively determine the effective electron occupancy at bulk and surface Sr and Ti sites. Our results confirm the essentially ionic character of Sr–O bond and the partially covalent character of Ti–O bond in bulk STO. The cation Ti and Sr electron occupation is greater for all the three surfaces than in the bulk. Surface covalency shifts the Madelung potential at the surface by ΔE_M. ΔE_M is a minimum for the (111) surface, and increases through (100), attaining a maximum for (110). The angle-resolved valence band spectra and the work function values also confirm this trend. The results are consistent with d–d charge fluctuations dominating at the surface, whereas metal-ligand charge transfers are more energetically favourable in the bulk.     

Study of Ag/Si(111) submonolayer interface: I. Electronic structure by angle-resolved UPS

Angle-resolved ultraviolet photoelectron spectra have been measured for well defined Ag/Si(111) submonolayer interfaces of (1) $\text{Si(111)(}\text{3}\text{×}\text{3}\text{)R30°-Ag}$, (2) “Si(111)(6 × 1)-Ag”, and (3) Ag/Si(111) as deposited at room temperature. Non-dispersive and very narrow (FWHM ～ 0.4–0.5 eV) Ag 4d derived peaks are found at 5.6 and 6.5 eV below the Fermi level for surface (1) and at 5.3 and 6.0 eV for surface (2). Dispersions of sp “binding” states in the energy range between E_F and Ag 4d states have been precisely determined for surface (1). Electronic structures similar to those of the Ag(111) surface, including the surface state near E_F, have been observed for surface (3).     

Comments on surface electronic properties of TiC and TiN

Some recent experimental and theoretical results on surface electronic properties of a special class of refractory transition metal compounds (TiC, TiN and ZrN) and particularly, on surface states, the surface charge self-consistency and the stability of the polar (111) surface, are discussed within a simple qualitative framework based on the use of energy weighted averages as the main tool of the analysis.     

Quantum size effects in the fermi energy and electronic density of states in a finite square well thin film model

The dependence of the Fermi energy, E_F, and the electronic density of states, ρ(E), of thin metallic films (L_z ≲ 50 Å) on film thickness, electron density, and potential well depth, is systematically investigated in a free-electron, finite square well model. Two size-dependent effects are observed: (1) oscillations in E_F and ρ(E) due to the size-quantization of the energy levels, and (2) changes in the mean values of these quantities, averaged over several oscillation periods, relative to their bulk values. The mean value of E_F is increased relative to its bulk value by as much as 5%–10% for physically reasonable well depths and typical metallic electron densities. For the special case in which the top energy level in the well is occupied, the mean value of E_F is equal to its bulk value. The mean value ofρ(E_F) can be either greater than or less than its bulk value, depending on the well depth. In contrast to the small amplitude oscillations in E_F, the oscillations in ρ(E_F) may have an amplitude as large as 25% of the mean value for sufficiently thin films. Accurate analytic expressions for the thickness dependence of the Fermi energy and density of states are derived.     

Spin-polarized metastable deexcitation spectroscopy study of iron films

A spin-polarized metastable deexcitation spectroscopy apparatus using a nozzle-skimmer pulsed discharge metastable atom source was developed. The oxygen adsorption dependence of the surface magnetism of thin iron films deposited on MgO(100) was investigated using this apparatus. The surface local density of states spilling towards the vacuum (SDOS) at around the Fermi energy, E_F, for the clean surface shows a negative polarization to the bulk. The oxygen derived SDOS for the lightly oxygen adsorbed surfaces (2–10 L) also shows a negative polarization while the SDOS at E_F changes its polarity to positive. The polarization of SDOS is not detected for the heavily oxidized surfaces (20–100 L).     

Electronic structures of the LaBO3 (B = Co, Fe, Al) perovskite oxides related to their catalysis

The discrete-variational (DV) X α molecular orbital method has been applied to investigate the bulk and surface electronic structures of perovskite oxides such as LaCoO₃, LaFeO₃ and LaAlO₃. The calculated XPS spectra for these oxides are in good agreement with the experimental ones and only the LaCoO₂ exhibited a rather high electron density of states near the Fermi level (E_F). The catalytic behavior of these oxides is discussed on the basis of their electronic structures; the marked catalysis by LaCoO₃ is associated with electron occupation of crystal field d states near E_F and with the buildup of surface charge so as to enhance the electron transfer between a surface cation and an interacting molecule.     

Effect of Cr on the electronic structure of Co3Al intermetallic compound: A first-principles study

The effect of doping with Cr on the electronic structure and magnetism of Co₃Al has been studied by density functional calculations. It has been found that the Cr atom has a strong site preference for the B-site in Co₃Al. With the substitution of Cr for Co, the total densities of states (DOS) change obviously: A DOS peak appears at E_F in the majority spin states and an energy gap is opened in the minority spin states. The effect of Cr in Co₃Al is mainly to push the antibonding peak of the Co (A,C) atoms high on the energy scale and to form the energy gap around E_F, and also to contribute to the large DOS peak at E_F in the majority spin direction. The calculations indicate a ferromagnetic alignment between the Co and Cr spin moments. The calculated total magnetic moment decreases and becomes closer to the Slater–Pauling curve with increasing Cr content. This is mainly due to the decrease of the Co (A,C) spin moments. At the same time, the moments of Co (B) and Cr (B) only change slightly.     

Ab initio calculations of the atomic and electronic structure of MgF<Subscript>2</Subscript> (011) and (111) surfaces

The results of ab initio slab calculations of surface relaxations, rumplings and charge distribution for the different terminations of the MgF₂ (011) and (111) polar surfaces are presented and discussed. We have employed the computer code CRYSTAL with the Gaussian basis set and the hybrid B3PW exchange-correlation functional. Despite the ionic nature of the chemical bonding at both surfaces, a considerable decrease of the optical band gap is predicted (1.3 eV or 10%) for the (111) surface as compared to the bulk.     

Surface photovoltage spectroscopy and surface piezoelectric effect in GaAs

“Real” (111) surfaces of n-type GaAs were investigated employing surface photovoltage spectroscopy and the surface piezoelectric effect. Surface states at the energy position E_c ? E_t ? 0.72 eV were found on both the Ga and the As surfaces. Both types of surfaces exhibited a barrier of about 0.55 V. No variations in the surface barrier or the energy position of the surface states were observed in various ambients at atmospheric pressure (dry air, wet air, ammonia and ozone). However, the capture cross-section of the surface states for electrons, as determined from the surface piezoelectric effect transients (of the order of 10^?13 cm²), was found to be sensitive to the ambient. It decreased in wet air and increased in ozone. This effect was more pronounced on the As than on the Ga surfaces. Additional surface states were found to be present in the energy region of 0.9 to 1.0 eV, below the bottom of the conduction band. However, their exact energy positions could not be determined due to interference caused by the carrier trapping of the surface states at E_c ? E_t ? 0.72 eV.     

Green function theory of chemisorption on sp-hybrid crystals

A self-consistent Green function calculation of the adatom charge transfer (Δq) and the chemisorption energy (ΔE) is performed for hydrogen and the halogens on Si and Ge substrates, which are modeled by sp-hybrid orbital chains. The effect of Shockley surface states on the hydrogen chemisorption process is studied in some detail. From the chemisorption point of view, the behaviour of Si resembles that of Ge, the (111) surfaces having a smaller value of |ΔE| than the (100) ones for all the adsorbates considered. In the case of the halogens, the strong reactivity of fluorine is confirmed by the large values obtained for Δq and |ΔF|.     

Surface electronic structure of the Cu(111)/Na adsorption system

An ab initio calculation of the surface electronic structure of the Cu(111)/Na adsorption is presented. The results are consistent with (a) the bulk Cu band structure, (b) the known properties of the clean Cu(111) surface and (c) the extremely narrow and intense photoemission peak observed at E_F for adsorbed Na.     

Unoccupied surface states on the (111) surface of silver: Evidence for broadening

We have studied Ag(111) withk-resolved inverse photoemission spectroscopy athv=9.7 eV. In normal incidence we find image-state emission atE _vac–(0.4±0.1) eV and the unoccupied part of an intrinsic surface-state band as a huge emission peak cut byE _F. The energy dispersion of the intrinsic surface-state band and in particular its crossing ofE _F predicted by Ho et al. cannot be observed because of broadening effects as is shown by a theoretical simulation. The broadening is due to the vicinity of the surface state to the bulk continuum nearE _F as suggested by Kevan.     

Surface states on the copper (111) face

Using Chodorow's potential in the bulk region and a z-dependent potential in the vacuum region, the surface states on Cu(1 11) were investigated. Fitting of a potential parameter to the well known energy of the surface state at $\text{Γ}$ resulted in a reasonable shape of the potential and in an effective mass of the surface state band in accordance with experiment. The state recently observed at $\text{M}$ about 2 eV below E_F could not be described with the present model.     

Unoccupied electronic states on Ni(100) induced by A c(2 × 2) oxygen overlayer

Bremsstrahlung isochromat spectra at hω₀ = 9.7 eV for electrons normally incident on a clean Ni(100) surface are compared to emission from Ni(100) with an ordered c(2 × 2) oxygen overlayer. We observe strong emission due to adsorbate induced antibonding electronic states near E_F and a simultaneous attenuation of previously identified direct bulk interband transitions. The results are in accord with theoretical predictions.