Surface Review and Letters

This study presents a theoretical framework for the propagation of a compound consisting of N interacting particles in a multicenter potential. A novel Green operator approach is proposed that disentangles the geometrical and dynamical properties of the scatterers from the internal evolution of the projectile compound. Furthermore, the transition operator for the scattering from the multicenter potential is expanded in terms of many-body scattering path operators, which in turn are expressed in terms of single site transition operators that are amenable to computations. To deduce the correlated many-body Green operator of the scattering compound, a cumulative method is designed that reduces the problem to the evaluation of Green operators of systems with a reduced number of interacting particles. This is particularly useful for efficient calculations and encompasses the usual perturbative approaches.

The possibility is shown of developing a new method based on the surface photo-charge effect (SPCE) to study liquids, gases and vapors. The main idea is that, due to the strong susceptibility of this effect to the state of the irradiated interface, each change in the liquid- or gas-contacting surfaces would cause changes in the observed signals. The experiments performed so far support such a possibility.

From the perspective of sp orbital hybrid bonding and its consequence on the behavior of valence electrons, new insight is presented into the nature of oxygen-derived features in the valence band and above of metals. It is suggested that the crystal geometry and the surface morphology may vary from surface to surface and from material to material but the oxygen-derived features in energy space are substantially the same in nature. The derived features can be consistently specified as oxygen–metal bonding (-5 to -8 eV), a nonbond lone pair of oxygen (-1 to -2 eV), holes of metal ions (~E_F) and antibonding metal dipole states (>E_F), other than the addition of the 2s or 2p states of the isolated oxygen additives.

We study the temperature and coverage dependence of an effective diffusion barrier assuming an Arrhenius shape of the chemical diffusion coefficient for a system of interacting particles. The previously published model of the diffusion of an fcc (111) surface with bivariate trap is used. The presence of two nonequivalent occupation sites and interaction result in a non-Arrhenius shape of the diffusion coefficient and a coverage- and temperature-dependent effective diffusion barrier. The temperature dependence of effective energy E_a shows a minimum at low temperatures (at approximately 150 K) for strong interactions. The coverage dependence of E_a has a deep minimum in the vicinity of Θ=1/2, even in a system without interaction.

The interfacial chemistry of an Al/Mg/PET sample, prepared by thermally evaporating first Mg and then Al on to a polyethyleneterephthalate (PET) film, was studied by X-ray photoelectron spectroscopy (XPS). The Mg/PET system showed two types of Mg species, namely an O–Mg–C complex, from reaction with the carbonyl group on PET, and metal-like clusters. The deposition of Al results in intermetallic mixing, which is manifested by alloy formation and Mg enrichment at the surface. It appears that some Al atoms penetrate the Mg region to react with carbonyl groups in the PET and form an interfacial complex with O–Al–C bonding. The stability of this sample was studied on exposing it to air and then rinsing it with water. The metal overlayers are strongly modified by these treatments, although the bonding at the PET interface appears relatively unchanged.

In this paper we study the growth of Cu_xSe_y and Na_xCu_ySe_z thin films on polycrystalline Cu substrates. The investigation took place in situ by means of photoemission spectroscopy with synchrotron radiation, in UHV. Deposition of elemental Se with a dense flux on a polycrystalline Cu substrate at RT and subsequent heating to 100°C causes the formation of a thick Cu_2-xSe film (with x equal or very close to zero). A phase transition of this copper selenide occurs by annealing at 200°C. Deposited Na on the Cu_2-xSe film is diffused into the bulk, resulting in the formation of a Na_xCu_ySe_z compound. Exposure to Cl₂ at 120°C causes the out-diffusion of Na to the surface, where NaCl is formed. The Na diffusion in and out of the copper selenide film is analogous to alkali intercalation–deintercalation from TX₂ layer compounds.

A previous two-step hydrodynamic model calculation of angle-resolved photoemission has been improved by modeling the surface with a soft analytic function. The same ground state potential is used to screen the photon field and to calculate the electronic wave functions. The implemented code can be used for arbitrary ground state densities along the perpendicular direction. Mathematical and physical aspects of the model are discussed extensively. The behavior of the main peak of the spectrum in response to the variation of some parameters leads us to the conclusion that the peak cannot be interpreted as the direct excitation of multipole modes. This conclusion was suggested in the previous two-step model calculation.

By the method of in situ Hall effect measurements at room temperature, it was shown that the formation of a clean Si(111)(7×7) surface at high temperature annealing (T=1250°, t=120–180 s) of n-type conductivity silicon results in a change of the majority carriers on a surface. But on a surface of p-type conductivity silicon it results in the formation of a hole accumulated layer. Various dynamics of changes in the Hall voltage and resistivity voltage within the limits of the first monolayer of room temperature adsorption of iron and chromium on substrates with a p–n junction and with a hole accumulated layer are not connected with conductivity along an absorbed layer. To understand the mechanism of conductivity in a system adsorbed layer Fe(Cr)/Si(111)(7×7), it is necessary to take into account two effects: (a) destruction of the superstructure Si(111)(7×7); (b) formation of donor-type surface states of Fe(Cr) in the band gap of silicon. Destruction of the localized surface states of the superstructure Si(111)(7×7) plays the greatest role, when the density of donor-type surface states is very small. It corresponds to adsorption of Cr on Si(111) of p-type conductivity. The greatest contribution of donor-type surface states to the conductivity of the surface space-charge layer was observed with adsorption of Fe on Si(111) of n-type conductivity. Conductivity through the adsorbed iron (chromium) layer began with a thickness more than three monolayers.

The atomic structure of the Si(111)-R30°-Ga surface has been studied by comparing measured low-energy electron diffraction (LEED) intensity (IV) curves with calculated IV spectra using the method of automated tensor LEED. The experimental LEED IV curves used in this work contain many beams and a wide energy range. The results show that the Ga atoms occupy T₄ sites, at 2.62 Å above the second-atomic-layer Si atoms. The Ga–Si vertical spacing is 1.44 Å and the bond length between the Ga atom and the first-layer Si atom is 2.52 Å. Large bucklings are found in the first and second Si bilayers below the adatom layer.

The adsorption of acetaldehyde has been studied on clean Ag(111) using temperature-programmed desorption (TPD) and reflection–absorption infrared (RAIRS) spectroscopies. Acetaldehyde adsorbs at 80 K via the oxygen lone pair in an η¹(O) configuration. It is found that a portion of the adsorbed acetaldehyde desorbs at low temperatures (~ 160 K) and also polymerizes. Based on this and other work, it is proposed that the surface polymer consists of an open chain with a desorption temperature that shifts with coverage.

The electronic structure and chemisorption properties of the surface of ionic crystals are reviewed, with emphasis on two topics: a critical overview of the experimental and theoretical studies of the adsorption of CO on single crystal and polycrystalline MgO, and a discussion on the most important defect centers at the MgO surface — low-coordinated sites, single oxygen and magnesium vacancies, divacancies, and impurity or substitutional atoms. The two subjects are to some extent interconnected. From the detailed theoretical and experimental study of the adsorption of a nonreactive molecule like CO and from the comparison of experiments done on single crystal or thin films and on powder samples, one can learn about the nature and concentration of the defects at the surface. A more precise characterization of defects requires, however, a careful spectroscopic investigation and a direct comparison with quantum-chemical calculations of both geometric structure and observable properties. The combined theoretical–experimental approach offers new opportunities for a better understanding of the complexity of oxide surfaces.

Near edge X-ray absorption spectroscopy (XAS), X-ray photoelectron diffraction (XPD) and Auger electron diffraction (AED) are powerful techniques for the qualitative study of the structural and electronic properties of several systems. The recent development of a multiple scattering approach to simulating experimental spectra opened a friendly way to the study of structural environments of solids and surfaces. This article reviews recent X-ray absorption experiments using synchrotron radiation which were performed at Ge L edges and core level electron diffraction measurements obtained using a traditional X-ray source from Ge core levels for ultrathin Ge films deposited on silicon substrates. Thermodynamics and surface reconstruction have been found to play a crucial role in the first stages of Ge growth on Si(001) and Si(111) surfaces. Both techniques show the occurrence of intermixing processes even for room-temperature-grown Ge/Si(001) samples and give a straightforward measurement of the overlayer tetragonal distortion. The effects of Sb as a surfactant on the Ge/Si(001) interface have also been investigated. In this case, evidence of layer-by-layer growth of the fully strained Ge overlayer with a reduced intermixing is obtained when one monolayer of Sb is predeposited on the surface.

In this review, we discuss the angular anisotropies in the scattering factors of electrons and positrons for scattering by atoms in solids. We show that, as a function of the angle, the maximum number of dips in the scattering factor's magnitude and jumps of near π in its phase are related to the angular momenta of the bound and resonance states of the potential. The effect of the scattering factor's anisotropies on low-energy electron and positron holographic wave-front reconstruction is discussed.

This brief review focuses on the nature, kinetics, dynamics and consequences of the sp-orbital hybrid bonding of C, N and O to the Ni/Rh(001) surfaces which give rise to the same kind of "radial and then the p4g clock" reconstruction. It is identified that the "radial" and the subsequent "clock" reconstruction result from the adsorbate–substrate bond formation with sp-orbital hybridization, and that the driving force behind the reconstruction originates from the electrostatic interaction along the <11> direction. At the initial stage, A^-1 (A=C, N or O adsorbate) sinks into the fourfold hollow site and forms one bond with a B (B = Ni or Rh host atom) underneath, giving rise to an AB₅ cluster with four dipoles at the surface. As A^-1 evolves into the hybridized-A^-n (n=4, 3, 2), the AB₅ cluster evolves into an AB₄ tetrahedron. Meanwhile, the AB₄ tetrahedron redefines three of the four surface dipoles as B⁺, B²⁺, B^{+/ dipole} or B^dipole, depending on the valence value of the adsorbate. The electrostatic force arises upon repopulating the valence electrons, which creates rhombus strings along the <11> direction. With the presence of nonbonding lone pairs, the clock rotation on Ni(00l)-(2×2)p4g-2N^-3 and Rh(00l)-(2×2)p4g-2O^-2 surfaces is initiated by the alternate attraction and repulsion in the <11> direction and the rotation is stabilized by bond tension; whereas the clock rotation on the Ni(00l)-(2×2)p4g-2C^-4 surface is driven by the nonequivalent electrostatic repulsion in the <11> direction and the rotation is balanced by strong bond compression. The findings so far have led to technical innovation for the adhesion between diamond and metals by designing a gradient TiCN transition layer to neutralize the bond stress.