FS and FS(OH) defect centers at the MgO(1 0 0) surface: cluster and periodic calculations

We present a model of a new paramagnetic defect center which results from the interaction of atomic hydrogen with the MgO(1 0 0) surface. DFT calculations have been performed using periodic supercells and embedded cluster models where long-range polarization effects are included explicitly. The H atom promotes the creation of an oxygen vacancy (F center) by formation of the F_S⁺(OH⁻) defect where an hydroxyl group is adsorbed near an electron trapped in an oxygen vacancy. This new center has some characteristics similar to those of the classical F_S⁺ centers but a smaller formation energy; furthermore, being globally neutral, it can be treated also with supercell methods.     

Dependence of surface properties on adsorbate-substrate distance: Work function changes and binding energy shifts for I/Pt(1 1 1)

This paper presents an analysis of the character of the bond of I adsorbed at on-top and 3-fold sites of Pt(1 1 1). At both sites, the bonding is dominated by an ionic interaction supplemented with some covalent character due to donation from the adsorbed I anion to the Pt surface. The way in which the I-Pt interaction affects observed properties has been established. In particular, the origins of the anomalous work function changes induced by the adsorption of I and the shifts of I core level binding energies are explained. It is shown that the magnitudes of the changes in these properties can be directly correlated with the distance of the I from the Pt surface. Thus, these shifts can be interpreted to indicate adsorbate height. The fact that the negatively charged I adsorbate leads to a work function decrease, rather than the increase expected due to the charge of the adsorbate, may appear to be an anomaly. However, it is shown that this decrease arises from electronic reorganizations that cancel the dipole due to the charge of the adsorbate. Furthermore, the electronic terms that contribute to a lowering of the work function are larger as the adsorbate moves closer to the surface.     

The atomic and electronic structure of the TiO2- and BaO-terminated BaTiO3(0 0 1) surfaces in a paraelectric phase

Relaxation and rumpling of BaTiO₃(0 0 1) surface with two different terminations have been investigated from ab initio local density approximation calculations. Large displacements of ions deviated from their crystalline sites have been obtained. These kinds of displacements lead to the formation the surface rumpling, dipole moments and electric field in the near-surface region. Band structures, density of states, bond population and electronic density redistributions have been obtained. Considerable enhancement of Ti–O chemical bond covalency nearby the surface, especially for the TiO₂ termination surface, has been found.     

Optical anisotropy at the W(1 1 0) surface

We report ab initio calculations of the anisotropic dielectric function of tungsten (1 1 0) surface using the linear muffin-tin-orbital method. The calculated anisotropy in the optical spectrum, for polarization of light parallel to the surface, exhibits three dominant broad structures at 3.00, 4.01 and 5.34 eV successively positive, negative and then positive. The first peak is clearly assigned to p → d interband transitions in surface atomic sites whereas the two others have their origin in interband transitions in bulk like atoms. Our results, including the interlayer relaxation effect on the surface optical response, are compared to recent reflectance anisotropy measurements.     

Bistability in a H-terminated Si(1 0 0)2 × 1 surface obtained by ab initio transport calculations

We have analyzed electron tunneling due to the electric field from a hydrogen-terminated Si(1 0 0)2 × 1 ultrathin film on a metal substrate by density functional transport calculations. We have obtained a hysteresis loop in the tunneling current, which comes from the existence of two electronic structures. Furthermore, we have clarified that, as a condition of bistable electron transport, a double-barrier potential structure is not necessarily required for zero field, because it can be induced by the electric field.     

Ab initio study of rumpled relaxation and core-level shift of barium titanate surface

The rumpled relaxation and the core-level shift of full-relaxed BaTiO₃ (0 0 1) surface have been investigated by first-principles calculation. Based on the work function and the electric-field gradient, the right size of vacuum and the slab have been evaluated. The large displacements of ions deviated from their crystalline sites to lead to the formation of the surface rumples have been found. Some fully occupied surface oxygen p states at the top M point of the valance band and the empty surface titanium d states at the edge of the bulk conduction band are observed on the TiO₂-terminated surface. In contrast, on the BaO-terminated surface, two different core levels of the Ba 5p states shifted about 1.29 eV are induced by the bulk perovskite Ba atoms and the relaxation of surface Ba atoms, respectively. Our calculations are consistent with the experimental data.     

First-principles study of the (0 0 1) surface of cubic SrZrO3

Density functional calculations have been used to investigate the (0 0 1) surface of cubic SrZrO₃ with both SrO and ZrO₂ termination. Surface structure and electronic structure have been obtained. The SrO surface is found to be similar to its counterpart in SrTiO₃, while there are marked differences between the ZrO₂ and TiO₂ terminations in SrZrO₃ and SrTiO₃, respectively, concerning surface relaxation and rumpling. For the ZrO₂-terminated surface of SrZrO₃, the covalency of the interaction between the outmost Zr and the O beneath is enhanced as a result of their bond contraction. The band gap reduction and the presence of the surface states are also discussed in relation with the behavior of the electrostatic potential.     

CO adsorption on Rh, Pd and Ag atoms deposited on the MgO surface: a comparative ab initio study

The adsorption properties of CO molecules adsorbed on Rh, Pd, and Ag atoms supported on various sites of the MgO surface have been studied by means of a density functional cluster model approach. The metal atoms are stabilized with different binding energies on the regular and morphological defect sites of the surface. Among others we considered oxide anions, neutral and charged anion vacancies (F centers) located at terraces, steps, edges, and corners. CO is used as a probe molecule to characterize where the metal atoms are located. This is done by analyzing how the metal-CO binding energy and the C-O stretching frequency change as function of the substrate site where the metal atom is bound.     

Secondary electron yield enhancement by MgO capping layers

The yield of secondary electrons emitted from an epitaxial three monolayer (3 ML) NiO(1 0 0)/Ag(1 0 0) film excited by soft X-ray linearly polarized synchrotron radiation at the Ni L_2,3 absorption threshold has been measured for different values of the thickness of a MgO(1 0 0) capping layer. Compared with the as grown 3 ML NiO(1 0 0)/Ag(1 0 0) film, we observe a significant enhancement by about a factor 1.2 of the secondary electron emission for the capped 8 ML MgO(1 0 0)/3 ML NiO(1 0 0)/Ag(1 0 0) sample. A further substantial yield enhancement by a factor 1.6 with respect to the uncapped NiO sample is observed after deposition of an additional 8 ML MgO(1 0 0) film, for a total capping layer thickness of 16 ML. The observed secondary electron yield enhancement is discussed in terms of modified electronic structure, surface work function changes, and characteristic electron propagation lengths.     

Ab initio investigation of the CdTe (001) surface

Based on first-principles density functional pseudopotential calculations, the CdTe (001) surface has been studied. Atomic configurations of different reconstructions are obtained with good accuracy. This reveals the efficiency of the force calculation for the surface relaxations. The surface energies were calculated on relaxed surface slabs as a function of the chemical potentials. The main result is that the energy of the Te-terminated dimerized surface with a (2 × 1) reconstruction is larger than the Cd-terminated c(2×2) reconstructed surface. This is in agreement with what was suspected by the equilibrium model introduced by F. Tinjod et al., which explains the formation of the quantum dots in CdTe/ZnTe superstructures.     

Kinetics of residual gas adsorption on Ge(1 1 1) surface in low-energy electron backscattering

Low-energy (0.4-1.2 eV) electron backscattering is applied for the investigation of kinetics of residual gas adsorption effect on the concentration and energy positions of surface electron states of Ge(1 1 1) surface. Chemosorption of residual gas molecules on Ge(1 1 1) at P ∼ 10⁻⁷ Pa and room temperature is shown to be most active during the first 48 h. Low concentration of dangling valence bonds on the reconstructed Ge(1 1 1) (2 × 8) surface is shown to determine its low activity to chemosorption.     

First-principles study on the electronic structures of iron phthalocyanine monolayer

The electronic band structure and magnetic properties of iron phthalocyanine (FePc) monolayer were investigated by using the first-principles all-electron full-potential linearized augmented plane wave energy band method. It is found that the ferromagnetic FePc monolayer is energetically more stable than the paramagnetic one. The exchange interaction, which splits the majority and minority bands, influences strongly on the electronic structure near the Fermi level (E_F). Magnetic moment of the central Fe atom is calculated to 1.95 μ_B. The range of the positive polarization of Fe site is larger in the out-of-plane than in the in-plane direction. The FePc ligand remains paramagnetic. The presence of states at E_F indicates the metallic character of FePc monolayer both for the paramagnetic and ferromagnetic states. However, the large density of states at E_F of the majority spins in the ferromagnetic state is expected to cause a phase transition to insulating antiferromagnetic state from the metallic ferromagnetic one.     

Charging and stabilization of Pd atoms and clusters on an electron-rich MgO surface

We report density functional theory calculations on the interaction of Pd atoms and small Pd clusters with an electron-rich MgO surface. This surface can be generated by forming a specific kind of defects, named (H⁺)(e⁻) centers, using well known chemical recipes. By deposition of gas-phase Pd atoms on the properly functionalized MgO surface, one can generate collections of small Pd cluster anions with peculiar chemical properties. The (H⁺)(e⁻) centers act as nucleation sites for diffusing Pd atoms and favor the formation of small, thermally stable clusters. The presence of an extra charge on the metal cluster results in a large vibrational red-shift of adsorbed CO molecules. The present results intend to stimulate experimental work to produce stable metal cluster anions on the surface of an ionic oxide.     

Mapping of the electron transmission through the wall of a quantum corral

We report on a theoretical study of the escape of confined surface states electrons from quantum corrals made of Cu adatoms on a Cu(1 1 1) surface. This study maps electron transmission through the corral wall and provides an extension of our earlier work focused on confinement in Cu corrals [S. Díaz-Tendero, F.E. Olsson, A.G. Borisov, J.P. Gauyacq, Phys. Rev. B 77 (2008) 205403]. The existence of two decay modes for the confined surface state is stressed: (i) non-resonant tunnelling through the corral wall concentrated on the Cu adatoms and (ii) a resonant-induced decay involving the transient formation of a resonant state localized on top of the corral wall. The present mapping of the electron transmission reveals how the interference between the two decay modes works: there exist regions where the electron leaves the corral, balanced by regions where it enters the corral, though the global behaviour of the quasi-stationary states is electron escape from the corral.     

Densities of states of low-index surfaces of tantalum by photofield emission

The total energy distributions (TED) of the true photofield emission current from clean (1 0 0), (1 1 0) and (1 1 1) facets of Ta field emitters have been measured using a new method to remove the thermocurrent associated with laser heating. Each TED exhibits one or more prominent peaks that are interpreted by comparing them with ab initio calculations of the TED of the emission current based on density functional theory. The generally good agreement with experiment indicates that the same method can be used for accurate calculations of the densities of states of low-index surfaces of Ta. Each of the experimental TEDs shows, in addition to the prominent peaks, a set of weaker peaks that are not predicted by the calculation and whose spacing depends on the sharpness of the field emitter. These weaker peaks are interpreted as arising from size-effect resonances in the microcrystal at the apex of the field emitter.     

Surface wave scattering by a nano-object situated on a surface

The scattering of the surface electromagnetic waves by a nano-defect (object) on a surface was calculated. The scattered field has been considered as a field caused by the current generated by the self-consistent local field inside the defect. In turn, the self-consistent local field has been determined as a result of solution of the integral Lippmann-Schwinger equation. The effective susceptibility of the object has been calculated using a self-consistent procedure. The corrections of self-energy part due to direct and indirect electromagnetic interactions, as well as due to interaction with surface wave field are taken into account. The self-energy part is calculated analytically within the framework of the near-field approximation. The scattering indicatrisses in reciprocal space have been computed for different shapes of the scatterer. Strong dependence of the scattered field on geometry of the scatterer has been found and explained.     

Quantum-well resonances and image states in the Ar/Cu(1 0 0) system

A theoretical study of the effect of an atomically thin rare gas layer on the dynamics of excited electronic states at metal surfaces is presented for the case of a few mono-layers of Ar on a Cu(1 0 0) surface. We develop a 3D-microscopic model with predictive capabilities of the interaction of an electron with an Ar layer physisorbed on a metal surface. It takes into account the 3D structure of the Ar layer as well as its dielectric character. The dynamics of the excited electron on the surface is treated within a wave-packet propagation approach. The calculations show that two different types of excited states are present at the Ar/Cu(1 0 0) surface. (i) Image states that are repelled into vacuum as compared to their position on clean Cu(1 0 0) surfaces, leading to a decrease of their binding energies and to an increase of their lifetimes. (ii) Quantum-well resonances, corresponding to quasi-stationary states localised inside the Ar layer; they are associated with the quantisation of the conduction band in the finite size Ar layer. The present results on image states nicely agree with very recent time-resolved two-photon-photo-emission experiments by Berthold, Feulner and Höfer.     

Tailoring surface electronic states via strain to control adsorption: O/Cu/Ru(0 0 0 1)

A possible relationship between population of the surface electronic state and chemical reactivity for Cu layers on Ru(0 0 0 1) is revealed by tunneling spectroscopy and microscopy. The surface state shifts down in energy with increasing thickness. Ab initio calculations indicate that the energy shift can be assigned to the decreasing tensile strain of the deposited film. The reactivity of the surface towards oxygen correlates with the population of this state, reaching a maximum at Cu thicknesses where the surface state is empty. These data provides an indication of the effect of strain on the reactivity of metal surfaces.