Contrasting bonding configurations of acetone on Pt(111) and Ru(001) surfaces.

The comparative chemistry of acetone adsorption on Pt(111) and Ru(001) has been studied by EELS. On the more easily oxidized Ru(001) surface, acetone bonded in a side-on, η²(O,C) configuration, whereas on the well-defined. close- packed regions of the Pt(111) surface acetone adopted a weak adduct-like, end-on, η¹(O) configuration. On Pt(111), some η²(O,C) was also observed and associated with adsorption at low coordination accidental step sites.     

Vibrational states of a cobalt dimer on the (111) and (001) copper surfaces

The results of calculations of the total (lateral and vertical) relaxation of the (001) and (111) copper surfaces in the presence of a small cluster of cobalt adatoms, local vibrational density of states and polarizations of these states are presented. The calculations were performed using the atomic interaction potentials in a tight binding approximation. An analysis of the results obtained showed that the presence of a cobalt dimer gives rise to modification of the vibrational states of the copper surface and generation of new modes localized both on the adatoms of the cluster and the surface atoms of the substrate. The revealed anisotropy of surface relaxation along [001] results in deformation of atomic bonding and splitting of the vibrational modes of the dimer. The lifetimes of the vibrational states of the dimer are found to be nearly equal for both surfaces under study, with a frequency shift being however observed. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 12, pp.73–78, December, 2008.     

Electronic structure of the (111) and (111) surfaces of GaAs

Inward relaxation effects of the outermost Ga layer on the electronic structure of GaAs (111) Ga and outward expansion effects of the outermost As layer on that of GaAs ($\text{111}$) As are studied by extended Hückel theory. Three different surface geometries are examined for the respective surfaces. It is shown that upon relaxation on GaAs (111) or upon expansion on GaAs ($\text{111}$) new surface states associated with dangling- and back-bonds are revealed. The character and dispersion behaviour of strongly localized surface states are described.     

Hydrogen adsorption on the silicon (001) surface and on a step on the (111) surface

Adsorption of atomic hydrogen on an ideal (001) silicon surface is investigated in the present paper. Saturation of one of the two dangling bonds of a silicon atom on this surface by hydrogen removes the interaction (hybridization) between them, resulting in the appearance of a bonding and an antibonding chemisorption state associated with the attacked dangling bond, and in the shift of the peak of the remaining unsaturated dangling bond to the energy typical of a surface state of the (111) surface. Further saturation leads to the disappearance of this peak from the energy spectrum. An analogous situation occurs for the silicon atom with two dangling bonds on a step on the (111) surface, when hydrogen is chemisorbed. Both examples testify to the local chemical nature of Shockley surface states in silicon.The authors thank A. N. Sorokin for useful discussions.     

On a rudimentary model of instability and (2 x 1) reconstruction of Si (001) and (111) surfaces

A recently proposed simple qualitative theory of instability and reconstruction of surfaces of solids exhibiting a certain covalent component of bonding is applied to (2 x 1) Si (001) and (111) surfaces with a possible extension to analogous surfaces of germanium and GaAs (001)., Shockley surface states from the vicinity of the Fermi energy are supposed to play a crucial role in the electron-phonon coupling. For the Si (001) surface the interaction between bridge and dangling bonds causes the dimerization (primary effect) and the buckling distortions to couple. If the latter effect is pronounced, the asymmetric dimer might exist as a stable or metastable surface phase. For the Si (111) surface the Pandey -bonded chain model seems to be a natural candidate for reconstruction from the present simple point of view.     

Subsurface strain in the Ge(001) and Ge(111) surfaces and comparison to silicon

The subsurface strain associated with surface reconstruction was measured for the Ge(001)-c(4×2) and Ge(111)-c(2×8) surfaces using high energy ion scattering. In the case of the Ge(001) surface we find the equivalent of ∼3 monolayers displaced by more than 0.12 Å, in accord with dimer models of the surface reconstruction. For the Ge(111) surface displacements are observed in off-normal incidence, indicating large displacements perpendicular to the surface or other reconstructions, such as a stacking fault configuration. The relationship between subsurface strain and stacking fault models is also discussed. The subsurface strain in these two Ge surfaces is remarkably similar to that of the corresponding Si surfaces, even though the details of the surface reconstruction are different. Measurements at low temperature indicate that the strain is essentially temperature independent, as expected. Measurements of the hydrogen covered surfaces show little change is strain, a surprising result when compared to the behavior of Si(001).     

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.     

Leed,aes and photoemission measurements of epitaxially grown GaAs(001), (111)A and (1̄1̄1̄)B surfaces and their behaviour upon cs adsorption

Epitaxially grown GaAs(001), (111) and (1?1?1?) surfaces and their behaviour on Cs adsorption are studied by LEED, AES and photoemission. Upon heat treatment the clean GaAs(001) surface shows all the structures of the As-stabilized to the Ga-stabilized surface. By careful annealing it is also possible to obtain the As-stabilized surface from the Ga-stabilized surface, which must be due to the diffusion of As from the bulk to the surface. The As-stabilized surface can be recovered from the Ga-stabilized surface by treating the surface at 400°C in an AsH₃ atmosphere. The Cs coverage of all these surfaces is linear with the dosage and shows a sharp breakpoint at 5.3 × 10¹⁴ atoms cm^?2. The photoemission reaches a maximum precisely at the dosage of this break point for the GaAs(001) and GaAs(1?1?1?) surface, whereas for the GaAs(111) surface the maximum in the photoemission is reached at a higher dosage of 6.5 × 10¹⁴ atoms cm^?2. The maximum photoemission from all surfaces is in the order of 50μA Im^?1 for white light (T = 2850 K). LEED measurements show that Cs adsorbs as an amorphous layer on these surfaces at room temperature. Heat treatment of the Cs-activated GaAs (001) surface shows a stability region of 4.7 × 10¹⁴ atoms cm^?2 at 260dgC and one of 2.7 × 10¹⁴ atoms cm^?2 at 340°C without any ordering of the Cs atoms. Heat treatment of the Cs-activated GaAs(111) crystal shows a gradual desorption of Cs up to a coverage of 1 × 10¹⁴ atoms cm^?2, which is stable at 360°C and where LEED shows the formation of the GaAs(111) (√7 × √7)Cs structure. Heat treatment of the Cs-activated GaAs(1?1?1?) crystal shows a stability region at 260°C with a coverage of 3.8 × 10¹⁴ atoms cm^?2 with ordering of the Cs atoms in a GaAs(1?1?1?) (4 × 4)Cs structure and at 340°C a further stability region with a coverage of 1 × 10¹⁴ at cm^?2 with the formation of a GaAs(1?1?1?) (√21 × √21)Cs structure. Possible models of the GaAs(1?1?1?) (4 × 4)Cs, GaAs(1?1?1?)(√21 × √21)Cs and GaAs(111) (√7 × √7)Cs structures are given.     

Electric-dipole layer on Au(111) surfaces

We measured the barrier height (BH) a UHV scanning tunneling microscope on Au(111) surfaces with Au, Pt, and carbon nanotube tips. The 222?{^[¯] }3^[¯]\sqrt\Box3\Box]] reconstruction was observed with all the tips, and the current-voltage relation reflected the density of states of the tips. The BH measured on the reconstructed Au(111) surface using a modulation method showed a bias-polarity dependence as 30%, at low currents (<100 pA) irrespective of the tip material, while on unreconstructed Au surfaces, BH values were independent of the bias polarity, suggesting a dipole layer originating from the reconstructed Au(111) surface.     

Ab-initio study of the coadsorption of Li and H on Pt(001), Pt(110) and Pt(111) surfaces

The coadsorption of Li and H atoms on Pt(001), Pt(110) and Pt(111) surfaces is studied using density functional theory with generalised gradient approximation. In all calculations Li, H and the two topmost layers of the metal were allowed to relax. At coverage of 0.25 mono-layer in a p(2×2) unit cell, lithium adsorption at the hollow site for the three surfaces is favoured over top and bridge sites. The most favoured adsorption sites for H atom on the Pt(001) and Pt(110) surfaces are the top and bridge sites, while on Pt(111) surface the fcc site appears to be slightly favoured over the hcp site. The coadsorption of Li and atomic hydrogen shows that the interaction between the two adsorbates is stabilising when they are far from each other. The analysis of Li, H and Pt local density of states shows that Li strongly interacts with the Pt surfaces.     

Elastic leed intensity-energy studies of clean (001), (110) and (111) nickel surfaces

Elastic low energy electron diffraction (LEED) intensity-energy (I-E) measurements for clean (001), (110), and (111) nickel surfaces were obtained at room temperature. Surface composition was monitored by Auger spectroscopy. I-E data from 15 to 220 eV were obtained at normal incidence for the non specular beams and for the specular beams at incidence angles from 4° to 20° on the 0° and 45° azimuths of (001), on the 0° and 90° azimuths of (110), and on the 0° azimuth of (111) nickel. Normalization of the data was performed electronically during data acquisition. Intensities were calibrated with the use of a shielded, biased Faraday collector. The effects of instrumental and experimental uncertainties were examined and minimized to obtain intensities accurate to ± 15 %, energy scales accurate to ± 0.35 eV, and incident and azimuthal angles accurate to ± 0.25° and ± 1.0° respectively.All nickel surfaces have I-E spectra which are characteristic of strong multiple scattering. Angular evolution features for (001) and (110) spectra may be correlated with intraplanar resonances associated with the onset of propagating beams. Only the (001) surfaces were found to have pronounced, sharp resonance features associated with surface barrier resonances and inelastic loss processes. Kinematic analysis of the Lorenzian-shaped I-E peaks on all surfaces in consistent with surface expansion using either an energy-dependent or a constant inner potential of 10.75 ± 0.5 eV. The widths of these same peaks on all surfaces were found to vary as $\text{E}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ above 40 eV and $\text{E}{}^{\mathrm{<mtext>1</mtext><mtext>3</mtext>}}$ below.     

Electronic properties of TiC(100) and polar TiC(111) surfaces

The energy bands of films of TiC have been calculated using the linear-combination-of-atomic-orbitals method with parameters obtained by a fit to the bulk band structure. The Madelung potentials and charge redistribution have been determined self-consistently. For the neutral TiC(100) surface, the density of states (DOS) is similar to that of the bulk. For the non-neutral Ti-covered TiC(111) surface, Ti 3d-derived surface states appear around the Fermi energy E_F. The long-range electric field produced by the polar surfaces is screened by the charge redistribution, and the polar surfaces are stabilized. Characteristic features of TiC(111) compared to other surfaces of TiC are attributed to the high surface DOS at E_F.     

A comment on surface states of the LaB6(001) face

An attempt is made to use the qualitative theory of surface states for the analysis of the nature of these states recently found by angle-resolved photoemission from the LaB₆ (001) surface. The character of chemical bonding in this compound is also discussed.     

Static sims studies of adsorbate structure: II. CO adsorption on Pd(111); adsorbate-adsorbate interactions on Ru(001), Ni(111) and Pd(111)

In a study of CO adsorption on Pd(111) it is shown that the secondary ion mass spectrum contains information on both adsorbate site geometry and adsorbate coverage. The fractional yields of PdCO⁺, Pd₂CO⁺ and Pd₃CO⁺, as a function of CO coverage are correlated with the changing site geometries suggested by reflection IR data. A relationship between secondary ion emission and the adsorbate-adsorbate interactions revealed by IR and EELS is also demonstrated for CO adsorption on Ru(001), Ni(111) and Pd(111).     

MD simulation of nanoindentation on (001) and (111) surfaces of Ag–Ni multilayers

We perform MD simulations of the nanoindentation on (001) and (111) surfaces of Ag–Ni multilayers with different modulation periods, and find that both the hardness and maximum force increase with the increase of modulation period, in agreement with the inverse Hall–Petch relation. A prismatic partial dislocation loop is observed in the Ni(111)/Ag(111) sample when the modulation period is relatively large. We also find that misfit dislocation network shows a square shape for the Ni(111)/Ag(111) interface, while a triangle shape for the Ni(001)/Ag(001) interface. The pyramidal defect zones are also observed in Ni(001)/Ag(001) sample, while the intersecting stacking faults are observed in Ni(111)/Ag(111) sample after dislocation traversing interface. The results offer insights into the nanoindentation behaviors in metallic multilayers, which should be important for clarifying strengthening mechanism in many other multilayers.     

Double diffraction spots in RHEED patterns from clean Ge(111) and Si(001) surfaces

In RHEED patterns from clean Ge(111) and Si(001) surfaces, extra diffraction spots have been observed with superlattice reflection spots due to Ge(111) 2 × 8 and Si(001) 2 × 1 surface structures. The extra spots have not been found out in many previous LEED and RHEED patterns of clean Ge(111) and Si(001) surfaces. When the Ge(111) and Si(001) samples were rotated about an axis normal to the surfaces so as to vary the incident direction of the primary electron beam, the intensity of the extra spots showed a remarkable dependence upon the incident direction and they became invisible in some incident directions, in spite of the experimental condition that an Ewald sphere intersected reciprocal lattice rods of the extra spots. In this study, the extra spots are understood as forbidden reflection spots resulting from double diffraction of superlattice reflections of the surface structures, and the remarkable dependence of their intensity upon the incident direction is explained in terms of excitation of the surface wave of the superlattice reflections. These results suggest that the intensity of diffraction spots in RHEED patterns may be greatly influenced by the surface wave excitation of fundamental and superlattice reflections.     

A model of ethylene and acetylene adsorption on the (111) surfaces of platinum and nickel

Despite the application of a variety of surface sensitive techniques to the adsorption of simple hydrocarbons on well characterized metallic surfaces, no consistent picture has appeared. We review briefly the published spectroscopic results of ultraviolet photoelectron spectroscopy (UPS) and electron energy loss spectroscopy (EELS) which probe, respectively, the electronic and vibrational structure of the surface-molecular complex, and we consider appropriate free molecular analogues, not only in their ground state but also in their first excited states. A simplified approach to determine the chemisorption geometry from UPS level shifts and EELS is presented. The technique allows an isolation of distortion induced shifts from the total relaxation shift, and we find that the true relaxation shift is rather constant, approximately 2.1 eV for the cases considered. These shifts can then be used to estimate the distance of the molecule to the surface. We concentrate primarily on four systems, C₂H₂ and C₂H₄ on Ni(111) and Pt(111), adsorbed at low temperature (below the onset of dissociation). Depending on the metal, the hydrocarbon can adsorb in a di-σ arrangement or with a distortion resembling the lowest energy configuration of the first excited state of the free molecule. We also consider briefly C₂H₄ on Ag and Cu in which no distortion occurs. The distortions that resemble the first excited states might occur as a consequence of donation of bonding (backbonding) electrons from (to) the normally filled π (empty π¹) to (from) the empty (filled) d-band states of the metal. The net effect on the hydrocarbon to partially empty the π level and fill the π¹ level, is analogous to a low excitation of the free molecule, π → π¹. For C₂H₄ (planar in the ground state), the lowest excitation is the triplet T-state (3–4 eV) of minimal energy for a 90° twisted configuration with a lengthened C-C bond. Acetylene is a linear molecule in the ground state, but cis- or trans-bent for the triplet excitations, ~a (5.2 eV) or ~b (6.0 eV), respectively. Chemisorbed geometries derived from these configurations seem possible for C₂H₄ on Ni(111) and C₂H₂ on Pt(111), while interchanging the adsorbates and substrates gives di-σ bonding, (sp³ hybridization), as proposed previously in the literature. For C₂H₄ on Ni(111), two of the hydrogens are twisted into the surface which leads to a softening of the CH vibrational frequency. For the four systems considered, the data are consistent with the C-C bond essentially parallel to the surface, but tilted orientations are not ruled out. While the models are clearly oversimplified, they suggest an interesting point of departure for likely chemisorption geometries. Also, some intriguing correspondences to the (presumed) location of the normally empty π¹ level and the d-band are noted.     

Tight-binding calculations of (111) surface densities of states of Ge and GaAs

We have used the tight-binding method to calculate the local densities of states of unreconstructed Ge (111) and GaAs (111), ($\text{111}$) surfaces. In the unrelaxed surface configuration we find two types of states for each surface. The effects of relaxation on Ge surface states are also discussed.     

Structural,electronic and magnetic properties of the (001), (110) and (111) surfaces of rocksalt sodium sulfide: A first-principles study

First principles study of the structural, electronic and magnetic properties of the (111), (110) and (001) surfaces of rocksalt sodium sulfide (rs-NaS) are reported. The results show that the bulk half-metallicity of this compound is well preserved on the surfaces considered here except for Na-terminated (111) surface. The spin-flip gap at the S-terminated (111), (001) and (110) surfaces are close to the bulk value. Using ab-initio atomistic thermodynamics, we calculate the surface energies as a function of chemical potential to find the most stable surface. We find that the Na-terminated (111) surface is the most stable one over the whole allowed range of chemical potential, while the surface energies of the (001) and (110) surfaces approach the most stable surface energy at the sulfur rich environment. We have also calculated the interlayer exchange interaction in bulk and Na-terminated (111) surface by classical Heisenberg model and we found that the surface effects do not change these kinds of interactions significantly.