LEED crystallographic analysis for the Rh(111)-(2 × 1)---O surface structure

A tensor LEED analysis is reported for the Rh(111)-(2 × 1)---O surface structure in which atoms in the O overlayer chemisorb close to the regular (fcc type) three-fold hollow sites for half-monolayer coverage. The structure shows significant relaxations: for example, a buckling of about 0.07 Å is indicated in the first metal layer and O appears to displace laterally by about 0.05 Å. The individual O---Rh bond lengths are around 2.01 and 1.92 Å to top layer Rh atoms, which bond to two and one O atoms, respectively, but the average value (1.98 Å) is close to that in bulk RhO₂ (1.96 Å). Comparison is also made with the previously determined O---Rh bond lengths in the Rh(110)-p2mg(2 × 1) surface structure.     

The atomic geometries of the ordered (2 × 2) and (√3 × √3)R30° phases of K, Rb, and Cs on Rh(111) in comparison with Ru(0001)

The local adsorption geometries of K, Rb and Cs in the (√3 × √3)R30° and (2 × 2) phases on a Rh(111) surface at coverages of 0.33 and 0.25 ML, respectively, are determined by analyzing LEED intensity data. For all (√3 × √3)R30° phases investigated, the three-fold hcp site is found. For the (2 × 2) overlayer, K remains in the hcp position, while Cs favors the on-top position. For the case of Rb-(2 × 2), LEED analysis suggests occupation of the unusual two-fold bridge site. Since LEED analysis of the Rb-(2 × 2) phase is not completely conclusive, additional experimental evidence is necessary to firmly establish this adsorption geometry.     

Metastable (1 × 2) and (1 × 3) reconstructions of Pd(110)

By adsorption and subsequent reduction of oxygen on Pd(110), metastable (1 × 2) and (1 × 3) reconstructed surfaces have been produced. Oxygen was not present after the reduction but a small amount of residual hydrogen (< 0.15 monolayers) remained. However this is not the origin of the reconstruction as adsorption of this amount of hydrogen on the clean surface did not cause reconstruction. The structures were stable up to ˜ 370 K, and at higher temperatures they reverted to (1 × 1). These results are compared with Rh(110) where similar reconstructions have been found.     

Growth mechanism of the Pd(100)-p(2×2)-p4g-Al surface alloy

We have studied the growth mechanism of a Pd(100)-p(2×2)-p4g-Al surface alloy by scanning tunneling microscopy (STM). The surface alloy has a bilayer structure and is formed by annealing at 450–700 K (depending on the initial aluminum coverage) after the deposition of aluminum on Pd(100) at room temperature. The ratio of the surface-alloy coverage to the initial aluminum coverage is found to be constant (0.44) irrespective of the initial aluminum coverage from 0.5 monolayers (ML) up to 2 ML. The growth mechanism of the surface alloy is proposed on the basis of the STM measurements at various annealing temperatures. Upon annealing at 450 K, some of the surface aluminum atoms migrate into the bulk and, instead, palladium atoms come out to the surface. These palladium atoms react with aluminum atoms remaining on the surface to form a surface alloy. When the initial aluminum coverage is less than 1 ML, bilayer-high islands of the surface alloy with an average area of 100 nm² are formed at 450–500 K, which diffuse on the terrace at 500–700 K and coalesce to form larger islands. A possible role of the percolation transition of aluminum islands in the formation of the surface alloy is discussed.     

Photon-stimulated desorption of methanol on Si(111)7 × 7 and Si(100)2 × 1 at the C 1s and O 1s thresholds

Photostimulated desorption experiments have been performed on deuterated methanol adsorbed on Si(111)7 × 7 and Si(100)2 × 1 at the C 1s and O 1s thresholds. D⁺ and the masses of the series CD⁺_x are produced in the photofragmentation process in both energy ranges. A comparison has been made with the photofragmentation spectra of methanol in the gas phase and two different desorption mechanisms have been hypothesized for the desorption of D⁺ and higher masses from the silicon surfaces at the C 1s threshold.     

A comparative infrared study of H2O reactivity on Si(1 0 0)-(2 × 1), (2 × 1)-H, (1 × 1)-H and (3 × 1)-H surfaces

The water adsorption on the bare and H-terminated Si(1 0 0) surfaces has been studied by the BML-IRRAS technique. It is found that H-terminated surfaces are much less reactive compared to the bare silicon surfaces. The (1 × 1)-H and (3 × 1)-H surfaces show similar and less reactivity pattern compared to the (2 × 1)-H surface. At higher exposures, the water reaction with coupled monohydride species provides an effective channel for oxygen insertion into the back bonds of dihydride species. It is not attributed to the H–Si–Si–H + H₂O → H–S–Si–OH + H₂, which could give rise to the characteristic Si–H and Si–OH modes, respectively at 2081 and 921 cm⁻¹. A more suitable reaction mechanism involving a metastable species, H–Si–Si–H + H₂O → H₂Si  HO–Si–H (metastable) explains well the bending modes of oxygen inserted silicon dihydride species which are observed relatively strongly in the reaction of water with H-terminated Si(1 0 0) surfaces.     

Structural analysis of the β-SiC(100)-(2 × 1) surface reconstruction by automated tensor LEED

The atomic structure of the β-SiC(100)-(2 × 1) surface was analyzed using dynamical calculations of low energy electron diffraction (LEED) intensities measured with a video camera. Surface composition was monitored using Auger electron spectroscopy (AES). The LEED calculations utilized our recently developed automated tensor LEED method. The results indicate that the surface is terminated by a monolayer of silicon with the topmost silicon atoms forming asymmetric, buckled dimers.     

X-ray diffraction study of Si(111)√3 × √3-Au

The structure of the Si(111)√3 × √3-Au surface has been investigated by the use of the surface X-ray diffraction with synchrotron radiation. The structure perpendicular to the surface was determined with respect to the Si bulk crystal. The results of least-squares analysis indicate that Au atoms are adsorbed on the Si substrate in which the first Si layer is missing. The heights of the Au layer and the Si second layer with respect to the intact Si third layer were estimated to be 3.09 ± 0.03 rA and 2.16 ± 0.10 rA, respectively. A possible model of the surface structure is proposed.     

Study of ultrathin Fe films on Pd{111)}, Ag{111} and A1{111}

The growth of epitaxial Fe films of 1 to 10 layer equivalents (LE) on Pd{111}, Al{111} and Ag{111} has been studied with quantitative low-energy electron diffraction and Auger electron spectroscopy. On Pd{111}, both at room temperature and at 200° C, the growth starts with pseudomorphic layers which may involve intermixing of Fe and Pd. At both temperatures, when the coverage reaches 6 LE, the Fe films develop into large bcc {110} domains related to the substrate by the Kurdjumov-Sachs orientation. On Ag{111} Fe grows initially in a very similar manner to Fe on Ag{001}, i.e., by way of small islands of bcc Fe. These islands then grow into bcc Fe{110} domains in the Nishiyama-Wassennan orientation. On Al{111} Fe behaves differently, despite the fact that the lattice mismatch for Fe on A1{111} is nearly identical to that for Fe on Ag{111}: for coverages of less than 1 LE the surface region becomes completely disordered and no LEED pattern is visible.     

The Si(111)−(5 × 1) Au structure

By combining recent results from STM, LEEM, LEED and X-ray diffraction a structure model is developed for the (5 × 1) structure observed in the Au/Si(111) system at low coverages.     

The kinetics of the (5 × 20) → (1 × 1) structural transition for Au(100)

The kinetics of the anion-induced (5 × 20) → (1 × 1) surface structural transition of reconstructed Au(100) electrodes was studied in sulfate-containing solutions by current transients. It is shown that lifting of the reconstruction follows a nucleation-and-growth type behavior which can be described by the Avrami equation. Moreover, for high positive potentials, i.e., high anion coverages, the current-transient analysis reveals instantaneous nucleation, whereas for low transition overpotentials and long transition times a mechanism with constant transition rate prevails. Apparent activation energies, which depend strongly on the electrode potential, are derived from the temperature dependence of the Avrami plots.     

Nucleation behaviour during silicon UHV-CVD on Si(111)7×7

We report a UHV-CVD homoepitaxy study on the Si(111)7×7 surface investigated with Scanning Tunnelling Microscopy (STM). We have investigated the two-dimensional island density in the temperature range from 450 to 550°C and silane pressure range from 2 to 8×10⁻⁴ Torr. Contrary to experiments using molecular beam epitaxy, we find that the two-dimensional island density in UHV-CVD cannot be directly described by the standard nucleation theory. We discuss this point and show that the variation of the steady-state hydrogen coverage on the surface during pressure- or temperature-dependent experiments can explain the observed two-dimensional island density behaviour.     

Thermodynamical Bethe Ansatz analysis in an SU(2)×U(1) symmetric σ-model

Four different types of free energies are computed by both thermodynamical Bethe Ansatz (TBA) techniques and by weak coupling perturbation theory in an integrable one-parameter deformation of the O(4) principal chiral σ-model (with SU(2)×U(1) symmetry). The model exhibits both ‘fermionic' and ‘bosonic' type free energies and in all cases the perturbative and the TBA results are in perfect agreement, strongly supporting the correctness of the proposed S matrix. The mass gap is also computed in terms of the Λ parameters of the modified minimal subtraction scheme and a lattice regularized version of the model.     

Reconstruction of the Ir(110) surface: A mixed faceted (1 × 3) and (1 × 1) structure

Time-of-flight scattering and recoiling spectrometry (TOF-SARS) has been used to show that the reconstructed Ir(110) surface, following annealing to 1400 ° C, consists primarily of domains of faceted (1 × 3) structures (with two missing first-layer rows and one missing second-layer row); the data are consistent with secondary domains of (1 × 1) structures (with no missing rows). This structure is determined from scans of (i) backscattering (BS) versus incidence angle , (ii) forwardscattering (FS) versus , and (iii) FS versus scattering angle Θ.     

HREELS and LEED studies on Cu deposited on 1 × 1 reconstructed α-Fe2O3(0001) surfaces

High-resolution electron energy-loss spectroscopy (HREELS), low-energy electron diffraction, and X-ray photoelectron spectroscopy have been used to study clean 825 K-preannealed α-Fe₂O₃-1 × 1 (haematite) surfaces, an α-Fe₂O₃-(0001)-1 × 1 surface reconstructed with Fe₃O₄(111)-1 × 1 and to study Cu deposited on room-temperature surfaces of those. Three pronounced losses, at 47.5, 55.5 and 78.0 meV, of the surface phonons for the clean α-Fe₂O₃(0001) were observed. By deposition of copper, Cu---O vibrational features observed by HREELS indicate formation of a Cu(I) state for the very low coverages. Increased submonoloayer amounts of Cu result in clustering of the copper, leading for both the α-Fe₂O₃(0001)-1 × 1 and the reconstructed composite substrate surfaces to Cu(111) epitaxial growth.     

Surface and subsurface distortions of the Au{110}-(1×2) structure

The reconstruction of the Au{110}-(1×2) missing-row surface has been studied by means of the new scattering and recoiling imaging spectrometry (SARIS) technique. The three-dimensional focusing patterns observed for scattering of 4 keV He⁺, Ne⁺ and Ar⁺ ions are highly sensitive to the structure of both the surface and subsurface layers. Classical ion trajectory simulations using the scattering and recoiling imaging code (SARIC) were used to simulate the scattering patterns. Using an R-factor comparison of the experimental and simulated images, it is demonstrated that SARIS is sensitive to changes of the order of 0.02 Å in the structural parameters of this Au surface. These parameters involve interlayer spacings, row pairing and row buckling in the first-through fifth atomic layers. Results for the shallow surface layers are in general agreement with the those of previous studies. The new results include structural parameters for the deeper subsurface layers and the observation of an oscillatory behavior of the layer spacings which is damped towards deeper layers.     

Distance dependence of noncontact-AFM image contrast on Si(111) × –Ag structure

Atomic resolution imaging of the Si(111) × R30°–Ag surface was investigated using a noncontact atomic force microscopy (NC-AFM) in ultrahigh vacuum. NC-AFM images showed three types of contrasts depending on the distance between an AFM tip and a sample surface. When the tip–sample distance was about 1–3 Å, the images showed the honeycomb arrangement with weak contrast. When the tip–sample distance was about 0–0.5 Å, the images showed the periodic structure composed of three bright spots with relatively strong contrast. On the other hand, the contrasts of images measured at the distance of 0.5–1 Å seemed to be composed of the above-mentioned two types of contrasts. By comparing the site of bright spots in the AFM images with honeycomb-chained trimer (HCT) model, we suggested the following models: when the tip is far from the sample surface, tip–sample interaction force contributing to imaging is dominated by physical bonding interaction such as Coulomb force and/or van der Waals (vdW) force between the tip apex Si atoms and Ag trimer on the sample surface. On the other hand, just before the contact, tip–sample interaction force contributing to imaging is dominated by chemical bonding such as the force due to hybridization between the dangling bond out of the tip apex Si atom and the orbit of Si–Ag covalent bond on the sample surface.     

X-ray and electron beam modification of thiophene overlayers on TiO2(100)1 × 1 and 1 × 3

The effects of electron and X-ray beams on thiophene overlayers on TiO₂(100) 1 × 1 and 1 × 3 surfaces have been investigated using AES, UPS and XPS. Mg K X-rays were found to polymerise a thiophene multilayer condensed at 120 K. The evidence points to a substrate-secondary-electron mediated process. A 3 keV electron beam also modifies a condensed thiophene overlayer, probably by polymerisation.     

Surface structures of S/Ni(111) in (√3 × √3 )R30° and (5√3 × 2) phases studied by surface extended X-ray absorption fine structure spectroscopy

The local surface structures of S/Ni(111) in the ( √3 × √3) R30° and (5√3 × 2) phases have been investigated by means of polarization-dependent sulfur K-edge surface EXAFS. In the (√3 × √3 ) R30° phase, sulfur adatoms are found to occupy threefold hollow sites with a S---Ni distance of 2.13 Å and an inclination angle ω of the Sz.sbnd;Ni bonds at 44° from the surface plane. In contrast, in the (5√3 × 2) phase, it is determined that the Sz.sbnd;Ni bond is longer, 2.18 Å, more inclined, ω = 31°, and that the coordination number is not 3 but 4. These results strongly support a picture involving reconstruction of the top nickel layer to form a rectangular structure. Consideration of several models proposed for the (5√3 × 2) phase leads to one which is compatible with both the present results and results recently reported using STM.     

Transition from SU(2)L × SU(2)R × U(1)B-L representation to SU(2)L × U(1)Y by q-deformation and the corresponding classical breaking term of chiral U(2)

The minimal Standard Model exhibits a nontrivial chiral U(2) symmetry if the VEV and the hypercharge splitting Δ = (y-y)/2 of right-handed leptons (quarks) in a family vanish and Q = T₀ + Y independently in each helicity sector. As a generalization, we start with SU(2)_L × SU(2)_R × U(1)_(B-L) and introduce Δ as a continuous parameter which is a measure of explicit symmetry breakdown. Values 0 ? Δ ? 1/2 take the neutral generator of the isospin ½ representation to the singlet representation, i.e. ‘deformes’ the LR representation into the minimal Standard one. The corresponding classical O(3)-breaking term is a magnetic field perpendicular to the x₃-axis. A simple mapping on the fundamental Drinfeld-Jimbo q-deformed SU(2) representation is given.