First-principles studies of the oxygen adsorption on unreconstructed and reconstructed Ni(1 1 0) surfaces

First-principles calculations have been performed to investigate the adsorption of oxygen on unreconstructed and reconstructed Ni(1 1 0) surfaces. The energetics, structural, electronic and magnetic properties are given in detail. For oxygen adsorption on unreconstructed surface, (n×1)(n=2,3) substrate with oxygen atom on short-bridge site is found to be the most stable adsorption configuration. Whereas energetically most favorable adsorption phase of reconstructed surface is p(n×1) substrate with oxygen atom located at long-bridge site. Our calculations suggest that the surface reconstruction is induced by the oxygen adsorption. We also find there are redistributions of electronic structure and electron transfer from the substrate to adsorbate. Our calculations also indicate surface magnetic moment is enhanced on clean surfaces and oxygen atoms are magnetized weakly after oxygen adsorption. Interestingly, adsorption on unreconstructed surface does not change surface magnetic moment. However, adsorbate leads to reduction of surface magnetic moment in reconstructed system remarkably.     

Atomic structure of β-SiC(100) surfaces: an ab initio study

We examine several different reconstructions of the β-SiC(100) surface by the ab initio Car-Parrinello method. Our results confirm that the lowest energy c(2 × 2) reconstructed surface consists of triply bonded carbon dimers in a bridging position between neighboring underlying silicon dimers. Added hydrogen atoms bond to the carbon dimers, resulting in a lengthened double-bonded dimer, and a larger separation for the underlying silicon dimers, although those Si bonds do not disappear. The most stable structure found for the (3 × 2) reconstructed surface with a 1/3 monolayer excess of silicon is an alternate dimer row structure rather than the added dimer row model proposed by others. The energetics of various surface reactions that may be involved in growth of SiC are discussed.     

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.     

Diffusion of particles on a heterogeneous surface: A case of adsorption-induced surface reconstruction

The influence of surface reconstruction on diffusion of particles adsorbed on the surface is investigated in the framework of symmetrical four-position model. The analytical expressions for free energy and diffusion coefficients are obtained assuming the lateral interaction between particles is negligibly small.The critical behavior of the system is described by the Ising spin model. The coverage dependencies of the tracer, jump and chemical diffusion coefficients are calculated for some representative temperatures. The dependencies show clearly strong influence of the surface reconstruction on the thermodynamic and kinetic phenomena: diffusion coefficients become anisotropic on the reconstructed surface. To check the analytical results we have used Monte Carlo simulations of the diffusion on this lattice.     

Photoemission studies of oxygen adsorbed on a LiAl(1 1 0) alloy surface: Role of Li segregation

We investigated that the effect of the number of segregated Li atoms on the rate of oxidation on a LiAl alloy surface. Oxygen molecules adsorbed on the LiAl alloy react with the surface atoms to form stable oxides. The segregated Li atoms at reconstructed surfaces (c(2×2) and (2×1)) enhance the oxidation rate and form stable LiAlO_x and Li₂O. The degree of enhancement of oxidation by segregated Li atoms varies as a function of O₂ exposure and annealing temperature, where the latter is directly related to the mode of surface reconstruction by Li segregation.     

UHV transmission electron microscopy on the reconstructed surface of (111) gold: I. General features

The reconstructed surface structure of (111) gold was studied by ultra-high vacuum transmission electron microscopy and diffraction and the results are described in this series of papers. In part I the observed fringes spaced about 6.3 nm in the images of (111) gold platelets grown on molybdenite, magnesium oxide and graphite are shown to be due to a reconstructed surface structure of unidirectional shrinkage of the surface layer by about 4% along one of the 〈1$\text{1}$0〉 directions on the (111) surface. The shrinkage of the surface layer of the same amount from the bulk lattice was found to take place on gold crystals of different lattice parameters, which were formed by pseudomorphic overgrowth of gold on gold containing various amounts of palladium and indium. Evidence is given for the fact that the shrinkage is not uniform and the observed fringes are not simple interference fringes between the shrunk surface layer and the underlying bulk lattice. At high temperatures the structure first transforms gradually and reversibly to an isotropically shrunk one, and finally the reconstruction disappears above about 900°C.     

Comment on the “cross-channel diffusion mechanism on the Pt(110) surface” BY S.H. Garofalini and T. Halicioglu

It is known that the Pt(110) surface exhibits in its stable state a 1 × 2 reconstruction. It seems therefore not appropriate to compare theoretical surface diffusion coefficients calculated for the regular, non-reconstructed Pt(110) surface with the experimental data obtained by field ion microscopy. In particular, the process of cross-channel diffusion by atom exchange which is observed on the regular (110) surface by molecular dynamics simulations may have no relationship to the experimental findings of cross-channel adatom diffusion on reconstructed Pt(110) and Ir(110) surfaces. An alternate mechanism for easy cross-channel adatom diffusion on reconstructed (110)?1 × 2 surfaces of Pt and Ir is proposed.     

On the structure of Si(113)

The structure of the (113) face of Si has received a large amount of interest recently because of its high stability, despite having a high index. There remains some controversy about the surface reconstruction, various groups having reported observations of both 3 × 1 and 3 × 2 structures. We have obtained atomically resolved STM images of this surface showing areas of 3 × 1 reconstructed surface, the structure of which matches a modified version of a previously proposed model. In addition we have also obtained CITS images showing the complex underlying reconstruction and spectroscopy measurements which are comparable with a recent ARUPS study of the same surface.     

Edge reconstruction limited electron transport of zigzag graphene nanoribbon

Recent experimental characterizations have clearly visualized edge reconstructions in graphene nanoribbon and stable defective configurations. We have performed first principles calculations to evaluate the effects of atomic edge arrangement on the electronic transport properties of zigzag graphene nanoribbons (ZGNR). It is found that different conductance behaviors and variation of resonant energies are influenced by atomic reconstruction among three defective edge configurations. It is predicted that the conductance in edge reconstructed ZGNR is not a monotonic function of the increasing concentration of defects in size, but the topology and the distribution of defects should be taken into account. Our findings suggest that the ability of tuning the electronic transport of ZGNR could be improved through edge reconstruction activated by energetic particle irradiation.     

Ordered arrangement of 9-aminoanthracene on Au(1 1 1) surfaces: A scanning tunneling microscopy study

We present a scanning tunneling microscopy (STM) investigation of 9-aminoanthracene (AA) on the reconstructed Au(1 1 1) surface. The bare Au(1 1 1) surface shows the herringbone reconstruction which is conserved upon deposition of the organic molecules. Most of the AA molecules are found to decorate the regions of fcc-stacking of the gold surface where a periodic linear arrangement is observed. The orientation of the long molecule axis of individual molecules is along the -directions of the Au substrate. In addition, for individual domains of the surface reconstruction, one of the three possible orientations is preferred. On substrate areas which exhibit a high step density, the steps are completely decorated by AA molecules. A detailed analysis of the STM images reveals that the molecules are located on top terrace levels. The fine structure of individual molecules on the terrace shows a clear dependence on the tunneling voltage and resembles the molecular orbitals of the free AA molecule.     

Hologram Recording in Various Glass Plates by Corona Charging

We propose techniques of hologram replication in glass plates and reconstruction on another film by corona charging. A surface relief hologram on an azobenzene polymer film was recorded in the form of electric polarization in glass plates that include alkali cations by corona charging at high temperature. The hologram recorded in the glass plate is stable for more than 1 month at room temperature. After removal of the azobenzene polymer film from the plate, poly(methyl methacrylate) (PMMA) film was spin-coated onto the plate. The hologram recorded in the plate was reconstructed with the same surface relief structure on the PMMA film by corona charging again at 110°C.     

Effect of surface reconstruction on the adsorption of Ge on clean Si(111)

The effect of ultrahigh vacuum deposition of Ge below and at monolayer (ML) coverage onto a 7 × 7 reconstructed clean Si(111) surface held at room temperature is studied by low energy electron diffraction (LEED), Auger electron spectroscopy (AES) and photoemission yield spectroscopy (PYS). The results are compared to those obtained on 2 × 1 reconstructed clean Si(111) : (i) the Si dangling bond states are replaced by Ge dangling bond states at submonolayer coverages in both cases; (ii) the 7 × 7 reconstruction persists below 1 ML, it is not replaced by a ? 3 × ? 3 R30° at 1/3ML as it was on the 2 × 1; and (iii) the coverage below 1 ML is not homogeneous on the 7 × 7 reconstruction. This behaviour can be explained by the influence of the inhomogeneties associated with the 7 × 7 reconstruction.     

Measurement of the surface strain induced by reconstructed surfaces of GaAs (001) using photoreflectance and reflectance-difference spectroscopies

We report photoreflectance-difference and reflectance-difference measurements on reconstructed GaAs (001) surfaces. From these data the linear and quadratic electro-optic coefficient spectra are determined in the important 2.8-3.4 eV spectral region. The surface strain and fields induced by the surface reconstruction are also determined. We show experimentally that between c(4x4) and (2x4) surfaces, there is an inversion of the surface electric field which we attribute to a direct piezo-electric effect related to the surface strain induced by reconstruction.     

稀疏阈值的超分辨率图像重建

为了解决基于字典学习的超分辨重构算法耗时过长的问题,提出了基于稀疏阈值模型的图像超分辨率重建方法。首先,将联合字典理论与图像块稀疏阈值方法相结合,训练得到高、低分辨率过完备图像字典对。接着,通过稀疏阈值OMP算法对图像特征块进行稀疏表示。然后,通过高分辨率字典重构出初始的超分辨图像。最后,通过改进迭代反投影算法对初始的超分辨图像进行全局优化,从而进一步提高图像重构质量。实验结果表明,超分辨图像重构平均峰值信噪比(PSNR)为30.1 d B,平均结构自相似度(SSIM)为0.937 9,平均计算时间为10.2 s。有效提高了超分辨重构的速度,改善了重构高分辨图像的质量。     

Reconstructed domains on a stepped W(100) surface

A model calculation is carried out for the scattering of low energy electron diffraction from reconstructed (√2 × √2)R45° domains on a stepped W(100) surface. The existence of monotonically increasing steps causes the integral order beams to split and the separation of the splitting oscillates with the incident electron energy. We show that due to the existence of an antiphase relationship among the randomly nucleated reconstructed domains, the half-order beams neither split nor oscillate with the incident electron energy. This nonsplitting of half order beams is in agreement with the observation by Debe and King (DK). We also show that the measured intensity profile of a half order beam is equal to the signal intensity profile from the individual finite size domains convoluted with the instrument response function. This gives a simple way to evaluate the reconstructed domain size quantitatively. From the angular distribution of the half-order beam intensity we deduce that the reconstructed domains are somewhat round in shape, instead of the “long strips” proposed by DK. Also, the long range inhibition (20 Å) of the reconstruction near the step edge suggested by DK does not necessarily follow from our analysis. As a matter of fact, there is evidence showing that the inhibition (if it exists at all) can be short range in nature. Our suggestion is in agreement with the observation of the reconstructed W(100) surface by Melmed, Tung, Graham and Smith using FIM technique.     

Antiferromagnetism in metals: from the cuprate superconductors to the heavy fermion materials

The critical theory of the onset of antiferromagnetism in metals, with concomitant Fermi surface reconstruction, has recently been shown to be strongly coupled in two spatial dimensions. The onset of unconventional superconductivity near this critical point is reviewed: it involves a subtle interplay between the breakdown of fermionic quasiparticle excitations on the Fermi surface and the strong pairing glue provided by the antiferromagnetic fluctuations. The net result is a logarithm-squared enhancement of the pairing vertex for generic Fermi surfaces, with a universal dimensionless coefficient independent of the strength of interactions, which is expected to lead to superconductivity at the scale of the Fermi energy. We also discuss the possibility that the antiferromagnetic critical point can be replaced by an intermediate 'fractionalized Fermi liquid' phase, in which there is Fermi surface reconstruction but no long-range antiferromagnetic order. We discuss the relevance of this phase to the underdoped cuprates and the heavy fermion materials.     

The surface structure of BaO on Pt(1 1 1): (2 × 2)-reconstructed BaO(1 1 1)

The surface structure of BaO(1 1 1) has been determined using STM and computer modelling. The BaO(1 1 1) surface was prepared in thin film form on Pt(1 1 1) and presents a surface with twice the lattice parameter expected for that of the bulk termination, i.e. a (2 × 2) reconstruction. Computer modelling indicates that the bulk termination is unstable, but that the (2 × 2) reconstructed BaO(1 1 1) surface has a low surface energy and is hence a stable surface reconstruction. The (2 × 2) reconstruction consists of small, three-sided pyramids with (1 0 0) oriented sides and either oxygen or barium ions at the apices. Less regular surface reconstructions containing the same pyramids are almost equally stable, indicating that we may also expect less regular regions to appear with a fairly random distribution of these surface species. The simulations further suggest that a regular (4 × 4) reconstruction built up of bigger pyramids is even more energetically favourable, and some evidence is found for such a structure in the STM.     

The Si (100) surface: Symmetric or asymmetric dimers?

We present new results obtained with medium energy ion beam crystallography on the clean Si (100)?(2 × 1) surface. This study, meant to discriminate between symmetric and asymmetric dimer models for the reconstruction, shows that symmetric dimers are not consistent with our results. We conclude that the Si (100)?(2 × 1) surface is reconstructed in asymmetric dimers, accompanied by extensive subsurface distortions.     

Imaging the surface and the interface atoms of an oxide film on Ag111 by scanning tunneling microscopy: experiment and theory

High-resolution images of the vacuum oxide surface atomic structure of Ag111-p(4x4)-O reveal large terraces of a perfect (4x4) reconstruction. Under certain conditions, the surface Ag atoms at the interface between the reconstructed oxide layer and the underlying Ag111 lattice are also imaged, providing the structural registry. Scanning tunneling microscopy simulations reveal a strong sensitivity to structure and comparison with the experimental images, therefore providing an atom-by-atom model for the entire metal-oxide-vacuum structure.     

Effects of the 2 × 1 reconstruction on the electronic structure of the (111) surface of Si and Ge

We have studied the electronic surface properties of the (2 × 1) reconstructed (111) surface of a diamond-structure semiconductor with a simple sp³ Hamiltonian. Different models for the reconstruction have been analysed. We conclude that, in order to explain experimental results, both relaxation and reconstruction effects have to be taken into account.