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.     

An investigation of the Si(111)−(√3 × √3)R30°−Ag surface by Li impact collision ion scattering spectroscopy

The Si(111)−(√3 × √3)R30°−Ag surface has been investigated using the technique of Li⁺ impact collision ion scattering spectroscopy. Typical LEED √3 domain sizes were estimated to be on the order of 150 Å for a 1 ML coverage of Ag, with the √3 structure persisting for coverages of Ag up to 35 ML. Silver islanding was found to influence the appearance of the 5 keV Li⁺ ICISS angular scans even for 1 ML coverages of Ag deposited at 480°C. A detailed structural analysis of the Si(111)−√3−Ag surface (0.25 ML deposition) involved the comparison of 5 keV Li⁺ ICISS experimental data along the [11 ], [ 10] and [2 1] azimuths with computer simulations of the scattered ion intensities based on previously proposed models for the √3 surface. Nine structurally different models were tested, and only the missing-top-layer (MTL) and the honeycomb-chained-trimer (HCT) models were found to be consistent with all the experimental results. An estimate of 0.4 Å for the maximum downward vertical displacement of the Ag atoms with resect to the surface Si atoms in the MTL model is made. The effects of increased thermal vibrational amplitude in the simulation of Si---Ag shadowing effects is also discussed. The interpretations of previous noble gas ICISS results are shown to be inconsistent with the present alkali metal ICISS study of the √3 surface.     

LEED structure determination of the Ni(1 1 1)(√3×√3)R30°-Sn surface

Quantitative low energy electron diffraction has been used to determine the structure of the Ni(1 1 1)(√3×√3)R30°-Sn surface phase. The results confirm that the surface layer comprises a substitutional alloy of composition Ni₂Sn as previously found by low energy ion scattering (LEIS), and also shows that there is no stacking fault at the substrate/alloy interface as has been found in (√3×√3)R30°-Sb surface alloys on Ag and Cu(1 1 1). The surface alloy layer is rumpled with the Sn atoms 0.45 ± 0.03 Å higher above the substrate than the surrounding Ni atoms. This rumpling amplitude is almost identical to that previously reported on the basis of the LEIS study. Comparison with similar results for Sn-induced surface alloy phases on Ni(1 0 0) and Ni(1 1 0) shows a clear trend to reduced rumpling with reduced surface atomic layer density, an effect which can be rationalised in terms of the different effects of valence electron charge smoothing at the surface.     

Surface state on the SiC(0001)-(√3 × √3) surface

An angle resolved photoemission study of a surface state on the SiC(0001)-(√3 × √3) surface is reported. Experiments carried out on the 6H and 4H polytypes give essentially identical results. A surface state band with semiconducting occupation is observed, centered around 1.0 eV above the valence band maximum (VBM) and with a width of about 0.2 eV. Recently calculated results for a Si-adatom-induced √3 × √3 reconstruction give a metallic surface state band centered about 1.2 eV above the VBM and with a width of 0.35 eV. The dispersion determined experimentally is smaller than calculated but exhibits the same trend, the surface state disperses downwards towards the VBM with increasing parallel wavevector component along both the qG--- and qG--- directions of the √3 × √3 surface Brillouin zone. The VBM is determined to be located at about 2.3(±0.2) eV below the Fermi level. The results indicate that Si adatoms on top of an outermost Si---C bilayer may be an inadequate structural model for explaining recent experimental findings for the SiC(0001)-(√3 × √3) surface.     

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.     

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.     

Formation of a √3 × √3 surface on Si/Ge(1 1 1) studied by STM and LEED

We have performed a detailed study of the formation and the atomic structure of a √3 × √3 surface on Si/Ge(1 1 1) using both scanning tunneling microscopy (STM) and low energy electron diffraction (LEED). Both experimental methods confirm the presence of a √3 × √3 periodicity but unlike the Sn/Ge(1 1 1) and the Sn/Si(1 1 1) surfaces, the Si/Ge(1 1 1) surface is not well ordered. There is no long range order on the surface and the √3 × √3 reconstruction is made up of double rows of silicon atoms separated by disordered areas composed of germanium atoms.     

Synchrotron radiation photoemission studies of the pentacene—Ag/Si(1 1 1) √3 × √3 interface

The interaction of monolayer coverages of pentacene with the √3 × √3 silver terminated Si(1 1 1) surface has been studies by high resolution core level and valence band photoemission spectroscopies. Core level Si 2p spectra reveal that there is only a very weak interaction between the pentacene and the underlying silicon, however, there is evidence of Fermi level movement. Valence band spectra acquired with both s and p polarised light indicate that for the surface coverages investigated, the molecular layers are oriented parallel to the plane of the surface. These results are in agreement with recent scanning tunneling microscopy (STM) studies which indicated that the pentacene molecules form highly ordered layers with the plane of the molecule parallel to the surface. Changes in the workfunction and Fermi level movements have been used to determine the energy level alignment at the interface. A 0.35 eV interface dipole forms between the pentacene and the silver terminated Si(1 1 1) surface within a two monolayer deposition. Photoemission measurements of the energy level alignment at the interface reveal that there is almost no barrier to charge injection from the conduction band of the semiconductor to the lowest unoccupied molecular orbital (LUMO) of the pentacene molecule.     

Surface restructuring process on a Ag/Ge(0 0 1) surface studied by photoelectron spectroscopy

The atomic structure and charge distribution of Ag adsorbed Ge(0 0 1) surfaces have been investigated by means of Ge 3d core- and Ag 4d core-levels photoelectron spectroscopy. A mono-atomic layer of Ag was deposited on the clean Ge(0 0 1) c(4×2) surface at 80 K. The Ge 3d spectrum measured at 80 K was deconvoluted into two surface components, which is consistent with the previously proposed Ag ad-dimer model. After annealing the surface at room temperature, the rearrangement of the charge distribution was revealed to include electron transfer from Ge to Ag in conjunction with the surface restructuring process by the annealing.     

Electronic structure of octane on Cu(1 1 1) and Ni(1 1 1) studied by near edge X-ray absorption fine structure

The electronic structure of an octane film grown on Cu(1 1 1) and Ni(1 1 1) was studied using C K-edge near edge X-ray absorption fine structure (NEXAFS). A pre-peak was observed on the bulk edge onset for the 1 ML thick octane films on the metal substrates. The pre-peak originated from metal induced gap states (MIGS) in the band gap of octane. The intensity of the pre-peak for octane/Ni(1 1 1) was the same as that of octane/Cu(1 1 1), suggesting that there was little difference in the density of unoccupied MIGS between the octane film on Ni(1 1 1) and Cu(1 1 1). We discuss the metal dependence of the density of unoccupied MIGS on the band structure of the metals.     

Structural study of Co/Pd(1 1 1) and CO/Co/Pd(1 1 1) by surface X-ray absorption fine structure spectroscopy

The structure of the Co thin films on Pd(1 1 1) and the effect of the CO adsorption on Co thin films were studied by Co K-edge surface X-ray absorption fine structure (XAFS). The polarization dependences of the X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectra indicate that Co thin films grow in the fcc stacking mode on Pd(1 1 1) up to 12 ML. The analysis of the nearest neighbor shell shows little mechanical strain at the interface, indicating that Co atom does not grow pseudomorphically on Pd(1 1 1). There is no alloy-like structure at the interface. CO adsorption causes no structural change of the Co thin films but modifies the Co surface electronic state. These structural studies provide deep insight in the magnetic property of the Co thin films on Pd(1 1 1).     

Buildup of the InSe/M interface (M Pd, Au) studied by X-ray photoemission and X-ray absorption spectroscopy

The electronic properties of InSe/M (M  Pd, Au) interfaces have been studied by X-ray photoemission measurements. For the InSe/Pd interface, it has been found that Pd atoms diffuse into the InSe lattice at early stages of Pd coverage, acting as acceptor centers. As the Pd coverage increases, a Pd-InSe reaction determines the electronic behaviour of the interface. However, for Pd coverages higher than 1 ML, the barrier formation tends to be controlled by an emerging bulklike Pd overlayer. Despite the atomic structure of this system is far from that expected for an ideal Schottky one, the final electronic barrier value is close to that expected for an abrupt InSe/Pd Schottky interface. On the contrary, the InSe/Au system appeared to behave as a quasi-ideal abrupt Schottky interface. Annealing processes performed at temperatures higher than 600 K alter this scheme, as revealed by X-ray absorption spectroscopy measurements, enhancing diffusion of Au atoms into InSe. In any case, the electronic barrier results to be determined by the Au overlayer formed.     

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.     

Surface structure and electron density dependence of scattered Ne ion fractions from the Si(1 0 0)-(2×1) surface

The magnitudes and azimuthal anisotropies of 4 keV Ne⁺ scattered ion fractions from the Si(1 0 0)-(2×1) two-domain surface have been measured by means of time-of-flight scattering and recoiling spectrometry. The absolute values of these ion fractions as well as their dependence on surface structure and electron density have been determined. By investigating the trajectories of the scattered Ne⁺, a clear correlation is demonstrated between these experimentally observed surviving ion fractions of Ne⁺ and the fraction of ions that scatters from the topmost layer of the surface. This is interpreted in terms of a model in which the neutralization probability of Ne⁺ is proportional to the local substrate electronic charge density.     

High temperature extended x-ray absorption fine structure study of multiferroic BiFeO(3)

Local atomic structure modifications around Fe atoms in polycrystalline multiferroic BiFeO(3) are studied by Fe K edge x-ray absorption spectroscopy as a function of temperature across the Néel temperature (T(N)?=?643?K) in order to reveal local structure modifications related to the magnetic transition. This work demonstrates that on crossing T(N) the local structure around Fe shows peculiar changes: the Fe-O bond lengths get shorter, the ligand symmetry increases and the Fe-O bond length disorder (σ(2)) deviates from Debye behaviour. These results suggest that the structural transition at the ferroelectric Curie temperature (T(C)?=?1103?K) is anticipated by early local rearrangement of the structure starting already at T(N).     

Si/Gen/Si(001)异质结薄膜的掠入射荧光X射线吸收精细结构研究

利用掠入射荧光X射线吸收精细结构(XAFS)方法研究了在400℃的温度下分子束外延生长的Si/Gen/Si(001)异质结薄膜(n=1,2,4和8个原子层)中Ge原子的局域环境结构.结果表明,在1至2个Ge原子层(ML)生长厚度的异质结薄膜中,Ge原子的第一近邻配位主要是Si原子.随着Ge原子层厚度增加到4 ML,Ge原子的最近邻配位壳层中的Ge-Ge配位的平均配位数增加到1.3.当Ge原子层厚度增加到8 ML时,第一配位壳层中的Ge-Ge配位占的比例只有55%.这表明在400℃的生长条件下,Ge原子有很强的迁移到Si覆盖层的能力.随着Ge层厚度从1增加到2,4和8 ML,Ge原子迁移到Si覆盖层的量由0.5 ML分别增加到1.5,2.0和3.0 ML.认为在覆盖Si过程中Ge原子的迁移主要是通过产生Ge原子表面偏析来降低表面能和Ge层的应变能.     

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.     

Structure investigation of the nitrogen induced Cu(110)−(2 × 3) phase with 180° low energy impact collision ion scattering spectroscopy

The 180° low energy impact collision ion scattering spectroscopy with detection of noble gas neutrals (180°-NICISS) has been used to investigate the nitrogen saturated Cu(110) surface, which is known to exhibit a (2 × 3) diffraction pattern. The nitrogen induced (2 × 3) phase appears to be the result of a surface reconstruction of a new missing row type, in which every third 100 row of Cu atoms of the first layer is missing. The 180° NICISS patterns further indicate within an accuracy of 0.1–0.2 Å, that the double periodicity in the [1 0] direction is not due to the reconstruction of the Cu surface. Its origin has to be found in the arrangement of the N atoms.