Bond polarization and image-potential screening in adsorbed C6F6 on Cu(1 1 1)

The orientation of hexafluorobenzene (C₆F₆) on the Cu(1 1 1) surface has been determined for different coverages with the help of near edge X-ray absorption fine structure (NEXAFS) spectroscopy and X-ray photoelectron spectroscopy (XPS). The adsorption geometry and the bonding mode of C₆F₆ differ significantly in comparison to its hydrocarbon analog C₆H₆. C₆F₆ is found to adsorb on Cu(1 1 1) with the ring plane parallel to the surface for coverages below 10 ML. Next to the distinct multilayer, bilayer and monolayer phases we also present evidence of sub-monolayer (i.e., 1/2 ML) coverage with different electronic structure. These findings are explained in a phenomenological model based on fluorine’s property as a σ-acceptor and a π-donor and the resulting bond polarization within the molecule, which is stabilized by image-potential screening within the substrate.     

Chemical bonding of PTCDA on Ag surfaces and the formation of interface states

We present a detailed investigation of the interface bonding of 3,4,9,10-perylene-tetracarboxylic acid dianhydride (PTCDA) on Ag(1 1 1) and Ag(1 1 0) surfaces by a combination of structural and electronic techniques (SPA-LEED, STM, TPD, UPS, HR-XPS, and NEXAFS) thus obtaining a consistent picture of the adsorption behaviour of PTCDA/Ag in the monolayer regime. The interaction with silver is strong and leads to the formation of new common hybrid orbitals in the monolayer, which are interface states for PTCDA films on Ag, involving at least LUMO, HOMO, and HOMO-1, and the Ag 5s- and 4d-states. This chemisorption is based on a covalent interaction between metal and molecular states, and can unambiguously be distinguished from mere van-der-Waals bonding.     

Diffusion versus sticking anisotropy: Anisotropic growth of organic molecular films

The molecular/crystal orientation and morphology of active molecular structures is a key determinant for the function of nanoscaled organic devices, yet little is known regarding the processes that govern thin film growth. Here, we show that either sticking or diffusion anisotropy can control the growth depending on preparation conditions. This is illustrated by an investigation into the growth of sexiphenyl (6P) on the anisotropic TiO₂(1 1 0)-(1 × 1) single crystal surface. The great differences in crystallite orientation and morphology observed are explained by the domination of the growth kinetics by either sticking or diffusion anisotropy depending on growth temperature.     

Photon-stimulated ion desorption from DCOO/Si(100) near the O K-edge

Photon-stimulated ion desorption from deuterated formic acid chemisorbed on Si(100) has been studied using pulsed synchrotron radiation in the energy region of the oxygen 1s electron excitation. The O 1s electrons of hydroxyl oxygen and carbonyl oxygen could be selectively excited in the O K-edge region because the chemical environments are different. It is found that the CDO⁺ yield is enhanced at the O 1s(C---O)→σ*(C---O) resonance and the CD⁺ yield at the O 1s(C=O)→σ*(C---O) resonance. The results indicate that ion desorption is related both to the antibonding character of excited molecular orbitals and the local character of core hole orbitals.     

Detection and switching of the oxidation state of Fe in a self-assembled monolayer

A self-assembled monolayer of ferrocene-capped alkanethiols is produced in two stable oxidation states of Fe (Fe²⁺ and Fe³⁺). These oxidation states are detected with sub-monolayer sensitivity by the fine structure of the Fe2p absorption edge. The native Fe²⁺ ferrocene layer is converted chemically to Fe³⁺, and the Fe³⁺ layer can be transformed back to Fe²⁺ or possibly Fe⁰ by irradiation with soft X-rays. The results have implications on switching mechanisms in molecular electronics.     

Adsorption and orientation kinetics of self‐assembled films of octadecyltrimethoxysilane on aluminium oxide surfaces

X‐ray photoelectron spectroscopy (XPS) and near‐edge x‐ray absorption fine structure (NEXAFS) spectroscopy have been used to study the time‐dependent adsorption and molecular orientation behaviour of octadecyltrimethoxysilane (ODTMS) on native aluminium oxide surfaces. By measuring the adsorption isotherm using XPS, we show that ODTMS molecules exhibit oscillatory adsorption. The oscillatory adsorption behaviour for ODTMS is analogous to that observed for its simpler short‐chain ‘cousin’—propyltrimethoxysilane (PTMS)—and suggests that the length of the functional alkyl chain on an organosilane does not have a significant influence upon the oscillatory adsorption mechanism. The oscillation in the ODTMS adsorption isotherm shows a maximum and a minimum in coverage at an adsorption time of ～30 and ～65 s, respectively, for a 0.75% ODTMS solution in a 90% ethanol–10% water mixture at pH 4. The time‐dependent orientation behaviour of the ODTMS molecules during adsorption was examined using angular‐dependent carbon K‐edge NEXAFS spectroscopy. We show that the alignment of the ODTMS film changes systematically with deposition time and appears to be correlated with coverage measurements obtained using XPS. In particular, by combining the XPS and NEXAFS results we demonstrate that the minimum ODTMS coverage corresponds to a film whose alignment appears to be predominantly randomized. Copyright © 2005 John Wiley & Sons, Ltd.     

Assessment of sulfur and iron speciation in a soil aggregate by combined S and Fe micro‐XANES: microspatial patterns and relationships

To test whether synchrotron‐based spectromicroscopy can be used to identify spatial patterns of sulfur (S) and iron (Fe) speciation as well as relationships between the speciation of S and Fe in soil colloids or aggregates at the micrometre and sub‐micrometre level, an anoxically prepared dissected soil aggregate (size ～1 mm³) was analyzed by µ‐XANES at the K‐edges of S (2472 eV) and Fe (7112 eV). The experiment included (i) elemental mapping at the S K‐edge (S, Si, Al) and the Fe K‐edge (Fe, Si), (ii) acquisition of 300 µm × 300 µm images of the region of interest with X‐ray energies of 2474 eV (addressing reduced organic and inorganic S), 2483 eV (total S), 7121 eV (divalent Fe) and 7200 eV (total Fe), as well as (iii) acquisition of S and Fe µ‐XANES spectra at two different positions, where image analysis suggested the dominance of reduced and oxidized S and Fe, respectively. Image analysis revealed a heterogeneous distribution of total Si, S and Fe as well as of different S and Fe species in the aggregate. Microregions which were either enriched in reduced or in oxidized S and Fe could be identified. A microregion with a large contribution of oxidized S (sulfate, sulfonate) to total S contained exclusively Fe(III) oxyhydroxides (probably ferrihydrite) as S‐bearing phase, whereas another microregion with a large contribution of reduced organic S (thiol, organic disulfide) to total S contained a small amount of Fe(II)‐bearing silicate in addition to the dominating Fe(III) oxyhydroxides. Our results show that combined S and Fe µ‐XANES is a powerful tool for studying microscale spatial patterns of S and Fe speciation as well as microscale relationships between the speciation of S and Fe in soil aggregates.     

The local adsorption geometry and electronic structure of alanine on Cu{1 1 0}

The adsorption of alanine on Cu{1 1 0} was studied by a combination of near edge X-ray absorption fine structure (NEXAFS) spectroscopy, X-ray photoelectron spectroscopy (XPS) and density functional theory (DFT). Large chemical shifts in the C 1s, N 1s, and O 1s XP spectra were found between the alanine multilayer and the chemisorbed and pseudo-(3 × 2) alaninate layers. From C, N, and O K-shell NEXAFS spectra the tilt angles of the carboxylate group (≈26° in plane with respect to and ≈45° out of plane) and the C-N bond angle with respect to could be determined for the pseudo-(3 × 2) overlayer. Using this information three adsorption geometries could be eliminated from five p(3 × 2) structures which lead to almost identical heats of adsorption in the DFT calculations between 1.40 and 1.47 eV/molecule. Due to the small energy difference between the remaining two structures it is not unlikely that these coexist on the surface at room temperature.     

Binding property and structure of aromatic isocyanide self‐assembly monolayers on Ag and Au surfaces

The binding property of p‐biphenyl isocyanide self‐assembled monolayers (SAMs) on Au and Ag was investigated by temperature‐dependent surface‐enhanced Raman spectroscopy (SERS). p‐Biphenyl isocyanide was found to desorb on Ag at a low temperature of ～393 K whereas it appeared to remain enduring at a high temperature of ～453 K for Au. Structures of p‐biphenyl isocyanide SAMs on Au and Ag flat films were checked by means of near‐edge x‐ray absorption fine structure spectroscopy (NEXAFS) at the two different normal (90° ) and grazing (20° ) angles of the incident x‐ray beam. Our results suggested that the SAMs prepared by p‐biphenyl isocyanide should have a relatively disordered structure even at room temperature on both Au and Ag, as indicated from an insubstantial change in NEXAFS spectra at the two different angles from those of p‐biphenyl thiolate and p‐biphenyl methanethiolate. The weakness of the isocyanide–metal bond in comparison with the sulphur–metal bond may result in both low surface coverage and orientational disorder. A density functional theory calculation method was employed to attempt to explain the difference in stability for phenyl isocyanide on Ag and Au surfaces. Our calculation result yielded a lower binding energy of phenyl isocyanide on Ag than that on Au, consistent with the temperature‐dependent Raman results. Copyright © 2005 John Wiley & Sons, Ltd.     

Cu-Ni alloy nanocluster formation by ion implantation in silicate glasses: Structure and optical properties

Sequential ion implantation (copper and nickel) in silica and soda-lime glasses has been performed. The formation of copper-nickel alloy nanocluster in the glass host has been evidenced by synchrotron radiation-based techniques, namely X-ray diffraction and absorption spectroscopy. The nanocrystals' lattice parameter value was estimated, indicating the formation of Cu₅₅Ni₄₅ alloy particles. Optical absorption spectra are also discussed. Received 6 May 1999 and Received in final form 22 September 1999