1,4-benzenedimethanethiol was chemisorbed from the vapor phase onto Au(111). The chemisorption geometry, molecular orientation, and bonding properties were studied at different degrees of surface coverage by photoelectron spectroscopy, metastable deexcitation spectroscopy, and near-edge x-ray absorption fine structure spectroscopy at the carbon K edge. Two main chemisorption regimes were identified: at low coverage the molecules adopt a flat configuration, then, as the molecular density of the first layer increases, the reduction of the available chemisorption sites induces the newly bonded molecules to assume a vertical alignment, with only one of the sulphur head groups interacting with the substrate. Experimental results were interpreted on the basis of theoretical calculations that we performed on the free molecule concerning the molecular orbitals' density of states and simulated x-ray absorption. (c) American Institute of Physics.

We report a comprehensive study of the self-assembly of a diindenoperylene (DIP) monolayer on Au(111) single crystals exploiting different electron probes ranging from STM and LEED to photoelectron spectroscopy and NEXAFS. By this multitechnique approach, we obtain a full picture of the crystallographic and electronic structure of the DIP layer as well as an insight into the assembly process and the role of the DIP-Au interactions. We contrast these experimental findings with theoretical calculations.

Thin films of pentacene (C22H14) have become widely used in the field of organic electronics. Here films of C22H14 of thickness ranging from submonolayer to multilayer were thermally deposited on Ag(1 1 1) surface. The determination of molecular geometry in pentacene films on Ag(1 1 1) studied by X-ray absorption at different stages of growth up to one monolayer is presented. XAS spectra at the C K-edge were collected as a function of the direction of the electric field at the surface. The different features of the spectra were assigned to resonances related to the various molecular unoccupied states by the comparison with the absorption coefficient of the pentacene gas phase. The transitions involving antibonding pi states show a pronounced angular dependence for all the measured coverages, from submonolayer to multilayer. The spectra analysis indicates a nearly planar chemisorption of the first pentacene layer with a tilt angle of 10 degrees. (C) 2007 Elsevier B.V. All rights reserved.

Molecular beam epitaxy has been used to grow SrF2 thin films on Si(001). The growth modes have been investigated by atomic force microscopy, electron diffraction, and photoemission. Two principal growth regimes have been identified: (i) when deposition is carried out with the substrate held at a temperature of 700-750 degrees C, SrF2 molecules react with the substrate giving rise to a Sr-rich wetting layer on top of which three dimensional bulklike fluoride ridges develop; (ii) when deposition is carried out with the substrate held at 400 degrees C, a nanopatterned film forms with characteristic triangular islands. Results are compared to the growth mode of CaF2 on Si(001) under analogous deposition conditions. Morphological and structural differences between the two systems are associated with the larger lattice parameter of SrF2 with respect to CaF2, resulting in a larger mismatch with the Si substrate.

We have determined the ytterbium valence as a function of temperature in the reported near-zero thermal expansion material YbGaGe using x-ray photoemission at various incident photon energies. The Yb 3d, 4d, and 4f levels, which directly yield the Yb valence, have been measured. Careful analysis enabled the clear separation of surface and bulk contributions. Resonant photoemission at the 4d-4f absorption edge was used to enhance the low contribution of the Yb3+ component. Contrary to the initially proposed Yb valence transition, we find no change in the valence from room temperature down to 115 K.

A detailed investigation of the chemistry and electronic structure during the formation of the interfaces between thin films of the archetypal organic molecular semiconductor copper phthalocyanine (CuPc) and Ag or Sn deposited on it was performed using photoemission and near-edge X-ray absorption spectroscopies with synchrotron light. Our study demonstrates the formation of sharp, abrupt interfaces, a behavior which is of particular importance for applications in organic devices. Moreover, for Ag on CuPc we demonstrate that this interface is free from any reaction, whereas there is slight interface reaction for Sn/CuPc.

Combined spatially resolved electron-energy loss spectroscopy (EELS) and high resolution near-edge X-ray absorption fine structure (NEXAFS) spectroscopy have been used to investigate the nitrogen doping of multi-walled carbon nanotubes (N-MWNT). EELS indicates that most of the tubes are nitrogen-doped. NEXAFS spectroscopy reveals pyridine-like and nitrile N structures. High resolution NEXAFS experiments show that the main nitrogen concentration originates from a high amount of molecular N-2 encapsulated into only a small quantity of tubes.

Functionalization of multi-walled carbon nanotubes (MWNTs) surface by sulfonated poly (ether ether ketone) SPEEK chains using a direct attachment reaction was investigated. A two step method was performed. MWNTs were oxidized by a nitric acid treatment to generate carboxyl groups on their surface. The grafting reaction of sulfonated groups of SPEEK with carboxyl groups present on the surface of oxidized MWNTs readily proceeds by using hexane diamine as an interlinking molecule. Transmission electron microscopy (TEM) shows that tubes are wrapped by polymer chains. Near edge X-ray absorption fine structure spectroscopy (NEXAFS) at the C K-edge, 0 K-edge, and N K-edge and X-ray photoelectron spectroscopy (XPS) were used to give evidence of covalent functionalization of MWNTs by SPEEK macromolecules.

The chemistry and electronic structure evolution were studied upon the formation of the interfaces between copper phthalocyanine and Ag and Sn deposited on it in ultra high vacuum conditions. The measurements were performed by means of high-resolution photoemission spectroscopy of core-levels and the valence band, and using synchrotron-radiation facilities. The investigations demonstrate the formation of chemically inert and abrupt interfaces. (C) 2007 Elsevier B.V. All rights reserved.

We have investigated the temperature-dependent changes in the near-E-F occupied and unoccupied states of Pr0.67Ca0.33MnO3 which shows the presence of ferromagnetic and antiferromagnetic phases. The temperature-dependent changes in the charge and orbital degrees of freedom and associated changes in the Mn 3d-O 2p hybridization result in varied O 2p contributions to the valence band. A quantitative estimate of the charge transfer energy (E-CT) shows a larger value compared to the earlier reported estimates. The charge localization causing the large E-CT is discussed in terms of different models including the electronic phase separation.

Epitaxial cobalt films on CaF2(110) buffer layers on Si(001) were grown by molecular beam epitaxy. It is found that Co grows in a face-centered cubic lattice forming nanodimensional islands. The islands tend to align along the parallel grooves which characterize the self-patterned CaF2(110) surface grown on Si(001). Photoemission was used to probe the uniformity of the film and the occurrence of chemical reactions with the substrate. X-ray magnetic circular dichroism at the Co L-2.3 edges was used to access the magnetic properties of the films. The thick Co film presents an in-plane easy magnetization axis along the directions of the grooves of the CaF2(110) surface. (c) 2006 Elsevier B.V. All rights reserved.

We present an analysis of the electronic structure of C-60 adsorbed on a vicinal Au(111) surface at different fullerene coverages using photoemission, x-ray absorption, and scanning tunneling microcopy/spectroscopy (STS). STS provides a straightforward determination of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) levels with respect to the Fermi energy. At C-60 coverages of 0.5 and 1 ML a 2.7 eV wide HOMO-LUMO gap is found. The near-edge x-ray absorption fine structure (NEXAFS) spectrum for the 0.5 ML C-60 nanomesh structure displays a significant intensity at the low energy side of the LUMO exciton peak, which is explained as due to absorption into HOMO-LUMO gap states localized at individual C-60 cluster edges. From 0.5 to 1 ML we observe a rigid shift of the HOMO-LUMO peaks in the STS spectra and an almost complete quenching of the gap states feature in NEXAFS. (c) 2006 American Institute of Physics.

The growth modes of SrF2 on Si(001) are investigated by AFM and ultraviolet photoemission. Two growth regimes are identified according to the substrate temperature during deposition, resulting in flat and ordered fluoride layers or in nano-patterned films with characteristic triangular islands. The flat layer growth obtained at high temperature is accompanied by molecular dissociation at the interface.