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International audience; The concept of combining x-ray absorption spectroscopy with suitable temperature scanning techniques has been sporadically exploited for various research purposes whenever structural or electronic transformations take place in the sample as a function of T. Fixed (single) energy 7-scanning techniques have been introduced about 10 years ago and their usefulness has been widely assessed. Not only they allow for a qualitative appreciation of the occurrence of a phase transition in a sample but they resulted a valuable source of information in the case of binary systems, in the presence of metastable states of matter, and for high pressure research.

International audience; A commercial fuel cell has been successfully modified to carry out X-ray absorption spectroscopy (XAS) measurements under optimized in operando conditions. The design is conceived for the performance of XAS experiments in transmission mode over a wide range of X-ray energies above 6 keV, owing to the reduced absorption of the cell. The wide angular aperture allows the collection of XAS in fluorescence mode and of X-ray diffraction patterns when needed. Details of the design of the cell and its performances are given. The quality of the extended X-ray absorption fine-structure spectra under working conditions has been verified at the ESRF and ELETTRA synchrotron radiation facilities, showing that relatively fast and low-noise transmission measurements on electrodes over a wide range of catalyst concentrations and energies are feasible.

The capability to monitor finely the physical properties of eumelanin, an important class of biopolymers, involved in melanoma cancer pathologies, whose function and intrinsic disorder still collects the interest of many investigators, was achieved by means of electrospray deposition (ESD). By alleviating the problem of the solubility of melanin through the realization of high-quality films it was possible to spread light on the unknown biopolymer supramolecular organization. In fact, on the basis of scanning probe microscopies, electron spectroscopies, and transport properties, it was possible to delineate peculiar features of the melanin organization varying from heteropolymeric to oligomeric in character and eventually turning in a cross-linked secondary molecular structure.

International audience; GNXAS is a suite of programs based on multiple scattering calculations which performs a structural refinement of EXAFS spectra. It can be used for any system although it has been mainly developed to determine the local structure of disordered substances. We developed a user-friendly graphical user interface (GUI) to facilitate use of the codes by using wxPython. The developed GUI and the codes are multiplatform running on Windows, Macintosh and Linux systems, and are free shareware (http://gnxas.unicam.it). In this work we illustrate features and potentials of this newly developed version of GNXAS (w-GNXAS).

We present a set of XPS (X-ray photoemission spectroscopy) measurements and detailed data-analysis of electrodes for polymer electrolyte membrane fuel cell (PEMFC) based on carbon supported catalysts (Pt and Pt–Co) subjected to a step-like potential cycling (accelerated degradation test). The results of the measurements complete and corroborate parallel TEM (transmission electron microscopy), XRD (X-ray diffraction) and XAS (X-ray absorption spectroscopy) results pointing at a modification of the nanostructure of the catalyst grains. The surface sensitivity of the XPS measurements is used to study the modification of the reactive surface structure of the nanoparticles and the interplay of reactivity and dissolution/precipitation/agglomeration of metal clusters during the catalyst work. The combination of XPS results with XRD, XAS and TEM ones allowed us to obtain a model for the structural changes in Pt–Co nanoparticles occurring during operation. The main structural modifications, in which predominantly Co atoms are involved, occur during the first hours of catalyst operation changing the stoichiometry and degree of disorder in external particles shell. In the last stages of the degradation process, core–shell particles structure, with ordered Pt4Co core and Pt-rich shell (and finally Pt-skin), is formed possibly explaining higher Pt–Co catalyst stability (structural and electrochemical) than pure Pt catalyst.

International audience; We report a detailed and systematic Raman scattering investigation of the high pressure - high temperature (350-620 K, 0-20 GPa) phase diagram of elemental selenium starting from its trigonal structure (Se-I). Phase transitions to the tetragonal Se-VII phase were identified by the measured Raman pattern along several isotherms, upon pressurization and depressurization. A wide hysteresis region (∼10-15 GPa) has been observed at 350 K, gradually reducing to about 3GPa at 620 K. These results extend previous scattered determinations of the phase diagram and shifts the Se-I Se-VII transition line up to about 3GPa at lower pressures and ∼150K at lower temperatures

Steady state photoconductivity and x-ray diffraction combined with optical and electronic spectroscopies were applied to study spin coated poly(3-hexylthiophene) (P3HT) films mechanically rubbed using a teflon bar. As expected, photoconductivity evidenced a strong increase in the photo-response along the rubbing direction. But, while rubbing during annealing at a temperature of 70 °C resulted in a sizeable photocurrent anisotropy along the direction parallel or perpendicular to the rubbing, the samples rubbed at room temperature (RT) showed an isotropic and comparatively stronger enhancement of absorption and photoconductivity spectral features. According to these observations, rubbing at RT resulted in a significant increase in local order and macroscopic alignment of the film, while a higher degree of global order but with a relaxed local structure was obtained after rubbing at 70 °C.

International audience; There is not a wide consensus on the role played by the icosahedral short range order on the stability of undercooled simple metals. The scenario is even less clear for undercooled metals under external pressure. Classical molecular dynamics simulations are performed to explain experimental results recently obtained on liquid and undercooled liquid copper under pressure. The atomic configurations are characterized by a common neighbor analysis to reveal the icosahedral short range order and its relation with external pressure. External pressure increases the probability to find atomic bonds with icosahedral symmetry both in the liquid and in the undercooled copper.

We present thick, uniform and rather flat melanin films obtained using spray deposition. The morphology of the films was investigated using Scanning Electron Microscopy (SEM) and Atomic Force Microscopy (AFM). Temperature-dependent electrical resistance of melanin thin films evidenced a semiconductor-like character and a hysteretic behavior linked to an irreversible process of water molecule desorption from the melanin film. X-ray Photoelectron Spectroscopy (XPS) was carried out to analyze the role of the functional groups in the primary and secondary structure of the macromolecule, showing that the contribution of the 5,6-dihydroxyindole-2-carboxylic acid (DHICA) subunit to the molecule is about 35%. Comparison of the optical absorption of the thick (800nm) and thin (80nm) films showed a spectral change when the thickness increases. From in vacuum photoconductivity (PC) measured at controlled temperatures, we suggest that the melanin films exhibit a possible charge transport mechanism by means of delocalized pi states along the stacked planar secondary structure.

International audience; In this work, we address the question of pressure-induced phase transitions in amorphous germanium.We discuss both the pressurization and depressurization cycles on an amorphous film, showing the appearance of various stable and metastable (LDA–HDA) phases by Raman scattering and X-ray absorption spectroscopy. In particular, we report evidence of recrystallization in the metastable ST12 phase driven by the density of voids of the film.

The nuclear break-up of 6He on a 208Pb target was studied at 20A MeV using a secondary beam of 6He produced by the SPIRAL facility at GANIL. $ \alpha$ -particles were detected in coincidence with two neutrons with a large angular coverage and the reaction mechanism was identified. From the distribution of the relative angles between the two neutrons the correlation function was extracted. It shows a strong correlation at small relative angles attributed to the contribution of the di-neutron configuration of 6He .