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Martins S.E.,University of Exeter | Withers F.,University of Exeter | Dubois M.,CNRS Laboratory of Inorganic Materials | Craciun M.F.,University of Exeter | Russo S.,University of Exeter
New Journal of Physics | Year: 2013

We demonstrate a novel method to tune the energy gap ε1 between the localized states and the mobility edge of the valence band in chemically functionalized graphene by changing the coverage of fluorine adatoms via electron-beam irradiation. From the temperature dependence of the electrical transport properties we show that ε1 in partially fluorinated graphene CF0.28 decreases upon electron irradiation up to a dose of 0.08 C cm-2. For low irradiation doses (<0.1 C cm-2) partially fluorinated graphene behaves as a lightly doped semiconductor with impurity bands close to the conduction and valence band edges, whereas for high irradiation doses (>0.2 C cm-2) the electrical conduction takes place via Mott variable range hopping. © IOP Publishing and Deutsche Physikalische Gesellschaft.

Eychenne J.,CNRS Magmas and Volcanoes Laboratory | Le Pennec J.-L.,CNRS Magmas and Volcanoes Laboratory | Troncoso L.,National Polytechnic School of Ecuador | Gouhier M.,CNRS Magmas and Volcanoes Laboratory | And 2 more authors.
Bulletin of Volcanology | Year: 2012

The violent August 16-17, 2006 Tungurahua eruption in Ecuador witnessed the emplacement of numerous scoria flows and the deposition of a widespread tephra layer west of the volcano. We assess the size of the eruption by determining a bulk tephra volume in the range 42-57 × 10 6 m 3, which supports a Volcanic Explosivity Index 3 event, consistent with calculated column height of 16-18 km above the vent and making it the strongest eruptive phase since the volcano's magmatic reactivation in 1999. Isopachs west of the volcano are sub-bilobate in shape, while sieve and laser diffraction grain-size analyses of tephra samples reveal strongly bimodal distributions. Based on a new grain-size deconvolution algorithm and extended sampling area, we propose here a mechanism to account for the bimodal grain-size distribution. The deconvolution procedure allows us to identify two particle subpopulations in the deposit with distinct characteristics that indicate dissimilar transport-depositional processes. The log-normal coarse-grained subpopulation is typical of particles transported downwind by the main volcanic plume. The positively skewed, fine-grained subpopulation in the tephra fall layer shares close similarities with the elutriated co-pyroclastic flow ash cloud layers preserved on top of the scoria flow deposits. The area with the higher fine particle content in the tephra layer coincides with the downwind prolongation of the pyroclastic flow deposits. These results indicate that the bimodal distribution of grain size in the Tungurahua fall deposit results from synchronous deposition of lapilli from the main plume and fine ash elutriated from scoria flows emplaced on the western flank of the volcano. Our study also reveals that inappropriate grain-size data processing may produce misleading determination of eruptive type. © 2011 Springer-Verlag.

Withers F.,University of Exeter | Bointon T.H.,University of Exeter | Dubois M.,CNRS Laboratory of Inorganic Materials | Russo S.,University of Exeter | Craciun M.F.,University of Exeter
Nano Letters | Year: 2011

We demonstrate the possibility to selectively reduce insulating fluorinated graphene to conducting and semiconducting graphene by electron beam irradiation. Electron-irradiated fluorinated graphene microstructures show 7 orders of magnitude decrease in resistivity (from 1 TΩ to 100 kΩ), whereas nanostructures show a transport gap in the source-drain bias voltage. In this transport gap, electrons are localized, and charge transport is dominated by variable range hopping. Our findings demonstrate a step forward to all-graphene transparent and flexible electronics. © 2011 American Chemical Society.

Claves D.,University Blaise Pascal | Claves D.,CNRS Laboratory of Inorganic Materials
New Journal of Chemistry | Year: 2011

Fluorocarbon nanostructures possess an outer halogenated crown that plays the role of an interface towards their environment and whose properties are then crucial for anticipating the integration of the latter materials in nanotechnology processes. Therefore, the result of fluorine addition to nanocarbon frames has been investigated here with the help of IR and XPS spectroscopies, and the role played by the pristine lattice curvature and the addend's addition scheme on the physicochemical characteristics of this outermost shell is experimentally established. The relevant paradigm of supposedly ionocovalent C-F terminations in these nanomaterials is shown to be obsolete. © 2011 The Royal Society of Chemistry and the Centre National de la Recherche Scientifique.

Renaudin G.,National Graduate School of Chemistry of Clermont Ferrand | Renaudin G.,CNRS Laboratory of Inorganic Materials | Filinchuk Y.,European Synchrotron Radiation Facility | Neubauer J.,Friedrich - Alexander - University, Erlangen - Nuremberg | Goetz-Neunhoeffer F.,Friedrich - Alexander - University, Erlangen - Nuremberg
Cement and Concrete Research | Year: 2010

Two different techniques were used to compare structural characteristics of "wet" ettringite (stored in the synthesis mother liquid) and "dried" ettringite (dried to 35% relative humidity over saturated CaCl2 solution). Lattice parameters and the water content in the channel region of the structure (site occupancy factor of the water molecule not bonded to cations) as well as microstructure parameters (size and strain) were determined from a Rietveld refinement on synchrotron powder diffraction data. Local environment of sulphate anions and of the hydrogen bonding network was characterized by Raman spectroscopy. Both techniques led to the same conclusion: the "wet" ettringite sample immersed in the mother solution from the synthesis presents similar structural features as ettringite dried to 35% relative humidity. An increase of the a lattice parameter combined with a decrease of the c lattice parameter occurs on drying. The amount of structural water, the point symmetry of sulphate and the hydrogen bond network are unchanged when passing from the wet to the dried ettringite powder. Ettringite does not form a high-hydrate polymorph in equilibrium with alkaline solution, in contrast to the AFm phases that lose water molecules on drying. According to these results we conclude that ettringite precipitated in aqueous solution at the early hydration stages is of the same chemical composition as ettringite present in the hardening concrete. © 2009 Elsevier Ltd. All rights reserved.

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