The electron mean free path (l(gr)) is "locally" evaluated by scanning capacitance spectroscopy on graphene obtained with different preparation methods and on different substrates, i.e., graphene exfoliated from highly oriented pyrolitic graphite (HOPG) and deposited (DG) on 4H-SiC(0001) and on SiO2 and epitaxial graphene grown on 4H-SiC (0001) (EG). l(gr) in DG on SiC was more than four times larger than in DG on SiO2. The improved mean free path is explained by the higher permittivity of SiC compared to SiO2, yielding a better dielectric screening of charged-impurities, and by the weaker coupling of graphene two-dimensional-electron-gas with surface polar phonons of SiC. On the other hand, l(gr) on EG is on average similar to 0.4 times that on DG-SiC and exhibits large variations from point to point, due to the presence of a laterally inhomogeneous positively charged layer at EG/SiC interface.Original Publication:S Sonde, F Giannazzo, C Vecchio, Rositsa Yakimova, E Rimini and V Raineri, Role of graphene/substrate interface on the local transport properties of the two-dimensional electron gas, 2010, APPLIED PHYSICS LETTERS, (97), 13, 132101.http://dx.doi.org/10.1063/1.3489942Copyright: American Institute of Physicshttp://www.aip.org

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In this work, we present a nanometer resolution structural characterization of epitaxial graphene (EG) layers grown on 4H-SiC (0001) 8° off-axis, by annealing in inert gas ambient (Ar) in a wide temperature range (Tgr from 1600 to 2000°C). For all the considered growth temperatures, few layers of graphene (FLG) conformally covering the 100 to 200-nm wide terraces of the SiC surface have been observed by high-resolution cross-sectional transmission electron microscopy (HR-XTEM). Tapping mode atomic force microscopy (t-AFM) showed the formation of wrinkles with approx. 1 to 2 nm height and 10 to 20 nm width in the FLG film, as a result of the release of the compressive strain, which builds up in FLG during the sample cooling due to the thermal expansion coefficients mismatch between graphene and SiC. While for EG grown on on-axis 4H-SiC an isotropic mesh-like network of wrinkles interconnected into nodes is commonly reported, in the present case of a vicinal SiC surface, wrinkles are preferentially oriented in the direction perpendicular to the step edges of the SiC terraces. For each Tgr, the number of graphene layers was determined on very small sample areas by HR-XTEM and, with high statistics and on several sample positions, by measuring the depth of selectively etched trenches in FLG by t-AFM. Both the density of wrinkles and the number of graphene layers are found to increase almost linearly as a function of the growth temperature in the considered temperature range.

Polyethersulphone (PES) is an aromatic thermoplastic, at low environmental impact, evaluated in this work as a promising candidate for new polymer electrolytes in the PEMFCs technology. A sulfonation procedure has been tuned in order to graft sulfonic acid groups on the polymer chains (sPES) and to make it hydrophilic. Homogeneous membranes with different polymer's sulfonation degrees (SD%) have demonstrated excellent mechanical properties and very low permeability toward methanol (important in the DMFCs), even if low proton conductivity. Nanocomposite sPES membranes were prepared by dispersion of highly hydrophilic lamellar particles such as layered double hydroxide (LDH) in the polymer. Deep investigations performed by a combination of PFG-NMR, EIS, XRD, DMA, and scanning electron microscopy have evidenced the exfoliation of the lamellae in polymer matrix. However, a certain anisotropy was evidenced both in the morphology and molecular diffusion, favored in the longitudinal direction (parallel to surface), while completely inhibited in the cross-section. This finding is most likely induced by the polymer structure, therefore particular attention must be paid to the choice of the filler and preparation of the composites.

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