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      Selective Chemical Modification of Graphene surfaces: Distinction between Single and Bilayer Graphene

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          Abstract

          Graphene modifications with oxygen or hydrogen are well known in contrast to carbon attachment to the graphene lattice. The chemical modification of graphene sheets with aromatic diazonium ions (carbon attachment) is analyzed by confocal Raman spectroscopy. The temporal and spatial evolution of surface adsorbed species allowed accurate tracking of the chemical reaction and identification of intermediates. The controlled transformation of sp2 to sp3 carbon proceeds in two separate steps. The presented derivatization is selective for single layer graphene and allows controlled transformation of adsorbed diazonium reagents into covalently bound surface derivatives with enhanced reactivity at the edge of single layer graphene. On bi-layer graphene the derivatization proceeds to an adsorbed intermediate without further reaction to form a covalent attachment on the carbon surface.

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          Born-Oppenheimer Breakdown in Graphene

          The Born-Oppenheimer approximation (BO) has proven effective for the accurate determination of chemical reactions, molecular dynamics and phonon frequencies in a wide range of metallic systems. Graphene, recently discovered in the free state, is a zero band-gap semiconductor, which becomes a metal if the Fermi energy is tuned applying a gate-voltage Vg. Graphene electrons near the Fermi energy have twodimensional massless dispersions, described by Dirac cones. Here we show that a change in Vg induces a stiffening of the Raman G peak (i.e. the zone-center E2g optical phonon), which cannot be described within BO. Indeed, the E2g vibrations cause rigid oscillations of the Dirac-cones in the reciprocal space. If the electrons followed adiabatically the Dirac-cone oscillations, no change in the phonon frequency would be observed. Instead, since the electron-momentum relaxation near the Fermi level is much slower than the phonon motion, the electrons do not follow the Dirac-cone displacements. This invalidates BO and results in the observed phonon stiffening. This spectacular failure of BO is quite significant since BO has been the fundamental paradigm to determine crystal vibrations from the early days of quantum mechanics.
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            The Effect of Structure upon the Reactions of Organic Compounds. Benzene Derivatives

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              Chemical modification of epitaxial graphene: spontaneous grafting of aryl groups.

              The addition of nitrophenyl groups to the surface of few-layer epitaxial graphene (EG) by the formation of covalent carbon-carbon bonds changed the electronic structure and transport properties of the EG from near-metallic to semiconducting.
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                Author and article information

                Journal
                15 January 2010
                Article
                10.1002/smll.200902370
                1001.2732

                http://arxiv.org/licenses/nonexclusive-distrib/1.0/

                Custom metadata
                Small 6 (2010) 1125-1130
                cond-mat.mtrl-sci

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