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Electronic correlation in pristine and doped graphene layers

Final Report Summary - ECO-GRAPHENE (Electronic correlation in pristine and doped graphene layers)

In this proposal, the spectroscopic investigation of functionalized mono– and few– layered graphene (FLG) hsa been performed. The samples were made by the proposer as highly crystalline layers grown by precipitation on SiC and by chemical vapour deposition on Ni(111) surfaces. Their electronic, vibronic and optical properties have been spectroscopically investigated by a combined experimental and theoretical approach. In low dimensional and strongly anisotropic systems, correlation effects play a crucial role in understanding and describing the electronic and vibronic properties. Therefore the particular focus of this proposal lies on correlation effects, i.e. the renormalization of the non–interacting electron and phonon dispersion relations of FLG and doped graphene layers. These correlation effects comprise electron–electron, electron–phonon and electron–plasmon coupling. These are the underlying processes that are key for unravelling (1) transport, (2) vibronic and (3) optical properties in FLG, GICs and related structures such as nanoribbons, nanotubes and fullerenes.
We have investigated the distinct changes of the electronic band structure upon (1) covalent, (2) substitutional and (3) ionic functionalization. To that end we have performed functionalization of graphene on metals by (1) hydrogen, (2) nitrogen and (3) potassium. It has been shown that the relevant physics in each case is quite different. The huge charge transfer of alkali metals results in an increase of the Fermi level, and the lattice distortion induced by hydrogenation causes defect scattering and bandgap opening. Regarding substitutional doping by nitrogen impurities, it was shown that sp2 bonded nitrogen transfers charge to graphene.
We have also found a new electronic state in H-graphene that is located between the π and π * bands. For undoped H-graphene this state is energetically situated within the gap around EF and is accessible with absorption spectroscopies such as NEXAFS. In the case of n-doped H-graphene the midgap state becomes available for electrons and directly observable with ARPES since it is then situated below EF . Therefore, the H impurity band likely acts as an electron acceptor level which provides the possibility to control the electron concentration in Hgraphene via the H/C ratio. An estimation of the Mott criterion and a calculation of the typical DOS suggests that above H/C 1% and below H/C 6%, the acceptor level can form an extended impurity band.