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8148 Graphene – Synthesis, Characterization, Properties anWdill-bAe-psept-lbiyc-IaN-tTiEoCnHs Fig. 5. The graphs show the self-consistently calculated band structures in trilayer graphene near the K point respectively in the first neighbour approximation (i.e., γ0 and γ1 only) and in the full parameter model, adapted from Koshino et al. (2009). The response of ABA multilayer graphene to the gate electric field perpendicular to the layers is unique and qualitatively different from that of AB-bilayer graphene (Craciun et al. (2009); Russo et al. (2009)). In trilayer graphene, for example, the gate field breaks mirror reflection symmetry with respect to the central layer and hybridizes the linear and parabolic low-energy bands, leaving the semi-metallic band overlap near zero energy (Craciun et al. (2009); Koshino et al. (2009)). Only the tight binding model based on the full set of hopping integrals (i.e. γ0 to γ5 see Fig. 5) can successfully describe the experimentally observed band-overlap (Craciun et al. (2009); Russo et al. (2009). The tuneable semimetallic nature of trilayers together with the tuneable band-gap in bilayers demonstrates that graphene materials are characterized by a unique range of physical properties not found in any other known material system. Theoretically, a similar band overlapping is expected in thicker ABA multilayers as well, though very little is known experimentally. Finally, graphene multilayers can also be constructed by carefully transferring layer by layer individual graphene sheets on a substrate. The rise of this new family of materials, the meta-few layers (μετα = "beyond" the few layer materials), offers the unique possibility to control the hopping parameters by stacking engineering. In particular, the interlayer hopping parameters are responsible for the rich low-energy band dispersion of these materials. Therefore, the ability to control these hopping parameters by means of graphene stacking engineering -for instance making thin tunnel barriers between two subsequent graphene layers- holds the promise for unprecedented functionalities as compared to natural few-layers. This layer-by-layer engineering of graphene materials has already been successfully employed to create transparent electrodes in organic solar cells, and an efficiency similar to the ITO electrodes was recently demonstrated (Wang et al. (2011)). The physics of graphene has expanded at a rapid pace mainly thanks to the easily accessible electronic properties in simple transistor geometries (Novoselov&Jiangetal. (2005); Zhang et al. (2005)) and the easy of nanostructures fabrication like nano-ribbons (Jiao et al. (2009); Li et al. (2008); Han et al. (2007); Oostinga et al. (2010)) and nano-rings (Russo et al. (2008)). In these devices, metallic contacts inject charge carriers into thePDF Image | GRAPHENE SYNTHESIS CHARACTERIZATION PROPERTIES
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