Graphene is an atomically thin sheet of sp2 bonded carbon. The very stable hexagonal structure is tightly bonded by σ bonds. The remaining electron forms π bands above and below the Fermi-level. Graphene is a semi-metall. The valence and conduction band touch only at two inequivalent points at the corners of the Brillouin zone in the so called Dirac cones. The slope of the cone viz. the Fermi velocity is an important material constant relevant to charge, spin and heat transport in the electronic channel. As graphene is an archetypical all surface material, actually every atom sits at two surfaces, the low energy electronic properties of graphene are easily modified by charge transfer and rehybridization. with a substrate or adsorbates.
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