In order to move the Fermi energy inside the gap region, they suggest to apply a gate voltage or to dope graphene with holes. The authors attribute this gap to the breaking of the sublattice symmetry due to the graphene-substrate interaction. It has been shown experimentally that, when graphene is epitaxially grown on SiC substrate, a gap below the Fermi energy is open in its band structure near the Dirac energy, that decreases with the increasing sample thickness. It is a huge experimental challenge to produce a gap in the dispersion relation of graphene due to the known robustness of the Dirac cone under deformations as well as to chemical modifications, ,, ,, ,, ]. However, the gapless nature of the Dirac cone is also a drawback for graphene application in optoelectronic devices. This dispersion relation contributes to its extremely high carrier mobility, which assuring graphene an important role in the carbon-based nanoelectronics. Close to the Fermi level, its energy dispersion curve is linear in momentum, forming the gapless Dirac cone structure. Graphene is one-atom-thick sheet of carbon in a two-dimensional hexagonal lattice, that surprised the scientific community for its unusual electronic and mechanical properties. Since the isolation of graphene from graphite by mechanical exfoliation, new perspectives were opened for the development of two-dimensional (2D) materials, with the aim to be applied in electronic devices.
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