Strain effects in the electron orbital coupling and electric structure of graphene

Graphene is not only a very strong two-dimensional material, but is also able to sustain reversible tensile elastic strain larger than 20%, which yields an interesting possibility to regulate the properties of graphene by applied strain. We have investigated the strain effects in the electron orbital coupling and electric structure of graphene adopting the density functional theory. We found that the Fermi level of graphene is elevated by compressive strain and degraded by tensile strain. But uniaxial strain can give rise to the symmetry breaking of graphene and open the band gap. Furthermore, the tensile uniaxial strain is more beneficial to the band gap opening than the compressive uniaxial strain when the uniaxial strain is perpendicular to the C-C bond, but the compressive uniaxial strain is more than the tensile uniaxial strain when the uniaxial strain is parallel to the C-C bond. Second, the symmetry breaking of graphene resulting from uniaxial strain can be illustrated in that the uniaxial strain weakens the electron orbital coupling of graphene between p and p orbitals and brings about the splitting of the peak of the p orbital density of states (DOS) on the left side of the Fermi level. Finally, whether uniaxial or biaxial strain, the compressive strain widens the pseudogap of graphene and the tensile strain narrows it. This would be useful for greatly broadening its applications in nanoelectronics and optoelectronics.