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 px and py orbitals and brings about the splitting of the peak of the pz 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.

Tunable mosaic structures in van der Waals layered materials.

Intrinsic mosaic structures composed of distinctive stacking domains separated by domain walls (DWs) show the potential to regulate many outstanding properties of van der Waals layered materials. A comprehensive simulation at the atomic scale is performed to explore how the lattice/twist mismatch and the interlayer interaction influence the mosaic configuration from the incommensurate Moiré pattern to commensurate mosaic structures by adapting a complex amplitude version of the phase field crystal method. It is found that after an incommensurate-commensurate transition occurs, the topology of the mosaic structure indicated by different domain wall (DW) patterns can be drastically changed. An experimentally observed intriguing spiral domain wall (SDW) network is revealed as result of the emergent mixed dislocation driven by minimizing the elastic and interlayer energies in the presence of both lattice and twist mismatches. The transition process from a herringbone domain wall (HBDW) network to a SDW network is also simulated, elucidated by a dislocation reaction and in good agreement with the experimental observations.