Structural Evolution of Flower Defects and Effects on the Electronic Structures of Epitaxial Graphene

The structural evolution processes of flower defects in epitaxial graphene at high temperature annealing are investigated by using scanning tunneling microscopy, Raman spectroscopy, and the density functional theory calculations. The experimental results reveal that flower defects can increase in amount with the increasing of annealing time until they nucleate and evolve into complex defect structures. As the direct evidence of the structural development for flower defects evolving into the complex structures, the conjoined-twin defect is detected in epitaxial graphene with two of 2/3 flower defects joined together. The theoretical calculations show that the conjoined-twin defect has a calculated energy 2.7 eV smaller than that of two isolated flower defects. And the conjoined-twin defect as a new topological defect in graphene can bring about a bandgap of 50 meV. The calculation results also demonstrate that new van Hove singularity states will be generated in the vicinity of + (0.4-0.7) and -0.4 eV in the density of states for the dislocated carbon rings of the conjoined-twin defects. These results can provide valuable insights into the growth evolution of topological defects and their effects on the electronic structures of graphene.