Vacancies and Stone-Wales defects in twisted bilayer graphene - A comparative theoretical study

The combination of miss-alignment and point defects on the electronic structure of bilayer graphene can lead to special properties, which deserve a theoretical investigation, since they are scarcely studied up to now. Although most graphene layers are nominally free of defects, they can be introduced in a simple way to achieve the desired properties. By performing density functional theory calculations implemented in two different computational approaches (SIESTA and QuantumEspresso) and a tight-binding hybrid method (DFTB+), we analyze the effects of single vacancies and Stone-Wale defects on the electronic structure of Twisted Bilayer Graphene (tBLG). Our results show that both kinds of defects can induce flat bands near the Fermi energy of tBLG, even for rotation angles quite larger than the well-known magic angles associated with the superconductivity of these systems. Additionally, with the aim to provide a theoretical background to identify defects on tBLG during experimental characterization, we simulate scanning tunneling microscopy (STM) images of these systems. As expected, such simulated STM images are dramatically influenced by the van Hove Singularities associated to the flat bands induced by defects. The results of our theoretical study may have an influence on the targeted introduction of defects in tBLG for the future design of tBLG with particular properties for some technological applications.