Effect of vacancies on the electronic and optical properties on twisted bilayer graphene

We systematically study the effects of a random distribution of vacancies on the electronic and optical properties of twisted bilayer graphene (TBLG), using a tight-binding method. We study the density of states and optical absorption for different concentrations of vacancies and different rotation angle between the layers.
The density of states and the optical absorption spectra are calculated by averaging over several vacancy configurations. The increase in the concentration of vacancies increases the number of midgap states near the Dirac point. For large twist angles, it is possible to observe that the intensity of the absorption peaks, typical of TBLG without vacancies, decrease and slightly shift when the vacancy concentration increases. In addition, peaks of lower intensity can be observed at low energies, coming from optical transitions between midgap states.
For small rotation angles and for a certain range of vacancy concentrations, the optical absorption peak is eliminated. This occurs when the energy range of the midgap states located at the Fermi level is comparable to the energy of Van Hove singularities where the optical transitions occur in the case of TBLG without vacancies.