Effect of Point Defects on Optical Properties of Graphene Fluoride: A First-Principles Study

The experimental optical gap of graphene fluoride has been measured between 3.1 and 3.8 eV, which is much smaller than the corresponding theoretical predictions (∼5.1 − 5.7 eV). To resolve this discrepancy, we have examined optical properties of graphene fluoride with several possible point defects (vacancies and substitution atoms). We employ a first-principles method for large-scale calculations of electronic excitations in solids based on time-dependent density functional theory (TDDFT) with optimally tuned and range-separated hybrid (OT-RSH) functionals. The first-order perturbation theory is applied to the solution of the OT-RSH functional Hamiltonian, from which the single- and two-particle excitation energies can be calculated. The method is validated for lithium fluoride, graphene fluoride, and phosphorene with excellent agreement to previous computational and experimental results. We reveal that the optoelectronic properties of graphene fluoride can be influenced profoundly by a small amount of fluorine vacancies and exciton binding energy in graphene fluoride can be doubled by a small concentration of oxygen substitutional defects. These point defects are believed to be responsible for the discrepancy in the optical gaps between the theory and experiments.