Electronic and optical properties of fluorinated graphene: A many-body perturbation theory study

Abstract

The electronic and optical properties of fully fluorinated graphene i.e. fluorographene (CF), and partially fluorinated graphene (C${}_{4}$F) are studied by means of the first-principles many-body Green's function method, i.e. $GW$--Bethe--Salpeter equation. In conformity with experimental observations, fluorination of graphene causes relatively wide quasiparticle band gaps, 7.01 eV for CF and 5.52 eV for C${}_{4}$F. The optical absorption properties of CF and C${}_{4}$F are of excitonic nature, leading to the formation of bound excitons with significantly high binding energies of 1.96 and 1.31 eV assigned to the first bright exciton of CF and C${}_{4}$F, respectively. The disagreement between the ab initio calculated electronic band gap and the experimentally optical gap seems to be mainly due to the binding energy of excitons rather than the presence of defects. Contrary to previous theoretical predictions, this paper demonstrates that CF seems to be less likely to realize the excitonic Bose--Einstein condensation because the exciton wave functions are relatively delocalized and not well separated. In the case of C${}_{4}$F, however, it indicates a charge-transfer excitation.