graphene, electronic structure, optical properties, exciton

We present structural, vibrational, electronic and optical properties of recently prepared fully fluorinated and hydrogenated graphene, called fluorographane. We consider 1:1 ratio of F and H adatoms and we carefully investigate possible structural conformers in C2FH stoichiometric material. The reported results show that all conformers are dynamically stable and homogeneous structures with F and H atoms on both sides of carbon honeycomb structure are more stable than the Janus structures with F atoms on one side and all H atoms on the other. While the main structural and vibrational features are roughly similar for various conformers, electronic and optical properties are very sensitive to local structure (namely specific combinations of F and H adatoms on the graphene surface) and differ significantly. Our highly accurate results based on many-body methods (GW and BSE) indicate that homogeneous fluorographane has unusually wide indirect electronic band gap of ~10 eV (larger than both pure graphane CH and pure fluorographene CF) and embodies a huge excitonic effect (~3 eV). Fluorographane C2FH is therefore a material with the widest electronic gap and a largest binding energy of exciton in the class of currently known 2D materials.