The electronic and optical properties of a new type of hydrogenated graphene in the C${}_4$H phase have been studied using the first-principles many-body Green's function method ($\mathit{GW}$ approximation and Bethe–Salpeter equation). It demonstrates that strong charge-transfer excitonic effects dominate the optical properties of C${}_4$H. The binding energy of the first exciton, which turns out to be strongly localized, is as large as 1.67 eV. The spatial separation of exited electrons and holes makes C${}_4$H a candidate for the realization of excitonic Bose–Einstein condensate. Dark state provides a traveling path for the bound excitons. As the situation in graphane with H vacancies, changing H coverage changes the electronic and optical properties and the associated applications.

• Received 20 July 2012

DOI:https://doi.org/10.1103/PhysRevB.86.165444