We present a density-functional theory study of low-density bromination of graphene and graphite, finding significantly different behavior in these two materials. In graphene, we find a new ${\mathrm{Br}}_2$ form where the molecule sits perpendicular to the graphene sheet with an extremely strong molecular dipole. The resultant ${\mathrm{Br}}^{+}$–${\mathrm{Br}}^{-}$ has an empty $p_z$ orbital located in the graphene electronic $\pi$ cloud. Bromination opens a small (86-meV) band gap and strongly dopes the graphene. In contrast, in graphite, we find ${\mathrm{Br}}_2$ is most stable parallel to the carbon layers with a slightly weaker associated charge transfer and no molecular dipole. We identify a minimum stable ${\mathrm{Br}}_2$ concentration in graphite, finding low-density bromination to be endothermic. Graphene may be a useful substrate for stabilizing normally unstable transient molecular states.

• Received 14 December 2010

DOI:https://doi.org/10.1103/PhysRevB.83.045411