Bromination of graphene and graphite

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 Br₂ form where the molecule sits perpendicular to the graphene sheet with an extremely strong molecular dipole. The resultant Br⁺--Br^- has an empty pz orbital located in the graphene electronic π cloud. Bromination opens a small (86-meV) band gap and strongly dopes the graphene. In contrast, in graphite, we find Br₂ is most stable parallel to the carbon layers with a slightly weaker associated charge transfer and no molecular dipole. We identify a minimum stable Br₂ concentration in graphite, finding low-density bromination to be endothermic. Graphene may be a useful substrate for stabilizing normally unstable transient molecular states.