Functionalizing Single- and Multi-layer Graphene with Br and Br2

The structural and electronic properties of Br-2/Br adsorption and intercalation of single-layer graphene (SLG) and multi-layer graphene (MLG) are studied by density-functional theory. As a result of charge transfer, the Br atom is found to be stable as adsorbed on the vertex or near bridge sites of graphene whereas the Br, molecule will be more stable when adsorbed perpendicularly on graphene. Because of the interactions between Br-2 molecules, the stable configurations of Br-2 on graphene or intercalated in MLG are parallel to graphene. With the analysis of charge difference, the experimental observation that the lowest stage of Br-2 intercalated graphite is the stage 2 compound is ascribed to the effect of localized dipoles on graphene induced by Br-2. Although only slightly disturbing the orbitals of graphene atoms, the existence of Br, molecules or Br atoms will still affect the electronic structures of both materials. As adsorbed on the single surface of graphene, Br-2 will open its bandgap at the K (K') point. While present on both surfaces, Br-2 molecules will induce a much larger bandgap of graphene with the Fermi level shifted down into the valence bands. If Br atoms are absorbed on graphene, the significant amount of charge will transfer from graphene to Br atoms because of the strong electronegativity of Br. More importantly, the electronic properties of SLG/MLG with the absorbed Br-2 molecules can be controlled by the ultraviolet light that decomposes the Br-2 on SLG/MLG.