Functionalizing Single- and Multi-layer Graphene with Br and Br 2 Xiaofeng Fan,* ,† Lei Liu,* ,† Jer-Lai Kuo, †,‡ and Zexiang Shen † DiVision of Physics and Applied Physics, School of Physical and Mathematical Sciences, Nanyang Technological UniVersity, Singapore, 637371, Singapore, and Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei, 10617, Taiwan ReceiVed: May 7, 2010; ReVised Manuscript ReceiVed: July 21, 2010 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 2 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 2 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. Introduction Graphene, a monolayer of carbon atoms with honeycomb lattice, has attracted enormous attention because of its extremely high mobility of carriers and other fascinating physical properties, 1-6 such as abnormal quantum Hall effects 7,8 and massless Dirac fermions. 9 In particular, the high mobility of electrical carriers in graphene may lead to the next generation of carbon-based nanoelectronics. 1,10-16 Some prototype graphene- based devices have been fabricated successfully. 17-22 However, for the wider electronic-device applications, such as as field- effect transistors, the control of type and concentration of carriers in graphene with an opened bandgap will be critical. 23 The modulation of band structure has spurred an intense scientific interest, and many methods have been tested to control the electronic behaviors of graphene, including electrostatic gating, 24,25 bilayer graphene, 11,12,15,26 graphene-substrate interaction, 27-29 contact with metals, 30 hydrogenation, 31 and intercalation/interac- tion with chemical molecules. 17,18,32-35 Furthermore, the nearly transparent optical properties of this semimetal membrane make it a good candidate material for the optoelectronic devices, for example, as a solar cell. 19 As a 2D membrane, the whole surface of graphene can be used to interact with metal atoms or chemical molecules, and this may functionalize graphene further with the desirable electronic properties. 36,37 Through chemical absorption, the Fermi level of graphene can be shifted because of the charge- transfer process, and the transport of its charge carriers can be controlled accordingly. 18 Without introducing substitutional impurities to disrupt the conjugated network, the adsorption- induced chemical doping would be an ideal way to scissor the band structure and control the π-electron conduction of graphene. 33 It has been known that intercalating alkaline metals into MLG can take up the Fermi level and result in electron doping. 17 On the other hand, with the intercalation/absorption of halogen molecules in MLG or on SLG, hole doping with opened gap is expected to be obtained. 33 The construction of intercalation compounds usually needs a host material with highly anisotropic layered structures. 38 In the early study of intercalation compounds, graphite, with weak interplanar interactions, has been considered as a ideal host for the insertion of atomic or molecular layers. 38 Actually, the notion of doping MLG or SLG with chemical absorption is the extension of the concept of intercalated graphite compounds. Halogen molecules, such as Cl 2 , Br 2 , and I 2 , are considered to be more electronegative than sp 2 -hybridized carbon, and they will induce a positive doping by charge transfer. For example, I 2 can be absorbed on fullerenes 39 and carbon nanotubes. 40,41 Charge transfer between Br 2 and double-walled carbon nanotube has been confirmed. 42-44 I 3 - anions can be obtained by induced iodide anions on the carbon substrate. However, Br 2 is the only diatomic homopolar halogen molecule that intercalates readily into MLG/graphite. 38 Therefore, the clear understanding of the structural and electronic properties of SLG/MLG intercalated or absorbed with Br atoms or Br 2 molecules would be helpful to explore a new way to chemically functionalize graphene layers. In this work, we report our systematic first-principles investigations on the absorption/insertion effect of SLG/MLG with Br atoms or Br 2 molecules. With the analysis of electronic structure, charge transfer, and redistribution of charges of graphene, we suggest that, with the activation of ultraviolet light, the conversion between Br atoms and Br 2 molecules adsorbed on graphene will result in the effective change of electronic properties of graphene. * Corresponding authors. E-mail: xffan@ntu.edu.sg, LiuLei@ntu.edu.sg. † Nanyang Technological University. ‡ Academia Sinica. J. Phys. Chem. C XXXX, xxx, 000 A 10.1021/jp1041537  XXXX American Chemical Society