Adsorption of Carbon Tetrahalides on Coronene and Graphene

Since carbon tetrahalides CX4 (X = Cl/Br) are adsorbed on carbon nanotubes and graphene sheets, we have studied the structures, adsorption energies, and electronic properties of CX4 adsorbed on benzene, coronene, and graphene using dispersion corrected density functional theory (DFT) with hybrid functionals. As compared with the benzene-CX4 complexes (with binding energy of similar to 14/15 kJ/mol) where electrostatic energy is significant due to the halogen bonding effect, the graphene-CX4 complexes show about three times the benzene-CX4 binding energy (similar to 40/45 kJ/mol) where the dispersion interaction is overwhelming with insignificant electrostatic energy. Since the X atoms in CX4 are slightly positively charged and the X atom's ends are particularly more positively charged due to the sigma-hole effect, CX4 behaves as an electron acceptor. This results in electron transfer from locally negatively charged C sites of benzene/coronene to CX4. In contrast, no electron transfer occurs from graphene to CX4 because of the large work function of graphene and significant electron affinity of CX4 and because homogeneously charge-neutral graphene has no locally charged sites. Nevertheless, due to the symmetry breaking upon adsorption, the CX4-adsorbed graphene shows a small band gap opening without p-doping. On the other hand, CF4 behaves as an electron donor due to the negatively charged F atoms, which results in electron transfer from CF4 to the locally negatively charged C sites of benzene/coronene. Again, no electron transfer occurs from CF4 to graphene because of high ionization energy of CF4, and so the CF4-adsorbed graphene opens only a small band gap without n-doping.