Besides the classical benzene ring we discussed above, the adsorption of heteroatom substituted aromatic molecules on graphene is also interesting. Triazine molecules have been found to form a uniform thin coating on the surface of a bilayer graphene, which efficiently blocks the accessible doping sites and prevents ambient p-doping on the top graphene layer [77]. The charge distribution asymmetry between the top and bottom layer can be used to open a bad-gap of up to 111 meV in a bilayer graphene sheet. The on/off current ratio for a bottom-gated bilayer transistor in ambient condition is improved by at least one order of magnitude. The surface GS967 and initial stages of growth of azabenzene 1,3,5-triazine molecules on graphene/Pt(1 1 1) have been studied using UHV-STM at a temperature ranging from 40 to 100 K [78]. As shown in Fig. 13, the binding of this molecule to graphene could be either through the nitrogen (N) lone pair or through the π orbitals in the molecular ring. STM data indicated that the initial adsorption of this molecule forms fractal shape islands, on which the molecules are arranged in a well-ordered hexagonal structure with the molecular plane parallel to the surface and a lattice parameter of 6.25 Å. Intramolecular resolution consisting of three lobes presenting the three-fold symmetry characteristic of the 1,3,5-triazine molecule that is compatible with a flat-lying configuration with respect to the substrate. The resulting distance of 2.49 Å between the N atom of one molecule and the H atom of the adjacent one is consistent with the formation of CH?N bonds. From nucleation experiments, the diffusion barrier for a single molecule adsorbed on graphene/Pt(1 1 1) surface has been measured as 68 ± 9 meV, which is higher than the previously reported value for this molecule on graphite surface. This result shows that even on the most weakly coupled graphene–Pt system, the dynamic process of organic molecules is affected by the interaction of graphene with the underlying metal [78].
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