Supplementary Information Tuning the Properties of Graphene by a Reversible Gas-Phase Reaction
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- Band structures of graphene after plasma treatment.
Fig. S5. AFM images of the same SLG sample before (a) and after plasma methylation (b). The average changes in graphene thickness after plasma methylation is 0.21nm. Theoretical computations. Our calculations were based on density functional theory (DFT) with generalized gradient approximation (GGA) for the exchange-correlation energy. We have used the Perdew-Burke-Ernzerhof (PBE)2 for the GGA and a plane-wave basis set with the projector-augmented-wave (PAW) method originally developed by Blochl3 and adapted by Kresse and Joubert in the Vienna ab initio simulation package (VASP)4. We used periodic boundary condition to simulate the two-dimensional methylated SLG along x and y direction, and along z direction we set vacuum space of 15 Å to avoid the interactions between two neighboring images. We also applied Monkhorst-Pack meshes5 of (7  71) to represent the reciprocal spaces. All the structures were relaxed through conjugated gradient method without any symmetric constraints. The energy cutoff and convergence criteria for energy and force were set to be 400 eV, 110-4 eV, and 0.01 eV/Å, respectively. The accuracy of the above procedure has been well tested in our previous work6.
Band structures of graphene after plasma treatment. In order to better understand the methylated SLG sheets, we performed first-principles density-functional calculations to investigate their band structure, orbital distribution, and elastic modulus using Vienna Ab initio Simulation package (VASP)3. Based on the experimental finding, the geometry of methylated SLG sheets is shown in Fig. S6(a). After geometric relaxation, the unit cell lattice constant is found to be 4.98 Å and the C-C bond lengths with sp3 and sp2 hybridizations in the graphene are 1.51 and 1.53 Å, respectively. Such a configuration is found to be energetically most stable with at least 0.913 eV lower in energy than other configurations (Supplementary Fig. S7). Due to the surface absorption, graphene could not retain its planar structure and the sp3 C atoms move outward. The formation energy, defined as Ef (=Emethylated SLG – ESLG – ECH4), is found to be 1.97 eV/unit cell. The positive value indicates that the reaction of surface decoration is endothermic, and thus the demethylation and dehydrogenation process can be achieved easily. It should be noted that the reaction of full hydrogenation on SLG (forming graphane) is exothermic, suggesting that it is more difficult to remove its surface hydrogen atoms. To gain some details of bonding features, let’s check the electron redistribution upon the surface decoration, as shown in Fig. S6(b) for the charge difference density △ρ (=ρmethylated SLG – ρCH3 – ρH – ρSLG). We can see that electrons are slightly accumulated around the functional groups (methyl and hydrogen). 
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