ARTICLE pubs.acs.org/JPCC
Improved Stability and Catalytic Properties of Au16 Cluster Supported on Graphane G. Chen,*,†,‡ S. J. Li,† Y. Su,† V. Wang,‡ H. Mizuseki,‡ and Y. Kawazoe‡ † ‡
School of Physics, University of Jinan, Jinan 250022, China Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan ABSTRACT: Using the density functional theory with the generalized gradient approximation, we have examined the stability and the catalytic properties of Au16 gold cage structure supported on graphane. The substantially improved stability is confirmed by the first-principles molecular dynamics simulation at the temperature above 500 K. The energy barrier is only 0.47 eV for the Langmuir Hinshelwood oxidation process for a CO coadsorbed on catalyst with an O2 molecule. The following Eley Rideal oxidation process can happen almost simultaneously for its low activation barrier of ∼0.13 eV. A simple model to mimic the situation of the full coverage of CO on gold catalyst can experience oxidation by overcoming 0.63 eV energy barrier, which suggests the CO tolerant property of the complex graphane-based gold catalyst.
I. INTRODUCTION Graphene is a single atomic layer of hexagonal sp2-bonded graphite that has attracted considerable interest because of its unique zero-gap electronic structure and the massless Dirac fermion behavior.1 4 The unusual electronic and structural properties make graphene a good candidate material for the generation of faster and smaller electronic devices. In the field of chemical functionalization to modify the electronic and crystal structure of graphene and its analogy boron nitride sheet material, the hydrogenation was approved to be a valid way and is of fundamental importance.5 29 The double-side hydrogenated graphene known as graphane was first proposed by Sluiter et al.,5 which has a band gap around 7 eV calculated with the accurate GW method.6,7 The modification on electronic properties from hydrogenation was later confirmed in experiment by Elias et al.22 In experiment, the graphene exposed to hydrogen plasma reacts with atomic hydrogen that transforms this highly conductive zero-overlap semimetal into an insulator. By completely removing the hydrogen on one side of graphane, Zhou et al.11 reported the semihydrogenated graphone to be ferromagnetic whose stability at room temperature was confirmed by a first-principles molecular dynamics (MD) simulation. By engineering the adsorbed hydrogen atoms on graphane, one can get conducting and semiconducting, magnetic and nonmagnetic sheet-based materials. Very recently, the experimental studies on controlled hydrogenation of graphene were reported,23 25 which provide facts for the tunable conductive properties by changing hydrogenation ratio proposed on theory. Jones et al.,23 based on their experimental studies, proposed the electron-beam lithography to be a useful technique for writing graphane and partially hydrogenated graphene nanostructures. On theory, an electric field was shown by Ao et al.12 with the r 2011 American Chemical Society
density functional theory calculations to be a potential way to hydrogenate graphene selectively by applying it at a local area on graphene sheet. Both the experimental22 28 and theoretical5 21 studies provide the possibility in engineering nanostructures on hydrogenated graphene, for example, the hydrogen vacancy cluster on graphane. Interestingly, the recent progresses reported by Peeters et al. show that the controlled or patterned hydrogenation of the graphene is a good technique to prepare the graphene-based nanostructures for application; for example, the graphene-based nanoribbon structure can be fulfilled by controlling the hydrogenation.8,9 Besides the numerous reports of conductivity and magnetism modification through controlling hydrogenation of graphene, we report another usage of the controlled hydrogenation of graphene, the graphane-based noble metal catalyst in this Article. By removing hydrogen atoms at a local area on one side of the graphane to form a hydrogen vacancy cluster, one can get a graphone fragment in which the delocalized π bonding network of graphene was broken to leave the electrons in the unhydrogenated carbon atoms localized and unpaired.11 The unpaired electrons in graphone area are ready to capture adsorbate. In this study, we use an inert Au16 cage adsorbed on graphone fragment to demonstrate the potential usage of engineered graphane for the nanostructured catalyst. The interaction was found to be analogous to the case of the gold adsorbed on Si(111)-(7 7) reconstruction surface on which the gold atom prefers to adsorb on a multicoordinate site to saturate maximum dangling bonds.30 The interaction in combination with the structural distortion of Received: August 10, 2011 Revised: September 11, 2011 Published: September 13, 2011 20168
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Au16 cluster as compared with its ideal cage structure results in a negative net charge of 0.7 electrons analyzed with the Bader technique31,32 on the upper gold atoms above the three basal ones that directly bonded to graphone fragment, which in turn alters the inert cage to be catalytic. In this Article, we first report the substantially enhanced stability of the Au16 cage anchored on graphane as compared with the free-standing one; then, we analyze the interaction between the small gas molecules and the gold cage anchored on graphane; last, we discuss the improved appreciable catalytic properties using the nudged elastic band method (NEB)33,34 and the dimer technique.35
II. COMPUTATIONAL DETAILS Our spin-polarized first principles calculations are carried out based on the density functional theory as implemented in the Vienna ab initio simulation package.36 We have employed the projector augmented-wave method37 and used the generalized gradient approximation with the Perdew, Burke, and Ernzerhof parametrization38 for the exchange-correlation energy. The solution of the Khon-Sham equations is calculated by an efficient matrix diagonalization technique based on a sequential band-byband residual minimization method.36 The gold cage catalyst supported on the graphane is placed in a super cell with a vacuum of 11 Å along the Z lattice, which keeps the hexagonal periodic conditions in XY plane with the edge length of 20.315 Å. The vacuum is large enough to eliminate the interaction of the complex catalyst with its periodic images. The unit cell consists of 128 C atoms, 121 H atoms, and 16 Au atoms. The wave functions are expanded in the plane wave basis with an energy cutoff of 400 eV. A Monkhost-Pack k-sampling of 3 3 1 is used for studying the equilibrium structures, and the Γ point is used for the NEB method33,34 and the dimer method35 calculations due to the intensive computing loading. The convergence tolerance of the structural optimization is set to 0.01 eV/Å. To test the accuracy of the method, we have calculated the lattice constants, which are 3.57 Å for diamond and 4.17 Å for Au fcc structure. These agree with the experimental structural parameters of 3.567 and 4.08 Å, respectively.39 The molecular bond lengths of Au2, C2, CO, O2, and H2 molecules are calculated to be 2.53, 1.31, 1.14, 1.23, and 0.75 Å, which are also in good agreement with the experimental data of 2.47, 1.24, 1.13, 1.21, and 0.74 Å, respectively.40 III. RESULTS AND DISCUSSION A. Substantially Enhanced Stability. By removing hydrogen atoms at a local area on one side of the graphane, one can get a graphone fragment on graphane. The hydrogen vacancy cluster at the graphone fragment leaves the unpaired electrons localized at the uncovered carbon atoms to capture adsorbate. Using the Au16 cluster as an example, we have confirmed by the MD simulations that the adsorbate can be stabilized at the hydrogen vacancy cluster. In Figure 1a, we show a schematic geometrical configuration for a graphone fragment where the C atoms in the hydrogen vacancy cluster are highlighted for illustration. The Au16 cluster stabilized at this vacancy cluster is presented in Figure 1b. The electronegativity of carbon and gold is 2.55 and 2.40, respectively.40 Therefore, when a gold atom contacts with the unhydrogenated C atoms in the hydrogen vacancy cluster, the orbital hybridization would probably dominate the chemical interaction, which is the fact for the case of a single gold atom
Figure 1. Schematic structural configuration for the hydrogen vacancy cluster (a) and the Au16 cage adsorbed on this vacancy cluster (b). C atoms in the hydrogen vacancy cluster are highlighted for illustration.
adsorbed at multicoordinate site on the Si(111)-(7 7) surface by developing maximum covalent-like bonds with the dangling bonds of the dimer-adatom stacking-fault model.30 For the gold cage anchored on the graphane, the gold atom prefers to sit upon a bridge site to develop bonds with two nearby C atoms. Because of the lattice mismatch between graphane and gold bulk materials, the bond length between Au atoms that directly bonded to the C atoms is elongated by 0.3 Å, which may contribute to its stability as in the case of MgO(111) enhanced stability of gold cages.41 For the Au16 adsorbed on graphane, we have studied the charge density distribution and the partial density of states (PDOS), which are shown in Figure 2a,b. In Figure 2a, the charge overlap between the gold cage and the hydrogen vacancy cluster can be seen. In Figure 2b,c, the orbital hybridization between the 6s orbital of Au and the s-p hybridization of C can be seen, inducing the p electron density for Au atom. This suggests the covalent-like bonding that accounts for the short Au C interatomic distance of ∼2.1 Å and the strong binding energy of ∼7 eV for the cage adsorption. A constant energy first-principles MD simulation was carried out for 3 ps with the time step of 1 fs. The temperature evolution versus simulation time for the gold cage anchored on graphane is shown in Figure 3, which suggests its thermodynamic stability at temperature >500 K. The slightly distorted geometry obtained at the end of the simulation was also presented in comparison with the initial one. This structure was further forwarded to perform a structure optimization, and it was found to converge to the initial structure quickly. We have also evaluated the thermodynamic stability of a free-standing Au16 cage structure using the firstprinciples MD simulation. The free-standing cage structure does not remain at the temperature above 300 K, which is in accordance with the fact that the cage structure of the isolated anionic Au16 obtained at low temperature in experiment42 can only be protected at the low temperature in neutral state because of a structure transformation barrier.43 The supporting materials, as the example of engineered graphane examined in this study, can play an important role in substantially enhancing the stability of gold cage, which is also verified for the gold cage supported on MgO(111) substrate.41 In Figure 4, we present the density of states (DOS) for the Au16 cage anchored on graphane and also that of the freestanding cage for a direct comparison. The broadening of the DOS can be attributed to the interaction with the unhydrogenated 20169
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Figure 4. DOS for the free-standing Au16 cage (a) and the cage anchored on the graphane (b). The Fermi energy was shifted to zero electronvolts.
Figure 2. Iso-value surface of 0.2 e/Å3 of charge density (a), the PDOS of the Au atom that bonded to the graphane (b), and that of the corresponding C atom (c) for the Au16 cluster supported on graphane. The black solid, gray solid (green solid), and gray dotted (cyan dotted) lines account for the s, p, and d electrons. The d electron density for Au was reduced by a factor of 5 to facilitate the comparison. The Fermi energy was shifted to zero electronvolt.
Figure 3. Temperature evolution as a function of the time of the MD simulation. The initial and final structures of the simulation are also presented for a comparison of the structural distortion.
C atoms on graphane, which also shifts the DOS downward from the Fermi energy. It is interesting to note that a band gap of ∼0.4 eV between the highest occupied molecular orbital and the
lowest unoccupied molecular orbital is opened, contributing to the enhanced stability also. The Bader charge analysis technique was also applied in our study. The cage anchored on the graphane gains ∼0.2 electrons from the graphane. It is surprising that the three basal Au atoms that directly bonded to C atoms were found to lose ∼0.5 electrons. Therefore, the upper part of gold cage above the three basal Au atoms gains ∼0.7 electrons. The interaction with the carbon atoms elongates the Au Au bond among the three basal Au atoms that bonded to the carbon, which could induce the charge redistribution among the Au16 cluster. This, in combination with the interaction between the Au and C atoms, may account for the charge gain of the upper part of the cage structure. B. Adsorption of Single Gas Molecule. The enhanced stable Au16 cluster supported on the graphane could be a potential functional nanostructured material. Because the gold particles have good catalytic properties whereas their bulk material is inert, we would like to examine whether the graphane-based Au16 nanostructure could be used as catalyst in this study. The adsorption of CO and O2 molecules on the gold particle could account for the initial stage for the catalyzed CO oxidation process. Therefore, we would concentrate on discussing the adsorption of CO and O2 gas molecules in this section. The CO molecule prefers to adhere upon a gold atom to develop a Au C bond with a binding energy of ∼1 eV. In Figure 5a, we presented the lowest energy configuration of the CO adsorption. The binding energy is calculated to be 1.07 eV. The Au C bond length is 1.94 Å, which is slightly shorter than the sum of the atomic radii of Au and C atoms, which are 1.37 and 0.77 Å, respectively.39 The bond length of CO is slightly elongated after adsorption because of fewer electrons localized in the C O bond. Some electrons of C atom would attend the bonding between C and Au atoms. In Figure 5b, the iso-surface of charge density of 0.2 e/Å3 is presented. Figure 5c,d shows the PDOS of the Au atom at the adsorption site and the C atom in CO molecule. A hybridization between Au orbitals and C orbitals can be seen, which induces the p electron density for Au atom and the d electron density for C atom. Besides the Au 6s orbital, the Au 5d orbital attends the hybridization also, as indicated by 20170
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Figure 5. Schematic structural configuration for the CO adsorbed on the complex graphane-based catalyst (a), the iso-value surface of 0.2 e/Å3 of its charge density (b), the PDOS of the Au atom that bonded to the CO (c), and that of the corresponding C atom (d). The black solid, gray solid (green solid), and gray dotted (cyan dotted) lines account for the s, p, and d electrons. The d electron density for Au was reduced by a factor of 5 to facilitate the comparison. The Fermi energy was shifted to zero electronvolt.
the states located around 8 eV below the Fermi energy. The bonding nature of Au C bond is mainly covalent-like. The Bader charge analysis technique shows that only 0.2 eV, whereas it is almost neglected for the adsorption on the inert intact cage,44 indicating the net charge effects on Au atoms. The small binding energy indicates weak interaction between O2 and gold cage. The lowest energy configuration for O2 adsorption is schematically shown in Figure 6a, and the corresponding binding energy is 0.25 eV. The interatomic distance between O and Au atoms is 2.20 Å, which is
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Figure 6. Schematic structural configuration for the O2 adsorbed on the complex graphane-based catalyst (a), the corresponding iso-value surface of 0.2 e/Å3 of its charge density (b), the PDOS of the Au atom that bonded to the O2 (c), and that of the O atom (d). The black solid, gray solid (green solid), and gray dotted (cyan dotted) lines account for the s, p, and d electrons. The d electron density for Au was reduced by a factor of 5 to facilitate the comparison. The Fermi energy was shifted to zero electronvolt.
larger than the sum of atomic radii of Au and O atoms by 0.25 Å. In Figure 6b, the charge density plot shows some overlap between O and Au atoms. The PDOS projected on the O atom and the bonded Au atoms shows a rather weak orbital hybridization, agreeing with the small binding energy. The interaction is in fact similar to the well-known peroxo dioxygen45 that bound to the transition-metal atom in a molecular compound. Some electrons are donated to O2 molecule from metal atom to enter the partially vacant antibonding orbitals πg*, resulting in an elongated O O bond. In our study, ∼0.5 electrons calculated with the Bader charge analysis technique are transferred from the gold cage to the O2 molecule to induce a 0.1 Å O O bond length increase. We have also integrated the density of the magnetism for the adsorbed O2 molecule to find a magnetism of ∼0.5 μB. 20171
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Figure 7. Minimum energy path obtained with the NEB method for the LH oxidation reaction. The inserts show the corresponding structural configurations. The structural parameters marked in the Figures are tabulated in Table 1.
This suggests that the interaction with the gold atoms shifts the electronic configuration of the triplet ground state 3O2 toward that of the doublet 2O2 , which is well known to be highly reactive. Therefore, the charge transfer coupled to the spin-state change of molecular oxygen may facilitate the CO oxidation. C. Improved Catalytic Properties for CO Oxidation. The above discussion shows that both the oxygen molecule and the carbon monoxide molecule can adsorb on the gold cage anchored on graphane. Also, considering the fact that the binding strength for CO adsorption is strong while it is weak for O2 adsorption, two configurations of the gas molecule adsorption as for the initial states for CO oxidation can be expected. One is the coadsorption of CO and O2 on the cage at the low CO concentration. The other one is the full coverage with CO at the presence of high CO concentration, on which the O2 molecule could not find adequate place to attach the gold cage. For the latter situation, the activation energy of the catalytic CO oxidation would determine whether the anchored gold cage on graphane would be CO-tolerant. In Figure 7, we show the studied minimum energy path obtained with the NEB method for the coadsorption configuration. The corresponding structural parameters marked in Figure 7 are tabulated in Table 1. The Langmuir Hinshelwood (LH) oxidation process46,47 would occur with a small activation energy of ∼0.47 eV. The structure of the initial coadsorption configuration (state A) would experience the close coadsorption
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Figure 8. Minimum energy path obtained with the NEB method for the ER oxidation reaction. The inserts show the corresponding structural configurations. The structural parameters marked in the Figures are tabulated in Table 2.
Table 1. Interatomic Distances (angstroms) for the Initial, Intermediate, and Final States As Shown in Figure 7 A
B
C
D
E
d1
3.38
1.51
1.32
1.18
1.18
d2
1.37
1.46
1.80
2.96
3.18
d3
2.09
2.44
2.42
2.94
2.92
d4
1.96
2.04
2.11
3.08
3.94
d5
1.15
1.19
1.20
1.18
1.18
configuration (state B), the dissociation of O2 molecule, and the formation of a CO2 molecule by combining the CO molecule with an O atom (state C), the desorption of the produced CO2 molecule (state D), and finally the movement of the single O atom to its best adsorption site on gold cage (state E). Meanwhile, the gold cage continues to play an important role in catalyzing the oxidation. The preadsorbed elongated peroxo oxygen molecule can be activated by a low energy cost to dissociate to offer oxygen atoms for the CO oxidation. The CO2 is quite inert and would desorb immediately in the structural optimization. An oxygen atom adsorbed on the anchored gold cage, the byproduct material of the LH oxidation, would be ready to oxidize a succeeding CO molecule. As shown in Figure 8, the Eley Rideal (ER) reaction46,47 can happen almost simultaneously after the LH reaction. A CO molecule approaching the preadsorbed O atom from the faraway site (state A) would first enter the physical adsorption configuration (state B), then 20172
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Table 2. Interatomic Distances (angstroms) for the Initial, Intermediate, and Final States As Shown in Figure 8 A
B
C
D
d1
1.14
1.15
1.16
1.18
d2 d3
5.71 2.16
2.80 2.16
1.92 2.21
1.18 7.24
Figure 9. Transition state (TS1) for a faraway O2 approaching the preadsorbed CO molecules to reach the metastable state (MS) and the transition state (TS2) for the dioxygen molecule to dissociate to oxidize the CO molecules finally. The transition states were obtained with the dimer method.
develop a bond with the O atom to form thr CO2 molecule (state C) and finally desorb from the gold catalyst (state D). The ER reaction can be driven by activation energy as low as 0.13 eV, which can be easily overcome at the room temperature. The corresponding interatomic distances, as illustrated in Figure 8, are presented in Table 2. It could be concluded that the CO and O2 coadsorption structure can process the oxidation by a small perturbation to start the LH reaction, followed by the ER reaction to reach the final production of an intact anchored gold cage on graphane with the desorbed CO2 molecules, completing a catalytic CO oxidation cycle. For the strong adsorption of CO molecule, the graphanebased gold catalyst exposed to the presence of the gas mixture with high CO concentration can have the chance to be fully covered by the CO molecules. We have confirmed this situation with the first-principles simulation in our study. So, it would be important whether the catalyst is CO tolerant. Otherwise, the catalyst would be poisoned to lose catalytic properties in the application. In this study, we used a simple model, as shown in Figure 9, with two CO molecules adsorbed on two adjacent Au atoms to check whether these molecules could react with an approaching O2. Because of the intensive computing loading, we have used the dimer method instead of the NEB. The dimer method was developed by Henkelman and Jonsson35 to search the saddle point (although not the entire minimum energy path) by defining a dimer with two neighboring images on the potential surface. If the images are away from the saddle point, then the dimer will be moved uphill on the potential surface. Along the way, the dimer is rotated to find the lowest curvature mode of the potential surface at the point where it is located. This method can give a good estimation of energy barrier, which was confirmed in the previous literatures.35,48 In Figure 9, we present the intermediate states of the reaction for a faraway O2 molecule approaching the preadsorbed CO molecules. As shown in Figure 9, the reaction consists of two stages. The first stage is the faraway O2 molecule to approach the CO molecules
adsorbed on the adjacent sites of the catalyst to pass the transition state TS1 and to get to the metastable state MS in which the dioxygen molecule bonds to the carbon atoms in the CO molecules. The second stage starts from the metastable state MS, passes the transition state TS2, and reaches the final state of two produced CO2 molecules to leave the catalyst. The energy barriers associated with these two reaction stages are calculated to be 0.63 and 0.42 eV, and the corresponding energy releases are 0.83 and 3.96 eV, respectively. The final state is 4.79 eV lower in total energy than the initial state. These results suggest that the CO molecules covered on the catalyst could be removed by oxidation. The reaction can be activated easily for the low activation energy in the first reaction stage, and the activation energy in the second reaction stage can be overcome by the energy release in the previous reaction. We have also analyzed the charge gain of the dioxygen molecule in the states TS1, MS, and TS2 by using the Bader charge analysis technique,31,32 which are 0.35, 0.72, and 2.27 electrons, respectively. This agrees with the corresponding O O bond elongations of 0.05, 0.26, and 0.47 Å. The studied partial density of states supports the orbital hybridization between O2 and CO molecules. The interaction between O2 and CO changes the CtO electronic configuration of CO toward the OdCdO configuration of CO2 where the C sp hybridization happens, which accounts for the charge gain of O2 calculated by the Bader method. The interatomic distances between the oxygen in O2 molecule with the nearest carbon in CO molecule are 2.07, 1.41, and 1.35 Å, showing the tendency to approach the length of 1.18 in CO2 molecule. The oxidation of two CO molecules adsorbed on the adjacent sites suggests the complex graphane-based gold catalyst to be CO- tolerant, making it an attractive recyclable nanostructured material for cleaning the CO pollution in automobile exhaust.
IV. CONCLUSIONS In this study, we have applied the first-principles method in studying the complex graphane-based gold catalyst. The stability of the cage is substantially enhanced by developing covalent-like bonds with C atoms in the hydrogen vacancy cluster on graphane. The interaction, in combination with the structural distortion, induces the charge redistribution and results in the negative charge of ∼0.7 electrons on the upper part of gold cage above the three basal Au atoms that directly bonded to the graphane. The O2 molecule is calculated to adsorb on the gold cage with the binding energy above 0.2 eV and gains ∼0.5 electrons. The bond length of O2 is elongated to make it analogous to the peroxo dioxgen. The coadsorption of CO and O2 molecules can be activated to process the oxidation by overcoming 0.47 eV energy barrier. The reaction would start with the LH process to release a CO2 molecule, to be followed by the ER process. The two CO molecules adsorbed on adjacent sites can be oxidized by overcoming 0.63 eV energy barrier with an approaching O2 molecule, suggesting the CO tolerant property of the studied complex catalyst. Our results are interesting not only to the theorist to study functional materials by anchoring metal particles on the hydrogenated or fluorinated sheet materials but also to the experimental scientist to synthesize this kind of nanostructured materials and to explore their catalytic properties. Also, for the hollow room inside the cage, the cage-like gold catalyst anchored on the graphane can also be doped with a guest atom to explore tailored properties for the potential usage, as already proved for the case of the free-standing Au16 gold cage.42,49 20173
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’ ACKNOWLEDGMENT We gratefully acknowledge the SR11000 supercomputing resources from the Center for Computational Materials Science of the Institute for Materials Research, Tohoku University, and the computing resources from the University of Jinan. G.C. also acknowledges the financial support from Shandong Province under the grants of TSHW20101004 and ZR2010AM027 and the National Natural Science Foundation of China under the grant of 11074100. ’ REFERENCES (1) Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Katsnelson, M. I.; Grigorieva, I. V.; Dubonos, S. V.; Firsov, A. A. Nature 2005, 438, 197. (2) Zhang, Y. B.; Tan, Y.-W.; Stormer, H. L.; Kim, P. Nature 2005, 438, 201. (3) Novoselov, K. S.; McCann, E.; Morozov, S. V.; Fal’ko, V. I.; Katsnelson, M. I.; Zeitler, U.; Jiang, D.; Schedin, F.; Geim, A. K. Nat. Phys. 2006, 2, 177. (4) Castro Neto, A. H.; Guinea, F.; Peres, N. M. R.; Novoselov, K. S.; Geim, A. K. Rev. Mod. Phys. 2009, 81, 109. (5) Sluiter, M. H. F.; Kawazoe, Y. Phys. Rev. B 2003, 68, 085410. (6) Klintenberg, M.; Lebegue, S.; Katsnelson, M. I.; Eriksson, O. Phys. Rev. B 2010, 81, 085433. (7) Leenaerts, O.; Peelaers, H.; Hernandez-Nieves, A. D.; Partoens, B.; Peeters, F. M. Phys. Rev. B 2010, 82, 195436. (8) Dzhurakhalov, A. A.; Peeters, F. M. Carbon 2011, 49, 3258. (9) Hernandez-Nieves, A. D.; Partoens, B.; Peeters, F. M. Phys. Rev. B 2010, 82, 165412. (10) Sofo, J. O.; Chaudhari, A. S.; Barder, G. D. Phys. Rev. B 2007, 75, 15340. (11) Zhou, J.; Wang, Q.; Sun, Q.; Chen, X. S.; Kawazoe, Y.; Jena, P. Nano Lett. 2009, 9, 3867. (12) Ao, Z. M.; Peeters, F. M. J. Phys. Chem. C 2010, 114, 14503. (13) Xiang, H. J.; Kan, E. J.; Wei, S.-H.; Whangbo, M.-H.; Yang, J. L. Nano Lett. 2009, 9, 4025. (14) Xiang, H. J.; Kan, E. J.; Wei, S.-H.; Gong, X. G.; Whangbo, M.H. Phys. Rev. B 2010, 82, 165425. (15) Gao, H.; Wang, L.; Zhao, J.; Ding, F.; Lu, J. J. Phys. Chem. C 2011, 115, 3236. (16) Huang, L. F.; Ni, M. Y.; Zheng, X. H.; Zhou, W. H.; Li, Y. G.; Zeng, Z. J. Phys. Chem. C 2010, 114, 22636. (17) Lu, N.; Li, Z.; Yang, J. J. Phys. Chem. C 2009, 113, 16741. (18) Wu, M.; Wu, X.; Gao, Y.; Zeng, X. C. J. Phys. Chem. C 2010, 114, 139. (19) Pujari, B. S.; Kanhere, D. G. J. Phys. Chem. C 2009, 113, 21063. (20) Pujari, B. S.; Gusarov, S.; Brett, M.; Kovalenko, A. Phys. Rev. B 2011, 84, 041402(R). (21) Berashevich, J.; Chakraborty, T. Nanotechnology 2010, 21, 355201. (22) Elias, D. C.; Nair, R. R.; Mohiuddin, T. M. G.; Morozov, S. V.; Blake, P.; Halsall, M. P.; Ferrari, A. C.; Boukhvalov, D. W.; Katsnelson, M. I.; Geim, A. K.; Novoselov, K. S. Science 2009, 323, 610. (23) Jones, J. D.; Mahajan, K. K.; Williams, W. H.; Ecton, P. A.; Mo, Y.; Perez, J. M. Carbon 2010, 48, 2335. (24) Jaiswal, M.; Lim, C. H. Y. X.; Bao, Q. L.; Toh, C. T.; Loh, K. P.; € Ozyilmaz, B. ACS Nano 2011, 5, 888. (25) Ng, M. L.; Balog, R.; Hornekæ, L.; Preobrajenski, A. B.; Vinogradov, N. A.; Martensson, N.; Schulte, K. J. Phys. Chem. C 2010, 114, 18559. (26) Ryu, S.; Han, M. Y.; Maultzsch, J.; Heinz, T. F.; Kim, P.; Steigerwald, M.; Brus, L. E. Nano Lett. 2008, 8, 4597. (27) Sessi, P.; Guest, J. R.; Bode, M.; Guisinger, N. P. Nano Lett. 2009, 9, 4343. (28) Luo, Z.; Yu, T.; Ni, Z.; Lim, S.; Hu, H.; Shang, J.; Liu, L.; Shen, Z.; Lin, J. J. Phys. Chem. C 2011, 115, 1422.
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