Graphene Self-Assemble into Core−Shelled Composite

ARTICLE pubs.acs.org/JPCC

How Do Metal/Graphene Self-Assemble into Core
Shelled Composite Nanostructures? Y. F. Li,† H. Q. Yu,† H. Li,†,* C. G. An,† K. Zhang,† K. M. Liew,‡ and X. F. Liu† †

Key Laboratory for Liquid
Solid Structural Evolution and Processing of Materials, Ministry of Education, Shandong University, Jinan 250061, People’s Republic of China ‡ Department of Building and Construction, City University of Hong Kong, Kowloon, Hong Kong

bS Supporting Information ABSTRACT: Molecular dynamics (MD) simulations were carried out to study the self-assembly of graphene and metallic particle. The metallic particle can help the graphene overcome the energy barrier, which leads to rapid self-scrolling of flat graphene and the formation of stable core
shelled composite nanostructure. The van der Waals interaction plays an important role in the self-assembly. The chirality of the graphene does not affect the selfscrolling process, which thus provides a simple way of controlling the chiralities and the physical properties of the resulting conformations. This work opens new and exciting possibilities for the fabrication of metal/ carbon core
shelled composite nanostructures through the self-scrolling of graphene.

1. INTRODUCTION Metallic nanowires (NWs) with small size and low dimensionality have exhibited extraordinary properties different from bulk metals1 and a wide range of potential applications in many areas.2
5 Metallic NWs have already been synthesized but are not suitable for handling since they are sensitive to oxidation.6,7 Furthermore, metallic wires with diameters smaller than 70 nm are very unstable at room temperature, resulting in fragmentation within a few hours during sample fabrication under condition of external pressure load.8 If the metallic NW were completely surrounded by a carbon nanotube (CNT)9,10 and therefore protected from oxidation and from shape fragmentation,11 then they would be ideal to study the fundamental physical and chemical properties that are essential prior to their implementation as nano-building blocks in nanoscale devices. Over the past decade, considerable effort has been made to explore various heterostructures with the combination of CNTs and NWs. On the one hand, these inner component materials alter the mechanical, electrical properties of pristine CNTs. On the other hand, inner component materials are completely surrounded by CNTs and therefore protected from oxidation and shape fragmentation. Various techniques such as chemical vapor deposition (CVD), wet chemical techniques, and plasma irradiation have been used to manufacture metallic particles encapsulated in CNTs.12
15 Actually, it is extremely difficult to insert metals into the CNTs, which needs the help of high temperature, catalyst, or other external energy12
14 because of the wetting and the small size effect. Therefore, exploration of a new method to fabricate the heterostructures with the combination of carbon and metals is currently in great demand. Recently, graphene sheets have r 2011 American Chemical Society

attracted intense interest due to their fascinating physical properties such as quantum electronic transport,16 tunable band gap,17 thermal conductivity,18 high elasticity,19,20 flexibility,21 and electromechanical modulation.22 Flat graphene may very well be a promising material in the nanoscale building blocks of new composite materials in the near future. Considerable effort has been made to reveal that graphene is thermodynamically unstable because it is a two-dimensional (2D) crystal (strictly twodimensional structure is not stable). Its thermodynamic stability is largely due to out-of-plane corrugations. A graphene sheet has an inclination to self-scroll to become a three-dimensional (3D) structure because 3D structures are thermodynamically stable.23 What we are concerned with is whether the extended graphene sheet can self-scroll spontaneously. It would be much better if people could trigger the self-scrolling of graphene in a controlled manner that ensures the flat graphenes and metallic nanoparticles self-assemble to produce core
shelled composite nanostructures instead of the insertion of the metallic particles into CNTs, which would guarantee a higher level of sophistication for future nanoscale devices. In this work, we reveal that graphenes can fully self-scroll onto the Ni particles to form the nickel/carbon core
shelled composite nanostructures using molecular dynamics (MD) simulations. Our findings are not only helpful for the better understanding of the properties of graphene at anatomistic level, but also provide crucial simulation input to help guide fabrication of Received: November 25, 2010 Revised: February 28, 2011 Published: March 23, 2011 6229

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Figure 1. Snapshots of core
shelled nanostructure produced by selfscrolling of a graphene motivated by the Ni NW located at on end.

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Figure 2. Variations of the scrolled length (L) and the self-scrolling velocity (V) of the graphene versus time.

the metal/carbon core
shelled composite structure with high performance.

2. COMPUTATIONAL METHODS In MD simulations, the force-field of condensed-phase optimized molecular potentials for atomistic simulation studies (Compass)24 was used to model the atomic interactions. This is an ab initio force field that is parametrized and validated using condensed-phase properties in addition to various ab initio and empirical data, which aims to achieve high accuracy in prediction of the properties of very complex mixtures25,26 and it has been proven to be applicable in describing the mechanical properties of CNTs.27,28 MD simulations were performed under an NVT (the volume and the temperature were constant) ensemble at room temperature, 298 K. The Nose method was employed in the thermostat to control the temperature. The thermodynamic temperature was kept constant by allowing the simulated system to exchange energy with a “heating bath”. The Verlet algorithm was adopted to integrate the motion equations for the whole system. The time step was chosen to be 1.0 fs, and data were recorded at intervals of 0.1 ps for further analysis. In this work, we considered different graphene sheets and Ni NWs. At all initial configurations, a series of Ni NWs were aligned parallel to one side of the graphene sheets with separations of 5.0 Å. Each system was simulated long enough to achieve an equilibrium state. The velocity of the selfscrolling was obtained as an average of each 1.0 ps. 3. RESULTS AND DISCUSSION Figure 1 illustrates the self-assembly process that an armchair graphene scrolls onto the Ni NW to produce core
shelled composite nanostructure spontaneously (See Video 1 of the Supporting Information for the detailed self-assembly process). The size of the armchair graphene ribbon is 88.6  318.0 Å2 and the diameter of the Ni NW is 11.2 Å. First, the Ni NW is placed on one end of the graphene with a separation distance of 5.0 Å above the surface as shown in Figure 1(a). Then, the graphene tightly adheres to the NW and begins to scroll. Figure 1(c) indicates that the graphene spontaneously wraps Ni NW completely to form a coil with a tail just like a tadpole. After the Ni

Figure 3. Snapshots of the self-scrolling of the graphene wrapped on a Ni NW located in the middle.

NW is totally wrapped, the self-scrolling does not stop but continues to accelerate. The tadpole-like part moves forward and the tail part winds in clockwise direction to overlap. At 57 ps, the remaining part does not wrap but starts to fold and slide (as shown in Figure 1(f)
(h)). Eventually, a coaxial core
shelled composite structure forms. To better understand the self-scrolling of graphene, Figure 2 gives the scrolled length (L) and the scrolling velocity (V) of the graphene with time. The scrolled length curve with time is nonlinear, indicating that the self-scrolling is not a uniform motion. The self-scrolling process has three distinct stages, as denoted by vertical lines. In the first stage (from beginning to 20 ps), the graphene is attracted on the Ni NW and begins to curl. The second stage (from 20 to 57 ps) is the self-scrolling in which the NW acts as axis and rotates together with graphene. The rotation accelerates and the speed of the self-scrolling reaches to a maximum. In the third stage, the rotation is completed and the slide starts. The inset is the snapshot at 57 ps. A turning point on the curve of the scrolled length at 57 ps indicates that the speed of the slide is smaller than the rotation. The average self-scrolling speed of the whole process can be up to 3.54 Å /ps (354 m/s). Figure 3 shows the snapshots of the self-scrolling of graphene when the Ni NW is located at the middle of the graphene (Video 2 of the Supporting Information). At 4 ps, the graphene starts to warp the NW. After the two sides meet, they contact each other 6230

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Figure 5. Double-core
shelled nanostructure prepared by self-scrolling of the graphene when two NWs are located at two ends and the corresponding change of the potential energy versus time. Figure 4. The changes of potential energy (a) and van der Waals energy (b) of the whole system with time when Ni NWs is at one end and in the middle of the graphenes. (c) The MSDs of the NWs in the direction vertical to the graphene surface during the simulation.

and fold tightly in opposite directions. The two ends move upward and downward until they form a knot structure with double graphene shells. The measured average adjacent layer distances of the scrolled graphenes in core
shelled nanostructures are about 3.45 Å, which are very close to the interlayer distance in the multiwalled CNTs. The average separation between the inner layer of the graphene and Ni NW is 2.96 Å, very close to the chemical bond lengths, indicating that the interaction between the graphenes and the Ni NWs is very strong. What we are concerned with is what makes the graphene sheet self-scroll. In principle, an isolated GPH can only lower its energy by bending to form corrugation to keep its 2D structure flat. According to the Mermin
Wagner theorem,29 longwavelength fluctuations destroy the long-range order of 2D crystals. These fluctuations can be suppressed by anharmonic coupling between the bending and stretching modes, meaning that a 2D membrane can exist but will exhibit strong ripples and corrugations.30 Theoretical investigations of 2D membranes have predicted their thermodynamic stability through static microscopic crumpling involving either bending or buckling. Although the corrugations are intrinsic to graphene membranes,30 the transition of an isolated, flat graphene membrane into a smaller, folded package is problematic. This is because the elastic energy of the graphene always tends to keep the graphene flat and generates an energy barrier for the structural transition. When Ni NWs come in, they can act as catalysts for graphene deformation: the van der Waals energy between the graphene and the Ni NW can help the graphene to overcome the energy barrier and provide attractive force to drive the graphene curl. Therefore, the self-scrolling of the graphenes is determined by the competition between the external C
Ni van der Waals interaction and the elastic potential which sustains the intrinsic planar state of graphene. In order to disclose the mechanism of the self-scrolling, Figure 4 gives the change of total potential energy ΔEP (a) and van der Waals energy ΔEvdW (b). It must be pointed out that, at 5 ps, each curve has an energy transition point at which the

graphene fully contacts the NW. The energy transition corresponds to the energy barrier generated by the elastic energy, which tries to prevent the self-scrolling of graphene. The energy barrier is 1.28 Mcal/mol when the NW is located at one end, higher than that of 1.17 Mcal/mol when the NW is placed in the middle. If the energy barrier is overcome, scrolling would take place. It happens that the Ni NWs help the graphene overcome the energy barrier and cause the scrolling. During the selfscrolling process, the intrinsic van der Waals energy of the system is partially converted into kinetic energy, which sustains the selfscrolling. This suggests that the van der Waals interaction should be the driving force. With the successive scrolling, the system potential energies decrease and the structures move to more stable states, indicating that the self-scrolling of the graphenes onto the Ni NWs is spontaneous. Therefore, the self-scrolling of the graphene provides a simple method to produce metal/carbon core
shelled nanostructure and it is far less energy and material demanding than the insertion of the metallic particles into CNTs. Figure 4(c) shows the mean square displacements (MSDs) of the Ni NWs in the direction perpendicular to the graphene surfaces. During the self-scrolling processes, the Ni NWs are found to be deformed owing to the strong interaction though they are solid cylinders and possess excellent geometric stability. After the selfscrolling of the graphenes finishes, the Ni NW recovers to its original shape to some extent due to the symmetrical forces along the radial direction. Figure 5 illustrates the self-scrolling of graphene triggered by two NWs located at two different sides (Video 3 in Supporting Information) and the corresponding change of potential energy versus time. The competition between two NWs results in the scrolls moving synchronously toward the middle of the graphene. Figure 5(c) shows that two scrolls form a barbell-like structure. Due to high speed, when two parts encounter at 53 ps, they bump and rebound. Finally, the system forms a stable double-core
shelled composite nanostructure. The energy change of this process is not very different from that of the self-scrolling process with a single Ni NW. The energy decreases with the successive self-scrolling of the graphene, and balances after the scrolling of graphene is over. The size effect of Ni NWs on the self-scrolling of the graphene is also simulated in this work due to the fact that the self-scrolling of the graphenes is determined by the competition between the 6231

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Figure 6. The snapshots of the same graphene self-scrolling onto Ni NWs with different sizes and shapes.

external C
Ni van der Waals interaction and the intrinsic elastic potential. We simulate the same graphene (46.74  83.797 Å2) scrolling onto different radii and sharp Ni NWs to calculate the size effect on the adhesion. However, when the radii of the Ni NWs are less than a threshold, namely 4 Å, the Ni NWs cannot keep their structural stability the integrality while simulating. So we choose Ni NWs with varying radii, from 4 to 13 Å, to simulate the interaction between the graphenes and the Ni NWs. The final configurations between the graphenes and five cylindrical Ni NWs with various radii and two rectangular Ni NWs are shown in Figure 6. It is obvious that all cylindrical and rectangular Ni NWs can help the graphenes to overcome the energy barrier and induce the graphenes self-scrolling onto them. The graphenes just scroll the cylindrical 10.0 Å radius NW and the rectangular NW with 5.6 Å in width and 21.1 Å in length around, while the graphenes self-scrolling onto the 12.8 Å radius NW form an arc, and onto the others are overlapped. Therefore, the size of the final composite structures can be controlled through the dimensions of Ni NW and graphene. From the size-effect study and the geometry-configuration analysis, the relation between the scroll shape and the NW position is given as follows: ( 0 e D e πðRNW þ dÞ RNW g 4Å scroll RNW g 4Å knot D > πðRNW þ dÞ where D is the axial position of the NW. RNW is the radius of the NW, and RNW g 4 Å indicates that Ni NWs can keep their own geometric stabilities. d = 2.96 Å is the average distance between the NW and the self-scrolled graphene sheet. The length of the graphene L must fit the equation: L g 2πðR þ dÞ If the above equation is not satisfied, then the graphene will paste to the NW to form an arc. When L = 2π (R þ d), a core
shelled structure with a single-layered shell forms. But when L > 2π (R þ d), the graphene shell would overlap and form multilayered scroll or knot nanostructure. It is well-known that the chirality of the graphene sheets has a significant effect on the properties of the graphene, which can change its type from quasi-metallic to semiconducting.31 To investigate the effect of the graphene chirality on the mechanical properties of the self-scrolling process and composite system, we simulate a 10-Å radius NW interacting with graphene sheets with different chiralities but with nearly the same sizes. Figure 7 shows the Ni/Graphene core
shelled composite nanostructures

Figure 7. Snapshots of the core
shelled composite nanostructures formed by the van der Waals force between Ni NWs and graphenes. The nanoscrolls formed with different chiralities.

Figure 8. The decreases of the potential energies after the self-scrolling of the different chiral graphene sheets onto the same Ni NWs.

formed by the self-scrolling of the graphene sheets with different chiralities after the MD simulations. The graphenes with finitesize sheets have special orthotropic behavior and positive Poisson ratios.32,33 From Figure 7, we can observe that different core
shelled nanostructures are formed by rolling up graphene sheets of different chiralities having nearly the same top views. The nanoscrolls formed exhibit the different chiralities like that of the CNT structures. There are no differences from the configurations of the core
shelled nanostructures. To further clarify the influence of the chirality on the adhesion of the Ni/graphene core
shelled composite structure, we calculate the decreases of the potential energies before and after the self-scrolling of different chiral graphene sheets, as shown in Figure 8. We can observe that the graphene chirality has a negligible influence on the adhesion between the Ni NW and the graphene. We also determine the average distances between Ni NWs and self-scrolled graphene sheets of all seven of these composites. All of these distances are about 3.0 Å, which is almost in the strong-adhesive binding region of the chemical bond. This also indicates that the graphene wrapping onto Ni NWs results in stable core
shelled structures with only little influence of the chirality of the graphene sheet. Because the physical properties of the graphene sheets can be controlled by varying their chirality, it 6232

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Figure 9. The self-scrolling of the graphene onto the Ni particles to build nanodumpling. (a)
(d) one and (e)
(h) two graphene sheets.

is important to form different kinds of metal/graphene core
shelled composites with different chiralities. Through this simple self-scrolling, we can produce different types of heterojunction materials, including the metal/semiconductor, and metal/metal types of metal/graphene core
shelled composites, which are promising candidates for various applications including nanomechanical devices or nanocircuits. To extend this work to the self-assembly of graphene and spherical particles, graphene with sizes of 134.1  133.3 Å2 and the spherical Ni particle containing 2700 atoms are selected for better understanding. The self-assembly of one and two graphene sheets with 3.4 Å separation and the Ni particles are shown in Figure 9. First, the graphenes are approaching the particles, and then self-scroll onto the particles like making dumplings, as shown in Figure 9(d) and (g). One layer nanodumpling is stable. While in the dumpling with two graphene shells, remarkably, after the Ni particle has done the work of folding the two graphene sheets, it tends to be expelled from the resulting structure [Figure 9(h)]. We also studied the interaction between graphene and some other metallic particles such as Cu, Al, Ag, Fe, Sn, and so forth. All results demonstrated that the graphenes can spontaneously selfscroll onto these metal particles by van der Waals forces. Depending on different species and positions, the metallic particles and graphenes can be controlled to self-assemble various heterogeneous core
shelled nanostructures with different properties, such as coaxial and double-core NWs, knots or dumplings. These core
shelled composite nanostructures have potential advantages for exploring new concepts and functional nanodevices, such as electronics, catalysis, sensor, optoelectronics, and thermoelectrics.

4. CONCLUSIONS MD simulations have been performed to study the selfassembly of graphene and the metallic nanoparticle, in which the graphene spontaneously self-scrolls onto the particle and forms a stable core
shelled composite nanostructure. The decline of the potential energy of the system suggests that the

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self-assembly is a spontaneous phenomenon and the system is increasingly stable during this process. The van der Waals interaction between graphene and the metallic particle is the driving force to drive the self-scrolling. The chirality of the graphene sheet is found to have no strong influence on the self-scrolling process. The size and the chirality of the final composite structure can be controlled through the dimensions of the metallic particles and graphenes and the chirality of the graphene sheets. These core
shelled composite nanostructures represent an important class of nanoscale building blocks for exploring functional nanodevices. This work provides a criterion for predicting and designing the final composite structure based on the individual requirements. The self-scrolling of the graphene is a new route to produce the metal/carbon core
shelled nanostructure, which is more convenient and simple than the encapsulation of the metallic particle into CNTs. More importantly, the above-mentioned discoveries are of great significance in the exploration of the properties of the self-scrolling of graphene on metallic NWs, which may expand the applications of graphene to some other extensive fields.

’ ASSOCIATED CONTENT

bS

Supporting Information. (Video 1) The graphene selfscrolls onto the Ni NW to produce core
shelled composite nanostructure. (Video 2) The self-scrolling of graphene when the Ni NW is placed in the middle. (Video 3) The self-scrolling of graphene triggered by two NWs located at two different sides. This material is available free of charge via the Internet at http:// pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT We would like to acknowledge support from the National Natural Science Foundation of China (Grant Nos. 50971081 and 50831003) and the National Basic Research Program of China (2007CB613901). We also thank the Natural Science Fund for Distinguished Young Scholars of Shandong (JQ200817) for support. This work is also supported by the Natural Science Fund of Shandong Province (ZR2009FM043), by the PhD. Dot Programs Foundation of the Ministry of Education of China (No. 20090131110025), by the National Science Fund for Distinguished Young Scholars (No. 2009JQ014), and by the Independent Innovation Foundation for Students (No. 31370070613157) from Shandong University. ’ REFERENCES (1) Rabkin, E.; Nam, H. S.; Srolovitz, D. J. Acta Mater. 2007, 55, 2085. (2) Duan, X. F.; Niu, C. M.; Sahi, V.; Chen, J.; Parce, J. W.; Empedocles, S.; Goldman, J. L. Nature 2003, 425, 274. (3) Tian, B.; Zheng, X. L.; Kempa, T. J.; Fang, Y.; Yu, N. F.; Yu, G. H.; Huang, J. L.; Lieber, C. M. Nature 2007, 449, 885. (4) Zhou, J. C.; Gao, Y.; Martinez-Molares, A. A.; Jing, X.; Yan, D.; Lau, J.; Hamasaki, T.; Ozkan, C. S.; Ozkan, M.; Hu, E.; Dunn, B. Small 2008, 4, 1507. (5) Tian, M.; Wang, J.; Snyder, J.; Kurtz, J.; Liu, Y.; Schiffer, P.; Mallouk, T. E.; Chan, M. H. W. Appl. Phys. Lett. 2003, 83, 1620. 6233

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