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Chemical Modification: an Effective Way of Avoiding the Collapse of SWNTs on Al Surface Revealed by Molecular Dynamics Simulations Jie Xie,† Qingzhong Xue,*,† Keyou Yan,† Huijuan Chen,† Dan Xia,† and Mingdong Dong*,‡ College of Physics Science and Technology, China UniVersity of Petroleum, Dongying, Shandong 257061, People’s Republic of China, and Rowland Institute at HarVard, HarVard UniVersity, 100 Edwin H. Land BouleVard, Cambridge, Massachusetts 02142 ReceiVed: May 19, 2009; ReVised Manuscript ReceiVed: June 25, 2009
The rapid collapse of intrinsic single-walled carbon nanotube (SWNT) on the aluminum surface is observed using molecular dynamics simulation. The collapsing threshold is ∼10 Å, and the length has no influence on its collapse. Furthermore, we report that the structural stability of cylindrical SWNTs on the aluminum surface can be improved through the surface modification method. The stability of SWNTs can be enhanced by increasing the modification coverage. When the modification coverage exceeds 3.3% and 3.8% coverage, respectively, both amidogen- and carboxyl-modified SWNTs can basically maintain the cylindrical structure in our described systems. The results also show that, to avoid SWNTs collapse by chemical modification, the longer and larger SWNTs are, the more modification coverage SWNTs require, and vice versa. Our method allows potentially used modified SWNTs as nanocontainers for maintaining or transporting molecules, etc. 1. Introduction 1
Cylindrical carbon nanotubes (CNTs) display a fascinating variety of new physical and chemical properties ranging from ultrahigh-strength mechanical properties, to thermal conductivity, to electronic properties, to optical properties, and to acting as catalyst supports. Both the fundamental interest in CNTs materials and the promise of technological applications have motivated many researchers from various fields. All of these aspects strongly rely on the structurally and chemically complex of CNTs. For the intrinsic CNTs are made of networks of carbon atoms, the properties of CNTs strongly depend on the highly specific structural shape and size of cylinders. In addition, the hollow structure of CNTs allows to be used for hydrogen storage,2 nanocontainer,3-5 which can protect molecules inside the tubes from external reactive species, molecules transport,6-8 etc. Besides, it has been investigated to fill many kinds of molecules, such as atoms,9 biological molecules,10-12 and polymers,13,14 into single-walled nanotubes (SWNTs), and they can be used for drug delivery and fabrication of CNTs/polymer composites, etc. However, both experimental and theoretical studies have demonstrated that CNTs, especially for the intrinsic CNTs with large diameters, can be easily deformed and collapsed under certain external forces, such as van der Waals force,15 electron beam pressure,16 and hydrostatic pressure.17 Theoretical studies also show that the collapse of CNTs can be governed by the number of walls, the radius of the innermost wall18 and the chirality,19 etc. The properties of CNTs will be dramatically affected due to the collapse.20,21 Hence, it is a great challenge to provide an effective method to avoid the CNTs’ collapse. In this paper, we proposed a new method to enhance the cylindrical structural stability of SWNTs by means of the chemical modification with simplicity and functionality. The influence of diameter and length on the collapse of intrinsic * To whom correspondence should be addressed. E-mail: xueqingzhong@ tsinghua.org.cn (Q.X.);
[email protected] (M.D.). † China University of Petroleum. ‡ Harvard University.
SWNTs and modified SWNTs on the Al surface is also investigated by a series of molecular dynamics (MD) simulations.
2. Experimental Section Molecular mechanics (MM) and MD simulations have been carried out using a commercial software package called Materials Studio (MS) developed by Accelrys Software Inc. All MD simulations are performed in the NVT ensemble and a fixed time step size of 1 fs is used in all cases. The Andersen thermostat method is employed to control the system at the temperature of 300 K, and the interactions are determined within a cutoff distance of 9.5 Å. MM simulations are performed to find the thermal stable morphology and achieve a conformation with minimum potential energy for all SWNTs. The primary goal of simulations of the systems containing a large number of particles is generally to obtain the systems’ bulk properties which are mainly decided by the location of atomic nuclei. If a reasonable, physically based approximation of the potential (force-field), which can be used to generate a set of system configurations that are statistically consistent with a fully quantum mechanical description, can be gained, we could have a good insight into the behavior of a system. As stated above, the force-field is one of the most essential concepts. The force-field we use here is the COMPASS force-field. It is a parametrized, tested, and validated first ab initio force-field,22,23 which enables an accurate prediction of various gas-phase and condensed-phase properties of most of the common organic and inorganic materials.24-26 A brief overview of the force-field is given in the following.14 In general, the total potential energy of a molecular system includes the following terms:
Etotal ) Evalence + Ecross-term + Enon-bond
10.1021/jp904670u CCC: $40.75 2009 American Chemical Society Published on Web 07/15/2009
(1)
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Evalence ) Ebond + Eangle + Etorsion + Eoop + EUB
Xie et al.
(2)
∑ ∑ Fb′θ(b′-b0′)(b′-b0′) ×
Emiddle-bond-torsion )
b′
Ecross-term ) Ebond-bond + Eangle-angle + Ebond-angle + Eend-bond-torsion + Emiddle-bond-torsion + Eangle-torsion + Eangle-angle-torsion Enon-bond ) EvdW + ECoulomb + EH-bond
(3)
Eangle-torsion )
∑ ∑ Fθφ(θ - θ0) × [V1 cos φ + θ
φ
V2 cos 2φ + V3 cos 3φ] (14)
(4)
Eangle-angle-torsion )
∑ ∑ ∑ Kφθθ′ cos φ(θ - θ0) × φ
The valence energy generally includes a bond stretching term, Ebond, a two-bond angle term, Eangle, a dihedral bond-torsion term, Etorsion, an inversion (or an out-of-plane interaction) term, Eoop, and a Urey-Bradlay term (involves interactions between two atoms bonded to a common atom), EUB. The cross-term interacting energy, Ecross-term, accounts for the effects such as bond lengths and angles changes caused by the surrounding atoms and generally includes: stretch-stretch interactions between two adjacent bonds, Ebond-bond, bend-bend interactions between two valence angles associated with a common vertex atom, Eangle-angle, stretch-bend interactions between a two-bond angle and one of its bonds, Ebond-angle, stretch-torsion interactions between a dihedral angle and one of its end bonds, Eend-bond-torsion, stretch-torsion interactions between a dihedral angle and its middle bond, Emiddle-bond-torsion, bend-torsion interactions between a dihedral angle and one of its valence angles, Eangle-torsion, and bend-bend-torsion interactions between a dihedral angle and its two valence angles, Eangle-angle-torsion. The nonbond interaction term Enon-bond, accounts for the interactions between nonbonded atoms and includes the van der Waals energy, EvdW, the Coulomb electrostatic energy, ECoulomb, and the hydrogen bond energy, EH-bond. The COMPASS force-field uses different expressions for various components of the potential energy as follows
Ebond )
∑ [K2(b - b0)2 + K3(b - b0)3 + K4(b - b0)4] b
(5)
∑ [H2(θ - θ0)2 + H3(θ - θ0)3 + H4(θ - θ0)4]
Eangle )
θ
(6) Etorsion )
∑ [V1[1 - cos(φ - φ01)] + φ
V2[1 - cos(2φ - φ02)] + V3[1 - cos(3φ - φ03)]] (7) Eoop )
∑ Kxχ2
(8)
x
Ebond-bond )
∑ ∑ Fbb(b - b0)(b′-b0′)
(9)
∑ ∑ Fθθ′(θ - θ0)(θ′-θ0′)
(10)
∑ ∑ Fbθ(b - b0)(θ - θ0)
(11)
b
Eangle-angle )
θ
Ebond-angle )
b
Eend-bond-torsion )
b′
θ′
θ
∑ ∑ Fbθ(b - b0) × [V1 cos φ + b
θ
[F1 cos φ + F2 cos 2φ + F3 cos 3φ] (13)
θ
V2 cos 2φ + V3 cos 3φ] (12)
θ
θ′
(θ′-θ0′) (15) ECoulomb ) EvDW )
∑ i>j
[
qq
∑ εri ijj i>j
Aij rij9
-
Bij rij6
(16)
]
(17)
where q is the atomic charge, ε is the dielectric constant, and rij is the i-j atomic separation distance, b and b′ are the lengths of two adjacent bonds, θ is the two-bond angle, φ is the dihedral torsion angle, and χ is the out-of-plane angle. b0, ki(i ) 2 - 4), θ0, Hi(i ) 2 - 4), φ0i (i ) 1 - 3), Vi(i ) 1 - 3), Fbb′, b0′, Fθθ′, θ0′, Fbθ, Fbφ, Fb′θ, Fi(i ) 1 - 3), Fθφ, Kφθθ′, Aij, and Bij are fitted from quantum mechanics calculations and are implemented into the Discover module of MS. Since there are many experiments have been conducted to fabricate the CNTs/Al composites,27-32 we selected Al surface as our studied surface in our simulations. A super cell of 57.3 × 57.3 × 88.2 Å3, which consists of a 18.3 Å Al (001) lattice plane, was built by MS. The Al surface here is used for providing SWNTs an external force. SWNT (14, 14) was built by MS and the unsaturated boundary effect of CNT was avoided by adding hydrogen atoms. Each C-C bond length was 1.42 Å and C-H bond length was 1.14 Å. The hydrogen atoms had charges of +0.1268 e and the carbon atoms connecting hydrogen atoms had charges of -0.1268 e; thus, the neutrally charged SWNTs were constructed. 3. Results and Discussion Similar to the behavior observed in Cu2O surface systems, when the diameter of SWNTs exceeds a threshold (about 10 Å), the SWNTs approach to the Al surface and collapse spontaneously by the van der Waals force.33 Using MD simulations, Figure 1 shows six snapshots of the temporal evolution of a CNT for an initial 3.7 nm long intrinsic SWNT (14, 14) placed on top of the Al surface with a distance of about 6 Å to achieve an equilibrium state. The real-time visualization of a CNT’s collapse (from 0 to 300 ps) has been recorded (Supporting Information, Movie S4). In the initial stage, the nanotube-substrate interaction makes nanotube approach the Al surface, and the cross-section of the SWNT (14, 14) stretches from a circle (0 ps) to an oval (34 ps) along the normal direction of the plane to try to contact the Al surface. Once the bottom of SWNT reaches the Al surface (40 ps), the interwall interaction (π-π interaction) begins to be the main force to induce the collapse of the SWNT, and the SWNT rapidly transforms into a linked double graphitic layers paralleling to the plane like a ribbon on top of the Al surface (55 ps). The details of the coordination profile (Supporting Information, Table S1) specifically reflect the geometric deformation of the SWNT, 4 in Table S1 denotes the difference between the coordinates along the three axis (x, y, z axis indicated in the
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Figure 1. Snapshots of intrinsic SWNT (14, 14) on the Al surface from approach to collapse at 0, 34, 40, 45, 55, and 300 ps.
TABLE 1: Geometric Sizes of Intrinsic SWNT (14, 14) for Both Initial and Final State on the Al Surface Obtained Based on Coordination Profiles of Figure 1 initial SWNT (14, 14)
4x (Å)
4y (Å)
4z (Å)
initial (0 ps) final (300 ps)
37.0 37.0
18.3 27.1
18.3 3.7
Figure 1) for initial SWNT and final equilibrium state SWNT, respectively. The detail of geometric size (Table 1) can be obtained from the coordination details of the profiles. 4yinitial and 4zinitial of the initial SWNT are approximately equal, which
indicates that the circular tube of the SWNT (14, 14) is about 18.3 Å in diameter, and 4xinitial ) 4xfinal ) 37.0 Å denotes equal length for the initial- and final-state SWNTs. Comparing with initial size of CNT, the final equilibrium size of SWNT along y axis (4yfinal - 4yinitial) increases by 8.8 Å and along z axis (4zfinal - 4zinitial) decreases by 14.6 Å, which means that the SWNT (14, 14) has fully collapsed on the Al surface showing a 37.0 × 27.1 × 3.7 Å3 ribbon structure. Using MD simulations, we put (14, 14) tubes with different length, which varies from 49.19 to 147.57 Å, on the Al surface to study the influence of length on the collapse of the SWNT. It is observed that all the SWNTs with different length collapse on the Al surface easily and quickly, and the final states are shown in Figure 2a. The adsorption energy (Eads) is estimated from the energy difference between the total energy (Etotal) and the energies of the Al surface (EAl) and the intrinsic SWNT (ESWNT) as the following equation: Eads ) Etotal - (EAl + ESWNT),14 and the adsorption energy per Å2 (Eper) is defined as the following equation: Eper ≈ (Eads)/(Lπr), where L and r denote the length and radius of the SWNT, respectively. From Figure 2b and c, we can find that the interaction energy between Al surface and the SWNTs linearly increases with increasing length and the Eper for all systems are approximately equal, both of which indicate that the length has no influence on the collapse of the intrinsic SWNTs on the Al surface at all. The ability to modify the surface of CNTs with chemical groups is important for their utilization in different applications. Covalently linking chemical groups to CNTs has been extensively used in many applications. To gain further insight into the chemical modification influencing CNTs, amidogen- (-NH2) or carboxyl- (-COOH) modified CNTs have been selected in this theoretical study. The chemical modification of the SWNTs has been performed by attaching functional groups to the surface of the SWNTs through chemical covalent bonding, and the functional groups were end-grafted to the surface of the SWNTs randomly. Initially, the modified SWNTs are placed at similar
Figure 2. (a) Collapse of SWNTs with different length on the Al surface, (b) the adsorption energy, and (c) the adsorption energy per Å2 between Al surface and SWNTs with different length.
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Figure 4. Interaction energy between Al surface and modified SWNT with different modification amount.
Figure 3. Final configurations of the SWNTs with different amounts of -NH2 groups modification in the Al surface system.
position on the Al surface with a distance of about 6 Å as the intrinsic SWNT simulation. When the system achieves an equilibrium state, the final configurations can be obtained. By changing the -NH2 functional group coverage on SWNTs (1.0%, 1.4%, 1.9%, 2.9%, 3.3%, 3.8%), the systemic study has been carried out. The final configurations of the -NH2-modified SWNTs with different modification amounts on the Al surface is shown in Figure 3. For the modification amount of ≈1.0% and 1.4%, it is found that both of them reach the Al surface and totally collapse which are consistent with intrinsic CNT’s results. However, the times to complete collapse dramatically increase. The real-time visualization of a 1.0% -NH2 functional CNT collapse (from 0 to 400 ps) has been recorded (Supporting Information, Movie S5). By further increasing the coverage until 1.9% and 2.9%, both of the modified SWNTs collapse partially after 1500 ps and behave stably on the Al surface in the shape shown as Figure 3. Surprisingly, the SWNTs with 3.3%, 3.8% -NH2 coverage do not fully collapse and their structures can maintain the cylindrical shape with slight deformation. For all of -NH2 coverage, Table 2 shows the detail of geometric size obtained from the coordination details of the profiles for both the initial- and final-stage SWNTs, respectively (Supporting Information, Table S2). In fact, the coordination differences in x, y, and z directions are very small for the 3.3%, 3.8%. The real-time visualization of a 3.8% -NH2-functional SWNT approaching to the Al surface (from 0 to 1500 ps) has been recorded (Supporting Information, Movie S3). The maximum
size difference is only 3.2 Å along the z direction, reflecting the strong geometric stability for 3.8% -NH2 coverage SWNTs. The chemical modification plays a vital role in maintaining SWNTs’ structures. According to MD simulation, the modification amount is above 3.3% -NH2 coverage, the collapse of SWNTs can be avoided. Therefore, surface modifications of CNTs not only introduce new functionality, but also effectively enhance the structural stability. By comparing the energy profile of the adsorption systems for the different amount of -NH2-modified SWNTs on Al, the SWNT and surface interaction can be determined. As shown in Figure 4, we can find that the interaction energy between modified SWNT and the Al surface becomes weaker with increasing amount of -NH2 group surface coverage, which well indicates that the modification increases the distance between the SWNTs and the Al surface thus weakens the influence of the external force on SWNTs, so that the small modification, ≈3.3% of -NH2, is already efficient for maintaining cylindrical structure of SWNTs in our described systems. Another reason for the fact that modified SWNTs have a higher stability than the intrinsic SWNT and avoid the collapse of the SWNTs on the Al surface maybe the functional groups allow the transformation of the hybrid orbital of the carbon atoms in the SWNTs. For the unmodified SWNT, the hybrid orbital of each carbon atom is sp2 and the configuration is planar (Figure 6a). After the modifications, the hybrid orbital of the carbon atom connected with functional group becomes sp3 and the planar configuration transforms into tetrahedron locally (Figure 6b, c). Therefore, chemical modifications of SWNT destabilize the π-π stacking of the linked collapsed CNT layers and the local tetrahedrons in the SWNTs provide the additional structure strength to the cylindrical structures. Similarly, SWNTs with different amount of -COOH groups are also simulated on the Al surface. By changing the -COOH functional group coverage on SWNTs (1.0%, 1.9%, 2.9%, 3.3%, 3.8%, and 4.3%), the systemic study has been carried out (shown
TABLE 2: Geometric Sizes of -NH2-Modified SWNT (14, 14) for Both Initial and Final State with Modification Amounts of 1.0%, 1.4%, 1.9%, 2.9%, 3.3%, and 3.8%, Respectively coverage (%) 1.0 1.4 1.9
initial final initial final initial final
4x (Å)
4y (Å)
4z (Å)
37.0 37.0 37.0 37.0 37.0 N/A
18.3 26.6 17.2 27.1 18.3 N/A
18.2 3.7 18.2 3.7 18.7 N/A
coverage (%) 2.9 3.3 3.8
initial final initial final initial final
4x (Å)
4y (Å)
4z (Å)
37.0 N/A 37.0 37.0 36.5 36.9
18.3 N/A 18.3 16.7 19.3 19.8
17.6 N/A 17.6 18.7 17.7 14.5
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Figure 5. Final configurations of the SWNTs with different modification amount of carboxyl groups on the Al surface.
J. Phys. Chem. C, Vol. 113, No. 33, 2009 14751 about 3.8%, the -COOH-modified SWNTs can maintain the cylindrical structures with slight deformation. Finally, take -NH2-modified SWNTs as an example, MD simulations are conducted to study the influence of diameter and length on the collapse of SWNTs on the Al surface. We modify SWNTs with different lengths or diameters with same modification of ≈3.3%. Figure 7 shows the final configuration of different sizes of the SWNTs on the Al surface at 1000 ps. It is found that, with same modification coverage and diameter, only the SWNT with a length of 24.60 Å can basically maintain SWNT’s cylindrical structure with slight deformation. For the other longer SWNTs, they collapse partially and are in the shape shown as Figure 7a. As shown in Figure 7b, with same modification coverage and length, the stability of SWNTs with large diameter is worse than that of SWNTs with small diameter. With increasing diameter, the stability of the SWNT decreases, which implies that the suitable modification coverage to well avoid the collapse of the SWNTs should increase with increasing diameter. This is maybe because that, as described above, the adsorption energy between SWNT and the Al surface becomes stronger with increasing length and diameter, thus more chemical modification coverage is required to weaken the interaction between them to avoid the SWNT’s collapse. Therefore, we can conclude that only SWNTs with certain length, diameter and certain modification coverage can well avoid their collapse. Generally, to avoid SWNTs collapse, the longer and larger SWNTs are, the more modification coverage SWNTs require, and vice versa. 4. Conclusions
Figure 6. Configuration of (a) the unmodified carbon atoms, the carbon atom connected with a (b) -NH2 group, (c) -COOH group.
in Figure 5). The details of the coordination profile (Supporting Information, Table S3) reflect the geometric deformation of the SWNTs. The simulations show that the -COOH-modified SWNTs exhibit the same trend as the -NH2-modified SWNTs on the Al substrate. When the modification amount exceeds
In summary, MD simulations were conducted to study the behaviors of the intrinsic SWNT and modified SWNTs on the Al surface. We find that the intrinsic SWNT (14, 14) collapses easily on the Al surface, and the length of the SWNT has no influence on its collapse. The influence of chemical modification on the SWNTs on Al surface has been systemically studied by MD simulations. The simulations show that the collapse degree of the SWNTs decreases with increasing modification amount of -NH2 groups. When the modification coverage is more than 3.3% on the carbon surface, the modified SWNTs can maintain their cylindrical structure with slight deformation. The -COOHmodified SWNTs with different coverage on the Al surface exhibit a similar trend as the -NH2 SWNTs, and also, the collapse of modified SWNT can be greatly influenced by its
Figure 7. Final configurations of the SWNTs with same modification coverage, (a) different L (length) and same diameter, (b) different D (diameter) and same length at 1000 ps.
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diameter and length; thus, we should properly select the suitable size of the SWNT with certain modification coverage to well avoid the collapse of SWNTs. All of our simulated systems described above well prove that few chemical modification can slow down SWNTs collapse process, and some suitable chemical modification can well avoid the collapse of SWNTs. This study provides an effective and simple way to avoid the collapse of SWNTs. The stable geometric structure of modified CNTs allows strongly controlling their electronic and mechanical properties. In addition, the chemically modified CNTs with the stable structural property show great potential for the use as a nanocontainer for many applications such as biomolecule transportation, drug delivery, hydrogen storage, etc. Acknowledgment. This work is supported by 973 National Basic Research Program (2008CB617508), Cultivation Fund of the Key Scientific and Technical Innovation Project, Ministry of Education of China (708061), and Program for New Century Excellent Talents in University (NCET- 08-0844). Supporting Information Available: Experimental method, video of the dynamic process of the intrinsic CNT landing on the Al surface (from 0 to 300 ps) in Figure 1, the real-time visualization of a -NH2-functional CNT (1.0%; 3.8%) approaching to the Al surface (from 0 to 400 ps; from 0 to 1500 ps). This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Iijima, S. Nature 1991, 354 (6348), 56–58. (2) Dillon, A. C.; Jones, K. M.; Bekkedahl, T. A.; Kiang, C. H.; Bethune, D. S.; Heben, M. J. Nature 1997, 386 (6623), 377–379. (3) Takenobu, T.; Takano, T.; Shiraishi, M.; Murakami, Y.; Ata, M.; Kataura, H.; Achiba, Y.; Iwasa, Y. Nat. Mater. 2003, 2 (10), 683–688. (4) Shiozawa, H.; Pichler, T.; Gruneis, A.; Pfeiffer, R.; Kuzmany, H.; Liu, Z.; Suenaga, K.; Kataura, H. AdV. Mater. 2008, 20 (8), 1443–1449. (5) Yanagi, K.; Miyata, Y.; Kataura, H. AdV. Mater. 2006, 18 (4), 437– 441. (6) Insepov, Z.; Wolf, D.; Hassanein, A. Nano Lett. 2006, 6 (9), 1893– 1895. (7) Whitby, M.; Cagnon, L.; Thanou, M.; Quirke, N. Nano Lett. 2008, 8 (9), 2632–2637.
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