First Principles Simulation of Molecular Oxygen Adsorption on SiC Nanotubes

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1 Commun. Theor. Phys. (Beijing, China) 53 (2010) pp c Chinese Physical Society and IOP Publishing Ltd Vol. 53, No. 4, April 15, 2010 First Principles Simulation of Molecular Oxygen Adsorption on SiC Nanotubes M.D. Ganji and B. Ahaz Department of Chemistry, Islamic Azad University, Ghaemshahr Branch, Mazandaran, Iran (Received April 8, 2009; revised manuscript received July 3, 2009) Abstract We study the binding of molecular oxygen to a (5, 0) single walled SiC nanotube, by means of density functional calculations. The center of a hexagon of silicon and carbon atoms in sites on SiCNT surfaces is the most stable adsorption site for O 2 molecule, with a binding energy of ev and an average Si O binding distance of Å. We have also tested the stability of the O 2 adsorbed SiCNT/CNT with ab initio molecular dynamics simulation which have been carried out at room temperature. Furthermore, the adsorption of O 2 on the single walled carbon nanotubes has been investigated. Our first-principles calculations predict that the O 2 adsorptive capability of silicon carbide nanotubes is much better than that of carbon nanotubes. This might have potential for gas detection and energy storage. PACS numbers: A, Fg, Mb, Pd Key words: adsorption, oxygen molecule, DFT, SiCNTs, CNTs, sensors 1 Introduction There is a strong interest in gas adsorption by carbon nanotubes. [1 12] Among them, the interactions of oxygen with single-wall carbon nanotubes (SWNTs) are widely studied. [13 23] Sensitivity of their electronic properties to oxygen exposure can be used as the basis for a chemical sensor. Recent experimental data [6 7] have shown that the transport properties of single-wall nanotubes (SWNT) change dramatically upon exposure to gas molecules such as O 2, NO 2, NH 3, and many other gases, at ambient temperature. Practical applications to the production of better gas sensors ( thermoelectric nano-nose ) have been envisioned. [24] Several mechanisms may explain such phenomena. The gas molecules could affect transport properties indirectly, by binding to donor or acceptor centers in the substrate [6] or at the contacts [25] or directly, by binding to the nanotube. [14] In the latter case, the gas could be physisorbed (bound by dispersive van der Waals forces) or chemisorbed (bound by formation of a chemical bond), and adsorption could take place either on perfect nanotube walls or at defect sites. If the gas is chemisorbed, a key factor affecting the transport properties would be the charge transfer from the gas molecule to the nanotube, or vice versa. Experimentally, a way to distinguish physisorbed from chemisorbed species is to check for a linear relation between the thermoelectric power and the additional resistivity induced by gas adsorption. According to such criterion, O 2 is chemisorbed. [24] However, a recent experimental study of the kinetics of O adsorption and desorption on SWNT and on graphite, finds that O is physisorbed on SWNT in molecular form [6] with an estimated binding energy E b 0.19 ev. This would be consistent with the well-established fact that molecular oxygen physisorbs on graphite [26 27] with a binding energy E b 0.1 ev. On the theoretical side, contradictory results have been reported. Calculations based on the local-density approximation (LDA) find [28] that O 2 binds to a semiconducting (8, 0) nanotube with a binding energy E b = 0.25 ev at a distance d = 2.7 Å from the nanotube. A weak hybridization between oxygen and carbon states occurs, with a charge transfer estimated at about 0.1ē, suggesting that the corresponding variation in the density-ofstates (DOS) at the Fermi energy E F is responsible for the observed behavior of the transport properties. [25,28] Similar results were reported by other groups as well. [1,12,29] Calculations based on gradient-corrected approximation (GGA) functionals, on the other hand, yield virtually no binding and no charge transfer for O 2 on both graphite and SWNT [30] and so does an earlier set of calculations for O 2 on graphite. [31] A recent quantum chemistry calculation at the MP2 level [32] also finds very weak binding, due to physisorption, and minimal charge transfer. Very recently, Xu et al. have investigated the adsorption of molecular oxygen [33] and nitrogen [34] at defective edge sites of zigzag and armchair graphite and SWNT surfaces, respectively. They showed that the defect edge sites exhibit the significant catalytic role toward the adsorption and activation of the adsorbed molecule. Their results also showed that the adsorbed molecule was not only able to strongly bind to these edge sites, but their bond strength was obviously weakened. [33 34] Silicon carbide nanotubes (SiCNTs), on the other hand, have been successfully synthesized with a Si to C ratio of 1:1 via the reaction of Si with multi-walled CNTs. [35] Theoretical studies of single-wall silicon carbide nanotubes (SWSiCNTs) have shown that the energetically favorable Corresponding author, ganji md@yahoo.com

2 No. 4 First Principles Simulation of Molecular Oxygen Adsorption on SiC Nanotubes 743 structure consists of alternating Si and C atoms with each Si (C) atom having only C (Si) atoms as their nearest neighbors. [36 38] All SiCNTs are semiconductors and the band gap increases with increasing tube diameter. SiC- NTs are considered to have the advantages over CNTs because they may possess high reactivity of exterior surface facilitating to sidewall decoration and stability against oxidation in air at high temperature [35,38 40] which may have potential applications in nanoelectronic devices. In this work, we investigate the interaction of a single O 2 molecule with SWSiCNTs using first-principles simulations. We also analyze the interaction between the O 2 molecule and SWCNTs. 2 Computational Methods The O 2 molecule interacting with nanotubes is studied by first-principles approaches using numerical atomic orbitals as basis set. We have performed ab initio calculations based on the generalized gradient approximation (GGA) with the Perdew Burke Ernzerhof (PBE) functional [41] in density functional theory and the standard norm-conserving Troullier Martins pseudopotentials. [42] We have used the SIESTA code which solves the standard Kohn Sham equations and has been demonstrated to be very efficient for large atomic systems. [43 45] In this code, the Kohn Sham orbitals are expanded using linear combination of numerical pseudoatomic orbitals for the valence electron wave functions. The calculations are done using a split-valence double-zeta plus polarization function (DZP) as basis set and with cutoff radii of 50 mev for all simulated atoms. The cutoffs of 150 Ry and 120 Ry for the grid integration were utilized to represent the charge density in the real space for the SiCNT and the CNT, respectively. Periodic boundary conditions and supercell approximations with a lateral separation of 14 Å between tube centers are used to make sure that the nanotubes plus O 2 do not interact with their periodic images. The unit is periodic in the direction of the tube and the lengths are Å for the SiCNT and Å for the CNT structures being studied. Along the tube axis, Monkhorst Pack k-points were used for the Brillouin zone integration. The relaxed atomic structures of the tubes were obtained by minimization of the total energy using Hellmann Feynman forces including Pullaylike corrections. Structural optimizations were performed using the conjugate gradient algorithm until the residual forces were smaller than 0.02 ev/å. 3 Results and Discussion To study the O 2 adsorption on the SiCNT we start with atomic structures of silicon carbide nanotube. We have considered a zigzag (5, 0) SiCNT which consists of alternating C and Si atoms, as depicted in Fig. 1. The calculated average Si C bond length of these tubes is about Å in agreement with the theoretical results in Refs. [42] and [40], but larger than the results in Refs. [46] and [47]. The interaction of the O 2 molecule with the exterior wall of the (5, 0) SiCNT is studied by performing a single point energy (SPE) calculation for several orientations of the axis of the molecule. The system includes 30 C atoms, 30 Si atoms, and one O 2 molecule. Six possible configurations, named A1 A6, are selected for the parallel/perpendicular approach of the molecule to the wall of the tube as represented in Figs. 1(a) 1(f). The optimized SiCNT (5,0) and O 2 molecule are used for the molecule adsorption. To find the approximate stable adsorption configuration, the structure of the tube and the O O bond lengths (1.228 Å) of the O 2 molecule are fixed, while the distance between the tube and the molecule is varied, to obtain the system energy as a function of the separation. Figure 2 shows the calculated adsorption energy (binding energy) of the system as a function of the distance between the O 2 molecule and the surface of the wall for various orientations. The binding energy is calculated from E b = E NT O2 E NT E O2, (1) where E NT O2, E NT, and E O2 are the total energies of the tube with an adsorbed O 2 molecule, the pure nanotube and the O 2 molecule, respectively. From these calculations, we know that the adsorption energies are slightly dependent on orientations and locations of the O 2 molecule and the interaction becomes rapidly repulsive as the molecule approaches the nanotube wall. The most stable configuration of O 2 is the parallel approach of the O 2 molecule to the (5, 0) SiC nanotube wall on the center of a hexagon of Si and C atoms (configuration A4). In this configuration, as we know, the molecule is able to fit optimally into the electron density valley that exists around the hexagon center. [48] Structural optimization of the energetically favorable configuration show that the O 2 molecule is dissociated and then oxygen atoms bond with both the C and Si atoms of the tube wall, as depicted in Fig. 3(a). Further movement in the optimization process show that one of the oxygen atoms and C atom form a CO molecule and then this molecule escapes from the tube surface while, another O atom resides in the tube lattice between two Si atoms (doped in the tube), as represented in Fig. 3(b). It can also be seen that the adsorption of O 2 on the SiC nanotubes, results in a distortion of the tube structure. The calculated binding energy E b and average Si O equilibrium distance, after optimization, are about ev ( kcal/mol) and Å, respectively. The small distance of adsorbed O atom from the plane and the negative adsorption energy of ev indicate strong interaction (chemisorption) of O 2 with the SiCNTs. [49 57]

3 744 M.D. Ganji and B. Ahaz Vol. 53 Fig. 1 Atomistic configurations of adsorption with the axis of an O 2 molecule perpendicular [(a), A1] and parallel ((b), A2) to the nanotube axis above a hexagon of silicon and carbon atoms on the (5, 0) SiCNT wall and above a Si/C atom of the tube with a molecular axis perpendicular to the nanotube surface [(c), A3)/((d), A4]. (e) and (f) represent the A5 and A6 configurations which correspond to the approach of an O 2 molecule to the Si-C bonds of the tube wall, with parallel and zig-zag orientations respect to the tube axis, respectively. Fig. 2 Binding energy of an oxygen molecule as a function of the separation distance of the closest oxygen atom to the pentagon plane of the (5, 0) SiCNT for the six orientations of Fig. 1. Having discussed the adsorption of O 2 molecule on the SiCNTs, we next present our ab initio molecular dynamics (MD) simulation on the most stable SiCNT/O 2 system in order to test whether the system under study is stable. We have accomplished MD simulation at room temperature for 1500 time steps, each step taking s. We observe that the system is quite stable at room temperature and the average Si O equilibrium distance changes to Å, which is the same as for the relaxed system. For comparison, the adsorption of the O 2 molecule on a (5, 0) single-wall CNT is studied using the same approach. Five possible configurations, named B1 B5, are selected for the parallel/perpendicular approach of the molecule to the wall upon the carbon atom(s) and the center of a hexagon of carbon atoms. The five configurations are given in Fig. 4. The optimized CNT (5, 0)

4 No. 4 First Principles Simulation of Molecular Oxygen Adsorption on SiC Nanotubes 745 and O 2 molecule are used for the molecule adsorption. To find the approximate stable adsorption configuration, the structure of the tube and the O O bond lengths (1.228 Å) of the O 2 molecule are fixed, while the distance between the tube and the molecule is varied, to obtain the system energy as a function of the separation. Figure 5 shows the calculated binding energy of the system as a function of the distance between the O 2 molecule and the surface of the CNT wall for various orientations. We find that configuration B2 is the most stable configuration, which corresponds to the parallel approach of the O 2 molecule to the nanotube wall up on the center of a hexagon of C atoms [Fig. 4(b)]. To further investigate the adsorption phenomenon of the O 2 on the CNTs we carry out the full structural optimization of the most stable configuration (B2). Our obtained results show that O 2 is dissociated and two C O bonds are formed, as depicted in Fig. 6. The calculated adsorption energy and the C O equilibrium distance are about 190 ev and Å, respectively All these indicate that there is strong interaction between small-diameter CNTs and oxygen molecule, very similar to those previously found for the similar diameter CNTs such as (4, 2) [2] and (3, 3) [58] tubes It should be noted that once such oxidation occurs and the chemisorbed product is formed, the (4, 2) tube will be significantly destroyed (the C C bond is broken and the tube s geometry is changed) [2] but this is however not the case for the (5, 0) tube as reported previously. [58] Fig. 3 The optimized geometric structure of SiCNT/O 2 system of Fig. 1(d). Fig. 4 Atomistic configurations of adsorption with the axis of an O 2 molecule perpendicular [(a), B1] and parallel [(b), B2] to the nanotube axis above a hexagon of carbon atoms on the (5, 0) CNT wall and above a C atom of the tube with a molecular axis perpendicular to the nanotube surface [(c), B3]. (d) and (e) represent the B4 and B5 configurations which correspond to the approach of an O 2 molecule to the C C bonds of the tube wall, with parallel and zig-zag orientations respect to the tube axis, respectively.

5 746 M.D. Ganji and B. Ahaz Vol. 53 Fig. 7 Snapshots of DFT molecular dynamics simulation of the most stable complex of CNT/O 2 system at 300 K and 1450 fs. Fig. 5 Binding energy of an oxygen molecule as a function of the separation distance of the closest oxygen atom to the pentagon plane of the (5, 0) CNT for the five orientations of Fig. 4. Fig. 6 The optimized geometric structure of CNT/O 2 system of Fig. 4(b). When comparing the results obtained here for chemisorption on the small-diameter CNTs considered with those from the previous study on large-diameter CNT, [1,17,29] we see that the interaction strength of oxygen is smaller for the large-diameter tube. Thus, it appears that introducing surface curvature reduces the binding energy between the oxygen molecule and the tube and the chemisorption of O 2 will be more difficult [2,59 60] Further details of discussion on the dependence of tube diameter can be found in [60]. Finally, to evaluate the stability of the O 2 /(5, 0) tube complex in ambient conditions we carry out similar ab initio MD simulation at room temperature. We find that O 2 will be physisorbed on the tube and the geometry of the tube is still kept when exposed to oxygen, which indicates that the (5, 0) tube is still stable in ambient conditions as can be seen from Fig. 7. Our first-principles simulation result is in good agreement with the experimental observation of Liu et al. [2] Their experimentally measured Raman spectra showed that the (4, 2) tube structure is slowly destroyed by oxidation while the (5, 0) tube is stable even after 50 h, which indicates that the (5, 0) tube is more stable than the (4, 2) tube in the sense of oxidation. It can be seen from comparison of the calculated binding energies and binding distances for O 2 on the SiCNTs and CNTs that the binding energy of the O 2 on the SiC- NTs is much larger than that on CNTs while, the binding distance of the O 2 on the SiCNT is much smaller than that on CNTs. The shorter binding distance and higher binding energy indicate that the O 2 adsorption capability of SiCNT is much better than that of carbon nanotubes. To further understanding the adsorption properties of O 2 on the SiCNTs, the calculations of the density of state (DOS) for the pristine SiCNT and the SiCNT/O 2 systems are performed. Figure 8 shows the total electronic densities of states (DOS) for the considered systems. The dotted lines represent the DOS of pristine (5, 0) SiCNT while, the solid lines represent the DOS of the relaxed SiCNT/O 2 system (configuration A4). It can be seen that the DOS near the Fermi level (E F = 3.96 ev) is affected by the adsorption of O 2 in the SiCNT surface. We can also see that the DOS of the SiCNTs where the O 2 is adsorbed shifts down by about 0.40 ev in comparison with a bare (5, 0) SiCNT. This substantial shift can be explained by the reduction in effective Coulomb potential due to the charge transfer. On the other hand, the difference in the Fermi level of the SiCNTs (E F = 4.36 ev) and the SiCNT/O 2 (E F = 3.96 ev) clearly shows a charge transfer between the O 2 and SiCNT in the adsorption process. Hence, we perform Mulliken charge analyses to evaluate the amount of electron transfers between the O 2 molecule and SiCNT. Charge analysis shows 0.36ē charge transferred from the adsorbed O 2 to the SiCNT surface. The above-mentioned charge transfer behavior, together with the transition in the electronic state of both Si/C and O atoms, is expected to affect the electronic structure and therefore the performance of the catalytic activities of the system. On this subject matter we are pursuing further investigations involving more computation-expensive calculations to address the effect of O 2 adsorption on catalytic activities of the SiCNT promising a suitable material for energy storage.

6 No. 4 First Principles Simulation of Molecular Oxygen Adsorption on SiC Nanotubes 747 Fig. 8 Projected density of states of (a) a pristine (5, 0) SiCNT (dotted curves) and the SiCNT/O 2 (solid curves) systems. The vertical solid lines and dotted lines denote the Fermi levels of the pristine nanotubes and nanotube/o 2 systems, respectively. 4 Conclusions We perform first-principles studies on the adsorption of an O 2 molecule on a (5, 0) single wall SiCNT surface and also on a (5, 0) single wall carbon nanotube. The best adsorption sites for O 2 are the parallel approach of the O 2 molecule to the nanotube wall on the center of a hexagon of silicon and carbon atoms in sites on SiCNT surfaces and the center of the carbon hexagon for the parallel configuration in sites on CNT surfaces. Our first-principles calculations predict that the O 2 adsorptive capability of silicon carbide nanotube is about 40 times stronger than that of carbon nanotubes. This might have potential for gas detection and energy storage. The adsorption results in strong Si O bondings and charge transfers from the O 2 molecule toward the SiC nanotube. The quantum MD simulation, carried out at room temperature, shows that the SiCNT/O 2 system is quite stable and that it is possible to adsorb O 2 molecules by silicon carbide nanotubes. The theoretical results should be confirmed experimentally. Acknowledgements We thank Dr. Mohammad Reza Gholami for many fruitful discussions. This work was supported by the Azad University of Ghaemshahr. References [1] P. Giannozzi, R. Car, and G. Scoles, J. Chem. Phys. 118 (2003) [2] H.J. Liu, J.P. Zhai, C.T. Chan, and Z.K. Tang, Nanotechnology 18 (2007) [3] A.C. Dillon, K.M. Jones, T.A. Bekkdeahl, C.H. Kiang, D.S. Bethune, and M.J. Heben, Nature (London) 386 (1997) 377. [4] G.E. Gadd, M. Blackford, S. Moricca, N. Webb, P.J. Evans, A.M. Smith, G. Jacobsen, S. Leung, A. Day, and Q. Hua, Science 277 (1997) 933. [5] K.A. Dean and B.R. Chalamala, Appl. Phys. Lett. 75 (1999) [6] J. Kong, N.R. Franklin, C.W. Zhou, M.G. Chapline, S. Peng, K.J. Cho, and H.J. Dai, Science 287 (2000) 622. [7] P.G. Collins, K. Bradley, M. Ishigami, and A. Zettl, Science 287 (2000) [8] X.P. Tang, A. Kleinhammes, H. Shimoda, L. Fleming, K.Y. Bennoune, S. Sinha, C. Bower, O. Zhou, and Y. Wu, Science 288 (2000) 492. [9] G.U. Sumanasekera, C.K.W. Adu, S. Fang, and P.C. Eklund, Phys. Rev. Lett. 85 (2000) [10] A. Fujiwara, K. Ishiia, H. Suematsua, H. Kataurab, Y. Maniwab, S. Suzukic, and Y. Achibac, Chem. Phys. Lett. 336 (2001) 205. [11] A. Wadhawan, R.E. II Stallcup, and J.M. Perez, Appl. Phys. Lett. 78 (2001) 108. [12] J. Zhao, A. Buldum, J. Han, and J.P. Lu, Nanotechnology 13 (2002) 195. [13] S.H. Jhi, S.G. Louie, and M.L. Cohen, Phys. Rev. Lett. 85 (2000) [14] K. Bradley, S.H. Jhi, P.G. Collins, J. Hone, M.L. Cohen, S.G. Louie, and A. Zettl, Phys. Rev. Lett. 85 (2000) [15] X.Y. Zhu, S.M. Lee, Y.H. Lee, and T. Frauenheim, Phys. Rev. Lett. 85 (2000) [16] N. Park, S.W. Han, and J. Ihm, Phys. Rev. B 64 ( 2001) [17] D.C. Sorescu, K.D. Jordan, and P. Avouris, J. Phys. Chem. B 105 (2001) [18] C.Y. Moon, Y.S. Kim, E.C. Lee, Y.G. Jin, and K.J. Chang, Phys. Rev. B 65 (2002) [19] S. Dag, O. Gülseren, T. Yildirim, and S. Ciraci, Phys. Rev. B 67 (2003) [20] G.E. Froudakis, M. Schnell, M. Muhlhauser, S.D. Peyerimhoff, A.N. Andriotis, M. Menou, and R.M. Sheetz, Phys. Rev. B 68 (2003) [21] A. Ricca, C.W. Bauschlicher, and A. Maiti, Phys. Rev. B 68 (2003) [22] T. Miyake and S. Satio, Phys. Rev. B 68 (2003) [23] V. Barone, J. Heyd, and G.E. Scuseria, Chem. Phys. Lett. 389 (2004) 289. [24] P.G. Collins, K. Bradley, M. Ishigami, and A. Zettl, Science 287 (2000) [25] J. Kong, N.R. Franklin, C. Zhou, M.G. Chapline, S. Peng, K. Cho, and H. Dai, Science 287 (2000) 622. [26] C.K.W. Adu, G.U. Sumanasekera, B.K. Pradhan, H.E. Romero, and P.C. Eklund, Chem. Phys. Lett. 337 (2001) 31. [27] V. Derycke, R. Martel, J. Appenzeller, and Ph. Avouris, Appl. Phys. Lett. 80 (2002) [28] K. Bradley, S.H. Jhi, P.G. Collins, J. Hone, M.L. Cohen, S.G. Louie, and A. Zettl, Phys. Rev. Lett. 85 (2000) 4361.

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