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C60 as a chemical Faraday cage for three ferromagnetic Fe atoms
Chemical Physics Letters 462 (2008) 72–74
Contents lists available at ScienceDirect
Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
C60 as a chemical Faraday cage for three ferromagnetic Fe atoms Guohua Gao a,b, Hong Seok Kang a,* a b
Department of Nano and Advanced Materials, College of Engineering, Jeonju University, Hyoja-dong, Wansan-ku, Chonju, Chonbuk 560-759, Republic of Korea Pohl Institute of Solid State Physics, Tongji University, Shanghai 200092, PR China
a r t i c l e
i n f o
Article history: Received 6 May 2008 In final form 11 July 2008 Available online 18 July 2008
a b s t r a c t Based on calculations using density functional theory, we show that C60 can act as a chemical Faraday cage in which a highly magnetic metal cluster with a high chemical reactivity can be encapsulated. As an example, we find that C60 can encapsulate a Fe3 cluster, while it is much less likely to encapsulate a Fe2 cluster. Spin multiplicity (=9) of the Fe3@C60 is very high, being comparable to that (=11) of a free Fe3 cluster. Geometrically, the triangular plane of the cluster is perpendicular to a S6 axis of the fullerene. Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction
2. Theoretical methods
Since the discovery of fullerene, various kinds of endohedral fullerenes have been prepared aiming at engineering electronic and chemical properties of fullerene. Recently, it has been shown that more than one metal atom can be encapsulated. The smallest fullerene found to be able to encase a metal dimer is C66 [1]. Larger fullerenes have been known to be able to encapsulate various kinds of metal dimmers: They are Sc2@C74 [2], Sc2@C84 [3], La2@C72 [4], Ti2@C80 [5], La2@C80 [6], Er2@C82 [7], and Ti2@C84 [8]. In addition, an endohedral complex of a metal trimer is known, i.e., Sc3@C82 [9]. Furthermore, a family of trimetallic nitride was also shown to be encapsulated [10]. However, there has been no report on the encapsulation of more than one metal atom in the fullerenes smaller than C66. This requires investigation. Fe atoms are of special interest as a possible candidate for encasement, since they form one of the mostly commonly known ferromagnetic materials. Fex clusters were sometimes known to possess large ground state spin values, and can occasionally function as single-molecule magnets [11]. In addition, Fe nanowires encapsulated in a carbon nanotube show excellent magnetic anisotropy. They also exhibit enhanced magnetic coercivity which is significantly higher than that for bulk iron [12]. In this respect, it will be very useful, if the fullerene can actually encapsulate many Fe atoms in such a way that their magnetic moments couple ferromagnetically. The present work is concerned on the investigation of such possibility using DFT calculations. Namely, we will investigate if a Fe trimer can be encapsulated in C60, a fullerene which can be produced in a macroscopic amount.
Our total energy calculations are performed using the Vienna ab initio simulation package (VASP) [13]. Electron-ion interaction is described by the projected augmented wave (PAW) method [14], which is basically a frozen-core all-electron calculation. Exchange–correlation effect is treated within the generalized gradient approximation presented by Perdew, Burke, and Ernzerhof (PBE) [15]. Solution of the Kohn–Sham (KS) equation is obtained using Davison blocked iteration scheme followed by the residual vector minimization method. All valence electrons of chemical elements are explicitly considered in the KS equation. For Fe, 3p electrons are also treated as valence electrons. We adopt a supercell geometry in which k-space sampling is done with C-point. For this, we use large supercells which guarantee interatomic distances between neighboring cells greater than 9.30 Å. Cut-off energy is set high (=400 eV) enough to guarantee accurate results. In regard to this, it is worth mentioning that the PAW does not reply on a localized orbital basis but on the plane wave basis. The conjugate gradient method is employed to optimize the geometry until Hellmann–Feynman force exerted on an atom is less than 0.03 eV/Å. All the results rely on the spin-polarized calculation, where the spin configuration was allowed to be able to be changed [16]. We note that our recent work on benzene–vanadium sandwich complex showed that our PBE calculation within PAW was successful in elucidating the origin of its magnetic coupling as well as in predicting its thermochemical data [17].
* Corresponding author. Fax: +82 63 220 2056. E-mail address: [email protected] (H.S. Kang). 0009-2614/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2008.07.044
3. Results Metallofullerenes are usually produced by arc discharge of a metal/carbon composite rod at the time of fullerene synthesis, not by the insertion of metal atoms at the post-production stage of fullerenes. This is particularly suitable for the encasement of metal clusters. In order to investigate this possibility, we consider the
G. Gao, H.S. Kang / Chemical Physics Letters 462 (2008) 72–74
encapsulation energy, DEn of the process Fen + C60 ? Fen@C60, where Fen is a cluster of n Fe atoms. Our geometry relaxations shows that the spin multiplicities (=7 and 11) of free clusters Fe2 and Fe3 are in agreements with recent DFT calculations [18,19]. Three Fe atoms in the latter cluster are assumed to form a triangular geometry. In addition, Fe–Fe bond lengths (=2.25 Å) of the trimer and bond dissociation energy (=2.65 eV) of the process (Fe3 ? Fe2 + Fe) are also in excellent agreements with a previous calculation (=2.24 Å and 2.67 eV) [19]. For Fen@C60, geometry optimization was done starting from various initial orientations of Fe clusters in the cage, and we will report our result on the structure which corresponds to the lowest energy. We find that the values of (DEn, spin multiplicity) are ( 0.95 eV, 2), (0.00 eV, 7), and ( 0.84 eV, 9) for n = 1, 2, and 3, respectively. This results shows that the trimer can be encapsulated, while the dimer is not highly likely to be encased. Considering that only C82 was experimentally known to encapsulate a metal trimer, it is surprising that Fe3 can be encased in C60, even though the fullerene has the volume which amounts to only 65% of that of C82 [20]. Our data indicate that the encapsulation of the trimer is energetically almost as favorable as that of an atom. In Fe3@C60, three Fe atoms are arranged in a triangular geometry in the cage in a way similar to that of the Sc trimer in Sc3@C82 (see Fig. 1. There is a fractional bond slightly weaker than a single bond among all pairs of Fe atoms, as indicated by the value of their Wiberg bond indices (=0.65) [21] obtained from a GAUSSIAN03 [22] calculation using a mixed basis, i.e., 6-31G(d) and Lanl2dz basis sets for C and Fe atoms, respectively. Fe–Fe distances (=2.36 Å) also support this. It is interesting to note that they are 0.11 Å longer than those in a free Fe3 cluster, being comparable to those of Sc– Sc distances (=2.3 Å) in Sc3@C82 [23]. Our analysis of the natural bond orbital [24] (NBO) shows that the electron configuration of each Fe atom is found to be (core) 4s0.623d6.774p0.034d0.02, giving rise to a natural charge of +0.55. (We may note that the charge is much larger that (=0.04) of the Fe atom in ferrocene, indicating the pure ionic character of Fe-cage binding in the metallofullerene. In fact, we find no appreciable amount of covalent interaction between Fe and carbon atoms of the cage, which is manifested in their WBI less than 0.01 [25]) One half valency of Fe atoms is lar-
73
gely different from the case of other metal atoms in metallofullerenes, such as the trivalency of Ga and Dy atoms in M@C82 (M = Ga and Dy) [26]. Charge analysis described above indicates that the fullerene cage becomes negatively charged with q = 1.65. Spin densities of Fe atoms and the fullerene cage couple antiferromagnetically, where total spin densities of the Fe3 cluster and the cage are 9.45 and 1.45, respectively. Therefore, most of the spin density of the cage is originated from the charge transferred from the cluster. This is different from the case of Li@C60 but similar to that of N@C60 in that much of the spin density resides on the encased atom [27]. In Fe3@C60, the amount of spin polarization on the encased atoms is 270% of that in N@C60. More interestingly, the complex is found to have the nonet spin configuration (multiplicity = 9), indicating a very large magnetic moment (l = 8lB). Our separate calculation shows that the triplet state is at least 0.28 eV less stable. This observation is not limited to the most stable orientation of the trimer in the cage. We find that another conformation, in which the trimer has different orientations with respect to the C3 axis, is energetically comparable to this one within 0.06 eV and show the nonet spin configurations too. Fe atoms adopt a distorted g5-hapticity with respect to specific pentagons, being slightly displaced from the center of the pentagons to the adjacent hexagon. Fe–C distances (=2.21–2.65 Å) are similar to those (=2.23 Å) in ferrocene. The triangular plane of the Fe3 cluster is perpendicular to one of S6 axes of C60, and the center of the plane coincides with the center of the fullerene cage. This is different from the case of Sc3@C82 in which the Sc3 trimer is not located at the center of the carbon cage. In short, Fe3@C60 adopts the overall symmetry of C3, where the C3 axis coincides with the S6 axis. Geometrical structure of the fullerene cage is only slightly changed after the encapsulation. For example, its diameter changes from 7.12 Å to 7.16 Å upon the encasement of the trimer. 4. Discussion Using density functional calculation, we have shown that a Fe3 trimer can be encapsulated in C60, resulting in nonet spin configuration. We believe that this is the first report on the possible confinement of such a high spin polarization in the fullerene of the radius of only 3.56 Å. In relation to this, we recall that calculations with explicit spin polarization were carried out very recently even for the transition metal trimers themselves [19]. The high spin configuration of Fe3@C60 is comparable to a recent report for an organometallic complex with a Fe6 center [28]. The symmetry of the trimer complex indicates the alignment of the magnetic moment along C3 axis, which will tend to rotate the whole complex under a strong applied magnetic field at sufficiently low temperature if the initial Fe3 triangle is not oriented properly. Therefore, the complex can act as a subnanometer-sized motor which can rotate continuously under an oscillating magnetic field. Or, the C60 cage can act as a chemical Faraday cage for a highly reactive metal cluster with a high magnetic moment. The complex may also have biomedical applications [29]. Furthermore, the endohedral metallofullerene can be also used in nanoelectromechanical memory cells when it is encased as a pea in carbon nanotube peopods [30,31]. Acknowledgments
Fig. 1. Optimized structure of Fe3@C60 viewed along the C3 axis.
We appreciate Jeonju University for a financial support. We also would like to acknowledge the support from KISTI (Korea Institute of Science and Technology Information) under the 11th Strategic Supercomputing Applications Support Program. The use of
74
G. Gao, H.S. Kang / Chemical Physics Letters 462 (2008) 72–74
computing system of the Supercomputing Center is also greatly appreciated. References [1] C.R. Wang et al., Nature 408 (2000) 426. [2] H. Shinohara, H. Yamaguchi, N. Hayashi, H. Sato, M. Ohkohchi, Y. Ando, Y. Saito, J. Phys. Chem. 97 (1993) 4259. [3] M. Takata, E. Nishibori, B. Umeda, M. Sakata, E. Yamamoto, H. Shinohara, Phys. Rev. Lett. 78 (1997) 3330. [4] H. Kato, A. Taninaka, T. Sugai, H. Shinohara, J. Am. Chem. Soc. 125 (2003) 7782. [5] B. Cao, M. Hasegawa, K. Okada, T. Tomiyama, T. Okazaki, K. Suenaga, H. Shinohara, J. Am. Chem. Soc. 123 (2001) 9679. [6] M.M. Alvares, E.G. Gillan, K. Holczer, R.B. Kaner, K.S. Min, L.R. Whetten, J. Phys. Chem. 95 (1991) 10561. [7] R.M. Macfarlane, G. Wittman, P.H.M. van Loosdrecht, M. de Vries, D.S. Bethune, Phys. Rev. Lett. 78 (1997) 1397. [8] B. Cao, K. Suenaga, T. Okazaki, H. Shinohara, J. Phys. Chem. B 106 (2002) 9295. [9] H. Shinohara, et al., Nature 357 (1992) 52. [10] S. Stevenson, et al., Nature 401 (1999) 55. [11] D. Gatteschi, R. Sessoli, A. Corina, Chem. Commun. 9 (2000) 725. [12] D.H. Quin, L. Cao, Q.Y. Huang, H.L. Li, Chem. Phys. Lett. 35 (2002) 484. [13] (a) G. Kresse, J. Hafner, Phys. Rev. B 47 (1993) RC558; (b) G. Kresse, J. Furthmuller, Phys. Rev. B 54 (1996) 11169. [14] G. Kresse, D. Joubert, Phys. Rev. B 59 (1999) 1758. [15] J.P. Perdew, K. Burke, M. Ernzerhof, Phys. Rev. Lett. 77 (1996) 3865. [16] The spin polarized calculation can be done only on a massively parallel supercomputer, since there are 282 valence electrons in the largest system
[17] [18] [19] [20]
[21]
[22] [23] [24] [25]
[26] [27] [28] [29] [30] [31]
considered. In fact, our calculation was done on an IBM p690 parallel supercomputer. H.S. Kang, J. Phys. Chem. A 109 (2005) 9293. C.J. Barden, J.C. Rienstra-Kiracofe, H.F. Schaefer, J. Chem. Phys. 113 (2000) 690. B.N. Papas, H.F. Schaefer, J. Chem. Phys. 123 (2005) 074321. The ratio of the volume of C60 with respect to that of C82 was estimated from (R60/R82), [3] where R60 and R82 are radii of C60 and C82 with C3v symmetry. Also see Ref. [23] below. (a) A.E. Reed, R.B. Weinstock, F. Weinhold, J. Chem. Phys. 83 (1985) 735; (b) A.E. Reed, F. Weinhold, Chem. Rev. 88 (1988) 899; (c) A.E. Reed, P.v.R. Schleyer, J. Am. Chem. Soc. 112 (1990) 1434. M.J. Frisch et al., GAUSSIAN03 Revision B. 05, Gaussian, Inc., Pittsburgh PA, 2003. M. Tanaka, E. Nishibori, M. Sakata, M. Inakuma, E. Yamamoto, H. Shinohara, Phys. Rev. Lett. 83 (1999) 2214. A.E. Reed, L.E. Curtiss, F. Weinhold, Chem. Rev. 88 (1988) 899. This is different from the case of ferrocene, where the WBI for each of the Fe–C bonds is 0.35. Therefore, there are significant amount of covalent interactions between the Fe atom and carbon atoms of the five-membered ring in ferrocene, while this is not the case in our complex. R. Kitaura, H. Okimoto, H. Shinohara, T. Nakamura, H. Osawa, Phys. Rev. B 76 (2007) 172409. T. Almeida Murphy, T. Pawlik, A. Weidinger, M. Hohne, R. Alcala, J.M. Spaeth, Phys. Rev. Lett. 77 (1996) 1075. C. Canada-Vilalta, T.A. O’Brien, E.K. Brechin, M. Pink, E.R. Davidson, G. Christou, Inorg. Chem. 43 (2004) 5505. E. Toth, et al., J. Am. Chem. Soc. 127 (2005) 799. E. Bichoutskaia, A.M. Popov, Y.E. Lozovik, Mater. Today 11 (2008) 38. A.P. Popov, E. Bichoutskaia, Y.E. Lozovik, A.S. Kulish, Phys. Stat. Sol. (a) 204 (2007) 1911.
Contents lists available at ScienceDirect
Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
C60 as a chemical Faraday cage for three ferromagnetic Fe atoms Guohua Gao a,b, Hong Seok Kang a,* a b
Department of Nano and Advanced Materials, College of Engineering, Jeonju University, Hyoja-dong, Wansan-ku, Chonju, Chonbuk 560-759, Republic of Korea Pohl Institute of Solid State Physics, Tongji University, Shanghai 200092, PR China
a r t i c l e
i n f o
Article history: Received 6 May 2008 In final form 11 July 2008 Available online 18 July 2008
a b s t r a c t Based on calculations using density functional theory, we show that C60 can act as a chemical Faraday cage in which a highly magnetic metal cluster with a high chemical reactivity can be encapsulated. As an example, we find that C60 can encapsulate a Fe3 cluster, while it is much less likely to encapsulate a Fe2 cluster. Spin multiplicity (=9) of the Fe3@C60 is very high, being comparable to that (=11) of a free Fe3 cluster. Geometrically, the triangular plane of the cluster is perpendicular to a S6 axis of the fullerene. Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction
2. Theoretical methods
Since the discovery of fullerene, various kinds of endohedral fullerenes have been prepared aiming at engineering electronic and chemical properties of fullerene. Recently, it has been shown that more than one metal atom can be encapsulated. The smallest fullerene found to be able to encase a metal dimer is C66 [1]. Larger fullerenes have been known to be able to encapsulate various kinds of metal dimmers: They are Sc2@C74 [2], Sc2@C84 [3], La2@C72 [4], Ti2@C80 [5], La2@C80 [6], Er2@C82 [7], and Ti2@C84 [8]. In addition, an endohedral complex of a metal trimer is known, i.e., Sc3@C82 [9]. Furthermore, a family of trimetallic nitride was also shown to be encapsulated [10]. However, there has been no report on the encapsulation of more than one metal atom in the fullerenes smaller than C66. This requires investigation. Fe atoms are of special interest as a possible candidate for encasement, since they form one of the mostly commonly known ferromagnetic materials. Fex clusters were sometimes known to possess large ground state spin values, and can occasionally function as single-molecule magnets [11]. In addition, Fe nanowires encapsulated in a carbon nanotube show excellent magnetic anisotropy. They also exhibit enhanced magnetic coercivity which is significantly higher than that for bulk iron [12]. In this respect, it will be very useful, if the fullerene can actually encapsulate many Fe atoms in such a way that their magnetic moments couple ferromagnetically. The present work is concerned on the investigation of such possibility using DFT calculations. Namely, we will investigate if a Fe trimer can be encapsulated in C60, a fullerene which can be produced in a macroscopic amount.
Our total energy calculations are performed using the Vienna ab initio simulation package (VASP) [13]. Electron-ion interaction is described by the projected augmented wave (PAW) method [14], which is basically a frozen-core all-electron calculation. Exchange–correlation effect is treated within the generalized gradient approximation presented by Perdew, Burke, and Ernzerhof (PBE) [15]. Solution of the Kohn–Sham (KS) equation is obtained using Davison blocked iteration scheme followed by the residual vector minimization method. All valence electrons of chemical elements are explicitly considered in the KS equation. For Fe, 3p electrons are also treated as valence electrons. We adopt a supercell geometry in which k-space sampling is done with C-point. For this, we use large supercells which guarantee interatomic distances between neighboring cells greater than 9.30 Å. Cut-off energy is set high (=400 eV) enough to guarantee accurate results. In regard to this, it is worth mentioning that the PAW does not reply on a localized orbital basis but on the plane wave basis. The conjugate gradient method is employed to optimize the geometry until Hellmann–Feynman force exerted on an atom is less than 0.03 eV/Å. All the results rely on the spin-polarized calculation, where the spin configuration was allowed to be able to be changed [16]. We note that our recent work on benzene–vanadium sandwich complex showed that our PBE calculation within PAW was successful in elucidating the origin of its magnetic coupling as well as in predicting its thermochemical data [17].
* Corresponding author. Fax: +82 63 220 2056. E-mail address: [email protected] (H.S. Kang). 0009-2614/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2008.07.044
3. Results Metallofullerenes are usually produced by arc discharge of a metal/carbon composite rod at the time of fullerene synthesis, not by the insertion of metal atoms at the post-production stage of fullerenes. This is particularly suitable for the encasement of metal clusters. In order to investigate this possibility, we consider the
G. Gao, H.S. Kang / Chemical Physics Letters 462 (2008) 72–74
encapsulation energy, DEn of the process Fen + C60 ? Fen@C60, where Fen is a cluster of n Fe atoms. Our geometry relaxations shows that the spin multiplicities (=7 and 11) of free clusters Fe2 and Fe3 are in agreements with recent DFT calculations [18,19]. Three Fe atoms in the latter cluster are assumed to form a triangular geometry. In addition, Fe–Fe bond lengths (=2.25 Å) of the trimer and bond dissociation energy (=2.65 eV) of the process (Fe3 ? Fe2 + Fe) are also in excellent agreements with a previous calculation (=2.24 Å and 2.67 eV) [19]. For Fen@C60, geometry optimization was done starting from various initial orientations of Fe clusters in the cage, and we will report our result on the structure which corresponds to the lowest energy. We find that the values of (DEn, spin multiplicity) are ( 0.95 eV, 2), (0.00 eV, 7), and ( 0.84 eV, 9) for n = 1, 2, and 3, respectively. This results shows that the trimer can be encapsulated, while the dimer is not highly likely to be encased. Considering that only C82 was experimentally known to encapsulate a metal trimer, it is surprising that Fe3 can be encased in C60, even though the fullerene has the volume which amounts to only 65% of that of C82 [20]. Our data indicate that the encapsulation of the trimer is energetically almost as favorable as that of an atom. In Fe3@C60, three Fe atoms are arranged in a triangular geometry in the cage in a way similar to that of the Sc trimer in Sc3@C82 (see Fig. 1. There is a fractional bond slightly weaker than a single bond among all pairs of Fe atoms, as indicated by the value of their Wiberg bond indices (=0.65) [21] obtained from a GAUSSIAN03 [22] calculation using a mixed basis, i.e., 6-31G(d) and Lanl2dz basis sets for C and Fe atoms, respectively. Fe–Fe distances (=2.36 Å) also support this. It is interesting to note that they are 0.11 Å longer than those in a free Fe3 cluster, being comparable to those of Sc– Sc distances (=2.3 Å) in Sc3@C82 [23]. Our analysis of the natural bond orbital [24] (NBO) shows that the electron configuration of each Fe atom is found to be (core) 4s0.623d6.774p0.034d0.02, giving rise to a natural charge of +0.55. (We may note that the charge is much larger that (=0.04) of the Fe atom in ferrocene, indicating the pure ionic character of Fe-cage binding in the metallofullerene. In fact, we find no appreciable amount of covalent interaction between Fe and carbon atoms of the cage, which is manifested in their WBI less than 0.01 [25]) One half valency of Fe atoms is lar-
73
gely different from the case of other metal atoms in metallofullerenes, such as the trivalency of Ga and Dy atoms in M@C82 (M = Ga and Dy) [26]. Charge analysis described above indicates that the fullerene cage becomes negatively charged with q = 1.65. Spin densities of Fe atoms and the fullerene cage couple antiferromagnetically, where total spin densities of the Fe3 cluster and the cage are 9.45 and 1.45, respectively. Therefore, most of the spin density of the cage is originated from the charge transferred from the cluster. This is different from the case of Li@C60 but similar to that of N@C60 in that much of the spin density resides on the encased atom [27]. In Fe3@C60, the amount of spin polarization on the encased atoms is 270% of that in N@C60. More interestingly, the complex is found to have the nonet spin configuration (multiplicity = 9), indicating a very large magnetic moment (l = 8lB). Our separate calculation shows that the triplet state is at least 0.28 eV less stable. This observation is not limited to the most stable orientation of the trimer in the cage. We find that another conformation, in which the trimer has different orientations with respect to the C3 axis, is energetically comparable to this one within 0.06 eV and show the nonet spin configurations too. Fe atoms adopt a distorted g5-hapticity with respect to specific pentagons, being slightly displaced from the center of the pentagons to the adjacent hexagon. Fe–C distances (=2.21–2.65 Å) are similar to those (=2.23 Å) in ferrocene. The triangular plane of the Fe3 cluster is perpendicular to one of S6 axes of C60, and the center of the plane coincides with the center of the fullerene cage. This is different from the case of Sc3@C82 in which the Sc3 trimer is not located at the center of the carbon cage. In short, Fe3@C60 adopts the overall symmetry of C3, where the C3 axis coincides with the S6 axis. Geometrical structure of the fullerene cage is only slightly changed after the encapsulation. For example, its diameter changes from 7.12 Å to 7.16 Å upon the encasement of the trimer. 4. Discussion Using density functional calculation, we have shown that a Fe3 trimer can be encapsulated in C60, resulting in nonet spin configuration. We believe that this is the first report on the possible confinement of such a high spin polarization in the fullerene of the radius of only 3.56 Å. In relation to this, we recall that calculations with explicit spin polarization were carried out very recently even for the transition metal trimers themselves [19]. The high spin configuration of Fe3@C60 is comparable to a recent report for an organometallic complex with a Fe6 center [28]. The symmetry of the trimer complex indicates the alignment of the magnetic moment along C3 axis, which will tend to rotate the whole complex under a strong applied magnetic field at sufficiently low temperature if the initial Fe3 triangle is not oriented properly. Therefore, the complex can act as a subnanometer-sized motor which can rotate continuously under an oscillating magnetic field. Or, the C60 cage can act as a chemical Faraday cage for a highly reactive metal cluster with a high magnetic moment. The complex may also have biomedical applications [29]. Furthermore, the endohedral metallofullerene can be also used in nanoelectromechanical memory cells when it is encased as a pea in carbon nanotube peopods [30,31]. Acknowledgments
Fig. 1. Optimized structure of Fe3@C60 viewed along the C3 axis.
We appreciate Jeonju University for a financial support. We also would like to acknowledge the support from KISTI (Korea Institute of Science and Technology Information) under the 11th Strategic Supercomputing Applications Support Program. The use of
74
G. Gao, H.S. Kang / Chemical Physics Letters 462 (2008) 72–74
computing system of the Supercomputing Center is also greatly appreciated. References [1] C.R. Wang et al., Nature 408 (2000) 426. [2] H. Shinohara, H. Yamaguchi, N. Hayashi, H. Sato, M. Ohkohchi, Y. Ando, Y. Saito, J. Phys. Chem. 97 (1993) 4259. [3] M. Takata, E. Nishibori, B. Umeda, M. Sakata, E. Yamamoto, H. Shinohara, Phys. Rev. Lett. 78 (1997) 3330. [4] H. Kato, A. Taninaka, T. Sugai, H. Shinohara, J. Am. Chem. Soc. 125 (2003) 7782. [5] B. Cao, M. Hasegawa, K. Okada, T. Tomiyama, T. Okazaki, K. Suenaga, H. Shinohara, J. Am. Chem. Soc. 123 (2001) 9679. [6] M.M. Alvares, E.G. Gillan, K. Holczer, R.B. Kaner, K.S. Min, L.R. Whetten, J. Phys. Chem. 95 (1991) 10561. [7] R.M. Macfarlane, G. Wittman, P.H.M. van Loosdrecht, M. de Vries, D.S. Bethune, Phys. Rev. Lett. 78 (1997) 1397. [8] B. Cao, K. Suenaga, T. Okazaki, H. Shinohara, J. Phys. Chem. B 106 (2002) 9295. [9] H. Shinohara, et al., Nature 357 (1992) 52. [10] S. Stevenson, et al., Nature 401 (1999) 55. [11] D. Gatteschi, R. Sessoli, A. Corina, Chem. Commun. 9 (2000) 725. [12] D.H. Quin, L. Cao, Q.Y. Huang, H.L. Li, Chem. Phys. Lett. 35 (2002) 484. [13] (a) G. Kresse, J. Hafner, Phys. Rev. B 47 (1993) RC558; (b) G. Kresse, J. Furthmuller, Phys. Rev. B 54 (1996) 11169. [14] G. Kresse, D. Joubert, Phys. Rev. B 59 (1999) 1758. [15] J.P. Perdew, K. Burke, M. Ernzerhof, Phys. Rev. Lett. 77 (1996) 3865. [16] The spin polarized calculation can be done only on a massively parallel supercomputer, since there are 282 valence electrons in the largest system
[17] [18] [19] [20]
[21]
[22] [23] [24] [25]
[26] [27] [28] [29] [30] [31]
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