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Bucky shuttle memory system based on boron-nitride nanopeapod
Physica E 23 (2004) 135 – 140 www.elsevier.com/locate/physe
Bucky shuttle memory system based on boron-nitride nanopeapod Won Young Choi, Jeong Won Kang∗ , Ho Jung Hwang Nanoelectronic and Future Technology Laboratory, School of Electrical and Electronic Engineering, Chung-Ang University, 221 HukSuk-Dong, DongJak-Ku, Seoul 156-756, South Korea Received 19 January 2004; accepted 30 January 2004
Abstract + We investigated the internal dynamics of a bucky shuttle memory system, consisting of three C60 s encapsulated in (10, 10) boron-nitride (BN) nanotubes and 3lled Cu electrode. Energetics and operating response of the shuttle-memory-element + proposed were examined by using classical molecular dynamics simulations. The C60 shuttle in the BN nanotube capsule was found under the external force 3elds. For the stable operations of the shuttle memory device, the periods and the magnitudes of the operating force 3elds were investigated. ? 2004 Elsevier B.V. All rights reserved.
PACS: 61.46.+w; 66.30.Pa; 83.10.Rs Keywords: Bulky shuttle memory device; Boron–nitride peapod; Boron–nitride nanotube; Fullerene; Molecular dynamics simulation; Nano nonvolatile memory
1. Introduction Nanostructure and nanoparticles are of great interest in materials science and technology because of their interesting structures, especially their size. However, it is hard to control their physical and chemical properties. Since fullerene-related materials have unique physical and chemical properties [1,2], they have attracted considerable attention this last decade. Compared to other nanostructures, fullerenes have found promising applications in a wide variety of very important technological processes such as in designing electronic devices, super-3bers, catalytic ∗ Corresponding author. Tel.: +82-2-820-5296; fax: +82-2-825-1584. E-mail address: [email protected] (J.W. Kang).
materials, etc. [3]. Especially the large empty space (particularly inside carbon nanotubes (CNTs)) also open new applications as storage materials with high capacity and stability [4]. These cavities are large enough to accommodate a wide variety of atomic and molecular species, the presence of which can significantly inBuence the properties of the materials. In particular, a new type of self-assembled hybrid structures called “nanopeapods”, consisting of fullerene arrays inside single-walled CNTs, have recently been reported [5–10]. The application of nanopeapods ranges from nanometer-sized containers of chemical reactant [8] to data storage [11] and high-temperature superconductor [12]. The encapsulation of fullerenes (such as C60 ) in nanotubes is favorable on energetic grounds and occurs rapidly by exposing nanotubes to sublimed fullerenes. Mickelson et al. [13] reported how to pack
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C60 in boron-nitride nanotubes (BNNTs), and Goldberg et al. [14] researched metal 3llings inside BNNTs. Okada et al. [15] showed the reaction energy of a BN nanopeapod such as (10, 10) BNNT + C60 → C60 at (10, 10) BNNT + 1:267 (eV). Although the quantity of the exothermic energy, 1:267 eV, obtained from the calculations of Okada et al. [15] has been doubtful [16], C60 at (10, 10) BNNT is obviously more stable than the structure that the (10, 10) BNNT is in3nitely separate with a C60 molecule. Kwon et al. [11] reported that multi-walled nanotubes called “bucky shuttle” [17] were synthesized from elemental carbon under speci3c conditions, and investigated bucky shuttle memory device, which acted as nanometer-sized memory element, using molecular dynamics (MD) simulations. Though CNTs are a representative nanostructure material, CNTs synthesized by experiments can be a semiconductor or a metal conductor by their chirality [2], because it is hard to control the chirality of CNT. However, since most of bandgap of BNNTs are about 5:5 eV, they are electrically insulators [18]. While aligned bucky shuttle structures are diLcult to be in self-assembly, nanopeapods can be synthesized in the aligned structures using bundles of single-walled BNNTs. Mickelson et al. [13] reported how to pack C60 in BNNTs, and Goldberg et al. [14] researched metal 3llings inside BNNTs. If some processes, such as C60 intercalation control, nanolithography, nanotube cutting and nanotube capping, are treated appropriately to the aligned nanopeapods, the aligned bucky shuttles can be synthesized. In this paper, we investigate the operations and the properties of proposed shuttle + memory system composed of three C60 s and a (10, 10) BNNT using classical MD simulations. 2. Methods and structure Endo-metallofullerenes have been investigated in experimental and theoretical studies. In this work, we assume that the charge of the endo metal encapsulated in fullerene is fully ionized such as F− , Na+ , K + , Mg2+ , and Al3+ . This assumption has been used in the previous works [11,19]. However, in our MD code, the endo metal were not included for the computational eLciency, whereas the charge of the C60 is assumed as +e and is uniformly distributed on the C60 , such
Fig. 1. Schematics of the fullerene-shuttle-memory-system based on BN nanopeapod : (a) BNNT opening, (b) one side copper 3lling, (c) fully ionized endo-fullerenes 3lling, (d) the other side copper 3lling and (e) shuttle memory system for MD simulations in this work.
as the previous work Kwon et al. [11]. Therefore, the charge per carbon atom is assumed as +e=60. For B–N interactions, we used the TersoM potential function that has been widely used [20]. BNNT wall and C60 interactions were modeled by the C60 –(10, 10) CNT interaction potential studied by Hodak and Girifalco [21]. Further work should include more realistic parameters of the interaction between BNNT and C60 . The C60 –C60 interactions were characterized with the Lennard-Jones 12-6 (LJ12-6) potential studied by Girifalco et al. [22]. For Cu–BN, we also used the P LJ12-6 potential with = 0:1448 eV and = 2:039 A [23]. For Cu used as the contacts, we used a LJ12-6 P potential [23] with = 0:415 eV and = 2:277 A. Fig. 1(a)–(d) shows the model schematics of the shuttle-memory-system proposed as follows: (1) opening both end of a nanotube, (2) 3lling conducting metal nanowires used as one-side contact by 3llings, (3) nanopeapod formation by fullerene encapsulation, and (4) 3lling conducting metal nanowires used as the other side contact by 3llings. In this work, we assumed the copper nanowires as the conductor material. However, to realize the proposed system, other materials that easily 3ll inside BNNTs should be used. Under external force 3elds, since fully ion+ + ized fullerene C60 can be accelerated, the C60 can
W.Y. Choi et al. / Physica E 23 (2004) 135 – 140
be shuttled by the alternative external force 3elds. Therefore, the bits of the proposed memory system can be de3ned by the positions of the fullerenes. We used both steepest descent (SD) and MD methods. The MD simulations used the same MD methods as were used in our previous works [24–28]. The MD code used the velocity Verlet algorithm, a Gunsteren –Berendsen thermostat to control temperature for all atoms except for fullerenes and neighbor lists to improve computing performance [29]. MD time step was 5 × 10−4 ps. The nanopeapod encapsulating three fullerenes was considered as the shuttle-memory-element as shown in Fig. 1(e). The structure of the copper nanowires was obtained from our previous work [30] that shows the copper nanowires encapsulated in armchair CNTs. The structure shown in Fig. 1(e) is consisted of a (10, + P length, three C60 10) BNNT with 54 A molecules, and both copper contacts composed of 60 atoms. At outside of the shuttle memory system, there is the alternative bias system to control the position of central fullerene. The structure was initially relaxed by the SD method; and then the atoms of both edges were 3xed during the MD simulations and for the other atoms, MD methods were applied.
3. Results and discussion Fig. 2 shows the energetics of the shuttle system as a function of the position of central C60 molecule.
Fig. 2. Energetics of the shuttle-memory-system as a function of the position of central C60 molecule.
137
Fig. 3. Velocities (Vz ) of the shuttle fullerene along the tube axis, the position variation (Pz ) of the shuttle fullerene along the tube axis as functions of MD step and external force 3eld (Fext ). The dashed and the solid lines indicate external force 3eld 0.09 and P respectively. MD temperature is 100 K. 0:1 eV= A,
Two C60 molecules at both ends were 3xed and the position of the central C60 molecule was displaced by P along the tube axis (the z-axis), and then the 0:01 A con3guration was relaxed by the SD method. Obviously, the van der Waals interaction stabilized the C60 molecules at either end of the nanopeapod, where the contact area is largest. This is reBected on the potential energy variation as shown in Fig. 2. The minimum potential energies are found near the both ends of the nanopeapod and the activation energy barrier is 0:1997 eV. For the structure that three C60 molecules are localized at either, the binding energy is higher (3:93 eV) than that for the structure in Fig. 2 in the empirical potential functions used. In our MD simulations, since the binding energy between the copper contact and one C60 molecule is higher than that between C60 molecules, after the left and the right force 3elds were successively applied, active elements were always changed into the structure as shown in Fig. 1(e). Two C60 molecules were always attached at the both end contacts and the shuttle medium was
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Fig. 4. Velocities (Vz ) of the shuttle fullerene along the tube axis, the position variation (Pz ) of the shuttle fullerene along the tube axis as functions of MD step and external force 3eld (Fext ). The dashed and the solid lines indicate external force 3eld 0.3 and P respectively. MD temperature is 100 K. 0:5 eV= A,
always the central C60 molecule. By the left-direction force 3eld, the left C60 was attached at the left copper contact, and by the right-direction force 3eld, the right C60 was attached at the right copper contact. The operations of the shuttle-memory-element proposed were investigated using classical MD simulations. Figs. 3–6 show the velocities (Vz ) of the shuttle fullerene along the tube axis, the position variation (Pz ) of the shuttle fullerene along the tube axis as functions of the MD step and the external force 3eld (Fext ). The MD temperature was 100 K in all cases. In Figs. 3 and 4, the MD simulations have been performed for the diMerent external force 3elds. ExP until 10,000 ternal force 3elds were initially 0 eV= A MD steps, and then successively changed to the objective external force 3eld in 100 steps. The external force 3eld continued in 10,000 MD step, and then, the external force 3eld decreased in the same way. Peaks + in the velocities (Vz ) are found when the shuttle C60 just before collides with the other fullerenes attached at both ends. After the collisions, the direction of
Fig. 5. Velocities (Vz ) of the shuttle fullerene along the tube axis, the position variation (Pz ) of the shuttle fullerene along the tube axis as functions of MD step and external force 3eld (Fext ). The solid line is a round-trip operation starting from bit 1 with external P The dashed line is opposite case with applied force 3eld 0:1 eV= A. P MD temperature is 100 K. force 3eld −0:1 eV= A.
+ the Vz of the shuttle C60 is changed in the opposite directions. In Fig. 3, when the external force 3eld + P C60 was below 0:09 eV= A, could not escape from left + end. The shuttle C60 under the external force 3eld P reached at the other side faster than that 0:3 eV= A P For the case under the external force 3eld 0:1 eV= A. P the time required of the external force 3eld 0:5 eV= A, to become entire bit Bip is similar with the case of P because of several the external force 3eld 0:3 eV= A, rebound events, although the accelerated speed of the + P is higher than that for 0:3 eV= A. P C60 for 0:5 eV= A For contemporary memory systems composed of ‘bit 0’ and ‘bit 1’, the storage data should be changeable from bit 0 to bit 1, and vice versa. We assign that the left and the right sides of the copper contacts are bit 0 and bit 1 as shown in Fig. 1(e). The external force P respec3eld in Figs. 5 and 6 were 0.1 and 0:2 eV= A, + tively. The solid lines indicate the case that the C60 was + initiated at bit 1 and the dashed lines indicate that C60
W.Y. Choi et al. / Physica E 23 (2004) 135 – 140
139
is achieved after a long time because of several rebounded events. Therefore, to design the system proposed in this work, the switching speed, the applied force 3eld, and the active region should be considered. 4. Summary
Fig. 6. Velocities (Vz ) of the shuttle fullerene along the tube axis, the position variation (Pz ) of the shuttle fullerene along the tube axis as functions of MD step and external force 3eld (Fext ). The solid line is a round-trip operation starting from bit 1 with external P The dashed line is opposite case with applied force 3eld 0:2 eV= A. P MD temperature is 100 K. force 3eld −0:2 eV= A.
We studied the energetics and the operations of the fullerene-shuttle-memory-system based on a BN nanopeapod using classical MD simulations. The system proposed in this work was composed of a (10, 10) + molecules, and two copper BN nanotube, three C60 contacts. The lowest energy con3gurations were found + in the both ends terminated by C60 s attached at cop+ per contacts. Obviously, the interactions between C60 + molecules stabilized the shuttle C60 at the both ends of the proposed system where the contact area was the largest. Therefore, the bit Bops could be classi3ed with both the position of the shuttle media and the potential energy of the system. To design the proposed system, the switching speed, the applied force 3eld, and the active region should be considered. Classical MD simulations showed that the fullerene shuttle memory system based on BN nanopeapod and metal-3lling technologies could be operated by an adequate external force 3eld. References
+ C60
was initiated at bit 0. The shuttle under the applied P worked well. Although the shuttle force 3eld 0:1 eV= A + P had much C60 under the external force 3eld 0:2 eV= A more rebound events, the switching speed was high. P However, when the external force 3eld was 0:2 eV= A, the operations of the shuttle-memory-device were not stable, because there were the cases that entire bit Bips were not achieved as showed in Fig. 6. In the dashed + line in Fig. 6, the rebounded C60 returned to the initial position. When the active region is suLciently long, the rebounded events may be unimportant in the operation of this system. When the applied force 3elds are very low in the conditions that the shuttle fullerene can be switched, these rebounded events are not found. The switching speed is very low under the very low applied 3elds. When external force 3eld is high, though the switching speed is very high, the entire bit Bip
[1] S. Iijima, Nature 354 (1991) 56. [2] M.S. Dresselhaus, G. Dresselhaus, P.C. Eklund, Science of Fullerenes and Carbon Nanotubes, Academic Press, San Diego, CA, 1996. [3] J. Zhao, A. Buldum, J. Han, J.P. Lu, Nanotechnology 13 (2002) 195. [4] R.E. Barajas-Barraza, R.A. Guirado-Lopez, Phys. Rev. B 66 (2002) 155426. [5] B.W. Smith, M. Monthioux, D.E. Luzzi, Nature 396 (1998) 323. [6] B. Burteaux, A. Claye, B.W. Smith, M. Monthioux, D.E. Luzzi, J.E. Fischer, Chem. Phys. Lett. 310 (1999) 21. [7] B.W. Smith, M. Monthioux, D.E. Luzzi, Chem. Phys. Lett. 315 (1999) 31. [8] B.W. Smith, D.E. Luzzi, Chem. Phys. Lett. 321 (2000) 169. [9] K. Hirahara, K. Suenaga, S. Bandow, H. Kato, T. Okazaki, H. Shinohara, S. Iijima, Phys. Rev. Lett. 85 (2000) 5384. [10] J. Sloan, R.E. Dunin-Borkowski, J.L. Hutchison, K.S. Coleman, V.C. Williams, J.B. Claridge, A.P.E. Yorka, C. Xu, S.R. Bailey, G. Brown, S. Friedrichs, M.K.H. Green, Chem. Phys. Lett. 316 (2000) 191.
140
W.Y. Choi et al. / Physica E 23 (2004) 135 – 140
[11] Y.K. Kwon, D. TomVanek, S. Iijima, Phys. Rev. Lett. 82 (1999) 1470. [12] R.F. Service, Science 292 (2001) 45. [13] W. Mickelson, S. Aloni, W.-Q Han, J. Cumings, A. Zettl, Science 300 (2003) 467. [14] D. Goldberg, F.-F. Xu, Y. Bando, Appl. Phys. A 76 (2003) 479. [15] S. Okada, S. Saito, A. Oshiyam, Phys. Rev. B 64 (2003) 201303. [16] L.A. Girifalco, M. Hodak, Phys. Rev. B 65 (2002) 125404. [17] B.I. Yakobson, R.E. Smalley, Am. Sci. 85 (1997) 324. [18] X. Blase, A. Rubio, S.G. Louie, M.L. Cohen, Europhys. Lett. 28 (1994) 335. [19] G. Gao, T. Cagin, W.A. Goddard III, Phys. Rev. Lett. 80 (1998) 5556. [20] K. Albe, W. Moller, Comp. Mater. Sci. 10 (1998) 111.
[21] M. Hodak, L.A. Girifalco, Phys. Rev. B 67 (2003) 075419. [22] L.A. Girifalco, M. Hodak, R.S. Lee, Phys. Rev. B 62 (2000) 13104. [23] S. Dorfman, K.C. Mundim, D. Fuks, A. Berner, D.E. Ellis, J.V. Humbeeck, Mater. Sci. Eng. C 15 (2001) 191. [24] J.W. Kang, H.J. Hwang, Nanotechnology 13 (2002) 524. [25] J.W. Kang, H.J. Hwang, Physica E 15 (2002) 82. [26] J.W. Kang, H.J. Hwang, J. Phys.: Condens. Matter 14 (2002) 2629. [27] J.W. Kang, H.J. Hwang, Comp. Mater. Sci. 23 (2002) 105. [28] J.W. Kang, H.J. Hwang, J. Korean Phys. Soc. 40 (2002) 946. [29] Z. Mao, A. Garg, S.B. Sinnott, Nanotechnology 10 (1999) 273. [30] W.Y. Choi, J.W. Kang, H.J. Hwang, Phys. Rev. B 68 (2003) 193405.
Bucky shuttle memory system based on boron-nitride nanopeapod Won Young Choi, Jeong Won Kang∗ , Ho Jung Hwang Nanoelectronic and Future Technology Laboratory, School of Electrical and Electronic Engineering, Chung-Ang University, 221 HukSuk-Dong, DongJak-Ku, Seoul 156-756, South Korea Received 19 January 2004; accepted 30 January 2004
Abstract + We investigated the internal dynamics of a bucky shuttle memory system, consisting of three C60 s encapsulated in (10, 10) boron-nitride (BN) nanotubes and 3lled Cu electrode. Energetics and operating response of the shuttle-memory-element + proposed were examined by using classical molecular dynamics simulations. The C60 shuttle in the BN nanotube capsule was found under the external force 3elds. For the stable operations of the shuttle memory device, the periods and the magnitudes of the operating force 3elds were investigated. ? 2004 Elsevier B.V. All rights reserved.
PACS: 61.46.+w; 66.30.Pa; 83.10.Rs Keywords: Bulky shuttle memory device; Boron–nitride peapod; Boron–nitride nanotube; Fullerene; Molecular dynamics simulation; Nano nonvolatile memory
1. Introduction Nanostructure and nanoparticles are of great interest in materials science and technology because of their interesting structures, especially their size. However, it is hard to control their physical and chemical properties. Since fullerene-related materials have unique physical and chemical properties [1,2], they have attracted considerable attention this last decade. Compared to other nanostructures, fullerenes have found promising applications in a wide variety of very important technological processes such as in designing electronic devices, super-3bers, catalytic ∗ Corresponding author. Tel.: +82-2-820-5296; fax: +82-2-825-1584. E-mail address: [email protected] (J.W. Kang).
materials, etc. [3]. Especially the large empty space (particularly inside carbon nanotubes (CNTs)) also open new applications as storage materials with high capacity and stability [4]. These cavities are large enough to accommodate a wide variety of atomic and molecular species, the presence of which can significantly inBuence the properties of the materials. In particular, a new type of self-assembled hybrid structures called “nanopeapods”, consisting of fullerene arrays inside single-walled CNTs, have recently been reported [5–10]. The application of nanopeapods ranges from nanometer-sized containers of chemical reactant [8] to data storage [11] and high-temperature superconductor [12]. The encapsulation of fullerenes (such as C60 ) in nanotubes is favorable on energetic grounds and occurs rapidly by exposing nanotubes to sublimed fullerenes. Mickelson et al. [13] reported how to pack
1386-9477/$ - see front matter ? 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.physe.2004.01.012
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W.Y. Choi et al. / Physica E 23 (2004) 135 – 140
C60 in boron-nitride nanotubes (BNNTs), and Goldberg et al. [14] researched metal 3llings inside BNNTs. Okada et al. [15] showed the reaction energy of a BN nanopeapod such as (10, 10) BNNT + C60 → C60 at (10, 10) BNNT + 1:267 (eV). Although the quantity of the exothermic energy, 1:267 eV, obtained from the calculations of Okada et al. [15] has been doubtful [16], C60 at (10, 10) BNNT is obviously more stable than the structure that the (10, 10) BNNT is in3nitely separate with a C60 molecule. Kwon et al. [11] reported that multi-walled nanotubes called “bucky shuttle” [17] were synthesized from elemental carbon under speci3c conditions, and investigated bucky shuttle memory device, which acted as nanometer-sized memory element, using molecular dynamics (MD) simulations. Though CNTs are a representative nanostructure material, CNTs synthesized by experiments can be a semiconductor or a metal conductor by their chirality [2], because it is hard to control the chirality of CNT. However, since most of bandgap of BNNTs are about 5:5 eV, they are electrically insulators [18]. While aligned bucky shuttle structures are diLcult to be in self-assembly, nanopeapods can be synthesized in the aligned structures using bundles of single-walled BNNTs. Mickelson et al. [13] reported how to pack C60 in BNNTs, and Goldberg et al. [14] researched metal 3llings inside BNNTs. If some processes, such as C60 intercalation control, nanolithography, nanotube cutting and nanotube capping, are treated appropriately to the aligned nanopeapods, the aligned bucky shuttles can be synthesized. In this paper, we investigate the operations and the properties of proposed shuttle + memory system composed of three C60 s and a (10, 10) BNNT using classical MD simulations. 2. Methods and structure Endo-metallofullerenes have been investigated in experimental and theoretical studies. In this work, we assume that the charge of the endo metal encapsulated in fullerene is fully ionized such as F− , Na+ , K + , Mg2+ , and Al3+ . This assumption has been used in the previous works [11,19]. However, in our MD code, the endo metal were not included for the computational eLciency, whereas the charge of the C60 is assumed as +e and is uniformly distributed on the C60 , such
Fig. 1. Schematics of the fullerene-shuttle-memory-system based on BN nanopeapod : (a) BNNT opening, (b) one side copper 3lling, (c) fully ionized endo-fullerenes 3lling, (d) the other side copper 3lling and (e) shuttle memory system for MD simulations in this work.
as the previous work Kwon et al. [11]. Therefore, the charge per carbon atom is assumed as +e=60. For B–N interactions, we used the TersoM potential function that has been widely used [20]. BNNT wall and C60 interactions were modeled by the C60 –(10, 10) CNT interaction potential studied by Hodak and Girifalco [21]. Further work should include more realistic parameters of the interaction between BNNT and C60 . The C60 –C60 interactions were characterized with the Lennard-Jones 12-6 (LJ12-6) potential studied by Girifalco et al. [22]. For Cu–BN, we also used the P LJ12-6 potential with = 0:1448 eV and = 2:039 A [23]. For Cu used as the contacts, we used a LJ12-6 P potential [23] with = 0:415 eV and = 2:277 A. Fig. 1(a)–(d) shows the model schematics of the shuttle-memory-system proposed as follows: (1) opening both end of a nanotube, (2) 3lling conducting metal nanowires used as one-side contact by 3llings, (3) nanopeapod formation by fullerene encapsulation, and (4) 3lling conducting metal nanowires used as the other side contact by 3llings. In this work, we assumed the copper nanowires as the conductor material. However, to realize the proposed system, other materials that easily 3ll inside BNNTs should be used. Under external force 3elds, since fully ion+ + ized fullerene C60 can be accelerated, the C60 can
W.Y. Choi et al. / Physica E 23 (2004) 135 – 140
be shuttled by the alternative external force 3elds. Therefore, the bits of the proposed memory system can be de3ned by the positions of the fullerenes. We used both steepest descent (SD) and MD methods. The MD simulations used the same MD methods as were used in our previous works [24–28]. The MD code used the velocity Verlet algorithm, a Gunsteren –Berendsen thermostat to control temperature for all atoms except for fullerenes and neighbor lists to improve computing performance [29]. MD time step was 5 × 10−4 ps. The nanopeapod encapsulating three fullerenes was considered as the shuttle-memory-element as shown in Fig. 1(e). The structure of the copper nanowires was obtained from our previous work [30] that shows the copper nanowires encapsulated in armchair CNTs. The structure shown in Fig. 1(e) is consisted of a (10, + P length, three C60 10) BNNT with 54 A molecules, and both copper contacts composed of 60 atoms. At outside of the shuttle memory system, there is the alternative bias system to control the position of central fullerene. The structure was initially relaxed by the SD method; and then the atoms of both edges were 3xed during the MD simulations and for the other atoms, MD methods were applied.
3. Results and discussion Fig. 2 shows the energetics of the shuttle system as a function of the position of central C60 molecule.
Fig. 2. Energetics of the shuttle-memory-system as a function of the position of central C60 molecule.
137
Fig. 3. Velocities (Vz ) of the shuttle fullerene along the tube axis, the position variation (Pz ) of the shuttle fullerene along the tube axis as functions of MD step and external force 3eld (Fext ). The dashed and the solid lines indicate external force 3eld 0.09 and P respectively. MD temperature is 100 K. 0:1 eV= A,
Two C60 molecules at both ends were 3xed and the position of the central C60 molecule was displaced by P along the tube axis (the z-axis), and then the 0:01 A con3guration was relaxed by the SD method. Obviously, the van der Waals interaction stabilized the C60 molecules at either end of the nanopeapod, where the contact area is largest. This is reBected on the potential energy variation as shown in Fig. 2. The minimum potential energies are found near the both ends of the nanopeapod and the activation energy barrier is 0:1997 eV. For the structure that three C60 molecules are localized at either, the binding energy is higher (3:93 eV) than that for the structure in Fig. 2 in the empirical potential functions used. In our MD simulations, since the binding energy between the copper contact and one C60 molecule is higher than that between C60 molecules, after the left and the right force 3elds were successively applied, active elements were always changed into the structure as shown in Fig. 1(e). Two C60 molecules were always attached at the both end contacts and the shuttle medium was
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Fig. 4. Velocities (Vz ) of the shuttle fullerene along the tube axis, the position variation (Pz ) of the shuttle fullerene along the tube axis as functions of MD step and external force 3eld (Fext ). The dashed and the solid lines indicate external force 3eld 0.3 and P respectively. MD temperature is 100 K. 0:5 eV= A,
always the central C60 molecule. By the left-direction force 3eld, the left C60 was attached at the left copper contact, and by the right-direction force 3eld, the right C60 was attached at the right copper contact. The operations of the shuttle-memory-element proposed were investigated using classical MD simulations. Figs. 3–6 show the velocities (Vz ) of the shuttle fullerene along the tube axis, the position variation (Pz ) of the shuttle fullerene along the tube axis as functions of the MD step and the external force 3eld (Fext ). The MD temperature was 100 K in all cases. In Figs. 3 and 4, the MD simulations have been performed for the diMerent external force 3elds. ExP until 10,000 ternal force 3elds were initially 0 eV= A MD steps, and then successively changed to the objective external force 3eld in 100 steps. The external force 3eld continued in 10,000 MD step, and then, the external force 3eld decreased in the same way. Peaks + in the velocities (Vz ) are found when the shuttle C60 just before collides with the other fullerenes attached at both ends. After the collisions, the direction of
Fig. 5. Velocities (Vz ) of the shuttle fullerene along the tube axis, the position variation (Pz ) of the shuttle fullerene along the tube axis as functions of MD step and external force 3eld (Fext ). The solid line is a round-trip operation starting from bit 1 with external P The dashed line is opposite case with applied force 3eld 0:1 eV= A. P MD temperature is 100 K. force 3eld −0:1 eV= A.
+ the Vz of the shuttle C60 is changed in the opposite directions. In Fig. 3, when the external force 3eld + P C60 was below 0:09 eV= A, could not escape from left + end. The shuttle C60 under the external force 3eld P reached at the other side faster than that 0:3 eV= A P For the case under the external force 3eld 0:1 eV= A. P the time required of the external force 3eld 0:5 eV= A, to become entire bit Bip is similar with the case of P because of several the external force 3eld 0:3 eV= A, rebound events, although the accelerated speed of the + P is higher than that for 0:3 eV= A. P C60 for 0:5 eV= A For contemporary memory systems composed of ‘bit 0’ and ‘bit 1’, the storage data should be changeable from bit 0 to bit 1, and vice versa. We assign that the left and the right sides of the copper contacts are bit 0 and bit 1 as shown in Fig. 1(e). The external force P respec3eld in Figs. 5 and 6 were 0.1 and 0:2 eV= A, + tively. The solid lines indicate the case that the C60 was + initiated at bit 1 and the dashed lines indicate that C60
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is achieved after a long time because of several rebounded events. Therefore, to design the system proposed in this work, the switching speed, the applied force 3eld, and the active region should be considered. 4. Summary
Fig. 6. Velocities (Vz ) of the shuttle fullerene along the tube axis, the position variation (Pz ) of the shuttle fullerene along the tube axis as functions of MD step and external force 3eld (Fext ). The solid line is a round-trip operation starting from bit 1 with external P The dashed line is opposite case with applied force 3eld 0:2 eV= A. P MD temperature is 100 K. force 3eld −0:2 eV= A.
We studied the energetics and the operations of the fullerene-shuttle-memory-system based on a BN nanopeapod using classical MD simulations. The system proposed in this work was composed of a (10, 10) + molecules, and two copper BN nanotube, three C60 contacts. The lowest energy con3gurations were found + in the both ends terminated by C60 s attached at cop+ per contacts. Obviously, the interactions between C60 + molecules stabilized the shuttle C60 at the both ends of the proposed system where the contact area was the largest. Therefore, the bit Bops could be classi3ed with both the position of the shuttle media and the potential energy of the system. To design the proposed system, the switching speed, the applied force 3eld, and the active region should be considered. Classical MD simulations showed that the fullerene shuttle memory system based on BN nanopeapod and metal-3lling technologies could be operated by an adequate external force 3eld. References
+ C60
was initiated at bit 0. The shuttle under the applied P worked well. Although the shuttle force 3eld 0:1 eV= A + P had much C60 under the external force 3eld 0:2 eV= A more rebound events, the switching speed was high. P However, when the external force 3eld was 0:2 eV= A, the operations of the shuttle-memory-device were not stable, because there were the cases that entire bit Bips were not achieved as showed in Fig. 6. In the dashed + line in Fig. 6, the rebounded C60 returned to the initial position. When the active region is suLciently long, the rebounded events may be unimportant in the operation of this system. When the applied force 3elds are very low in the conditions that the shuttle fullerene can be switched, these rebounded events are not found. The switching speed is very low under the very low applied 3elds. When external force 3eld is high, though the switching speed is very high, the entire bit Bip
[1] S. Iijima, Nature 354 (1991) 56. [2] M.S. Dresselhaus, G. Dresselhaus, P.C. Eklund, Science of Fullerenes and Carbon Nanotubes, Academic Press, San Diego, CA, 1996. [3] J. Zhao, A. Buldum, J. Han, J.P. Lu, Nanotechnology 13 (2002) 195. [4] R.E. Barajas-Barraza, R.A. Guirado-Lopez, Phys. Rev. B 66 (2002) 155426. [5] B.W. Smith, M. Monthioux, D.E. Luzzi, Nature 396 (1998) 323. [6] B. Burteaux, A. Claye, B.W. Smith, M. Monthioux, D.E. Luzzi, J.E. Fischer, Chem. Phys. Lett. 310 (1999) 21. [7] B.W. Smith, M. Monthioux, D.E. Luzzi, Chem. Phys. Lett. 315 (1999) 31. [8] B.W. Smith, D.E. Luzzi, Chem. Phys. Lett. 321 (2000) 169. [9] K. Hirahara, K. Suenaga, S. Bandow, H. Kato, T. Okazaki, H. Shinohara, S. Iijima, Phys. Rev. Lett. 85 (2000) 5384. [10] J. Sloan, R.E. Dunin-Borkowski, J.L. Hutchison, K.S. Coleman, V.C. Williams, J.B. Claridge, A.P.E. Yorka, C. Xu, S.R. Bailey, G. Brown, S. Friedrichs, M.K.H. Green, Chem. Phys. Lett. 316 (2000) 191.
140
W.Y. Choi et al. / Physica E 23 (2004) 135 – 140
[11] Y.K. Kwon, D. TomVanek, S. Iijima, Phys. Rev. Lett. 82 (1999) 1470. [12] R.F. Service, Science 292 (2001) 45. [13] W. Mickelson, S. Aloni, W.-Q Han, J. Cumings, A. Zettl, Science 300 (2003) 467. [14] D. Goldberg, F.-F. Xu, Y. Bando, Appl. Phys. A 76 (2003) 479. [15] S. Okada, S. Saito, A. Oshiyam, Phys. Rev. B 64 (2003) 201303. [16] L.A. Girifalco, M. Hodak, Phys. Rev. B 65 (2002) 125404. [17] B.I. Yakobson, R.E. Smalley, Am. Sci. 85 (1997) 324. [18] X. Blase, A. Rubio, S.G. Louie, M.L. Cohen, Europhys. Lett. 28 (1994) 335. [19] G. Gao, T. Cagin, W.A. Goddard III, Phys. Rev. Lett. 80 (1998) 5556. [20] K. Albe, W. Moller, Comp. Mater. Sci. 10 (1998) 111.
[21] M. Hodak, L.A. Girifalco, Phys. Rev. B 67 (2003) 075419. [22] L.A. Girifalco, M. Hodak, R.S. Lee, Phys. Rev. B 62 (2000) 13104. [23] S. Dorfman, K.C. Mundim, D. Fuks, A. Berner, D.E. Ellis, J.V. Humbeeck, Mater. Sci. Eng. C 15 (2001) 191. [24] J.W. Kang, H.J. Hwang, Nanotechnology 13 (2002) 524. [25] J.W. Kang, H.J. Hwang, Physica E 15 (2002) 82. [26] J.W. Kang, H.J. Hwang, J. Phys.: Condens. Matter 14 (2002) 2629. [27] J.W. Kang, H.J. Hwang, Comp. Mater. Sci. 23 (2002) 105. [28] J.W. Kang, H.J. Hwang, J. Korean Phys. Soc. 40 (2002) 946. [29] Z. Mao, A. Garg, S.B. Sinnott, Nanotechnology 10 (1999) 273. [30] W.Y. Choi, J.W. Kang, H.J. Hwang, Phys. Rev. B 68 (2003) 193405.