Journal of the Korean Physical Society, Vol. 45, No. 2, August 2004, pp. 416∼422

Nanoscale Data Storage Device of Metallofullerene Peapods

Ki Ryang Byun, Jeong Won Kang∗ and Ho Jung Hwang

Nano Electronic Future Technology Laboratory, School of Electricaland Electronic Engineering, Chung-Ang University, Seoul 156-756

(Received 25 February 2004)

We studied the energetics and the operations of nonvolatile nano-memory elements based onmetallofullerene peapods by using classical molecular dynamics simulations. The system proposedin this work was composed of two carbon nanotubes and metallofullerenes. In classical moleculardynamics simulations, the open ends of two (10, 10) carbon nanotubes were face to face with aseparation of 8 A, and the metallofullerenes shuttled between two carbon nanotubes under alterna-tively applied force fields. When the metallofullerenes were encapsulated in the carbon nanotubes,the electronic properties of the metallofullerene peapods were different from those of the carbonnanotubes. Therefore, these changes could be detected by a current flow change; then, the bitclassifications could be done by using metallofullerene shuttle between two carbon nanotubes underapplied force fields. Since the metallofullerenes encapsulated in the carbon nanotubes can hardlyescape from the carbon nanotubes without external force fields, the proposed system can operate anon-volatile memory device.

PACS numbers: 61.46.+w, 66.30.Pa, 83.10.RsKeywords: Nanopeapod memory device, Nano non-volatile memory, Bucky shuttle memory device, Metallo-fullerene, Molecular dynamics simulation

I. INTRODUCTION

Fullerene-related materials have attracted consider-able attention this last decade due to their unique physi-cal and chemical properties [1,2]. Compared to other car-bon structures, fullerenes have revealed promising appli-cations in a wide variety of very important technologicalprocesses such as in designing electronic devices, super-fibers, catalytic materials, etc. [3]. Especially, the largeempty space inside carbon nanotubes (CNTs) has led tonew applications such as storage materials with high ca-pacity and stability [4]. These cavities are large enoughto accommodate a wide variety of atomic and molecularspecies, the presence of which can significantly influencethe properties of the materials. In particular, a new typeof self-assembled hybrid structure called “nanopeapods”,consisting of fullerene arrays inside single-walled CNTs,have recently been reported [5–10]. The applicationsof nanopeapods range from nanometer-sized containersof chemical reactants [8] to data storage [11] and high-temperature superconductors [12]. The encapsulation offullerenes (such as C60) in nanotubes is favorable on ener-getic grounds and occurs rapidly by exposing nanotubesto sublimed fullerenes. Kwon et al. [11] reported thatmulti-walled nanotubes called “bucky shuttles” [13] were

∗E-mail: gardenriver@korea.com;Tel: 82-2-820-5296; Fax: 82-2-825-1584

synthesized from elemental carbon under specific con-ditions and investigated bucky-shuttle memory devices,which acted as nanometer-sized memory elements, byusing molecular dynamics (MD) simulations. Cioslowskiand Fleischmann [14] investigated endohedral fullerene(metallofullerene) complexes, including F−, Ne, Na+,Mg2+ and Al3+. Many metallofullerenes and metallo-fullerene peapods have been synthesized and investigatedby several experimental and theoretical methods, andthese recent works have been reviewed in Refs. 15 ∼17. Micro-fluidic memory devices were recently demon-strated through the use of an aqueous viscoelastic poly-mer solution as a working fluid [18].

In our previous work [19–22], several electro-fluidic shuttle memory elements were investigated us-ing classical MD simulations, and the proposed sys-tems were bucky-shuttle memory elements (C+

36@C640

and C+60@C640), encapsulated-ion shuttle memory el-

ements (13K+)@C640, (3K+-C60-2 K+)@C640, (5K+-C60)@C640, and the metallofullerene shuttle memory el-ement ((K+@C60-F−@C60)@C640). However, for theabove shuttle memory elements, the bit classificationsare very complicated to define and to detect by usingthe connected electrodes because the shuttle media arealways inside the same nanocapsule. For the bucky-shuttle memory of Kwon et al. [11], the stored data canbe measured by using the current pulse in the connectingwires, which current is caused by the motion of chargedmolecules due to the applied probing voltage. Therefore,

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Nanoscale Data Storage Device of Metallofullerene Peapods – Ki Ryang Byun et al. -417-

if the stored data is to be read, the K+@C60 should beshuttled. For example, when the K+@C60 settles in theposition of ‘bit 1’, the K+@C60 has to moved towardthe position of ‘bit 0’ during the data reading processes;then ‘bit 1’ can be measured. After the data reading, theK+@C60 should be returned to the position of ‘bit 1’ tomaintain its original data. Therefore, the data readingprocesses have to include the data erasing/writing pro-cesses; thus, both power consumption and data process-ing time increase. However, when the K+@C60 comesand goes between separate nanocapsules, simpler data-reading processing can be achieved. Our previous workused MD simulations to show nanotube-to-nanopeapodtransitions under external electrostatic force fields [22].For the nanoscale memory device proposed in this work,the nonvolatility and the data writing/erasing processesare the same as those for the bucky-shuttle device ofKwon et al. However, for the data reading processes,our proposed system is different from that of Kwon etal, and this will be discussed in the next section.

While aligned bucky-shuttle structures are difficult toachieve in self-assembly, nanopeapods can be synthe-sized in aligned structures by using bundles of single-walled CNTs. If some processes, such as nanolithog-raphy, carbon nanotube etching or cutting, metallo-fullerene intercalation control, carbon nanotube capping,and metal fillings for electrodes, are applied appropri-ately to the aligned nanopeapods, aligned bucky-shuttleelements can be synthesized. The electronic proper-ties of CNTs are changed by the intercalation of met-allofullerenes into CNTs [23,24]. Therefore, if metallo-fullerene intercalation into CNTs can be controlled byusing external force fields, the digit composed of ‘bit 0’and ‘bit 1’, which is widely used in electronic memory de-vices, can be considered by using a nanotube (bit 0) and ananopeapod (bit 1). In this paper, we present schemat-ics of a nano-memory device based on metallofullerenepeapods. We expect that a memory element based onnanopeapods can be realized by using nanotechnologybased on carbon nanosciences. Therefore, in this paper,using classical MD simulations, we investigate the opera-tions and the properties of a memory element composedof fully ionized metallofullerenes and (10, 10) CNTs.

II. SCHEMATICS

Many metallofullerene nanopeapods have been investi-gated in experimental and theoretical studies. If the en-capsulated metallofullerenes are to be moved by an elec-trostatic force field, the metallofullerenes should carrya net charge. As the number of carbon atoms compos-ing the fullerenes increase, the electron affinity gener-ally increases [25]. The electron affinities of the met-allofullerenes are generally higher than those of emptyfullerenes [26, 27]. Some ab-initio calculations for met-allofullerene peapods have shown charge transfer from a

nanotube to a metallofullerene or fullerene [24, 28, 29].Chen and Lue [30] showed that the electron affinities ofthe CNTs were 1.0 ∼ 1.5 eV, which were lower than thevalues ∼3 eV for the electron affinity of fullerenes [25],by using the theoretical fittings based on field-emissionexperiments on CNTs. The above results imply that thecharge of the encapsulated metallofullerene can be neg-ative due to charge transfer from a nanotube to a met-allofullerene. However, contrary results and discussion ofthe charge transfer have been also found. Cioslowski etal. [31] showed that the electron affinities of nanotubeswith finite lengths were generally higher than those ofempty fullerenes. Kazaoui et al. [32] estimated thatthe average electron affinity of semiconductor single-wallCNTs was 4.8 eV by using in-situ measurements of opti-cal absorption spectra. Hirahara et al. [33] discussed thepossibility of charge transfer from the Gd@C82 metallo-fullerene to a single-wall CNT. These results imply thatthe charge of the encapsulated metallofullerene can bepositive. Therefore, we think that charge transfer of met-allofullerene peapods should be investigated in furtherworks. Although there are opposing results for chargetransfer, the metallofullerene can have either a negativeor a positive net charge.

In this work, we follow the assumption by Kwon etal. [11] for the charge of the metallofullerene encapsu-lated in CNTs. In the K@C60 metallofullerene, which isknown to form spontaneously under synthesis conditionsin the presence of potassium, the valence electron of theencapsulated potassium atom is completely transferredto the C60 shell; then, K+@C60 is formed [34,35]. Kwonet al. assumed that the C60 shell was likely to transferthe extra electron to the graphitic outer capsule. The ex-tra electron on the outer capsule would likely be furthertransferred to the structure that held the bucky-shuttledevice. Therefore, Kwon et al. modeled the dynamicsof the K@C+

60 ion in the neutral carbon capsule by uni-formly distributing a static charge of +1e over the C60

shell. However, in our MD code, the endo metal was notincluded for computational efficiency whereas the massof the C60 was increased by the mass of the potassiumatom and the charge of the C60 shell was assumed to be+1e and was uniformly distributed on the C60 shell, suchas in the previous work by Kwon et al. Therefore, thecharge per carbon atom was assumed to be +e/60.

Figure 1 shows the schematics of the proposednanopeapod memory elements (if technologies are sup-portive); (a) alignment of single-walled CNTs, (b) lithog-raphy using mask and etching of CNTs, (c) formation ofnanopeapods by filling of fully ionized metallofullerenes,(d) formation of nanopeapods for only one side by usingexternal force fields and evaporations and (e), in orderthat the metallofullerenes cannot escape from the sys-tem, reduction of the gaps between both CNTs by usingmechanical controls. For the processes from (c) to (d),our previous work showed that the structure shown inFig. 1(d) could be realized when the external force fieldswere adequately applied. The structure consisted of one-

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Fig. 1. Process schematics of proposed nanopeapod mem-ory: (a) aligning of single-walled CNTs, (b) lithography us-ing mask and etching of CNTs, (c) formation of nanopeapodsby filling of fully ionized metallofullerenes, (d) formation ofnanopeapods for only one side using external force fields andevaporations, and (e) in order that the metallofullerenes cannot escape from the system, reduction of the gaps betweenboth CNTs by mechanical controls.

side-capped two (10, 10) CNTs including six C+60’s. The

open ends of the two (10, 10) CNTs were face to facewith a separation of 16 A on the same tube axis. Theexternal force fields were applied along the direction ofthe tube axis, 0.54 eV/A, as well as perpendicular tothe tube axis, 0.6 eV/A. Three C+

60’s were encapsulatedin a one-side CNT whereas the other C+

60’s escaped theother CNT and was not encapsulated in any CNT. In or-der that the three C+

60’s cannot escape from the system,the gap between both CNTs should be reduced by usingmechanical controls. In this work, the reduced gap of 8A was used.

The electronic properties of nanopeapods are very dif-ferent from those of empty CNTs [23,24,26,28,33]. Underexternal force fields, since metallofullerenes with a netcharge can be accelerated, the metallofullerenes can beshuttled by the alternate external force fields. Therefore,the changes in the electronic properties can be used todefine or classify bits in the change in the current con-duction. For example, for the K@C60 metallofullerenesconsidered in this work, when the K@C+

60’s come andgo between separate nanotubes, a nanoscale memory de-vice as shown in Fig. 2 is possible. When the K@C60

is encapsulated in a (17, 0) single-wall CNT, the dop-ing energy level of the K@C60@(17, 0) peapod is about0.3 eV lower than the conduction band energy lever ofthe (17, 0) CNT, and when it is encapsulated in a (16,0) single-wall CNT, the K@C60@(16, 0) peapod is an n-type semiconductor [26]. Although we don’t present the

Fig. 2. Under external force fields due to the switchingelectrodes, since metallofullerenes with net charge can be ac-celerated, the metallofullerenes can be shuttled by using thealternate external force fields. The electronic properties ofthe nanopeapods are very different from those of the emptyCNTs. Therefore, changes in the electronic properties can de-fine or classify of bits in the change of the current conduction(Id) under the applied bias (VA) by using the bit detectioncircuit.

electronic properties of the system in Figs. 1 and 2, wethink that the recent results for metallofullerene peapodsby Du and Cheng [26] show that the electronic proper-ties of empty carbon nanotubes are different from thoseof carbon nanotubes filled with K@C60. Since the infi-nite nanotube structure investigated by Du and Chengis different from the finite nanotube structure consideredin this work, the electronic properties of the carbon nan-otube with both a one-side spherical cap and an openend in our simulation must be different from those of aninfinite carbon nanotube. However, the electronic prop-erties and the current tunneling of a finite nanotube en-capsulating metallofullerenes can be clearly understoodto be different from those of an empty finite nanotube.

The nonvolatility and the data writing/erasing pro-cesses are the same as those for the bucky-shuttle de-vice of Kwon et al. [11] whereas, the system shown inFigs. 1 and 2 is different from that of Kwon et al. forthe data reading processes. For the bucky-shuttle mem-ory suggested by Kown et al., the reading processes canbe measured by using the current pulse in the connect-ing wires, the current pulse being caused by the mo-tion of the K@C+

60 due to the applied probing voltage.Therefore, the data-reading processes have to include thedata erasing/writing processes, the K@C+

60 shuttles in ananocapsule. However, in our proposed element, whenthe K@C+

60’s come and go between separate nanocap-sules, as shown in Fig. 2, electrodes to detect the storeddata are connected to the left-hand carbon nanotube.The right-hand CNTs are connected to ground, and theleft-hand CNTs are connected to the bit writing or eras-ing lines and the bit reading lines. For bit writing, whenforward force fields are applied, the K@C+

60’s transferfrom the left- to the right-hand CNTs; then, the bits can

Nanoscale Data Storage Device of Metallofullerene Peapods – Ki Ryang Byun et al. -419-

be read through the bit detection circuit. For bit eras-ing, when backward force fields are applied, the K@C+

60’stransfer from the right to the left CNTs; then, the bitscan be read through the bit detection circuit. When anapplied bias (VA) is applied to the data-reading gate,the current flow (Id) of the empty nanotube is differentfrom the current flow (Id) of the peapod [26,29]. There-fore, semiconducting nanotubes are more effective thanmetallic nanotubes for application to the proposed mem-ory device.

III. SIMULATION METHODS

For carbon-carbon interactions, we used the Tersoff-Brenner potential function [37–39] that has been widelyapplied to carbon systems. The long-range interactionswere characterized with the Lennard-Jones 12-6 poten-tial [40]. Detailed descriptions of the empirical potentialsused in this work can be found in our previous papers[19–22]. We used both steepest descent (SD) and MDmethods. The MD simulations used the same MD meth-ods as were used in our previous works [41–45]. TheMD code used the velocity Verlet algorithm and neigh-bor lists to improve computing performance [46]. TheMD time step was 5 × 10−4 ps. Initial velocities wereassigned from the Maxwell distribution, and the magni-tudes were adjusted so as to maintain the temperaturein the system. A Gunsteren-Berendsen thermostat wasused to control the temperature for all atoms, except forthe fullerenes [46].

In this work, the structure of an element consisted oftwo one-side-capped (10, 10) CNTs with spherical capsand one or three C60’s. The open ends of the two (10,10) CNTs were face to face with an initial separationof 8 A; then, the structures were relaxed by using theSD method. We assumed that the CNTs interacted withthe substrate and that the materials used as the maskstill covered the CNTs. Therefore, the atoms of the cen-tral carbon-ring along the tube axis were fixed duringthe MD simulations, and the MD methods were appliedto the other atoms to avoid shifts of the CNTs due tothe collisions between the fullerenes and the caps of theCNTs. For the MD simulations in this work, we usedmetallic (10, 10) nanotubes because the schematics andthe energetic operations of the proposed system were fo-cused on this work and because (10, 10) nanotube couldeasily be connected with spherical caps. However, if theproposed memory element is to be realized, semiconduct-ing nanotubes, such as a (17, 0) CNTs, should be used.The binding energy of a C60@(17, 0) peapod is similarto that of a C60@(10, 10) peapod [36].

IV. MD SIMULATION RESULTS ANDDISCUSSION

Fig. 3. (a) Position as a function of the MD time for vari-ous temperatures and (b) the initial and the final structuressucked into the CNTs.

Figure 3(a) shows the position variations of a C60

fullerene as a function of the MD time for various tem-peratures. The fullerene was initially located at the mid-dle position between both CNTs. Since the initial ve-locities and displacements of the atoms were randomlyallocated using a Gaussian random number generator,the fullerene could be sucked in any CNTs under thegiven conditions. Figure 3(b) shows the initial and thefinal structures. Figure 4 shows the energy as a func-tion of the position of the fullerene for the system shownin Fig. 3(b). After the system had been fully relaxedunder the fixed C60 fullerene, the positions of the C60

fullerene were displaced by an increase or a decrease of0.1 A; then, the configurations were relaxed by using theSD method. The potential energy where the fullerene issucked into a (10, 10) CNT is 3.543 eV lower than thepotential energy where the fullerene is settled betweenboth CNTs. This value is slightly higher than the value,3.01 eV, obtained from the work by Ulbricht et al. [47]and the value, 3.26 eV, obtained from the work by Gir-ifalco et al. [48]. The potential well depth used in thiswork was slightly higher than those used by Ulbricht etal. [47] and Girifalco et al. [48]. The binding energyof a C60 fullerene at a spherical cap of a (10, 10) CNTobtained from Girifalco et al. [48] was 4.40 eV; then, theenergy to escape from the spherical cap was 1.14 eV. Inthis work, these values were 5.004 and 1.461 eV, respec-tively. Also, the activation energy of the C60 fullereneencapsulated in the (10, 10) CNT was zero in this workas in the previous works [47,49]. Berber et al. [49] alsoshowed that the encapsulation process did not involvean activation energy barrier by using tight-binding MDsimulations. Qian et al. [50] showed that C60 was sucked

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Fig. 4. Energy as a function of the position of the fullerenefor the system shown in Fig. 3(b).

Fig. 5. Atomic structures in the MD simulation at 10 Kwhen a force field of 0.6 eV/A per C+

60 was applied to thestructure of the left nanopeapod encapsulating three C+

60’s:atomic configurations at (a) 0, (b) 5, and (c) 50 ps.

into the (10, 10) CNT due to the sharp surface-tensionforce present at the front of the open end and then os-cillated between the two open ends of the CNT, neverescaping. The above results show that for the operationof the proposed memory device, an external force fieldunder the condition in order that a C60 escapes from a(10, 10) CNT should be applied to the C60. When anyexternal force field is not applied, the C60 fullerene al-ways settles the both end caps of the CNTs where thecontact areas are large and the binding energies are high,as shown in Figs. 3 and 4. Since it is very difficult forthe fullerene to naturally escape from the (10, 10) CNT,the proposed element can be applied to a non-volatilememory device on a nanometer scale.

Figure 5(a) shows the left-side nanopeapod encapsu-

Fig. 6. Central position of three C+60’s as a function of the

MD time for various temperatures. The solid line, the filledcircles, and the diamonds indicate applied force fields of 0.6eV/A per C+

60 at 10 K, 0.06 eV/A per C+60 at 300 K, and 0.06

eV/A per C+60 at 10 K, respectively.

lating three C+60’s. The structure was the same as that

shown in Fig. 3, except that the open ends of the two(10, 10) CNTs are face to face with a separation of 8A in the same tube axis and that the right CNT is justvacant. Thus structure is the structure after the processfrom Fig. 1(d) to Fig. 1(e). We performed the MD sim-ulation at 10 K for an applied force field of 0.6 eV/A perC+

60. Figures 5(b) and 4(c) show the structures at 5 andat 50 ps, respectively. We found that the transitions ofC+

60’s from the left- to the right-hand CNTs, and changesof metallofullerene positions were achieved by using theexternal force fields. Figure 6 shows the central positionof the three C+

60’s as a function of the MD time for vari-ous temperatures. When the applied force field was 0.6eV/A per C+

60 at 10 K, all C+60’s escaped from the orig-

inal CNT; then, they were sucked into the other CNT.When the applied force field was 0.06 eV/A per C+

60 at10 K, no C+

60 could escape from the original CNT. Whenthe applied force field was 0.06 eV/A per C+

60 at 300 K,two C+

60’s escaped from the original CNT and were thenjust sucked into the other CNT whereas one C+

60 couldnot escape from the cap of the CNT. In the MD simula-tions at 10 K, when the applied force fields were above0.54 eV/A per C+

60, all C+60’s escaped from the original

CNT and were then sucked into the other CNT. Whenthe applied force fields were 0.24 to 0.50 eV/A per C+

60,only one or two C+

60’s escaped from the original CNT andwere then sucked into the other CNT. In the MD simula-tions at 300 K, when the applied force fields were above0.36 eV/A per C+

60, all C+60’s escaped from the original

CNT and were then sucked into the other CNT. When

Nanoscale Data Storage Device of Metallofullerene Peapods – Ki Ryang Byun et al. -421-

the applied force fields were 0.06 to 0.3 eV/A per C+60,

only one or two C+60’s escaped from the original CNT and

then were sucked into the other CNT. As Fig. 4 shows,since the binding energy of a C60 fullerene in a (10, 10)CNT is highest at either end cap of CNTs, the externalforce fields should be applied so that the C60 fullerenecan escape from the caps of CNTs. For the fullerenes,except for the fullerene captured at the cap, since thebinding energy is constant inside CNTs as shown in Fig.4, the fullerenes are easily accelerated. Therefore, the ac-celerated fullerenes can escape from the CNT easier thanthe fullerene captured at the cap. Our MD simulationsshowed that the fullerene shuttle-memory elements basedon carbon nanopeapods could be operated by using ad-equate external force fields. When several technologiesare supportive, we expect that the proposed systems willbe realized and be used as an active device.

V. SUMMARY AND FUTURE WORK

This work showed the schematics, the energetics, andthe operations of a nonvolatile nano-memory element.The system proposed in this paper was composed ofone-side-capped (10, 10) carbon nanotubes and metallo-fullerenes. The open ends of two (10, 10) carbon nan-otubes were face to face with the separation of 8 A dis-tance, and the metallofullerenes shuttled between twocarbon nanotubes under alternately applied force fields.Our idea was as follows: when the metallofullerenes areencapsulated in the carbon nanotubes, the electronicproperties of the carbon nanotubes are changed. Whenthese changes can be controlled by using external forcefields, then bit classifications can be achieved. Since it iswell known that fullerenes encapsulated in carbon nan-otubes can hardly escape from carbon nanotubes withoutexternal force fields, the proposed system can operate asa non-volatile memory device. Since the interactions be-tween the fullerene and the cap of carbon nanotube sta-bilized the shuttle fullerene at both ends of the proposedsystem where the contact area was largest, the externalforce fields should be applied so that the fullerene canescape from the caps of the carbon nanotubes. There-fore, if the proposed system is to be operated, metallo-fullerenes with a net charge should be involved. Theswitching processes were investigated for external forcefields and temperatures. Classical molecular dynam-ics simulations showed that a metallofullerene shuttle-memory element based on carbon peapods could be op-erated by using an adequate external force field. Theproposed system can also be applied to a nano-electro-mechanical system as a non-volatile memory device.

This work shows the possibility of a shuttle memory el-ement based on metallofullerene peapods in the nanome-ter range. For the proposed system to effectively operate,the switching speed, the applied force field, the activeregion, the length of the CNTs, and the number of op-

erating metallofullerenes should be considered in futureworks. The authors expect the proposed system to berealized.

ACKNOWLEDGMENTS

This work was supported by the Research ProfessorGrant of the Information Telecommunication ResearchInstitute (ITRI), Chung-Ang University.

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