NANO LETTERS
A Molecular Linear Motor Consisting of Carbon Nanotubes
2009 Vol. 9, No. 1 62-65
Hiroshi Somada,†,‡ Kaori Hirahara,*,†,§ Seiji Akita,‡ and Yoshikazu Nakayama†,‡ Department of Mechanical Engineering, Graduate School of Engineering, Osaka UniVersity, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan, Department of Physics and Electronics, Osaka Prefecture UniVersity, 1-1 Gakuen-cho, Sakai, Osaka 599-8531, Japan, and Frontier Research Base for Global Young Researchers, Graduate School of Engineering, Osaka UniVersity, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan Received July 31, 2008
ABSTRACT We experimentally investigated a “molecular-linear-motor” system consisting of a capsule-like carbon nanotube (CNT) in the interior space of a host CNT. Transmission electron microscopy revealed the capsule traveled back and forth between both ends of the hollow space along the axial direction and rotated simultaneously. The mechanism was well explained with molecular dynamics simulation by considering the driving force supplied from thermal energy. The present system operates around room temperature and this opens up the possibility of designing novel nanodevices such as oscillators and switching memory devices.
The molecular motor, which transfers given energy to mechanical motion on a single molecule scale, plays an important role in bioengineering, for example, in transferring cellular materials and contracting muscles. Such a system would have valuable application in nanotechnology; that is, it would evolve into components of information transportation in a nanodevice. Therefore, it is essential to design synthetic molecular motor systems. There are two types of motions in molecular motors consisting of natural proteins: rotary motion provided by proteins such as bacterial flagella and linear motion provided by other proteins such as myosin and kinesin. Some synthetic molecular motors providing rotary motion have been reported and are constructed using organic molecules1 and carbon nanotubes (CNTs).2 CNTs are one of the most promising materials for components of nanodevices owing to the unique electronic properties originating from their atomic structures.3 Synthetic linear motion also has been reported recently;4 Cargo made of the outermost layer of a multiwall CNT (MWCNT) traveled forth and back along the inner coaxial CNT. Here we have studied the other fabrication process of molecular linear motors comprising much tiny sized CNTs, with diameters up to only a few nanometers. Among various types of CNT materials, capsule-like short CNTs are thought to be good components of a linear transportation system, as theoreticians have * Corresponding author,
[email protected]. † Department of Mechanical Engineering, Graduate School of Engineering, Osaka University. ‡ Department of Physics and Electronics, Osaka Prefecture University. § Frontier Research Base for Global Young Researchers, Graduate School of Engineering, Osaka University. 10.1021/nl802323n CCC: $40.75 Published on Web 11/25/2008
2009 American Chemical Society
proposed many types of linear motor systems applicable to devices such as memory devices.5-8 In this paper, we present experimental evidence that such a CNT capsule works as a molecular linear motor as well as a molecular rotary motor inside the hollow spaces of host CNTs, by means of transmission electron microscopy. CNT capsules were obtained by coalescence of C60 molecules encapsulated in carbon nanotubes assisted by heat treatment in vacuo. In the preparation of double-wall CNTs, fullerene molecules coalesce and form a long CNT newly along the host CNT with heating in vacuo at 1473 K.9 Since the length of the CNT formed is determined by the number of C60 molecules coalesced, some control is possible by changing the temperature and treating time. In the present study, we obtained CNT capsules with 3-10 nm lengths encapsulated in the interior of host CNTs by heating at 1073-1373 K for 8 h. The CNT capsules were observed with a transmission electron microscope (TEM), JEM-2500SE from JEOL Co. Ltd., at room temperature. The equipment was tuned with a 90 kV acceleration voltage to reduce additional structural changes in the carbon materials due to electron irradiation. Panels a-g of Figure 1 show a series of TEM images of a CNT capsule encapsulated in an interior hollow space of a single-wall CNT (SWNT) and Figure 1h is a schematic illustration of the encapsulation. The TEM images are picked up from of a sequential movie for 175 s, which are shown in Supporting Information. The CNT capsule is ∼0.95 nm in diameter and ∼3.2 nm in length. The diameter of the host SWNT is ∼1.6 nm, and the length of the hollow space is
Figure 1. (a-g) A series of TEM images showing linear motion of a CNT capsule. The exposure time for each image is 500 ms, and the total recording time was ∼10 min. The CNT capsule is ∼0.95 nm in diameter and ∼3.2 nm in length. The diameter of the host SWNT is ∼1.6 nm, and the length of the hollow space is about 8.5 nm. The CNT capsule is encapsulated in a hollow space between A and B as illustrated in (h). (i) Capturing intervals at sides A and B for seven laps.
∼8.5 nm. The end of the hollow space indicated by arrow A is closed by a hemisphere-like cap of another encapsulated CNT, the length of which is more than 100 nm. The other end, indicated by arrow B, seems also to be closed by a hemisphere-like cap of a capsule-like material. Initially, the CNT capsule is located on side A of the hollow space (Figure 1a). After several seconds, we find the capsule quickly travels to side B (Figure 1b). The capsule then travels back to side A after a couple of seconds (Figure 1c). In an observation of about 170 s, the capsule travels back and forth seven times. It is noted that the CNT capsule continued to travel after the recording stopped. The “linear-motor”-like motion of the CNT capsule observed in this study has two characteristics. One is the second-scale time interval. Time intervals for the CNT capsule being at sides A and B for the seven laps are presented in Figure 1i. We see the CNT capsule usually takes rest (is captured) at either end for 1-5 s, and sometimes up to 40-50 s. The other is the very quick movement of the CNT capsule. It was difficult to capture the moment of movement in the present experiment, so the traveling time from one end to the other is less than the frame rate (0.5 s). We consider two questions regarding the mechanism of the linear motion in the present system: why the CNT capsule remains stationary at the ends and what the driving force to Nano Lett., Vol. 9, No. 1, 2009
start traveling is. As an answer to the questions, we propose here the CNT capsule becomes stationary at the ends owing to interactions with the caps based on the van der Waals (vdW) interactions and its travel is initiated by thermal energy. The detail is as follows. First, it is reasonable to consider the entire hollow space of the host SWNT is energetically equivalent, except at the ends owing to the caps of other encapsulated tubes. Since the vdW interactions between a cap and the CNT capsule gain the total energy of the whole system, the capsule can become stationary. The total potential energy calculated by molecular dynamical (MD) simulation has “valleys” at the ends of the hollow space (Figure 2). The vdW energy W is very sensitive to the diameters of CNTs comprising the system and can be estimated by W ) -(A ⁄ 12d)D1D2 ⁄ (D1 + D2)
(1)
where D1 and D2 are the diameters of two hemispheres facing each other at a distance d. The diameter of the CNT capsule shown in Figure 1 is approximately 0.95 nm, and that of the cap at side A is approximately 1.0 nm. The value of d approximately corresponds to the interlayer distance of graphite as 0.34 nm. A is the so-called Hamaker constant, which is ∼6 × 10-19 J (for multiwall CNTs10). Accordingly, W is estimated as -0.71 × 10-19 J or -450 meV. 63
Figure 2. (a) Structural model of the system used for calculation. A (12, 0) capsule is encapsulated in a (22, 0) host SWNT, and both ends are sealed by two fixed (12, 0) CNTs. (b) Calculated total potential energy showing the existence of energy “valleys”.
We now consider the activation energy for movement of the capsule travel. Assuming the activation energy is thermal energy at room temperature, we can estimate the activation energy E on the basis of the Arrhenius equation E ) kBT ln νt
(2)
where kB, T, ν, and t represent the Boltzmann constant, absolute temperature, lattice vibration in carbon (1012-1013 Hz), and activation time, respectively. According to this equation, when the capsule leaves the energy valley with 450 meV depth as estimated above, it is estimated to take only ∼0.3 ms at 300 K. This estimated value is not consistent with the experimental result, however, since the capsule actually traveled in intervals of the order of seconds at room temperature. This suggests not only the apparent energy valley originates from the simple vdW interaction but also there are additional elements. In results from MD calculations, for the capsule traveling at rather high temperature, the host CNT had a wavelike deformation owing to thermal fluctuation of the graphitic lattices. Such a waving feature was remarkable in the hollow portion rather than in the portion encapsulating the capsule. Moreover, further calculations at far higher temperatures revealed the capsule did not travel because the hollow space deformed too greatly to maintain a tubular path for the capsule. Therefore, we can say in the present system the thermal energy not only activates the capsule travel from the energy valley due to the vdW interaction but also likely obstructs the travel of the capsule by deforming the host space. Since the magnitude of E becomes approximately (1.15-1.23) × 10-19 J or 720-770 meV at T ) 300 K and t ) 1 s, there is an additional energy valley probably up to ∼300 meV due to thermal fluctuation in the present system. Accordingly, the mechanism of the CNT linear motor fabricated in this study can be explained by considering an equilibrium among the thermal energy and the energy gain due to the vdW energy as well as resistance originating in lattice fluctuation due to heating; that is, the thermal-energyderived motor. Figure 1g indicates the time intervals of the capsule being on side A are likely to be longer than the time intervals on side B. This can be explained by considering 64
the heat conducting between the CNT capsule and the other long tube at side B. Since the long tube is only partially irradiated by the electron beam, which is probably able to supply a small amount of additional thermal energy,11 it can rather easily transport heat to outside the irradiation area. This may support the mechanism discussed in this paper that the driving force is the thermal energy. Alternatively, the cap-cap interactions may slightly differ for sides A and B because of different cap shapes or sizes of the sealing. One of the most intriguing results for the linear motor system in the present study is the system operates around room temperature. The thermal energy obtained during ∼1s at room temperature is the appropriate amount to extricate the CNT capsule from the energy valleys at the ends of the hollow space; that is, there is thermal equilibrium. Control of the equilibrium between these two energies is essential in realizing effective control of the linear motion. The depth of the energy valley due to vdW interaction strongly depends on the diameters of CNT capsules and caps of CNTs sealing both ends of the hollow space. According to MD simulation, for example, the energy gain between two caps of (12, 0) tubes with 0.95 nm diameters encapsulated in a (22, 0) host tube is approximately 450 meV, while that for two caps of (5, 5) tubes with 0.7 nm diameters in a (10, 10) host tube is approximately 60 meV. These results suggest that ranges of oscillation frequencies at room temperature can be tuned by selecting certain diameters of CNTs. A (12, 0) CNT capsule sealed by two other (12, 0) tubes in a host CNT will work as a kilohertz oscillator. A very thin CNT capsule such as a (5, 5) tube sealed by two other (5, 5) tubes will have up to sub-terahertz oscillation. Further fine-tuning of the frequencies is also possible by slightly changing the temperature of the whole system following eq 2. It is also interesting to employ a thicker capsule than that in the present system. If we choose CNT capsules such that they are activated at a temperature slightly higher than the atmosphere temperature, we can switch their movement on/off by providing/stopping additional energies such as those of heat, light, and electron beams. When such energies are provided to the whole system, linear motion will occur. On the other hand, by providing energy only to the small local area on one trapping side of a CNT capsule using a pulsed convergent electron beam, the CNT capsule can only travel one way, not being able to return. This suggests the possibility to control the location of the CNT capsule by moving it to one end of the hollow space; hence we have a switching memory device. However, we should consider detecting the switching from “bit 1” to “bit 0” by a method other than observation by TEM, which might not be a practical design. Concerning this problem, some theoretical approaches have been reported.5,6 Kwon et al. have theoretically proposed the utilization of ionized fullerene molecules as the bucky shuttle memory devices.5 This idea may be practical for applying to the present system, since we can dope ionic metals together with C60 molecules into the host CNTs in the preparation of peapods, or dope metallofullerene molecules instead of C60 molecules. In the latter case, it has been reported a metal nanowire can be fabricated inside CNTs newly formed by heating peapods.12 In addition, Legoas et al. predicted the CNT capsule shows gigahertz frequency when both ends of the host CNT are open or connected to thick CNTs.7 Although the system is different Nano Lett., Vol. 9, No. 1, 2009
motor but also as a switching memory device by controlledsupply of thermal energy, and the amount of required energy depends on the diameters of the CNTs. We should note again that the greatest advantage of a nanoscale motor system comprising CNT capsules is the operational temperature range is around room temperature. In addition, techniques for handling, processing, and attaching a single CNT in SEM and TEM have been developed, and we may install CNT components such as motors or switches by hand.16-20 Accordingly, these finding and technologies give great expectation to realizing CNT devices.
Figure 3. Enlarged TEM images of the CNT capsule in Figure 1, indicating rotary motion.
from the present one in this paper, it will also operate as an oscillator of an electromagnetic wave if the CNT capsule has a charge. In addition, we found the CNT capsule has not only linear motion but also simultaneous rotary motion. Closer views of the capsule in Figure 1 are given in Figure 3. The apparent shapes of the caps on sides A and B differ, although they are the same portion of the CNT capsule. At side A, the cap left of the capsule has a rather sharp feature and the tips in individual images, as indicated by arrows, seem upturned by about 0.1 nm above the center axis (dotted lines) of the host SWNT. On the other hand, the same cap seems rather rounded when the capsule travels to side A, and the tip is almost on the center line of the hollow space. CNTs with approximately 0.9 nm diameters often have asymmetric caps because of their geometry, so the projected shapes change with rotation. Therefore, the changing in apparent shapes of the caps observed in the present study directly shows the CNT capsule rotates in the interior of the host CNT. This rotation could potentially be applied in a rotary motor. Kwon et al. have theoretically predicted that a DWNT structure can work as a Feynman’s ratchet motor; the difference in chiralities between inner and outer tubes corresponds to an asymmetric gear.13 Saito et al. have also theoretically proposed the atomic corrugations between inner and outer tubes in 2001.14 Experimentally, another type of synthetic CNT motor reported by Barreiro et al. showed rotary motion, and the mechanism was discussed on the basis of a similar idea.4 Although we have not yet obtained direct evidence experimentally proving that the rotary motion of coaxial CNT system originated in the atomic corrugation with different chiralities, it will be examined in the very near future. Such behavior may be examined by means of nanobeam electron diffraction of the capsule and host tube, which can uniquely determine the chiral indices individually, if a rather long capsule (at least 5 nm) is obtained.15 In summary, the present study investigated the linear and rotary motions of a CNT capsule at room temperature when it is sealed by other CNTs in a hollow space of a host CNT. It is the first observation of linear motion of CNT capsules. Such a system can be obtained by simply heating C60 peapods, and its size is comparable or much smaller than wellknown protein-based molecular motors in the bioengineering field. The experimental results were well explained by considering the driving force as simply thermal energy. The CNT capsule can be worked not only as a frequency-tunable Nano Lett., Vol. 9, No. 1, 2009
Acknowledgment. The authors thank H. Mori for theoretical studies on CNT motors. The authors also thank Y. Ueno for helping with the MD calculations. K.H. benefitted from fruitful conversation with K. Okazaki-Maeda on the interaction between electrons and materials. This work was supported by a Grant-in-Aid for Scientific Research on the Priority Area ”Carbon Nanotube Nano-Electronics” from the Ministry of Education, Science and Culture of Japan. Supporting Information Available: Movies showing linear motion of a CNT capsule. One consists of sequential TEM images experimentally taken every 0.5 s, and some of them were picked up and shown in Figure 1. The others indicate MD simulations of a (12, 0) capsule encapsulated in a (21, 0) tube at 3000 and 300 K. These materials are available free of charge via the Internet at http://pubs.acs.org. References (1) Kelly, T. R.; Cai, X.; Damkaci, F.; Panicker, S. B.; Tu, B.; Bushell, S. M.; Cornella, I.; Piggott, M. J.; Salives, R.; Cavero, M.; Zhao, Y.; Jasmin, S. J. Am. Chem. Soc. 2007, 129, 376–386. (2) Fennimore, A. M.; Fennimore, A. M.; Yuzvinsky, T. D.; Han, W.-Q.; Fuhrer, M. S.; Cumings, J.; Zettl, A. Nature 2003, 424, 408–410. (3) Saito, R.; Dresselhaus, G.; Dresselhaus, M. S. Physical properties of carbon nanotubes; World Scientific: Singapore, 1998; Chapter 4. (4) Barreiro, A.; Rurali, R.; Herna´ndez, E. R.; Moser, J.; Pichler, T.; Forro´, L.; Bachtold, A. Science 2008, 775, 320. (5) Kwon, Y. K.; Toma´nek, D.; Iijima, S. Phys. ReV. Lett. 1999, 82, 1470. (6) Kang, J. W.; Hwang, H. J. Comput. Mater. Sci. 2005, 33, 338–345. (7) Legoas, S. B.; Coluci, V. R.; Braga, S. F.; Coura, P. Z.; Dantas, S. O.; Galva˜o, D. S. Phys. ReV. Lett. 2003, 90, 055504. (8) Kang, J. W.; Song, K. O.; Kwon, O. K.; Hwang, H. J. Nanotechnology 2005, 16, 2670–2676. (9) Bandow, S.; Takizawa, M.; Hirahara, K.; Yudasaka, M.; Iijima, S. Chem. Phys. Lett. 2001, 337, 48. (10) Akita, S.; Nishijima, H.; Nakayama, Y. J. Phys. D: Appl. Phys. 2000, 33, 2673–2677. (11) Williams, D. B.; Carter, C. B. Transmission Electron Microscopy I; Springer Science and Business Media: Berlin, 1996; p 62. (12) Kitaura, R.; Imazu, N.; Kobayashi, K.; Shinohara, H. Nano Lett. 2008, 8 (2), 693–699. (13) Tu, Z. C.; Ou-Yang, Z. C. J. Phys.: Condens. Matter 2004, 16, 706. (14) Saito, R.; Matsuo, R.; Kimura, T.; Dresselhaus, G.; Dressenhaus, M. S. Chem. Phys. Lett. 2001, 348, 187–193. (15) Hirahara, K.; Kociak, M.; Bandow, S.; Nakahira, T.; Itoh, K.; Saito, Y.; Iijima, S. Phys. ReV. B 2006, 73, 195420. (16) Suenkane, O.; Nagataki, A.; Nakayama, Y. Appl. Phys. Lett. 2006, 89, 183110. (17) Nakayama, Y.; Nagataki, A.; Suekane, O.; Cai, X.; Akita, S. Jpn. J. Appl. Phys. 2005, 44, L720–L722. (18) Somada, H.; Yoshikawa, Y.; Nagataki, A.; Hirahara, K.; Seiji, A.; Nakayama, Y. Jpn. J. Appl. Phys. 2007, 46, L1055–L1057. (19) Jin, C.; Suenaga, K.; Iijima, S. Nat. Nanotechnol. 2008, 3, 17–21. (20) Suekane, O.; Nagataki, A.; Mori, H.; Nakayama, Y. Appl. Phys. Express 2008, 1, 064001.
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