Machine Phase Fullerene Nanotechnology

Authors: Al Globus¹, Charles Bauschlicher², Jie Han¹, Richard Jaffe², Creon Levit², Deepak Srivastava¹

1. MRJ Technology Solutions, Inc. at NASA Ames Research Center
2. NASA Ames Research Center

Abstract

Recent advances in fullerene science and technology suggest that it may be possible, in the far future, to design and build atomically precise programmable machines composed largely of functionalized fullerenes. Large numbers of such machines with appropriate interconnections could conceivably create a material able to react to the environment and repair itself. This paper reviews some of the experimental and theoretical work relating to these materials, sometimes called machine phase, including the fullerene gears and high density memory recently designed and simulated in our laboratory.

Introduction

Advanced materials are routinely used in the construction of aerospace vehicles. Despite many advances, transportation to space still costs about $10,000 per pound. [Drexler 92] proposed a nanotechnology based on diamond and investigated its potential properties. These studies and others suggest enormous potential for the role of diamonoid nanotechnology in aerospace systems [McKendree 95]. In particular, [McKendree 95] estimates $150-400 per pound transportation cost to orbit assuming naive use of diamonoid molecular nanotechnology to improve existing launch vehicle designs.

Unfortunately, methods to realize diamonoid nanotechnology are at best highly speculative. Recent computational efforts at NASA Ames Research Center [Globus 96] and computation and experiment elsewhere suggest that a nanotechnology based on machine phase functionalized fullerenes may be synthetically relatively accessible and possess great potential for aerospace applications. This nanotechnology might use carbon nanotubes and related components as the building blocks of molecular machines. Carbon nanotubes [Iijima 91] are rolled up sheets of hexagonal graphite that form single or multi-walled tubes. Typically, these tubes have caps with six pentagons each that add curvature to form closed molecules.

Machine phase materials are (often hypothetical) materials consisting entirely or in large part of microscopic machines. In a sense, most living tissue fits this definition since it is composed in large part of protein 'machines'. As a result, for example, our skin is able to sense its environment, react to it, change shape as we grow, and repair itself. Thus, although skin is not particularly strong, it routinely lasts 80 or more years in an often hostile environment.

A viable general purpose machine phase technology requires, at a minimum, mechanical motion, cooling, power, support structures, control, a variety of physical components, a system architecture, and some approach to manufacture. Except for the system architecture and manufacturing, there is some experimental or simulation basis in all these areas. The rest of this paper investigates each area in turn; but first we briefly review the known and computed properties of carbon nanotubes relevant to machine design. Note that the references in this paper are not meant to be a complete review of the field of fullerene nanotubes.

Carbon nanotubes are extremely strong and flexible. [Treacy 96] observed an exceptionally high Young's modulus for individual multi-walled carbon nanotubes (0.40-3.7 terapascals). [Yacobson 96] calculated a Young's modulus of 5.5 terapascals for single-walled carbon nanotubes. Apparently, when calculating Young's modulus some authors use the full disk as the cross sectional area and others use a ring with an open center. This, of course, results in different values. Care should be used when reading the literature ([Yacobson 96] uses a hollow core). In any case, these strength studies should not be considered definitive.

[Yacobson 96] made a theoretical study placing single-walled carbon nanotubes in axial compression. They found that the tubes compressed continuously with occasional singularities corresponding to shape changes as buckling adds waves to the system. [Srivastava 97a] found that axially compressed multi-walled nanotubes behave differently than single-walled nanotubes because of the long range tube-tube interactions which are not present in single-walled nanotubes.

[Iijima 96] reports on experiments and simulations of bending carbon nanotubes. They notice that nanotubes bent to a 30 degree angle develop kinks but the hexagonal bond pattern remains intact up to about 110 degrees. This suggests that even when tubes are substantially deformed, they can return to their original state.

[Satishkumar 96], [Ugarte 96] and others have opened the ends of nanotubes, filled them with various metal compounds, and closed the ends.

[Lee 97] and [Rao 97] report in companion papers that single-walled carbon nanotubes doped with bromine or potassium exhibit increased conductivity at 300K by a factor of 30, suggesting that doped nanotubes represent a new family of synthetic metals. Raman scattering measurements suggest that the doping increases carrier concentration.

Carbon nanotubes with different diameter and helicity are commonly described by two numbers, for example, (10,10), (9,0), etc. To understand this notation, "start with a point on a graphitic sheet, take an integral number of steps along one crystallographic axis, followed by another (and typically different) integral number of steps along the second crystallographic axis, reaching an endpoint. The straight line connecting the start point and the end point is then defined as the circumference" of the carbon nanotube -- Ralph Merkle.

For an excellent overview of fullerene science including carbon nanotubes see [Dresselhaus 95]. For a more recent review of carbon nanotubes see [Ebbesen 97].

Mechanical Motion

[Tuzun 95a] used molecular dynamics to investigate the properties of bearings consisting of an inner and an outer carbon nanotube. They found that performance was dominated by vibrational effects but smooth rotation could be achieved with careful choice of temperature, velocity and size.

To begin investigation of fullerene nanotechnology at Ames, we used molecular dynamics to study the properties of carbon nanotube-based gears and gear/shaft configurations [Han 97a]. Experiments on C60 [Hoke 92] have shown that C60 combines with benzyne in a 2+2 cycloaddition reaction under mild conditions. [Jaffe 97a] matched these results with quantum calculations, matched the 2+4 cycloaddition reaction of benzyne and naphthalene [Hoffmann 67], and went on to show that benzyne attached to the side of a (14,0) carbon nanotube with 2+2 and 2+4 cycloadditions is stable, the 2+4 addition is somewhat more stable. Jaffe later determined that when benzyne is added to the side of a (9,9) carbon nanotube, only the 2+2 cycloaddition is stable, not the 2+4 cycloaddition. Our fullerene gears are formed (in software) by adding these relatively stiff benzyne fragments around the tube to make teeth. See figure 1.

[Han 97a] used Brenner's potential [Brenner 90] to computationally demonstrate that molecular gears fashioned from (14,0) single-walled carbon nanotubes with benzyne teeth spaced every two rings around the tube's circumference should operate well at 50-100 gigahertz. Brenner's potential is a classical, reactive hydrocarbon potential parameterized to fit diamond, graphite, and small hydrocarbon molecules. A software thermostat kept the temperature at 200 or 300 Kelvin (depending on the simulation run), software springs were attached to atoms at the end of each tube to simulate a support system, and atoms near the ends of one (powered) tube were given an angular velocity increment each time step to simulate a motor. At rotation rates below about 100 gigahertz, rotation of the powered gear induces rotation of the other (driven) gear. At rotation rates above 100 gigahertz the teeth slipped past each other. However, bonds were not broken (Brenner's potential is reactive) so that when rotation rates were reduced the gears began functioning again.

[Han 97a] also investigated systems involving large (18,0) and small (10,0) gears. When power was applied to the large gear, the system worked well. However, when power was applied to the smaller gear, a very complex motion resulted and the larger gear did not rotate substantially, perhaps because the larger gear had too much inertia. See figure 2.

[Han 97a] also investigated a rack and pinion system. Here, a gear meshes its teeth with teeth spaced every three rings along the long axis of a (9,9) tube. Spacing every three rings, rather than every other ring as works on the gears, is necessary for the opening between teeth to be large enough (the teeth are parallel rather than divergent). The rack and pinion system was able to convert rotation of the gear into linear motion of the (9,9) tube and linear motion of the (9,9) tube into rotation of the gear. See figure 2.1.

While the gears appear to work well, at least in simulations, synthesizing them presents problems. First, the gear teeth must be added at precise positions but there is no particular reason for a reaction to prefer one site over another. Worse, fullerene nanotubes are much like the surface of a graphite sheet, which is not very reactive. The curvature of the tubes gives the primarily SP2 carbon atoms some SP3 character -- particularly for small radius tubes, but apparently not enough to easily functionalize them. SP3 character for a carbon atom with only three neighbors creates a reactive radical site. However, computation and experiment have shown that bending [Ruoff 95] or compressing [Yacobson 96] tubes causes buckling. Some of the atoms where the tube buckles must have substantial SP3 character. Therefore, we predict that bending carbon nanotubes sufficiently to cause buckling will allow fairly precise functionalization of the nanotubes. This may be a route to adding gear teeth to nanotubes.

Robertson et. al. designed a gear system based on fullerenes and described it on the WWW. [Cagin 97] investigated a diamonoid planetary gear.

Our group has conceptually designed a number of other components, including hinges, springs, universal joints and other systems. However, we have not yet investigated these systems in detail.

Cooling

[Han 97b] investigated cooling fullerene gears in an inert atmosphere. A neon or helium atmosphere was added to the (14,0) gear system. The software thermostat was removed from the gears and applied to the atmosphere. In initial simulations, the gears no longer turned so the software motor was modified. Instead of adding angular momentum to the end atoms of the powered gear, their position was updated each simulation step and this position was not allowed to change in response to inter-atomic forces. This simulates a more powerful motor. The gears turned and the temperature eventually stabilized, i.e., the atmosphere was able to control the temperature of the gears.

Power

[Tuzun 95b] simulated using a laser to turn carbon nanotubes. In these simulations, unit positive and negative charges were added to two carbon atoms on opposite sides of a nanotube, and an electric field was added simulating one or more lasers. The tubes turned but the direction alternated, undesirable behavior for a motor. [Srivastava 97b] simulated using alternating electric fields generated by a single simulated laser to power Han's fullerene gears. In these simulations, the software motor was removed, unit positive and negative charges were added to two carbon atoms on opposite sides of the powered nanotube, and a 140 gigahertz alternating electric field was added simulating a laser. The proper frequency was found using a linearized approximation of a phenomenological equation describing the system. The laser powered gear system rotates consistently in one direction, although one was unable to predict whether rotation will be clockwise or counter-clockwise. Interestingly, [Srivastava 97c] discovered that a pulsed laser worked better than a continuous laser. When the laser was off, the gears slowed down and cooled but rapidly sped up when the laser came on again. Since the initial start-up of the gears must overcome static friction, a great deal of heat was generated. On the other hand, adding power to moving gears must only overcome dynamic friction which generates less heat. The pulsed laser resulted in a cooler system.

[Tsai 93] and [Tahmasebi 95] proposed a six degree of freedom minimanipulator of remarkably simple design. See figure 3. In particular, this manipulator requires only one dimensional linear motion for power and control. This power must be applied to components mounted on a planar base on which the rest of the manipulator is mounted. If the powered components have a small tab (for example, a benzyne ring attached to a carbon nanotube) protruding from the base of the manipulator on the opposite side from the rest of the unit, then a single carbon nanotube tipped scanning probe microscope (SPM) [Dai 96] could be used to power and control the system. For this to work, the powered components would need to stay put once positioned. This could be accomplished by meshing another benzyne ring on the powered tube with a series of benzyne teeth on a support structure. The teeth can be made to slip when pushed hard enough, but will hold the powered tube in position while the SPM is used to control other powered elements. Tooth slip can be enhanced by minimally overlapping the meshed teeth. See figure 4. Such a system, although very slow and as yet poorly analyzed, would use only existing SPM technology for both power and control.

Support Structures

Theory [Dunlap 92], [Dunlap 94a], [Dunlap 94b] and experiment [Zhang 95] suggest that fullerene tubes may be joined at 30 degree angles to create complex structures including helices. Theoretical models of three way joined tubes have appeared in the literature [Colbert 95], but there is no experimental evidence of such structures. Such evidence would be of great value in the design of support structures.

Multi-walled carbon nanotubes have been observed with broken outer walls [Sattler 96]. This suggests that an open-ended nanotube can exist surrounding a longer nanotube. This would allow components to be stabilized in all directions except along the tube axis. Strategically placed benzyne rings added to the inner tube could be used to constrain motion of the outer tube along the tube axis.

Control

First, we remind the reader of the discussion in the power section suggesting that control of a primitive six DOF manipulator appears possible using existing SPM technology. More complex control systems usually require computers at their heart. While no one has built a fullerene computer, computational studies suggest that a number of promising computer components could be built, at least in theory. Thus, we will investigate fullerene computers first.

[Joachim 97] has created an electromechanical switch using a single C₆₀ molecule held between an STM tip and a conducting substrate. Current passed through C₆₀ by the STM changes substantially and reversably when the STM tip deforms the molecule. The speed of the switch is a function of the speed of mechanical deformation, limited only by the vibrational frequency of C₆₀ -- approximately 10 terahertz.

Theory [Dresselhaus 95, pp. 802-814] suggests that single-walled carbon nanotubes can have metallic or semiconductor properties depending on the helical winding of the tube. [Dresselhaus 95, pp. 903-904] proposes a number of computer device components based on this property. [Dunlap 92] and [Dunlap 94a] have supplied a theoretical foundation for one way to join nanotubes with different electronic properties together using pentagon and heptagon defects on opposite sides of a nanotube to change the helical winding. Recent calculations [Chico 96] suggest that tubes with different helical windings joined by a pentagon-heptagon pair can have different electrical properties at different positions. Experiment has shown that single-walled carbon nanotubes are quantum wires [Bockrath 97]. [Langer 96] demonstrated that the conductance of multi-walled nanotubes can be increased by applying a magnetic field perpendicular to the tube axis; which may have applications in data storage. This effect has been demonstrated at temperatures below 4.2 K. Combining these components into a computer architecture is a significant challenge for the future.

See figure 5. [Bauschlicher 97a] computationally studied storing data in a pattern of fluorine and hydrogen atoms on the (111) diamond surface using a one dimensional model. If (presumably write-once) data could be stored this way, 10¹⁵ bytes/cm² is theoretically possible. [Bauschlicher 97a] compared the interaction of different probe molecules with a one-dimensional model of the diamond surface. They found that some molecules have sufficiently different interaction energies with H and F. Such a difference in force should be detectable by an SPM. These studies were extended to include a two dimensional model of the diamond surface and two other systems besides F/H [Bauschlicher 97b]. Other surfaces, such as Si, and other probes, such as those including transitional metal atoms, have also been investigated [Bauschlicher 97c].

Among the better probes was C₅H₅N (pyridine). We have shown that pyridine attached to C60 in the orientation necessary for sensing the difference between hydrogen and fluorine should be stable. Half of C₆₀ can form the end cap of a (9,0) or (5,5) carbon nanotube and carbon nanotubes have been attached to an SPM tip [Dai 96]. Thus, it should be possible using today's most advanced laboratory techniques to build a system to read the diamond memory surface.

[Avouris 96] has shown that individual hydrogen atoms can be removed from a silicon surface. If this could be accomplished on diamond in a gas that donates fluorine to radicals created on a diamond surface, the memory system could be built. [Thummel 97] computationally investigated methods for adding a fluorine at the radical sites where a hydrogen atom had been removed from a diamond surface.

Mitre maintains an excellent www page with links to their survey papers on nanoelectronics, which includes brief discussions of fullerene based electronic components.

Physical Components

Fullerenes such as C₆₀ have been functionalized by a wide variety of molecular fragments [Taylor 93]. [Satishkumar 96] has functionalized nanotubes with an acid solution.

Carbon nanotubes have been observed with a wide variety of ends, including a variety of cap shapes [Ajayan 93], and tubes that first reduce diameter for some distance before ending [Iijima 92].

[Iijima 93a] and [Sattler 96] observed cone-like fullerene objects. [Liu 97] observed fullerene tori. [Endo 95] observed spindle shaped objects. [Ebbesen 95] observed multi-walled nanotubes whose diameter gradually increased, presumably from the presence of many pentagon-heptagon defects.

[Amelinckx 94] observed multi-walled fullerene helices, [Zhong-can 97] explained their ratio of pitch to radius on energetic grounds. [Dunlap 94b] provided a theoretical basis for single-walled helices with pentagon and heptagon defects. [Ihara 95] developed several theoretical ways to construct fullerene tori and helices and computationally determined that the spring constant for helical C ₃₆₀ is 4.09 meV/nm and for helical C ₅₄₀ is 0.16 meV/nm. We have derived a generized topological construction method to join nanotubes into bends with angles of 0-30 degrees by introducing pentagon-heptagon pairs at different separations.

It therefore seems reasonable that a variety of shapes could be designed and synthesized. When we designed the gears in [Han 97a], very little searching of the design space was necessary. Actually, the second design attempted was eventually published. We were either very lucky, or the design space is well populated with potentially useful devices.

System Architecture

See figure 6. We have an extremely preliminary concept called the replicating swarm. The swarm consists of roughly spherical nodes capable of attaching to five edges (for a tetrahedral geometry with one free edge per node) and rotating each of them in pitch and yaw. The linear edges are capable of changing length, rotating around their long axis, and attaching/detaching to/from nodes. Both components have internal computers, sense force, and can pass data and power to each other. The swarm grows by assembling synthetically generated fullerene components into nodes and edges. When a swarm is large enough, it divides in two by letting go of the appropriate edge/node connections.

Besides the obvious and severe difficulties of building the components and physically connecting them, the software problems in planning and controlling the swarm's actions are daunting. However, these problems could be addressed in simulators with simulated swarm components. Such a simulator could also be used for research into the range of capabilities swarm components should use. One author suspects that genetic programming may be a fruitful approach to the software control problem.

Manufacture

Multi-walled carbon nanotubes were discovered in 1991 by [Iijima 91]. [Bethune 93] and [Iijima 93b] reported observing single-walled carbon nanotubes in the same issue of Nature. [Ebbesen 92] and [Thess 96] demonstrated high yield synthesis of multi-walled and single-walled nanotubes respectively. As mentioned above, [Satishkumar 96] has functionalized nanotubes with an acid solution. Given the history of C₆₀, the next few years should witness an explosion of functionalized nanotube syntheses; some, perhaps, with properties amenable to machine phase materials.

[Li 96] developed a method to grow 0.05 mm multi-walled nanotubes in aligned arrays on a silica surface. The spacing is controlled by the spacing of iron nanoparticles embedded in the mesoporous silica. Most of the iron particles are believed to be near the bottom of holes in the silica. These cavities apparently orient the nanotubes to be approximately normal to the silica surface. Growing nanotubes in predictable, regular arrays should make modification to useful products easier.

Conceptually, we envision a reasonably mature fullerene nanotechnology manufacturing system in which small molecular components are generated synthetically in bulk and fed to one of the swarms described above. The swarm assembles swarm edges and nodes from the molecular components thereby growing and eventually dividing. Of course, one needs an initial swarm to begin this process.

[Dai 96] demonstrated that individual carbon nanotubes can be attached to a scanning probe microscope (SPM) tip. SPM's can manipulate their tips with sub-angstrom accuracy. The end of carbon nanotubes should have a chemistry similar to C₆₀. C₆₀ can be functionalized with a wide variety of molecular fragments [Taylor 93]. Thus, with some further development it should be possible to synthesize a wide variety of molecular structures using mechanical control to guide reactions of individual molecules. This should allow construction of extremely small quantities of many otherwise inaccessible atomically precise products; for example, swarm components.

Conclusions

We see that there is some evidence that fullerene based machines and, conceivably, machine phase materials based on them may be possible. Combined with the apparently remarkable mechanical and electrical properties of carbon nanotubes, there is some reason to believe that a focused effort to develop fullerene nanotechnology could yield materials with remarkable properties. Materials with electrical properties that could revolutionize circuit design and increased strength-of-materials leading to, among other things, opening the space frontier by radically lowering the cost of launch to orbit. We hope that others will join us in a long range, high risk, potentially enormous payoff effort to develop machine phase fullerene materials.

Acknowledgments

We would like to thank our colleagues at NASA Ames Research Center for their support and advice, particularly Chris Henze and David Kenwright for reviewing this manuscript. Thanks to Bonnie Klein for her invaluable proof reading. Ralph Merkle, Xerox PARC, has been a constant source of interesting ideas. Todd Wipke's group at the University of California at Santa Cruz was very helpful in many molecular design discussions.

References

[Ajayan 93] P. M. Ajayan, T. Ichihashi and S. Iijima, "Distribution of pentagons and shapes in carbon nano-tubes and nano-particles," Chemical Physics Letters, volume 202, number 5, 29 January 1993, pp. 384-388.

[Amelinckx 94] S. Amelinckx, X. B. Zhang, D. Bernaets, X. F. Zhang, V. Ivanov and J. B. Nagy, "A Formation Mechanism for Catalytically Grown Helix-Shaped Graphite Nanotubes," Science 265, 29 July 1994, p. 635-639.

[Avouris 96] Ph. Avouris, R. E. Walkup, A. R. Rossi, H. C. Akpati, P. Nordlander, P.-C. Shen, G. G. Ablen and J. W. Wyding, "Breaking Individual Chemical Bonds via STM-Induced Exitations," Surface Science, 1996 August 1, V363 N1-3:368-377.

[Bauschlicher 97a] Charles W.Bauschlicher Jr., Alessandra Ricca and Ralph Merkle, "Chemical storage of data," Nanotechnology, volume 8, number 1, March 1997 pp. 1-5.

[Bauschlicher 97b] C. W. Bauschlicher and M. Rosi, "Differentiating between hydrogen and fluorine on a diamond surface", Theor. Chem. Acta, volume 96, pages 213-216 (1997).

[Bauschlicher 97c] C. W. Bauschlicher and M. Rosi, in preparation.

[Bethune 93] D. S. Bethune, C. H. Kiang, M. S. de Vries, G. Gorman, R. Savoy, J. Vazquez and R. Beyers, Nature, volume 363, 17 June 1993, pp. 605-607.

[Bockrath 97] M. Bockrath, D. H. Cobden, P. L. McEuen, N. Chopra, A. Zettl, A. Thess, and R. E. Smalley, "Single-Electron Transport in Ropes of Carbon Nanotubes," Science 275, March 28, 1997, pp. 1922-1925.

[Brenner 90] Don W. Brenner, "Empirical potential for hydrocarbons for use in simulating the chemical vapor deposition of diamond films," Physical Review B-Condensed Matter, 15 November 1990, V42 N15:9458-9471.

[Cagin 97] T. Cagin, A. Jaramillo-Botero, G. Gao, and W. A. Goddard, III, "Molecular Mechanics and Molecular Dynamics Analysis of Drexler-Merkle Gears and Neon Pump," The Fifth Foresight Conference on Molecular Nanotechnology, November 5-8, 1997; Palo Alto, CA.

[Chico 96] L. Chico, Vincent H. Crespi, Lorin X. Benedict, Steven G. Louie and Marvin L. Cohen, "Pure Carbon Nanoscale Devices: Nanotube Heterojunctions," Physical Review Letters, volume 76, number 6, 5 February 1996, pp. 971-974.

[Colbert 95] D. T. Colbert and R. E. Smalley, "Fullerene Tinkertoys," Proceedings from NATO Advanced Research Workshop on Modular Chemistry, Estes Park, Colorado, (September 1995).

[Dai 96] H. Dai, J. H. Hafner, A. G. Rinzler, D. T. Colbert and R. E. Smalley, "Nanotubes as Nanoprobes in Scanning Probe Microscopy," Nature 384, pp. 147-151, (1996).

[Dresselhaus 95] M. S. Dresselhaus, G. Dresselhaus and P. C. Eklund, Science of Fullerenes and Carbon Nanotubes, Academic Press (1995).

[Drexler 92] K. Eric Drexler, Nanosystems: Molecular Machinery, Manufacturing, and Computation, John Wiley & Sons, Inc. (1992).

[Dunlap 92] Brett I. Dunlap, "Connecting carbon tubules," Physical Review B, volume 46, number 3, 15 July 1992-I, pp. 1933-1936.

[Dunlap 94a] Brett I. Dunlap, "Relating carbon tubules," Physical Review B, volume 49, number 8, 15 February 1994-II, pp. 5643-5649.

[Dunlap 94b] Brett I. Dunlap, "Constraints on small graphitic helices," Physical Review B, volume 50, number 11, 15 September 1994-I, pp. 8134-8137.

[Ebbesen 92] T. W. Ebbesen and P. M. Ajayan, "Large-scale synthesis of carbon nanotubes," Nature, volume 358, 16 July 1992, pp. 220-222.

[Ebbesen 95] T. W. Ebbesen and T. Takada, "Topological and SP³ Defect Structures in Nanotubes," Carbon, V33 N7:973-978 (1995).

[Ebbesen 97] T. W. Ebbesen, editor, Carbon Nanotubes: Preparation and Properties, CRC Press, 1997.

[Endo 95] Morinobu Endo, Kenji Takeuchi, Kiyoharu Kobori, Katsushi Takashi, Harold W. Kroto, and A. Sarkar, "Pyrolytic Carbon Nanotubes from Vapor-Grown Carbon Fibers," Carbon, V33 N7:873-881 (1995).

[Globus 96] Al Globus, David Bailey, Steve Langhoff, Andrew Pohorille and Creon Levit, "Computational Nanotechnology at NASA Ames Research Center, 1996," First Electronic Molecular Modelling & Graphics Society Conference and NAS technical report NAS-96-020.

[Han 97a] Jie Han, Al Globus, Richard Jaffe and Glenn Deardorff, "Molecular Dynamics Simulation of Carbon Nanotube Based Gears," Nanotechnology, in press.

[Han 97b] Jie Han, Al Globus, and Richard Jaffe, "The Molecular Dynamics of Carbon Nanotube Gears in He and Ne Atomspheres," The Fifth Foresight Conference on Molecular Nanotechnology, November 5-8, 1997; Palo Alto, CA.

[Hoffmann 67] R. W. Hoffmann, Dehydrobenzene and Cycloalkynes, Verlag Chemie-Academic Press (1967).

[Hoke 92] Steven H. Hoke, Jay Molstad, Dominique Dilettato, Mary Jennifer Jay, Dean Carlson, Bart Kahr and R. Graham Cooks, "Reaction of Fullerenes and Benzyne," Journal of Organic Chemistry, 11 September 1992, V57 N19:5069-5071.

[Ihara 95] Sigeo Ihara and Satoshi Itoh, "Helically Coiled and Toroidal Cage Forms of Graphitic Carbon," Carbon, V33 N7:931-939 (1995).

[Iijima 91] Sumio Iijima, Nature, "Helical microtubules of graphitic carbon," 7 November 1991, volume 354, N6348:56-58.

[Iijima 92] Sumio Iijima, Toshinari Ichihashi, and Yoshinori Ando, "Pentagons, heptagons and negative curvature in graphite microtubule growth," Nature, volume 356, 30 April 1992, pp. 756-778.

[Iijima 93a] Sumio Iijima, "Growth of carbon nanotubes," Materials Science and Engineering, B19, pp. 172-180, (1993).

[Iijima 93b] Sumio Iijima and Toshinari Ichihashi, "Single-shell carbon nanotubes of 1-nm diameter," Nature, volume 363, 17 June 1993, pp. 603-605.

[Iijima 96] Sumio Iijima, Charles Brabec, Amitesh Maiti, and Jerzy Bernholc, "Structural flexibility of carbon nanotubes," Journal of Chemical Physics, volume 104, number 5, 1 February 1996, pp. 2089-2092.

[Jaffe 97a] Richard Jaffe, Jie Han and Al Globus, "Formation of Carbon Nantoube Based Gears: Quantum Chemistry and Molecular Dynamics Simulations of the Electrophilic Addition of o-Benzyne to Fullerenes, Graphene, and Nanotubes," First Electronic Molecular Modelling & Graphics Society Conference and presented to the American Physical Society 1997 March Meeting, 17-21 March, Kansas City, MO Division of Materials Physics Focused Session on Fullerenes, Carbon Nanotubes, and Related Materials.

[Joachim 97] C. Joachim and J. Gimzewski, "An Electromechanical Amplifier Using a Single Molecule," Chemical Physics Letters, volume 265, pp. 353-357, 1997.

[Langer 96] L. Langer, V. Bayot, E. Grivei, J-P. Issi, J. P. Heremans, C. H. Olk, L. Stockman, C. Van Haesendonck, Y. Bruynseraede, "Quantum Transport in a Multiwalled Carbon Nanotube," Physical Review Letters 15 January 1996, V76 N3:479-482.

[Lee 97] R. S. Lee, H. J. Kim, J. E. Fischer, A. Thess, and Richard E. Smalley, "Conductivity enhancement in single-walled Carbon Nanotube Bundles Doped with K and Br," Nature, volume 388, 17 July 1997, pp. 255-257.

[Li 96] W. Z. Li, S. S. Xie, L. X. Qian, B. H. Chang, B. S. Zou, W. Y. Zhou, R. A. Zhao, G. Wang, "Large-Scale Synthesis of Aligned Carbon Nanotubes," Science, 6 December 1996, volume 274, pp. 1701-1703.

[Liu 97] Jie Liu, Hongjie Dai, Jason H. Hafner, Daniel T. Colbert, Richard E. Smalley, Sander J. Tans and Cees Dekker, "Fullerene 'crop circles,'" Nature, volume 385, 27 February 1997, pp. 780-781.

[McKendree 95] Tom McKendree, "Implications of Molecular Nanotechnology: Technical Performance Parameters on Previously Defined Space System Architectures," The Fourth Foresight Conference on Molecular Nanotechnology, Palo Alto, CA. (November 1995).

[Rao 97] A. M. Rao, P. C. Eklund, Shunjl Bandow, A. Thess, and Richard E. Smalley, "Evidence for Charge Transfer in Doped Carbon Nanotube Bundles from Raman Scattering", Nature, volume 388, 17 July 1997, pp. 257-259.

[Ruoff 95] Rodney S. Ruoff and Donald C. Lorents, "Mechanical and Thermal Properties of Carbon Nanotubes," Carbon V33 N7:925-930, (1995).

[Satishkumar 96] B. C. Satishkumar, A. Govindaraj, J. Mofokeng, G. N. Subbanna and C. N. R. Rao, "Novel experiments with carbon nanotubes: opening, filling, closing and functionalizing nanotubes," J. Phys. B: At. Mol. Opt. Phys., volume 29, pp. 4925-4934, (1996).

[Sattler 96] Klaus Sattler, "Scanning tunneling microscopy of carbon nanotubes and nanocones," Carbon, V33 N7:915-920 (1995).

[Srivastava 97a] Deepak Srivastava and Steve Barnard, "Molecular Dynamics Simulation of Large-Scale Carbon Nanotubes on a Shared Memory Architecture", SuperComputing 97 (1997).

[Srivastava 97b] Deepak Srivastava, "Molecular Dynamics Simulations of Laser Powered Carbon Nanotube Gears," submitted to Nanotechnology.

[Srivastava 97c] Deepak Srivastava, in preparation (May 1997).

[Tahmasebi 95] Farhad Tahmasebi and Lung-Wen Tsai, "On the Stiffness of a Novel Six-Degree-of-Freedom Parallel Minimanipulator," Journal of Robotic Systems, 12(12), 845-856 (1995).

[Taylor 93] R. Taylor and D. R. M. Walton, "The Chemistry of Fullerenes," Nature, volume 363, N6431, 24 June 1993, pp. 685-693.

[Thess 96] Andreas Thess, Roland Lee, Pavel Nikolaev, Hongjie Dai, Pierre Petit, Jerome Robert, Chunhui Xu, Young Hee Lee, Seong Gon Kim, Andrew G. Rinzler, Daniel T. Colbert, Gustavo E. Scuseria, David Tomanek, John E. Fischer and Richard E. Smalley, "Crystalline Ropes of Metallic Carbon Nanotubes," Science, volume 273, 26 July 1996, pp. 483-487.

[Thummel 97] H. T. Thummel and C. W. Bauschlicher, "On the reaction of FNO₂ with CH₃, t-butyl, and C₁₃H₂₁," J. Phys. Chem., 101, 1188 (1997).

[Treacy 96] M. M. J. Treacy, T. W. Ebbesen and J. M. Gibson, "Exceptionally High Young's Modulus Observed for Individual Carbon Nanotubes," Nature, 381, 678 (1996).

[Tsai 93] Lung-Wen Tsai and Farhad Tahmasebi, "Synthesis and Analysis of a New Class of Six-Degree-of-Freedom Parallel Minimanipulators," Journal of Robotic Systems, 10(5), 561-580 (1993).

[Tuzun 95a] Robert E. Tuzun, Donald W. Noid and Bobby G. Sumpter, "The Dynamics of Molecular Bearings," Nanotechnology Nanotechnology 6, 64-74 (1995).

[Tuzun 95b] Robert E. Tuzun, Donald W. Noid and Bobby G. Sumpter, "Dynamics of a Laser Driven Molecular Motor," Nanotechnology 6, pp. 52-63 (1995).

[Ugarte 96] D. Ugarte, A. Chatelain, W. A. de Heer, "Nanocapillarity and Chemistry in Carbon Nanotubes," Science, volume 274, 13 December 1996, pp. 1897-1899.

[Yacobson 96] B. I. Yacobson, C. J. Brabec and J. Bernholc, "Nanomechanics of Carbon Tubes - Instabilities Beyond Linear Response," Physical Review Letters, 1 April 1996, V76 N14:2511-2514.

[Zhang 95] Xiao Feng Zhang and Ze Zhang, "Polygonal spiral of coil-shaped carbon nanotubes," Physical Review B, volume 52, number 7, 15 August 1995-I, pp. 5313-5317.

[Zhong-can 97] Ou-Yang Zhong-can, Zhao-Bin Su, Chui-Lin Wang, "Coil Formation in Multishell Carbon Nanotubes: Competition between Curvature Elasticity and Interlayer Adhesion. Physical Review Letters , 26 May 1997, V78 N21:4055-4058.

To the NAS computational nanotechnology home page.