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
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].
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.
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.
[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.
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.
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.
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.
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.
[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.
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