Fullerenes Add Motion to Micromachines
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RESEARCH/RESEARCHERS Fullerenes Add Motion to Micromachines In order to make machines on the molecular scale, researchers need to be able to fabricate components that are analogous to those of macromachines, such as gears, wheels, pistons, valves, and bearings. Fullerenes, with their sphere-like structure, would seem to be ideal for use as bearings, but early work involving these C60 molecules between two surfaces was less than encouraging. Multilayers of C60 between plates deformed elastically, leading to high friction. However, results reported in the February 7 issue of Physical Review Letters suggest that monolayers of C60 sandwiched between silicon surfaces coated with graphite may produce a nearly frictionless bearing system for use in micromachines. While graphite is a well-known lubricant, its function in this bearing system is not one of lubrication. Rather, the hexagonal carbon faces on the top and bottom of the C60 molecule “mesh” with the sixmember carbon rings that form the sheets of the graphite sandwich to form nanogears; these nanogears allow the C60 molecules to roll between the graphite layers, acting as bearings. The action is
similar to rolling a ball between the hands by moving them in parallel but opposite directions. K. Miura and S. Kamiya of Aichi University of Education in Kariya, Japan, along with N. Sasaki of Sekei University in Tokyo, used frictional force-mapping techniques to show that C 60 forms a close-packed monolayer on graphite. Furthermore, the monolayer can be formed initially with hexagonal faces of C60 maintaining AB stacking with the hexagonal carbon rings of the graphite on both sides of the sandwich. When a small torque is applied to the system, the C60 molecules roll, ideally in a direction that will find their hexagonal faces once again aligned with the hexagonal rings of the graphite sheets on either side. However, because the C60 molecule is not composed exclusively of hexagonal faces but includes pentagonal faces as well, there is no guarantee that this ideal alignment will occur each time. If a pentagonal face of C60 comes into contact with the graphite hexagon, the gear will briefly stick until thermal energy causes rotation of the C60 so that a hexagonal face once again matches up with the graphite sheet. From this position, the C60
bearing can then turn again. The bearing system thus moves by a stick-slip mechanism at room temperature. Because the energy required to overcome the sticking is relatively low, the researchers believe that at slightly elevated temperatures, the bearings will roll smoothly. This first demonstration of a practical molecular bearing system, coming as it does with a fundamental understanding of the mechanism involved, could help add the long-awaited capability of movement to previously static micromachines. “Silicon substrates covered with graphite should be useful as movable parts of nano- and micromachines,” said Miura. “This system is expected to be useful in MEMS [microelectromechanical systems] devices.” Jean Michel Martin, head of t
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