Chapter 29: Molecules and Solids Applications

C60Courtesy of the Fullerene group, University of Sussex

What are Buckyballs? The image to the right shows a basic form of Buckyball, C₆₀. It is a molecule composed of 60 carbon atoms arranged to form a nearly spherical object. Actually, the "sphere" is more like a soccer ball; it has 32 sides, 20 hexagons and 12 pentagons. The carbon atoms are at the vertices. Technically, the shape is a truncated icosahedron. This molecule was discovered in 1985 by Richard Smalley, and Robert Curl of Rice University in Texas, and Harold Kroto of the University of Sussex in England. They were rewarded for their efforts with the 1996 Nobel prize in chemistry.

The story of their discovery is an interesting, and provides a wonderful example of the often unexpected turns that a scientific study can take. In the 1970's, Dr. Kroto was interested in carbon-chain molecules that occur in interstellar space, and in certain carbon rich stars. He and his research group had even detected a few forms such as HC₅N, HC₇N. However, they still had many questions about how these molecules formed. Dr. Smalley had a system (used for studying silicon and other semiconductors) that used a laser to vaporize materials. In such an apparatus, the conditions are in many ways similar to those in stellar atmospheres. In the mid 1980's

C70Courtesy of the Fullerene group, University of Sussex

Curl told Smalley of Kroto's interests, and they decided to collaborate, using Smalley's apparatus to see if they could produce molecules of the kind Kroto had been studying. Smalley invited Kroto to Texas, and what they found surprised them, and everyone else in the scientific community.

In general, the term "Buckyball" has been used to describe any roughly spherical molecule built entirely of carbon atoms. C₆₀ was the first to be discovered, but there are many more as well. Since the discovery of C₆₀, many other similar structures have been found. Among the other Buckyballs, are C₇₀, C₈₄, and the list goes on... the largest I have heard reference to is 960! However, it is because of the perfect symmetry of C₆₀ that these materials were named after the engineer, architect, mathematician, and 1960's icon R. Buckminster Fuller, who invented (among other things) the geodesic dome. The scientifically correct name for C₆₀ is "Buckminsterfullerene."

In addition, to balls, other shapes are possible. The first "non-ball" structures to be found were the "Buckytubes." Often called carbon nanotubes, these are tubes with a carbon mesh surface and, in some cases, spherical end caps.

Nanotube, Courtesy of Daniel T. Colbert, Center for Nanoscale Science and Technology, Rice University

The tubes can have bends in them, and several tubes can even be joined. What other possibilities are there? Well, if a tube can be distorted so that the ends touch, a "donut" shape is formed (more technically, a torus). The same distortion can be applied to a carbon nanotube resulting in a carbon nanotorus. Even the Y shaped structure show at the right is possible. Clearly, tubes, tori,

"Y-NOT", Courtesy of Gustavo Scuseria, Rice University

These materials turn out to be remarkably strong. Buckyballs smashed into a steel surface at 17,000 miles/hour bounce of undamaged. Of course, it is possible to destroy them. If your browser can handle mpeg movies, have a look at a page that shows simulations of collisions between a C₆₀ and a C₂₄₀. You can also have a look at some collisions between oneC₆₀ and a Si surface. Depending on the energy, the C₆₀ may bounce off or get stuck. There have literally been hundreds of applications suggested for using fullerenes. To see a few, check out the fullerene patent database.

C₆₀ would have been surprising in its own right. Then, it was found that some compounds of C₆₀ were superconductors! What sorts of compounds? well, it turns out that if you condense a bunch of C₆₀ on a glass plate it will form a film, the molecules stack up on an fcc lattice, much the way copper atoms stack up in a solid.

fcc C60. Courtesy of Lazlo Mihaly, SUNY Stony Brook

One such stack is shown at the right. Looking at the picture, one almost cannot help but ask, what if some small atoms were placed in the gaps between the spheres? Well it was tried, successfully, with the alkalai metals, K, Rb, and Cs. Some of these compunds turn out to be superconductors at fairly high temperatures. For instance, T_c = 16 K for K₃C60, 30 K for Rb₃C60, and 47 K for Cs₃C60.

Incidentally, Buckyballs are not difficult to make. You can read a very good, readable account of a buckyball synthesis project by two undergraduates at Evergreen State College. The end of their page has links to more Buckyball sites. Or, if you would prefer to buy the materials you can do that too. The price for 1 gram of C₆₀ is about $40.

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