Chemists discovered buckyballs—cagelike molecules of 60 carbon atoms—more than 20 years ago. Members of a family of carbon cages known as fullerenes, buckyballs form spontaneously in a hot gas of vaporized carbon. But the exact mechanics of their formation have remained somewhat hazy.

One theory holds that larger fullerenes form first, then shed atoms as they cool, shrinking to become buckyballs.

Researchers have now filmed giant fullerenes in the act. Jianyu Huang of the Sandia National Laboratories in Albuquerque and his colleagues ran an electric current through multiwalled carbon nanotubes and filmed the results with atomic resolution using a transmission electron microscope.

The current heated the nanotubes to more than 2,000°C. When the high temperature broke up the innermost layer of a nanotube, fullerenes appeared in its place. At first, the newly formed giant fullerenes, composed of thousands of atoms, were much larger than buckyballs. But the fullerenes, still trapped inside the heated nanotubes, kept shedding atoms, and they eventually turned into buckyballs.

Huang says that this is the first time that anyone has observed such a "shrink-wrap" mechanism in action. While admitting that other pathways might exist for creating the molecules, he says, "This is one way that carbon-60 can form." The team describes the results in the Oct. 26 Physical Review Letters.

Huang says that refinement of the technique could allow scientists to create fullerenes of specific sizes for applications such as drug delivery and energy storage.

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