Big fullerene clusters form an onion shape.

A scientist investigating recently discovered microscopic carbon tubules (SN: 7/18/92, p.36) has inadvertently created yet another form of this common element.

Carbon soot, when subjected to intense high-energy beams of electrons, transforms into nested layers of spheres, says Daniel Ugarte, an electron microscopist at the Federal Polytechnic School of Lausanne in Switzerland.

This onion-like structure represents the latest twist in the developing story of hollow, all-carbon molecules called fullerenes, and it challenges the notion that graphite represents the most stable configuration for carbon, Ugarte reports in the Oct. 22 NATURE.

Electron microscopists typically use electron beams instead of light to image their samples. But for this study, Ugarte turned up the intensity of the beam and irradiated soot samples for up to one-half hour. Periodically he adjusted the beam so that he could see how this irradiation affected the carbon particles.

"I was sure I would get just disordered graphite," Ugarte told SCIENCE NEWS. "Instead I saw I was getting spherical, shelled particles."

He observed that the beam causes the carbon atoms to move about. Tubules break apart and rearrange as concentric spheres. "The higher the [energy] dose, the quicker the transformation," Ugarte adds. Some spheres contained approximately 70 shells and measured 47 nanometers in diameter.

Theorists have asserted that graphite--in which atoms arrange like chicken wire, in a flat sheet of hexagons -- represents the most stable form of carbon. But that may hold true only for infinitely large graphite sheets, says Harold W. Kroto of the University of Sussex in Brighton, England. Sheets with a finite number of atoms have edges with dangling bonds: The carbon atoms along these borders need to attach to something else in order to become stable.

Thus, when 60 to 600 carbon atoms link up, the dangling bonds encourage the formation of hollow fullerenes, Kroto says. Ugarte's work now shows that collections containing millions of carbon atoms also curl and seem most stable in the form of multilayered spheres, he adds.

"[The new results] cast light on the mechanism by which carbon atoms can rearrange themselves," Kroto says.

Ugarte hopes researchers will eventually learn to manipulate the spacing between these concentric layers of carbon by putting other atoms between the layers. The resulting materials would have new properties, he says, "but we are not yet at the point that we can control this very well."