Simple recipe yields fullerene tubules.

Scientists who first observed hollow, all-carbon molecules, called fullerenes, in the shape of microscopic tubes have now developed a method to produce large quantities of this interesting new form of the material.

Since the discovery of fullerene tubules last November (SN: 11/16/91, p.310), several theorists have predicted that these new carbon fibers could strengthen materials better than graphite fibers and could work as molecular-scale wires or as catalysts -- if they could be produced in large quantities.

Now they can be. While trying to chemically modify fullerenes by producing them in helium, Thomas W. Ebbesen and P.M. Ajayan, materials scientists with NEC Corporation's Fundamental Research Laboratories in Tsukuba, Japan, found they could make several grams of tubules all at once. They describe their simple recipe in the July 16 NATURE.

"It's an invitation for people to find out anything they can about these materials," says John W. Mintmire, a theorist at the Naval Research Laboratory in Washington, D.C.

"Anyone who has a [fullerene] generator can make these things very easily," adds Lai-Sheng Wang, a physical chemist at Rice University in Houston. "The key now is to be able to control the process to make longer fibers."

Ebbesen and Ajayan make the tubules with a procedure similar to that used to produce fullerenes. They place two graphite rods in a helium-filled reaction vessel. Keeping the pressure at two-thirds that of the atmosphere, they apply a voltage between the carbon rods and cause the smaller rod to emit carbon, up to 75 percent of which builds up on the larger rod, they say. That carbon deposits as a cylinder that grows a few millimeters per minute.

When the scientists first examined a cross-section of this inch-long, quarter-inch-wide cylinder with an electron microscope, they saw it consisted of a hard, gray, metallic shell that was chock full of black fullerene nanotubes and microscopic carbon particles. Those insides represent about 25 percent of the starting carbon, a higher yield than fullerene syntheses, they report. The nanotubes range from 2 to 20 nanometers in diameter and measure several thousand nanometers long.

Because the tubes form only inside the cylinder, temperature and cooling rate must be critical to their formation, Ebbesen and Ajayan say.

These tubules should be stiffer than carbon fibers and should possess interesting electronic properties. Already, the NEC researchers have shown that their sample conducts electricity better than other carbon materials, Mintmire notes.

Theorists have concluded that some tubules should conduct electricity, while others of different diameters will act as semiconductors. "This is very surprising," says Mildred S. Dresselhaus, a solid-state physicist at the Massachusetts Institute of Technology.

According to Dresselhaus, a second Japanese group is working out another way to grow nanotubes from the vapor phase of carbon. Neither approach makes single tubes, however, and the typical tubes-within-tubes arrangement may prove difficult to study. "We don't have a lot of techniques to work with these nanometer-[size] things," says Dresselhaus.