Nanotubes get another glowing review.

Many scientists say that carbon nanotubes--tiny tubes of graphite that are cousins to the buckyball--have a promising future as a new generation of microscopic wires, probes, and sensors. Now, a new study shows that their future isn't the only thing that's bright.

Researchers at the Swiss Federal Institute of Technology (EPFL) in Lausanne, Switzerland, have found that sending a current through nanotubes not only causes them to give off electrons--a process called field emission--but also to luminesce.

"You can actually see the light with the naked eye when you darken the room," says EPFL's Jean-Marc Bonard. Even a single nanotube generates a faint but visible glow.

The nanotubes emit just one photon of light for every million electrons. "It's something that happens on the side, but it tells us a lot," says Bonard. The luminescence offers insight into the tubes' electronic properties, the group reports in the Aug. 17 Physical Review Letters.

In 1995, Richard E. Smalley's group at Rice University in Houston reported seeing nanotubes give off light but attributed the glow to rapid unraveling of carbon chains from the surface (SN: 9/16/95, p. 183). The tubes were essentially burning up, the group said.

"That work stimulated the entire work that we did," says Walt A. de Heer, a coauthor of the current study who is now at the Georgia Institute of Technology in Atlanta. The Swiss team decided to take a careful look at the origins of light emission, working at voltages that wouldn't destroy the tubes.

The researchers balanced a single nanotube on the tip of a gold wire electrode and placed another electrode nearby to detect emitted electrons. When they sent a current through the tube, the tip glowed with light of wave-lengths around 700 nanometers.

"Luminescence tells us something special is happening at the tip," says Bonard. "Our interpretation of these results is that we have emission from different electronic levels at the tip of the tube."

These different levels might result from the tube's structure, de Heer suspects. Electrons flow through the shaft of the nanotube much as they do in a metal wire, sharing one broad energy level. However, the dome-shaped tip, like a molecule, has discrete energy states. Occasionally, an electron moving between these energy levels triggers the release of a photon, de Heer suggests.

The team also looked at thin films containing many nanotubes, which also glowed in response to an electric current.

"It looks like an interesting observation," says R.P.H. Chang of Northwestern University in Evanston, Ill., whose group recently used nanotubes to generate electrons for a flat panel display. "By understanding the mechanism of field emission, one also will understand the electronic properties of nanotubes."

Studying luminescence will probably yield more basic knowledge than practical applications, says Bonard. "But who knows? Maybe someone will need a very tiny device that will at the same time emit light and electrons."