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Nanotubes Offer New Light

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Hank Hogan

It is not often that researchers reinvent the lightbulb. Scientists at École Polytechnique Fédérale de Lausanne have constructed a luminescent tube with the output of commercial fluorescent offerings, based on the field emission of electrons from carbon nanotubes.

The new light offers significant advantages over current lamps, said researcher Jean-Marc Bonard. "We get rid of the mercury, we have a dimmable tube that starts up instantly, and we can deliver high electronic currents," he said.

Researchers have demonstrated a lighting element based on a carbon nanotube cathode. While lacking the energy efficiency of a fluorescent tube, the light source offers a comparable luminance, without the use of mercury or limitations in warm-up time.

The researchers deposited an iron catalyst over wires of Kanthal -- an alloy of iron, aluminum and chromium -- that were 1 mm in diameter and 7 mm long. They put the Kanthal wires in a tube reactor and exposed them to a flow of high-temperature acetylene, which covered the wires with curved carbon nanotubes roughly 20 nm in diameter. The resulting film's small and relatively uniform nanotubes, along with a low level of defects, made it an efficient emitter of electrons.

In the next step, the group suspended this cathode in a 5-cm-long glass tube. The tube's inner wall was coated with a conductive layer of indium tin oxide and, on top of that, a phosphor. The entire assembly was put into a vacuum chamber at a pressure of 10-7 mbar.

Applying a voltage to the inner cathode caused the carbon nanotubes to produce electrons. And when these electrons struck the phosphor on the outer tube, they caused it to glow. The measured luminance from the tube was 10,000 cd/m2, comparable to 11,000 cd/m2 from a commercial tube.

Viable devices

The nanotube-based light source is less energy efficient than fluorescent tubes, but Bonard believes that the thinner and different phosphor layers under development will substantially improve that. Sealing the tube will eliminate the need for a vacuum pump.

The technique could be used to produce other devices, Bonard said, including vacuum gauges, microwave tubes, x-ray sources and magnetic-field sensors.

The researchers presented their findings in the April 30 issue of Applied Physics Letters.

Photonics Spectra
Aug 2001
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