Accidental Find to Signal "Lights Out" for Incandescent Bulbs?

Joab Jackson
for National Geographic News

November 1, 2005

Microscopic crystals called quantum dots, which measure less than a millionth of a meter wide, might soon replace technology that has existed largely unchanged since 1879—the incandescent lightbulb.

New research shows that applying a thin layer of these dots over light emitting diodes (LEDs) causes the diodes to emit white light with the warm hue of a typical household bulb.

The work might be the final step in helping replace today's incandescent bulbs with LEDs, which can last longer and use much less electricity.

The breakthrough discovery was made by 26-year-old chemistry graduate student Michael Bowers of Vanderbilt University in Nashville, Tennessee.

"We didn't set out to make a white-light LED. It happened by accident," said Sandra Rosenthal, the Vanderbilt associate professor who oversees Bowers' work.

Bowers and Rosenthal, along with fellow graduate student James McBride, describe their find in the October 18 issue of the Journal of the American Chemical Society.

Small Crystals

The discovery started out as a favor for a friend. Bowers is the Vanderbilt chemistry department's resident quantum-dot chef. His job is to work up batches of the tiny crystals for researchers in the department to use.

McBride asked Bowers to produce a batch of quantum dots that were smaller than ones he had ever made before. McBride was interested in the relationship between the size and shape of nanocrystals and the types of energy they emit when stimulated.

Bowers made the dots by injecting molecules of the metals cadmium and selenide with a hot solvent, a chemical that loosens the metals' atoms so that bits break off and join into small clumps to form crystals.

After Bowers produced the crystals, he was curious to see what kind of light his newly minted dots would emit when a laser passed through them. He was surprised at the results.

"I expected [the light] to be blue or violet, but what came out was white," Bowers said. "I said, I've got to get a picture of this, because I may never be able to reproduce this."

Bowers and McBride brought the resulting picture to Rosenthal, who called it "the most beautiful thing I've ever seen."

According to Rosenthal, researchers had long known different-size quantum dots produce different colors. Few scientists, however, had predicted that a very small nanocrystal would give off white light, which is a combination of all colors.

"It wasn't proven before. It wasn't even really predicted, that we know of," Rosenthal said. The team soon realized the huge potential Bowers' nanocrystals had for the lightbulb industry.

Edison's Invention

The lightbulbs in your home are still very similar to the ones first invented by Thomas Edison over a century ago.

An incandescent bulb is a glass tube filled with a non-reactive gas that contains a thin metal filament. When electric current runs through the filament, the atoms within the filament vibrate, causing it to emit both light and lots of heat.

The heat is wasted energy. Researchers have long anticipated that LEDs, found in everything from digital clocks to car dashboards, could replace lightbulbs.

LEDs use a greater proportion of the electricity flowing through them, so they emit less heat. In fact, LEDs are cool to the touch.

This translates into savings for consumers. Widespread adoption of LEDs could cut U.S. consumption of electricity for lighting by 29 percent, according to the U.S. Department of Energy.

And unlike fragile glass bulbs, LEDS are made from sturdy wafers of semiconducting materials. An LED can last up to 50,000 hours—50 times as long as a 60-watt bulb.

But creating white-light LEDs has been problematic. Only recently have researchers come up with LEDs as powerful as incandescent and fluorescent bulbs. And these news LEDs give off light with a harsh, bluish tinge that can be unpleasant to human eyes.

"The holy grail for the [LED] industry is to penetrate the lighting market. The industry is working very hard on that," said Robert Steele, director of optoelectronics research for technology firm Strategies Unlimited in Mountain View, California.

Lightbulb purchases add up to 15 billion U.S. dollars in sales a year, Steele said. Bowers' coating may help put LEDs next to those bulbs on hardware store shelves.

"It is a good step forward. Any work along these lines is worthy of attention," Steele said.

He noted that similar, ongoing research is funded by countries such as the United States, Korea, China, and Taiwan. A number of private companies are also developing their own quantum-dot technologies.

Meanwhile, the Vanderbilt team admits that much work remains to make the crystals cheap and sturdy enough for commercial use.

"We're cautiously optimistic," Rosenthal said.

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