LEDs could light future

You can read by glow of zinc oxide

A revolution is under way in lighting: The incandescent bulb -- the familiar one Thomas Edison invented more than a century ago -- is quickly giving way to compact fluorescent lights. But even before that transition is completed, energy-efficient light-emitting diodes, or LEDs, may outshine fluorescent lights.

"Someday you won't have a light bulb," said Henry Everitt, an adjunct physics professor at Duke University. "You'll have an LED. They would be just as bright, but more efficient. ... It would be like a computer chip that light comes off of."

Everitt and Jie Liu, a chemistry professor at Duke, hope to contribute to the lighting revolution. They've produced a new kind of illumination by adding sulfur at high temperatures to zinc oxide, an inexpensive chemical compound commonly used to prevent sunburn and diaper rash.

The fine zinc oxide powder the Duke scientists are studying may eventually coat an LED chip and convert invisible ultraviolet light into a brilliant white light -- a light better suited for reading than LEDs now produce. It would work much the same way the fluorescent coatings, called phosphors, work inside a fluorescent vacuum tube, absorbing ultraviolet light and re-emitting it as light that brightens offices and homes.

The researchers are excited about the commercial potential for the discovery because the phosphor they've developed is compatible with the light-emitting semiconductors currently being made. It also would be a less toxic and cheaper light source than the combinations used in today's commercial LEDs.

"What we're proposing is an alternative phosphor that could be used to make the white light," said Everitt, who also works as a senior research scientist at the Army's Redstone Arsenal in Huntsville, Ala. "It is an alternative that is cheap and very environmentally friendly. People put it on their baby's bottoms and on their noses."

The Army selected the research project for priority funding because of its potential as a safe, energy-efficient illumination source.

"One of the objectives is to give soldiers efficient lighting that doesn't run their batteries down," Everitt said. "They need efficiency, brightness, longevity and ruggedness, and this helps with all of those things.

"As a tank driver or helicopter pilot, you can't afford to have fluorescent light as a lighting source," he said. "If they get shot and the bulb breaks, then what are they going to do? But a solid state bulb could withstand that kind of attack and keep working."

LEDs are already used in traffic signals, cell phones, dashboard lighting and other automotive applications.

"They are good enough for decoration and for use in traffic lights, but they don't make good reading lights because they are not of a white color that our eyes use best," Liu said.

But the researchers hope their phosphor may help LED manufacturers produce a more usable brilliant white light.

A laboratory surprise

The initial discovery came about by accident when a graduate student working with Everitt focused a laser on some nanoparticles of zinc oxide mixed with sulfur prepared by Liu. They were interested in examining the chemical structure of the microscopic particles, but they got a surprise.

"We hit it with a laser, and bang, all this white light came off," Everitt said. "We didn't expect that."

The surprising white light highlights the fascination with the field of nanotechnology: that materials in very small quantities can exhibit unexpected and unobserved changes in optical, electronic and magnetic properties. Opaque materials may be transparent, or materials may take on different colors. For example, a tiny five-nanometer particle of gold looks pink.

Liu's research group focuses on the chemistry of nanomaterials that are tens to hundreds of atoms in diameter. It takes about 50,000 nanometers to make up the diameter of a human hair.

When they stumbled onto the white light, Liu said, they quickly decided it was an interesting direction to explore.

"It's a new kind of phosphor," Liu said. "It can be used to make white-light LEDs. It's the next generation light source."

While scientists have long known that zinc oxide has interesting optical properties, the Duke researchers have advanced the use of ultraviolet light to produce visible light from the zinc oxide and sulfur mixture and are investigating how nanostructuring improves the performance.

Everitt and Liu have applied for a patent on using the preparations as a light source.