Home » Rigor and RelevanceSpring 2008

A Little Matter of Light

by David F. Salisbury No Comment

A group of Vanderbilt chemists didn’t set out to make traditional light bulbs obsolete and cut carbon emissions, but that may be what they have done. 

Then-chemistry graduate student Michael Bowers, PhD’07, was working with Associate Professor of Chemistry Sandra Rosenthal when he discovered a new way to make solid-state lights (light-emitting
diodes—LEDs) that produce white light. Use of solid-state lights could halve lighting electricity consumption, cutting carbon dioxide emissions by 258 million metric tons per year. The discovery was published by The Journal of the American Chemical Society and received a Breakthrough Award from Popular Mechanics magazine.

While more expensive than ordinary lights, LEDs can produce about twice as much light per watt as incandescent bulbs. LEDs last up to 50,000 hours or 50 times as long as 60-watt bulbs, and they are very tough and hard to break. Although color LEDs have been used for decades in consumer electronics, the LEDs available today produce a bluish-white light, not quite white enough for general use. Rosenthal’s group discovered that microscopic semiconductor nanocrystals, called quantum dots, can absorb the blue light and emit a warm white light. If the quantum dots can produce white light more efficiently, then quantum-dot coated LEDs could replace light bulbs. 

Eureka Moment

At the time of the discovery, the Rosenthal group was exploring two applications for semiconducting nanocrystals, one for medical use and one for photovoltaic (lighting or solar cell) use. Bowers was working on making small-sized quantum dots. He pumped a solution containing the nanocrystals into a small glass cell and illuminated it with a laser. “I was surprised when a white glow covered the table,” Bowers says. “The quantum dots were supposed to emit blue light, but instead they were giving off a beautiful white glow.”

The discovery has led the Rosenthal group to study this new application. “The exciting thing about this is that it is a nano-nanoscience phenomenon,” Rosenthal says. In larger nanocrystals, the light originates in the center of the crystal. But as the size of the crystal shrinks—becomes even more nano—the light emission region appears to move to the surface of the crystal and broadens out into a full spectrum, producing white light.

Photo by Daniel Dubois.

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