Please note that this article has been updated from its original text, correcting a couple of mistakes. -- JamaisAn accidental discovery at Vanderbilt University may well be the key to making light-emitting diodes the dominant lighting technology of the century. Up until very recently, the only way to make "white" light was to add yellow phosphors to bright blue LEDs. It wasn't quite right, though, as even the best "white" LED retained a blue tint. This week, we got the news that a chemistry grad student at Vanderbilt has stumbled on a way to make broad-spectrum white LEDs using quantum dots -- and in doing so, he may well have kicked off a revolution.Michael Bowers was making quantum dots, tiny nanocrystals just a few dozen atoms across. Crystals at that scale often have unusual properties, and the ones that Bowers created were no exception. When he illuminated his batch with a laser, rather than the blue glow he expected, out came a rich white light, similar in spectrum to sunlight. Bowers then took a polyurethane sealing liquid, mixed in some of his dots, and coated a blue LED. Although the resulting bulb -- pictured above -- is crude, it puts out white light. Its visible spectrum is similar to a typical incandescent bulb, but it puts out twice the light-per-watt, and lasts fifty times longer. One key reason for its efficiency is that it doesn't put out the infrared light typical of a regular light bulb; despite being much brighter, it's still far cooler to the touch. (The LED assembly still gets hot, however.) Completely by accident, Bowers had come up with a technology that possessed the quality of incandescent light, but none of its drawbacks.
The discovery was reported in the October 18 web publication of the Journal of the American Chemical Society; an abstract is available here.These "hybrid quantum dot LED" lights should be easier and far less expensive to make than current "white" LEDs, a big step towards light-emitting diodes becoming the dominant illumination technology. The Vanderbilt researchers are the first to report making quantum dots that spontaneously emit white light, but they aren’t the first to report using quantum dots to produce hybrid, white-light LEDs. The other reports – one by a group at the University of St. Andrews in Scotland and one by a group at Sandia National Laboratories – describe achieving this effect by adding additional compounds that interact with the tiny crystals to produce a white-light spectrum. The magic-sized quantum dots, by contrast, produce white light without any extra chemical treatment: The full spectrum emission is an intrinsic effect.
One difference between the Vanderbilt approach and the others is the process they used to make the quantum dots, Bowers observes. They use synthesis methods that take between a week and a month to complete; whereas, the Vanderbilt method takes less than an hour.
A second significant difference, according to [Bower's advisor Sandra] Rosenthal, is that it should be considerably easier to use the magic-sized quantum dots to make an “electroluminescent device” – a light source powered directly by electricity – because they can be used with a wider selection of binding compounds without affecting their emissions characteristics. [...]
The light bulb is made out of metal and glass using primarily mechanical processes. Current LEDs are made using semiconductor manufacturing techniques developed in the last 50 years. But, if the quantum dot approach pans out, it could transform lighting production into a primarily chemical process. Such a fundamental change could open up a wide range of new possibilities, such as making almost any object into a light source by coating it with luminescent paint capable of producing light in a rainbow of different shades, including white.There are a few remaining hurdles before we start seeing white LED lightbulbs on the shelves at your local grocery store, or as commonplace tools for leapfrog lighting. LEDs are still more expensive than common incandescent and fluorescent lights, although production costs are dropping. While LEDs are very efficient, putting out more light-per-watt than incandescent bulbs, they're still not as efficient as fluorescent bulbs; the maximum efficiency possible for LEDs, however, exceeds that of fluorescents. Still, the light quality was the biggest stumbling block to broader use of LEDs, as it was an aesthetic, not a practical, deterrent. Although replacing incandescent bulbs with white LEDs has great potential for improving energy use efficiency, I find the possibility of embedding light-emitting quantum dots into our industrial materials to be even more interesting. The breakthrough here isn't just in making it possible for LEDs to replace standard light bulbs -- although that's certainly big. The real breakthrough will come in the ways designers can start to rethink how illumination is used in our physical environment.