Tuesday, September 25, 2007

On the White Path

Continued from page 1

By Neil Savage

A number of other researchers are also developing nanocrystals to produce white-light LEDs. James McBride, for one, is an assistant professor at Vanderbilt University who as a graduate student found a way to make white-emitting nanocrystals. Instead of using cadmium sulfide like Sarma, McBride makes his nanocrystals out of cadmium selenide. And while Sarma dopes his crystals with manganese to get enough red in the white light, McBride uses no dopants. But Sarma says that his approach gives better control over the shade of white coming out and requires less uniformity of size among the nanocrystals.

Neither Sarma nor McBride is on the verge of producing marketable nanocrystal coatings. Sarma reported that only 2 percent of the energy going into one of his coated LEDs was coming out as white light. McBride says that he's now up to about 8 percent. But the technologies will need to reach 40 or 50 percent before the other advantages of nanocrystals make them competitive with existing phosphors.

And the cadmium could be a problem as well. It's highly toxic, and the lighting industry would rather avoid it. McBride thinks that if the nanocrystals prove superior in other ways, the industry might take the steps necessary to make processing the material safer. Meanwhile, both researchers hope to take what they're learning with cadmium-based semiconductors and see if it works with less-toxic material.

Steven DenBaars, codirector of the Solid-State Lighting and Display Center at the University of California, Santa Barbara, says that nanocrystals may have some advantages over current phosphors--for example, providing better white light and improving manufacturing yield--if researchers can substantially increase the efficiency and deal with the toxicity problem. But at the moment, DenBaars says, "they've got a long way to go."

"This is proof of concept only," says Sarma. He notes that it should take two or three months to figure out if he and his colleagues are thinking along the right lines, and another six months of development to reach high efficiency if they are. If not, the researchers will have to start over again. If that's the case, Edison, who said that genius is 99 percent perspiration, may be smiling somewhere.