On the White Path
Nanocrystals could light the way to using LEDs to replace the lightbulb.
Thomas Edison’s lightbulb has ruled the world’s lighting for more than a century, but numerous researchers are trying to replace the incandescent bulb with more energy-efficient solid-state lighting. One problem has been producing bright white light. To address the problem, D.D. Sarma, a materials scientist at the Indian Institute of Science, in Bangalore, has made tiny crystals of semiconductor material that, when coated onto a light-emitting diode (LED), give off a white glow just the right color for illuminating a living room. So far, it’s only a weak light, but Sarma hopes to make it much brighter.
Sarma says that his approach gives better control over the whiteness and is simpler than other research efforts that use nanocrystals to produce white-light LEDs. Sarma grows tiny crystals of cadmium sulfide. He then paints them onto an LED that emits ultraviolet wavelengths, and the crystals produce the mix of colors that we perceive as white light. It’s the extremely small size of the nanocrystals–each crystal is only five nanometers in diameter–that gives them their remarkable properties, says Sarma.
Single-color LEDs have largely taken over for lightbulbs in uses such as traffic signals. There’s a big push to replace incandescent and fluorescent bulbs for general illumination as well. Sandia National Laboratory estimates that if half of all lighting is based on LEDs by 2025, the world would use 120 gigawatts less electricity, saving $100 billion a year and cutting the carbon-dioxide emissions from power plants by 350 megatons annually.
But to light up a room, single-color LEDs won’t do. LED makers typically coat on a mix of phosphors to get white light out of an ultraviolet LED–the same method used in fluorescent bulbs. However, the molecules in the phosphors are so big that they scatter the light in unpredictable directions, and a good deal of it bounces back toward where it came from, never providing useful illumination. Scattering becomes a nonissue with the nanocrystals because they have less surface area for photons to bounce off. “The nanomaterials in general are so small they don’t scatter light,” Sarma says. “It is one of the reasons we get so excited about these materials.”
Another potential advantage of nanocrystals over current materials, Sarma says, is that his nanocrystals produce a uniform shade of white. Traditional phosphors individually produce red, green, and blue light; they have to be mixed in the right ratios to create white light. But the phosphors that emit red light also absorb some of the green and blue light, making the mix more complex, so different LEDs wind up producing different shades of white. And the different phosphors age at different rates, so the color of the light could change over the lifetime of the product. Neither is a problem with the nanocrystals, Sarma says.
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.
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