Tuesday, September 25, 2007

On the White Path

Nanocrystals could light the way to using LEDs to replace the lightbulb.

By Neil Savage

White light: An LED coated with nanocrystals (top) gives off a faint white light. The nanocrystals can emit white light in shades within the ellipse shown on a standard lighting color diagram. Credit: D.D. Sarma

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.