A diode of many colors.

At a typical intersection, cars take cues from a traffic light, whose colors turn from green to yellow to red.

Mimicking such color-changing sequences in light-emitting diodes (LEDs), commonly used to display information on electronic devices, offers practical advantages. Though LEDs already come in a range of colors, each hue requires a different kind of LED. So scientists have eagerly sought a LED that can produce a wide spectrum.

In the Aug. 4 NATURE, Vicki L. Colvin, a chemist at the University of California, Berkeley, and her colleagues report making a new kind of variable-color LED by blending cadmiumselenide nanocrystals with the semiconducting polymer p-paraphenylene vinylene.

By adjusting the size of the nanocrystals, the researchers can prompt the LED to shine yellow, orange, and red. By changing the voltage, they can get it to shine green.

"A traffic sequence," says Michael C. Schlamp, a chemist at Berkeley and a coauthor. "In theory, we should be able to get other colors too. We just haven't made the other nanocrystal clusters yet. But we will."

This LED has two unique features, says Schlamp. It represents the first use of nanocrystals in an electroluminescent device. And its color "can be adjusted without changing the method of material synthesis, just the size of the nanocrystals. One technique makes all the crystals. Their size varies based on how long they're brewed."

"That ability to tune the color of the electroluminescence by varying the nanocrystals' size is something that can't be done with polymers. To get a different color from a polymer electroluminescing device, you have to change the synthesis and make a different material," says Schlamp. "This is a big advantage for this kind of hybrid device."