Nanocrystals Show Potential for Cheap Lasers and New Lighting

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Researchers at the University of Rochester and the Eastman Kodak Compay report in Nature they've created a nanocrystal that constantly emits light.

The breakthrough may open the door to dramatically less expensive and more versatile lasers, brighter LED lighting, and biological markers that track how a drug interact with a cell at a level never before possible.

For more than a decade, scientists have been frustrated in their attempts to create continuously emitting light sources from individual molecules because of an optical quirk called "blinking." By uncovering the basic physics behind the phenomenon, lasers and lighting could be incredibly cheap and easy to fabricate. Currently, different color laser light is created using different materials and processes, but with the new nanocrystals a single fabrication process can create any color laser. To alter the light color, an engineer needs only to alter the size of the nanocrystal, which is a relatively simple task.

Many molecules, as well as crystals just a billionth of a meter in size, can absorb or radiate photons. But they also experience random periods when they absorb a photon, but instead of the photon radiating away, its energy is transformed into heat. These "dark" periods alternate with periods when the molecule can radiate normally, leading to the appearance of them turning on and off, or blinking.

"A nanocrystal that has just absorbed the energy from a photon has two choices to rid itself of the excess energy—emission of light or of heat," says Todd Krauss, associate professor of chemistry at the University of Rochester and lead author on the study. "If the nanocrystal emits that energy as heat, you've essentially lost that energy."

The reason the blinking didn't occur, the authors report, was due to the unusual structure of the nanocrystal. Normally, nanocrystals have a core of one semiconductor material wrapped in a protective shell of another, with a sharp boundary dividing the two. The new nanocrystal, however, has a continuous gradient from a core of cadmium and selenium to a shell of zinc and selenium. That gradient squelches the processes that prevent photons from radiating, and the result is a stream of emitted photons as steady as the stream of absorbed photons.

Researchers at the Naval Research Laboratory and Cornell University also contributed to the discovery.