Nanocrystals key to cheap lasers

Scientists think they may have opened 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.

Nanocrytal  Lasers could be much more versatile

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," but now scientists in the US have uncovered the basic physics behind the phenomenon and created a nanocrystal that constantly emits light.

"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, professor of chemistry at the University of Rochester and lead author on the study published in Nature. "If the nanocrystal emits that energy as heat, you've essentially lost that energy."

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.

The team synthesized nanocrystals of various compositions and on inspection of one of these new nanocrystals and saw no evidence of the expected blinking phenomenon. Remarkably, even after four hours of monitoring, the new nanocrystal showed no sign of a single blink - unheard of when blinks usually happen on a scale of miliseconds to minutes.

The reason the blinking didn't occur 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.

With blink-free nanocrystals, Krauss believes lasers and lighting could be incredibly cheap and easy to fabricate.