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14 May 2009

Nanocrystals see the light

As the old saying goes, blink and you miss it, and the same holds true for continuously emitted light sources from individual molecules.

Now, however, there is a plan under development that can create a nanocrystal that constantly emits light.

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Krauss’ new “non-blinking” nanocrystals
Source: University of Rochester

For more than a decade, researchers grew frustrated in their attempts to create continuously emitting light sources from individual molecules. An optical quirk called “blinking” was the culprit.

It took a while, but scientists at the University of Rochester uncovered the basic physics behind the phenomenon, and along with researchers at the Eastman Kodak Company, created this constant light emitting nanocrystal.

This may lead to less expensive and more versatile lasers, brighter LED lighting, and biological markers that track how a drug interacts with a cell at a level never before possible.

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 transforms 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,” said 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.”

Krauss worked with engineers at Kodak and researchers at the Naval Research Laboratory and Cornell University to discover the new, non-blinking nanocrystals.

Krauss, an expert in nanocrystals, and Keith Kahen, senior principal scientist of Kodak and an expert in optoelectronic materials and devices, were investigating new types of low-cost lighting similar to organic light-emitting diodes. They wanted to find something that did not suffer from the short lifespans and manufacturing challenges inherent in these diodes. Kahen and Megan Hahn, a postdoctoral fellow in Krauss’ laboratory, tried synthesizing nanocrystals of various compositions.

Xiaoyong Wang, another postdoctoral fellow in Krauss laboratory, inspected one of these new nanocrystals and saw no evidence of the blinking phenomenon. Even after four hours of monitoring, the new nanocrystal showed no sign of a single blink, which was unheard of when blinks usually happen on a scale of miliseconds to minutes.

After an investigation, Krauss and Alexander Efros from the Naval Research Laboratory found the reason the blinking did not 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 said he believes lasers and lighting could be cheap and easy to fabricate. Currently, different color laser light comes from using different materials and processes, but with 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 Krauss said is a relatively simple task.

The same is true of what could one day be OLED’s successor, Krauss said. Essentially, “painting” a grid of differently sized nanocrystals onto a flat surface could create computer displays as thin as paper, or a wall that lights a room in any desired color.

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