May 14, 2009 Organic light emitting diodes (OLEDs) are just starting to appear in consumer electronics, such as Sony's OLED XEL-1 TV and the United Keys OLED Keypad, but barely have we been there five seconds and already scientists are talking up what could be OLED’s successor – a nanocrystal that constantly emits light, which may open the door to dramatically less expensive and more versatile lasers, brighter LED lighting and biological markers tracking how a drug interacts with a cell at a level never before possible.
The creation of the nanocrystal was made possible by uncovering the basic physics behind the phenomenon of “blinking”, an optical quirk that has so far frustrated attempts to create continuously emitting light sources from individual molecules. 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 blinks usually happen too fast for the human eye to detect, on a scale of milliseconds to minutes, but even after four hours of monitoring, the new nanocrystal showed no sign of a single blink.
The researchers attributed the blink-free characteristic of the nanocrystal to its unusual structure. 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.
The blink-free nanocrystals offer a cheap and easy way to fabricate lasers and lighting. At the moment, 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 the team says is a relatively simple task. Paper-thin computer displays made by “painting” a grid of differently sized nanocrystals onto a flat surface, or a wall, which lights a room in any desired color are other possibilities for the technology.
To develop the nanocrystals, Todd Krauss, associate professor of chemistry at the University of Rochester, worked with researchers at the Naval Research Laboratory and Cornell University, as well as engineers at Kodak Eastman, who were exploring new types of low-cost lighting. The Kodak engineers were investigating lighting similar to OLEDs, but didn't suffer from the short lifespans and manufacturing challenges inherent in these diodes. Poor old OLED.
It seems a technology doesn't even have time to find its feet before the next young buck comes along looking to steal its thunder.
The nanocrystals are detailed in the team's study published online recently in Nature.
Darren Quick
