Nanocrystal sheds new light on future illumination

A new source of white light has been discovered by accident at a US university, promising innovative ways to create and use illumination. Crystals just nanometres across have be used to coat a blue LED to produce "a warm white light with a slightly yellow cast".

These nanocrystals can theoretically be encased in compounds that conduct electricity — effectively creating electroluminescent suspensions. In the experiment, the blue LED was coated in transparent wood varnish that contained the suspended nanocrystals.

LEDs can be more cost effective than traditional light bulbs, and are already replacing them in automotive and signalling applications. Although LEDs cost more to make they are less fragile, last longer and use less electricity than traditional bulbs, because they produce minimal radiation in the infrared portion of the spectrum.

The cadmium selenide nanocrystals are 'magic-sized' — they generally form molecules that have just 33 or 34 pairs of atoms — which could lead to production cheaper than traditional chemical synthesis, mechanical production techniques used for making light-bulbs and semiconductor techniques used for making LEDs.

The white-light emitting nanocrystals were discovered by graduate student Michael Bowers, who was trying to make blue-light emitting quantum dots. Quantum dots are nanometre-sized collections of atoms that contain and control small numbers of electrons, in effect acting like artificial atoms with unique electronic properties. Until now they have been resolutely monochromatic, emitting one frequency of light.

Bowers pumped a solution containing the nanocrystals into a small glass cell and illuminated it with a laser. "I was surprised when a white glow covered the table," Bowers said. "The quantum dots were supposed to emit blue light, but instead they were giving off a beautiful white glow," he said in Exploration, the online research journal of Vanderbilt University.

Red, green and blue LEDs can already be combined to produce white light, but most often with a bluish tinge.

When cadmium selenide nanocrystals are illuminated by a laser or LED they "produce a smoother distribution of wavelengths in the visible spectrum with a warmer, more yellow tint." said Bowers.

An expert described the results of the research as "very surprising". Professor Keith Barnham, head of the experimental solid state physics group at Imperial College London, said: "I suppose the question really is: How efficient would the whole thing be? I imagine the light from the blue dot is illuminating impurities — that is how you usually mimic white light."

Barnham was interested in the possible application of illuminating the nanocrystals using solar cells, "or maybe even stimulating them directly with sunlight". He thought that electroluminescence would have to be used in order to have enough power to light a room.

Professor Colin Humphreys of Cambridge University has reservations about the commercial viability of this latest breakthrough.

"The new discovery from Vanderbilt is of considerable scientific interest but the quality of the white light does not seem to be easily controllable. Hence I doubt whether this new discovery could be used to produce really high-quality white lighting that would sell commercially," Humphreys said.

The crystals also raise the possibility of electroluminescent paint — a light source powered directly by electricity. Research groups at different universities have reported stimulating quantum dots to produce light by applying an electrical current, according to Vanderbilt.

In order to make the quantum dots emit light as a paint you'd need three layers of transparent paint — a back contact, the suspension containing the nanocrystals, and a front contact.

Bloggers were excited by the possibility of light emitting paint.

"If they can make the nanocrystals electroluminesce then LEDs aren't needed and lighting becomes a very different and interesting task. I'd like a softly glowing ceiling rather than point source lights. And if it could just be painted on it seems like it would be quite easy and cheap to do as well as energy efficient. Using different sized quantum dots would allow patterns, signs, even art." said one blogger on Crumbtrail.

Bowers' research paper, "White-Light Emission from Magic-Sized Cadmium Selenide Nanocrystals", was published online on 18 October by the Journal of the American Chemical Society.