Published online 24 December 2008 | Nature | doi:10.1038/news.2008.1334

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Synthetic opals show their colours

Tunable material promises cheap route to electronic books and advertising displays.

Could synthetic opals build the next generation of advertising displays?

A material that can readily switch between a rainbow of colours has cleared a key hurdle to commercialisation, according to a group of entrepreneurial chemists.

The developers of 'photonic ink' (P-Ink) say that the material could be used in electronic books or advertising displays. Their finished product, described in the latest issue of Angewandte Chemie1, can display any colour when just a few volts of electricity are applied.

Electronic inks are already used in commercial products such as Amazon's Kindle reader. Most current technologies use an electric field to manipulate drops of oil or pigment particles. The presence or absence of a voltage makes pixels on the display appear light or dark, and most displays are confined to monochrome. P-Ink, however, can display any colour without using pigments. Instead, it relies on the same effect that generates shimmering colours in the semi-precious stone, opal.

Shimmering success

Opal is made from stacks of silica spheres, each just a few hundred nanometres across. When light bounces off the spheres, interference and diffraction eliminates some wavelengths, giving the material an apparent colour.

Geoffrey Ozin, a chemist at the University of Toronto, Canada, and Ian Manners of the University of Bristol, UK, have spent years recreating the effect using synthetic materials whose optical properties can be tuned with electricity (see 'Ink changes colour at flick of a switch')

The scientists make the materials by first stacking silica spheres 180 nanometres wide onto a glass plate to form a template. They then add an electroactive polymer containing iron atoms, which forms a gel around the spheres. When the material is incorporated into an electrical cell with a liquid electrolyte, applying a voltage causes electrons to move between the iron atoms and the electrolyte. This moves liquid around within the polymer gel, causing it to swell or contract, and altering its optical properties so that different wavelengths of light are reflected.

The team has now improved the device by dissolving away the silica template with hydrofluoric acid. This leaves a honeycomb of tiny pores through the material which can accommodate the liquid electrolyte, enabling much faster electron transfer.

This has made the P-Ink brighter and more responsive, while lowering power requirements. The latest version can span the entire visible spectrum using just under three volts, and can change colour in seconds.

Colour factor

The technology is now being developed by Opalux, a Toronto-based start-up company led by André Arsenault, one of Ozin's former students. Already, Ozin says, Opalux has caught the eye of advertisers. Unlike the expensive colour billboards that appear in Times Square or Piccadilly Circus, signs made of P-Ink would be cheap and energy efficient. "We believe that cost is our winner," says Ozin. The material could, in principle, be used to create rooms or even entire buildings that could change colour.

"It's a beautiful technology," says Johan Feenstra, chief technology officer at Liquavista, an electronic-ink firm based in Eindhoven, the Netherlands. But, Feenstra adds, the next challenge will be figuring out how to mass produce the material. "It's key to get a manufacturing process that will give well-defined colours," he says. Because the colour depends on perfectly producing the gel's microstructure, that could be a "daunting task", he says.

Ozin says that Opalux hopes the material can be manufactured using modified versions of existing techniques to make flexible solar cells. 

  • References

    1. Puzzo, D. P., Arsenault, A. C., Manners, I. & Ozin, G. A. Angew. Chem. Int. Edn doi:10.1002/anie.200804391 (2008).
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