Information technology, nanotechnology

New displays possible from colour-changing magnetised liquid

25 July 2007

Researchers at the University of California, Riverside (UCR) have discovered that it is possible to change the colour of a liquid consisting of very small particles of iron oxide suspended in water simply by varying the strength of a magnetic field.

The discovery has potential to greatly improve the quality and size of electronic display screens and to enable the manufacture of products such as erasable and rewritable electronic paper and ink that can change colour electromagnetically.

Iron oxide in water changing colour under a magnetic field
A suspension of iron oxide in water changing colour under a magnetic field, with increasing strength of the field from left to right. Credit: Yin laboratory, UCR.

The results were published in Angewandte Chemie International Edition’s online edition and in print in issue 34 of the journal.

“The key is to design the structure of iron oxide nanoparticles through chemical synthesis so that these nanoparticles self-assemble into three-dimensionally ordered colloidal crystals in a magnetic field,” said Yadong Yin, an assistant professor of chemistry who led the research.

A nanoparticle is a microscopic particle whose size is measured in nanometers, a nanometer being a billionth of a meter. A colloid is a substance comprised of small particles uniformly distributed in a liiquid — milk, paint and blood are examples of colloids.

“By reflecting light, these crystals, also called photonic crystals, show brilliant colours,” Yin said. “Ours is the first report of a photonic crystal that is fully tunable in the visible range of the electromagnetic spectrum, from violet light to red light.”

Iron oxide (formula: Fe3O4) nanoparticles are “superparamagnetic,” meaning that they turn magnetic only in the presence of an external magnetic field. In contrast, “ferromagnetic” materials become magnetized in a magnetic field and retain their magnetism when the field is removed.

The researchers used the superparamagnetic property of iron oxide particles to tune the spacing between nanoparticles, and therefore the wavelength and colour of the light reflected, by changing the strength of the external magnetic field.

“Other reported photonic crystals can only reflect light with a fixed wavelength,” Yin said. “Our crystals, on the other hand, show a rapid, wide and fully reversible optical response to the external magnetic field.”

Photonic materials such as those used by Yin and his team could help in the fabrication of new optical microelectromechanical systems and reflective colour display units. They also have applications in telecommunication (fibre optics), sensors and lasers.

“What should make the technology commercially attractive is that iron oxide is cheap, non-toxic and available in plenty,” Yin said. "A different colour for each pixel can be assigned using a magnetic field,” he said. “The advantage is that you need just one material for all the pixels. Moreover, you don’t need to generate light in each pixel. You would be using reflected light to create the images.”

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