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David Bradley ISSUE #67
July-August 2007
Yongxing Hu, Jianping Ge, and Yadong Yin (Credit: Yin laboratory)
Attractive Changing Colors

Yadong Yin and colleagues at the University of California, Riverside, have discovered that a simple magnet can be used to change the color of nanoparticles of iron oxide in aqueous suspension. The discovery could lead to a new class of low-power electronic displays. It also has the potential to be exploited in rewritable electronic paper and e-ink products.

Yin explains that the key was to design the structure of superparamagnetic iron oxide (Fe3O4) nanoparticles, just a few billionths of a meter in diameter, through chemical synthesis so that they self-assemble into three-dimensionally ordered crystals floating in the water and held in a magnetic field. Unlike certain other designer nanoparticle materials, such as coated gold particles, iron oxide is cheap, non-toxic and anything but rare.

The team found that by changing the strength of the magnetic field applied to their colloidal suspension they were able to change the color of the iron oxide solution. The magnetic field disturbs the arrangement of the spherical iron oxide particles in solution, altering how light passes through the solution.

Iron oxide nanoparticles are superparamagnetic, which means they become magnetic only in the presence of an external magnetic field. Ferromagnetic materials, in contrast, such as those used in horseshoe magnets and compass needles become magnetized and stay magnetized, producing their own magnetic field. The applied magnetic field magnetizes the nanoparticles, altering the spacing between them as they become attracted to each other. This space then affects the wavelength, and so the color, of light that is reflected by the aqueous suspension of the particles.

Magnetized iron oxide nanoparticles suspended in water change color with increasing magnetic field strength (Credit: Yin laboratory)

"By reflecting light, these crystals—also called photonic crystals—show brilliant colors," Yin explains. "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."

Photonic crystals have attracted a great deal of attention from researchers and technologists because they can control the flow of photons, in a manner analogous to the control of electron flow by semiconductor materials, such as silicon.

"Other reported photonic crystals can only reflect light with a fixed wavelength," Yin explains. "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 color display units. They also have applications in telecommunication (fiber optics), sensors and lasers.

Independent but related research from scientists at the U.S. Department of Energy's Argonne National Laboratory and Ames National Laboratory recently demonstrated novel materials that change temperature in magnetic fields. These materials could lead to new energy-efficient refrigeration technologies.

Angew Chem, Int Edn, 2007, in press; http://dx.doi.org/10.1002/anie.200701992