Magnets Change Crystals' Tint

Nanotechnologists can change the color of tiny particles of iron oxide suspended in water by applying an external magnetic field to the solution. The discovery could lead to proved electronic displays and the manufacture of products such as erasable and rewritable electronic paper and ink that can change color electromagnetically.

University of California, Riverside's Yongxing Hu (l-r), Jianping Ge and Yadong Yin of the Department of Chemistry. The team discovered they could change the color of photonic crystals in solution by applying an external magnetic field. (Photos courtesy Yin laboratory, UCR)

A research team headed by Yadong Yin at the University of California, Riverside, made the discovery that nanoscopic particles made of tiny magnetic crystals coated with a plastic shell self-assemble in solution to form photonic crystals -- semiconductors for light. When a magnetic field is applied, the optical properties of the crystals change, allowing their color to be very precisely adjusted through variation of the strength of the field.

Photonic crystals are the optical analog of electronic semiconductor materials. Like their electronic counterparts, they have photonic band gaps, forbidden energy levels, or wavelengths, at which the photonic crystal does not transmit light. These optical properties depend on the spatial relationships within the crystal.

"By reflecting light, these crystals -- also called photonic crystals -- show brilliant colors," 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 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.

“What should make the technology commercially attractive is that iron oxide is cheap, non-toxic and available in plenty,” Yin said.

A solution of iron oxide in water changes color under a magnetic field, with increasing strength of the field from left to right.

Yin said that the new technology can be used to make an inexpensive color display by forming millions of small pixels using the photonic crystals. “A different color for each pixel can be assigned using a magnetic field,” he said. “The advantage is that you need just one material -- for example, photonic crystals like iron oxide -- 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 -- a form of recycling.”

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. These switchable "optical semiconductors" also have applications in telecommunications (fiber optics), sensors and lasers.

Yin was joined in the research by UC Riverside’s Jianping Ge, a postdoctoral researcher, and Yongxing Hu, a first-year graduate student in the Department of Chemistry. The UCR Office of Technology Commercialization has filed a patent application on the technology. The work was published this month in the journal Angewandte Chemie.

For more information, visit: www.ucr.edu