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Magnets Change Crystals' Tint

Photonics.com
Jul 2007
RIVERSIDE, Calif., July 16, 2007 -- 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.
YinTeam.jpg
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.
PhotonicCrystals.jpg
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


GLOSSARY
color
The attribute of visual experience that can be described as having quantitatively specifiable dimensions of hue, saturation, and brightness or lightness. The visual experience, not including aspects of extent (e.g., size, shape, texture, etc.) and duration (e.g., movement, flicker, etc.).
light
Electromagnetic radiation detectable by the eye, ranging in wavelength from about 400 to 750 nm. In photonic applications light can be considered to cover the nonvisible portion of the spectrum which includes the ultraviolet and the infrared.
nanoparticle
A small object that behaves as a whole unit or entity in terms of it’s transport and it’s properties, as opposed to an individual molecule which on it’s own is not considered a nanoparticle.. Nanoparticles range between 100 and 2500 nanometers in diameter.  
optical
Pertaining to optics and the phenomena of light.
photonic crystal
photonics
The technology of generating and harnessing light and other forms of radiant energy whose quantum unit is the photon. The science includes light emission, transmission, deflection, amplification and detection by optical components and instruments, lasers and other light sources, fiber optics, electro-optical instrumentation, related hardware and electronics, and sophisticated systems. The range of applications of photonics extends from energy generation to detection to communications and...
sensor
1. A generic term for detector. 2. A complete optical/mechanical/electronic system that contains some form of radiation detector.
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