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Color changing crystals use magnetic fields

23 Jul 2007

Photonic crystals that can change color thanks to a simple magnet could benefit display screen technology.

Colloidal photonic crystals that can be made to change color by applying a magnetic field have been developed by researchers in the US. The crystals could be used to dramatically improve the quality and size of electronic display screens and enable the manufacture of flexible color displays. (Angewandte Chemie International Edition 46 1)

"It is highly desirable to develop photonic crystals with tunable band gaps, so that their optical property can be changed by external stimuli," Yadong Yin, a researcher from the University of California, told optics.org. "Our approach employs simple and inexpensive synthetic methods and yields photonic crystals with wide and reversible tunability and an instant response to external magnetic fields."

The team believes that this research will lead to applications in chemical and biological sensors, highly integrated optical switches, and products such as erasable and rewritable electronic paper. They speculate that the system could also be used to replace expensive LCD monitors as extra-large displays or posters.

"Unlike a conventional flat-panel display, which uses a backlight to illuminate its pixels, reflective displays emit light in the same way as ordinary objects. This means they are better for our eyes," explained Yin. "The reflective displays will also be brighter under the exposure of strong illumination such as direct sunlight. This can solve the low-contrast problem experienced by current LCD displays and make them especially suitable for outdoor uses."

The team made the photonic crystals by using high-temperature hydrolysis to prepare colloidal nanocrystals of iron oxide (Fe3O4). Each cluster is composed of magnetite crystallites about 10nm across that retain their superparamagnetic properties at room temperature.

"This has never been achieved before because of the difficulty in synthesizing superparamagnetic particles with strong magnetization per particle and high density surface charges," commented Yin.

"In our case, each particle has the same charge so they repel each other in solution. However, because the particles are magnetic, we found that applying a magnetic field can bring them together," he continued. "The electrostatic repulsive force and the magnetic attractive force balance so that the particles arrange themselves into a three-dimensionally-ordered crystal."

The color emitted by the crystals is determined by the spacing between the particles. "This spacing can be conveniently changed by altering the strength of the magnetic field," explained Yin. "Therefore, the system emits colors across the entire visible spectrum by moving the magnet closer or further away from it."

The next step for the team is to look into further applications of the technology. "We are looking for all the possible uses of these materials, but the main focus will be on developing color display units," concluded Yin. "The key problems that have to be solved will depend on the application. For example, for their use as real-time displays, the switching rates may need to be further improved."

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