US scientists from the University of California, Riverside, were able to tune the color of colloidal systems by simply moving a magnet closer or further away from the mixtures. According to Yadong Yin and colleagues, color tuning of the colloids was achieved over the whole visible spectrum. The optical response to external magnetic stimuli was rapid and fully reversible. The authors believe that the photonic crystals could provide a new way to materials for color display units. Their results have been published online in the journal Angewandte Chemie.
Photo: Colloidal photonic crystals (Courtesy of Yadong Yin, University of California, Riverside)
Photonic crystals have attracted great attention because of their potential applications in the development of optoelectronic devices. However, the use of these crystals is hampered by their limited tunability and their slow response to external stimuli. Yin's team synthesized polyacrylate-capped superparamagnetic magnetite (Fe3O4) nanoparticles and suspended them in water. In a magnetic field, the particles assembled into brightly colored colloidal photonic crystals with wide and reversible tunability and an instant response to external magnetic fields. According to the authors, their approach employs simple and inexpensive synthetic methods that could be used for developing low-cost sensors and display screens.
When light passes through a photonic crystal, only certain wavelengths can be reflected. This phenomenon is related to the spacing in the crystal. Yin and co-workers changed the spacing in their colloidal crystals by simply moving the magnet closer or further away from them. In this way, they were able to tune the color, thereby covering the entire visible spectrum.
Superparamagnetic particles have been synthesized and used before; for example, Asher and co-workers, from the University of Pittsburgh, fabricated colloidal photonic crystals using highly charged polystyrene microspheres. However, according to the California team, their new nanoparticles are more responsive to the magnetic field. The authors also point to the broad tunability of their system and the low cost of the materials used.
Although some concerns exist regarding the stability of the crystals and their practical implementation for the development of new devices, Yin's work surely represents an important contribution in the field of optoelectronics. The other authors of the paper are Jianping Ge and Yongxing Hu. July 20, 2007 Source: Angewandte Chemie International Edition
