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PHOTONICS
A redox-active silica-polymer composite material may herald the development of a new kind of digital display technology, according to scientists at the University of Toronto.
"The ultimate idea is to put the film of the material and a solvent onto an electrode and deliver different amounts of charge to the matrix so that the polymer will expand or contract accordingly," Manners tells C&EN. "With electrodes, it should be possible to pixelate and change the color of individual pixels." Ozin says the idea is "founded upon a specially designed metallopolymer-silica colloidal crystal nanocomposite film whose lattice dimensions are tunable by the application of a solvent or by a charge change to the film. Light incident on the film is optically Bragg diffracted at a wavelength related to the lattice dimensions of the film and is, in principle, continuously tunable across the entire visible spectral range." The optical response of the film to a change in solvent is exceptionally fast, according to the authors. It switches from the dry shrunken state to a fully swollen state in less than half a second. Each repeat unit of PFS contains a redox-active ferrocene group. In a solvent, the PFS-opal composite gel contracts on oxidation since the polymer-solvent interactions are less favorable when the iron centers are oxidized. As a result, the Bragg diffraction shifts to a shorter wavelength. According to chemistry professor Thomas E. Mallouk at Pennsylvania State University, University Park, the electrochemical stop-band tuning reported in the paper is a clever piece of new science. Stop bands are photon band gaps that stop the passage of light only in certain crystal directions. "The dimensional and refractive index changes induced by redox cycling of the ferrocene polymer cause the stop band to be tunable between about 700 and 820 nm," he says. "Presumably, by using smaller spheres, the same kind of change could be made to happen in the visible." The team is now working to develop an electrochemical cell using the system. The aim is to incorporate the cell into prototype devices such as visible-light pixelated displays and near-infrared optical telecommunication switches and attenuators. |
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