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Electrochemically oxidizing and reducing an inverse polymer-gel opal causes it to swell and shrink, which alters the wavelength of the light it diffracts brightly from ultraviolet through the visible to the near infrared. Chemists Andre Arsenault, Daniel Puzzo and Geoffrey Ozin of the University of Toronto, Canada, together with Ian Manners of the University of Bristol, UK, and explain the interest in photonic crystals. These materials, made either by a chemical "bottom-up self-assembly or by a "top-down" nanofabrication approach are being envisioned as useful for a wide range of technological applications. These might include the optical transistors and waveguides of light-driven devices to new, high-efficiency and controllable light-emitting diodes (LEDs) and lasers. Photonic crystals are also being touted as components for chemical and biochemical sensors, and in data storage media. "A challenge in the field has been the realization of photonic crystals for full-colour reflective displays, which could be used for electronic books, billboards, shelf-edge labels, and state-of-health fuel gauges for batteries," the researchers say. Previously, Ozin and colleagues have developed techniques that allow them to control the thickness, area, topology, orientation and registry of patterned single crystals in a silicon wafer easily and cheaply in their lab. These 'opal chips' were first mooted as components of microphotonic chips and for lab-on-chip technologies. Now, the team has turned to photonic crystals that can be controlled reversibly and tuned to produce a full spectrum of colours. They chose polyferrocenylsilane (PFS) derivatives polyferrocenylmethylvinylsilane (PFMVS), and polyferrocenyldivinylsilane (PFDVS). "Narrow polydispersity index and molecular weight control are achieved through anionic ring-opening polymerization from the appropriate silaferrocenophanes," the researchers explain. "Under anionic polymerization conditions, ring-opening of the silaferrocenophane is favoured over addition to the carbon-carbon double bonds present in each monomer unit leaving the latter intact." Cross-linking is possible through the pendant carbon-carbon double bonds along the polymer backbone. The opaline material itself is then created by first depositing a layer of silica spheres on glass. The researchers then bathe this monodisperse layer in a solution of the polymer. Ultraviolet light "cures" the material by allowing cross-links to form via a thiol-ene reaction. Excess polymer is removed and then etching with hydrofluoric acid releases the inverse polymer-gel opals which can then be mounted on glass coated with indium-tin oxide. The team used scanning electron microscopy to image the composite materials and reflectance spectroscopy to gain structural insights into their product. "The desired polymer inverse opal exhibits a Bragg reflectance peak centred at 538 nm which is blue-shifted by 65 nm and 150 nm relative to the analogous peaks of the bare silica opal and polymer-gel/silica opal composite, respectively," they explain. They add that the differences in the spectra are due to changes in the materials' refractive index between opal and non-opaline material. The next step was to create a sealed "button" cell to allow electrical tuning of the opaline material immersed in electrolyte to be carried out. Application of an oxidative potential to the cell extracts electrons from the iron ions in the polymer backbone, while anions diffuse into the polymer to maintain neutrality. This influx has the effect of causing the gel to swell and concomitantly red-shifts its optical diffraction peak. A reducing potential reverses the process. The colour of the material can thus be changed continuously through the complete visible spectrum and beyond by changing the applied voltage. "The impressive performance of the inverse polymer-gel opal is attributed primarily to its highly porous structure which increases the specific surface area of the film in contact with electrolyte," the team explains. This is the first example of an inverse opal that can be controlled electrically at low drive voltages and be shifted from ultraviolet, visible, and near infrared spectral ranges. The team points out that there are several technical hurdles yet to be overcome including improving reflectivity, boosting the speed of the reverse scan, and increasing the cycle lifetime. They suggest that the addition of nanoparticles to the polymer gel might be used to enhance colour contrast and provide control over the viewing angle. Additionally, fine-tuning the device components should allow them to reduce button cell resistance and boost full-colour tuning below 2 volts. Related links:
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