Self-Assembly at a Liquid-Liquid Interface Yields High-Quality Photonic Crystals
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Self-Assembly at a Liquid–Liquid Interface Yields High-Quality Photonic Crystals Photonic crystals, a key component in optical devices, can be fabricated using colloidal particles with diameters on the order of the wavelength of light. In the November 28, 2006, issue of Chemistry of Materials (p. 5643; DOI: 10.1021/cm0615525), S. Takeda and P. Wiltzius of the University of Illinois at Urbana-Champaign demonstrated a method for fabricating colloidal polystyrene (PS) photonic crystals using self-assembly at the interface of water and a fluorinated solvent. This method is more robust than existing dip-coating methods and produces crystals with fewer defects. In this method, a droplet of water containing PS microspheres (~466 nm diameter) was added to the surface of a fluorinated solvent bath. The solvent was chosen so
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that it had density lower than water and surface tension higher than water. Consequently, the two liquids remained separated and the colloidal particles selfassembled into close-packed structures at the interface between them. After the water droplet and the ordered microspheres were transferred to a glass slide and allowed to dry, scanning electron micrographs and optical reflectance measurements revealed highly ordered crystals covering large areas of the substrate. Because of the high mobility at a liquid– liquid interface, the degree of ordering achieved using this method surpassed that of standard dip-coating methods in which the self-assembly occurs at a liquid–solid interface. Furthermore, using separate droplets containing PS microspheres of two different sizes (i.e., one droplet with 466 nm diameter microspheres and one
droplet with 240 nm diameter microspheres), the researchers coated both sides of a glass slide with crystals of different periodicities. The reflectance spectrum at visible and near-infrared wavelengths of this photonic crystal depends upon which side the light is initially incident. When light was incident on the side coated with the smaller microspheres, strong reflectance peaks were observed near 580 nm and 980 nm in wavelength. However, when the light was incident on the side coated with the larger microspheres, the shorter wavelength reflectance peak vanished. The researchers said that this asymmetry is due to the stronger scattering of the short-wavelength region by the 466 nm diameter microspheres. They also said that this structure could form the basis of an optical switch in which the switching is obtained by simply rotating the photonic
MRS BULLETIN • VOLUME 32 • JANUARY 2007 • www.mrs.org/bulletin
RESEARCH/RESEARCHERS
crystal device to present the different faces to the incident light. This technique offers a new, potentially easier, and more effective approach for the fabrication of highly ordered photonic crystals than currently used. According to the researchers, further refinement of this method may yield even greater advantages by shortening fabrication time and further reducing defects. KRISTA NIECE
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