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DISPLAY TECHNOLOGY
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Groovy self-assembly process used to make liquid-crystal displays
The method relies on supramolecular chemistry to amplify small islands of nanogrooves on the surfaces of indium tin oxide (ITO)-covered plates that are currently used in industry to manufacture LCDs [Angew. Chem. Int. Ed., 42, 1812 (2003)]. "Our procedure eliminates the need for clean room conditions that are required for LCD manufacture," explains Ph.D. student Johan Hoogboom. He developed the process with professor of organic chemistry Roeland J. M. Nolte and coworkers at the University of Nijmegen, in association with Philips Research Laboratories, Eindhoven, and the Dutch Technology Foundation. LCDs are currently produced using a technology that has been around for more than 25 years. The liquid crystals are sandwiched in cells between two glass plates that are coated with electrodes, polarizers, color filters, and polyimide layers spin-coated onto ITO. Microscopic grooves in the polymer surface align the liquid crystals. "The most common method for creating the grooves is mechanical rubbing of the polymer surface with a rotating velvet cloth," Hoogboom says. "Rubbing, however, is not only labor intensive but also causes faulty pixels due to static charges created during the process and dust rubbed off the polymer surface." The Nijmegen group coats ITO-covered plates by immersing them in a solution of siloxane monomers. Within six minutes, oligomers form and covalently link to the ITO surface. Scanning electron microscopy and tapping-mode atomic force microscopy reveal that the surface contains large domains of groovelike structures that resemble those present in conventionally rubbed layers. "Nanogrooves on the ITO plate act as seeds of alignment, which are amplified over 1,000- fold by a hierarchical self-assembly process," Hoogboom explains. "The grooves, which are hundreds of micrometers long, are formed by molecular recognition between the aromatic oligomers grafted onto the ITO surface." The group used the plates to construct twisted nematic LCDs and claims they rival industrially created LCDs in liquid-crystal-surface interaction energy. "This work is a most elegant example of how supramolecular chemistry might be exploited in important technological applications," comments Christoph Weder, associate professor of macromolecular science at Case Western Reserve University. "The concept of alignment amplification by self-assembly appears to be a powerful tool. "What intrigues me," he continues, "is that the approach relies on simple, yet carefully designed, organic molecules, and that good orientation layers can be produced by simply immersing a weakly oriented ITO substrate in a solution of these molecules. Clearly, the extent and quality of the preorientation of the ITO substrate are crucial. The immediate success of this system will depend on how well this preorientation can be controlled."
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