NÎW5fîFÏHE«f€£K SCIENCE
PAINTABLE LIQUIDCRYSTAL DISPLAYS Liquid-crystal coating technique could lead to cheaper, thinner,flexibledisplays NEW LIQUID-CRYSTAL Display (LCD) technology may facilitate the fabrication of displays that can be mounted onto walls or integrated into clothing, researchers in the Netherlands suggest. The technique, developed at Philips Research Laboratories, Eindhoven, employs a wet coating formulation that is painted onto a single substrate. Aprocess known as photo-enforced stratification (PES) converts the film into micrometer-sized polymer boxes filled with liquid crystals {Atown>417,55(2002)}. "The PES-LCD technology opens the way to make portable equipment smaller, lighter, and more robust, and also provides possibilities for making displays large enough to cover walls," says Dirk J. Broer, professor of polymer chemistry at Eindhoven University of Technology and a research fellow at Philips. The formulation, which contains a blend of liquid crystals, a mixture of monomers, an ultraviolet-light-absorbing dye, and a photoinitiator, can be painted ontoavariety of substrates using standard coating technology Closed cells of liquid crystals are formed by exposing the film to UV radiation in a two-step procedure. Exposure of the layer to high-intensity UV light through a mask induces polymerization and phase separation of the mixture, resulting in the formation of polymer walls. In the second step, a stratified polymer layer that covers the film is formed by exposing the film to low-intensity UV light at a lower wavelength. The polymer walls
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mechanically connect the underlying substrate to the cover and stabilize the structure. LCDs currently are produced using a technology that has been around for 25 years. In these displays, the liquid crystals are sandwiched in cells between two glass plates that are coated with electrodes, polarizers, color filters, and layers that align the liquid crystals. Vacuum suction is used to fill the cells with the liquid crystals. The whole process is time-consuming and expensive. Although there are hurdles to be overcome before the new technique can be applied on an industrial scale, it is a promising approach for LCD fabrica-
MATERIALS
tion, comments Peter Raynes, professor of optoelectronic engineering at the University of Oxford, in the same issue ofNature (page 28). The main challenge, he notes, is device lifetime. "One of the main reasons that plastic substrates have not yet replaced glass in the conventional LCD sandwich is their reduced lifetime, owing to the permeability of plastics to atmospheric contaminants such as water and oxygen," he writes. —MICHAEL
OPEN SANDWICH Liquid-crystal display is painted onto a single substrate.
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SCIENCE
Nanotube Strands Are Centimeters Long
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rowing carbon nanotubes as long as 1 mm was once considered a big deal For many applications, though, much longer tubes are required. Such tubes are now a step doter. A CWna-U.S. team has synthesized single-walled carbon nanotube (SWNT) strands tome 20 cm long [Science, 2*4,884 (2002)]. These are the longest and strongest assemblies thus far made entirely of carbon nanotubes, according to one scientist familiar with the report. The strands, which look and feel much like human hair, were prepared by mechanical engineering professor Dehai Wu of Tsmghua university, in Beijing, and coworkers by pyrotyzing n-hexane at 1,150 °C in the presence of ferrocene catalyst and thiophene in flowing hydrogen gas. The strand shown here consists of hundreds of nanotubes and is about 10 μπ\ wide. Characterization of the nanotube strands was car ried out by materials engineer Pulkkel M. Ajayan and coworkers at Rensselaer Polytechnic In stitute. They find that the strands are about five times stiffer than SWNT fibers and ribbons spun from nanotube/surfactant slurries reported in 2000 by a French group led by Philippe PouUn, Nevertheless, the properties of Wu's strands still fall far short of the ideal predicted values, undoubtedly because of defects in the strands. Researchers would get closer to the predicted values for nanotube properties if they could just figure out how to make aligned nanotubes ex tend continuously from one end of a strand to the other, Ajayan says. Now that would be a big deal—RON 0A6ANI
C & E N / MAY 6. 2002
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