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Guiding crystallization in thin films around bends and corners ( Nanowerk Spotlight ) Thin films comprising carbon-based molecules and polymers havepromising technological applications, such as biosensors, solarcells, electrically-active and light-emitting layers for displays,etc. Oftentimes, properties, such as luminescence and conductivity,depend on the orientation of crystals within the film. In organicthin films deposited on substrates, crystallization most oftenoccurs isotropically in the plane of the film. Much research hasthus focused on controlling the orientation of crystals in theplane of organic thin films. The use of temperature gradients andgravitational flow have been successfully employed to orientcrystals unidirectionally. Two-dimensional control of theorientation of crystals spatially within organic thin films,however, remains exceedingly difficult to achieve. In new work, researchers have now demonstrated a method to guidecrystallization along arbitrary patterns in the plane of organicthin films (ca. 100 nm thick), using an organic semiconductor,triethylsilylethynyl anthradithiophene (TES ADT). "TES ADT thin films are largely amorphous when spun cast fromsolution onto substrates," Lynn Loo , Professor of Chemical and Biological Engineering, and DeputyDirector of the Andlinger Center for Energy and the Environment atPrinceton University, explains to Nanowerk. "When exposed tosolvent vapors, crystallization is induced through the nucleationand subsequent growth of spherulites (a common crystallinesuperstructure seen in a wide variety of materials). We found thatthe growth rate of these spherulites depends strongly on thesurface energy of the underlying substrate. By patterning theunderlying substrate to have regions of different surface energiesprior to TES ADT deposition, we can guide crystallization alongpre-specified, nonlinear paths." As the team, which also included researchers at the University ofKentucky and the Cornell High Energy Synchrotron Source, explainsin their paper in the April 17, 2012 online edition of Advanced Materials ( "Guiding Crystallization around Bends and Sharp Corners" ), the surface energy of the path is selected such that fastcrystallization is promoted along the path, while the surfaceenergy of the region surrounding the path is selected such thatcrystallization is suppressed. Stephanie Lee, a graduate student in Loo's group and first authorof the paper, points out that central to this technique is the factthat the crystallization fronts of spherulites sample a widedistribution of orientation – unlike single crystals in whichmolecules are uniquely oriented. Crystals with orientations thatare parallel to the path direction propagate rapidly, whilecrystals with non-parallel orientations propagate until they reachthe edge of the path, at which point their growth is significantlysuppressed. In this manner, the crystallization front follows thepre-defined path, even around bends and sharp corners. Time-lapsed optical microscopy images of a-c) TES ADTcrystallization on PFBT-treated Au/SiO 2 substrates preferentially progressing along the SiO 2 patterns, and d) TES ADT crystallization on untreated Au/SiO 2 substrates in which spherulites grow indiscriminately across bothAu and SiO 2 surfaces. The arrows indicate the progression of crystallizationfronts and the time point for each frame is provided for clarity.(Reprinted with permission from Wiley-VCH Verlag) With this approach, it now has become possible to introducematerials properties anisotropy along paths and patterns on asingle substrate and over arbitrarily large areas. "The ability to guide crystallization allows us to control theorientation of crystals along pre-specified, arbitrary paths"says Lee. "In this manner, we can spatially pattern theproperties of organic thin films. For example, we can fabricatehighly conductive, nonlinear pathways within organic-semiconductorthin films for electronic applications. This technique of guidingcrystallization may also have implications in polarized lightapplications by allowing one to spatially control the luminescenceof materials within the plane of the films." According to the scientists, future directions of this researchwill include expanding this method of guiding crystallization toother materials systems. To achieve this goal, it will be necessaryto identify conditions under which spherulitic growth in the thinfilms can be induced, as well as surface chemistries that canaffect the growth rate of the spherulites. "Although each system will need to be optimized individually,we believe our method to be widely applicable due to the ubiquityof spherulitic growth in many different materials," says Loo. By Michael Berger. Copyright Nanowerk. I am an expert from custompaperbagprinting.com, while we provides the quality product, such as Custom Playing Card Printing , Custom Roll Labels Manufacturer, Custom Paper Bag Printing,and more.
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