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July 6th, 2007
Engineers provide insight into the dynamics of molecular self-assembly
By studying how a layer of molecules grows into an ordered layer from the edge of a rectangular silicon wafer, engineers at North Carolina State University, working with researchers from the National Institute of Standards and Technology (NIST), have established the time evolution of self-propagating self-assembly fronts. The team is the first to confirm the phenomenon in a real physical system.
The NC State researchers, Dr. Jan Genzer, professor of chemical and biomolecular engineering, and Dr. Kirill Efimenko, research assistant professor of chemical and biomolecular engineering, and NIST researchers, Dr. Jack Douglas, Dr. Daniel Fischer and Dr. Frederick Phelan, examined the spontaneous assembly of organosilane molecules into a monolayer film formed on an oxidized silicon surface.
They found that if a supply of the carbon-silicon-based molecule is placed along one edge of a treated silicon wafer, under controlled conditions, the organosilane molecules spontaneously organize themselves into a well-ordered layer, creating a carpet-like layer on the silicon that advances from the edge of the wafer at a constant velocity where the ordering initiates, ultimately covering the surface at long times. By following this process using a high resolution synchrotron X-ray technique and computer simulations, the NC State/NIST team established that the propagating wavefronts did not follow the constant width predicted by the classical mean-field theory that is widely believed to govern reaction-diffusion and self-assembly processes. (A wavefront is the leading edge of a wave or line of points that have the same phase or stage in a process.) What actually occurred is described as a "power-law broadening in time" when an autocatalyst is present.
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