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Organic materials hold tremendous promise for inexpensive electronic and optoelectronic devices such as solar cells, but these applications require the challenging step of coaxing molecules into crystalline form. Covalent organic frameworks (COFs) are a general approach to overcoming this challenge, in which molecules are specifically designed to form predictable crystalline structures. However, until now, these structures could be synthesized only as powders -- randomly oriented crystallites of limited use for electronic applications.
In the April 8th issue of Science, researchers John Colson, Arnab Mukherjee and others from Professor William Dichtel's group in the Cornell University Department of Chemistry and Chemical Biology, the Department of Electrical Engineering, and the Cornell High Energy Synchrotron Source (CHESS), report a substantial push forward in this area by fabricating thin, oriented COF films for the first time. The crystallites in the film are composed of stacks of layers, each of which resembles a microscopic chain-link fence (left panel of figure). The "fence" itself is composed of periodic arrays of molecules which can be selected to best suit the application. Grazing incidence x-ray diffraction measurements (upper right), performed at the G2 station at CHESS, show that the layers are stacked perpendicular to the substrate surface. UV spectroscopy (lower right) shows that the molecules of interest retains the light-absorbing properties essential for applications. The group reports successful fabrication of three distinct COF films, demonstrating the general nature of their approach. Moreover, the films are grown on graphene -- a single layer of carbon atoms -- which may act as a thin, transparent electronic interface to the films.
Science Magazine: http://www.sciencemag.org/content/332/6026/228
Cornell Chronicle:
http://www.news.cornell.edu/stories/April11/DichtelCofs.html
Submitted by: Arthur Woll, CHESS, Cornell University