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August 19, 2002 Nanoscale Self-Assembly of Thin-Film Molecular Materials for Electro-optic SwitchingM.E. van der Boom1, P. Zhu2, G. Evmenenko2, J.E. Malinsky2, W. Lin2, P. Dutta2, and T.J. Marks2 Scientists from Northwestern University in Evanston, Illinois and the Weizmann Institute of Science, Rehovot, Israel, have devised a two-step assembly technique to make highly ordered, intrinsically acentric organic materials which can be integrated into electro-optic (EO) and related devices, such as light modulators and switches. The scientists have shown that the self-assembled photonically/electronically functional materials are competitive in terms of EO responses with the highest efficient polar films reported to date, and are more efficient than inorganic systems, such as LiNbO3.
A general applicable method has been developed generating thermally robust multilayered materials. This new synthetic approach involves two alternating deposition steps, as shown in Figure 1. First, monolayers (one-molecule-sized layers) of chromophores are covalently bound on hydrophilic substrates (step (i)). The siloxy removal step (ii) renders the surface hydrophilic, thus allowing the rapid build-up of a covalently-bound siloxane-based capping layer. The resulting films are intrinsically acentric, so no post-deposition steps such as high-voltage poling to align the molecular building blocks are necessary, as in other film growth techniques.
We have developed a film growth process based on chemically reliable steps, amenable to automation – by using a single reaction vessel or dip-coating – and allowing an excellent control of material properties – which is of great interest for optical telecommunications and electronic applications. The high degree of control over film dimensions, texture, and properties has been unambiguously demonstrated using various physico-chemical analytical tools, including second harmonic generation measurements and synchrotron x-ray reflectivity measurements (XRR) performed at NSLS beamline X23B (Figure 2).
The XRR experiments afforded crystal-clear structural information on the chromophore density (~50 Å2/chromophore), film thickness (~2.8 nm for each chromophore + siloxane-based capping layer), and surface morphology. The robust capping layer is ~8 Å thick. The streamlined two-step assembly process shown in Figure 1 could be extended to a wide range of molecular building blocks, and become a major synthetic route for the formation of various functional sub-micrometer-sized solids with superb control of material characteristics at the nanoscale level. This assembly process is also part of an ongoing investigation aimed at creating "all-organic" electro-optical modulators (Figure 3).
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The National Synchrotron Light Source at Brookhaven National Laboratory in New York, is a national user research
facility funded by the U.S. Department of Energy's Office of Basic Energy Science. The NSLS operates two electron
storage rings: an X-Ray ring and a Vacuum UltraViolet (VUV) ring which provide intense light spanning the electromagnetic
spectrum from the infrared through x-rays. Each year over 2300 scientists from universities, industries and government
labs perform research at the NSLS.
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Forming nanoscale organic films and integrating them into
semiconductor electronics and all-organic microphotonic circuits has
stimulated intense academic and industrial research, but progress is
currently hampered by the lack of device-quality functional
molecule-based thin films, driving the need for new reliable
film-growth methods.
