Atomic Resolution X-ray Standing Wave Microstructural Characterization of NLO-Active Self-Assembled Chromophoric Superlattices

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• 作者：Wenbin Lin ; Tien-Lin Lee ; Paul F. Lyman ; Jaejin Lee ; Michael J. Bedzyk ; and Tobin J. Marks
• 刊名：Journal of the American Chemical Society
• 出版年：1997
• 出版时间：March 5, 1997
• 年：1997
• 卷：119
• 期：9
• 页码：2205 - 2211
• 全文大小：311K
• 年卷期：v.119,no.9(March 5, 1997)
• ISSN：1520-5126

文摘

This contribution reports the first X-ray standing wave structuralcharacterization of self-assembled NLO-active chromophoric multilayers (SAMs). These siloxane-basedself-assembled stilbazolium multilayers are intrinsicallyacentric and exhibit very large second-order nonlinear opticalresponses. The locations of bromide ions within theSAMs were precisely determined using X-ray standing waves generated bytotal external reflection from mirrorsurfaces as well as by Bragg diffraction from layered syntheticmicrostructures. The large coherent fraction (i.e.,small Gaussian distribution width) of the Br^- ionsprovides direct evidence for the high structural regularity oftheseself-assembled multilayers along the surface normal direction.These results are supported by atomic forcemicroscopic(AFM) and X-ray photoelectron spectroscopic (XPS) studies which probethe structural regularity and chemicalcomposition of SAMs, respectively. The anion surfacecoverage has also been measured in this study (2.5(5)×10¹⁴ Br^-/cm²) and is in excellentagreement with the cation surface coverage measured bysecond harmonic generation(2(1) × 10¹⁴ molecules/cm²). Theseresults clearly demonstrate the utility of X-ray standing wave analysesas aquantitative microstructural probe for self-assembled mono- andmultilayers, especially for SAMs with incommensuratestructures.