Profile Structures of Macromolecular Monolayers on Solid Substrates By X-Ray Interferometry/Holography
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PROFILE STRUCTURES OF MACROMOLECULAR MONOLAYERS ON SOLID SUBSTRATES BY X-RAY INTERFEROMETRY/HOLOGRAPHY J. P. C.
KENT BLASIE, SONTAO XU, MARGARET MURPHY, JANINE CHUPA, JOHN MCCAULEY, AMOS B. SMITH III*; LAWRENCE J. PETACOLAS AND JOHN BEAN** *Department of Chemistry and Laboratory for Research on the Structure of Matter, University of Pennsylvania, Philadelphia, PA 19104 **AT & T Bell Laboratories, Murray Hill, NJ 07974 ABSTRACT Previous work has recently employed x-ray interferometry for the unique determination of the profile structures of ultrathin Langmuir-Blodgett multilayer films of Cd-arachidate and of tethered protein monolayers on the surface of Ge/Si multilayer substrates [1,2]. These studies utilized the inorganic substrate as the reference structure for the interferometric phasing of the meridional x-ray diffraction I (Qxy=0A- 1 ,Qz) from the inorganic-organic composite structure. The substrates, fabricated by magnetron sputtering, contained only broad profile features (Ž20 A), thereby limiting the spatial resolution of the organic profile structures sodetermined. Molecular beam epitaxy (MBE) permits the fabrication of the multilayer reference structure with profile features as narrow as a single atomic monolayer, thereby providing delta-function-like features in the reference structure. The reference structure can then be tailored such that the autocorrelation function of the inorganic-organic composite profile structure [obtained by a Fourier transform of its meridional diffraction I(Qxy=0A-iQz) data without phase information] contains only the organic profile structure itself over a particular range of the profile coordinate z. This approach for uniquely determining the unknown profile structure of the organic overlayer is x-ray holography by analogy to simple off-axis holography with much longer wavelength radiation. We have initially utilized MBE fabricated Ge/Si multilayer substrates of the type N(Ge 2 Si 30 ), e.g., for N=2 or 3 superlattice unit cells, each containing two Ge monolayers and thirty Si monolayers, to thereby determine the profile structures of four different organic overlayers, namely a) selfassembled alkylsiloxane monolayers, b) Langmuir-Blodgett cadmium alkylcarboxylate monolayers, c) a Langmuir-Blodgett Cdarachidate head-to-head bilayer deposited on a)-above and, d) a covalently tethered protein monolayer. INTRODUCTION The profile and in-plane structures of a single macromolecular monolayer, either chemisorbed or physisorbed on the surface of a solid substrate, together provide substantial information concerning the intramolecular configurations and intermolecular correlations present in the monolayer (3]. Elastic x-ray scattering, for photon momentum transfer along Q, perpendicular to the substrate surface (at Qxy=0A- 1 ) and along Qxy parallel to the substrate surface (at Q,=0A-1), can be used, in principle, to determine these profile and in-plane Mat. Res. Soc. Symp. Proc. Vol. 237. 01992 Materials Research Society
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projections [4] of the monolayer structure, respectivel
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