pubs.acs.org/Langmuir © 2009 American Chemical Society
Structure and Growth of Vapor-Deposited n-Dotriacontane Films Studied by X-ray Reflectivity )
Valeria del Campo,† Edgardo Cisternas,† Haskell Taub,*,‡ Ignacio Vergara,† Tomas Corrales,† Pamela Soza,† Ulrich G. Volkmann,† Mengjun Bai,‡ Siao-Kwan Wang,‡ Flemming Y. Hansen,§ Haiding Mo, and Steven N. Ehrlich^ Facultad de Fı´sica, Pontificia Universidad Cat olica de Chile, Santiago 22, Chile, ‡Department of Physics and Astronomy and University of Missouri Research Reactor, University of Missouri, Columbia, Missouri 65211, § Department of Chemistry, Technical University of Denmark, IK-207 DTU, DK-2800 Lyngby, Denmark, Advanced Optowave Corporation, 1170 Lincoln Avenue, Unit# 10, Holbrook, New York 11741, and ^ National Synchrotron Light Source, Brookhaven National Laboratory, Upton, New York 11973 )
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Received May 20, 2009. Revised Manuscript Received June 12, 2009 We have used synchrotron X-ray reflectivity measurements to investigate the structure of n-dotriacontane (n-C32H66 or C32) films deposited from the vapor phase onto a SiO2-coated Si(100) surface. Our primary motivation was to determine whether the structure and growth mode of these films differ from those deposited from solution on the same substrate. The vapor-deposited films had a thickness of ∼50 A˚ thick as monitored in situ by high-resolution ellipsometry and were stable in air. Similar to the case of solution-deposited C32 films, we find that film growth in vacuum begins with a nearly complete bilayer adjacent to the SiO2 surface formed by C32 molecules aligned with their long axis parallel to the interface followed by one or more partial layers of perpendicular molecules. These molecular layers coexist with bulk particles at higher coverages. Furthermore, after thermally cycling our vapor-deposited samples at atmospheric pressure above the bulk C32 melting point, we find the structure of our films as a function of temperature to be consistent with a phase diagram inferred previously for similarly treated solution-deposited films. Our results resolve some of the discrepancies that Basu and Satija (Basu, S.; Satija, S. K. Langmuir 2007, 23, 8331) found between the structure of vapordeposited and solution-deposited films of intermediate-length alkanes at room temperature.
I. Introduction As the principal constituents of commercial lubricants, linear alkane molecules of intermediate length (n-CnH2nþ2, 15
