High-resolution ellipsometric study of an n-alkane film, dotriacontane, adsorbed on a SiO2 surface

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High-resolution ellipsometric study of an n-alkane film, dotriacontane, adsorbed on a SiO2 surface

Please use this identifier to cite or link to this item: http://hdl.handle.net/10355/8763

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Title: High-resolution ellipsometric study of an n-alkane film, dotriacontane, adsorbed on a SiO2 surface
Author: Volkmann, Ulrich G.; Pino, M.; Altamirano, L. A.; Taub, Haskell; Hansen, Flemming Y.
Date: 2002-02-01
Publisher: American Institute of Physics
Citation: J. Chem. Phys. 116, 2107 (2002)
Abstract: Using high-resolution ellipsometry and stray light intensity measurements, we have investigated during successive heating-cooling cycles the optical thickness and surface roughness of thin dotriacontane (n-C32H66) films adsorbed from a heptane (n-C7H16) solution onto SiO2-coated Si(100) single-crystal substrates. Our results suggest a model of a solid dotriacontane film that has a phase closest to the SiO2 surface in which the long-axis of the molecules is oriented parallel to the interface. Above this “parallel film” phase, a solid monolayer adsorbs in which the molecules are oriented perpendicular to the interface. At still higher coverages and at temperatures below the bulk melting point at Tb = 341 K, solid bulk particles coexist on top of the “perpendicular film.” For higher temperatures in the range Tb<T<Ts where Ts = 345 K is the wetting temperature of the bulk phase, the coexisting bulk particles melt into droplets; and for T>Ts, a uniformly thick fluid film wets to the parallel film phase. This structure of the alkane/SiO2 interfacial region differs qualitatively from that which occurs in the surface freezing effect at the bulk alkane fluid/vapor interface. In that case, there is again a perpendicular film phase adjacent to the air interface but no parallel film phase intervenes between it and the bulk alkane fluid. Similarities and differences between our model of the alkane/SiO2 interface and one proposed recently will be discussed. Our ellipsometric measurements also show evidence of a crystalline-to-plastic transition in the perpendicular film phase similar to that occurring in the solid bulk particles present at higher coverages. In addition, we have performed high-resolution ellipsometry and stray-light measurements on dotriacontane films deposited from solution onto highly oriented pyrolytic graphite substrates. After film deposition, these substrates proved to be less stable in air than SiO2.
URI: http://hdl.handle.net/10355/8763
ISSN: 0021-9606

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