High-resolution ellipsometric study of an n-alkane film, dotriacontane, adsorbed on a SiO_2 surface

摘要

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-C_32H_66) films adsorbed from a heptane (n-C_7H_16) solution onto SiO_2-coated Si(100) single-crystal substrates. Our results suggest a model of a solid dotriacontane film that has a phase closest to the SiO_2 surface in which trhe 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 e interface. At still higher coverages and at temperatures below the bulk melting point at T_b=341 K, solid bulk particles coexist on top of the "perpendicular films." For higher temperature in the range T_bT_s, a uniformly thick fluid film wets to the parallel film phase. This structure of the alkane/SiO_2 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/SiO_2 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 SiO_2.