• Taub, H.
• Diama, A.
• Herwig, K. W.
• Hansen, F. Y.
• Criswell, L.

Abstract

The structure of monolayer tetracosane (n-C_24H_50) is of interest as a first step toward investigating the interfacial dynamics of intermediate-length alkanes. We report neutron diffraction experiments on tetracosane adsorbed on the (0001) surfaces of an exfoliated graphite substrate as a function of temperature which reveal three different monolayer phases. Below ~210 K, the tetracosane monolayer forms a rectangular-centered partially-commensurate structure with the lattice constants a = 65.0 Åand b = 4.26 ÅAt ~210 K, the monolayer undergoes a transition from the crystalline to a smectic phase in which the lattice constant b increases to 4.53 ÅA second transition in the monolayer occurs at ~340 K from a smectic to an isotropic fluid phase.footnote F. Y. Hansen, K. W. Herwig, B. Matthies, and H. Taub, Phys. Rev. Lett. 83, 2362 (1999). The crystalline phase has been observed previously in STM studies;footnote G. C. McGonigal, R. H. Bernhardt, and D. J. Thomson, Appl. Phys. Lett. 57, 28 (1990). however, the smectic and fluid phases have not been observed by other techniques. These experimentally inferred structures provide a check on molecular dynamics simulations modeling the molecular diffusive motions within the monolayers. <P />...

Topics

• impedance spectroscopy
• surface
• experiment
• simulation
• crystalline phase
• molecular dynamics
• neutron diffraction
• isotropic
• alkane
• scanning tunneling microscopy