Chain Length Dependence of the Striped Phases of Alkanethiol Monolayers Self-Assembled on Au(111): An Atomic Beam Diffraction Study

Abstract

Low-energy helium atom diffraction measurements of the surface structure of n-alkanethiol films deposited from a molecular beam on to the (111) face of gold single crystals (at an impingement rate on the order of 1011±1 molecules cm-2 s-1) show that the thiols form “striped” overlayers. These structures are similar to those previously seen by Dubois et al. in a recent low energy electron diffraction (LEED) study of vapor-deposited overlayers (J. Chem. Phys. 1993, 98, 678), by Poirier et al. in a scanning tunneling microscope (STM) study of low coverage solution-grown short-chain thiols monolayers (Langmuir 1994, 10, 3383), and more recently by us, Poirier, and Tarlov by thermal treatment of the full-coverage c(4√3×2√3)R30° phase formed in the standard way by self-assembly from solution (J. Chem. Phys. 1994, 101, 11031). The surface periodicity of the monolayer structures observed in the present study can be described (with respect to the Au(111) surface lattice) in terms of a rectangular p×√3 unit mesh where p, the periodicity of the stripes, scales linearly with the length of the adsorbed thiol. The absolute value of the stripes' period is, with a maximum deviation of 3%, 1.9 times the length of the corresponding fully stretched thiolate fragment which coincides with the length of the corresponding fully stretched dialkyl disulfide. The present results, analyzed in the context of the others, confirm the presence of coverage-dependent and chain-length-dependent phase behavior in these systems and suggest that, at the lowest “full” coverages, the molecules may assume a near-flat configuration on the gold substrate.