Comparison of the Structures and Wetting Properties of Self-Assembled Monolayers of Normal-Alkanethiols on the Coinage Metal-Surfaces, Cu, Ag, Au.


Paul E. Laibinis, George M. Whitesides, David L. Allara, Yu-Tai Tao, Atul N. Parikh, Ralph G. Nuzzo, Journal of the American Chemical Society 113, 7152-7167 (1991)

Long-chain alkanethiols, HS(CH2),CH3, adsorb from solution onto the surfaces of gold, silver, and copper and form monolayers. Reflection infrared spectroscopy indicates that monolayers on silver and on copper (when carefully prepared) have the chains in well-defined molecular orientations and in crystalline-like phase states, as has been observed on gold. Monolayers on silver are structurally related to those formed by adsorption on gold, but different in details of orientation. The monolayers formed on copper are structurally more complex and show a pronounced sensitivity to the details of the sample preparation. Quantitative analysis of the IR data using numerical simulations based on an average single chain model suggests that the alkyl chains in monolayers on silver are all-trans zig-zag and canted by - 12 degree from the normal to the surface. The analysis also suggests a twist of the plane containing the carbon backbone of -45 degree from the plane defined by the tilt and surface normal vectors. For comparison, the monolayers that form on adsorption of alkanethiols on gold surfaces, as judged by their vibrational spectra, are also trans zig-zag extended but, when interpreted in the context of the same single chain model, have a cant angle of - 2 7 O and a twist of the plane of the carbon backbone of -53 degree. The monolayers formed on copper (when they are obtained in high quality) exhibit infrared spectra effectively indistinguishable from those on silver and thus appear to have the same structure. Films on copper are also commonly obtained that are structurally ill-defined and appear to contain significant densities of gauche conformations. These spectroscopically based interpretations are compatible with inferences from wetting and XPS measurements. The structure of the substrate-sulfur interface appears to control molecular orientations of the alkyl groups in these films. An improved structural model, incorporating a two-chain unit cell and allowing for the temperature-dependent population of gauche conformations, is presented and applied to the specific case of the structures formed on gold.

DOI: 10.1021/ja00019a011