Self-Assembled n-Alkanethiolate Monolayers on Underpotentially Deposited Adlayers of Silver and Copper on Gold G. Kane Jennings and Paul E. Laibinis* Contribution from the Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 ReceiVed August 23, 1996 X Abstract: This paper describes the use of gold films that contain underpotentially deposited (upd) metal layers of copper or silver as substrates in the generation of self-assembled monolayers (SAMs). The assembly of alkanethiols to form SAMs is compatible with the presence of the upd layer and forms a system that contains an interlayer of the upd metal that is between the gold substrate and the adsorbed organic monolayer. The assembly on these substrates can accommodate both polar and nonpolar tail groups, and the resulting SAMs span the range of wettabilities (θ a - (H 2 O) )<15° to 113°). The SAMs on the upd substrates have highly organized structures that are distinct from those that form on the parent bulk metal surfaces. In addition, the upd metal has a more noble redox potential than the corresponding bulk metal and allows an expanded potential window in cyclic voltammetry. For example, ferrocene- terminated alkanethiolssdespite having redox potentials that are positive of bulk silverscan be assembled onto silver upd substrates and form stable electroactive SAMs. The presence of the upd layer improves the stability of alkanethiolate monolayers against both desorption at elevated temperatures and molecular exchange within thiol- containing solutions. Introduction Advances in materials chemistry rely on the ability to tailor the structure and composition within the bulk and at interfaces at the nanoscopic level. 1 Indeed, the ability to generate systems with higher levels of organization and structural complexity is a focus of many areas of current research. Supramolecular assemblies, 2 self-assembled structures, 3 heteroelement compos- ites, 4 and bimetallic catalysts 5 are examples where molecular (or atomic-scale) engineering has produced species with novel architectures, synthetic flexibility, and tailored properties. At metal surfaces, two strategiessthe formation of self- assembled monolayers (SAMs) 6 and the process of underpo- tential deposition (upd) 7 shave provided useful means for functionalizing the surface and tailoring its properties. These methods functionalize the surface with a highly organized single layer of material, where the composition of the layer is readily controlled. The difference between these methods is that the self-assembly method produces a thin organic layer on the metal surface and the upd procedure coats the metal surface with a one-atom-thick layer of a dissimilar metal. Self-assembled monolayers (SAMs) form by the spontaneous adsorption of organic molecules onto a metal or metal oxide surface. 6 Various systems are presently available, with the assembly of n-alkanethiols onto copper, 8,9 silver, 8-12 and (particularly) gold 6,8,13 being the most investigated. On these metals, the thiols form a densely packed, oriented monolayer and the hydrocarbon chains pack in a trans-zigzag-extended structure. 8,10 The assemblies are the product of strong metal- sulfur interactions that are also responsible for the robust nature of the SAM in liquid and vacuum environments. A notable feature of these assemblies is their ability to accommodate a wide range of polar and nonpolar functionalities in the tail group of the adsorbate. 9,13 This flexibility in their synthesis has allowed the formation of tailored organic surfaces for studies of wetting, 9,12,14 adhesion, 15 biocompatibility, 16 friction, 17 and interfacial electron transfer. 18 X Abstract published in AdVance ACS Abstracts, May 1, 1997. (1) (a) Bard, A. J. Integrated Chemical Systems: A Chemical Approach to Nanotechnology; Wiley: New York, 1994. (b) Ball, P. Designing the Molecular World: Chemistry at the Frontier; Princeton University Press: Princeton, 1994. (2) (a) Vo ¨gtle, F. Supramolecular Chemistry; Wiley: Chichester, 1991. (b) Dietrich, B.; Viout, P.; Lehn, J.-M. Macrocyclic Chemistry; VCH: Weinheim, 1992. (c) Lehn, J.-M. Supramolecular Chemistry: Concepts and PerspectiVes; VCH: Weinheim, 1995. (3) (a) Whitesides, G. M.; Mathias, J. P.; Seto, C. T. Science 1991, 254, 1312-1319. 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