Comparison of the Structures and Wetting Properties of Self - Assembled Monolayers of n - Alkanethiols on the Coinage Metal Surfaces , Cu , Ag , Aul

Introduction This paper describes the preparation and characterization of self-assembled monolayers (SAMs) formed by the chemisorption of alkanethiols (HS(CHz),R) on surfaces of copper and silver. Both of these metal surfaces are highly active in the chemisorption of organosulfur compounds.H They offer an excellent opportunitl to study the influences of the substrate in def rning and controlling the order of self-assembled monolayer films. A central objective of this study was to compare the structures of the monolayers formed on silver and copper with those formed by chemisorption of alkanethiols on Bold.r-rr The molecular orientation and ordering of long-chain alkyl thiolate monolayers on gold reflects the epitaxial bonding of the sulfur atoms (or, perhaps more precisely, of gold thiolates) to the surface lattice plane. For long-chain alkyl disulfides and thiols chemisorbed on Au( I l I ) (the preferred crystallographic texture of typical polycrystalline samples), electron diffraction (TEM and LEED)r0 has established that the sulfur atoms in the adsorbates form (V3 x y'3)R30' werlaycn. This epitaxialoverlayer results in an interchain spacing of 4.99 A and is well suited to the formation of close-packed, and therefore ordered, organic surface phases when the alkyl chain is long (n > 15) and the R group roughly commensurate in size with the polymethylene chain.ll'12 Several recent studies discuss these issues.?-13 Copper, silver, and gold have nearest neighbor spacings in the ( l l l ) p lane o f 2 .56 A, 2 .89 A, and 2 .88 A, respec t ive ly . ra I f structures form on copper and silver that are analogous to those on gold (i.e., (y'3 x /3)R30o), we expect the molecular orien-