Structural analysis of saturated alkanethiolate monolayers on Cu(100): Coexistence of thiolate and sulfide species

The adsorption of alkanethiols [CH3(CH2)(n-1)SH] of various chain lengths ( n = 1, 2, 4, 7, 10) on a Cu(100) surface has been studied by means of low-e nergy electron diffraction, He atom scattering, thermal desorption spectros copy, and X-ray photoelectron spectroscopy. Highly ordered thiolate films w ere prepared by vapor-phase deposition which formed, at room temperature, a n intermediate (2 x 2) phase and, upon further exposure, a c(6 x 2) saturat ion structure. An extended XPS analysis unveiled that, in contrast to the i ntermediate phase, the saturation phase is metastable and decomposes upon p artial desorption of alkyl chains into a previously unobserved thermodynami cally stable mixture of thiolate and sulfide species under retention of its c(6 x 2) structure. As a consequence, the density of the alkyl chains is s ignificantly reduced, demonstrating that such alkanethiolate monolayers on copper are not as densely packed as in the case of gold.