Surface adsorption structures in 1-octanethiol self-assembled on Cu(111)

Abstract

A normal incidence X-ray standing wave (NIXSW) and near-edge X-ray absorption fine structure (NEXAFS) study was performed on saturated monolayers of 1-octanethiol on Cu(111). NEXAFS around the S K-edge taken in normal and grazing incidence revealed that the S-C bond was oriented with respect to the Cu(111) surface to give a calculated orientation of the pendant hydrocarbon which was almost normal to the surface, similar to self-assembled alkanethiols on gold surfaces. Modulation in the S(1s) photoemission with variation in the energy of the X-ray standing wave about the (111) and (1¯11) Bragg energies was modelled to yield values of structural parameters: the coherent position, d hkl , and coherent fraction, f hkl , for the adsorbate with respect to the bulk Cu lattice planes. For the (111) reflection, d 111 = 1.19 ± 0.10A˚, f 111 = 0.70 ± 0.03, but for the (1¯11) reflection, f 1¯11 was close to zero ( f 111 = 0.10 ± 0.05) and d 1¯11 was indeterminate. The coherent fraction for the Cu substrate was high for both reflections studied ( f 111 = 0.95 ± 0.03; 1¯11 = 0.92 ± 0.02). The structural parameters are rationalised in terms of penetration of the Cu by thiolate sulfur atoms to cause a surface layer reconstruction. The resultant adsorbate structure is either incommensurate or else consists of a large-mesh commensurate superlattice in which the S atoms exist at a number of different positions with respect to the (1¯11) planes of the bulk.