An XPS study of heterocyclic thiol self-assembly on Au(111)

The adsorption of the heterocyclic thiol 2-mercaptobenzimidazole (2MBI) self-assembled on Au(lll) from the liquid phase has been studied using high resolution X-ray photoelectron spectroscopy (XPS). On the basis of chemical shifts of the nitrogen (1s) and sulphur (2p) electron binding energies for liquid assembled layers, it is concluded that the molecule bonds to Au in the thiol form. A secondary co-ordination with the surface via the unprotonated nitrogen heteroatom is also possible. Angular dependence of core level intensities and a packing density/layer thickness estimation is consistent with a single monolayer of 2MBI adopting a 'flat-lying' orientation, although slight inclination of the plane of the molecule cannot be ruled out. Liquid phase self-assembly resulted in films which also contained oxygen and excess carbon. Direct in situ sublimation of 2MBI onto Au(lll) yielded oxygen-free layers, however, partial decomposition of the molecule occurs resulting in the formation of atomic sulphur. In the case of ex situ sublimed films, atomic and higher oxidation state sulphur are observed. (C) 1998 Elsevier Science B.V. All rights reserved.