Spontaneously organized molecular assembly of an aromatic organic disulfide on silver/platinum alloy surfaces: an angle dependent X-ray photoemission investigation

Abstract

This article reports an angle dependent X-ray photoemission investigation on the formation of a self-assembled monolayer of a small organic disulfide molecule (Naphtho[1,8-cd]-1,2-dithiole) on silver/platinum alloy surfaces. In addition to the S 2p signal normally associated with thiolate bond formation, a high binding energy S 2p signal was observed and is assigned to a sulfonic acid moiety. This is the first such report of sulfonic acid formation in disulfide systems and is quite similar to the process occurring on copper and silver surfaces for n-alkane thiols. The angle dependent intensities of the C 1s, S 2p and Ag 3d core levels were used to make a quantitative estimate of the surface composition and thus obtain information on the molecular orientation. The molecule was oriented almost normal to the silver/platinum alloy surface with the sulfonic acid moiety being closer to the monolayer-air interface than the thiolate sulfur which was very close to the alloy surface. This is explained as arising due to steric effects associated with the larger sulfonic acid group.