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

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 Is, 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. (C) 1997 Elsevier Science B.V.