ADSORPTION OF DOCOSANETHIOL FROM SOLUTION ON POLYCRYSTALLINE SILVER SURFACES - AN XPS AND NEXAFS STUDY

Adsorption and molecular ordering of docosanethiolate (CH3(CH2)(21)SH) self-assembled monolayers prepared by immersing polycrystalline silve r substates in a 3 mu mol ethanol solution were investigated as a func tion of immersion time by X-ray Photoelectron Spectroscopy (XPS) and N ear Edge X-ray Absorption Fine Structure Spectroscopy (NEXAFS). The ch anges in the adsorbate and substrate core level spectra show that init ial physisorption of docosanethiol on the air oxidized surface is foll owed by reduction of the surface oxide by the thiol, resulting in chem isorbed silver alkanethiolates. The NEXAFS data reveal, contrary to pr eviously reported observations for the adsorption of docosanethiol on gold surfaces, that on Ag substrates even after the shortest immersion times a preferential orientation of the hydrocarbon backbones exists in the film. These results indicate a domain growth adsorption mechani sm, where alkanethiolate molecules nucleate around the initial alkanet hiolate islands. A high oxygen precoverage promotes alkanethiolate fil m formation. A two-step adsorption mechanism explains the experimental results and, using simple thermodynamic arguments, the catalytic effe ct of surface oxygen on film formation. (C) 1998 Elsevier Science B.V. All rights reserved.