Self-assembled monolayers from organosulfur compounds: A comparison between sulfides, disulfides, and thiols

Film formation of decanethiol [CH3(CH2)(9)SH], didecyl disulfide [CH3(CH2)(9)S-S(CH2)(9)CH3], and didecyl sulfide [CH3(CH2)(9)-S-(CH2)(9)CH3] on polycrystalline gold from ethanol solutions was studied in situ by second harmonic generation. The completed films were characterized by X-ray photoelectron spectroscopy and voltammetry. Film formation is described by Langmuir absorption kinetics and is found to be fastest for the thiol. Referenced to the number of thiolate units the disulfide adsorbs about 40% more slowly. In sharp contrast, the rate of alkyl sulfide adsorption is smaller by more than 3 orders of magnitude. Films formed from the thiol and disulfide are indistinguishable and virtually free of contaminations. In contrast; the maximum coverage achieved with the sulfide is considerably lower and substantial amounts of contamination remain on the substrate. Differences between thiol/disulfide films and sulfide films are also indicated by their electrochemical behavior.