Nanoscopic evidence for dissociative adsorption of asymmetric disulfide self-assembled monolayers on Au(111)

We have monitored the adsorption process of 11-hydroxyundecyl octadecyl disulfide (CH3(CH2)(17)SS(CH2)(11)OH, HUOD) self-assembled monolayers (SAMs) on the Au(lll) surface during the initial SAM growth stage using scanning tunneling microscopy (STM). STM imaging clearly exhibits two types of phase-separated domains having different corrugation periodicities which are consistent with the lengths of CH3(CH2)(17)S and HO(CH2)(11)S molecules, respectively. This is the first direct observation of the dissociative adsorption of disulfides on the nanometer scale. The self-assembly process of HUOD molecules physisorbed on graphite, on the other hand, is mainly governed by a hydrogen bond with a hydroxyl group facing another hydroxyl group of adjacent molecules without any S - S bond cleavage of the disulfide group unlike the adsorption process of HUOD molecules on gold. We have demonstrated a new and simple insightful method of comparing monolayers chemisorbed on Au(lll) with those physisorbed on graphite.