Adsorption behaviors of hydroxyl-terminated alkanethiol, 3-mercapto-1-propanol (MPO), and arenethiol, 4-mercaptophenol (MPH), on Au(111) electrodes were studied by cyclic voltammetry (CV) and in situ scanning tunneling microscopy (STM). The effects of the molecular structure and the chain length on the characteristics of the self-assembled monolayers (SAMs) were investigated by comparison with the results of 11-mercapto-1-undecanol (MUO) and 6-mercapto-1-hexanol (MHO) reported in the literature. All the alkanethiol SAMs have the same coverage ratio (0.33 ML). For MUO which has a longer chain length, a hexagonal lattice, the (√3 ×√3) structure was observed. However, for shorter thiols (MHO and MPO), the adsorbed molecules exhibit different contrasts under the imaging of STM, ascribed to the different conformations of adsorbed molecules. The CV results indicated that a longer chain length triggers a SAM with higher adhesion and higher resistance to the charge transfer across a SAM. For the SAM of arenethiol, MPH, the π-π stacking interaction of the phenyl ring leads to a lower surface mobility of thiol/Au complexes, lower coverage ratio, and less uniform structure of the adlayer. Furthermore, the vacancy islands commonly observed on alkanethiol-modified Au(111) electrodes do not appear on the MPH-modified surface. Instead, 2-dimensional patch islands formed on the terrace due to the aggregation of moveable MPH/Au complexes.
This journal is © The Royal Society of Chemistry 2012