Assembly behavior and monolayer characteristics of OH-terminated alkanethiol on Au(111):: in situ scanning tunneling microscopy and electrochemical studies

Self-assembled monolayers (SAMs) of 6-mercapto-1-hexanol (MHO) on an Au(111) electrode were prepared in an electrochemical system. The adsorption behavior of MHO and the time-dependent organization of the SAM were investigated by in situ scanning tunneling microscopy (STM) and cyclic voltammetry (CV). The results show that a potential higher than 0.28 V (relative to RHE) is required to induce the adsorption of MHO. At 0.28 V, the MHO molecules adsorb in a flat-lying orientation, forming an ordered striped phase with a molecular arrangement of (8 x v 3). However, the adlayer is not stable at this potential. The adsorbed striped phase may recover to the herringbone feature of the gold substrate due to the desorption of adsorbed MHO. At a higher potential (0.35 V), the adlayer becomes stable and can undergo a phase evolution from the striped phase to a condensed structure, identified as c(3 x 2 root 3). This structure can also be described as a c(4 x 2) superlattice of a (root 3 x root 3) R30 degrees hexagonal adlattice. The surface coverage of the MHO SAM is identical to the saturated structure of an 11-mercapto-1-undecanol (MUO) SAM reported in a previous work, (root 3 x root 3) R30 degrees. However, the STM image of MHO adlayer shows a modulation in intensity, reflecting the presence of various conformations of adsorbed molecules. This result is attributed to the shorter chain length of MHO, which gives a weaker van der Waals interaction between adsorbed molecules. This effect also results in a higher charge permeability across the adlayer and a lower striping potential to an MHO SAM.