Molecular-Scale Investigation of Structural Changes in Cyclohexanethiol Self-Assembled Monolayers on Au(111)

Two-dimensional ordered structures of cyclohexanethiol (CHT) self-assembled monolayers (SAMs) on Au(111) formed from different solution concentrations were investigated using scanning tunneling microscopy (STM). Molecular-scale STM imaging clearly revealed the 2D phase transitions of CHT SAMs from a (2 x 4 root 2)R20 degrees structure to a (5 x 2 root 3)R35 degrees structure via a intermediate phase of a (5 x root 15)R25 degrees structure depending on solution concentration used for SAM fabrication. The (5 x 2 root 3)R35 degrees phase was observed for high concentrations, after deposition at low temperature or after long-time immersion. Based on these results, we confirmed that the molecular packing arrangement can be regarded as a fully covered SAM structure. In addition, 2D SAM formation of CHT SAMs is markedly different from that of alkanethiol SAMs, which is a result of structural dynamics of the CHT ring in the process of molecular self-assembly. The molecular-scale STM results for CHT SAMs on Au(111) can provide very useful information on understanding structural behaviors of cyclic thiol SAMs.