Probing Phase Evolutions of Au-Methyl-Propyl-Thiolate Self-Assembled Monolayers on Au(111) at the Molecular Level

A self-assembled monolayer (SAM) consisting of a mixture of CH₃S-Au-SCH₃, CH₃S-Au-S(CH₂)₂CH₃, and CH₃(CH₂)₂S-Au-S(CH₂)₂CH₃ was studied systematically using scanning tunneling microscopy and density functional calculations. We find that the SAM is subjected to frequent changes at the molecular level on the time scale of ∼minutes. The presence of CH₃S or CH₃S-Au as a dissociation product of CH₃S-Au-SCH₃ plays a key role in the dynamical behavior of the mixed SAM. Slow phase separation takes place at room temperature over hours to days, leading to the formation of methyl-thiolate-rich and propyl-thiolate-rich phases. Our results provide new insights into the chemistry of the thiolate-Au interface, especially for ligand exchange reaction in the RS-Au-SR staple motif.