Homogeneous Dispersion of Aromatic Thiolates in the Binary Self-Assembled Monolayer on Au(111) via Displacement Revealed by Tip-Enhanced Raman Spectroscopy

Self-assembled monolayers (SAMs) of aromatic thiols on metal surfaces are of interest as covering functional layers. Unlike alkanethiols, multicomponent SAMs of aromatic thiols have not been comprehensively studied, especially regarding formation processes and the surface distribution of the components. In this work, we investigated the surface structures and chemical states of binary SAMs on Au(111) made from simple aromatic thiols, benzenethiol (BT), and benzenemethanethiol (BMT), via a displacement method. We utilized tip-enhanced Raman spectroscopy (TERS) based on scanning tunneling microscopy (STM) under ambient conditions for probing local surface information regarding vibrational fingerprints as well as morphologies. STM visualized morphological changes upon adsorption of BMT and desorption of BT as adatom islands were destructed and vacancy islands emerged on the underlying Au(111). The mixture states of two thiolates in the SAM were investigated using TERS, which demonstrates the facile displacement of BT by BMT and their homogeneous dispersion in the binary SAMs. Thus, the complementary analytical results using TERS and STM analyses provided fundamental information on the binary aromatic thiolate SAMs on Au(111).