Nanoscale Chemical Imaging of Coadsorbed Thiolate Self-assembled Monolayers on Au(111) by Tip-Enhanced Raman Spectroscopy

Self-assembled monolayers (SAMs) of thiolates on metal surfaces are of key importance for engineering surfaces with tunable properties. However, it remains challenging to understand binary thiolate SAMs on metals at the nanoscale under ambient conditions. Here we employ tip-enhanced Raman spectroscopy (TERS) and density functional theory (DFT) calculations to investigate local information of binary SAMs on Au(111) coadsorbed from an equimolar mixture of p-cyanobenzenethiol (pCTP) and p-aminothiophenol (pATP), including chemical composition, coadsorption behavior, phase segregation, plasmon-induced photocatalysis, and solvation effects. We found that upon competitive adsorption of pCTP and pATP on Au(111) from a methanolic solution, the coadsorption initially occurs randomly and homogeneously; eventually, pATP is replaced by pCTP through gradual growth of pCTP nanodomains. TERS imaging also allows for visualization of the plasmon-induced coupling of pATP to p,p’-dimercaptoazobenzene (DMAB) and the solvation-induced phase segregation of the binary SAMs into nanodomains, with a spatial resolution of ~9 nm under ambient conditions. According to DFT calculations, these aromatic thiolates differing only in their functional groups, -CN versus –NH2, show different adsorption energy on Au(111) in vacuum and methanol, and thus the solvation effect on adsorption energy of these thiolates in methanol can determine the dispersion state and replacement order of the binary thiolates on Au(111).