Binary Monolayers Formed from the Sequential Adsorption of Terphenylthiol and Dodecanethiol on Gold

Binary self-assembled monolayers (SAMs) of 1-dodecanethiol (DDT) and 1,1',4',1″-terphenyl-4-thiol (TPT) were prepared via sequential deposition on the surface of Au(111) on mica. The SAMs were studied using scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), cyclic voltammetry (CV), and reductive desorption (RD). Varying the sequence of deposition and the deposition temperature of DDT resulted in four distinct binary monolayer systems. Characterization of DDT SAMs with STM exhibited a combination of ordered and disordered domains, whereas the TPT monolayers were well-ordered into various phases and aligned with extensive characterization in the literature. Binary SAMs displayed varied behaviors, including retention of the initial SAM structure, adsorption of the secondary compound at domain boundaries, or temperature-dependent replacement of the initial SAM. Elemental analysis via XPS revealed the presence of oxygen in single-component DDT monolayers, which was absent in TPT monolayers, and the binary SAMs exhibited nearly identical elemental compositions. RD results indicate well-ordered domains for DDT, less ordered and less strongly bound TPT regions, and distinct domains for binary SAMs that were consistent with the binary STM results. CVs for single-component and binary SAMs of DDT and TPT indicate an absence of significant defects in the molecular layers. The inhibited electron transfer observed aligns with prior studies for DDT and is more moderate for TPT and mixed SAMs. This study addresses the need for an experimental understanding of the phase behavior of binary SAMs.