Thermal-Processing-Induced Structural Dynamics of Thiol Self-Assembly in Solution

Abstract

Using ultrahigh vacuum scanning tunneling microscopy (UHV-STM), we studied the thermal-processing-induced structural changes that occur during formation of a 1-octanethiol (OT) self-assembled monolayer (SAM) on Au(111) near melting temperature in an OT solution. A favorable ordered phase of a c(4√3 × 2√3) structure was achieved at high temperature (e.g., 343 K) below the melting temperature of (e.g., 353 K) alkanethiol SAMs, while a favorably ordered phase of a (√3 × √3) structure was achieved at the melting temperature. The high resolution STM observation indicated the following: (1) the growth process of OT SAMs in an OT solution induced a change in the structural phase via diffusion of OT molecules on gold by thermal energy at a high temperature below the melting temperature; (2) application of the melting temperature resulted in partial desorption of OT molecules from the surface due to melting of OT SAMs, showing a striped phase and a disordered phase; and (3) after melting of the SAM, time-dependent rearrangement of OT molecules adsorbed on Au(111) occurred through relaxation of surface Au atoms, which caused thiolate–gold complexes to diffuse and reform a (√3 × √3) phase. Furthermore, the ultimate structural transition to a c(4√3 × 2√3) structure at 343 K revealed the translational transition of molecular adsorption sites induced by lateral movements of OT–Au complexes at a high temperature in a solution. Also, the absence of domain boundaries among the three mobile phases (i.e., disordered, striped, and ordered (√3 × √3)) at 353 K revealed that there is no transition of molecular adsorption sites after melting of the SAM.