Time-dependent phase transition of self-assembled monolayers formed by thioacetyl-terminated tolanes on Au(111)

The binding conditions and time-dependent phase transition of self-assembled monolayers (SAMs) formed by thioacetyl-terminated tolane molecules on Au(111) were examined by X-ray photoelectron spectroscopy (XPS) and scanning tunneling microscopy (STM). XPS measurements revealed that two S 2p(3/2) XPS peaks of the tolanethioacetate (TTA) SAMs on Au(111) were observed at 162 eV (bound sulfur) and 163.4 eV (unbound sulfur). On the other hand, an additional strong S 2p(3/2) XPS peak for the SAMs of tolane methylthioacetate (TMTA) that contained a methylene spacer between the tolane group and the sulfur atom was observed at 161.2 eV (bound sulfur). This suggested that the adsorption conditions of the tolane SAMs were strongly influenced by the difference in the chemical structure of the tolanes. It was found that TTA and TMTA molecules did not form ordered SAMs after the immersion of the Au(111) substrate in a 0.5 mM N,N'-dimethylformamide solution for 1 day. However, we observed a time-dependent phase transition of the TTA and TMTA SAMs from the disordered phase to the ordered phase as a result of a long-term structural rearrangement after incubation at room temperature for 10 days. These results will be very useful in understanding the formation and binding conditions of thioacetyl-terminated SAMs on Au(111) and in controlling the two-dimensional SAM structure.