Influence of Surface Morphology and Substrate on Thermal Stability and Desorption Behavior of Octanethiol Self-Assembled Monolayers: Cu, Ag, and Au

The formation and thermal desorption behaviors of octanethiol (OT) SAMs on single crystalline Au(111) and polycrystalline Au, Ag, and Cu substrates Were examined by X-ray photoelectron microscopy (XPS), thermal desorption spectroscopy (TDS), and contact angle (CA) measurements.,XPS and CA measurements revealed that the adsorption of OT Molecules on these metals led to the formation of chemisorbed self-assembled monolayers (SAMs). Three main desorption fragments for dioctyl disulfide (C8SSC8(+), dimer), octanethiolate (C8S(+)), and octanethiol (C8SH(+)) were monitored using TDS to understand the effects of surface morphology and the nature of metal substrates on the thermal desorption behavior of alkanethiols. TDS measurements showed that a sharp dimer peak with a very strong intensity on single crystalline Au(111) surface was dominantly observed at 370 K, whereas a broad peak on the polycrystalline Au surface was observed at 405 K. On the other hand, desorption behaviors of octanethiolates and octanethiols were quite similar. We concluded that substrate morphology strongly affects the dimerization process of alkanethiolates on Au surfaces. We also found that desorption intensity of the dimer is in the order of Au >> Ag > Cu, suggesting that the dimerization process occurs efficiently when the sulfur-metal bond has a more covalent character (Au) rather than an ionic character (Ag and Cu). The relative desorption intensity of the octanethiolates to the octanethiols follows the order of bond strength, Cu > Au > Ag. Alkanethiolates may be a dominant desorption product as metal-sulfur bonds become stronger. In this study, we clearly demonstrate that the thermal stability and desorption behaviors of alkanethiol SAMs are strongly influenced by the surface morphologies of metal substrates, bonding character of the sulfurs, the bond strength of metal-sulfur, and van der Waals interactions. Our results provide new insights into understanding the thermal stability and desorption behaviors of alkanethiols on Au, Ag, and Cu surfaces.