Surface-conditioning effect of gold substrates on octadecanethiol self-assembled monolayer growth

The surface-conditioning effect of gold substrates on n-octadecanethiol (ODT) self-assembled monolayer (SAM) growth was studied by X-ray photoelectron spectroscopy (XPS). The gold substrates were immersed in an ethanol solution containing 1 mM ODT for 1 h. The C/Au ratios of the ODT SAMs estimated by XPS formed on gold substrates depended on the amount of contamination on the gold surface. A full-coverage ODT SAM was successfully obtained on the gold substrate after ultraviolet light (UV)/ozone treatment for 1 h and rinsing with pure ethanol for 1 h. XPS measurements indicated that low contamination levels and a sulfur-free gold surface were obtained using this pretreatment and, hence, the adsorption of ODT molecules on gold proceeded efficiently. On the other hand, the C/Au ratio of an ODT SAM on a contaminated gold substrate was less following immersion in an ODT solution for 1 h, because surface contamination may reduce the adsorption rate of ODT molecules on the gold surface. When the gold substrate was rinsed with a chloroform solution for 1 h before SAM formation, the lowest C/Au ratio was obtained. XPS results showed that sulfur atoms quickly adsorbed onto the gold surface after chloroform rinsing and prevented the adsorption of ODT molecules onto the gold surface.