Surface damage characteristics of self-assembled monolayers of alkanethiols on metal surfaces

The effect of metal oxide layer on the nano-tribological characteristics of single chain alkanethiol CH₃(CH₂)_nSH) self-assembled monolayers (SAMs) on various metal surfaces (gold, silver, copper) was investigated. In order to correlate the surface structures with the tribological characteristics, various surface analysis techniques such as Scanning Probe Microscopy, X-ray Photoelectron Spectroscopy, Secondary Ion Mass Spectrometry and Spectroscopic Ellipsometry were used. Results of surface analyses showed that thiols on a metal surface were susceptible to forming multilayers if the metal surface was oxidized before the thiol assembly process. From the friction and wear tests conducted using an Atomic Force Microscope and a Lateral Force Microscope, it was found that thiols on copper oxide surface could be easily removed even under a few nano-Newton normal load. On the other hand, thiols on gold and copper fresh surfaces (the surface which was made by minimizing oxide formation) could endure up to micro-Newton level loads. Based on these findings, it could be concluded that the nano-tribological characteristics of alkanethiol SAMs on various metal surfaces were largely dependent on the oxide layer that already formed on the metal surface before the thiol adsorption process.