The topography and tribology of pure and mixed alkanethiol (CH₃(CH₂)_n-1SH) monolayers self-assembled on Au (111) are studied experimentally with scanning tunneling microscope (STM) and atomic force microscope/friction force microscope (AFM/FFM) respectively. From STM images, the two thiol molecules in the mixture at the ratio of 1:1 is found to distribute in domains. The domain size is proportional to the chain length difference. For the thiol mixture at the ratio of 1:9, a pure-thiol-monolayer-like phase is observed over large areas and domains with stripes are observed occasionally....[ Read more ]

The topography and tribology of pure and mixed alkanethiol (CH₃(CH₂)_n-1SH) monolayers self-assembled on Au (111) are studied experimentally with scanning tunneling microscope (STM) and atomic force microscope/friction force microscope (AFM/FFM) respectively. From STM images, the two thiol molecules in the mixture at the ratio of 1:1 is found to distribute in domains. The domain size is proportional to the chain length difference. For the thiol mixture at the ratio of 1:9, a pure-thiol-monolayer-like phase is observed over large areas and domains with stripes are observed occasionally.

In the friction measurement with AFM, some technical difficulties, such as tip modification, damage of the monolayer and humidity and temperature dependence have been encountered. These difficulties can be solved under some given experimental conditions for long chain thiols molecules.

The friction measurement with AFM is thus carried out on the long chain mixed alkanethiols self-assembled monolayers (SAMs). For the pure thiol SAM, there is no obvious chain length dependence for the friction but the adhesion increases with the increase of the chain length. For the mixed SAMs at the ratio of 1: 1, the chain length difference dependence exists for both the friction and the adhesion. For the mixed SAM at the different ratios, a ratio dependence is observed. These tribological properties support the gauche defects density dependence, i.e. the gauche defect works as the lubricant in the friction and lead to a reduced friction for alkanethiol self-assembled monolayers.