Tribological properties of self-assembled monolayers of fluorocarbon and hydrocarbon thiols and disulfides on Au(111) studied by scanning force microscopy

The tribological properties of highly ordered self-assembled monolayers (SAMs) of fluorocarbon and hydrocarbon thiols and disulfides formed on triangular Au(111) terraces were studied by scanning force microscopy (SFM). Molecular resolution SFM images proved the high degree of lateral order in the films. The magnitude of friction between Si3N4 tips and the monolayers measured by SFM was related to the order of the alkane chains. The disorder of the hydrocarbon chains in SAMs of mixed hydrocarbon–fluorocarbon disulfides, or short chain hydrocarbon disulfides, was determined by Fourier transfrom infrared (FTIR) spectroscopy. Conformational and structural disorder was found to correlate with a significant increase in the magnitude of friction. The correlation between disorder (transition from `solid-likeʹ to `amorphous-likeʹ monolayers) and increase in observed friction force can be interpreted based on models by Yoshizawa et al. [H. Yoshizawa, Y.-L. Chen, J. Israelachvili, J. Phys. Chem. 97 (1993) 4128].