Comparative study of the adhesion, friction, and mechanical properties of CF3- and CH3-terminated alkanethiol monolayers.

We report the results of a direct comparison of the adhesion, friction, and mechanical properties between alkanethiol self-assembled monolayer films terminated by either CH(3) or CF(3) end groups using both interfacial force (IFM) and atomic force (AFM) microscopies. The purpose of this work is to gain insight into the detailed origins of the differing frictional behavior previously observed with AFM. The IFM results reveal an increased adhesive interaction for the CF(3)-terminated film due to the highly polar nature of the end groups. In agreement with earlier studies, the AFM results show two linear regions with differing frictional slopes for the CH(3)-terminated film but only a single slope for the CF(3)-terminated film. We contrast the differences between these techniques, approximately 100 times smaller tips for the AFM, and discuss the role of the mechanical properties, the increased adhesive interaction, and the amount of disorder present in the film in creating differences in frictional behavior between the two systems. We conclude that increased adhesion for the CF(3)-terminated film plays an important role in the observed differences in frictional behavior, while the differences between the two techniques can be traced to the different tip sizes and the consequent responses to the presence of disorder in the films.

Local packing environment strongly influences the frictional properties of mixed CH3- and CF3-terminated alkanethiol SAMs on Au(111).

Compositionally mixed, self-assembled monolayers (SAMs) derived from 16,16,16-trifluorohexadecanethiol and a normal alkanethiol, either hexadecanethiol or pentadecanethiol, were formed on Au(111) substrates. The relative composition of the films was determined using X-ray photoelectron spectroscopy and was found to approximately equal the equimolar composition of the isooctane solution from which they were formed. The frictional properties of the mixed films were measured on the nanometer scale using atomic force microscopy and were observed to decrease when the chain length of the CH(3)-terminated component was shortened by one methylene unit (i.e., when hexadecanethiol was replaced by pentadecanethiol). For comparison, the frictional properties of a mixed-chain-length CH(3)-terminated SAM derived from hexadecanethiol and pentadecanethiol in a 1:1 ratio was also examined. In contrast to the mixed CF(3)/CH(3) system, the latter mixed-chain-length system exhibited relatively higher friction when compared to single-component SAMs derived solely from either hexadecanethiol or pentadecanethiol. For both types of mixed films, the change in frictional properties that occurs as a result of modifying the position of neighboring terminal groups with respect to the surface plane is discussed in terms of the influence of local packing environments on interfacial energy dissipation (friction).