Friction-anisotropy dependence in organic self-assembled monolayers

A study in order to understand the origin of the friction at the nanometer scale has been performed. Atomic force microscopy (AFM) has been used to measure the mechanical and frictional properties of self-assembled monolayers of the oligothiophene derivative 4-(5″″-tetradecyl-[2,2′;5′,2″;5″,2‴;5‴,2″″] pentathiophen-5-yl)-butyric acid (C14-5TBA) on mica. The films were prepared by drop casting a dilute THF solution (1 mM) of the oligothiophene on mica. The molecules adsorb on mica through the carboxylic group, and expose the alkyl chain (CH₂)₁₃CH₃. Instead of complete monolayers, islands were produced to facilitate measurements of the film height. By applying load to the tip a compression of the molecules from an initial height of 4.1 nm to a final height of 2.6 nm was achieved which we attribute to deformation of the alkyl chains. We previously observed that the oligothiophene derivative 4-(5″″-decyl-[2,2′;5′,2″;5″,2‴;5‴,2″″] pentathiophen-5-yl)-butyric acid (C10-5TBA), exhibits strong friction anisotropies showing different domains of friction. The topographic images for both of these films reveals that they form molecularly flat films. However, the longer alkyl chains of the C14-5TBA induces better organization of the molecules as shown by the lattice resolved AFM images obtained. Well-ordered molecular structures were seen with measured unit cell dimensions of 0.65 and 0.46 nm. The ability to resolve the lattice structure of these films allows us to measure friction in different lattice structure domains, and the results show strong friction anisotropy.