Atomic force microscopy nanotribology study of oligothiophene self-assembled films

The oligothiophene derivative 4-(5″″-tetradecyl-[2, 2′; 5′, 2″; 5″, 2‴; 5‴, 2″″] pentathiophen-5-yl)-butyric acid (C14-5TBA) was synthesized and the structural and mechanical properties of self-assembled monolayers on mica have been studied by atomic force microscopy (AFM). The films were prepared by drop casting a dilute THF solution (1 mM) of the oligothiophene on mica. Islands containing primarily monolayers with a very small percentage of multilayers were formed. The molecules adsorb through the carboxylic group, and expose the alkyl chain (CH₂)₁₃CH₃. High resolution AFM scans reveal a well ordered structure of molecules with unit cell dimensions of 0.65 and 0.46 nm. Applying load to the tip, the molecular film was gradually compressed from an initial height of 4.1 nm to a final one of 2.6 nm, corresponding to a tilt of the alkyl chains. In regions covered with bilayers the molecules in the second layer were oriented opposite to those in the first layer, thus exposing the carboxylic end group to the air. This second layer was easily removed as the tip pressure increased.