Lateral Force Modulation Atomic Force Microscopy of Langmuir-Blodgett Film in Water

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Abstract
The mechanical properties of a Langmuir-Blodgett (LB) film in water were investigated using an atomic force microscope with lateral force modulation. During the force curve measurement, the dynamic friction force is measured by observing the amplitude and phase of the cantilever torsion vibration, as the sample is deformed by a tip with gradually increasing normal force. In contrast to conventional friction force microscopy, the friction measurement is performed without x–y scanning, and therefore is in real time. This method was applied to a LB film consisting of an arachidic acid (AA) single layer on top of a single layer of partially fluorinated ether of carboxylic acid (PFECA), and interesting results were obtained. In particular, the film was found to be stable for a short period (<120 min) and the structure and ratio of the friction forces between the AA and PFECA areas are identical with those observed in air. Moreover, a friction force transition was observed at a normal force of around 14 nN on loading and an almost reversible decrease was observed on unloading. This result indicates penetration of the AA film by the tip and recovery of the penetrated hole on unloading.