Nanoscale Chemical Effect on Friction Force

Self-assembled alkylsilane monolayers reduce friction on silicon surfaces. Bias-assisted nanolithography can be used to create chemical patterns on such films by the process of local oxidation, whereby an atomic force microscope is used to scan a biased tip across the surface in a pre-defined pattern. The chemistry of this process involves a redox reaction that oxidizes the terminal methyl groups of the film forming carboxyl groups in their place. In this study, we have prepared a sample designed specifically for measuring nanoscale chemical friction on a silicon substrate without topography effects. This unique sample possesses wide regions of both oxidized and unmodified film within a small area, so that direct measurement of the relative friction between the two films can be made within a single 1 μm scan, eliminating tip and sample inconsistencies that are common when comparing friction force measurements in consecutive scans or on different samples. Further, since the oxidation process modifies the film chemically, there is almost no contribution from the surface topography. We found that friction force increases as a function of applied load for both types of terminal groups and that the coefficient of friction for the carboxyl terminated region is five times greater than for the methyl terminated region. Moreover, friction force decreases for both surfaces as the tip velocity increases, though much more dramatically for the carboxyl terminated film. Both of these observations are consistent with a model that includes tip/sample bonding and localized condensation as the significant factors influencing chemically induced friction.