Self-Assembled Monolayers: Effect of Chain Length on Nanofriction
S Hsieh, S Sambasivan, J V. Sengers, Stephen M. Hsu
We measured the interfacial friction of a series of alkylsilanes with chain lengths from C5 to C30 on silicon (100) surfaces using an Atomic Force Microscope (AFM). The dependence of the friction on chain length exhibits an optimum curvature with C12 having the lowest friction, and C5 and C30 having higher friction. This result is contrary to current views that longer chain lengths yield lower friction. Using Fourier transform infrared (FTIR) spectroscopy and Near Edge X-ray Absorption Fine Structure (NEXAFS) spectroscopy, we found that the friction variations of SAMs as a function of chain length can be attributed to the degree of order (defect population) of the film structure. Highly ordered films tend to have lower friction, while more disorder in the films leads to increased friction. This experimental conclusion is supported by the results of a molecular dynamics simulation by Chandross et al., which suggests that defect population in SAMs affects friction.
Journal of Vacuum Science and Technology A
AFM, defect population, film order, nanofriction, NEXAFS, SAM