Pull-off forces measured between hexadecanethiol self-assembled monolayers in air using an atomic force microscope: analysis of surface free energy

The pull-off forces were measured between hexadecanethiol monolayers, self-assembled on gold-coated silicon nitride cantilever tip and silicon wafer, using atomic force microscopy (AFM). The blind tip reconstruction technique was used for determination of the curvature of the AFM tip. The measured pull-off force value remained practically unaffected by a variation of the maximum applied load in a range of 5 - 80 nN. This result suggests that the use of continuum elastic contact mechanics in the analysis of AFM pull-off force measurements is not as straightforward as usually assumed in the literature reports on similar systems. The surface free energy of hexadecanethiol monolayer on a gold film of gamma = 24-27 mJ/m(2) was calculated based on the pull-off forces (F), measured using R = 60-80 nm radius tips, and next applying the Derjaguin approximation: F = 4pi Rgamma. These gamma-values were found to match the surface free energy value calculated from contact angle data and using Lewis acid-base interfacial free energy theory.