Adhesive and friction forces between chemically modified silicon and silicon nitride surfaces

We report the results of probing adhesion and friction forces between surfaces with functional terminal groups with chemically modified scanning probe microscopy (SPM) tips. Surfaces with terminal groups of CH3, NH2, and SO3H were obtained by direct chemisorption of silane-based compounds on silicon/silicon nitride surfaces. We studied surface properties of the resulting self-assembled monolayers (SAMs) in air and aqueous solutions with different pHs. Work of adhesion, "residual forces", and friction coefficients was obtained for four different types of modified tips and surfaces. Absolute values of the work of adhesion between various surfaces, W-ad, were in the range 0.5 - 8 mJ/m(2). The work of adhesion for different modified surfaces correlated with changes of solid-liquid surface energy estimated from macroscopic contact-angle measurements. Friction properties varied with pH in a register with adhesive forces showing a broad maximum at intermediate pH values for a silicon nitride/silicon nitride mating pair. Similar broad maxima were observed in the acid range for a NH2-terminated SAM and in the basic range for a SO3H-terminated SAM. This behavior can be understood considering the changes of the surface charge state determined by the zwitterionic nature of silicon nitride surfaces with multiple isoelectric points.