Chemical Force Microscopy: Probing and Imaging Interactions between Functional Groups

Adhesion and friction forces between surfaces with pre-determined terminal groups were measured by chemically modified scanning probe microscopy tips. Surfaces composed of various terminal groups (CH₃, NH₂, and SO₃H) were obtained by direct chemisorption of silane based compounds on silicon/silicon nitrides. Work of adhesion and friction coefficients were obtained for different types of modified tips and surfaces in aqueous solutions with variations in pH. Absolute values of work of adhesion between various surfaces, W_ad, are in the range of 0.5 to 8 mJ/m². Friction properties vary with pH in a register with adhesive forces showing a broad maximum at intermediate pH values. This behavior can be understood by considering changes in surface charge state which is determined by the zwitterionic nature of silicon nitride surfaces with multiple isoelectric points.