Effect of sample topography on adhesive force in atomic force spectroscopy measurements in air

The effect of local curvature of a sample surface on capillary and interface tension adhesion forces of the water meniscus formed between the AFM tip and sample in air is demonstrated both theoretically and experimentally. We propose an analytical solution for capillary and tension adhesive force derived from approximation of the thermodynamic equilibrium of a symmetric water meniscus formed at the tip-sample contact region. It is shown that the sample local curvature strongly affects the water meniscus geometry and the adhesion force. Compared to the force computed for a flat sample surface, the theoretical model predicts a larger/smaller adhesive force for a concave/convex local curvature. Our theoretical predictions were confirmed by experiment. Atomic force spectroscopy measurements were performed for a silicon nitride cantilever and a standard sample of platinum-covered quartz patterned with 1.5 x 1.5 mu m square, 20 nm deep pits. Maps of sample surface topography and adhesive force showed a much larger force at the bottom of the pit edge, where the local curvature was concave. This was contrasted with a much smaller force observed at the top of the pit edge, where the local curvature was convex.