Influence of surface roughness on the pull-off force in atomic force microscopy

We investigate how the pull-off force in atomic force microscopy (AFM), which arises from a nanoscale water bridge between the AFM tip and the surface, is influenced by atomic scale (smaller than 0.6 nm) roughness in the surface. Adopting a lattice gas model for water, we have simulated the adhesion of a silicon-nitride tip (with a 20nm diameter) to mica under ambient humidity. The pull-off force responds sensitively to both surface and tip roughness, and its humidity dependence changes significantly with slight variation in the tip and surface morphology. The change in the pull-off force due to roughness smaller than 0.6 nm can be larger than the change from doubling the tip radius. The roughness effect is large at low humidities and diminishes as humidity increases. Even at 80 percent humidity, the pull-off force varies considerably with changes in tip-surface geometry. On average, the pull-off force decreases with increasing tip roughness. However it decreases with surface roughness for small roughness (<0.2 nm), and then it increases for larger roughness. The pull-off force is also found to decrease with increasing average tip-surface distance at the point of initial contact, which shows the importance of spatial confinement of the water droplet.