Contrast mechanism in non-contact AFM on reactive surfaces

Total-energy pseudopotential calculations are used to study the imaging process in non-contact atomic force microscopy on Si(111) surfaces. The atomic resolution seen in the experiments is attributed to the onset of covalent bonding between a localised dangling bond on the atom at the apex of the tip and the dangling bonds on the adatoms in the surface. This interaction dominates the force gradients, which drive the frequency changes used to create the experimental images. Force vs. tip displacement curves provide information about the optimum operation range and show the importance of the relaxation of the tip apex and surface atoms in the understanding of the damping images.