SURFACE-TIP INTERACTIONS IN NONCONTACT ATOMIC-FORCE MICROSCOPY ON REACTIVE SURFACES - SI(111)

Total-energy pseudopotential calculations are used to study the imagin g process in noncontact atomic-force microscopy on Si(111) surfaces. A t the distance of closest approach between the tip and the surface, th ere is an onset of covalent chemical bonding between the dangling bond s of the tip and the surface. Displacement curves and lateral scans on the surface show that this interaction energy and force are comparabl e to the macroscopic Van der Waals interaction. However, the covalent interaction completely dominates the force gradients probed in the exp eriments. Hence, this covalent interaction is responsible for the atom ic resolution obtained on reactive surfaces and it should play a role in improving the resolution in other systems. Our results provide a cl ear understanding of a number of issues such as (i) the experimental d ifficulty in achieving stable operation, (ii) the quality of the image s obtained in different experiments and the role of tip preparations a nd (iii) recently observed discontinuities in the force gradient curve s. [S0163-1829(98)08336-2].