Ionic and electronic processes at ionic surfaces induced by atomic-force-microscope tips

Several ion and electron processes near contact between two insulators are predicted, which should be observable following recent developments in atomic force microscopy. When two solids approach each other closer than about two interatomic distances, instabilities and strong outward displacement of the surface atoms occur. This has been known for metals, and is shown in this paper for ionic systems. Using periodic density-functional calculations, we demonstrate that the ionic model holds for relatively large (<1.5) displacements of individual Mg²⁺ and O^2- ions from the MgO surface. It breaks at further displacements where the electron redistribution is strong, and neutralizes both the displaced ion and the vacancy left at the surface. The interaction of the sharp and blunt MgO tips with the LiF, NaCl, and MgO surfaces was studied using an embedded-cluster Hartree-Fock method. Conditions for trapping of surface ions on the tip during the tip and surface separation are determined. It is demonstrated that this trapping can initiate formation of one-dimensional strings of ions stretching out from the surfaces as they separate. The latter result is confirmed by the classical molecular-dynamics simulations of the contact formation and separation of the plane MgO and LiF surfaces. These simulations have also demonstrated that the junction between two dissimilar ionic surfaces breaks not at the original but at a new interface accompanied by contamination of one of the surfaces. Analysis of electronic processes reveals that electrons can tunnel from the tip to the anion vacancies left in the surface.