Role of image forces in non-contact scanning force microscope images of ionic surfaces

We consider the effect of the image interaction on the force acting between tip and surface in non-contact scanning force microscope experiments. This interaction is relevant when a conducting tip interacts with either a polar bulk sample or with a thick film grown on a conducting substrate. We compare the atomistic contribution due to the interaction between the microscopic tip apex and the sample with the macroscopic van der Waals and image contributions to the force on the tip for several representative NaCl clusters adsorbed on a metal substrate. We show that the microscopic force dominates above the plain (001) terrace sites and is solely responsible for image contrast. However, the image force becomes comparable to the microscopic force above the surface di-vacancy and dominates the interaction above a charged step.