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 pola r bulk sample or with a thick film grown on a conducting substrate. We comp are the atomistic contribution due to the interaction between the microscop ic tip apex and the sample with the macroscopic van der Weals and image con tributions to the force on the tip for several representative NaCl clusters adsorbed on a metal substrate. We show that the microscopic force dominate s above the plain (001) terrace sites and is solely responsible for image c ontrast. However, the image force becomes comparable to the microscopic for ce above the surface di-vacancy and dominates the interaction above a charg ed step. (C) 2000 Published by Elsevier Science B.V. All rights reserved.