A model of the interaction of ionic tips with ionic surfaces for interpretation of scanning force microscope images

Scanning force microscopy (SFM) is an increasingly popular tool in surface studies. With the promise of lateral as well as vertical atomic resolution, its use is sure to become widespread in fields such as catalysis and crystal growth. However, the interpretation of the observed images is still unclear and therefore theoretical models are very important for an understanding of the imaging mechanism. We present a review of our recent calculations on the interaction between ionic tips and ionic surfaces and its effects on the scanning process. Our theoretical model of the SFM experiment combines an atomistic treatment of the interaction between a crystalline sample and the nanoasperity at the end of the tip with a semiempirical treatment of the mesoscopic van der Waals attraction between tip and surface and the macroscopic parameter of cantilever deflection. These static calculations based on total energy minimisation were used to determine the surface and tip geometries and energy as a function of tip height at each point of a scan. Scanlines of the perfect (001) surfaces of LiF and MgO were studied at different constant vertical forces exerted on the tip with and without jump to contact. Although scanlines showing lattice periodicity were obtained in some cases, the calculations demonstrate that the tip-surface interaction is a collective phenomenon with the tip probing several rows of surface ions. The calculations demonstrated a wide range of deformations of the tip and sample during a surface scan, and the (often reversible) interchange of material between the tip and sample. The conditions required for the resolution of point defects as well as the mechanism of friction are discussed.