Role of tip contamination in scanning force microscopy imaging of ionic surfaces

Abstract

We present a theoretical model of the scanning force microscope (SFM) using a new quantum-mechanical embedding technique and a molecular dynamics method for calculation of the interaction between a crystalline sample and a tip nanoasperity. These are combined with a semi-empirical treatment of the mesoscopic van der Waals attraction between tip and surface, and the macroscopic parameter of cantilever deflection. The main features of the SFM experiment were modelled, including force vs. distance curves at various tip positions on the surface and scanning of a perfect LiF surface in contact regime. It is shown that tip contamination due to adhesion to the surface atoms may promote periodic SFM imaging, if the adsorbed surface material makes stable structures on the tip. We demonstrate that the adsorbed cluster can adjust itself to conditions of scanning by exchanging atoms with the surface and changing its structure. We believe that this dynamic ‘self-organisation’ of the surface material on the tip during scanning could be a general effect which may explain why periodic surface images are often obtained using a variety of tips and large tip loads.