Lateral and friction forces originating during force microscope scanning of ionic surfaces

We present a simple theoretical model of the scanning force microscope (SFM) experiment using an atomistic treatment of the interaction between a crystalline sample and a tip nanoasperity combined with a semiempirical treatment of the mesoscopic van der Waals attraction between tip and surface and the macroscopic parameter of cantilever deflection. For the nanoasperity at the end of the tip we used a corner of a neutral and a protonated (MgO)(32) cube, which models a hard tip made of oxide material. The scanning of the perfect (100) surfaces of NaCl and LiF were modelled at different constant vertical forces exerted on the tip. Lateral forces originating in the quasistatic regime of scanning are calculated. The results demonstrate that at relatively ''soft'' engagement force but still realistic conditions of the tip-surface interaction, the lateral force SFM image can be periodic and correspond to low or zero friction. For the particular example of a locally charged protonated MgO tip scanning the (001) NaCl surface in hard contact, we have studied the tip and surface distortion and have observed several characteristic instabilities in the behaviour of the surface ions. These instabilities are manifested in sudden ionic motion within the surface plane and are accompanied by adsorption of some of the ions onto the tip, which is one of the micro-mechanisms of friction with wear in these systems.