Site-specific force-distance characteristics on NaCl(001):: Measurements versus atomistic simulations

A scanning force microscope was used to measure the frequency shift above various atomic sites on a NaCl(001) surface at 7 K. The data was converted to force and compared to the results of atomistic simulations using model NaCl and MgO tips. We find that the NaCl tip demonstrates better agreement in the magnitude of the forces in experiments, supporting the observation that the tip first came into contact with the sample. Using the MgO tip as a model of the originally oxidized silicon tip, we further demonstrate a possible mechanism for tip contamination at low temperatures.