Atomistic theory of the interaction between AFM tips and ionic surfaces

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Abstract
The interactions between an atomic force microscope (AFM) tip and the perfect and defective (001) surfaces of LiF, NaCl and CaO have been studied by quantum-chemical and atomistic simulation techniques. The liquid which is usually present on surfaces in experimental conditions, is considered to be inert and not contributing to the imaging. However, its chemical interaction with the tip is taken into account via the specific microscopic structure of the very end of the tip, reflecting the possibility of its oxidation and protonation. Calculations were performed for three models representing the nano-asperity at the end of SiO2 and MgO tips consisting of up to 66 atoms. The tip-surface interaction and related forces were calculated as a function of the chemical structure of the tip, its shape, and its distance from the surface. The associated tip and surface distortions caused by this interaction were investigated, We studied the atomic structure of Mg and O impurity defects near the (001) LiF surface, and OH- molecular ion substituting for Cl on the (001) surface of NaCl, and calculated their stability adiabatic barriers for diffusion, and AFM images. It is demonstrated that the optimal tip-surface distance for 'atomic resolution' is about 3-5 AA, which corresponds to the presence of one or two liquid layers between the tip and the surface. The surface and defect distortion by the tip is small in this distance range and greatly increases at smaller distances, leading to creation of surface defects. The electrostatic contribution to the tip-surface interaction makes a basis for 'atomic resolution' at large distances, whereas much stronger 'chemical' interactions dominate at small distances. The results suggest that it should be possible to image charged impurities such as Mg or O ions substituting for the host ions in alkali halides by AFM.

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