Model of noncontact scanning force microscopy on ionic surfaces

We analyze the mechanisms of contrast formation in noncontact SFM imaging of ionic surfaces and calculate constant frequency shift scanlines of the perfect surfaces of NaCl, MgO, and LiF. The noncontact scanning force microscopy (SFM) operation is modeled by a perturbed oscillator using atomistic static and molecular-dynamics techniques for the force-field calculations. The electrostatic potentials of silicon tips contaminated by various atoms and that of a MgO tip are calculated using a periodic density-functional theory (DFT) method. Their analysis demonstrates that the presence of polar groups or chemisorbed species, such as oxygen atoms, makes the electrostatic forces acting on the surface ions from the Si tip one of the most important contributions to the image contrast. The (MgO)(32) cube model of the nanotip was found to be representative of a wide class of polar tips and used in the image calculations. The results of these calculations demonstrate that the contrast in noncontact SFM imaging of ionic surfaces is based on an interplay of the electrostatic and van der Waals forces. The main contributions to the contrast formation result from the interaction of the tip with the alternating surface potential and with the surface polarization induced by the electric field of the tip. The results emphasize the importance of the tip-induced relaxation of the surface ions in the tip-surface interaction and in image contrast. The noncontact SFM image of the Mg2+-cation vacancy defect on the LiF surface is calculated using the same method. [S0163-1829(99)08803-7].