Model of noncontact scanning force microscopy on ionic surfaces

We analyze the mechanisms of contrast formation in noncontact SFM imaging o f ionic surfaces and calculate constant frequency shift scanlines of the pe rfect surfaces of NaCl, MgO, and LiF. The noncontact scanning force microsc opy (SFM) operation is modeled by a perturbed oscillator using atomistic st atic and molecular-dynamics techniques for the force-field calculations. Th e electrostatic potentials of silicon tips contaminated by various atoms an d that of a MgO tip are calculated using a periodic density-functional theo ry (DFT) method. Their analysis demonstrates that the presence of polar gro ups or chemisorbed species, such as oxygen atoms, makes the electrostatic f orces 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 nanoti p 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 int erplay of the electrostatic and van der Waals forces. The main contribution s 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].