  A numerical analysis of the origin of the atomic-scale contrast in Kelvin
probe force microscopy (KPFM) is presented. Atomistic simulations of the
tip-sample interaction force field have been combined with a non-contact Atomic
Force Microscope/KPFM simulator. The implementation mimics recent experimental
results on the (001) surface of a bulk alkali halide crystal for which
simultaneous atomic-scale topographical and Contact Potential Difference (CPD)
contrasts were reported. The local CPD does reflect the periodicity of the
ionic crystal, but not the magnitude of its Madelung surface potential. The
imaging mechanism relies on the induced polarization of the ions at the
tip-surface interface owing to the modulation of the applied bias voltage. Our
findings are in excellent agreement with previous theoretical expectations and
experimental observations.
