  Noncontact atomic force microscopy (NC-AFM) and Kelvin probe force microscopy
(KPFM) have become important tools for nanotechnology; however, their contrast
mechanisms on the atomic scale are not entirely understood. Here we used
chlorine vacancies in NaCl bilayers on Cu(111) as a model system to investigate
atomic contrast as a function of applied voltage, tip height, and tip
functionalization. We demonstrate that the AFM contrast on the atomic scale
decisively depends on both the tip termination and the sample voltage. On the
contrary, the local contact potential difference (LCPD) acquired with KPFM
showed the same qualitative contrast for all tip terminations investigated,
which resembled the contrast of the electric field of the sample. We find that
the AFM contrast stems mainly from electrostatic interactions but its tip
dependence cannot be explained by the tip dipole alone. With the aid of a
simple electrostatic model and by density functional theory we investigate the
underlying contrast mechanisms.
