We probed the polar CaF2(111) surface by non-contact atomic force microscopy using
a bare metal tip, a CuO tip and a CO-terminated tip. The tips are engineered by poking, CO or CuO pickup and subsequent COFI
imaging (see Welker et al., Science 336, 444 (2012)). In the non-contact regime far from the surface,
the contrast is entirely determined by electrostatics and can be calculated at a precision of about 10 percent using
Coulomb's law and assuming a point charge at the tip apex (positive for the metal tip, negative for CO and CuO tips).
While metal tips turn unstable when reaching the repulsive regime, using CO or CuO tips allow to image the surface at very high resolution
as well. Interestingly, the CO tips provide less van-der-Waals background force, while CuO tips show less artifacts due to tip bending.