  We investigated insulating Cu$_2$N islands grown on Cu(100) by means of
combined scanning tunneling microscopy and atomic force microscopy with two
vastly different tips: a bare metal tip and a CO-terminated tip. We use
scanning tunneling microscopy data as proposed by Choi et al. [T. Choi et al.,
PRB 78, 035430 (2008).] to unambiguously identify atomic positions. Atomic
force microscopy images taken with the two different tips show an inverted
contrast over Cu$_2$N. The observed force contrast can be explained with an
electrostatic model, where the two tips have dipole moments of opposite
directions. This highlights the importance of short-range electrostatic forces
in the formation of atomic contrast on polar surfaces in non-contact atomic
force microscopy.
