  The complex atomic structures and defects of metal-oxide surfaces are vital
for a variety of applications in material science and chemistry. While scanning
probe microscopy allows accessing atomic-scale structures in real space,
elemental discrimination and defect characterization usually rely on indirect
assumptions and extensive theoretical modelling. By investigating a variety of
different sample systems with increasing structural complexity and inherent
defects, we demonstrate that noncontact atomic force microscopy with an
O-terminated copper tip allows imaging metal-oxide surfaces with a clear
elemental contrast. This universal approach provides not only immediate access
to the metal- and oxygen sub-lattices, but also to the chemical and structural
configuration of atomic-scale defect structures. The observed contrast can be
explained by purely electrostatic interactions between the negatively charged
tip apex and the strongly varying electrostatic potential between metal- and
oxygen surface sites. These results offer a standardized methodology for the
direct structural characterization of even most-complex metal-oxide surfaces,
which is highly relevant for a fundamental understanding of atomic-scale
processes on these material systems.
