Oxides and minerals are often electrically insulating and thus out of reach for many experimental techniques. Despite its direct impact on surface chemistry, the detailed surface structure of many of these materials has not been conclusively determined. Recent advances in noncontact atomic force microscopy (nc-AFM) have revolutionized imaging of surfaces including those of insulators. [1]. Moreover, a suitably terminated tip apex can reveal the chemical identity of individual surface atoms.

In recent work, we employed oxygen-terminated CuOx tips [2] to solve a complex surface reconstruction of a notorious insulator – corundum Al₂O₃ [3]. At close proximity, the oxygen atom on the tip is repelled by oxygen atoms on the surface and attracted to aluminum atoms of the Al₂O₃ surface. Mapping this local repulsion and attraction allows us to directly determine the distribution of atoms within the reconstructed surface. The atomically resolved images directly show that the reconstructed Al₂O₃(0001) surface retains stoichiometric composition, disproving the previously hypothesized metallic Al termination.

[1] F. J. Giessibl, Rev. Sci. Instrum. 90, 011101 (2019)

[2] B. Schulze Lammers, et al., Nanoscale 13, 13617–13623 (2021)

[3] J. I. Hütner, et al., Science 385, 1241–1244 (2024)