On the basis of non-contact atomic force microscopy experiments and simulations, a detailed account was given of atomic-scale contrast encountered in force microscopy of the typical metal-oxide surface TiO2(110). It had previously been shown, for this surface, how the atomic-scale atomic force microscopy contrast depended critically upon the tip-termination polarity. Here, this finding was extended by also taking account of the influence of the tip-surface imaging distance as controlled by the scanning parameters. Atomic-resolution imaging was shown to be possible in three distinctly different types of contrast modes corresponding to three different types of tip-apex terminations. In the two predominant modes, the atomic force microscopy contrast was found to be dominated principally by the polarity of the electrostatic interactions between the tip-apex atoms and the O and Ti surface sub-lattices. A negatively (presumably Oδ−) terminated tip generated atomic force microscopy images in which the positive sub-lattice (Ti) and bridging hydroxyl (OH) adsorbates were imaged as bright protrusions, whereas a positively terminated tip (Tiδ+) results in atomic force microscopy images with inverted contrast. Experiments show that the qualitative details of the imaging contrast of the surface signatures were retained at all realistic tip-surface imaging distances for both tips, but a detailed comparison of atomic force microscopy images recorded at different scanning parameters with calculated site-specific force-distance curves illustrated how the quantitative appearance may change as the surface was probed at closer distances. The third observed imaging mode, which, however, was obtained quite seldom, reflects a tip having a predominantly covalent interaction with the surface atoms, since the resulting imaging contrast was very close to the real topographic structure of the surface. It was also expected that, for other surfaces with an ionic or semi-ionic character, atomic-scale atomic force microscopy contrast would depend strongly upon the exact nature of the tip apex in a similar way, and the present analysis outlined how all imaging modes could be included in an atomic-scale analysis so as to reveal the chemical identity of defects and adsorbates on such surfaces.

Noncontact Atomic Force Microscopy Studies of Vacancies and Hydroxyls of TiO2(110) - Experiments and Atomistic Simulations. Enevoldsen, G.H., Foster, A.S., Christensen, M.C., Lauritsen, J.V., Besenbacher, F.: Physical Review B, 2007, 76[20], 205415