Noncontact atomic force microscopy studies of vacancies and hydroxyls of TiO2(110):: Experiments and atomistic simulations

From an interplay of noncontact atomic force microscopy experiments and simulations, we present here a detailed account of atomic-scale contrast encountered in force microscopy of the prototypical metal-oxide surface TiO2(110). We have previously shown for this surface how the atomic-scale atomic force microscopy (AFM) contrast depends crucially on the tip-termination polarity. Here, we extend this finding by also taking into account the influence of the tip-surface imaging distance as controlled by the scanning parameters. Atomic-resolution imaging is 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 AFM contrast is found to be dominated principally by the polarity of the electrostatic interactions between the tip-apex atoms and the O and Ti surface sublattices. A negatively (presumably O delta-) terminated tip generates AFM images in which the positive sublattice (Ti) and bridging hydroxyl (OH) adsorbates are imaged as bright protrusions, whereas a positively terminated tip (Ti delta+) results in AFM images with inverted contrast. Experiments show that the qualitative details of the imaging contrast of the surface signatures are retained at all realistic tip-surface imaging distances for both tips, but a detailed comparison of AFM images recorded at different scanning parameters with calculated site-specific force-distance curves illustrates how the quantitative appearance may change as the surface is probed at closer distances. The third observed imaging mode, which, however, is obtained quite seldom, reflects a tip having a predominantly covalent interaction with the surface atoms, since the resulting imaging contrast is very close to the real topographic structure of the surface. We expect that also for other surfaces with an ionic or semi-ionic character that the atomic-scale AFM contrast depends strongly on the exact nature of the tip apex in a similar way, and the present analysis outlines how all imaging modes can be included in an atomic-scale analysis to reveal the chemical identity of defects and adsorbates on such surfaces.