Contrast inversion of O adatom on rutile TiO₂(1 1 0)-(1 × 1) surface by atomic force microscopy imaging

Oxygen adatom on rutile TiO₂(1 1 0)-(1 × 1) surface was imaged by atomic force microscopy (AFM) with atomic resolution. The atomic contrast inversion of oxygen adatom is dependent on tip-sample distance and can be obtained in both constant frequency shift and constant height mode. The origin of contrast inversion was analyzed, combining the effect of adsorbed species, feedback loop and tip apex structure. Experimental frequency shift versus distance curves around O adatom show that there is a turning-point in the constant frequency shift mode and a cross-point in the constant height mode. The repulsive force is thought to contribute to the contrast inversion of oxygen adatom. The present results show the origin of contrast inversion of oxygen adatom on rutile TiO₂(1 1 0)-(1 × 1) surface and illustrate the importance of tip-height for the atomic contrast inversion of the surface adsorbate on the metal oxide surface.