Tip-Induced Control of Charge and Molecular Bonding of Oxygen Atoms on the Rutile TiO₂ (110) Surface with Atomic Force Microscopy

We study a low-temperature on-surface reversible chemical reaction of oxygen atoms to molecules in ultrahigh vacuum on the semiconducting rutile TiO₂(110)-(1-1) surface. The reaction is activated by charge transfer from two sources, natural surface/subsurface polarons and experimental Kelvin probe force spectroscopy as a tool for electronic charge manipulation with single electron precision. We demonstrate a complete control over the oxygen species not attainable previously, allowing us to deliberately discriminate in favor of charge or bond manipulation, using either direct charge injection/removal through the tip-oxygen adatom junction or indirectly via polarons. Comparing our ab initio calculations with experiment, we speculate that we may have also manipulated the spin on the oxygens, allowing us to deal with the singlet/triplet complexities associated with the oxygen molecule formation. We show that the manipulation outcome is fully governed by three experimental parameters, vertical and lateral tip positions and the bias voltage.