Removal of Adsorbed Organic Molecules with Scanning Tunneling Microscope: Formate Anions on TiO₂(110) Surface

A scanning tunneling microscope (STM) operated under ultrahigh vacuum enabled us to eliminate individual formate anions ( DCOO - ) adsorbed on a TiO 2 (110) surface. When bias voltages of +3.7 V or more were applied on the sample, the formate anions were removed under the STM tip. In contrast, much higher voltages, +5.0 V or more, were required to damage the underlying TiO 2 surface. Thus we could strip off the formate anions in a desired area without damage to the substrate, to create patchworks of uncovered substrate in the monolayer of adsorbed formate anions. The threshold bias voltage for the elimination showed a small dependence on tunneling current, suggesting that the removal process is through field evaporation rather than direct excitation or thermal activation by tunneling electrons.