Atomic resolution Imaging on CeO2(111) with hydroxylated probes

The formation of atomic contrast on the stoichiometric CeO2(111) surface imaged by dynamic scanning force microscopy (SFM) operated in the noncontact mode (NC-AFM) is investigated. We reproducibly obtain two stable contrast patterns, namely a pattern of hexagonally ordered disk-like features and a honeycomb type structure. In series of measurements where we systematically vary the tip-surface, distance, we find at a large distance exclusively disk-like contrast while the honeycomb pattern occasionally appears at a small tip-surface distance. We explain the contrast features within a model assuming that both types of contrast are created by the attractive interaction of first and third layer surface oxygen ions with the terminating cluster of the tip. We propose a tip model involving single or vicinal hydroxyl groups at the apex of a silicon oxide tip. Within this model, the disk like contrast is produced by the most protruding hydroxyl group while the reversible change in contrast found for a small tip-surface distance is caused by its relaxation and possibly further hydroxyl groups contributing to the tip sample interaction.