Received: August 31, 2007

In Final Form: November 7, 2007

Abstract:

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.

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