Direct observation of hopping and merging of single Au adatoms to form dimers on Si(111)-(7 × 7)

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Au single adatoms and dimers were imaged on Si(111)-(7 × 7) at different temperatures and bias voltages using a variable-temperature scanning tunneling microscope. At room temperature (RT), a single Au adatom induces sharp highlighted triangular features in the half unit cells (HUCs) of Si(111)-(7 × 7). These triangular features become fuzzy at temperatures lower than 225 K, as a result of the reduced moving speed of the single Au adatoms inside the HUCs. The formation of an Au adatom dimer was directly observed at RT when a single Au adatom in a HUC jumped into a neighboring HUC that already contained a single Au adatom. The Au adatom dimer appears either as a noisy feature in the central area of the HUC defined by three Si center adatoms or as a bright protrusion located close to a corner Si adatom site at RT. It was observed that a noisy feature also can change into a bright protrusion, inducing charge redistribution in the nearby Si adatoms in both the occupied and neighboring HUCs.