We present an in situ scanning tunneling microscopy study on Sn underpotential deposition (UPD) on
reconstructed and unreconstructed Au(111) electrodes. Unusual behaviors in the formation of Sn (sub)monolayer at high underpotential shift are observed on both surfaces. On the unreconstructed Au(111)
surface, Sn forms size-confined two-dimensional clusters of 1−2 nm. Decreasing potential leads to immersion
of the Sn clusters into the substrate surface lattice forming surface alloying. While on reconstructed
Au(111) surface, Sn prefers to nucleate in the face-centered cubic regions followed by anisotropic growth
along the [112̄] direction of the reconstruction rows. The nuclei expand toward the hexagonal close packed
regions to build up deposit domains which develop until a final gap of 3−6 nm is reached between two
adjacent domains. Decreasing potential leads to a structural transformation to cordlike structures aligning
all in one direction by rotation of 18−21° with respect to the [112̄] direction, involving surface alloying.
The results are in contrast to the previously observed UPD adlayer structure and dynamics. The nanoscale
properties of the Sn deposit and the influence from the UPD alloying are discussed as the two possible
causes for the unusual Sn UPD phenomena.
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