Direct observation of the basic mechanisms of Pd island nucleation on Au(111)

TL;DR

This study uses STM to directly observe how Pd islands nucleate on Au(111), revealing the roles of place exchange and surface distortions in the early stages of growth.

Contribution

It provides the first atomically resolved images of Pd nucleation mechanisms on Au(111), highlighting the importance of substitution and surface distortions in island formation.

Findings

Pd atoms preferentially nucleate at elbows of the herringbone surface

Substitution of Pd atoms in lattice sites occurs at very low coverage

Distortion of the surface appears at higher Pd coverages (>0.25 ML)

Abstract

The formation mechanisms of evaporated Pd islands on the reconstructed Au(111) $22 /times /sqrt{3}$ herringbone surface have been here studied by Scanning Tunneling Microscopy (STM) at room temperature. Atomically resolved STM images at the very early stages of growth provide a direct observation of the mechanisms involved in preferential Pd islands nucleation at the elbows of the herringbone structure. At low Pd coverage the Au(111) herringbone structure remains substantially unperturbed and isolated Pd atoms settled in hollow sites between Au atoms are found nearby the elbows and the distortions of the reconstructed surface. In the same regions, at extremely low coverage (0.003 ML), substituted Pd atoms in lattice sites of the Au(111) surface are also observed, revealing the occurrence of a place exchange mechanism. Substitution seems to play a fundamental role in the nucleation process, forming aggregation centers for incoming atoms and thus leading to the ordered growth of Pd islands on Au(111). Atomically resolved STM images of Pd islands reveal a close-packed arrangement with lattice parameter close to the interatomic distance between gold atoms in the fcc regions of the Au(111) surface. Distortion of the herringbone structure for Pd coverages higher than 0.25 ML indicates strong interaction between the growing islands and the topmost Au(111) layer.