Abstract
Gold electrodeposition on C(0001) from aqueous 0.5 mM AuCl3·HCl, in the range of apparent charge density (6 ≤ q ≤ 10 mC/cm2) and under mass transport kinetic control from the solution side, results in the formation of nanometer to micrometer size Au(111) islands. These islands consist of a small three-dimensional (3D) central core and large quasi-two-dimensional (2D) dendritic branches. Ex situ scanning tunneling microscopy (STM) imaging data show that the shape of islands is determined by anisotropic surface diffusion contributions. The addition of citric acid to the plating solution hinders branching and promotes 3D island growth. The adsorption of citric acid at step edges hinders interterrace gold atom surface diffusion, as concluded from in situ STM imaging. Conversely, the addition of an excess of sodium chloride enhances 2D island growth and dense branching formation. These results can be explained considering that the presence of adsorbates either increases or decreases the height of activation energy barriers for interterrace surface diffusion. In this way, the change of the island aspect ratio with the nature of the additive can be explained.
Highlights
The heteroepitaxial growth of a solid phase on a welldefined crystalline foreign substrate occurring far from conditions of equilibrium is a complex process involving the initial formation of nuclei, their growth forming either two-dimensional (2D) or three-dimensional (3D) crystals, and the overlapping of these crystals covering most of the substrate surface.1 The first and second steps lead to a solid surface consisting of patches of the substrate and islands of the new solid phase
Ex situ scanning tunneling microscopy (STM) imaging data show that the shape of islands is determined by anisotropic surface diffusion contributions
The same conclusion comes out from the analysis of the corresponding j vs t-1/2 plots. These results indicate that there is no influence of those species added to the solution on the mass transport rate control from the solution side
Summary
The heteroepitaxial growth of a solid phase on a welldefined crystalline foreign substrate occurring far from conditions of equilibrium is a complex process involving the initial formation of nuclei, their growth forming either two-dimensional (2D) or three-dimensional (3D) crystals, and the overlapping of these crystals covering most of the substrate surface. The first and second steps lead to a solid surface consisting of patches of the substrate and islands of the new solid phase. In contrast to the presence of sodium chloride, citric acid eliminates branching and promotes the growth of rounded quasi-3D islands This effect of citric acid on the gold island growth mode is due to an increase in the height of activation energy barriers for gold atom surface diffusion at step edges that interfere with interterrace and corner surface diffusion. The change in the growth mode induced by the presence of different adsorbates can be explained either by a hindrance to the interterrace gold atom surface diffusion by molecular adsorption at step edges in the case of citric acid or by its enhancement in the presence of sodium chloride
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