Electrochemical copper deposition on Au(100): a combined in situ STM and in situ surface X-ray diffraction study

The electrochemical deposition of copper onto Au(100) has been investigated using in situ scanning tunneling microscopy (STM) and in situ surface X-ray scattering (SXS). Copper deposition from sulfuric acid solutions starts at potentials positive of the reversible Nernst potential, where a pseudomorphic (1×1) copper monolayer forms. Negative of the Nernst potential, STM measurements have revealed that bulk copper nucleates at the surface defects, step edges or island rims of the gold substrate. A bilayer of copper is formed on top of the underlying copper monolayer and further growth proceeds in a layer-by-layer fashion. Atomic resolution STM images have shown that the copper film has a pseudomorphic structure for films less than or equal to 10 layers. Surface X-ray scattering measurements have confirmed the pseudomorphic arrangement and have provided a precise measure of the copper layer spacing (1.45±0.02 Å), in good agreement with the value obtained from the copper–copper step height in the STM measurements. This layer spacing is much smaller than the value expected for fcc copper. The combined structural analysis leads to the conclusion of a thermodynamically stable bcc-like structure. Both the X-ray and STM results suggest alloying at the copper–gold interface.