Factors influencing the interfacial width of copper gradients on gold produced by spatiotemporal control of the in-plane electrochemical potential distribution: electrode geometry and plating solution composition

Injecting an in-plane current through a thin Au film results in the formation of a linear electrochemical potential gradient at the surface of the electrode. Coupling the surface potentials to a redox species in solution then effects spatially dependent redox chemistry. This strategy has been used to map the electrochemical deposition of Cu0 onto the surface of Au. Because the local electrochemical potential, V(x), varies spatially in the plane of the electrode, a transition region is established on the surface in an area corresponding to the anodic peak (strip-out) potential. The width of this transition region is of both technological and theoretical interest, and the effects of electrode geometry and plating solution modifiers on the interfacial width of electrochemically deposited thin Cu films have been investigated. Careful calibration of the surface electrochemical potential gradient shows that the Au–Cu transition occurs at a spatial location corresponding to V(x)=125±14 mV in agreement with the observation of an anodic peak potential centered at ca. 125 mV vs. Ag|AgCl from cyclic voltammetry. Interfacial widths are dramatically affected by different solution modifiers, with l-(+)-tartaric acid and NaCl producing much sharper transitions and different morphologies in comparison to Cu gradients prepared in the presence of citric acid or malonic acid. Data reported here strongly support the notion that nucleation/growth, not electrical effects, play the dominant role in determining interfacial width and final morphology of the deposited film.