Galvanic Copper Deposition on Thiol-Modified Gold Electrodes

Galvanic deposition of copper from 10^-2 MCuSO₄ on self-assembled X-terminated alkanethiol monolayers (HS(CH₂)„-₁X) on gold is studied. The influence of the chain length (X = CH₃, n = 12, 18, 22) and of the type of terminal group (X = OH (n = 12)) on the deposition process is determined using cyclovoltammetry and optical and scanning electron microscopy. The presence of a thiol layer on gold, independent of the terminal group or thiol chain length, results in deposition of hemispherical nuclei, whereas deposition on bare gold gives rather homogeneous flat copper films. The difference in morphology is ascribed to the difference in surface tension between copper and the substrate. Nucleation occuss on top of the thiol layer when the self-assembled monolayer is highly ordered. For nucleation to occur, an overpotential is required due to the potential drop across the dielectric of the thiol layer. As a consequence, the overpotential is found to increase with increasing thiol chain length: it is about -210 mV for a HS(CH₂)₁₇CH₃ layer and —270 mV for a HS(CH₂)₂₁CH₃ layer. The structure of the monolayer affects the potential drop across the thiol layer, and therefore the magnitude of the deposition overpotential. This makes cyclovoltammetry to a very sensitive method to test the quality of self-assembly. Although the overpotential for nucleation on OH-terminated thiol is about the same as for CH₃-terminated thiol, about 100 times moee particles are deposited on the OH-terminated thiol. This is ascribed to a combination of a smaller potential drop across the OH-thiol layer and a higher chemical affinity of Cu adatoms for OH groups.