Effect of Self-Assembled Monolayers on the Locally Electrodeposited Silver Thin Layers

The localized electrodeposition of silver on gold coated with self-assembled monolayers (SAMs) by scanning electrochemical microscopy (SECM) is reported. The SAMs were -functionalized alkanethiols of the form X-(CH₂)₂SH, X = OH, NH₂, CO₂H, SO₃H, as well as 4-mercaptobenzoic acid. The SAMs were characterized by X-ray photoelectron spectroscopy (XPS) and cycling voltammetry (CV). The anodic dissolution of a Ag microelectrode, which was held within a few microns from the Au surface, formed a well-controlled flux of Ag⁺. Deposition of silver nanostructures was driven by the electrochemical reduction of the Ag⁺ on the Au surface. The effect of the functional group on the Ag local deposition was studied and compared with bulk deposition on the same SAMs. For bulk deposition, we found that the interaction between Ag⁺ ions and the functional group of the alkanethiols slowed the kinetics of Ag deposition, shifting the deposition to potential that is more negative and caused the formation of large, well-faceted Ag crystals. A clear correlation between the potential shift value and the morphology of deposited Ag was observed. The local deposition of Ag showed distinct difference compared to bulk deposition. We found that a continuous and homogeneous Ag film was formed locally below the Ag microelectrode in the presence of a 3-mercaptopropionic acid monolayer. This was observed when a 120 s delay between the electrogeneration of the silver ions and the application of a negative potential to the gold surface was applied. Moreover, the potential applied to the Au surface also affected deposition. The deposited silver was recollected by the Ag microelectrode by stripping the silver from the Au surface while holding the microelectrode in the same position. This enabled calculating the thickness of the Ag film deposited on the Au coated with 3-mercaptopropionic acid. Additional experiments clearly indicated that the mechanism of deposition involved complexation of silver ions by the SAM and their local reduction, which commenced prior to applying a negative potential to the Au surface.