Underpotentially-deposited silver substrates reverse the odd–even interfacial properties of CF3-terminated SAMs

Abstract

Modifications to the metal substrate used in alkanethiol self-assembled monolayer (SAM) formation impact the structural characteristics of the thin films and their macroscopic interfacial properties. In this study, evaporated gold surfaces underwent electrochemical modification, where a monolayer of silver was deposited through underpotential deposition (UPD). The effects of such a modification of the gold substrate on the structural and interfacial properties of n-alkanethiol and CF₃-terminated SAMs, wherein the latter bears an interfacial dipole at the CF₃–CH₂ transition, were explored. Structural analysis of the films revealed well-ordered monolayers on both gold and UPD Ag surfaces. Ellipsometric thickness assessment and X-ray photoelectron spectroscopy (XPS) of UPD Ag surfaces showed that the adsorbates formed densely packed monolayers that were ∼4 Å thicker than their counterparts on gold. These variations were attributed to the different binding geometries adopted by the sulfur atoms on the respective metals, which in turn dictates the tilt angles and the orientation of the terminal moiety. Polarization modulation infrared reflection−absorption spectroscopy (PM-IRRAS) revealed a shift in the orientation of the chain termini, likely due to differences in the mobility of underlying methylene units between substrates. Moreover, odd–even effects in the contact angle data of both polar and nonpolar liquids show changes in interfacial wettability further highlighting the impact of the subtle change to the substrate on the film structure.