Molecular Dynamics Simulations of the Structural Arrangement and Density of Alkylamine Surfactants on Copper Surfaces: Implications for Anisotropic Growth of Copper Nanowires

Understanding the role of alkylamine surfactants in the anisotropic growth mechanism of copper nanowires (CuNWs) in solution-phase synthesis has implications for tuning of the properties that can be employed in diverse, important applications. In this paper, we used molecular dynamics simulations to show that the hexadecylamine (HDA) adsorption structure can, depending on the HDA density, form a surfactant monolayer or many stacked layers parallel to the copper surface. We showed that the layers are similar on the Cu(111) and Cu(100) facets, independent of the HDA density, and that the presence of Cl- ions appears to have no significant effect on the HDA/HDA(+) stacked layer structure. Within the simulation time frame, Cl- ions do not pass through a regular, uniform HDA monolayer, but they can pass through a monolayer with some protonated HDA molecules (HDA(+)). Our results suggest that Cl- ions not only affect the HDA/HDA(+) monolayer stability in CuNW synthesis, causing a disruption of the monolayer and formation of the stacked layer, but also participate in copper(II) alkylamine complex formation. Moreover, our results stress the importance of a reliable model of the surfactant layer on the metal particle, explicitly considering the protonation of part of the surfactant. Our results show a completely innovative perspective of the HDA adsorption structure and its role in the growth mechanism of CuNWs in solution-phase synthesis, allowing improvements in control of the properties and the quality of the CuNWs obtained.