Derek A. Triplett1, Lisa M. Dillenback2, Benjamin D. Smith2, Christine D. Keating2, and Kristen A. Fichthorn1. (1) Dept. of Chemical Engineering, The Pennsylvania State University, University Park, PA 16802, (2) Dept. of Chemistry, The Pennsylvania State University, University Park, PA 16802
The assembly of metallic nanowires at surfaces is important in the construction of nanoelectronic circuits as well as in many other higher-order structures. The ability to understand and control the assembly of these anisotropic building blocks is fundamental to the creation of properly ordered structures. Here, we investigate the ordering of aqueous mixed-metal, 50% Au/ 50% Ag nanowires that possess a gold end and a silver end and are coated with 2-mercapto-ethanesulfonic acid (MESA). The nanowires settle from solution onto a glass cover slip and are visualized with an optical microscope. The wires form a smectic-like layer on the substrate and exhibit a slight preference for the gold ends of the wires to be aligned next to one another. The alignment of Au/Au, Au/Ag, and Ag/Ag ends is characterized using an order parameter that compares the observed alignment of the wires against a random distribution.
In an effort to understand assembly in this system and ultimately exercise control over it, we use canonical Monte Carlo (MC) simulations to model the ordering of the wires on the substrate. We account for van der Waals and electrostatic interactions between the wires. The MC simulations correctly reproduce the experimentally observed ordering and demonstrate that the preference for alignment of the Au ends stems from differences between the Hamaker constants for Au and Ag. We study how general differences between the Hamaker constants of the two materials, as well as differences in the zeta potentials can alter the value of the order parameter. We show that the propensity of the wires to exhibit a smectic vs. a nematic phase is governed by a balance between van der Waals attraction and electrostatic repulsion. If the zeta potential is sufficiently weak, van der Waals attraction leads to a smectic phase, as is observed experimentally. On the other hand, if the zeta potential is sufficiently strong, a nematic phase is predicted that mitigates electrostatic repulsion. The ordering of the nanowires is also shown to depend on their density and length.