(105b) Self-Assembly of Long-Chain Alkylamines in the Growth of Fivefold-Twinned Cu Nanoseed

Metal nanocrystals are promising in a broad range of novel applications. They can take on isotropic shapes, such as nanocubes, octahedra, icosahedra, as well as anisotropic shapes such as nanorods and nanobars. Metal nanowires originate from anisotropic growth, and their aspect ratio can reach ∼1000. Fivefold-twinned Cu nanowires (CuNWs) are widely used in electronic, optical, and catalytic applications. Long-chain alkylamine molecules have been employed as capping agents in the shape-controlled synthesis of Cu nanocrystals in aqueous solutions. To understand how tetradecylamine (TDA) might function as a capping agent in the growth of CuNWs, we study the adsorption of solution-phase TDA around a small Cu nanoseed using a many-body metal-organic force field with molecular-dynamics (MD) simulations run with the LAMMPS code.

Given sufficient TDA, the density of TDA molecules on the Cu nanoseed surfaces is lower than that of the TDA self-assembled monolayer (SAM) on a planar Cu surface. In addition to a diffuse near-surface layer, in which molecules tend to orient normal to the Cu surfaces, the excess TDA forms an outer layer, in which TDA is prone to be parallel to the surface. This bilayer act as a dense “web” to protect the Cu surfaces against the solution phase. The relatively weak TDA alky-tail interactions around the curved nanoseed allow the exchange of TDA molecules within the bilayer, while the TDA population in each layer remains dynamically stable. By fitting the exchanging probabilities into Poisson forms, we obtained the diffusion coefficient of TDA, which is in good agreement with an experimental NMR measurement. Compared to the TDA SAM on the planar Cu surfaces, the TDA exchanges much more rapidly around the small nanoseed. This exchange could serve as a special mechanism for the growth of nanowire: the solution-phase Cu²⁺ complexes first attach to the outer layer, and then reach the Cu surface through the exchange with the inner layer TDA.

The significance of corners and edges, which are dominant in the nanoseed, determines if TDA forms a SAM or a bilayer around the Cu surfaces. To reproduce features in both an infinite planar surface and a small nanoseed, we create stepped Cu surfaces. We can reproduce features of seeds and planar surfaces by tuning the dimensions of the stepped surfaces. As expected, we found TDA can form SAM on the stepped surface when the effect of the planar surface overwhelms that of the edges and corners. In contrast, when the planar surface is relatively smaller, the effect of edge and corners dominates. Part of the TDA molecules escape from the SAM to form a second layer. Interestingly, with a long but thin step to reproduce a CuNW, we observe SAM formation indicating that relatively thin CuNW are well protected by alkylamine SAMS.