(334cb) Seseed

Metal nanocrystals are promising in a broad range of novel applications. They can bear 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 structure-directing agents (SDAs) in the shape-controlled synthesis of Cu nanocrystals in aqueous solutions. To understand how tetradecylamine (TDA) might function as an SDA in the growth of CuNWs, we study the adsorption of solution-phase TDA around a small and curved 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 the planar Cu surfaces. Instead, TDA molecules form a distinct “bilayer” around the seed: the surface molecules (inner layer) tend to stand normal to the Cu surfaces while TDA closer to the aqueous phase (outer layer) is prone to be parallel to the surface. The 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 resembles a good agreement with NMR measurement from experimental works. Compare 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 TDA in the out layer, and then reach the Cu surface through the exchange with the inner layer TDA.

The significance of corners and edges, which is dominated by the size of the nanoseed, determines if TDA could form SAM or a bilayer around the Cu surfaces. To mind the gap between an infinite planar surface and a small and curved nanoseed, we create stepped Cu surfaces, which combine the features of a planar surface and a curved seed. The extent to which feature can be tuned by the dimensions of 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.