(552b) Role of Long-Chain Alkylamines in the Growth of Fivefold-Twinned Cu Nanowires

Fivefold-twinned Cu nanowires (CuNWs) are widely used in various electronic, optical and catalytic applications. Long-chain alkylamine molecules have been employed as structure-directing agents (SDAs) or capping agents in the shape-controlled synthesis of Cu nanocrystals. To understand how tetradecylamine (TDA) might function as an SDA, we study the adsorption of TDA on planar surfaces and CuNW surfaces using a recently-fitted many-body metal-organic force field with molecular-dynamics (MD) simulations run with the LAMMPS code.

Our MD simulations and energy minimizations of various possible Cu nanowire seeds show that CuNWs grow from fivefold-twinned seeds, which contain {111}“notches” at each twin boundary of the {100} side facets, and {110} steps at the between {111} and{100} facets. Except for the {110} facets, this structure resembles the Marks decahedron. Our calculations indicate that this structure persists as seeds grow to nanowires.

To understand how TDA adsorbs on nanowires, we simulated various TDA adsorption patterns on Cu surfaces and obtain the surface energy as a function of the chemical potential of solution-phase TDA. For a given facet orientation [Cu(100), Cu(111) or Cu(110)], we identify the surface structures with minimum free energy. We achieve similar dense TDA packing on each facet and note that such dense packings are sufficient to prevent water from adsorbing on the facets.

To understand the role of corners, edges, etc. in the growth of CuNW, we investigate the packing of aqueous solution-phase TDA molecules on fivefold-twinned nanowire seeds. On each facet, the coverage of TDA molecules on the CuNW surfaces is lower than that on the flat surface. This can be understood by the tendency of TDA of form a “shell” around the facet edges and corners of the CuNW, which requires TDA molecules to bend and flex. Even at a lower coverage, TDA capping agents can effectively shield water from approaching the CuNW surfaces, which is analogous to our observation with perfect Cu surfaces.