(368f) Diffusion Growth Mechanism of Penta-Twinned Ag/Cu Nanowires: Multiscale Theory

Penta-twinned metal nanowires are finding applications in many different technologies, ranging from health monitoring and human-machine interfaces to smart fabrics to windows that change color or tint in response to light, provide shielding from electromagnetic interference, and something about heating. These nanowires find applications in catalysis, photothermal desalination, transparent and flexible electronics applications, such as touch screens, and various elements of solar cells.

The penta-twinned nanowires can be produced with high aspect ratios from diverse face-centered cubic (fcc) metals, with {100} as sides and {111} as ends. However, the mechanism of their anisotropic growth is still poorly understood. We develop a model combining deposition and surface diffusion to predict the final aspect ratio of Ag and Cu nanowires. We describe two aspects of nanowire growth, denoted as the seed stage and the wire stage. In the seed stage, the deposition is much slower than the surface diffusion, so that the seed shape only depends on the difference between interfacet diffusion rates. In the wire stage, the nanowires continue to grow longer until the diffusion is slower than deposition.

The structure of nanowire seeds is similar to Marks decahedra. In the uncapped Ag case, the diffusion from {100} to {111} is comparable to the opposite and both inter-facet diffusion processes are faster than deposition in seed stage. The result shows that the uncapped Ag seeds could lead to three kinds of products with different strucutures as nanowires, decahedron particles and giant decahedron seeds. For the nanowire products, our predictions well match the experimental results. In the uncapped Cu case, we found out that the diffusion from {100} to {111} is slower than the opposite, which impies that the seeds would prefer to grow to large decahedron particles. In experiments, the Cu nanowires were always synthesized with some surfactants. In the case of Cu seeds with Cl on the surface, the relationship between the inter-facet diffusion processes are inversed. The diffusion from {100} to {111} is the fastest and the diffussion from {111} to {100} is even slower than the deposition, which means that the Cu seeds with Cl would prefer to grow to wire products. Based on our model, the predictions are able to represent the average sizes from experimental results.