Modulating the Growth Rate, Aspect Ratio, and Yield of Copper Nanowires with Alkylamines

This article shows how the chain length of alkylamine capping agents and the corresponding stability of their self-assembled monolayers on a Cu surface determines the growth rate, yield, and dimensions of Cu nanowires produced in a solution-phase synthesis. Of the 10 linear alkylamines that were tested, only those with 12 or more carbon atoms induced growth of nanowires. The length, yield, and growth rate of nanowires were larger for shorter alkylamines. As the Cu nanowire growth rates were up to 1050 times smaller than the calculated diffusion-limited growth rates—and the alkylamine chain length had no significant effect on the in situ generation of the reducing agent—we conclude the rate of alkylamine-mediated Cu nanowire growth is limited by charge transfer. Electrochemical measurements indicate longer alkylamines form more effective passivation layers that greatly decrease the rate at which Cu–alkylamine complexes are reduced onto a Cu surface. Molecular dynamics simulations show that the energy required for removal of octadecylamine from a self-assembled monolayer on the Cu surface is much larger (3.59 eV) than for removal of tetradecylamine (2.06 eV). Thus, the more stable self-assembled monolayer formed by longer-chain alkylamines leads to greater inhibition of Cu addition, slower growth, reduced yield, and reduced nanowire aspect ratio.