(423a) Dielectrophoretic Assembly of Nanowires in Shear Flows

Fluid flow in conjunction with an electric field has been shown to be a promising approach to precisely assemble nanowires or carbon nanotubes over pre-patterned electrode sites on a substrate [1]. When the flow rate of the suspension of nanowires and the strength of the electric field are tuned correctly, individual nanowires are deposited across each electrode, aligned with the electric field. While the proof of concept is complete, several open questions remain for the practical application of this technique for controlled assembly of nanowires. What is the quantitative relationship between flow and dielectrophoretic forces that lead to assembly? What is the effect of Brownian motion? What is the smallest nanowire that can be assembled in this manner?

To address these questions, we present simulations and analysis to clarify the interplay among dielectrophoresis, hydrodynamics and Brownian motion that determines whether nanowires will successfully assemble for a given set of experimental parameters, including flow rate, potential differences across the electrodes, size of electrodes and nanowires, and electrical properties of the nanowires. It is found that within a finite distance from the substrate, there exists a depletion layer largely void of nanowires (due to hydrodynamics, steric effects and nanowire anisotropy) and a capture zone (due to dielectrophoresis competing with hydrodynamics). Nanowires are captured when the capture width exceeds the depletion width; otherwise they are carried away with the flow. Successful assembly is aided by strong electric fields and low flowrates. Scaling arguments are presented to identify two dimensionless numbers that are ratios of hydrodynamic, Brownian and dielectrophoretic forces and mediate particle dynamics and predict flowrate-voltage combinations for successful assembly. With decreasing nanowire length, increasing diffusion distributes particles more broadly, thereby aiding assembly by moving some nanowires into the region where the electric field is strong enough for capture. However, for very short nanowires, the strength of diffusion eventually becomes limiting and retards assembly. Diffusion in conjunction with the practical necessity to avoid dielectric breakdown of solvent is shown to impose a minimum nanowire length for which assembly is feasible. Depending upon material properties and field frequency, this is about 20 nm for semiconducting nanowires.

1. Freer, E. M.; Grachev, O.; Duan, X.; Martin, S.; Stumbo, D. P. High-yield self-limiting single-nanowire assembly with dielectrophoresis. Nat Nano 5, 525-530, 2010.