461623 Dielectrophoretic Assembly of Nanowires in Shear Flows
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.
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