Text

theses

One-dimensional materials have attracted growing interest for the past two decades due to their fascinating physical properties. Researchers have focused their investigations on two broad areas: (1) Exploration of the unique physical properties and applications of individual nanowires and nanotubes, and (2) Manipulation of vast ensemble of 1D nanoparticles via various scalable techniques to create macroscopic composites with unique functionality. For individual nanowires, their physical properties can vary significantly from the bulk. Nanowires of the same composition, and even fabricated within the same batch, often exhibit electrical conductivities that can vary by orders of magnitude. Unfortunately, existing electrical-characterization methods are time consuming, making the statistical survey of highly variable samples impractical. We propose and demonstrate a contactless, solution-based method to efficiently measure the electrical conductivity of 1D nanomaterials based on their transient alignment behavior in AC electric fields of different frequencies. Comparison with direct transport measurements shows that this new technique, electro-orientation spectroscopy (EOS), can quantitatively measure nanowire conductivity over a 6-order-of-magnitude range, 10^(-5)-10 Ω^(-1) m^(-1). With the new EOS method, we statistically characterize the conductivity of a variety of nanowires, and find significant variability in those even from the same wafer. We further integrate this technique into a microfluidic device and automate the electrical-characterization process to enable continuous-flow measurement of the electrical conductivity of an individual nanowire in less than a minute. We make a proof-of-concept demonstration of conductivity-based sorting, as a first step towards enabling the fabrication of functional nanodevices from post-growth sorted and assembled nanowires. For the efficient manipulations of a vast ensemble of 1D nanomaterials, we develop another solution-based, electric-field-assisted approach as a cost-effective and scalable method to produce large-area vertically aligned carbon nanotube (VACNT) composites. Multiwall-carbon nanotubes are dispersed in a polymeric matrix, aligned with an AC electric field, and electrophoretically concentrated to one side of the thin film with a DC component to the electric field. The composite AC + DC field also introduces complex fluid motion associated with AC electro-osmosis and the electrochemistry of the fluid/electrode interface. We experimentally probe the electric-field parameters behind these electrokinetic phenomena, and demonstrate, with suitable choices of processing parameters, the ability to scalably produce large-area composites containing VACNTs at number densities up to 10^10 nanotubes/cm2. This VACNT number density exceeds that of previous electric-field-fabricated composites by an order of magnitude, and the surface-area coverage of the 40 nm VACNTs is comparable to that of chemical-vapor-deposition-grown arrays of smaller-diameter nanotubes.

ETD_6891

Ph.D.

Includes bibliographical references

by Cevat Akin

doi:10.7282/T3W0980W

ETD doctoral

The author owns the copyright to this work.