Single Walled Carbon Nanotube Networks as Transparent Conductors

• Main Author: Garrett, Matthew Patrick
• Format: Others
• Published: Trace: Tennessee Research and Creative Exchange 2009
• Subjects: Physics
• Online Access: http://trace.tennessee.edu/utk_graddiss/50

A variety of technological applications depend on transparent conducting films, and carbon nanotubes have the properties required to serve in that role. Single-walled carbon nanotube networks have been studied as transparent conductors in order to understand and optimize their electrical and optical properties. Nanotube films are complex networks of semiconducting and metallic nanotubes, bundled and branched in multifarious directions, with different strength connections between bundles. Chemical modification of nanotubes and inclusion of non-nanotube material can further alter network properties. Separating the contributions of all aspects of the network is a necessary but daunting task in order to optimize nanotube films. To understand and optimize film conductivity, the effect of sonication on nanotubes in several dispersants was examined. Film transmittance and resistance was found to depend on the method of dispersion of the starting material, and the characteristics of a good dispersant were outlined. A purification method was developed specifically for SWNT transparent conductors, eliminating the problems of more complicated purification procedures which were developed for general purification. Our single-step centrifugation purification procedure was shown to achieve similar yield and purity films as multi-step acid-oxidation purification. The films were doped, lowering network conductivity by a multiplicative factor, dependent on the method of dispersion and dopant. A linear relationship was found between the intensity of absorbance spectrum’s first semiconducting transition and the change in film conductivity upon doping. The contributions of doping to bundle and junction conductivity have been separated by impedance spectroscopy modeling. Networks of transmittances up to 98%T have been prepared to determine the lower limit of conductivity for a SWNT film. It was shown that the percolation threshold of the networks depend on nanotube purity and dispersant. A model has been proposed which not only determines the percolation threshold of a network, but also describes the distribution of conductivities that arise from the variety of nanotube bundles and junctions. Films have also been incorporated into functioning photovoltaic and LED devices, proving the effectiveness of these networks as transparent electrodes. This demonstrates the necessity of understanding network behavior and of developing methods for producing films for technological application.