Electrochemistry and self-assembly of complex single-walled carbon nanotube (SWNT) nanostructures
Duque, Juan G.
Doctor of Philosophy
This dissertation explores the optical and electrochemical properties of surfactant stabilized and individually---suspended SWNTs in aqueous media via application of various external stimuli. Resulting effects such as nanostructure formation, photocurrent generation, and inherent nanotube electronic and optical properties are then analyzed. The goal is to engineer SWNT systems which can be tuned by understanding the mechanism of the electrochemical and environmental reactions so that applications in nanophotonics, photovoltaics, and electronics can be effectively exploited. A strategy to obtain a surfactant---polymer protective "shell" that improves the stability and luminescence signal of individual SWNTs is presented. We used literature evidence of emission shifts to understand the interactions between polymers and surfactants and show how morphological changes induced by extrinsic factors distort the SWNT luminescence. We developed an in-situ polymerization which creates an outer shell around the SWNT micelle that resulted in suspensions with stable luminescence at all pH, in saline buffers, and on the surface of living cells. Single molecule imaging and time resolved spectroscopy of individual (6,5) SWNTs demonstrated that SWNT luminescence depends strongly on intrinsic and extrinsic factors such as sample preparation, sample inhomogeneities, defects, and tube synthesis conditions. Moreover, we found compelling spectroscopic evidence of substantial differences in chirality distribution and luminescence properties within HiPco batches. Nanoparticle-nanotube Structures (nanoPaNTs) were fabricated by exploiting the electrochemical properties of SWNTs upon activation with alternating electromagnetic fields. The incident field polarizes the SWNTs at the ends, antenna-like-behavior, which readily drives electrochemical reactions. This process is shown to activate redox reactions preferentially with metallic SWNT and proceed at or near diffusion-limited rates. Electrochemical photocathodes with optical rectifying antennae were developed from an array of vertically grown CNT forests. We demonstrated rectification of AC signals by associating anionic surfactant molecules around the CNT and charge separation in the optical regime that generates measurable, wavelength dependent, electrical current. We show that charge separation drives redox reactions with transition metal salts in SWNT suspensions. The results of this research provide key information on the interaction between SWNTs and electromagnetic fields and insight into the extrinsic and intrinsic factors that affect the optical properties.
Physical chemistry; Chemical engineering; Optics