Controlled structures and properties of single-walled carbon nanotubes custom-produced by chemical vapor deposition method.

Single walled carbon nanotubes (SWNT) are considered as one of the most promising nanomaterials for a large variety of applications that require SWNTs with controlled structures and properties, which is the main focus of this dissertation. The first approach to tackle this problem is to develop appropriate methods to synthesize SWNTs of controlled structure. To achieve this goal, a number of techniques have been developed to selectively grow SWNTs on different support from porous silica to flat substrates. It is demonstrated that a precise control over chirality, diameter and bundle size can be obtained by tuning the reaction temperature in the growth of SWNT over Co-Mo/silica powder by CO disproportionation. In addition, a novel method for selective growth of SWNT on flat substrates has been developed. In this method, SWNTs can be grown either in random direction or vertical alignment on the surface under standard CoMoCATRTM reaction conditions. The second trust of this dissertation is to investigate the properties of as-produced SWNTS with their controlled structural parameters (i.e., diameter, bundle size, chirality, and alignment). Field emission measurements have been conducted to evaluate the dependence of the emission characteristics on the SWNT structure. For the nanotubes grown on flat substrates, the response of the vertically aligned SWNT to polarization of both X-rays (in XANES) and visible light (in Raman) clearly revealed the anisotropic optical properties of V-SWNT. Finally, efforts have been made to explore the growth mechanism of VSWNT on flat substrate. X-ray photoelectron spectroscopy and atomic force microscopy conducted on the flat surface with deposited catalyst gave detailed information about the chemical status of Co-Mo catalyst and their morphological distribution. The evolution of the growth of VSWNT with time was visualized by scanning electron the chemical status of Co-Mo catalyst and their morphological distribution. The evolution of the growth of VSWNT with time was visualized by scanning electron microscopy and clearly demonstrated a two-step process involving the formation of a crust layer followed by a concerted growth constrained by crust. Then a kinetic study with fitted growth data has been derived and the maximum growth rate estimated (i.e. 12.5 nm/sec). In addition to the growth of VSWNT, oxidation and transferring of VSWNT has been investigated for future applications

Polyacrylonitrile/carbon nanotube composite fibers: reinforcement efficiency and carbonization studies

Polyacrylonitrile (PAN)/carbon nanotube (CNT) composite fibers were made using various processing methods such as conventional solution spinning, gel spinning, and bi-component gel spinning. The detailed characterization exhibited that the smaller and longer CNT will reinforce polymer matrix mostly in tensile strength and modulus, respectively. Gel spinning combined with CNT also showed the promising potential of PAN/CNT composite fiber as precursor fiber of the next generation carbon fiber. High resolution transmission electron microscopy showed the highly ordered PAN crystal layer on the CNT, which attributed to the enhanced physical properties. The subsequent carbonization study revealed that carbonized PAN/CNT fibers have at least 50% higher tensile strength and modulus as compared to those of carbonized PAN fibers. Electrical conductivity of CNT containing carbon fiber was also 50% higher than that of carbonized PAN fiber. In order to have carbon fiber with high tensile strength, the smaller diameter precursor fiber is preferable. Bi-component gel spinning produced 1-2 µm precursor fiber, resulting in ~1 µm carbon fiber. The tensile strength of the carbonized bi-component fiber (islands fibers) is as high as 6 GPa with tensile modulus of ~500 GPa. Further processing optimization may lead to the next generation carbon fiber.Ph.D.Committee Chair: Satish Kumar; Committee Member: Anselm Griffin; Committee Member: Dong Yao; Committee Member: Naresh Thadhani; Committee Member: Samuel Graha

Carbon nanotubes for ultrafast fibre lasers

Carbon nanotubes (CNTs) possess both remarkable optical properties and high potential for integration in various photonic devices. We overview, here, recent progress in CNT applications in fibre optics putting particular emphasis on fibre lasers. We discuss fabrication and characterisation of different CNTs, development of CNT-based saturable absorbers (CNT-SA), their integration and operation in fibre laser cavities putting emphasis on state-of-the-art fibre lasers, mode locked using CNT-SA. We discuss new design concepts of high-performance ultrafast operation fibre lasers covering ytterbium (Yb), bismuth (Bi), erbium (Er), thulium (Tm) and holmium (Ho)-doped fibre lasers

Highly conductive carbon nanotube fibers for biomedical applications

We developed a wet spinning process for the formation of polymeric fibers with high loading of single walled carbon nanotubes. The dissertation consists of five chapters. In the first chapter, the research goals were formulated and the art and technologies of fiber spinning from carbon nanotubes were critically analyzed. The next three chapters report the original results. Last chapter summarizes all the findings. In the second chapter, we describe a surfactant based method of stabilization of carbon nanotube dispersions. The conditions of stability of nanotube dispersions in aqueous solutions of sodium dodecyl sulfate were analyzed. Using surface tension isotherms, the phase diagram was experimentally constructed. The diagram covers a broad range of nanotube concentrations. The proposed method allowed us to analyze highly concentrated opaque dispersions, which are hard to study using traditional optical techniques. In the third chapter, we explain the process of electrostatic assembling of polyelectrolytes and nanotubes coated with sodium dodecyl sulfate. Taking sodium alginate as an example of a suitable polymer, we successfully wet spun fibers with various carbon nanotube loadings. The maximum concentration of nanotubes in the spun polymer fibers was 23 wt %, which is significantly greater than the percolation limit. It was shown that the Young\u27s modulus of these fibers non-monotonically depends on nanotube concentration. The dependence was explained using Halpin-Tsai and Voigt models. Scanning electron microscope micrographs and resistivity analysis of the fibers suggest that the nanotube-alginate system undergoes a morphological transition from a composite structure of discrete nanotube bundles embedded in an alginate matrix to a complex continuous structure consisting of a nanotube network interwoven into a macro-molecular network of alginate. These nanotube - alginate fibers have unprecedented high flexibility and very high electrical conductivity - similar to semimetals (between germanium and carbon). In the fourth chapter, we report on a method to stabilize single walled carbon nanotube-alginate fibers in aqueous solutions enriched with Na+ and K+ ions through covalent crosslinking of alginate. The unmodified wet spun nanotube-alginate fibers are unstable in electrolyte solutions such as phosphate buffered saline. This instability makes them unsuitable for biomedical applications as biosensor platforms or actuators. Therefore, these fibers were chemically modified through incorporation of covalent crosslinking to provide stability in solutions enriched with Na+ and K+ ions. Nano-pores were also introduced in the chemically modified fibers. We demonstrated that the modified alginate-nanotube fibers are stable in electrolyte solutions and achieve volumetric swelling up to 16 times their original volume in buffer solutions in 10 minutes. Loading the fibers with nanotubes, we achieved much better tensile and compression properties compared to the covalently crosslinked alginate fibers without nanotubes. The chemically modified nanotube-alginate fibers also show instantaneous pH-dependent swelling, promising interesting sensory applications

Metal-Insulator Transition in Doped Single-Wall Carbon Nanotubes

We find strong evidence for a metal-insulator (MI) transition in macroscopic single wall carbon nanotube conductors. This is revealed by systematic measurements of resistivity and transverse magnetoresistance (MR) in the ranges 1.9-300 K and 0-9 Tesla, as a function of p-type redox doping. Strongly H2SO4-doped samples exhibit small negative MR, and the resistivity is low and only weakly temperature dependent. Stepwise de-doping by annealing in vacuum induces a MI transition. Critical behavior is observed near the transition, with ρ(T) obeying power-law temperature dependence, ρ(T) ∝ T -β. In the insulating regime (high annealing temperatures) the ρ(T) behavior ranges from Mott-like 3-dimensional (3D) variable-range hopping (VRH), ρ(T) ∝ exp[(-T0/T)-1/4], to Coulomb-gap (CGVRH) behavior, ρ(T) ∝ exp[(-T0/T)-1/2]. Concurrently, MR(B) becomes positive for large B, exhibiting a minimum at magnetic field Bmin. The temperature dependence of Bmin can be characterized by Bmin(T) = Bc(1 - T/Tc) for a large number of samples prepared by different methods. Below a sample-dependent crossover temperature Tc, MR(B) is positive for all B. The observed changes in transport properties are explained by the effect of doping on semiconducting SWNTs and tube-tube coupling