Dispersion of Single-Walled Carbon Nanotubes in Organic Solvents

This thesis contains a systematic study of the dispersion of pristine HiPco Single Walled Carbon Nanotubes (SWNTs) in a series of organic solvents. A double beamed UV-Vis-NIR absorption spectrometer coupled with an integrating sphere was employed to demonstrate the dispersibility of SWNTs in different solvents. Raman Spectroscopy and Atomic Force Microscopy (AFM) were used to confirm the debundling and exfoliation of SWNTs aggregates. An investigation of the solubility of SWNTs in four chlorinated aromatic solvents demonstrated that the similarity in structure between solvent molecules and nanotube sidewall is not a dominant factor to obtain stable SWNT solutions. A comparative study of the solubility of SWNTs between the aromatic solvents and other reported solvents was then conducted, in terms of the solvent solubility parameters, including Hildebrand and Hansen solubility parameters. Although the established correlation between extinction/absorption coefficients as a function of Hildebrand/Hansen solubility parameters indicated there may be a selective debundling of metallic and semiconducting SWNTs in different solvents, this was not confirmed by a detailed Raman investigation. A further study of the dispersion limit of SWNTs in different solvents as a function of the solvent solubility parameters was carried out. Good agreement with literature is demonstrated here in terms of Hildebrand parameters, but not in terms of the Hansen solubility parameters. It has been demonstrated that the degree of dispersion is critically dependent on sample preparation conditions, in particular sonication. Finally, the effect of sonication parameters and solvent properties during the dispersion of SWNTs was investigated. The results indicated that the sonication process is closely dependent on many of the physical parameters of the solvent, including vapour pressure, viscosity, surface tension, density and molecular weight. Longer sonication time and higher sonication power help debundling SWNTs in organic solvents but significantly damage the nanotubes. The choice of solvent should be guided by minimisation of sonication requirements

Atomic and electronic structure studies of nano-structured systems : Carbon and related materials

Modeling in the framework of density functional theory has been conducted on carbon nanotubes and graphene nano-structures. The results have been extended to non-carbon systems such as boron nanostructures. Computational studies are complemented by experimental methods to refine the structural models and obtain a better understanding of the electronic structure. It is observed that the zigzag edged bilayered graphene nanoribbons are highly unstable as compared to their armchair counterparts. A novel approach has been proposed for the patterning of chirality/diameter controlled single walled carbon nanotubes. Nanotube formation is found to be assisted by edge ripples along with the intrinsic edge reactivity of different types of bilayered GNRs. The effect of bundling on the electronic structure of single walled carbon nanotubes in zigzag single walled carbon nanotubes has been studied. Hydrostatic pressure effects were examined on bundled single walled carbon nanotubes. Nanotubes with chiral indices (3n + 3, 3n + 3) acquire hexagonal cross-sections on application of hydrostatic pressures. The formation of a novel quasi two-dimensional phase of carbon during hydrostatic compression of small and large nanotubes under extreme conditions of pressure is modeled and is understood to be dictated by breaking of symmetry during compression. Nanoscale materials with anisotropic compressibility do not exhibit symmetric compression as in bulk materials. Structural stability of boron nanoribbons derived from \u27α-sheet\u27 and reconstructed {1221} sheets was studied. Antiaromatic instabilities were found to destabilize nanoribbons constructed from reconstructed {1221} sheets when compared to those obtained from the \u27α-sheet\u27. The stability of the nanoribbons was found to increase with increasing width and increase in the hole density (η) of the boron nanoribbons. The study of electronic structure reveals the presence of semiconducting nanostructures. The presence of nanoscale crystalline domains due to random functionalization has made it difficult to resolve the chemical structure of graphene oxide and it remains a much debated topic to date. A combination of analytical, spectroscopic and density functional techniques have been used to determine the structure and properties of such nano materials. Graphene oxide has unusual exotic properties and belongs to this class of materials. Investigations reveal that the chemical structure of graphene oxide can be visualized as puckered graphene sheets linked by oxygen atoms. Density functional theory has been used to calculate the site projected partial density of states for carbon and oxygen atoms involved in different types of bonding. A comparison of these simulations with carbon and oxygen K-edge absorption spectra has led to an understanding of the basic electronic structure of this material

Terahertz Spectroscopy of Charge-Carrier Dynamics in One-Dimensional Nanomaterials

One-dimensional (1D) nanomaterials are of great importance for a number of potential applications. However, in order to realize this potential a thorough understanding of the charge-carrier dynamics in these materials is required, since these largely determine the optoelectronic properties of the materials in question. This thesis investigates the charge-carrier dynamics of two 1D nanomaterials, single-walled carbon nanotubes (CNTs) and tungsten-oxide nanowires (WOxNWs), with the goal of better understanding the nature of their optoelectronic responses, and how nanomaterial geometry and morphology influence these responses. We do this using terahertz time-domain spectroscopy (THz-TDS) and optical pump - terahertz probe time-domain spectroscopy (OPTP). Firstly, we discuss how to properly analyse and interpret the data obtained from these experiments when measuring 1D nanomaterials. While the data obtained from THz-TDS is fairly straight-forward to analyse, OPTP experimental data can be far from trivial. Depending on the relative size of the sample geometry compared to the probe wavelength, various approximations can be used to simplify the extraction of their ultrafast response. We present a general method, based on the transfer matrix method, for evaluating the applicability of these approximations for a given multilayer structure, and show the limitations of the most commonly used approximations. We find that these approximations are only valid in extreme cases where the thickness of the sample is several orders of magnitude smaller or larger than the wavelength, which highlight the danger originating from improper use of these approximations. We then move on to investigate how the charge-carrier dynamics of our CNTs is influenced by nanotube length and density. This is done through studying the nature of the broad THz resonance observed in finite-length CNTs, and how the nanotube length and density affects this resonance. We do this by measuring the conductivity spectra of thin films comprising bundled CNTs of different average lengths in the frequency range 0.3-1000 THz and temperature interval 10-530 K. From this we show that the observed temperature-induced changes in the terahertz conductivity spectra depend strongly on the average CNT length, with a conductivity around 1 THz that increases/decreases as the temperature increases for short/long tubes. This behaviour originates from the temperature dependence of the electron scattering rate, which results in a subsequent broadening of the observed THz conductivity peak at higher temperatures and a shift to lower frequencies for increasing CNT length. Finally, we show that the change in conductivity with temperature depends not only on tube length, but also varies with tube density. We record the effective conductivities of composite films comprising mixtures of WS2 nanotubes and CNTs vs CNT density for frequencies in the range 0.3-1 THz, finding that the conductivity increases/decreases for low/high density films as the temperature increases. This effect arises due to the density dependence of the effective length of conducting pathways in the composite films, which again leads to a shift and temperature dependent broadening of the THz conductivity peak. Next, we investigate the conflicting reports regarding the ultrafast photoconductive response of films of CNTs, which apparently exhibit photoconductivities that can vastly differ, even in sign. Here we observe explicitly that the THz photoconductivity of CNT films is a highly variable quantity which correlates with the length of the CNTs, while the specific type of CNT has little influence. Moreover, by comparing the photo-induced change in THz conductivity with heat-induced changes, we show that both occur primarily due to heat-generated modification of the Drude electron relaxation rate, resulting in a broadening of the plasmonic resonance present in finite-length metallic and doped semiconducting CNTs. This clarifies the nature of the photo-response of CNT films and demonstrates the need to carefully consider the geometry of the CNTs, specifically the length, when considering them for application in optoelectronic devices. We then move on to consider our WOxNWs. We measure the terahertz conductivity and photoconductivity spectra of thin films compromising tungsten-oxide (WOx) nanowires of average diameters 4 nm and 100 nm, and oxygen deficiencies WO2.72 and WO3 using THz-TDS and OPTP. From this we present the first experimental evidence of a metal-to-insulator transition in WOx nanowires, which occurs when the oxygen content is increased from x=2.72->3 and manifests itself as a massive drop in the THz conductivity due to a shift in the Fermi level from the conduction band down into the bandgap. Furthermore we present the first experimental measurements of the photoexcited charge-carrier dynamics of WOx nanowires on a picosecond timescale and map the influence of oxygen-content and nanowire diameter. From this we show that the decay-dynamics of the nanowires is characterized by a fast decay of <1 ps, followed by slow decay of 3-10 ps, which we attribute to saturable carrier trapping at the surface of the nanowires.European Union’s Seventh Framework Programme for research, technological development and demonstratio

Functional carbon nanotubes for electrical conductors

Carbon nanotube (CNT) conductors are an enabling technology for advancing the efficacy of sustainable energy systems. In parallel, proactive consideration for each of the phases in the material life cycle can enhance device performance while minimizing unwanted impacts. Increasing the yield of CNTs through advances in synthesis will help reduce the electricity, chemicals, and costs associated with their production. Modifications to the nanoscale morphology (alignment, bundling, density and lower contact resistances) are needed to improve the CNT material properties to meet or exceed those of conventional metallic conductors. Also, a robust evaluation of methods for contacting carbon-based wires is needed when interfacing with metallic contacts. Finally, it\u27s important to begin looking at upstream options for proper treatment of waste streams containing CNT conductors when they reach the end of their useable life. Therefore, the subject of this dissertation focuses on the development of functional CNT conductors and considers approaches to improve each phase of their life cycle. Specifically, progress towards using more efficient catalysts in the laser vaporization process has led to a 50% increase in SWCNT yield and simplified the purification procedure. The use of chemical dopants such as KAuCl4 has increased the electrical conductivity up to 1x106 S/m which is over an order of magnitude higher than the pre-doping baseline value. Alternatively, chlorosulfonic acid was used to disperse high weight loadings of SWCNTs and modify the nanoscale morphology through the use of selective coagulation and mechanical extrusions of binder free SWCNT wires. The highly dense and aligned wires have electrical conductivities as high as 4.9x106 S/m and are in agreement with the highest CNT conductivities reported. The ability to contact bulk CNT conductors through ultrasonic welding was demonstrated for the first time and exhibit low carbon-copper contact resistances of 4.3 mΩ-cm2. Finally, a refunctionalization procedure was developed for upcycling end-of-life CNT electrodes from lithium ion anodes. This is the first reported recycling procedure developed for CNT materials and was successful in reducing the direct electricity consumption by 75 % and the volumetric waste generation by 66 % compared to synthesizing new CNT materials. Overall, CNT based conductors have been enhanced at each point in their life cycle the results presented in this dissertation represent a significant step forward towards manufacturing of next generation carbon conductors

SPECTROSCOPIC SIGNATURE FOR BUNDLING, EDGE STATES AND IMPURITIES IN 1D AND 0D MATERIALS

Study of nanomaterials has gained interest of researchers from various fields of science and technology due to their unique electronic and vibrational properties as compared to their bulk counterparts. In particular, carbon nanotechnology has evolved rapidly over the past few decades and nowadays, carbon nanotubes are used in various fields such as energy storage, electronics etc. However, the quest for new properties of this material is never ending and the invention of graphene generated enormous interest in the scientific community due to its excellent properties such as strength, high electron mobility, thermal conductivity etc. In this thesis, I aim at gaining better understanding of the electronic properties of carbon nanostructures and also discuss the effect of impurities on the vibrational properties of Bismuth nanorods. In the case of SWNTs, I have studied the effect of surrounding environment on their electronic properties, in particular Sub-nm SWNTs. Due to their unique electronic and vibrational properties, single walled carbon nanotubes (SWNTs) with sub-nanometer diameters d ∼ 0.5-0.9 nm have recently gained interest in the carbon community. Using UV-Vis-NIR spectroscopy and ultra-centrifugation, we have conducted a detailed study of the π plasmon energy (present at∼5-7 eV) in sub-nm SWCNTs as a function of the size of the bundle. We find that the energy of the π plasmon peak E varies with the bundle diameter Dh as E = (0.023 eV )∗ln(Dh/do) + 5.3 7 eV, where do = 0.5 nm and corresponds to the smallest tube diameter. This is compared with the same data for HiPCo and Carbolex SWCNTs of larger diameter (1-1.4 nm) confirming a clear dependence of E on the bundle size, which is present in addition to the previously reported dependence of E on SWCNT diameter d. In case of graphene, the carbon atoms at the edges of graphene sheet contribute to its electronic properties. This effect becomes more prominent in confined structures such as graphene nanoribbons (GNRs) and graphene quantum dots (GQDs). In case of GQDs, previous reports showed that they exhibit a strong photoluminescence (PL) in visible region upon excitation. However, currently no experimental evidence is reported for the origin of PL in these quantum dots. In this work, based on a combination of synthesis, annealing and PL measurements of GQDs, carbon nano-onions (CNOs) and GNRs, we found the PL of GQDs to be independent of its suspension medium and the chirality of its edges. Since GQDs can also be understood as a highly conjugated aromatic molecules, for comparison, this study also discusses the PL spectra of aromatic molecules, which collectively with the PL spectra of GQDs, GNRs and carbon nano-onions serve as a basis for future theoretical and experimental studies of PL in carbon nanostructures. Lastly, we studied the effect of impurities on the vibrational properties of Bismuth nanorods. Theoretical calculations predict that Bi should undergo a semimetal-to-semiconductor transition as at least one of its dimensions becomes \u3c 50 nm. This prediction was experimentally confirmed by infrared (IR) absorption spectra, which is largely underlain by transitions between the L (electron) and T (hole) pockets of the Fermi surface. In this work, however, we report that in our nanosize samples, the observed IR peak positions are practically independent of temperature, which is hard to reconcile with the predicted behavior of the L-T transition. To help elucidate the origin of these IR peaks, we performed a careful analysis of the IR spectra of Bi nanorods, as well as those of bulk Bi, Bi samples prepared under different conditions and Bi2(CO3)O2 using Fourier transform infrared and photoacoustic spectroscopy measurements. We propose that the observed IR peaks in Bi nanorods arise from the oxygen-carbon containing secondary phases formed on the surface of Bi rather than from the Bi itself. We believe that secondary phases must be taken into account on a general basis in modeling the IR spectra of Bi and that the scenario that ascribes these IR peaks solely to the L-T transitions may not be correct. The results reported herein may also impact the research of Bi-based thermoelectric nanostructures and bulk material

CHEMICAL FUNCTIONALIZATION OF CARBON NANOTUBES FOR CONTROLLED OPTICAL, ELECTRICAL AND DISPERSION PROPERTIES

A carbon nanotube is a graphitic sheet, rolled into a one-dimensional, hollow tube. This structure provides certain individual nanotubes with high conductivity and near-infrared optical activity. These properties are not necessarily translated at the macroscale, however, due to strong van der Waals attractive forces that determine the behavior at the bulk level - exemplified by aggregation of nanotubes into bundles with significantly attenuated functionality. Different methods of carbon nanotube covalent functionalization are studied to improve dispersion while simultaneously maintaining intrinsic electrical and optical properties. In addition to retention of known behavior, new carbon nanotube photoluminescence pathways are also revealed as a result of this same covalent functionalization strategy. With various wet chemistries, including super-acid oxidation, the Billups-Birch reaction, and various diazonium based reactions, that utilize strong reducing or oxidizing conditions to spontaneously exfoliate aggregated carbon nanotubes, we are able to covalently functionalize individually dispersed nanotubes in a highly scalable manner. Covalent addition to the nanotube sidewalls converts the native sp2 hybridized carbon atoms to sp3 hybridization, which helps disrupt inter-tube van der Waals forces. However, this change in hybridization also perturbs the carbon nanotube electronic structure, resulting in an undesired loss of electrical conductivity and optical activity. We observe that controlling the location of functionalization, such as to the outer-walls of double-walled carbon nanotubes or as discrete functional "bands," we avert the loss of desirable properties by leaving significant tracts of sp2 carbon atoms unperturbed. We also demonstrate that such functional groups can act as electron and hole traps through the creation of a potential well deviation in the carbon nanotube electronic structure. This defect-activated carrier trapping primes the formation of charged excitons (trions) which are observed as redshifted photoluminescence in the near-infrared region. Implications and impacts of these covalent functionalization strategies will be discussed

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