Even changing one atom in nanoscale materials is expected to alter their properties due to their small physical sizes. Such sensitivity can be utilized to modify materials' properties from bottom up and is essential for the utility of nanoscale materials. As such, the impact of extrinsic atomic adsorbates was measured on pristine graphene and a network of carbon nanotubes using atomic hydrogen, cesium atoms, and dye molecules. In order to further quantify such an atomic influence, the resistance induced by a single potassium atom on metallic and semiconducting carbon nanotubes was measured for the first time. Carbon nanotubes are sensitive to adsorbates due to their high surface-to-volume ratio. The resistance arising from the presence of extrinsic impurity atoms depends on the types of nanotubes. Metallic carbon nanotubes are resilient to a long-ranged, Coulomb-like potential, whereas semiconducting carbon nanotubes are susceptible to these impurities. The difference in the scattering strength originates from the chirality of carbon nanotubes, which defines their unique electronic properties. This difference had not directly measured experimentally because of the issue of contact resistance, the difficulty of chirality identification, and the uncertainty in the number of impurity atoms introduced on carbon nanotubes. We synthesized atomically clean, long ( > 100 ?m) carbon nanotubes, and their chirality was identified by Rayleigh scattering spectroscopy. We introduced potassium atoms on the nanotubes to impose a long-range, Coulomb potential and measured the change in resistivity, excluding the contact resistance, by plotting the resistance as a function of the carbon nanotube length. The flux of potassium atoms coming onto the nanotubes was monitored by quartz crystal microbalance, and the scattering strength of a single potassium atom was deduced from the change in resistivity and the density of potassium atoms on the nanotubes. We found that the scattering strength of potassium atoms on semiconducting nanotubes depends on the charge carrier type (holes or electrons). Metallic nanotubes were found to be less affected by the presence of potassium atoms than semiconducting nanotubes, but the scattering strength showed a large dependence on Fermi energy. These experimental results were compared to theoretical simulations, and we found a good agreement with the experiments. Our findings provide crucial information for the application of carbon nanotubes for electronic devices, such as transistors and sensors.

Safe, lightweight, and cost-effective materials are required to practically store hydrogen for use in portable fuel cell applications. Compressed hydrogen and on-board hydrocarbon reforming present certain advantages, but their limitations must ultimately render them insufficient. Storage in hydrides and adsorption systems show promise in modeling and experimentation, but a practical medium remains unavailable. Since the earliest report of adsorption on single-walled carbon nanotubes (SWNT) in 1997, a number of controversial publications have claimed the hydrogen capacity of these materials to be between 0.1 to 10 wt. %. However, no study has yet demonstrated a plateau of adsorption with pressure that would verify the reported capacity. A volumetric adsorption measurement instrument was designed and constructed to resolve this controversy. The instrument is capable of degassing samples under high vacuum and offers unprecedented measurements of hydrogen storage up to a pressure of 300 atm and a broad range of temperatures. In addition, an electrical probe within the sample cell was designed to study the mechanism of adsorption in situ. The best hydrogen storage observed on bundles of purified SWNT was 1.6 wt. % at 264 atm and 200 K. At room temperature, a high-pressure plateau was found corresponding to an adsorption of 0.9 wt. % at a pressure of 300 atm, which equates to an adsorption to surface area ratio of 1.14 wt. %/l 000 m2/g. Contrary to the claim by the Caltech Group [Ye et al., 1999], resistance measurements of purified SWNT bundles revealed that bundles do not separate under high pressure. Instead, the bundles were found to compress under the action of external pressure, leading to an increase in conductivity with pressure. A simple geometrical model suggests that without this bundle separation the volume displaced by the sample may counteract the benefit gained by adsorption because of the increase in gas density at high pressure. The isosteric heat of adsorption on SWNT bundles was measured to be between 3.9 and 5.0 kJ/mol at low levels of adsorption, and the activation energy for adsorption determined by the Langmuir model was found to be 1.9 kJ/mol. These low energy parameters are indicative of weak physisorption. / Thesis / Master of Applied Science (MASc)

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Latently-reactive conjugated polymer-coated single-walled carbon nanotubes / Single-walled carbon nanotubes (SWNTs) are intensely investigated nanomaterials that exhibit intriguing physical and optoelectronic properties. Although SWNTs are highly regarded in terms of their potential societal impact, commercialization of SWNT applications has been dampened by the difficulty in SWNT processability and purification. Current commercially viable carbon nanotube syntheses produce complex mixtures of metallic and semiconducting SWNTs, as well as amorphous carbon and metal catalyst particles. Furthermore, the ability to decorate carbon nanotube surfaces to modulate their properties is non-trivial, especially if concurrent preservation of optoelectronic properties is desired. To date, the issues of SWNT solubilization, sorting, and functionalization have been approached in a piecemeal fashion. Conjugated polymers, which are macromolecules that possess extended π-systems, have the potential to address all of these issues simultaneously. In my Thesis, I explore conjugated polymer structures to investigate (i) factors that influence dispersion selectivity, and (ii) the decoration of polymer-SWNT complexes by incorporating reactive moieties into the polymer structure. The work presented in this Thesis begins by examining the ability of conjugated polymers to sort SWNTs. To date, the selective dispersion of metallic SWNTs is unrealized. In Chapter 2, I examine the effect of the electronic nature of the conjugated backbone on the selective dispersion of SWNTs by preparing SWNT dispersions pre- and post-methylation of a pyridine-containing conjugated polymer. In doing so, I prepare a series of polymers with identical degrees of polymerization and dispersity (to minimize extraneous selectivity factors) and find that electron rich π-systems disperse only semiconducting SWNTs, while electron poor π-systems disperse relatively more metallic SWNTs. In Chapter 3, I challenge the conventional wisdom that complete backbone conjugation is required to selectively disperse semiconducting SWNTs by introducing non-conjugated linkers into the polymer backbone and demonstrating that nanotube sorting is still possible. I next examine conjugated polymers as tools that can simultaneously sort SWNTs and impart reactivity to the polymer-SWNT complex, while preserving SWNT optoelectronic properties. In Chapter 4, I incorporate azides into polyfluorene side chains and perform solution-phase Strain-Promoted Azide-Alkyne Cycloaddition (SPAAC). I show that the polymer-SWNT complex can be rapidly decorated with strained cyclooctyne derivatives, and that only pre-clicked polymer enables for sorting of semiconducting SWNTs. The sorted SWNT population can then be made water soluble post-SPAAC, enabling for the study of SWNT emission in solvents with very different polarity. In Chapter 5, I examine the reactivity of azide-containing polymer-SWNT thin films and show that thin film properties can be drastically altered. Interfacial chemistry enables for the spatially-resolved patterning of a Janus polymer-SWNT thin film containing both hydrophilic and hydrophobic regions. In Chapter 6, I devise a system to perform aqueous solution-phase chemistry on the polymer-SWNT complex. The water soluble polymer-SWNT complex allows for functionalization of the hydrophobic SWNT scaffold with polar and charged molecules. Clicking an acidochromic switch onto the polymer-SWNT surface enables for control over the SWNT emission properties. Lastly, in Chapter 7 I develop a conjugated polymer whose backbone can be functionalized using visible light. The visible-light mediated photoclick coupling of a conjugated polymer backbone enables for rapid polymer modification and is the first example of spatially-resolved conjugated polymer backbone functionalization. / Thesis / Doctor of Philosophy (PhD) / Carbon nanotubes are cylindrical shells of carbon that possess fascinating physical, optical, and electrical properties. Commercial syntheses of carbon nanotubes produce complex mixtures of impure material, and raw carbon nanotube samples further suffer from insolubility. A grand challenge preventing commercialization of carbon nanotube applications is simultaneously solubilizing, sorting, and functionalizing carbon nanotube structures while avoiding damage to the nanotube properties. To date, these issues have been tackled in a piecemeal fashion. In my Thesis, I explore conjugated polymer coatings as a solution to address these problems all at once. I investigate how modifying conjugated polymer structure can (i) influence carbon nanotube purification and (ii) produce latently-reactive polymer-nanotube complexes that can be used to decorate carbon nanotubes without damaging nanotube properties.

Single-walled carbon nanotubes (SWNTs) are a new class of materials that have recently attracted a great deal of interest because of their unique structural, mechanical, and electronic properties. Also, SWNTs have a high potential for a number of technological applications, including molecular electronics, emissive devices, and photovoltaic devices. To fully utilize their unique properties, control of the solubility, processibility, and functionality of SWNTs is required. Therefore chemical functionalization of SWNTs using a variety of methods, in either covalent or noncovalent manner, has been developed to produce soluble nanotube composites coupled with various chemical moieties. To explore the possibility of making potential soluble nanotube-based materials for solar cells, SWNTs were functionalized with organic chromophore-labeled polymers via a radical coupling process. The organic chromophore was used to absorb light to produce photo-induced electrons, while the polymer chains were used for improving the solubility of SWNTs. These novel chromophore-labeled polymers were made by stable free radical polymerization (SFRP), either using a synthetic chromophore-functionalized styrenic monomer or by derivatizing well-defined polystyrenes. Specifically, the chromophores employed in this investigation were commercially available 7-hydroxycoumarin and coumarin-343. In order to carry out fluorescence studies of SWNT-coumarin composites systematically, various factors were probed by (1) altering polystyrene lengths between the SWNT and the coumarin; (2) changing the distribution of coumarins along the polymer chain, in the form of either a block or random copolymer; (3) placing single coumarins on the surface of SWNTs. All of these resulting polymer functionalized SWNTs were found to be soluble in certain organic solvents such as CHCl3. Different absorption behaviors have been observed for SWNTs functionalized with 7-hydroxycoumarin containing copolymers. Fluorescence was still observable for all of these composites, and the pi-pi interactions between coumarins and nanotubes were believed to be responsible for the broadening of emission bands of the resulting composites. / Thesis / Master of Science (MS)

Carbon Nanotubes (CNTs) hold great promise as an important engineering material for future applications. To fully exploit CNTs to their full potential, it is important to characterize their material response and ascertain their material properties. We have used molecular mechanics (MM) simulations to conduct virtual experiments on single-wall and multi-wall carbon nanotubes (SWNTs and MWNTs respectively) similar to those performed in the mechanics of materials laboratory on a continuum structure. The output (energy and deformation rather than the load and deflection) is used to understand the material response and formulate macroscopic constitutive relations. From results of MM simulations of axial and torsional deformations on SWNTs, Young's modulus, the shear modulus and the wall thickness of an equivalent continuum tube made of a linear elastic isotropic material were found. These values were used to compare the response of the continuum tube, modeled as an Euler-Bernoulli beam, in bending and buckling with those obtained from the MM simulations. MM simulations have been carried out to find energetically favorable double-walled carbon nanotube (DWNT) configurations, and analyze their responses to extensional, torsional, radial expansion/contraction, bending, and buckling deformations. Loads were applied either to one wall or simultaneously to both walls of an open-ended DWNT. These results were compared against SWNT results. It was found that for simple tension and torsional deformations, results for a DWNT can be derived from those for its constituent SWNTs within 3% error. Radial deformations of a SWNT were achieved by considering a DWNT with the SWNT as one of its walls and moving radially through the same distance all atoms of the other wall of the DWNT thereby causing a pseudo-pressure through changes in the cumulative van der Waals forces which deform the desired wall. Results of radial expansion/contraction of a SWNT were used to deduce an expression for the van der Waals forces, and find through-the-thickness elastic moduli (Young's modulus in the radial direction, Er, and Poisson's ratio ?r?) of the SWNT. We have found four out of the five elastic constants of a SWNT taken to be transversely isotropic about a radial line. MWNTs were studied using the same testing procedures as those used SWNTs. Based on the results from those simulations a continuum model is proposed for a MWNT whose response to mechanical deformations is the same as that of the MWNT. The continuum structure is comprised of concentric cylindrical tubes interconnected by truss elements. Young's modulus, Poisson's ratio, the thickness of each concentric tube, and the stiffness of the truss elements are given. The proposed continuum model is validated by studying its bending and buckling deformations and comparing these results to those from MM simulations. The major contributions to the field on nanotubes and the scientific literature is a simple and robust continuum model for nanotubes. This model can be used to study both SWNTs and MWNTs in either global or local responses by applying different analytic techniques. This model was developed using a consistent engineering methodology that mimicked traditional engineering testing, assumptions and constraints. / Ph. D.

The overall research theme of this dissertation was the study of nanoparticle/polymer composites. Two types of nanoparticles were utilized: Single-Walled Carbon Nanotubes and quantum dots. Chapter 1 of this thesis comprises an extensive literature review on Carbon Nanotubes, which presents theoretical aspects relevant to the structure and properties of CNTs, methods of purifying and solubilizing CNTs in aqueous and organic solvents and selected applications. This literature review is followed by the study and comparison of the optical limiting performances of different Single-Walled Carbon Nanotubes/conjugated polymer dispersions (Chapter 2). The results obtained are discussed in terms of dispersion of the SWNTs in the polymer solutions and resulting SWNT bundle diameters. Chapter 3 presents the spontaneous assembly of dendrimer patterns induced by SWNTs. Finally, chapter 4 presents a new method for fabricating quantum dot/polymer composites, which uses the extraction of positively charged quantum dot into a hydrophobic liquid. The resulting solution is used as a compatible polymerization medium for poly(methylmethacrylate ) networks enabling the formation of transparent and fluorescent composites. / Ph. D.

The solution rheological and electrospinning behavior of a series of charge-containing polymers, surface-active agents, and carbon nanotube composites was studied to investigate the effect of intermolecular interactions, including electrostatic interactions, hydrogen bonding, surface activity, and surface functionalization of carbon nanotubes. The synthesis of novel polyelectrolytes with varied topologies, charge content, and counterions tailored the charged macromolecules to elucidate structure-rheology and structure-processing relationships. In addition, the use of additives for electrospinning, including surfactants and nanofillers, allows us to tailor the functionality of electrospun nanofibers for high-performance applications. Novel polyelectrolytes based on poly(2-(N,N-dimethyl)aminoethyl methacrylate) (DMAEMA) were synthesized with the counteranions Cl-, NO3-, (CN)2N-, BF4-, PF6-, triflate (TfO-), and bis(trifluoromethanesulfonyl)imide (Tf2N-). The counteranion selection controlled the thermal transitions and degradation; the larger and more charge-delocalized anions typically resulted in lower Tg and higher decomposition temperature. The polyelectrolyte behavior in solution was nearly independent of anion choice, though solution conductivity depended on the electrophoretic mobility of the counterion. Charge containing copolymers of DMAEMA and di(ethylene glycol) methyl ether methacrylate (MEO2MA) were synthesized and demonstrated that polyelectrolyte behavior in solution was also nearly independent of charge content. Low ionic contents resulted in extended solution conformations and high conductivities. Controlled atom-transfer radical polymerization allowed the synthesis of star-shaped polyelectrolytes with varying arm numbers and lengths. The solution behavior of the stars deviated slightly from the linear polyelectrolytes due to significant counterion condensation within the star core and constrained polymer conformations. The linear and star-shaped polyelectrolytes were electrospun to understand the interplay between polyelectrolyte structure and electrospinnability. Similar to other strong polyelectrolytes described in the literature, PDMAEMA-based polyelectrolytes with polar anions (e.g. Cl-) experienced significant instabilities during electrospinning, requiring high concentrations and viscosities to stabilize the electrospinning jet. The use of large, more hydrophobic anions (BF4-, TfO-) led to increased electrospinnability. Unlike neutral branched polymers, which electrospin nearly identically to linear polymers of similar molecular weight, the star-shaped PDMAEMA-based polyelectrolytes required even higher viscosities than linear polyelectrolytes for stable electrospinning. The correlations between electrospinnability and solution rheological analysis are detailed. The use of surfactants facilitates the electrospinning of neutral polymers at lower concentrations. However, we have demonstrated that specific cylindrical aggregates of surfactants (wormlike micelles) can be electrospun into microfibers under the proper conditions. Ammonium and phospholipids surfactants as well as organogelators were studied using solution rheology and DLS to determine the effects of micellar structure and solution viscosity on the electrospinnability of low molar mass surfactants. In addition, the effects of charged and uncharged surfactants on the electrospinning behavior of poly(methyl methacrylate) were determined. Added surfactant facilitated uniform fiber formation at lower PMMA concentrations. XPS analysis demonstrated the formation of core-shell fibrous structures resulting from the self-migration of surfactants to the fiber surface. Hydrogen bonding also influences fiber formation through electrospinning. Star-shaped poly(D,L-lactide)s (PDLLAs) were end-functionalized with adenine (A) or thymine (T) units. The complementary hydrogen bonding between the adenine and thymine lead to thermoresponsive rheological behavior for mixtures of PDLLA-A and PDLLA-T. The mixtures could be electrospun above the hydrogen bond dissociation temperature and resulted in thicker fibers compared to unfunctionalized PDLLA stars. The hydrogen bonding allows the preparation of polymers with a combination desirable solid-state properties and very low processing viscosities. The effects of carbon nanotube incorporation on electrospinning behavior and fiber morphology were also investigated. Nonfuntionalized and carboxylic-acid functionalized carbon nanotubes were electrospun into polyurethane nanofibers. The nonfunctionalized nanotubes required high-shear melt mixing to disperse within the polyurethane, but remained well dispersed through electrospinning. The surface functionalization with acid groups produced nanotubes which dispersed more readily into the polyurethane solutions. TEM analysis revealed that nanotube dispersion and alignment within the nanofibers was similar for both nonfunctionalized and acid-functionalized nanotubes. / Ph. D.