Magister Scientiae - MSc / The spectroscopic, SEM and Raman measurements on carbon nanotubes under the exact conditions of which OES analysis were made showed that at a pressure of 400 Torr and a flow rate of 200 sccm, the quality and quantity of single-walled carbon nanotubes was the highest. / South Africa

The thesis contains two parts. Part 1 describes the investigations on graphene and contains five sections. Section 1, gives a brief overview of graphene and other nanocarbons. The other four sections deal with various aspects of single-layer and few-layer graphene such as functionalization and solubilization, surface properties and gas adsorption, molecular charge transfer interaction and some properties and applications. Section 2 describes covalent and noncovalent functionalization and solubilization of few-layer graphene samples prepared by different methods as well as of single-walled carbon nanotubes (SWNTs). It includes covalent functionalization of graphene with organometallic reagents, noncovalent functionalization of graphene and SWNTs with surfactants as well as large aromatic molecules, and exfoliation of few-layer graphene by a water-soluble coronene carboxylate. Section 3 deals with surface properties and gas adsorption (mainly H2 and CO2) of few-layer graphenes. It is found that graphene samples with high surface area can adsorb even more than 3 wt% of H2 at high pressure which makes it promising material for gas-storage applications. Section 4 describes the molecular charge-transfer interaction of single and few-layered graphenes and SWNTs with different electron-donor and -acceptor molecules probed by both ITC measurements and Raman spectroscopy. Electron–acceptor molecules interact more strongly with graphene and SWNTs than the -donor molecules and nature of interaction of metallic SWNTs are different than the as-prepared ones. A Raman study of the interaction of single-layer graphene, prepared by micromechanical cleavage as well as chemical route, with an electron donor molecule such as tetrathiofulvalene (TTF) and an electron acceptor molecule such as tetracyanoethylene (TCNE) is examined. In Section 5, some properties and applications of graphene are discussed. These include fluorescence quenching phenomena observed with few-layer graphene samples on two fluorescent molecules such as coronene and perylene derivatives. Fabrication of a sensing device as well as of FETs prepared from doped and undoped few-layer and single-layer graphene samples forms part of this section. Part 2 of the thesis includes a brief introduction of hybrid open-framework material and synthesis, characterization and crystal structure of various open-framework metal carboxylates, starting with different transition and main group metals. The carboxylic acids used to form these frameworks vary such as simple aliphatic amino acids such as beta-alanine and aspartic acid or simple aliphatic hydroxyl carboxylic acid such as malic acid in its chiral and achiral forms or five-membered heterocyclic aromatic acid, such as imidazole dicarboxylic acid.

No description available.

Folien aus mehrwandigen Kohlenstoffnanoröhren, auch Buckypaper genannt, stellen eine vielversprechende Alternative zu herkömmlichen Schirmmaterialien für elektromagnetische Strahlung dar. Zum Aufbau eines grundlegenden Verständnisses und zur Verbesserung der Verarbeitbarkeit gegenüber kommerziell erhältlichem Buckypaper, wurden nach Auswahl eines geeigneten Herstellungsverfahrens Buckypaper aus unterschiedlichen Nanomaterialien hergestellt. Zur Untersuchung der Auswirkung ausgewählter Herstellungsparameter erfolgte dabei eine Parametervariation mittels statistischer Versuchsplanung. Zusätzlich wurden im Vorfeld weitere Einflussfaktoren betrachtet. Dadurch konnten unterschiedliche Einwaagemengen und Rohmaterialien, verschiedene Lösungsmittel und Konzentrationen der Nanomaterialien sowie diverse Prozessparameter des angewandten Nassprozesses untersucht werden. Im Rahmen der Charakterisierung der hergestellten Proben, mittels unter anderem Vierleiter- und Schirmdämpfungsmessung sowie Rasterelektronenmikroskopie, wurden die signifikanten Einflüsse der untersuchten Parameter identifiziert und beschrieben. Es konnten dadurch sehr homogene Buckypaper mit hoher Leitfähigkeit und guter Schirmungseffektivität hergestellt werden, die eine verbesserte Grundlage für die weitere Entwicklung mikrostrukturierter Schutzfolien bilden. Aus dem Ergebnis der Arbeit lassen sich optimale Herstellungsparameter von Buckypaper für den Einsatz als Schirmmaterial im Bereich der elektromagnetischen Verträglichkeit ermitteln. / Freestanding multiwalled carbon nanotube sheets, also known as buckypaper, represent a promising alternative for various applications, especially for electromagnetic interference shielding. In order to develop a fundamental understanding and improve the processability compared to commercially available buckypaper, sheets from different nanotube materials were produced after a suitable manufacturing process had been selected. Design of experiments was used to investigate the effects of various manufacturing parameters. Other influencing factors were considered in advance. Several raw materials of different weights, varying solvents and concentrations of the nanomaterials as well as various processing parameters of the applied wet process were thereby examined. Significant influences on the properties of produced buckypaper were identified during characterization by, among other means, four-point probe, shielding attenuation measurements and scanning electron microscopy. From the result, optimal manufacturing parameters can be determined. Thus, very homogeneous buckypaper with high electrical conductivity as well as good mechanical strength and shielding effectiveness could be produced. This provides a solid foundation for further development of frequency selective electromagnetic interference shields.

Two major projects involving the use of solution processed blended semiconductors for organic field effect transistors (OFET) were explored. The first incorporated unsorted single walled carbon nanotubes (SWCNTs) into a diketopyrrolopyrrole-quarterthiophene (DPP-QT) semiconductor to enhance the mobility of the OFET. 2 wt % SWCNT was found to be the optimal blend ratio, nearly doubling the device mobility (0.6 to 0.98 cm^2/V·s). Beyond this ratio, the metallic content of the SWCNT’s dropped the on/off ratio below acceptable levels. When source drain metals who’s work function poorly matched that of the DPP-QT semiconductors highest occupied molecular orbital (HOMO) were used, the SWCNT could dramatically reduce the charge injection ratio with best results achieved for Al, dropping the contact resistance from 10^5 to 45 MΩ. The second project explored the addition of small molecule additives into a UV-sensitive semiconductor 2,7-dipentyl[1]benzothieno[3,2-b][1] benzothiophene (C5-BTBT) mixed with a polymethyl methacrylate (PMMA) polymer binder. We generated a C5-BTBT based phototransistor sensitive to UV-A light. The HOMO and lowest unoccupied molecular orbital (LUMO) of C5-BTBT and the various additives were measured and discovered to play a critical role in how the device operates. We discovered if an additive has a LUMO lower in energy than C5-BTBT, it can act as a charge trap for a photogenerated electron. Electron deficient additives were found to retain a trapped electron for an extended period of time, allowing the device to remain in a high current state for an extended period of time (>1 hour). This provides an opportunity for the device to be used as an optical memory system or photoswitch. The best system could detect UV-A with a Pill > 10^5 and a photoresponsivity of 40 A/W at a Pinc of 0.0427 mW/cm^2. / Thesis / Doctor of Philosophy (PhD) / An emerging field of electronics is the use of organic materials that can be solution processed, to reduce manufacturing costs and make new and interesting products. Here we used unsorted carbon nanotubes blended into the semiconductor layer of a transistor, providing a bridge for the energy mismatch between the electrodes and the semiconductor. This allowed us the freedom to choose different metals to act as our electrodes when making electronic devices. Additionally through the correct choice of semiconductor, we added device functionality, making it responsive to UV-A light. This produced a device that could act as a UV-A sensor, logic switch or memory device. These devices are air stable and solution processable, a necessity if they are to be used in real world applications.

This dissertation summarises the study of different aspects of the aerosol-assisted chemical vapour deposition (AACVD) technique for the production of multi-wall carbon nanotubes (MWCNTs). Upscaling the synthesis while retaining the quality of MWCNTs has been a prime objective throughout the work. A key aspect of this work was the study of different growth parameters and their influence on the homogeneity of the products across the reactor. The effect of the precursor composition on the yield and quality of MWCNTs were also investigated. It was shown that the synthesis rate can be significantly (60 – 80 %) increased by tuning the composition of the precursor. Moreover, by optimising the synthesis recipe and using a larger reactor, the synthesis rate and efficiency of the precursor were increased fivefold (up to 14 g/hr) and twice (up to 88 %) respectively. Large area (up to 90 cm²), mm-thick carpets of MWCNTs which were both free-standing and on substrate were produced. The carpets could withstand normal handlings without tearing apart, making them suitable for macroscopic characterisations and applications. By in-situ qualitative and quantitative gas analysis of the atmosphere of the reactor, the thermocatalytic cracking behaviour of 25 precursors was investigated and a mechanism for successive formation of different hydrocarbon fragments inside the reactor was proposed. A number of dedicated gas analysis methods and apparatuses such as a probe for zone-by-zone gas analysis of reactor and a heated chamber for preparation of standard gas analysis samples were developed to explore some of the least investigated aspects of the thermocatalytic cracking of precursors. Mapping the reactor revealed that some single-wall and double-wall carbon nanotubes (SWCNTs and DWCNTs) were also produced near the exhaust of the reactor. The SWCNTs were partly covered by fullerene-like species and resembled different forms of carbon nanobuds. In addition, the effect of the electron beam on the interaction of the SWCNTs and the fullerene-like species was studied in situ using high-resolution transmission electron microscopy (HRTEM).

The rise of nanotechnology industries has led to the design and production of new nano-scaled materials such as quantum dots, nano-metals, carbon nanotubes, fullerenes and a myriad of functionalised derivatives. Extensive work concerning well characterised pathogenic fibres has led to the development of a fibre paradigm that suggests respirable fibres vary in their ability to cause disease based on length and pulmonary bio-persistence. Induction of oxidative stress is also a central plank of the mechanism used to explain inflammatory, fibrotic and carcinogenic effects of fibres. The toxicity of different particle types has consistently been shown to depend upon particle size and surface area, reactive surface molecular groups, metal content, organic content and the presence of endotoxins. A growing body of work has begun to examine the potential pathogenicity of carbon nanotubes to the pulmonary system as a consequence of superficial similarities to known pathogenic particle and fibres. The aim of this thesis was to investigate the potential toxicity of two commercially manufactured multi-walled carbon nanotubes (MWCNT) compared to a panel of low and high toxicity particles and fibres. The pro-inflammatory nature of MWCNT was examined in vitro and in vivo to determine the effects they may exert in the pulmonary system. In aqueous solutions of phosphate buffered saline, saline and cell culture medium (with or without foetal calf serum supplementation) MWCNT were found to exist as tight aggregates even after sonication. Analysis of metal content of MWCNT by ICP-AES revealed the presence of a low percentage of non extractable residual iron. From analysis of MWCNT by electron spin resonance (ESR) the CNT were found to be ready producers of a free radical species, despite this MWCNT were not able to cleave plasmid DNA. Upon incubation with the alveolar epithelial cell line A549 MWCNTs did not cause noticeable toxicity but did dose dependently deplete total glutathione levels. No increase in production of the pro-inflammatory cytokine IL-8 could be detected at the level of protein or at the level of mRNA. Analysis of the levels (protein and mRNA) of the pro-fibrotic mediator TGF-β did not indicate induction of a fibrotic response to MWCNT. Neither were MWCNTs found to consistently activate the pro-inflammatory associated transcription factor nuclear factor kappa B (NF-κB). Upon instillation into the peritoneal cavity of mice MWCNT failed to induce a pro-inflammatory response in contrast to long amosite asbestos that induced an extensive inflammatory reaction. Analysis of the diaphragms of exposed animals revealed the induction by MWCNT of an apparent foreign body type reaction. Overall with limited processing and dispersion MWCNT were morphologically more akin to particles than fibres. Although apparently able to spontaneously generate ROS in aqueous solution this did not translate into a capacity to cause toxicity or a capacity to induce inflammation either in vitro or in vivo.

Carbon nanotubes (CNT) are a new form of industrially relevant nano-scale particle and are seen as the cutting edge of the burgeoning nanotechnology revolution which promises to impact on all our lives. Due to high length to diameter ratio, CNT are perhaps the most well known of a growing collection of high aspect ratio nanoparticles (HARN). However the production and use of carbon nanotubes has presented an interesting toxicological question based on its structure and raised the question ‘are carbon nanotubes like asbestos?’. Few people are unaware of the devastating global pandemic of diseases caused by asbestos and similarities in needle-like shape between asbestos and nanotubes have raised fears that nanotubes may mimic asbestos-type disease. The purpose of this study was to investigate this link, based on the wealth of information known about the toxic effects of certain forms of fibre on the respiratory system. From this we hope to identify those carbon nanotubes which are hazardous whilst not prejudicing the use of those industrially relevant materials which can be used safely. Within fibre toxicology there exists a central paradigm which outlines the main properties a fibrous particle must possess if it is to exert pathogenic effects in the body. This paradigm outlines the importance of length, thinness and biopersistence to a fibre and an absence of one or more of these attributes results in a loss of pathogenicity. We took this paradigm and, using suitable asbestos and non-asbestos controls, applied it various morphological forms of carbon nanotubes using an in vivo model. The resultant data demonstrates for the first time that asbestos-like pathogenic behaviour associated with carbon nanotubes is closely linked to the morphology of the nanotubes and their aggregates. Specifically our results showed that CNT which possessed a long, straight length were highly inflammogenic and fibrogenic within the peritoneal cavity of mice; a model sensitive to the pathogenic effects of fibres. As well as length, the importance of biopersistence in the pathogenesis of fibrous particles has been known for many years and is a central attribute affecting the pathogenicity of fibres. Amphibole asbestos is known to be durable, a commercially exploited attribute and as such is biopersistent in the lung which is a key feature of its pathogenicity. Glass fibre on the other hand is bio-soluble, and whilst long and inhalable, does not cause significant disease due to its lack of biopersistence. Based on the grapheme structure of CNT which impart exceptional strength and rigidity and the chemical inertness of carbon we would hypothesis that CNT would be biopersistent and therefore fulfil another of the criteria of the fibre pathogenicity paradigm (FPP). Our aim therefore has been to establish the durability of CNT against fibrous particles of known durability using a synthetic solution maintained at a pH to simulate the lung environment. Using a range of 4 CNT and using both durable and non-durable fibres such as amphibole asbestos and glass fibre to bench mark our result; we demonstrated that 3 of the 4 CNT tested displayed exceptional durability whilst the fourth lost approximately 30% of its mass during the experiment with concomitant reduction in pathogenicity. As well as length and biopersistence, the surface of a particle has been shown to contribute to the overall toxicity of a particle and in certain circumstances, such as that of quartz, the surface of the particle can be the biologically active component. In the case of carbon nanotubes, surface functionalisation is commonly used for various endpoints including the addition of various tags and labels for tracking. As such our further aim was to investigate the relationship between the length-dependent pathogenicity of a fibre sample and the surface of the fibre. By using different forms of functional groups attached to the surface of a pathogenic carbon nanotube we aim to critically test if the level of inflammation and fibrosis triggered in vivo can be altered by simple alteration of the surface. Our results showed that surface modification of CNT could alter the inflammogenic and fibrogenic effects of CNT which may have important implications when considering the hazard assessment of functionalised HARN. As CNT are not the only form of fibrous nanomaterial and within this project we also attempted to determine the applicability of the FPP to further high aspect ratio nanomaterials. In order to do this we set out to determine the generality of this hypothesis by asking whether nickel nanowires, a radically different form of HARN to CNT, show length-dependent pathogenicity. Nickel oxide nanowires synthesised to be predominantly long (>20 μm) act similarly to amphibole asbestos in showing the ability to elicit strong inflammation in the mouse peritoneal model in a dose dependent manner; inflammation was not seen with the short (<5 μm) nanowires. In summation, the results from this study are the first to show that long HARN can indeed behave like asbestos when in contact with the sensitive mesothelium. This study suggests a potential link between inhalation exposure to long nanotubes and asbestos-related disease, especially mesothelioma and as such this may have immediate implications across many disciplines if care is to be taken to avoid a long term legacy of harm.

Carbon nanotubes are carbon based structures with outstanding electronical and mechanical properties. They are used in a wide range of applications, usually embedded in polymer in the form of composites, in order to affect the electronic behavior of the matrix material. However, as the nanotubes properties are directly dependent on their intrinsic structure, it is necessary to select specific nanotubes depending on the application, which can be a complicated and inefficient process. This makes it attractive to be able to reduce the amount of material used in the composites. In this thesis, focus is placed on the electrical properties of the composites. A simple patterning method is presented which allows the use of extremely low amounts of nanotubes in order to increase the electrical conductivity of diverse polymers such as polystyrene (PS) or poly(3-hexylthiophene) (P3HT). This method is called nanoimprint lithography and uses a flexible mold in order to pattern composite films, leading to the creation of conducting nanotube networks, resulting in vertically conducting samples (from the bottom of the film to the top of the imprinted patterns). In parallel, X-ray diffraction measurements have been conducted on thin P3HT polymer films. These were prepared on either silicon substrate or on graphene, and the influence of the processing conditions as well as of the substrate on the crystallinity of the polymer have been investigated. The knowledge of the crystalline structure of P3HT is of great importance as it influences its electronic properties. Establishing a link between the processing conditions and the resulting crystallinity is therefore vital in order to be able to make opto-electronic devices such as transistor or photovoltaic cells.

Thesis (MSc)--Stellenbosch University, 2014. / ENGLISH ABSTRACT: Since the discovery of carbon nanotubes (CNTs), a large interest has developed around the incorporation of these into polymeric matrices in order to introduce the excellent mechanical, thermal and electrical properties of CNTs into the resultant composites. Nanocomposites of polymer/CNT composition allow for the favourable combination of the physical properties of the polymeric matrix and of the CNT filler. The biggest existing challenge of producing such nanocomposites is presented by the tendency of CNTs to occur in bundles or aggregates which are difficult to break up and to disperse in solution which leads to non-uniform distributions within the polymeric matrix. This problem has been combated through the use of CNT surface functionalization. However, a disadvantage exists with this solution. Since covalent functionalization of the CNT surface disrupts the electronic π-electron cloud which is responsible for the excellent electronic properties which CNTs are often desired for, an alternative method of functionalization must be employed in order to maintain the excellent intrinsic properties of CNTs yet create uniform dispersion of the nanotubes upon compatibilization with the polymeric matrix. Two alternative methods for the production of noncovalent compatibilization of multi-walled carbon nanotubes (MWNTs) with polystyrene were investigated and compared. These two methods involved the synthesis of a pyrene-functional macroinitiators for reversible addition fragmentation chain transfer (RAFT) and atom transfer radical polymerization (ATRP). Both of these methods allow for the controlled polymerisation of pyrene functional polystyrene chains. For comparison, the direct covalently functional MWNTs were also synthesised first by oxidation of the MWNT surface and conversion of the MWNT into the multifunctional RAFT and ATRP macroinitiator in which the styrene chains were controllably directly grafted from the surface of the MWNTs. The interaction of the pyrene chains with MWCNTs was monitored by using NMR, TGA and fluorescence spectroscopy. The NMR results showed the broadening and weakening of the pyrene protons as well as the polystyrene (PS) protons. TGA showed the loss of the pyrene-functional PS portion throughout the heating process. Fluorescence provided the conclusive result that the noncovalent compatibilization had occurred through the quenching of the emission and excitation signals as a result of electron transfer being facilitated by the π-stacking interactions. Finally, the MWNT nanocomposite polymer nanofibres are produced via the electrospinning technique with the various covalent and non-covalent compatibilized MWNT. The fibre morphology for the different compatibilization methodologies is compared as a function of the MWNT content. Distinct differences are observed for the different composites. / AFRIKAANSE OPSOMMING: Sedert die ontdekking van koolstof-nanobuisies (KNBs), het ʼn groot belangstelling ontwikkel rondom die betrekking van KNBs in polimeriese matrikse om samestellings met uitstekende meganiese, termiese en elektriese eienskappe te vervaardig. Nanosamestellings van polimeer/KNB komposisie laat toe dat gunstige kombinasies van fisiese eienskappe van die polimeer en die KNB vuller gerealiseer kan word. Die grootste uitdaging van die vervaardiging van sulke nanosamestellings is die neiging van KNBs om gebondelde formasie te vorm wat baie moeilik is om op te breek. Dit maak hulle verspreiding in oplossings en in polimeer matrikse oneweredig. Hierdie probleem word deur funksionalisering opgelos. Nogtans, ʼn nadeel van hierdie oplossing is dat kovalente funksionalisering verander die elektroniese struktuur van die KNB oppervlakte deur die ontwrigting van die π-elektron wolk wat vir die uitstekende elektroniese eienskappe verantwoordelik is. Dus moet ʼn alternatiewe funksionalisering metode gebruik word om die inherente eienskappe van die KNBs te behou en terselfde tyd ʼn uniforme verspreiding te bewerkstellig gedurende die vermenging met die polimeer matriks. Twee alternatiewe metodes vir die vervaardiging van nie-kovalente gefunksionaliserde multiommuurde koolstof-nanobuisies (Eng: MWNTs) met polistireen (PS) was ondersoek en vergelyk. Hierdie twee metodes was uitgevoer deur die sintese van ʼn pyreen-funksionele omkeerbare addisie-fragmentasie-kettingoordrag (OAFO) en atoomorrdragradikaaladdisie (AORA) makromiddel. Al twee van hierdie metodes lei tot ʼn gekontrollerde polimerisasie van pyreen-gefunksionaliserde stireen. Vir vergelyking was ʼn kovalente- gefunksionaliserde MWNT vervaardig deur die oksidasie van die MWNT oppervlakte en die daaropvolgende immobilisasie van dieselfde AORA en OAFO middel aan hierde aktiewe punte. Daarvan af was stireen gekontroleerd gepolimeriseer deur middel van die AORA en OAFO middel. Die interaksie was gekarakteriseer deur TGO, KMR en fluoressensie spektroskopie. Die KMR resultate het seine gewys van die verspreiding en verswakking van die pyreen en PS protone. TGO het die verlies van die pyreen-funksionele PS deel van die nie-kovalente produk gewys. Fluoressensie het beslissende bewyse gelewer dat die nie-kovalente funksionalisiering plaas gevind het deur die onderdrukking van die stralende en opwekkings seine as ʼn gevolg van die elektron oordrag wat deur die π-stapel interaksies gefasiliteer word. Uiteindelik was die nanosamestellings vermeng met PS en geelektrospin. Die vesel morfologie vir die verskillende gefunksionaliserde MWNT nanosamestellings metodes was vergelyk as ʼn funksie van MWNT inhoud. Duidelike verskille is waargeneem vir die verskillende samestellings.