Electronic properties of single walled carbon nanotubes synthesized by laser ablation

Current research in the field of nano-electronics is directed towards device miniaturization in order to find ways to increase the speed of electronic devices. The work presented in this dissertation is on the electronic transport properties of single walled carbon nanotube (SWNT) ropes synthesized by laser ablation. The measurements were performed on devices with different geometries; namely SWNT mats, metal incorporated (aligned individual and bundled) SWNTs and lastly on aligned pure SWNTs from low temperatures up to room temperature. The work was performed so as to gain an understanding on how best to utilize SWNTs in the semiconductor industry towards miniaturization. Such an understanding would ultimately highlight if SWNTs can be considered as a viable alternative to the current silicon-based technology, which seems to be approaching its physical limit. For a mat of SWNTs, 3D-Variable range hopping is the principal conduction mechanism from 2 K – 300 K. The magneto-resistance was found to be predominantly negative with a parabolic nature which converts to a linear nature as the temperature is increased. The negative MR is a consequence of quantum interference and the positive upturn is attributed to wave function shrinkage at low temperatures as described by the Efros-Shklovskii model. The hopping ranges of the electrons for a SWNT mat increases as the temperature decreases due to manifestation of quantum effects and reduced scattering. It was also found that metal incorporation does not alter the properties of the SWNT significantly. SWNT ropes aligned by di-electrophoresis across a 1 micron gap between gold micro-electrodes, exhibit Tomonaga-Luttinger liquid (TLL) like behaviour, within the 80 K – 300 K temperature range. The effects of confinement and electron-electron interaction unique to one dimension were identified in electronic transport as a non-universal power law dependence of the differential conductance on temperature and source-drain voltage. Ballistic conductance at room temperature was confirmed from the high frequency transport of the SWNT devices. The complex impedance showed some oscillatory behaviour in the frequency range 6 to 30 GHz, as has been predicted theoretically in the Tomonaga-Luttinger Liquid model. The observation of Luttinger Liquid behaviour demonstrates the outstanding nature of these one-dimensional molecular systems. In these devices the charging Coulomb energy of a single particle played a critical role in the overall device performance. This study can be used to understand the nature of dynamics of plasmons which are the charge carriers in a TLL system and how Coulomb interactions can be used to design highly tuneable systems for fabrication of single molecule devices. The incorporation of metal onto individual SWNT ropes does not alter its electronic properties significantly but the properties of the bundled metal incorporated SWNT ropes are altered. This study has found that under optimized conditions SWNTs might be a viable option for incorporation in nano electronics devices. Individual SWNT ropes promise better devices compared to SWNT mats and further work should be done on individual SWNTs

Fundamental properties of thermoset resin with boron nitride nanotube reinforcement for radiation shielding applications

Boron nitride nanotubes (BNNT's), like carbon nanotubes (CNT's), have properties beneficial for the application in various fields of science including materials, electronics, and medicine. B10 has one of the largest neutron capture cross sections of any isotope and presents an opportunity to incorporate radiation shielding in composite materials by infusing the matrix with BNNT's. However, due to the challenges in synthesizing quality BNNT's, little research has been done to further the technology. The aim of this research is to: 1) Create theoretical models to substantiate that there is no detrimental effects on the fundamental properties, such as: modulus, strength and glass transition temperature. 2) Acquire structural information on the BNNT's and the resin system infused with BNNT's and 3) Generate experimental data which will verify the computational models. Structural information has been obtained on the BNNT's and nanocomposites by analytical and microscopic techniques. Calculations of the fundamental mechanical material properties of BNNT's are performed utilizing molecular dynamics simulations via Material Studio by Accelrys Inc. After the full characterization of the BNNT's, BNNT's have been dispersed into the Epon862/W thermoset resin system. Glass transition temperature has been predicted by simulating the annealing process and monitoring the density of the material at various temperatures. Also, interfacial information between the BNNT's and resin system has been described to provide a foundation for engineers in the fabrication of nanocomposites. Experimental data, from the differential scanning calorimetry (DSC), of glass transition temperature confirms the accuracy of the computational models. Also, models in which the BNNT's undergo hydrogenation have been performed to understand the effects of hydrogenation on the properties of the BNNT's and the nanocomposite. Previous studies have demonstrated that CNT's have improved the mechanical and thermal properties of nanocomposites. Thus, it has been demonstrated that BNNT's will have advantageous effects on the fundamental properties of composites while incorporating radiation shielding

Modeling and simulation to investigate effects of static mixer, carrier gas, temperature and pressure on the mixing ratio of carbon nanotubes growth reactors

The problem of this study was to investigate the effects of static mixer, carrier gas, carrier gas inlet pressures, and reactor operating temperatures on the mixing ratio of carbon nanotube synthesizing reactor. The methodology included design of static mixers, mathematical modeling, and computer modeling and simulation experiments. The simulation experiment was performed based on single phase carrier gas modeling due to difficulty and time for three phase fluid modeling. First only nitrogen carrier gas in addition to the other three factors under constant inlet flow velocity and inlet temperature was simulated. Secondly, the same procedure was applied to argon carrier gas. Three temperature values were extracted at exit of model reactors with internal configuration varied with types of static mixers. The bulk temperature and temperature deviations were calculated. The deviations were then divided by the bulk temperature to obtain the mixing ratios from which the mixing indices were determined. In addition, the stream lines for each treatment were obtained to validate the quantitative mixing indices. A 4-way analysis of variance (ANOVA) was completed, and the diagnostics check on the transformed data showed that the statistical assumptions were met. Thus, the inferential statistics and conclusions confirming or disconfirming the original research questions and research hypotheses were then determined at significant level of .05. In conclusion, the baffle static mixer showed significant improvement over the existing reactor in the mixing ratio using single phase buffer gas flow. Also the reactor temperature showed significant effect on the mixing ratio. On the other hand, the type of carrier gas and pressure did not show significant effect on the mixing ratio. This indicated that the appropriate reactor temperatures in addition to improving the inner configuration of the carbon nanotube growth reactors with static mixers can improve achieving uniform atomic distances between carrier gases, carbon and metal catalyst vapors. In the case of laser and solar methods this can then lead to uniform plume formation, cooling, nucleation, growth, diameter and length of carbon nanotubes. The purity of carbon nanotubes can improve and consequently lead to higher yield and improved productivity of the laser vapor method and other methods of growing carbon nanotubes such as the solar, arc, flame and chemical vapor deposition. This will further contribute to cheaper purification cost and hence the overall price of carbon nanotubes

CFD-based tool to support CNT synthesis via CVD

Tese de Doutoramento (Programa Doutoral em Líderes para as Indústrias Tecnológicas)Ao longo dos últimos anos, tem havido bastante interesse na aplicação de Nanotubos de Carbono (CNTs) devido às suas propriedades únicas, principalmente a nível mecânico e elétrico. Contudo, os processos de síntese, como a Deposição Química a Vapor (CVD), são bastante imprevisíveis e inconsistentes, levando a uma metodologia de tentativa-erro quando se pretende extrapolar resultados. Como alternativa, nesta tese é proposta e desenvolvida uma ferramenta, baseada em métodos computacionais de dinâmica de fluidos (CFD), de suporte à compreensão do processo e à transição entre diferentes setups de CVD. O desenvolvimento desta ferramenta começa com uma análise de sensibilidade, baseada em métodos CFD, a modelos computacionais de quatro setups CVD reais para a síntese de CNTs. Nesta análise foi pretendido avaliar que parâmetros de processo mais influenciam as condições de síntese. Tal informação permite ajustar os parâmetros de processo de forma a obter as condições de síntese desejadas e, consequentemente, desenvolver a ferramenta de suporte à transição entre diferentes setups de CVD. Definindo esta transição como o processo de replicar as condições de síntese medidas num dado (primeiro) setup, num outro (segundo) setup, a metodologia proposta é baseada num problema de otimização, onde é pretendido reduzir o erro percentual entre as condições de síntese de ambos os setups. Os resultados mostraram um erro percentual abaixo dos 2% para a maior parte dos casos testados e nunca superior a 7% para os restantes casos, o que valida a metodologia proposta do ponto de vista do modelo computacional. A ferramenta de transição foi ainda melhorada com a integração dos efeitos da temperatura do forno, parâmetro do processo CVD ainda não considerado, nas condições de síntese. Estes efeitos foram avaliados através de uma análise de sensibilidade. Uma vez validada a hipótese, a ferramenta de transição foi alterada considerando a temperatura do forno e foi testada para vários casos. Os resultados obtidos mostraram uma redução média de 63% do erro percentual anterior. Para explorar e entender as capacidades dos métodos CFD, foi criado um caso de estudo, onde reações químicas foram incluídas num modelo para resolver problemas existentes na síntese de CNTs quando é usado um novo catalisador. Através do estudo das reações químicas e interações existentes entre os vários componentes, é possível analisar outras dependências existentes no processo de síntese, permitindo ultrapassar as limitações encontradas no setup experimental. Por último, a inclusão das reações químicas no modelo foi proposta como forma de estudar a transição entre setups de CVD, onde o processo de síntese de CNTs é feito com diferentes hidrocarbonetos. A integração destas capacidades possibilitaria a análise de condições de síntese, resultantes da interação química dos diversos gases. Adicionalmente, é espectável a validação experimental da metodologia de transição desenvolvida. Tal validação, quer seja feita em dois setups existentes no mesmo grupo de investigação ou entre diferentes grupos, potenciaria a metodologia como uma ferramenta robusta na transferência de conhecimento e resultados, como ainda em técnicas de scale up da síntese de nanotubos de carbono. O desenvolvimento deste trabalho de investigação resultou numa melhor compreensão acerca da dinâmica de fluidos in das interações entre gases que ocorrem durante a fase de crescimento da síntese de CNTs por CVD. Esta compreensão foi usada para o desenvolvimento duma metodologia de transição, baseada em mimica de condições, para suporte à transferência de conhecimento entre diferentes setups de CVD. Foi ainda analisada a integração de reações químicas nos modelos CFD, que potenciariam a metodologia proposta para abordar a transição entre setups CVD que usem diferentes gases. No entanto, este trabalho computacional deve ser validado, uma vez que ainda existem questões cientificas a abordar. Por exemplo, que reações químicas ocorrem, quando usados diferentes gases? Ou, que condições de síntese devem ser replicadas entre estes setups? Estas e outras questões devem ser abordadas em trabalho futuro.Over the last years, there has been a high interest in Carbon Nanotubes' (CNTs) applications due to their unique properties, mainly at mechanical and electrical levels. However, current synthesis processes, such as Chemical Vapor Deposition (CVD), are highly unpredictable and inconsistent, which leads to an exhaustive trial-and-error methodology when extrapolating results. Alternatively, a Computational Fluid Dynamics (CFD) based tool to support the transition process between two distinct CVD setups is here proposed and developed. For the correct development of such tool, a CFD-based sensitivity analysis was first performed to the models of four distinct and real CVD processes to synthesize CNTs. Such analysis intended to give a better understanding of the whole process by evaluating which process parameters affect the most the synthesis conditions. Such understanding of the process’ fluid dynamics would enable the targeting of specific synthesis conditions by adjusting the process parameters. With such insights of the process’ fluid dynamics, the model to support the transition between two different CVD setups was designed. Defining this transition as the act of mimicking the synthesis conditions obtained in one tube in the other, the proposed methodology was based in an optimization problem, intended to minimize the percentual error between the conditions measured in both setups. Results have shown a total percentual error less than 2% for most of the tested cases and never higher than 7% for the remaining ones, which validates the proposed methodology. The transition model was then improved with the integration of the furnace temperature effects on the synthesis conditions. These effects were evaluated by a sensitivity analysis. Once the proposed hypothesis was validated, the transition model was altered considering the furnace temperature. The obtained results showed an average reduction of 63% of the previously achieved percentual error. Further capabilities of the CVD setups modelling via CFD tools were assessed by a case study, where chemical reactions kinetics were included in the model in order to solve a few uniformity issues in the CNT synthesis process, when addressing the usage of a different catalyst. Including the interactions between the compounds, the chemical reactions kinetics enable the analysis of further dependencies existent in the CVD process, which gave some insights to be considered for overcoming the encountered issues in the experimental work. Finally, the inclusion of the chemical reactions kinetics in the transition model was proposed as a way for it to tackle transition between CVD setups, whose CNT synthesis is based in different hydrocarbons. The inclusion of such capabilities would enable the analysis of other synthesis conditions, resulted from the interaction between the different compounds. Moreover, experimental validation of the designed transition methodology is envisioned. Such validation, either within the same research group or between different groups, would potentiate the methodology as a robust tool to support knowledge transfer as well as scale-up techniques in the carbon nanotubes synthesis. The development of this research work resulted in a better understanding of the fluid dynamics and compounds interactions occurring during the growth phase of the CNT synthesis by CVD. Such understanding was used to develop a conditions mimicking based transition methodology to support knowledge transfer between different CVD setups. It was also analyzed the integration of chemical reactions in the CFD model, which would potentiate the proposed methodology to tackle the transition between CVD setups using different compounds. Nonetheless, this computational work should be validated, as there are still research questions to be addressed. For instance, what chemical reactions occur when using different compounds? Or, what would be a suitable synthesis condition to be mimicked between these setups? These and other issues should be tackled in future research work.The author, Carlos José Fortunas Teixeira, was supported by a national Portuguese scholarship from FCT - scholarship reference: SFRH/BD/52350/2013. This work was supported by the FCT - Fundação para a Ciência e a Tecnologia - with the reference project UID/EEA/04436/2013, by FEDER funds through the COMPETE 2020 - Programa Operacional Competitividade e Internacionalização (POCI) - with the reference project POCI-01-0145-FEDER-006941; and by the project “IAMAT – Introduction of advanced materials technologies into new product development for the mobility industries”, with reference MITP-TB/PFM/0005/2013, under the MIT-Portugal program exclusively financed by FCT

Production of carbon nanotubes by PECVD and their applications to supercapacitors

Màster en Nanociència i NanotecnologiaPlasma enhanced chemical vapor deposition (PECVD) is a versatile technique to obtain vertically dense-aligned carbon nanotubes (CNTs) at lower temperatures than chemical vapor deposition (CVD). In this work, we used magnetron sputtering to deposit iron layer as a catalyst on silicon wafers. After that, radio frequency (rf) assisted PECVD reactor was used to grow CNTs. They were treated with water plasma and finally covered by MnO2 as dielectric layer in order to use CNTs as electrode for supercapacitors. Optimization of annealing time, reaction time and temperature, water plasma time and MnO2 deposition time were performed to find appropriate conditions to improve the characteristics of supercapacitors. SEM (Scanning Electron Microscopy), TEM (Transmission Electron Microscopy), AFM (Atomic Force Microscopy) and Raman spectroscopy were used to characterize obtained electrodes

Novel thermal and electron-beam approaches for the fabrication of boron-rich nanowires

Pursuing the development and implementation of novel synthesis techniques to produce nanostructures with an interesting set of properties is a goal that advances the frontiers of nanotechnology. Also of fundamental importance is to revisit well-established synthesis techniques employing a new set of materials as precursors, substrates and catalysts. Fundamental breakthroughs in the field of nanotechnology can be achieved by developing new synthesis procedures as well as by adapting known procedures to new materials. This thesis focuses on both kinds of experiments. A variant of chemical vapor deposition (CVD) has been used to produce Al5BO9 nanowires out of sapphire wafers without the need of a catalyst material. The novelty of the work relies on the formation mechanism of the Al5BO9 nanowires. Essentially, the process can be described as a large-scale topological transformation taking place on the substrate’s surface as its chemical composition changes due to the arrival of precursor molecules. Dense mats of Al5BO9 nanowires cover large areas of the substrate that were previously relatively flat. The process is enhanced by a high temperature and the presence of pre-existing superficial defects (cracks, terraces, etc.) on the substrates. Al5BO9 nanowires as well as B/BOX nanowires and BOX nanotubes were also produced via a novel in-situ electron beam-induced synthesis technique. The process was carried out at room temperature and inside a transmission electron microscope. Au nanoparticles were used as catalyst for the case of B/BOX nanowires and BOX nanotubes, while the Al5BO9 nanowires were synthesized without the need of a catalyst material. The formation and growth of the nanostructures is solely driven by the electron beam. The growth mechanism of the B/BOX nanowires and BOX nanotubes relies on interplay between electrostatic charging of the precursor material (to produce and transport feedstock material) and electron stimulated desorption of oxygen which is able to activate the catalytic properties of the Au nanoparticles. For the case Al5BO9 nanowires a nucleation process based on massive atomic rearrangement in the precursor is instigated by the e-beam, afterwards, the length of some of the nanowires can be extended by a mechanism analogous to that of the growth of the B/BOX nanowires

Pulsed laser ablation in liquid (PLAL) for nanoparticle generation

Nanoparticles, broadly spherical pieces of material with diameters in the nanoscale range, have a number of advantageous physical, chemical, electrical, and optical properties. These unique properties make them suitable for a wide range of applications including sensing, medical therapeutics, printed electronics, and anti-fouling/anti-microbial surfaces. Pulsed laser ablation in liquid (PLAL), also known as laser ablation synthesis in solution (LASIS), is an attractive, green method for producing ligand-free nanoparticles in solution. These nanoparticles can be produced from a wide range of target materials and avoids the use of hazardous, environmentally-unfriendly chemicals. In this chapter, the key applications, conventional generation methods of nanoparticles, as well as the background and cutting edge of PLAL are reviewe

Production, processing and assembly of carbon nanotubes

This dissertation reports the development of a method to achieve continuous production of carbon nanotubes. The as produced carbon nanotubes were purified and further processed to tune their properties. Then dielectrophoresis as a versatile technique was used to manipulate and assemble carbon nanotubes into functional structures. In the processing part, purified carbon nanotubes are treated with strong acid then annealed at different temperatures. Combined TEM and NMR studies show that tips of SWNTs in bulk quantities can be uniformly opened by oxidation and closed by vacuum annealing at a surprisingly low temperature. The results provide a guideline on how the SWNTs should be processed for potential energy storage applications. In the assembly part, first, the results of a systematic study on the interactions of CNTs suspended in media of various viscosities and ionic conductivities with an AC field of different frequencies were reported. Then the feasibility of utilizing the dielectrophoresis for controlled assembling of functional CNT structures was explored. Finally, the automated process of assembling CNT fibrils unto sharp probes was realized and a precise control over the process was especially studied

Laser and carbon : nanotube synthesis and annealing

Thanks to their predicted and measured properties, carbon nanotubes (CNTs) are becoming viable and superior alternatives to many of materials science’s established materials. Yet, although the divide between model and reality has narrowed, insufficient CNT quality and purity remain major hindrances to the performance of most CNT-based materials. Furthermore, CNT precursors are overwhelmingly high-purity petrochemical substances, hampering sustainable and widespread adoption of CNTs. These two identified challenges were addressed towards synthesis energy and cost efficiency, sustainability and material performance. To this end, single-wall CNTs (SWCNTs) were synthesised in a custom-built oven laser apparatus as the object of this study. Laser annealing was found to quickly and drastically recrystallise defects and remove impurities as measured by Raman spectroscopy, thermogravimetry, electrical resistance and hydrogen adsorption measurements. Results could be reproduced at the micro- and millimetre scale. Composite processing related damage, artificially introduced into SWCNTs, was almost completely reversed by laser annealing. Quality and purity levels equal to that of commercial tubes could be achieved through this technique. A waste product of petroleum refining, fluid catalytic cracking catalyst residue, was successfully employed as carbon precursor for SWCNT synthesis, as well as silica nanowires, onion-like carbons and carbon nanodiamonds.Aufgrund Ihrer vielfach prognostizierten aber auch bekannten Eigenschaften, werden Kohlenstoff-Nanoröhrchen (CNTs) zu konkurrenzfähigen und teilweise überlegenen Alternativen zu vielen in der Materialforschung etablierten Materialien. Obwohl sich die „Kluft“ zwischen Modellvorstellungen und Realität verkleinert hat, sind die unzureichende CNT-Qualität und Reinheit noch immer wesentliche Hindernisse für die Performance der meisten CNT-basierten Materialien. Darüber hinaus sind CNT-Vorgänger überwiegend hochreine petrochemische Substanzen, die eine Akzeptanz von CNTs erschweren. Diese Arbeit befasst sich mit den damit verknüpften Herausforderungen nämlich den Zielen Energie-/Kosteneffizienz, Nachhaltigkeit und Performance. Zu diesem Zweck wurden einwandige CNTs (SWCNTs) in einem speziellen Ofen, der mit einem Hochleistungslaser kombiniert wurde, synthetisiert und näher studiert. Die Laserbehandlung heilt Defekte und entfernt Verunreinigungen schnell und effizient. Dies wurde durch Raman-Spektroskopie, Thermogravimetrie, elektrische Widerstandsmessung und schließlich Wasserstoff-Adsorption bestätigt. Die Ergebnisse konnten im Mikro- und Millimeterbereich reproduziert werden. Herstellungsbedingte Defekte in den SWCNTs konnten durch Laserglühen fast vollständig beseitigt werden. Durch diese Technik konnten Qualität und Reinheitsgrade erreicht werden, die denen von handelsüblichen Nanoröhrchen entsprechen. Katalysatorrückstände des Cracking Prozesses während der Erdölraffination wurden dabei für die Synthese von SWCNTs, sowie Silica-Nanodrähten, „Onion-like Carbon“ und Kohlenstoff- Nanodiamanten eingesetzt