This dissertation presents results of a project bringing Scanning Tunneling Microscope (STM) into a regime of unlimited operational time at cryogenic conditions. Freedom from liquid helium consumption was achieved and technical characteristics of the instrument are reported, including record low noise for a scanning probe instrument coupled to a close-cycle cryostat, which allows for atomically resolved imaging, and record low thermal drift. Subsequent studies showed that the new STM opened new prospects in nanoscience research by enabling Scanning Tunneling Spectroscopic (STS) spatial mapping to reveal details of the electronic structure in real space for molecules and low-dimensional nanomaterials, for which this depth of investigation was previously prohibitively expensive. Quantum-confined electronic states were studied in single-walled carbon nanotubes (SWCNTs) deposited on the Au(111) surface. Localization on the nanometer-scale was discovered to produce a local vibronic manifold resulting from the localization-enhanced electron-vibrational coupling. STS showed the vibrational overtones, identified as D-band Kekulé vibrational modes and K-point transverse out-of plane phonons. This study experimentally connected the properties of well-defined localized electronic states to the properties of associated vibronic states. Electronic structures of alkyl-substituted oligothiophenes with different backbone lengths were studied and correlated with torsional conformations assumed on the Au(111) surface. The molecules adopted distinct planar conformations with alkyl ligands forming cis- or trans- mutual orientations and at higher coverage self-assembled into ordered structures, binding to each other via interdigitated alkyl ligands. STS maps visualized, in real space, particle-in-a-box-like molecular orbitals. Shorter quaterthiophenes have substantially varying orbital energies because of local variations in surface reactivity. Different conformers of longer oligothiophenes with significant geometrical distortions of the oligothiophene backbones surprisingly exhibited similar electronic structures, indicating insensitivity of interaction with the surface to molecular conformation. Electronic states for annealed ligand-free lead sulfide nanocrystals were investigated, as well as hydrogen-passivated silicon nanocrystals, supported on the Au(111) surface. Delocalized quantum-confined states and localized defect-related states were identified, for the first time, via STS spatial mapping. Physical mechanisms, involving surface reconstruction or single-atom defects, were proposed for surface state formation to explain the observed spatial behavior of the electronic density of states. This dissertation includes previously published co-authored material.

Ce mémoire est consacré à l'optimisation de la connexion enzymatique d'enzymes pour l'oxydation du glucose et la réduction de O2 sur matrices de nanotube de carbone (CNT) dans les biopiles à glucose.Premièrement, le transfert électronique indirect de la glucose oxydase (GOx) est optimisé dans une matrice nanostructurée de CNT contenant la 1,4-naphtoquinone comme médiateur rédox. Cette bioanode a ensuite été combinée avec des biocathodes similaires à bases d'enzymes à cuivre (laccase et tyrosinase). La biopile GOx-NQ/Lac a permis d'obtenir des puissances maximales de l'ordre de 1,5 mW.cm-2. Les utilisations de cette pile en décharge courte, longue et sa stabilité dans le temps ont également été étudiées. La seconde partie présente la préparation d'une autre anode basée sur la connexion indirecte d'une glucose déshydrogènase NAD+-dépendante (GDH-NAD+) comme alternative pour l'oxydation du glucose. La GDH-NAD+ a été combinée avec un catalyseur d'oxydation de NADH par différentes méthodes. Tout d'abord, elle a été encapsulée au sein du métallopolymère rédox, puis, la modification supramoléculaire a dans un second temps permis d'immobiliser le catalyseur moléculaire et l'enzyme à la surface des CNTs. Ces deux bioanodes ont permis respectivement l'obtention de courants catalytiques d'oxydation du glucose de 1,04 et 6 mA.cm-2. La seconde bioanode a été combinée avec une biocathode à base de BOD et a permis l'obtention de densités de courants maximales de l'ordre de 140 µW.cm-2 La dernière partie concerne l'élaboration d'une biocathode bienzymatique pour la réduction de O2. Le DET de la HRP sur CNTs a dans un premier temps été optimisé par modification de la surface par différents dérivés pyrène. Ensuite, la combinaison de la GOx et de la HRP sur la même électrode a permis de réduire efficacement O2 en 2 étapes. La biocathode est capable de délivrer une densité de courant maximale de l'ordre de 200 µA.cm-2. Cette dernière, combinée avec la bioanode GDH présentée précédemment a permis d'obtenir une biopile opérationnelle en conditions physiologiques et 10 mM de NAD+, en étant capable de débiter une densité de puissance maximale de l'ordre de 57 µW.cm-2. / This work focuses on the optimization of the electrical wiring of glucose oxidizing and dioxygen reducing enzymes on carbon nanotube (CNT) matrixes for glucose biofuel cells.In the first part, glucose oxidase (GOx) mediated electron transfer (MET) is optimized in nanostructured CNTs matrixes by mechanical compression of a CNTs/GOx composite containing 1,4-naphtoquinone as redox mediator. This bioanode was then combined with MCOs (laccase and tyrosinase) based biocathodes. The GOx-NQ/Lac biofuel cell was able to deliver a maximum power density of 1.5 mW.cm-2. The use of this biofuel cell in short/long time discharge and in storage has also been studied. The second part presents the preparation of another bioanode based on the indirect wiring of a NAD+-dependant glucose dehydrogenase (GDH-NAD+) as an alternative for glucose oxidation. The GDH-NAD+ has been combined with an NADH oxidation catalyst by two different techniques. The first one involves the encapsulation of the protein in the metallopolymer redox film, whereas the second one relies on the supramolecular modification of the CNTs by the molecular catalyst and the enzyme. Both bioanodes showed good catalytic properties toward glucose oxidation in presence of NAD+ with respectively 1.04 mA cm-2 and 6 mA cm-2. The latter has been combined with a BOD based biocathode to form a biofuel cell exhibiting maximum power densities of 140 µW cm-2. The last part of this work focuses on the design of a bienzymatic biocathode for O2 reduction. The DET of horseradish peroxidase (HRP) was first investigated and optimized by modification of the CNTs with pyrenes derivatives. The combination of the HRP with the GOx on the same electrode enables an efficient reduction of O2 in a 2-step process. The biocathode could exhibit maximum currents densities of 200 µA cm-2. This cathode along with the previous GDH bioanode formed a biofuel cell functional in physiological conditions and 10 mM NAD+ showing maximum power densities of 57 µW cm-2.

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / In this Thesis we studied through first principles methods the effects of uniform transversal electric fields on the structural and electronic properties of carbon nanotubes. We study the formation of Si-doped carbon nanotubes through the interaction of Si with single-vacant nanotubes and investigate the action of electric fields on semiconducting and metallic carbon nanotubes, both pure carbon, carboxylated or Si-doped nanotubes. All investigations were done using computational first principles simulations based on the density functional theory using the SIESTA code. Firstly, we show that carbon nanotubes with single vacancies are highly reactive centers, which can be used as adsorbing sites for selected substances to form covalent bonds on surface. This feature is used to investigate the possibility of forming Si-doped carbon nanotube, analyzing possible energetic barriers and observing the steps of the structural rearrangement to the complete stabilization. The effect of the transversal electric field applied on carbon nanotubes was also investigated. The applied field has a remarkable influence on electronic structure of the semiconductor carbon nanotubes, leading to a band gap decreasing which is dependent on the field intensity and nanotube radius, including a possible semiconductormetal transition. It is also observed that the electric field induces electric polarization and a comparison between semiconductor and metal tubes is presented. In the same way, the action of transversal electric fields on semiconductor and metallic carbon nanotubes functionalized with carboxylic group was estimated. We observe that the presence of carboxyl in the nanotube surface modifies the response to the electric field, leading to considerable alterations on the electronic structure of the original system, such as energy bands, inducing an electric polarization and charge transfers between the adsorbed molecules and the carbon nanotube. Therefore, we also study the electric fields action on carboxylated Si-doped carbon nanotubes. The electronic and structural properties of these systems are analyzed and the results discussed. / Na presente Tese estudamos através de métodos de primeiros princípios os efeitos de campos elétricos uniformes transversais sobre as propriedades eletrônicas e estruturais de nanotubos de carbono. Apresentamos o estudo da formação de nanotubos de carbono dopados com Si através da interação com nanotubos apresentando vacâncias simples e investigamos a ação de campos elétricos sobre nanotubos de carbono semicondutores e metálicos, sejam eles puros, funcionalizados por ácido carboxílico ou dopados com Si. Toda investigação foi realizada através de simulações computacionais baseadas na teoria do funcional da densidade, utilizando-se, para tanto, o código computacional SIESTA. Uma revisão teórica dos métodos é apresentada, bem como as justificativas de emprego desta metodologia. Mostramos inicialmente que nanotubos de carbono apresentando vacâncias simples constituem um centro de alta reatividade, que pode ser empregado para adsorver substâncias de interesse e formar ligações covalentes com sua superfície. Utilizamos esta característica para investigar a possível formação de um nanotubo dopado com Si, analisando possíveis barreiras de energia e observando os diversos passos de rearranjo estrutural até a estabilização do sistema. Apresentamos, em seguida, os efeitos de campos elétricos transversais sobre nanotubos de carbono. Mostramos que campos elétricos transversais têm efeitos marcantes sobre a estrutura eletrônica de nanotubos semicondutores, provocando uma diminuição do gap de banda que depende da intensidade do campo aplicado e do raio do nanotubo, podendo ocorrer uma transição semicondutor-metal. Observamos que estes campos provocam uma polarização dos nanotubos, e apresentamos as diferenças observadas nesta resposta por nanotubos semicondutores e metálicos. Estudamos a ação de campos elétricos transversais sobre nanotubos semicondutores e metálicos funcionalizados com carboxila. Observamos que a presença deste grupo na superfície permite uma nova resposta aos campos elétricos, levando a alterações substanciais nas propriedades eletrônicas do sistema, como bandas de energia e densidades de estado, além de modificar a população eletrônica, levando à polarização do sistema. Finalizamos esta Tese com o estudo da ação de campos elétricos sobre nanotubos dopados com Si e com o sistema formado por um nanotubo dopado com Si e funcionalizados com ácido carboxílico. A influência destas modificações estruturais sobre as propriedades eletrônicas são apresentadas e os resultados, discutidos. Apresentamos, finalmente, as conclusões pertinentes a este trabalho, com suas implicações e possíveis conexões com a prática de realização experimental.

abstract: Flame retardants (FRs) are applied to variety of consumer products such as textiles and polymers for fire prevention and fire safety. Substantial research is ongoing to replace traditional FRs with alternative materials that are less toxic, present higher flame retardancy and result in lower overall exposure as there are potential health concerns in case of exposure to popular FRs. Carbonaceous nanomaterials (CNMs) such as carbon nanotubes (CNTs) and graphene oxide (GO) have been studied and applied to polymer composites and electronics extensively due to their remarkable properties. Hence CNMs are considered as potential alternative materials that present high flame retardancy. In this research, different kinds of CNMs coatings on polyester fabric are produced and evaluated for their use as flame retardants. To monitor the mass loading of CNMs coated on the fabric, a two-step analytical method for quantifying CNMs embedded in polymer composites was developed. This method consisted of polymer dissolution process using organic solvents followed by subsequent programmed thermal analysis (PTA). This quantification technique was applicable to CNTs with and without high metal impurities in a broad range of polymers. Various types of CNMs were coated on polyester fabric and the efficacy of coatings as flame retardant was evaluated. The oxygen content of CNMs emerged as a critical parameter impacting flame retardancy with higher oxygen content resulting in less FR efficacy. The most performant nanomaterials, multi-walled carbon nanotubes (MWCNTs) and amine functionalized multi-walled carbon nantoubes (NH2-MWCNT) showed similar FR properties to current flame retardants with low mass loading (0.18 g/m2) and hence are promising alternatives that warrant further investigation. Chemical/physical modification of MWCNTs was conducted to produce well-dispersed MWCNT solutions without involving oxygen for uniform FR coating. The MWCNTs coating was studied to evaluate the durability of the coating and the impact on the efficacy during use phase by conducting mechanical abrasion and washing test. Approximately 50% and 40% of MWCNTs were released from 1 set of mechanical abrasion and washing test respectively. The losses during simulated usage impacted the flame retardancy negatively. / Dissertation/Thesis / Doctoral Dissertation Materials Science and Engineering 2018

abstract: Nitrate is the most prevalent water pollutant limiting the use of groundwater as a potable water source. The overarching goal of this dissertation was to leverage advances in nanotechnology to improve nitrate photocatalysis and transition treatment to the full-scale. The research objectives were to (1) examine commercial and synthesized photocatalysts, (2) determine the effect of water quality parameters (e.g., pH), (3) conduct responsible engineering by ensuring detection methods were in place for novel materials, and (4) develop a conceptual framework for designing nitrate-specific photocatalysts. The key issues for implementing photocatalysis for nitrate drinking water treatment were efficient nitrate removal at neutral pH and by-product selectivity toward nitrogen gases, rather than by-products that pose a human health concern (e.g., nitrite). Photocatalytic nitrate reduction was found to follow a series of proton-coupled electron transfers. The nitrate reduction rate was limited by the electron-hole recombination rate, and the addition of an electron donor (e.g., formate) was necessary to reduce the recombination rate and achieve efficient nitrate removal. Nano-sized photocatalysts with high surface areas mitigated the negative effects of competing aqueous anions. The key water quality parameter impacting by-product selectivity was pH. For pH < 4, the by-product selectivity was mostly N-gas with some NH4+, but this shifted to NO2- above pH = 4, which suggests the need for proton localization to move beyond NO2-. Co-catalysts that form a Schottky barrier, allowing for localization of electrons, were best for nitrate reduction. Silver was optimal in heterogeneous systems because of its ability to improve nitrate reduction activity and N-gas by-product selectivity, and graphene was optimal in two-electrode systems because of its ability to shuttle electrons to the working electrode. "Environmentally responsible use of nanomaterials" is to ensure that detection methods are in place for the nanomaterials tested. While methods exist for the metals and metal oxides examined, there are currently none for carbon nanotubes (CNTs) and graphene. Acknowledging that risk assessment encompasses dose-response and exposure, new analytical methods were developed for extracting and detecting CNTs and graphene in complex organic environmental (e.g., urban air) and biological matrices (e.g. rat lungs). / Dissertation/Thesis / Ph.D. Civil and Environmental Engineering 2013

La croissance de nanotubes de carbone (NTC) sur particules micrometriques d'alumine (Al2O3) permet d'obtenir une dispersion uniforme des NTC dans des matrices sans enchevêtrement de NTC. Ce type de structure hybride NTC-Al2O3 fournit également une solution prometteuse au problème de sécurité de NTC car ils intègrent NTC avec des particules micrométriques, étendant la dimension des NTC à partir de nano-échelle au micro-échelle. Cependant, l'adhérence entre les NTC et les Al2O3 particules ne permet pas de fixer efficacement les NTC. Par ailleurs, une autre préoccupation essentielle de NTC est de savoir comment créer une forte adhésion interfaciale entre les NTC et les matrices polymères pour obtenir de bonnes propriétés mécaniques et de ne pas diminuer la conductivité électrique de NTC. Motivé par ces considérations, cette thèse vise à proposer plusieurs techniques concernant le dépôt d'une nano-couche conductrice sur la surface des structures hybrides NTC-Al2O3. De plus, les impacts de la nano-couche déposée sur la fixation des NTC à la surface d’Al2O3, sur la conductivité électrique des hybrides NTC-Al2O3, et sur l'adhésion interfaciale des systèmes NTC-Al2O3/epoxy composites sont étudiés en détail. / Growth of carbon nanotubes (CNTs) on micro-sized alumina (Al2O3) particles helps to achieve a uniform dispersion of CNTs in matrices without CNT entanglement. This kind of CNT-Al2O3 hybrids also provides a promising solution to the CNT safety problem since they integrate CNTs with micrometric particles, extending dimension of CNT fillers from nano-scale to micro-scale. However, the adhesion between the CNTs and the Al2O3 particles doesn’t sufficiently enable to fix the CNTs firmly and stably. Besides, another crucial concern of CNTs is how to create a strong interfacial adhesion between CNTs and polymer matrices for good mechanical properties and meanwhile not to degrade CNTs’ electrical conductivity. Motivated by these considerations, this PhD thesis aims to establishing several techniques for deposition of an electrically conductive nano-layer on the surface of CNT-Al2O3 hybrids. And the impacts of the deposited nano-layer on the fixing the CNTs on the Al2O3 surface, on bulk electrical conductivity of the CNT-Al2O3 hybrids, and on the interfacial adhesion of CNT-Al2O3/epoxy composite systems are investigated in detail.

L'augmentation croissante de la demande en énergie et l'épuisement des combustibles fossiles exigent l'amélioration de l'efficacité de l'utilisation de l'énergie ainsi que la recherche de ressources durables et renouvelables. Les condensateurs sont des systèmes de stockage de l'énergie qui se sont imposés comme une des solutions aux problèmes énergétiques en raison de leurs avantages, tels que le respect de l'environnement, et la charge et la décharge d'énergie très rapides. Des besoins se sont créés pour un système de stockage capacitif à faible coût et à haut rendement, il est donc nécessaire de développer des matériaux avec une forte permittivité diélectrique. A ce propos, les composites diélectriques à matrice polymère suscitent une attention croissante en raison de leurs bonnes performances diélectriques. Des composites à matrice polymère chargée de particules céramiques ont par exemple été utilisés dans certains condensateurs pour le stockage d'énergie. Pourtant, en pratique, l'applicabilité de tels dispositifs est sérieusement entravée par la faible permittivité diélectrique des matériaux d'une part, et par la détérioration des propriétés mécaniques en raison de la forte teneur en particules céramiques rigides dans la matrice polymère flexible d'autre part. En remplaçant ces particules céramiques par des particules conductrices dans la matrice polymère, des composites percolatifs peuvent être réalisés avec une permittivité diélectrique nettement accrue au voisinage du seuil de percolation. Parmi les charges conductrices, les nanotubes de carbone (NTC) ont été les plus étudiés. En effet, du fait de leur facteur d'aspect et de leur conductivité élevée, ces derniers peuvent conduire à des niveaux de percolation dans les composites pour une très faible quantité de charges. Un des plus grands défis pour l'utilisation des NTC dans les matériaux composites est de séparer les NTC les uns des autres afin de réaliser une dispersion uniforme dans les polymères. Les travaux présentés dans cette thèse ont porté sur l'augmentation de la permittivité diélectrique de matériaux composites à matrice polymère à base de NTC en optimisant à la fois la dispersion des nanotubes ainsi que le contrôle de la microstructure des matériaux composites finaux. L'augmentation de la permittivité diélectrique dans les composites précédents provient de la formation de microcondensateurs au sein du matériau. Toutefois, les NTC ont toujours tendance à s'agglomérer dans le composite, ce qui n'est pas bénéfique à la formation de microcondensateurs. Pour surmonter ce problème, nous avons proposé une microarchitecture hybride SiC-NTC comme charge conductrice. Ces hybrides mulit-échelles ont été produits par dépôt chimique catalytique en phase vapeur assisté par aérosol. L'organisation des NTC sur les particules de SiC peut être efficacement contrôlée en ajustant les différents paramètres de synthèse. Les résultats ont montré que les propriétés de surface asymétriques des micro-particules de SiC étaient plutôt favorables à la croissance des NTC sur SiC selon "une direction unique", alors que certaines conditions expérimentales particulières peuvent aboutir à une croissance "multi-directionnelle". Les particules hybrides de SiC-NTC ainsi obtenues ont également été incorporées dans le PVDF pour préparer des composites percolatifs. Il a été constaté que les hybrides SiC-NTC permettent d'améliorer considérablement la permittivité diélectrique du composite avec un chargement extrêmement faible en NTC. Les NTC sont orientés le long d'un axe sur chaque microplaque de SiC et séparés par une couche mince de polymère, donnant lieu à un réseau de microcondensateurs. Par conséquent, de grandes permittivités diélectriques de plus de 8700 et 2100 à 100 Hz peuvent être obtenues pour un faible taux de chargement de NTC de l'ordre de 2,30% et 1,48% en volume pour les composites à renforts "multi-directionnel" et "unidirectionnel" respectivement. / Ever-increasing energy requirement and exhaustion of fossil fuels demands improving efficiency of energy usage as well as seeking sustainable and renewable resources. Energy storage capacitors are devices that could take this responsibility, and have been the focus of increasing attention due to their advantages such as environment friendliness and very fast energy uptake and delivery. As the requirements grow for a low-cost and high-efficiency capacitive storage system, there is great need for the development of materials with high dielectric permittivity. Polymer composite dielectrics are arousing increasing attention due to their large tunability in dielectric performances. Polymer composites filled with ceramic particles have been used in some energy storage capacitors. Still, their applicability for practical devices is severely hindered by the low dielectric permittivity and deteriorated mechanical and processing properties due to the high content of rigid ceramic particles in the flexible polymer matrix. By replacing ceramic particles with conductive particles in the polymer composites, the percolative polymer composites can be made with the dielectric permittivity dramatically increased in the vicinity of the percolation threshold. Among the conductive fillers, carbon nanotubes (CNTs) have been most intensively studied, as their large aspect ratio coupled with high conductivity can lead to percolation levels in composites at much low loading. One of the greatest challenges for CNT usage in composites is to debundle pristine CNTs and realize uniform dispersion into polymers. This thesis focused on increasing the dielectric permittivity of CNT-based polymer composites by both carefully optimizing the dispersion of nanotubes as well as controlling the microstructure of the composites. The dielectric permittivity increment in the previous composite systems originated from the formation of microcapacitors. However, CNTs were always frizzy in the CNT/polymer composites, which was not beneficial in forming parallel pair electrodes of microcapacitors. To overcome this problem, we proposed a microarchitecture of hybrid SiC-CNT as conductive filler. Such micro/nano hybrids were produced by floating catalytic chemical vapor deposition. The organization mode of CNTs on SiC particles could be effectively tuned by adjusting synthesis conditions. The results showed that asymmetric surface properties of 6H-SiC were prone to led to “single-direction” growth of CNTs on SiC particles, while the competition between the substrate nature and the experimental conditions can resulted in a “multi-directions” hybrid structure. Resultant SiC-CNT hybrids were further incorporated into PVDF to prepare percolative composites. It was found that the SiC-CNT hybrid can significantly improve the dielectric permittivity of SiC-CNT/PVDF composite with an extremely low CNT loading. CNTs on each SiC microplate are oriented along an axis and separated by a thin polymer matrix, giving rise to a network of microcapacitors. As a result, a large dielectric permittivity of more than 8700 and 2100 at 100 Hz could be obtained at a low CNT loading of 2.30 vol% and 1.48 vol% in the “multi-directions” and “single-direction” composites respectively.

Field emission is the emission of electrons from a solid under an intense electric field, of the order of 109 V/m. Emission occurs by the quantum mechanical tunneling of electrons through a potential barrier to vacuum. Field emission sources offer several attractive features such as instantaneous response to field variation, resistance to temperature fluctuation and radiation, a high degree of focusing ability in electron optics, good on/off ratio, ballistic transport, and a nonlinear current-voltage relationship. Carbon nanotubes (CNTs) are potential candidates as field emitters since they possess high aspect ratio and are chemically inert to poisoning, and physically inert to sputtering during field emission. They can carry a very high current density and do not suffer field-induced tip sharpening like metallic tips. In addition, the CNT field emitters have the advantage of charge transport through 1D channels and electron emission at the sharp tips due to large enhancement. But the injection of electrons from the back contact remains a technical challenge which requires binding of CNT emitters to metallic substrate. Also, detachment of the CNT from the substrate tends to occur with time. The electrically conducting mixtures of CNTs and polymer can provide an alternative route to address these issues in the field emission of CNTs. The composites can be casted on any substrate in desired shape and the polymer matrix provides necessary support. The research work reported in this thesis includes the preparation of high quality multiwall carbon nanotubes (MWCNTs), MWCNT-polystyrene (PS) composites, and experimental investigation on field emission properties of MWCNT¬PS composites in two different configurations. Electrical conductivity and percolation threshold of the MWCNT-PS composites are also investigated to ensure their high quality prior to the field emission studies. The study has been further extended to reduced graphene oxide (rGO) coated on polymer substrate. The main results obtained in present work are briefly summarized below. This thesis contains eight chapters. Chapter 1 provides an overview of basics of field emission, and the potential of CNT and CNT-polymer composites as field emitters. Chapter 2 deals with the concise introduction of various structural characterization tools and experimental techniques employed in this study. Chapter 3 describes the synthesis of MWCNTs and characterization by using electron microscopy and Raman spectroscopy. MWCNTs are synthesized by chemical vapor deposition (CVD) of toluene [(C6H5) CH3] and ferrocene [(C5H5)2 Fe] mixture at 980 °C. Here toluene acts as carbon source material and ferrocene provides catalytic iron (Fe) particles. The MWCNT formation is based on the thermal decomposition of the precursor mixture. Scanning electron microscopy (SEM) characterization shows that the MWCNTs are closely packed and quite aligned in one direction. The average length of MWCNTs is about 200 μm and outer diameter lies in the range of 50-80 nm. The high quality of as-prepared MWCNT sample is confirmed by Raman spectroscopy. The as-grown MWCNTs are encapsulated with catalytic Fe nanoparticles, revealed by transmission electron microscopy. The Fe nanoparticles trapped within the MWCNT serve as fantastic system for studying the magnetic properties. Three types of MWCNT samples filled with Fe nanoparticles of different aspect ratio (~10, 5 and 2) are synthesized by varying the amount of ferrocene in the precursor material, and their magnetic properties are investigated. Enhanced values of coercivity (Hc) are observed for all samples, Hc being maximum (~2.6 kOe) at 10 K. The enhancement in Hc values is attributed to the strong shape anisotropy of Fe nanoparticles and significant dipolar interactions between Fe nanoparticles. Chapter 4 deals with the field emission studies of MWCNT-PS composites in the parallel configuration. By incorporating as-prepared MWCNTs in PS matrix in a specific ratio, composites with varying loading from 0.01-0.45 weight (wt.) fraction are prepared using solution mixing and casting. High degree of dispersion of MWCNTs in PS matrix without employing any surfactant is achieved by ultrasonication. Low percolation threshold (~0.0025 wt. fraction) in the MWCNT-PS composites ensures the good connectivity of filler in the fabricated samples. Field emission of MWCNT¬PS composites is studied in two different configurations: along the top surface of the film (parallel configuration) and along the cross section of the sample (perpendicular configuration). In this chapter field emission results of the MWCNT-PS composites in parallel configuration are presented. The effect of charge transport in limiting the field emission of MWCNT-PS composite is discussed. Field emission results of MWCNT-PS composites in parallel configuration indicate that the emission performance can be maximized at moderate wt. fraction of MWCNT (0.15). The obtained current densities are ~10 µA/cm2 in the parallel configuration. Chapter 5 presents the study of field emission characteristics of MWCNT¬PS composites of various wt. fractions in the perpendicular configuration. Till date most studies using nanotube composites tend to have the nanotubes lying in two dimensional plane, perpendicular to the applied electric field. In the perpendicular configuration, the nanotubes are nearly aligned parallel to the direction of the applied electric field which results in high field enhancement, and electron emission at lower applied fields. SEM micrographs in cross-sectional view reveal that MWCNTs are homogeneously distributed across the thickness and the density of protruding tubes can be scaled with wt. fraction of the composite film. Field emission from composites has been observed to vary considerably with density of MWCNTs in the polymer matrix. High emission current density of 100 mA/cm2 is achieved at a field of 2.2 V/µm for 0.15 wt. fraction. The field emission is observed to follow the Fowler– Nordheim tunneling mechanism, however, electrostatic screening plays a role in limiting the current density at higher wt. fractions. Chapter 6 highlights the field emission response of rGO coated on a flexible PS film. Field emission of rGO coated PS film along the cross section of the sample is studied in addition to the top film surface of the film. The effect of geometry on the improved field emission efficiency of rGO coated polymer film is demonstrated. The emission characteristics are analyzed by Fowler–Nordheim tunneling for field emission. Low turn-on field (~0.6 V/µm) and high emission current (~200 mA/cm2) in the perpendicular configuration ensure that rGO can be a potential field emitter. Furthermore, stability and repeatability of the field emission characteristics are also presented. Chapter 7 deals with the synthesis, characterization, and field emission of two different kinds of hybrid materials: (1) MWCNT coated with zinc oxide (ZnO) nanoparticles (2) ZnO/graphitic carbon (g-C) core-shell nanowires. The field emission from the bucky paper is improved by anchoring ZnO nanoparticles on the surface of MWCNT. A shift in turn on field from 3.5 V/µm (bucky paper) to 1.0 V/µm is observed by increasing the ZnO nanoparticle loading on the surface of MWCNT with an increase in enhancement factor from 1921 to 4894. Field emission properties of a new type of field emitter ZnO/g-C core-shell nanowires are also presented in this chapter. ZnO/g-C core/shell nanowires are synthesized by CVD of zinc acetate at 1300 °C. Overcoming the problems of ZnO nanowire field emitters, which in general possess high turn on fields and low current densities, the core-shell nanowires exhibit excellent field emission performance with low turn on field of 2.75 V/µm and high current density of 1 mA/cm2. Chapter 8 presents a brief summary of the important results and future perspectives of the work reported in the thesis.

Neste trabalho foram realizadas simulações computacionais para investigar as propriedades eletrônicas de nanossistemas baseados em nanotubos de carbono e grafeno por meio de cálculos de primeiros princípios. Um dos nanossistemas investigados é formado por um nanotubo de carbono acoplado a eletrodos de nanofios de paládio encapsulados. Foi mostrado que estados provenientes dos eletrodos interagem fortemente com os estados do nanotubo de carbono. Cálculos de transporte eletrônico foram realizados para investigar a potencialidade desse nanossistema em aplicações como transistor de efeito de campo. Foi mostrado que a intensidade da corrente elétrica desse nanossistema pode ser variada com o campo elétrico de gate. Outro trabalho desenvolvido no presente trabalho tem como base um nanossistema formado pelo grafeno depositado nos substratos SiO2 amorfo e h-BN. Foi determinada a energia de adsorção e a quantidade de carga transferida para investigar a influênicas desses substratos na adsorção da molécula de H2 pelo grafeno. Foi mostrado que a energia de adsorção da molécula de H2 adsorivda na interface grafeno/SiO2 amorfo é menor em comparação com o grafeno suspenso ou disposto sobre o substrato h-BN. Além disso, a adsorção do H2 nessa região resulta em uma transferência de carga de uma ordem de grandeza maior em comparação com a adsorção no grafeno suspenso, sendo observado um deslocamento do Cone de Dirac em relação ao nível de Fermi. Esse estudo poderá contribuir para a construção de futuros sensores de H2 à base de grafeno. / In this work, ab initio calculations were performed within DFT framework to analyse electronic properties of Carbon nanotubes and grapheme nano systems. In this work, computer simulations were performed to investigate the electronic properties of nanosystems based on carbon nanotubes and graphene within DFT framework. One of these systems studied is a Carbon nanotube semiconductor coupled to encapsulated leads of Pd nanowires. It has been shown that leads states interact strongly with the carbon nanotube states. Electronic transport calculations were performed to unfold new applications of this system, such as the field effect transistor. We noticed that charge current intensity can be tuned by electrical field. We also described the influence of amorphous SiO2 and h-BN, in H2 energy adsorption and charge transfer, where both materials are used as graphene substrates. It was shown that the latter adsorption energy in the graphene/Si02 is smaller than graphene/h-Bn and the graphene suspended itself. In fact this adsorption results in a charge transference one order greater than in the suspended graphene, which can be seen as a vertical shift of the Dirac Cone. This study may improve the construction of future H2 sensors based on graphene.

Orientador: Ana Flavia Nogueira / Dissertação (mestrado) - Universidade Estadual de Campinas, Instituto de Quimica / Made available in DSpace on 2018-08-10T16:04:48Z (GMT). No. of bitstreams: 1 Lomba_BrunoStelutti_M.pdf: 2789085 bytes, checksum: c5cf56c110af4564ce46cde2fd239d4f (MD5) Previous issue date: 2007 / Resumo: Nanotubos de carbono de paredes simples (SWNT) têm atraído grande interesse devido a sua aplicação em diversas áreas de pesquisa, incluindo novos materiais e dispositivos optoeletrônicos. Entretanto uma boa dispersão destes materiais é um fator necessário na obtenção de filmes homogêneos, com menor grau de agregação para se obter dispositivos com maior desempenho. Neste trabalho, foi realizada uma modificação química das extremidades e defeitos dos SWNT com grupos tiofenos com a finalidade de melhorar a interação do nanotubo de carbono com a matriz polimérica de poli(3-octiltiofeno) (POT). De fato, células solares com melhor desempenho foram obtidas. O SWNT modificado e seu compósito com POT foram caracterizados por espectroscopia Raman, espectroscopia no infravermelho, espectroscopia UV-VIS e voltametria cíclica. A melhor célula solar de heterojunção dispersa foi obtida usando 5 % m m de SWNT modificado (SWNT-TIOF), e apresentou potencial de circuito aberto (Voc), fotocorrente (Isc) e eficiência (h) de 0,75 V, 9,5 mA cm e 0,184 %, respectivamente. O uso de um derivado de indenofluoreno (DPIF) como camada transportadora de buraco no lugar do polietilenodioxitiofeno dopado com sulfato de poliestireno (PEDOT:PPS) também foi investigado. Os resultados iniciais indicam que o uso dessa camada pode ser uma tentativa interessante para melhorar o Voc de células solares orgânicas / Abstract: Single-wall carbon nanotubes (SWNT) have attracted great interst for applications in a variety of research areas, including electronics and functional materials. However, a good dispersion of these materials is a demanding factor in order to obtain more homogeneous and less bundled films for constructing devices. In this report we describe how a covalent modification with thiophene groups at the edges and defects of SWNT can improve interaction with a polymer matrix, resulting in solar cells with improved performance. The modified SWNT and its composite with poly(3-octylthiophene) were characterized by Raman, Infrared and UV-VIS spectroscopies and cyclic voltammetry. The best bulk heterojunction solar cell was obtained using 5 wt % of the modified carbon nanotube (SWNT-THIOP) and shows open circuit voltage (Voc), photocurrent (Isc) and efficiency (h) of 0.75 V, 9.5 mA cm e 0,184 %, respectively. The use of an idenofluorene derivative as a hole transport layer in replacement of the PEDOT:PPS was also investigated. The primary results indicate that the use of this layer can be an interesting approach to improve the open-circuit voltage in polymer/nanotube solar cells / Mestrado / Quimica Inorganica / Mestre em Química