Dentre as formas alotrópicas do carbono, o grafeno merece destaque. Este material consiste de uma monocamada atômica de carbono sp2 disposta em uma rede bidimensional cristalina hexagonal tipo favo de mel. Uma das formas de se obter o grafeno é pela esfoliação química, que consiste da oxigenação do grafite policristalino em um ambiente altamente oxigenado, formando um produto intermediário chamado de grafeno oxidado (GO). Por este método, gera-se um produto altamente estável e solúvel em água, podendo ser utilizado na construção de filmes ultrafinos. Uma das aplicações possíveis do grafeno oxidado é em sensores e biossensores, área de aplicações de novos materiais que viabilizem o diagnostico rápido, preciso, seletivo e de baixo custo, principalmente os do tipo, point-of-care. O grafeno é um forte candidato, nessa área, devido a sua alta performace e seu baixo custo de processamento. Neste trabalho, a técnica de automontagem eletrostatic layer-by-layer (ELBL) foi utilizada na construção de filmes de grafeno oxidado, juntamente com o policátion dendritico poli(amido amina) geração 4 (PAMAM-G4) em uma plataforma potenciométrica, utilizando um amplificador de instrumentação comercial AD620 como transdutor em um sensor de pH para caracterização da sensibilidade do dispositivo. Diversas técnicas espectroscópicas, óticas e morfológicas foram utilizadas para caracterizar a esfoliação química do grafite e dos filmes automontados. Através destas caracterizações mostrou-se que a esfoliação ocorreu satisfatoriamente concordando com resultados já existentes na literatura. A construção dos filmes ultrafinos ocorreu pela rápida adsorção do GO em camadas alternadas de PAMAM-G4, e o monitoramento do crescimento dos filmes foi realizado utilizando espectroscopia no UV-Vis, onde o crescimento é linear a partir da décima camada. Por microscopia de força atômica, mostrou-se que a adsorção das folhas de grafeno se dá primeiramente pelas folhas de menor número de camada (1 e 2 camadas) de GO e para camadas de número superior tem-se uma morfologia totalmente diferente da primeira. Por espectroscopia no infravermelho, mostrou-se a interação entre os dois polieletrólitos, onde há formação de ligações do tipo cross-linking entre as camadas. Medidas de potenciometria indicam a sensibilidade máxima em torno de 57 mV/pH em um substrato de ouro( sensibilidade Nerstiana), o que evidencia a sua aplicabilidade em sensoriamento e também a sua aplicação em biossensores. / Among the allotropic forms of carbon, graphene deserves special attention. This material consists of one atomic monolayer of sp2 carbon arranged in a two-dimensional hexagonal lattice type, namely \"honeycomb\". One common method to obtain graphene is -=by chemical exfoliation, which consists of oxygenation of polycrystalline graphite in a highly oxygenated medium, forming an intermediate product namely graphene oxide (GO). This method generates a highly stable and water soluble GO that can be used in the fabrication of ultrathin films. Graphene is a strong candidate for sensing, due to its high performance and low cost processing. In this dissertation, GO was obtained via chemical routs and processed in the form of ultrathin films in conjunction with polyamidoamine dendrimer (PAMAM G4) in a layered fashion using the Electrostatic Layer-by-Layer (ELBL). The GO/PAMAM films were deposited on potentiometric platforms and used as pH sensors using a commercial amplifier AD620 instrumentation as the detection technique. GO and the LbL films were characterized via spectroscopic, optical and morphological techniques. Film growth was monitored via UV-Vis spectroscopy and revealed a linear adsorption up to the tenth GO/PAMAM bilayer. AFM analyses revealed that graphene sheets containing 1 or 2 layers occurred adsorbed first on the substrates. Potentiometric measurements indicated a maximum sensitivity of ca. 57 mV/pH for GO/PAMAM films deposited on gold substrates (Nernstian Sensitivity), which demonstrates the applicability of the films in sensing and biosensing.

Le graphène est un matériau prometteur pour de nombreuses applications du fait de ses propriétés exceptionnelles. Cependant, à la fois les propriétés et les rendements des graphènes et dérivés sont très variables et dépendants de la méthode de production utilisée. De ce fait, la méthode de production doit être développée en accord avec les besoins liés à l'application visée. Dans cette thèse, différents nanomatériaux dérivés du graphène ont été préparés en employant deux méthodes différentes, l'exfoliation en milieu liquide et la synthèse directe par dépôt chimique catalytique en phase vapeur (CVD), et une compréhension fondamentale sur la production de graphène a été acquise. Ces deux méthodes sont prometteuses en termes de qualité, de possibilité de production à grande échelle, de cout de production et enfin d'applications des différents "graphènes" obtenus. D'une part, l'exfoliation en milieu liquide permet de produire des dérivés du graphène tels que le few-layer graphene (FLG (<5 feuillets)) à relativement grande échelle, pour des applications telles que les nanocomposites. Le défi principal consiste à augmenter la concentration en FLG autant que possible, tout en conservant une bonne qualité. De ce fait, une étude comparative de l'exfoliation dans l'alcool isopropylique de 3 poudres de graphite différentes a été entreprise. Des suspensions de FLG de concentration élevée (?1,1 mg/ml) ont été obtenues en partant de poudre de graphite de surface spécifique élevée, par sonication dans l'alcool isopropylique (cuve, 90 min). Ces nanoparticules ont ensuite été incorporées dans des nanocomposites à matrice alumine. Ainsi, nous avons préparé par frittage SPS des nanocomposites à matrice céramique possédant des propriétés mécaniques, thermiques et électriques anisotropes. D'autre part, des films de graphène alliant une grande surface, prometteurs pour des applications dans le domaine de l'électronique, ont été synthétisés par CVD basse pression sur des feuilles de Cu. L'influence de l'atmosphère pendant le traitement thermique et la nature du catalyseur sur le niveau d'impuretés, la qualité des films de graphène synthétisés et leur uniformité en termes de nombre de feuillets a été étudiée. Des films de graphène CVD de haute qualité (très peu de désordre de structure) ont été utilisés pour la réalisation de dispositifs. Ces derniers ont été caractérisés à température ambiante en mesurant leur résistance électrique en fonction de la tension grille ainsi que par des mesures de transport en régime d'effet Hall quantique (basse température et forts champs magnétiques - pulsés). Les résultats préliminaires obtenus confirment que du graphène mono-feuillet possédant une bonne mobilité électronique atteignant jusqu'à 46500 cm²/Vs à température ambiante a pu être synthétisé avec succès. / Graphene is a promising material for many applications due to its unique properties. However, properties and yield of graphene-based materials show variations depending on which production route is used. Therefore, an appropriate production method has to be preferred according to the requirements of a specific application. In this thesis study, graphene-based materials have been successfully produced by liquid phase exfoliation (LPE) and chemical vapor deposition (CVD), which are promising graphene production methods in terms of quality, scalability, cost and applicability of the produced material to relevant applications, and a fundamental understanding on graphene production has been developed. LPE route allows one to produce graphene-based materials at a large-scale for applications such as nanocomposites. The challenge of this method is to increase graphene concentration as much as possible while maintaining the quality of the graphene flakes. Therefore, a comparative study, at which three different graphite-based powders were investigated as starting materials for an effective exfoliation process in isopropyl alcohol (IPA), was performed. High concentration (?1.1 mg/ml), few-layer (?5 layers) graphene-based dispersions were prepared by sonication in IPA within 90 min by utilizing a high surface area graphite nano-powder. This graphene-based material was then incorporated into Al2O3 matrix nanocomposites as a reinforcing/filler phase. Ceramic matrix nanocomposites which exhibit anisotropic mechanical, thermal and electrical properties have been successfully prepared by spark plasma sintering. On the other hand, large-area graphene films, which are promising for electronic applications, were synthesized via low-pressure CVD method over Cu-foils. The influence of ramping atmosphere and Cu foil characteristics on the impurity level, quality of the synthesized graphene films and their thickness uniformity was investigated. High quality CVD-grown graphene films with a significantly reduced disorder level were used for device fabrication and characterized in terms of their electrical resistance at room temperature as a function of gate voltage and transport property measurements in Quantum Hall Effect (QHE) regime (at low temperature and high magnetic field) by pulsed magnetic field experiments. The preliminary results confirmed that single layer graphene with a relatively high electronic mobility reaching up to ?46500 cm2/Vs at room temperature was successfully produced.

Conventional processes for the manufacture of exfoliated natural vermiculite employ temperatures exceeding 800 °C and the onset temperature is above 450 °C. In many applications, notably fire retardants, it is desirable that exfoliation of vermiculite should occur at temperatures in the range of 200 °C to 350 °C. For this purpose, South African Palabora vermiculite was modified by ion exchange with ammonium and selected alkali metal and alkaline earth metal ions. These expeiments were performed at room temperature using an orbital shaker for 15 days. Another set of experiments was performed by immersing vermiculite in saturated sodium chloride solution for periods up to six months. At the end of every month, sodium-exchanged vermiculite samples were taken for analysis. The thermal expansion and degradation of modified vermiculites were studied using thermo-mechanical analysis (TMA) and thermogravimetric analysis (TGA) respectively. Interlayer composition was studied using cation exchange with ammonium acetate. The leached cations were quantitatively determined using inductively coupled plasma optical emission spectroscopy (ICP-OES). Scanning electron microscopy (SEM) was applied to study the samples’ morphology. X-ray diffraction (XRD), infrared (FT-IR) and Raman spectroscopy confirmed that the basic vermiculite crystal structure was left unchanged during inorganic modification in both sets of experiments. It was confirmed that the blowing agent in the neat vermiculite was the interlayer water. In the exchanged vermiculite with ammonium ions, it was found that ammonia and water were the blowing agents and they were simultaneously released during thermal degradation. However, ammoniumvermiculite did not show a relatively better thermal expansion ratio, despite the presence of both ammonia and water in the interlayer. In the increasing ionic potential series: Na-vermiculite, Ba-vermiculite, Ca-vermiculite and Mg-vermiculite, it was found that the onset expansion temperature also increased dramatically. Exchange with relatively low ionic potential species, namely sodium, potassium and ammonium, lowered the exfoliation onset temperature of vermiculite to below 300 °C. PORTUGUESE : Processos convencionais de produção da vermiculita natural exfoliada empregam temperaturas que excedem 800 °C e a temperatura inicial de expansão está acima de 450 °C. Em muitas aplicações, particularmente em áreas onde se deseja extinção de fogo, é desejável que a exfoliação da vermiculite ocorra na região entre 200 °C e 350 °C. Assim, a vermiculita proveniente de Palabora, África do Sul, foi quimicamente modificada com cloretos de amónio, de metais alcalinos e de metais alcalino-terrosos previamente seleccionados. As experiências foram realizadas à temperatura ambiente sob agitação constante por 15 dias. Outro desenho experimental foi executado por imersão da vermiculita em solução saturada de cloreto de sódio durante 6 meses. Mensalmente foram retiradas alíquotas de vermiculita modificada para a devida análise. A expansão e degradação térmicas das amostras da vermiculita modificada foram estudadas usando análises termo-mecânica (TMA) e termogravimétrica (TGA). O método de extracção de catiões usando acetato de amónio foi aplicado em todas as amostras e os extractos foram analisados usando plasma inductivamente acoplado ligado ao espectroscópio óptico de emissão (ICP-OES). A morfologia das amostras foi comparada usando microscopia electronica (SEM). Difracção por raios-X (XRD), espectroscopia de infravermelho (FT-IR) e Raman, confirmaram a manutenção da estrutura cristalina da vermiculita durante a reacção de troca iónica em ambas experiências. Na vermiculita nativa foi confirmada a libertação de água antes e durante a exfoliação. Na amostra de vermiculita-NH4 foi detectado amoníaco, para além da água, e ambos foram libertos simultaneamente durante a degradação térmica. Entretanto, vermiculita-NH4 não exibe uma relativamente melhor expansão térmica apesar da presença de água e amónia entre as camadas da vermiculita. Na seguinte série crescente the potencial iónico: vermiculita-Na, vermiculita-Ba, vermiculita-Ca e vermiculita-Mg, a temperatura inicial de exfoliação também é proporcionalmente crescente. Trocas catiónicas da vermiculita natural pelos seguintes iões de baixo potencial iónico: sódio, potássio e amónio resultaram na redução da temperatura inicial de exfoliação para temperaturas abaixo de 300 °C. / Dissertation (MSc)--University of Pretoria, 2011. / Chemistry / unrestricted

abstract: Liquid-phase exfoliation (LPE) is a straightforward and scalable method of producing two-dimensional nanomaterials. The LPE process has typical been applied to layered van der Waals (vdW) solids, such as graphite and transition metal dichalcogenides, which have layers held together by weak van der Waals interactions. However, recent research has shown that solids with stronger bonds and non-layered structures can be converted to solution-stabilized nanosheets via LPE, some of which have shown to have interesting optical, magnetic, and photocatalytic properties. In this work, two classes of non-vdW solids – hexagonal metal diborides and boron carbide – are investigated for their morphological features, their chemical and crystallographic compositions, and their solvent preference for exfoliation. Spectroscopic and microscopic techniques are used to verify the composition and crystal structure of metal diboride nanosheets. Their application as mechanical fillers is demonstrated by incorporation into polymer nanocomposite films of polyvinyl alcohol and by successful integration into liquid photocurable 3D printing resins. Application of Hansen solubility theory to two metal diboride compositions enables extrapolation of their affinities for certain solvents and is also used to find solvent blends suitable for the nanosheets. Boron carbide nanosheets are examined for their size and thickness and their exfoliation planes are computationally analyzed and experimentally investigated using high-resolution transmission electron microscopy. The resulting analyses indicate that the exfoliation of boron carbide leads to multiple observed exfoliation planes upon LPE processing. Overall, these studies provide insight into the production and applications of LPE-produced nanosheets derived from non-vdW solids and suggest their potential application as mechanical fillers in polymer nanocomposites. / Dissertation/Thesis / Doctoral Dissertation Chemistry 2020

In this work, general properties of fundamental gas sensors are described. Thesis is mainly focused on chemoresistive and ChemFET types, which are further used in experimental part. Subsequently, properties, preparation and transfer methods of chosen 1D and 2D materials are described. Experimental part of this work describes design and fabrication of chips, which combine the sensing principals mentioned above for utilization of 1D and 2D materials as an active layer. Transfer methods of individual materials on fabricated chips are described and these materials are characterized by Raman spectroscopy and field effect transistor characteristics measurements. Finally, the response of chosen materials to oxidative and reductive gases is measured.

This diploma thesis deals with the preparation of the graphene membranes for depo-sition of gallium atoms by the molecular beam epitaxy. In the first part properties ofgraphene and methods of its production are described. Second part focuses on the gra-phene membranes, their specific properties, applications and methods of production. Thirdpart describes growth theory of the thin films. Practical part is focused on preparationof graphene membranes, which consists of covering the holes in the silicon substrate bygraphene layer. For that mechanical exfoliated and chemical vapor deposited graphenewere used. Subsequently, gallium atoms were deposited on these membranes by molecularbeam epitaxy and in situ observed by scanning electron microscopy.

Les dispositifs à base de silicium, industrialisés aujourd’hui pour les systèmes électroniques, atteignent leurs limites en termes de miniaturisation et de performances. La course à l’innovation et à la miniaturisation vise aujourd’hui à dépasser cette limite en intégrant de nouveaux matériaux dans les dispositifs, en couplant d’autres phénomènes physiques de l’optique à l’électronique haute fréquence. Le travail conduit pendant cette thèse porte sur la caractérisation du phosphore noir (bP) pour des applications dans le domaine de l'optoélectronique hyperfréquence avec une application spécifique aux interrupteurs microondes pilotés optiquement à 1,55 µm. La caractérisation du bP passe par le développement de techniques de fabrication de couches bidimensionnels de bP et également par la détermination de l'influence des matériaux annexes utilisés sur les propriétés de la couche. Cela a été couplé à une étude optique pour connaitre la réponse du bP à une excitation laser à 1,55 µm. La détermination de paramètres intrinsèques spécifiques du matériau tels que le temps de vie des photoporteurs, la résistivité et la permittivité a été conduite par l'intermédiaire d'expériences de caractérisation dans le domaine optique, radiofréquence et électronique (DC). Les résultats obtenus confirment l’intérêt du bP pour ce genre d'application et ont permis l'intégration du matériau dans le dispositif hyperfréquence visé. Les résultats obtenus lors de tests préliminaires présentés dans ce mémoire sont très encourageants et ouvre la voie à de nombreuses applications ultra-rapide à haute fréquence. / The research project conducted focuses on the optoelectronic and high frequency characterization of black phosphorus. The context of this project is the trend of downscaling and multi-physical coupling seen today in industrial electronics. The characterization carried out is directed for a specific application, the realisation of microwave photoswitch controlled by a laser optical excitation at 1.55 µm. For this purpose, during this PhD a production process of thin and large bi-dimensional layers of black phosphorus has been performed, along with the fabrication of characterization devices, and a discussion to determine suitable appendices for substrate, capping layer and metallization. The technological development is coupled with optical, electronic (DC) and radiofrequency characterizations of the bi-dimensional layers for the determination of inherent black phosphorus properties like the photogenerated carrier lifetime, the material permittivity, the resistivity and the mobility of the carriers. Those parameters are essential to understanding design and simulate high frequency optoelectronic devices on black phosphorus such as the microwave photoswitch controlled at 1.55 µm. The obtained results assert black phosphorus as a promising material for this kind of application. The first performances obtained with the use of bP as an active material for photoconductive switching are very encouraging and open the way for high frequency and high speed applications.

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