Le graphène est un matériau constitué d'une seule couche atomique de carbone et représente un sujet majeur de la physique de la matière condensée. Le graphène possède de nombreuses propriétés remarquables : structure électronique décrite par une equation de Dirac sans masse, forte mobilité électronique, effet Hall quantique anormal, résistance ,rigidité et conductivité thermique élevée. Cette these concerne la structure électronique et le transport dans le graphène. Nous considérons en particulier le cas des bicouches tournées de graphène. Ces systèmes ont été découverts en particulier dans le graphène produit sur le carbure de silicium et présentent des propriétés originales par rapport aux bicouches dans l' empilement AB qui existe par exemple dans le graphite. Nous analysons au moyen d'une théorie perturbative et aussi par des approches numériques la densité d'états dans ces systèmes.Nous montrons que la densité d'états présente des oscillations avec la même période que celle du Moiré produit par ces bicouches. Nous analysons aussi le rôle des défauts sur les propriétés de transport en particulier dans le cas ou les défauts sont répartis uniquement sur une des deux couches. Ici aussi notre approche combine théorie perturbative du couplage interplans et approches purement numérique en liaisons fortes. Nous considérons aussi le role joué par les adatomes comme l'hydrogène par exemple. Nous analysons la modification de la densité d'états induite autour de l'adatome et les variations correspondantes de densité de charge et de potentiel électrostatique. Ces systèmes tendent à produire des états resonants près de l'énergie de Dirac qui dependent beaucoup aussi de la position top ou hollow de l' adsorbat. Pour des orbitales de type “s” la resonance est plus marquée si l'adatome est en position hollow. Nous montrons que l'image par experience STM (microscopie à effet tunnel) depend beaucoup de la distance entre l'adsorbat et la pointe du STM. Dans un régime de champ proche la résonance de l'adsorbat peut même apparaître comme un creux dans le signal dI/dV du STM. / Graphene, a material made of a one-atom-thick carbon layer, is a major topic of modern condensed-matter research. Graphene exhibits exciting properties such as massless Dirac electronic structure, high mobility anomalous quantum Hall effects, strength, stiffness and extraordinary high thermal conductivity. This thesis deals with electronic structure and transport properties of graphene. We consider in particular the case of twisted bilayers of graphene. These systems have been discovered especially in graphene produced on Silicon Carbide and present original properties when compared with standard AB bilayers that occur for example in graphite. We analyze by perturbative theory and by numerical methods the density of states. We show that the electronic density of states presents periodic oscillations with the period of the geometric Moiré produced by these systems. We analyze also the role of defects on transport properties and in particular we consider the case where the defects are on one layer only : the layer exposed to the air. We show how defects on this layer affects the conductivity of the bilayer. Here also we use simple analytical models and numerical approaches. We consider also the role played by atomic impurities like Hydrogen adatom on the graphene plane. We analyze the modification of density of states induced around the adatom and the corresponding modifications of charge density and electrostatic potential. These systems tend to produce a resonant state close to the Dirac energy which depends much on whether the adatom is in a top or hollow position. For hydrogen like orbital (s orbital) the resonance is stronger in the hollow position. We show that the image obtained through STM experiments for these resonant state depends very much on the distance of the STM tip to the adatom. In a near field regime the resonance can even appear as a dip in the STM signal dI/dV.

Neste estudo foi investigado o processo de redução térmica do óxido de grafeno em médio vácuo como uma rota viável de baixo custo econômico para obtenção do óxido de grafeno reduzido para aplicação em supercapacitores. O objetivo principal foi estudar a influência da temperatura de processamento no grau de redução do óxido de grafeno utilizando um sistema de vácuo com bomba mecânica de duplo estágio. O processamento constituiu na exposição do óxido de grafeno em várias temperaturas (200, 400, 600, 800 e 1000 °C) com pressão reduzida (10-2mbar) condição de médio vácuo. Foram utilizadas técnicas convencionais para caracterização dos materiais precursores e processados, tais como: microscopia eletrônica de varredura (MEV), difração de raios-X (DRX) e espectroscopia no infravermelho com transformada de Fourier (FTIR). Com os resultados deste estudo foi demostrado que é possível obter o óxido de grafeno reduzido utilizando um sistema de vácuo com bomba mecânica de duplo estágio e temperaturas de processamento superiores a 200°C. / In this study the process of medium vacuum thermal reduction of the graphene oxide as a low cost route for obtaining reduced graphene oxide has been investigated. The main objective was to study the influence of the processing temperature on the degree of reduction of the graphene oxide using a vacuum system with two stage backing pump. The processing was carried out by exposing the graphene oxide at various temperatures (200, 400, 600, 800 e 1000 °C) with reduced pressure (10-2 mbar). Conventional techniques have been employed to the characterization of the starting and processed materials, such as: scanning electron microscopy (SEM), X-ray diffraction and Fourier transformed infrared spectroscopy (FTIR). With the results of this study it has been demonstrated that it is possible to obtain the reduced graphene oxide using a vacuum system with a two stage backing pump and processing temperatures superior to 200°C.

Since its discovery, the one atom thick material graphene has been at the centre of growing interest in two-dimensional materials. Due to its exceptional properties, graphene is a rich topic to explore by physicists, chemists, engineers and materials scientists. In addition to its use in the fundamental research, graphene is also a promising candidate for future electronics, photonics and energy storage devices.The project presented in this thesis was carried out to explore the structure of suspended graphene in particular in order to probe the metal-graphene interaction via Transmission Electron Microscopy, as most graphene applications require interfacing with metals. As the work was based on free standing graphene, graphene layers obtained by mechanical cleavage or growth on a substrate were transferred onto TEM-grids. Therefore, fabrication, suspended sample preparation and identification of graphene layers were first discussed for a better understanding of how to obtain high quality graphene, as this was essential for the rest of the project.Structural, topographic and chemical analysis of pristine suspended graphene layers were investigated in detail via Transmission Electron Microscopy and Scanning Tunnelling Microscopy. The latter technique was also employed for graphene on a substrate along with establishing annealing conditions for residue free graphene.Metal deposited suspended graphene layers were then investigated in the electron microscopes. Different metal behaviours were observed on the graphene surfaces for the same amount of metal evaporation. Generally, metals interact only weakly with graphene as they are not observed on clean (residue free) parts and are mainly clustered. On the other hand, graphene etching has been observed in the presence of metals. The etching was initiated with graphene vacancy formation as a result of the interaction between metal and carbon atoms on clean graphene. Once a vacancy was created, a hole quickly formed and eventually the graphene layers were destroyed. However, those holes created by metals were healed spontaneously either by non-hexagonal or perfect hexagonal rings. The possible etching and healing mechanisms of the suspended graphene were also discussed.

Graphene nanocomposites have been successfully prepared in this study in the form of a sandwich structure of PMMA/graphene/SU-8. It has been proved that Raman spectroscopy is a powerful technique in the characterisation of the structure and deformation of graphene. The 2D band of the monolayer graphene has been used in the investigation of stress transfer in the graphene reinforced nanocomposites. It has been demonstrated that the 2D band moves towards low frequency linearly under tensile stress, which is shown to be significant method of monitoring the strain in graphene in a deformed specimen. The Raman spectroscopy behaviour under deformation validates that the monolayer graphene acts as a reinforcing role in nanocomposites although it is only one atom thick.A systematic investigation of the deformation of bilayer, trilayer and few-layer graphene has been undertaken with a view to determine the optimum number of layers for the reinforcement of nanocomposites. It has been demonstrated that monolayer graphene is not necessarily the optimum material to use for reinforcement in graphene-based polymer nanocomposites and bilayer graphene will be equally as good as monolayer graphene. There is therefore a balance to be struck in the design of graphene-based nanocomposites between the ability to achieve higher loadings of reinforcement and the reduction in effective Young’s modulus of the reinforcement, as the number of layers in the graphene is increased.Both the G and 2D bands have been found to undergo splitting under high strain levels or asymmetric band broadening in lower strain deformation. The G band polarisation property has been utilized to determine the crystallographic orientation of monolayer graphene by measuring the intensity ratio of G-/G+ bands. Analogously, the 2D band also undergoes strain-induced splitting where the 2D- band has higher Raman shift rate than that of the 2D+ band.

Résumé : Des feuillets de graphène fonction-nalisés FGSx avec un rapport C/O~x ont été post-fonctionnalisés avec des dérivés de tétrazine (3,6-bis(2-pyridyl)-1,2,4,5-tétrazine) par cycloa-ddition de Diels-Alder à demande inverse. Des applications potentielles de ces graphènes fonctionnalisés ont été explorées sur le stockage d'énergie (supercondensateur) et l’électrocatalyse (réaction de réduction de l'oxygène).Pour les applications de supercondensateurs, des nanocomposites constitués de FGS20 fonction-nalisé et de polypyrrole ont été synthétisés en deux étapes en incorporant d’abord les fonctions pyridine-pyridazine à la surface du FGS20 par cycloaddition suivie de l’électropolymérisation du pyrrole dans l’acétonitrile. La capacité spéci-fique du matériau a été mesurée par des cycles de charge-décharge galvanique et la stabilité au cyclage a été étudiée dans divers milieux électro-lytiques (acétonitrile, liquide ionique, eau acide et eau neutre) et les résultats comparés par rapport au FGS20 non fonctionnalisé avec ou sans polypyrrole. Alors que l'acétonitrile révèle un comportement capacitif pur pour tous les matériaux étudiés, l'eau acide est le milieu où les valeurs de capacité sont les plus élevées et, de manière surprenante, où les nanocomposites contenant du polypyrrole présentent une meilleure rétention de capacité lors du cyclage que le graphène seul. Un impact positif de la fonctionnalisation du graphène avant l'électropoplymérisation a été mis en évidence dans tous les milieux électrolytiques (pertes de capacité limitées à moins de 8% après 1500 cycles dans tous les milieux sauf l'eau neutre), soulignant l'intérêt du contrôle d'interface dans ce type de nanocomposites. / Abstract: Functionalized graphene sheets with a ratio C/O~x (FGSx) was further functionalized with a tetrazine derivatives (3,6-bis(2-pyridyl)-1,2,4,5-tetrazine) by inverse demand Diels-Alder cycloaddition reactions. Functionalized FGS20 and FGS13 potential applications were explored on energy storage (super-capacitor) and catalysis (oxygen reduction reaction).In supercapacitor applications, nanocomposites made of functionalized FGS20 with polypyrrole were synthesized in two steps by first incorporating pyridine-pyridazine functions on FGS20 surface through cycloaddition followed by electropolymerization of pyrrole in acetonitrile. The specific capacitance of the material was measured by galvanic charge-discharge cycles and the stability upon cycling investigated in various electrolytic media (acetonitrile, ionic liquid, acidic, and neutral water) in comparison with non-functionalized FGS20 with or without polypyrrole. While acetonitrile reveals pure capacitive behaviour for all investigated mate-rials, acidic water is the medium where the capacitance values are the highest and surprisingly where nano-composites with polypyrrole show better capacitance retention upon cycling than graphene alone. A positive impact of graphene functionalization prior to electropolymerization was demonstrated in all electrolytic media (capacitance losses limited to less than 8% after 1500 cycles in all media but neutral water), highlighting the interest of interface control in this kind of nanocomposites.

Reduced graphene oxide as outstanding candidate electrode material for supercapacitor has been investigated. This thesis includes two topics. One is that three kinds of reduced graphene oxides were prepared by hydrothermal reduction under different pH conditions. The pH values were found to have great influence on the reduction of graphene oxides. Acidic and neutral media yielded reduced graphene oxides with more oxygen-functional groups, lower specific surface areas but broader pore size distributions than those in basic medium. Variations induced by the pH changes resulted in great differences in the supercapacitor performance. The graphene produced in the basic solution presented mainly electric double layer behavior with specific capacitance of 185 F/g, while the other two showed additional pseudocapacitance behavior with specific capacitance of 225 F/g (acidic) and 230 F/g (neutral), all at a constant current density of 1A/g. The other one is that different reduced graphene oxides were prepared via solution based hydrazine reduction, low temperature thermal reduction, and hydrothermal reduction. The as- prepared samples were then investigated by UV-vis spectroscopy, X-ray diffraction, Raman spectroscopy, and Scanning electron microscope. The supercapacitor performances were also studied and the hydrothermally reduced graphene oxide exhibited the highest specific capacitance.

Se ha estudiado la actividad catalítica de materiales derivados de grafeno como soporte de nanopartículas de paladio así como de complejos de paladio(II) en reacciones de acoplamiento carbono-carbono y carbono-heteroátomo. Así mismo, se ha llevado a cabo su actividad catalítica como carbocatalizador en diversas transformaciones orgánicas.

>Magister Scientiae - MSc / This project focuses on the development of simple, highly sensitive, accurate, and low cost electrochemical sensors based on the modification of pencil graphite electrodes by the electrochemical reduction of graphene oxide-metal salts as nanocomposites (ERGO-MC-PGE; MC = Sb or Au nanocomposite). The electrochemical sensors ERGO-Sb-PGE and ERGO-Au-PGE were used in the determination of paracetamol (PC) in pharmaceutical formulations using adsorptive stripping differential pulse voltammetry. The GO was prepared from graphite via a modified Hummers’ method and characterized by FTIR and Raman spectroscopy to confirm the presence of oxygen functional groups in the conjugated carbon-based structure whilst, changes in crystalline structure was observed after XRD analysis of graphite and GO. / 2023-10-07

One of the challenges in graphene fabrication is the production of large scale, high quality sheets. To study a possible approach to achieve quasi-freestanding graphene on a substrate by the intercalation of alkali metal atoms, Cs intercalation between graphene and Ni(111) is investigated. It is known that direct contact between graphene and Ni(111) perturbs the Dirac states. Cs intercalation restores the linear dispersion characteristic of Dirac fermions, which agrees with experiments, but the Dirac cone is shifted to lower energy, i.e., the graphene sheet is n-doped. Cs decouples the graphene sheet, while the spin polarization of Ni(111) does not extend through the intercalated atoms to the graphene sheet, for which we find virtually spin-degeneracy. In order to employ graphene in electronic applications, one requires a finite band gap. We engineer a band gap in metallic bilayer graphene by substitutional B and/or N doping. Specifically, the introduction of B-N pairs into bilayer graphene can be used to create a band gap that is stable against thermal fluctuations at room temperature. Introduction of B-N pairs into B and/or N doped bilayer graphene likewise hardly modifies the band dispersions, however, the size of the band gap is effectively tuned. We also study the influence of terrace edges on the electronic properties of graphene, considering bare edges and H, F, Cl, NH2 terminations. Periodic structural reconstruction is observed for the Cl and NH2 edge terminations due to interaction between the terminating atoms/groups. We observe that Cl edge termination p-dopes the terraces, while NH2 edge termination results in n-doping.

Les attentes autour du graphene peuvent être expliquées par les fortes potentialités applicatives de ce matériau (transistors RF, (bio)capteurs, TCL…). Sa grande surface spécifique, sa forte résistance mécanique ainsi que sa bonne conductivité permettraient de cibler des applications dans le domaine du stockage électrochimique tel que les pseudo-condensateurs. Il a été mis en évidence que des structures 3D de graphène définies comme des hydro- ou des aero- gels et présentant des surfaces spécifiques très élevées peuvent être obtenues. Ce type d’architecture présente un fort potentiel dans le domaine des matériaux d’électrodes carbonées pour le stockage de l’énergie et notamment pour les super-condensateurs. Leur obtention repose soit sur l’utilisation de molécules pontantes et de la création d’interactions covalentes ou électrostatiques, soit sur la réalisation d’aérogel à partir d’oxyde de graphène dans des conditions de pression/température établies. Basé sur ce contexte, le sujet de thèse présenté propose de développer des matériaux structurés à base de graphene (Fig.) pouvant être utilisés pour des applications en stockage électrochimique. La grande surface spécifique ainsi que la bonne conductivité du graphène permettraient de cibler de hautes densités d’énergie et de fortes puissances. Les différentes méthodes de préparation de ces architectures aérogels seront testées et les matériaux obtenus seront caractérisés morphologiquement (MEB, TEM, BET), structurellement (DRX) et chimiquement (XPS, IR, Raman, ATG). Les propriétés électrochimiques de ces espèces - correspondant à de nouvelles formes de carbones nanoporeux - seront ensuite évaluées. Ces structures seront également fonctionnalisées pour favoriser la formation d’une SEI stable et l’intercalation de certains ions. L’immobilisation de composés redox actifs sera testée afin d’insérer une composante pseudocapacitive au système. Les performances de ces matrices modifiées - s’apparentant à des matériaux d’électrodes pour pseudo-supercondensateur - seront enfin analysées. Les tâches que le thésard réalisera au cours de sa thèse seront donc variées et iront de la fabrication du matériau de base jusqu’à l’évaluation de son utilisation en cellule électrochimique. / The expectations around graphene come from huge potentialities for various applications (RF transistor, (bio)sensors…). Graphene high specific surface, mechanical resistance and conductivity make it specifically attractive for electrochemical storage applications. It has been shown that graphene sheets could be assembled to form structured graphene frameworks to translate the properties of individual sheets to functional materials and allow practical applications. The key features of these frameworks in terms of electrochemical storage applications are their graphitization level, their structural or textural disorder, and their porosity. On another hand, graphene has been interfaced with molecules to tune its electrical/optical properties or to make it more processable. The goal of the PhD project is to develop such structured graphene matrices following different methods such as the functionalization with bi-functional pilar molecules or the aerogel formation techniques. These assemblies will then be modified with different molecules to favor the formation of a stable SEI (solid electrolyte interface) and the intercalation of ions. The immobilization of redox compounds will also be attempted in order to bring a pseudo capacitive component to the system and target pseudo-capacitor applications. This thesis work will include material synthesis, assembling and functionalization steps combined to an important characterization tasks (IR, XRD, XPS, MEB, TEM, BET, electrochemistry...) The added-value of such work is that it represents an approach going from the basic material develop to the proof of concept.