The thesis consists of four parts of which part 1 presents a brief overview of nanomaterials. Parts 2, 3 and 4 contain results of investigations of graphene, nanofilms of noble metal nanoparticles and ZnO nanostructures respectively. Investigations of graphene are described in Part 2 which consists of six chapters. In Chapter 2.1, changes in the electronic structure and properties of graphene induced by molecular charge-transfer have been discussed. Chapter 2.2 deals with the results of a study of the interaction of metal and metal oxide nanoparticles with graphene. Electrical and dielectric properties of graphene-polymer composites are presented in Chapter 2.3. Chapter 2.4 presents photo-thermal effects observed in laser-induced chemical transformations in graphene and other nanocarbons system. Chapter 2.5 describes the mechanical properties of polymer matrix composites reinforced by fewlayer graphene investigated by nano-indentation. The extraordinary synergy found in the mechanical properties of polymer matrix composites reinforced with two nanocarbons of different dimensionalities constitute the subject matter of Chapter 2.6. Investigations of noble metal nanoparticles have been described in Part 3. In Chapter 3.1, ferromagnetism exhibited by nanoparticles of noble metals is discussed in detail while Chapter 3.2 deals with surface-enhanced Raman scattering (SERS) of molecules adsorbed on nanocrystalline Au and Ag films formed at the organic–aqueous interface. Factors affecting laser-excited photoluminescence from ZnO nanostructures are examined in great detail in Part 4.

Graphene, a single sheet of carbon atoms sp2-bonded in a honeycomb lattice, is a possible all-carbon successor to silicon electronics. Ballistic conduction at room temperature and a linear dispersion relation that causes carriers to behave as massless Dirac fermions are features that make graphene promising for high-speed, low-power devices. The critical advantage of epitaxial graphene (EG) grown on SiC is its compatibility with standard lithographic procedures. Surface X-ray diffraction (SXRD) and scanning tunneling microscopy (STM) results are presented on the domain structure, interface composition and stacking character of graphene grown on both polar faces of semi-insulating 4H-SiC. The data reveal intriguing differences between graphene grown on these two faces. Substrate roughening is more pronounced and graphene domain sizes are significantly smaller on the SiC (0001) Si-face. Specular X-ray reflectivity measurements show that both faces have a carbon rich, extended interface that is tightly bound to the first graphene layer, leading to a buffering effect that shields the first graphene layer from the bulk SiC, as predicted by ab initio calculations. In-plane X-ray crystal truncation rod analysis indicates that rotated graphene layers are interleaved in C-face graphene films and corresponding superstructures are observed in STM topographs. These rotational stacking faults in multilayer C-face graphene preserve the linear dispersion found in single layer graphene, making EG electronics possible even for a multilayer material.

Comme le quatrième élément le plus abondant dans l’univers, le carbone joue un rôle important dans l’émergence de la vie sur la terre comme nous la connaissons aujourd’hui. L’ère industrielle a vu cet élément au cœur des applications technologiques en raison des différentes façons dont les atomes forment les liaisons chimiques, ce qui donne lieu à une série d’allotropies chacun ayant des propriétés physiques extraordinaires. Par exemple, l’allotrope le plus thermodynamiquement stable du carbone, le cristal de graphite, est connu pour être un très bon conducteur électrique, tandis que le diamant, très apprécié pour sa dureté et sa conductivité thermique, est néanmoins considéré comme un isolant électrique en raison de sa structure cristallographique différente par rapport au graphite. Les progrès de la recherche scientifique ont montré que les considérations cristallographiques ne sont pas le seul facteur déterminant pour une telle variété dans les propriétés physiques des structures à base de carbone. Ces dernières années ont vu l’émergence de nouvelles formes allotropiques de structures de carbone qui sont stables dans les conditions ambiantes, mais avec dimensionnalité réduite, ce qui entraîne des propriétés largement différentes par rapport aux structures en trois dimensions. Parmi ces nouvelles classes d’allotropes il y a le graphene, qui est le premier matériau à deux dimensions. L’isolation réussi de monocouches de graphène a contesté une croyance établie depuis longtemps en physique : le fait que les matériaux purement 2D ne peuvent pas exister dans les conditions ambiantes parce qu'ils sont instables en raison de l’augmentation des fluctuations thermiques lorsqu’ils se prolongent dans les 2D. Afin de minimiser son énergie, un matériau se brisera en îlots coagulées. Le graphène arrive cependant à surmonter cette barrière en formant des ondulations continues sur la surface du substrat et est stable même à température ambiante et pression atmosphérique. Une grande intention dans la communauté scientifique a été donnée au graphène, après les premiers résultats publiés sur les propriétés électroniques de ce matériau. Les propriétés fondamentales et mécaniques du graphène sont fascinants. Grace aux atomes de carbone qui sont emballés dans un mode sp2 hybridé, formant ainsi une structure de réseau hexagonal, le graphène possède le plus grand module de Young et la plus grande capacité d’étirement, en même temps des centaines de fois plus dur que l’acier. Il conduit la chaleur et l’électricité de manière très efficace. L’aspect le plus fascinant à propos du graphène est surement la nature de ses porteurs de charge à basse énergie. En effet, le graphène présente des bandes d’énergie linéaires au point de neutralité de charge, donnant aux porteurs de charge une nature relativiste. De nombreux phénomènes observés dans ce matériau sont des conséquences de la nature relativiste de ses porteurs. Transport balistique, conductivité optique universelle, absence de rétrodiffusion, et une nouvelle classe d’effet Hall quantique sont de bons exemples de phénomènes nouvellement découverts dans ce matériau. Il est cependant encore trop tôt pour affirmer que toutes les propriétés physiques du graphene sont bien comprises. Dans cette thèse, nous avons mené des expériences de spectroscopie magnéto-Raman pour répondre à certaines des questions ouvertes dans la physique du graphène, notamment l’effet de couplage de Coulomb sur le spectre d’énergie du graphène, et le changement dans les propriétés physiques du graphène multicouche en fonction de sa cristallographie. Nos echantillions ont été soumis à de forts champs magnétiques, appliqués perpendiculairement aux plans atomiques. Le spectre d’excitation sous champ magnétique montre un couplage entre ces excitations et les modes de vibratoires. Cette approche expérimentale permet de remonter à la structure de bande du graphene en champs nul, ainsi que de nombreuses autres propriétés du matériau. / As the fourth most abundant element in the universe, Carbon plays an important rolein the emerging of life in earth as we know it today. The industrial era has seen this element at the heart of technological applications due to the different ways in which carbon forms chemical bonds, giving rise to a series of allotropes each with extraordinary physical properties. For instance, the most thermodynamically stable allotrope of carbon, graphite crystal, is known to be a very good electrical conductor, while diamond very appreciated for its hardness and thermal conductivity is nevertheless considered as an electrical insulator due to different crystallographic structure compared to graphite. The advances in scientific research have shown that crystallographic considerations are not the only determining factor for such a variety in the physical properties of carbon based structures. Recent years have seen the emergence of new allotropes of carbon structures that are stable at ambient conditions but with reduced dimensionality, resulting in largely different properties compared to the three dimensional structures. Among these new classes of carbon allotropes is the first two-dimensional material: graphene.The successful isolation of monolayers of graphene challenged a long established belief in the scientific community: the fact that purely 2D materials cannot exist at ambient conditions. The Landau-Peierls instability theorem states that purely 2D materials are very unstable due to increasing thermal fluctuations when the material in question extends in both dimensions. To minimize its energy, the material will break into coagulated islands, an effect known as island growth. Graphene happens to overcome such barrier by forming continuous ripples on the surface of its substrate and thus is stable even at room temperature and atmospheric pressure.A great intention from the scientific community has been given to graphene, since 2004. Both fundamental and mechanical properties of graphene are fascinating. Thanks to its carbon atoms that are packed in a sp2 hybridized fashion, thus forming a hexagonal lattice structure, graphene has the largest young modulus and stretching power, yet it is hundreds of times stronger than steel. It conducts heat and electricity very efficiently, achieving an electron mobility as high as 107 cm−2V−1 s−1 when suspended over the substrate. The most fascinating aspect about graphene is the nature of its low energy charge carriers. Indeed, graphene has a linear energy dispersion at the charge neutrality, giving the charge carriers in graphene a relativistic nature. Many phenomena observed in this material are consequences of this relativistic nature of its carriers. Ballistic transport, universal optical conductivity, absence of back-scattering, and a new class of room temperaturequantum Hall effect are good examples of newly discovered phenomena in thismaterial. Graphene has become an active research area in condensed matter physics since 2004. It is however still early to state that all the physical properties of this material are well understood. In this thesis we conducted magneto-Raman spectroscopy experiments to address some of the open questions in the physics of graphene, such as the effect of electron-electron coupling on the energy spectrum of monolayer graphene, and the change in the physical properties of multilayer graphene as a function of the crystallographic stacking order. In all our experiments, the graphene-based systems have been subject to strong continuous magnetic fields, applied normal to the graphene layers. We study the evolution of its energy excitation spectra in the presence of the magnetic field, and also the coupling between these excitations and specific vibrational modes that are already in the system. This experimental approach allows us to deduce the band structure of the studied system at zero field, as well as many other lowenergy properties.

[pt] Nesta tese de doutoramento apresentamos a fabricação e a caracterização de dispositivos fotovoltaicos orgânicos (OPVs) fabricados a partir de eletrodos de grafeno e de substratos híbridos flexíveis à base de polímeros recobertos com um filme fino condutor. Para isso, inicialmente sintetizamos filmes de grafeno através da técnica de deposição química em fase de vapor (CVD), seguido de modificações no processo de transferência do grafeno para o substrato desejado. Nesta etapa, desenvolvemos uma nova metodologia utilizando uma blenda condutora de EPDM-PAni que simplifica o processo de transferência e melhora as propriedades elétricas do grafeno. Em outro momento, otimizamos diferentes substratos híbridos à base de polímeros de PVC, PVA e celulose bacteriana (BC) recobertos com um filme fino condutor de ITO. Tanto os substratos híbridos flexíveis, quanto os filmes de grafeno, foram investigados por transmitância ótica e resistência de folha a fim de avaliar os seus potenciais uso para as aplicações em OPVs. Por fim, fabricamos diversas estruturas de OPVs, tanto com o grafeno como eletrodo condutor, quanto usando os substratos híbridos flexíveis. Estes dispositivos foram caracterizados principalmente através das suas curvas características JxV, no escuro e sob iluminação. Além disso, realizamos ciclos de flexão/extensão de alguns dispositivos a fim de avaliar seu comportamento frente aos esforços mecânicos a estes submetidos. Os resultados obtidos mostraram que os filmes de grafeno fabricados são promissores para a aplicação como eletrodo condutor transparente em OPVs e que os substratos híbridos investigados podem ser utilizados em dispositivos flexíveis, visto que apresentaram comportamento semelhante aos substratos inorgânicos comumente utilizados. / [en] In this doctoral thesis we present the fabrication and characterization of organic photovoltaic devices (OPVs) assembled onto graphene electrodes and flexible hybrid polymers-based substrates coated with a conductive thin film. For this, initially the graphene films were synthesized by chemical vapor deposition (CVD) technique, followed by modifications in the transfer process of the graphene to the desired substrate. In this step, we developed a new methodology using an EPDM-PAni conductive blend that simplifies the transfer process and improves the electric properties of graphene. We also used another approach which consists in optimizing different hybrid substrates based on PVC, PVA and bacterial cellulose (BC) polymers coated with an ITO conductive thin film. The flexible hybrid substrates as well as the graphene films were investigated by optical transmittance and sheet resistance in order to evaluate their potential use for OPVs applications. Finally, we fabricate various structures of OPVs, using graphene as a conducting electrode, well as using flexible hybrid substrates. Such devices were characterized mainly through their dark and light J×V characteristic curves. In addition, we performed flexion/extension cycles in some devices in order to evaluate their behavior against the mechanical stresses submitted to them. The results showed that the graphene films are a promising material for the application as a transparent conductive electrode in OPVs and the hybrid substrates investigated can be used in flexible devices, since they presented similar behavior to the commonly used inorganic substrates.

Graphene is a two dimensional carbon material whose outstanding properties have been envisaged for a variety of applications. Cu-catalyzed chemical vapour deposition (Cu-CVD) is promising for large scale production of high quality monolayer graphene. But the existing Cu-CVD technology is not ready for industry-level production. It still needs to be improved on some aspects, three of which include synthesizing industrially useable graphene films under safe conditions, visualizing the domain boundaries of the continuous graphene, and understanding the kinetic features of the Cu-CVD process. This thesis presents the research aiming at these three objectives. By optimizing the Cu pre-treatments and the CVD process parameters, continuous graphene monolayers with the millimetre-scale domain sizes have been synthesized. The process safety has been ensured by delicately diluting the flammable gases. Through a novel optical microscope set up, the spatial distributions of the domains in the continuous Cu-CVD graphene films have been directly imaged and the domain boundaries visualised. This technique is non-destructive to the graphene and hence could help manage the domain boundaries of the large area graphene. By establishing the novel rate equations for graphene nucleation and growth, this study has revealed the essential kinetic characteristics of general Cu-CVD processes. For both the edge-attachment-controlled and the surface-diffusion-controlled growth, the rate equations for the time-evolutions of the domain size, the nucleation density, and the coverage are solved, interpreted, and used to explain various Cu-CVD experimental results. The continuous nucleation and inter-domain competitions prove to have non-trivial influences over the growth process. This work further examines the temperature-dependence of the graphene formation kinetics leading to a discovery of the internal correlations of the associated energy barriers. The complicated effects of temperature on the nucleation density are explored. The criteria for identifying the rate-limiting step is proposed. The model also elucidates the kinetics-dependent formation of the characteristic domain outlines. By accomplishing these three objectives, this research has brought the current Cu-CVD technology a large step forward towards practical implementation in the industry level and hence made high quality graphene closer to being commercially viable.

Les dernières années ont vu l'avènement des couches cristallines bidimensionnelles, appelées matériaux 2D. L'exemple le plus connu est le graphène, d'autres étant le nitrure de bore hexagonal isolant et le diséléniure de niobium supraconducteur. Ces matériaux 2D peuvent être empilés de manière contrôlée sous la forme d'hétérostructures de van der Waals pour obtenir les propriétés électroniques désirées. L’une des plus simples hétérostructures de van der Waals est l'empilement de deux couches de graphène tournées. Cet empilement donne naissance à un moiré qui peut être vu comme un potentiel superpériodique dépendant de l'angle entre les deux couches. Les propriétés électroniques des couches tournées de graphène sont intimement liées à ce moiré.Le sujet de cette thèse est l'étude expérimentale du lien entre la structure et les propriétés électroniques des couches tournées de graphène par Microscopie et Spectroscopie à effet tunnel à basse température.Alors que l'effet de l'angle entre les couches sur les propriétés électroniques a déjà été étudié en détail, la modification de celles-ci par une déformation des couches n'a été envisagée que récemment. La première partie de ce travail expérimental étudie la modification par la déformation des propriétés électroniques de couches de graphène tournées d'un angle de 1.26° crûes sur carbure de silicium. La déformation en question est différente dans les deux couches et son effet apparait clairement dans la densité locale d'états électroniques du moiré. Contrairement à une déformation appliquée identiquement aux deux couches, une différence de déformations entre les couches (déformation relative) modifie fortement la structure de bandes même à faibles valeurs de déformations. Alors que la déformation relative était spontanément présente, la deuxième partie de cette thèse s'intéresse à l'effet d'une déformation appliquée directement aux couches de graphène. Cette déformation vient d'une interaction induite par l'approche de la pointe STM vers la surface de graphène. La modification active de la densité d'états qui en résulte dépend de la position de la pointe dans le moiré avec l'apparition d'instabilités périodiques lorsque la distance entre la pointe et l'échantillon est très faible.La troisième partie de cette thèse concerne l'étude d'un autre type de modification des propriétés électroniques consistant en l'induction de supraconductivité dans les couches de graphène. Cette modification est effectuée par une croissance du graphène en une seule étape sur du carbure de tantale supraconducteur. Les résultats montrent la formation d'une couche de carbure de tantale de grande qualité sur laquelle les couches de graphène forment des moirés. La mesure à basse température de la densité d'états de ces moirés montre la présence d'un effet de proximité supraconducteur induit par le carbure de tantale. / Recent years have seen the emergence of two-dimensional crystalline layers, called 2D materials. Examples include the well-known graphene, insulating hexagonal boron nitride and superconducting niobium diselenide. The stacking of these 2D materials can be controlled to achieve desirable electronic properties under the form of van der Waals heterostructures. One of the simplest van der Waals heterostructures is the misaligned stacking of two graphene layers. Twisted graphene layers show a moiré pattern which can be viewed as a superperiodic potential that depends on the twist angle. The electronic properties of the twisted graphene layers are strongly linked to this moiré pattern.The subject of the present thesis is the experimental study of the link between the structural and the electronic properties of twisted graphene layers by means of low-temperature Scanning Tunneling Microscopy and Spectroscopy (STM/STS).While the effect of the twist angle has already been studied in great details, the modulation of the electronic properties by the deformation of the layers has been explored only recently. In the first part of this experimental work, a strain-driven modification of the electronic properties is probed in graphene layers with a twist angle of 1.26° grown on silicon carbide. The determined strain is found to be different in the two layers leading to a clear signature in the local electronic density of states of the moiré even at low strain magnitudes. Contrary to a strain applied in the two layers, this difference of strain between the layers (relative strain) modifies strongly the electronic band structure even at low strain magnitudes. While this relative strain is natively present, the second part of the work explores the effect of an applied strain in the layers. This is realized by approaching the STM tip to the graphene surface to trigger an interaction between the two. The resulting active modification of the density of states is shown to depend on the position on the moiré, leading to periodic instabilities at very low tip-sample distances.In the third part of the work, another type of modification of the electronic properties is studied when superconductivity was induced in the graphene layers. This is done by growing graphene on superconducting tantalum carbide in a single-step annealing. The results show the formation of a high-quality tantalum carbide layer on which graphene layers form moiré patterns. The low-temperature density of states of these moirés show evidence of a superconducting proximity effect induced by the tantalum carbide.

CASTRO, Manuela Oliveira de. Síntese de grafeno pelo método CVD. 2011. 84 f. Dissertação (Mestrado em Física) - Programa de Pós-Graduação em Física, Departamento de Física, Centro de Ciências, Universidade Federal do Ceará, Fortaleza, 2011. / Submitted by Edvander Pires (edvanderpires@gmail.com) on 2014-11-13T20:03:00Z No. of bitstreams: 1 2011_dis_mocastro.pdf: 3411512 bytes, checksum: 64f6579c926c263cd9391dfb058954bb (MD5) / Approved for entry into archive by Edvander Pires(edvanderpires@gmail.com) on 2014-11-13T20:35:29Z (GMT) No. of bitstreams: 1 2011_dis_mocastro.pdf: 3411512 bytes, checksum: 64f6579c926c263cd9391dfb058954bb (MD5) / Made available in DSpace on 2014-11-13T20:35:29Z (GMT). No. of bitstreams: 1 2011_dis_mocastro.pdf: 3411512 bytes, checksum: 64f6579c926c263cd9391dfb058954bb (MD5) Previous issue date: 2011 / The advancement and improvement of synthesis techniques and handling of materials are fundamental to understand their properties and possible forms of production and use. However, in the case of nanomaterials, problems such as structural defects, high cost and difficulty of achieving production on a large scale have yet to be solved. Inserted in this panorama is graphene, a two-dimensional nanomaterial whose morphology, consisting of carbon atoms arranged in hexagonal form, is responsible for unprecedented properties that have revolutionary relevance for both basic and applied research. There are different methods of synthesis of graphene. The method of Chemical Vapor Deposition (CVD) is among the most advantageous ones. This method consists in breaking the bonds of the molecules of a gas subjected to high temperatures so that the atoms from the gas are deposited on a given substrate. In this work, we used the CVD method for the synthesis of graphene on oxidized silicon substrates (Si/SiO2) coated with a 500 nm thick film of nickel (Ni), which served as the catalyst. Methane gas (CH4) was used as the source of the carbon atoms and the synthesis was carried out using different sets of parameters. Experiments were performed, firstly, using parameters es-tablished in the literature and the results were compared with those obtained by other authors. The influence of the synthesis parameters and the characteristics of the films of Ni catalysts on the properties of the graphene films was studied. The samples were characterized using Scanning Electron Microscopy, Confocal Raman and Optical Microscopy, and Atomic Force Microscopy. In agreement with results from the literature, it could be observed that thin films are synthesized and they are composed of graphitic flakes with a non-uniform thickness, which is strongly dependent of the morphology of catalyst film. Larger regions with characteristic Raman spectra of monolayer and few layer graphene could be obtained by combining thermal treatment of Ni film during the sputtering process with low gas flow and time of exposure to CH4 in the CVD experiment. Variations in the Raman spectra of the flakes could be observed, including the emergence of the D-band and the displacement of the peaks. These variations, which reveal the influence of substrates on the synthesized films, were more intense the smaller the number of graphene layers. Next, we combined methods reported in the literature for estimating the number of layers on the basis of the characteristics of the Raman spectra with AFM analysis to obtain the thickness of the graphene layer. The results obtained from our analysis show that monolayer graphene could be successfully synthesized in the experiments. / O avanço e o aperfeiçoamento das técnicas de síntese e manipulação de materiais são fundamentais para o entendimento de suas propriedades e das possíveis formas de produção e utilização. Porém, no caso dos nanomateriais, principalmente, cujas extraordinárias capacidades são bastante celebradas, problemas como defeitos estruturais, alto custo de obtenção e dificuldade de produção em larga escala ainda necessitam ser solucionados. Inserido neste panorama está o grafeno, um nanomaterial cuja morfologia bidimensional, constituída por átomos de carbono dispostos de forma hexagonal, é responsável por propriedades sem precedentes que apresentam revolucionária relevância, tanto para a pesquisa básica quanto para a pesquisa aplicada. Neste sentido, existem diferentes métodos de síntese de grafeno, estando entre os mais vantajosos o método de deposição química em fase de vapor (Chemical Vapor Deposition - CVD). Este método consiste na quebra das ligações das moléculas de um gás submetido a altas temperaturas de modo que os átomos provenientes do gás sejam depositados sobre um determinado substrato. Neste trabalho, utilizou-se o método CVD para a síntese de grafeno sobre substratos de silício oxidado (Si/SiO2) recobertos por filmes de níquel (Ni) com, aproximadamente, 500nm de espessura, os quais funcionaram como catalisadores. O gás metano (CH4) foi utilizado como a fonte dos átomos de carbono depositados e os processos de síntese tiveram diferentes conjuntos de parâmetros executados. A síntese de grafeno pelo método CVD teve como objetivo geral verificar os resultados divulgados na literatura e aperfeiçoá-los, relacionando os parâmetros utilizados nas sínteses e as características dos filmes de Ni catalisadores com aquelas apresentadas pelos filmes de grafeno obtidos nos experimentos. As amostras foram caracterizadas por meio de Microscopia Eletrônica de Varredura, Microscopia Óptica e Raman Confocal e Microscopia de Força Atômica. Em consistência com os resultados publicados na literatura, observou-se que são sintetizados filmes finos compostos por flakes de material grafítico com espessura não uniforme, e que a obtenção de filmes mais uniformes é fortemente dependente da morfologia do filme catalisador. Regiões apresentando espectro Raman característico de monocamadas de grafeno e de grafeno de poucas camadas foram maiores quando combinados o tratamento térmico do filme de Ni com o baixo fluxo e menor tempo de exposição ao CH4. Verificaram-se, ainda, variações nos espectros Raman dos flakes. Estas variações apresentaram-se mais intensas, quanto mais reduzido é o número de camadas de grafeno e incluem o aparecimento da banda D, além do deslocamento dos picos, revelando a influência dos substratos sobre os filmes sintetizados. Esta pesquisa considerou métodos de estimativa do número de camadas por características do espectro Raman, divulgados na literatura, aliados à análise da espessura por AFM que mostraram ser possível a síntese de monocamadas de grafeno.

Conselho Nacional de Desenvolvimento CientÃfico e TecnolÃgico / O avanÃo e o aperfeiÃoamento das tÃcnicas de sÃntese e manipulaÃÃo de materiais sÃo fundamentais para o entendimento de suas propriedades e das possÃveis formas de produÃÃo e utilizaÃÃo. PorÃm, no caso dos nanomateriais, principalmente, cujas extraordinÃrias capacidades sÃo bastante celebradas, problemas como defeitos estruturais, alto custo de obtenÃÃo e dificuldade de produÃÃo em larga escala ainda necessitam ser solucionados. Inserido neste panorama està o grafeno, um nanomaterial cuja morfologia bidimensional, constituÃda por Ãtomos de carbono dispostos de forma hexagonal, à responsÃvel por propriedades sem precedentes que apresentam revolucionÃria relevÃncia, tanto para a pesquisa bÃsica quanto para a pesquisa aplicada. Neste sentido, existem diferentes mÃtodos de sÃntese de grafeno, estando entre os mais vantajosos o mÃtodo de deposiÃÃo quÃmica em fase de vapor (Chemical Vapor Deposition - CVD). Este mÃtodo consiste na quebra das ligaÃÃes das molÃculas de um gÃs submetido a altas temperaturas de modo que os Ãtomos provenientes do gÃs sejam depositados sobre um determinado substrato. Neste trabalho, utilizou-se o mÃtodo CVD para a sÃntese de grafeno sobre substratos de silÃcio oxidado (Si/SiO2) recobertos por filmes de nÃquel (Ni) com, aproximadamente, 500nm de espessura, os quais funcionaram como catalisadores. O gÃs metano (CH4) foi utilizado como a fonte dos Ãtomos de carbono depositados e os processos de sÃntese tiveram diferentes conjuntos de parÃmetros executados. A sÃntese de grafeno pelo mÃtodo CVD teve como objetivo geral verificar os resultados divulgados na literatura e aperfeiÃoÃ-los, relacionando os parÃmetros utilizados nas sÃnteses e as caracterÃsticas dos filmes de Ni catalisadores com aquelas apresentadas pelos filmes de grafeno obtidos nos experimentos. As amostras foram caracterizadas por meio de Microscopia EletrÃnica de Varredura, Microscopia Ãptica e Raman Confocal e Microscopia de ForÃa AtÃmica. Em consistÃncia com os resultados publicados na literatura, observou-se que sÃo sintetizados filmes finos compostos por flakes de material grafÃtico com espessura nÃo uniforme, e que a obtenÃÃo de filmes mais uniformes à fortemente dependente da morfologia do filme catalisador. RegiÃes apresentando espectro Raman caracterÃstico de monocamadas de grafeno e de grafeno de poucas camadas foram maiores quando combinados o tratamento tÃrmico do filme de Ni com o baixo fluxo e menor tempo de exposiÃÃo ao CH4. Verificaram-se, ainda, variaÃÃes nos espectros Raman dos flakes. Estas variaÃÃes apresentaram-se mais intensas, quanto mais reduzido à o nÃmero de camadas de grafeno e incluem o aparecimento da banda D, alÃm do deslocamento dos picos, revelando a influÃncia dos substratos sobre os filmes sintetizados. Esta pesquisa considerou mÃtodos de estimativa do nÃmero de camadas por caracterÃsticas do espectro Raman, divulgados na literatura, aliados à anÃlise da espessura por AFM que mostraram ser possÃvel a sÃntese de monocamadas de grafeno. / The advancement and improvement of synthesis techniques and handling of materials are fundamental to understand their properties and possible forms of production and use. However, in the case of nanomaterials, problems such as structural defects, high cost and difficulty of achieving production on a large scale have yet to be solved. Inserted in this panorama is graphene, a two-dimensional nanomaterial whose morphology, consisting of carbon atoms arranged in hexagonal form, is responsible for unprecedented properties that have revolutionary relevance for both basic and applied research. There are different methods of synthesis of graphene. The method of Chemical Vapor Deposition (CVD) is among the most advantageous ones. This method consists in breaking the bonds of the molecules of a gas subjected to high temperatures so that the atoms from the gas are deposited on a given substrate. In this work, we used the CVD method for the synthesis of graphene on oxidized silicon substrates (Si/SiO2) coated with a 500 nm thick film of nickel (Ni), which served as the catalyst. Methane gas (CH4) was used as the source of the carbon atoms and the synthesis was carried out using different sets of parameters. Experiments were performed, firstly, using parameters es-tablished in the literature and the results were compared with those obtained by other authors. The influence of the synthesis parameters and the characteristics of the films of Ni catalysts on the properties of the graphene films was studied. The samples were characterized using Scanning Electron Microscopy, Confocal Raman and Optical Microscopy, and Atomic Force Microscopy. In agreement with results from the literature, it could be observed that thin films are synthesized and they are composed of graphitic flakes with a non-uniform thickness, which is strongly dependent of the morphology of catalyst film. Larger regions with characteristic Raman spectra of monolayer and few layer graphene could be obtained by combining thermal treatment of Ni film during the sputtering process with low gas flow and time of exposure to CH4 in the CVD experiment. Variations in the Raman spectra of the flakes could be observed, including the emergence of the D-band and the displacement of the peaks. These variations, which reveal the influence of substrates on the synthesized films, were more intense the smaller the number of graphene layers. Next, we combined methods reported in the literature for estimating the number of layers on the basis of the characteristics of the Raman spectra with AFM analysis to obtain the thickness of the graphene layer. The results obtained from our analysis show that monolayer graphene could be successfully synthesized in the experiments.

The uniform dispersion of the nano fillers without agglomeration in a polymeric matrix is widely adapted for the purpose of mechanical properties enhancement. In the context to biomedical applications, the type and amount of nanoparticles can potentially influence the biocompatibility. In order to address these issues, High Density Polyethylene (HDPE) based composites reinforced with graphene oxide (GO) were prepared by melt mixing followed by compression moulding. In an attempt to tailor the dispersion and to improve the interfacial adhesion, polyethylene (PE) was immobilized onto GO sheets by nucleophilic addition-elimination reaction. A good combination of yield strength (ca. 20 MPa), elastic modulus (ca. 600 MPa) and an outstanding elongation at failure (ca. 70 %) were recorded with 3 wt % polyethylene grafted graphene oxide (PE-g-GO) reinforced HDPE composites. Considering the relevance of protein adsorption as a biophysical precursor to cell adhesion, the protein adsorption isotherms of bovine serum albumin (BSA) were determined to realize three times higher equilibrium constant (Keq) for PE-g-GO reinforced HDPE composites as compared to GO reinforced composites. In order to assess the cytocompatibility, osteoblast cells (MC3T3) were grown on HDPE/GO and HDPE/PE-g-GO composites, in vitro. The statistically significant increase in metabolically active cell was observed, irrespective of the substrate composition. Such observation indicated that HDPE with GO or PE-g-GO addition (upto 3 wt %) can be used as cell growth substrate. The extensive proliferation of cells with oriented growth pattern also supported the fact that tailored GO addition can support cellular functionality, in vitro. Taken together, the experimental results suggest that the PE-g-GO in HDPE can effectively be utilized to enhance both mechanical and cytocompatibility properties and can further be explored for potential biomedical applications.

Electronic systems are an indivisible part of modern life. Every day, new materials, devices, passive components, antennas for wireless communication are needed to be designed and developed. In particular, flexible and biocompatible wearable devices are urgent required for medical and industrial applications. The great hope lies in the materials with high crystalline quality and flexibility such as graphene and other 2D semiconductors and insulators. Doping is a conventional tool for tailoring of the electronic properties of the functional materials. Here we examine application of the widely used the electron donor species to the graphene and hexagonal boron nitride monolayer (h-BN). For each we determine surface-interface properties and the full electronic band structure using the combination of the surface science methods such as angle-integrated and angle resolved photoemission (XPS, ARPES), electron diffraction (LEED) and photo absorption (XAS). As the result we provided insight into mechanisms underlying the doping gating of the graphene h-BN monolayer by the alkali metals. We fully characterized their surface and interface structure. Finally we studied the interplay between electrons and phonons in the doped graphene and we demonstrated that Ca-doped graphene is the promising candidate for realizing superconductivity in graphene.