First-principles calculations have been carried out to investigate structural and electronic properties of graphene on SiC and diamond substrates and for a study of doping of fluorographene with various surface adsorbates. New insight is given into the problem of the decoupling of the graphene layers from SiC substrates after epitaxial growth. Mechanisms of hydrogen penetration between graphene and SiC(0001) surface, and properties of hydrogen and fluorine intercalated structures have been studied. Energy barriers for diffusion of atomic and molecular hydrogen through the interface graphene layer with no defects and graphene layers containing Stone-Wales defect or two- and four-vacancy clusters have been calculated. It is argued that diffusion of hydrogen towards the SiC surface occurs through the hollow defects in the interface graphene layer. It is further shown that hydrogen easily migrates between the graphene layer and the SiC substrate and passivates the surface Si bonds, thus causing the graphene layer decoupling. According to the band structure calculations the graphene layer decoupled from the SiC(0001) surface by hydrogen intercalation is undoped, while that obtained by the fluorine intercalation is p-type doped. Further, structure and the electronic properties of single and double layer graphene on H-, OH-, and F- passivated (111) diamond surface have been studied. It is shown that graphene only weakly interacts with the underlying substrates and the linear dispersion of graphene pi-bands is preserved. For graphene on the hydrogenated diamond surfaces the charge transfer results in n-type doping of graphene layers and the splitting of conduction and valence bands in bilayer graphene. For the F- and OH-terminated surfaces, charge transfer and doping of graphene do not occur. Finally, the possibility of doping fluorographene by surface adsorbates have been investigated. The structure and electronic properties of fluorographene with adsorbed K, Li, Au atoms, and F4-TCNQ molecule are described. It is shown that adsorption of K or Li atoms results in electron doping of fluorographene, while Au atoms and F4-TCNQ introduce deep levels inside the band gap. The calculated value of the fluorographene work function is extremely high, 7.3 eV, suggesting that p-type doping is difficult to achieve.

Orientadores: Abner de Siervo, Richard Landers / Tese (doutorado) - Universidade Estadual de Campinas, Instituto de Física Gleb Wataghin / Made available in DSpace on 2018-08-25T18:41:09Z (GMT). No. of bitstreams: 1 DeLima_LuisHenrique_D.pdf: 24142455 bytes, checksum: b13ea70fe8434614a9b9ec40d25b3770 (MD5) Previous issue date: 2014 / Resumo: Monocamadas sp2 de grafeno e nitreto de boro hexagonal (h-BN) têm atraído muita atenção devido ao descobrimento de importantes propriedades, por exemplo, alta resistência mecânica, boa condutividade térmica e excelente estabilidade química e térmica. Porém, enquanto o grafeno é um semicondutor de gap nulo com alta mobilidade dos portadores de carga; o h-BN é um isolante com um largo band gap. Além disso, quando estas monocamadas sp2 são obtidas na superfície de uma variedade de substratos, elas podem exibir superestruturas corrugadas com parâmetro de rede no plano da ordem de nanômetros. Estas superestruturas são importantes para o autoordenamento de moléculas, átomos ou aglomerados de átomos e também para a intercalação de partículas e átomos em posições específicas na interface entre a monocamada sp2 e o substrato. Nesta tese, realizou-se um estudo, básico e aplicado, de monocamadas sp2 de grafeno e h-BN obtidas sobre a superfície do SiC(0001) e do Rh(111), respectivamente. Do ponto de vista básico, foi aplicada a técnica de Difração de Fotoelétrons (XPD) para um estudo da estrutura atômica do grafeno obtido por aquecimento na superfície do SiC(0001) e para a camada de interface entre o grafeno e o SiC, denominada buffer layer (BL). Os resultados de XPD mostraram particularidades distintas na estrutura atômica dessas monocamadas, o que explicaria a diferen_ca na estrutura eletrônica entre a BL e o grafeno. Do ponto de vista aplicado, foi mostrada a viabilidade do nanotemplate de grafeno/BL/SiC(0001) para a obtenção de aglomerados de Co e subsequente estudo das suas propriedades magnéticas por Dicroísmo Circular Magnético de Raios X (XMCD). Os aglomerados de Co foram obtidos sobre a camada de grafeno e os resultados evidenciam uma possível interação cluster-cluster de longo alcance, com influência nas propriedades magnéticas das partículas. Foi investigada também a intercalação dos átomos de Co entre o grafeno e a BL, formando uma rede quase periódica de clusters 2D. O grafeno forma uma barreira de proteção contra oxidação, preservando o caráter metálico das partículas. A monocamada de h-BN sobre a superfície do Rh(111) foi utilizada para a implantação de átomos de Rb. Para a implantação, foi construída uma evaporadora de íons de Rb que permite acelera-los numa faixa de energia possível para penetrar a monocamada de h-BN. Imagens de STM mostraram que os átomos de Rb termalizam entre a monocamada de h-BN e a superfície do Rh(111) em posições especificas da superestrutura, formando o que se denominou de nanotent. A formação dos nanotents e dos defeitos de vacância gerados pelo choque dos íons é uma forma de funcionalização do h-BN, sendo estas estruturas possíveis pontos de ancoragem de moléculas, átomos ou clusters de átomos / Abstract: Graphene and hexagonal boron nitride (h-BN) sp2 monolayers have attracted much attention due the discoveries of their important properties, such as high mechanical strength, good thermal conductivity and excellent chemical and thermal stability. However, while graphene is a zero band gap semiconductor with high carrier mobility; h-BN is a wide band gap insulator. Furthermore, when these sp2 monolayers are obtained on the surface of a variety of substrates, they can exhibit corrugated superstructures with a few nanometers in-plane lattice constants. Such superstructures are important for the self-assembly of molecules, atoms or clusters of atoms and also for the intercalation of these structures at specific positions between the sp2 monolayer and the substrate. In this thesis, we performed a study, fundamental and applied, of sp2 monolayers of graphene and h-BN obtained on the surface of SiC(0001) and Rh(111), respectively. From a fundamental point of view, the Photoelectron Di_raction (XPD) technique was applied for the study of the atomic structure of graphene obtained by heating the surface of the SiC(0001) and for the interface layer between the SiC and graphene, named buffer layer (BL). The XPD results showed distinct peculiarities in the atomic structure of these monolayers, which would explain the difference in electronic structure between BL and graphene. From the applied point of view, it has shown the feasibility of graphene/BL/SiC(0001) nanotemplate to obtain Co clusters and subsequent study of their magnetic properties by X-ray Magnetic Circular Dichroism (XMCD). The Co clusters were obtained on the graphene layer and the results suggest a possible clustercluster long-range interaction, that has influence on the magnetic properties of the particles. It was also investigated the intercalation of Co atoms between graphene, forming a quasi-periodic lattice of 2D-clusters. Moreover, graphene acts as a barrier to oxidation, preserving the metallic character of the clusters. The h-BN monolayer on the surface of Rh(111) was used for the implantation of Rb atoms. For the implantation, it was constructed an evaporator that allows the acceleration of Rb ions to an energy that enables the penetration through the h-BN monolayer. STM images show that the Rb atoms thermalize between the h-BN monolayer and the surface of the Rh(111) at specific positions of the superstructure, forming what is called a \\nanotent\". The formation of the nanotents and the vacancy defects generated by the collision of the ions is a form to functionalize the h-BN, with these structures being possible points for the anchoring of molecules, atoms or clusters of atoms / Doutorado / Física / Doutor em Ciências

Des mesures de magnétorésistances locales et non locales dans des monocouches de graphène obtenues par sublimation sur la face silicium du carbure de silicium (SiC) sont présentées dans cette thèse. L’objectif est d’étudier les phénomènes physiques qui apparaissent proche de point de neutralité de charge (dopage faible en trous) dans ces monocouches. Or, celles-ci sont généralement fortement dopées en électrons à cause de l’interaction avec la couche d’interface et le substrat. Des dispositifs en forme de barre de Hall encapsulés par une résine sont utilisés. Le contrôle du niveau de Fermi dans ces dispositifs est réalisé en utilisant la méthode de décharge corona. L’amplitude du désordre est évaluée dans ces monocouches de graphène en : (i) ajustant la courbe de résistivité en fonction du coefficient de Hall obtenue à température ambiante ; (ii) ajustant les courbes de dépendance en température de la densité de Hall pour les échantillons proche de point de neutralité de charge. Toutes ces analyses donnent une amplitude du désordre de l’ordre de (20 ±10) meV. Les échantillons préparés avec un faible dopage en trous sont ensuite étudiés en régime d'effet Hall quantique. Les mesures de magnétorésistances montrent que la résistance de Hall présente un comportement ambipolaire en fonction du champ magnétique. Ce comportement coïncide avec l’apparition d’un maximum local dans la résistance longitudinale. Ces résultats sont expliqués via un modèle de transfert de charge entre régions de différents dopages dans le graphène. Néanmoins, l’origine microscopique de ces régions est mal connue. Finalement, des mesures non locales sont effectuées sur ces mêmes échantillons et montrent l’apparition des résistances non locales importantes dont la valeur peut, dans certains cas, dépasser les résistances longitudinales correspondantes. L'analyse de ces résultats montre que la contribution du courant de spin et des effets thermiques dans l’apparition de ces tensions non locales est négligeable. Cependant, les données expérimentales sont raisonnablement reproduites par un modèle de conduction basé sur des états de bord rétrodiffusés par le "bulk" isolant. / Local and nonlocal magnetoresistances measurements on monolayer graphene grown on the silicon face of silicon carbide (SiC) are reported. The purpose of this work is to understand the physical phenomena appearing close to the charge neutrality point in these monolayers. The first issue to overcome was that graphene is generally strongly doped with electrons due to the interaction with the substrate. The control of the Fermi level has been realised using the corona discharge method. The disorder amplitude has been evaluated in these structures by : (i) fitting the resistivity dependence curve of the Hall coefficient obtained at room temperature; (ii) fitting the temperature dependence of the Hall density for samples that were prepared near the charge neutrality point. All these analyses gave a disorder strength equal to (20 ± 10) meV. It is then shown that for samples with low hole doping, the Hall resistance exhibits an ambipolar behavior as a function of the magnetic field. This behavior is accompanied by the appearance of a local maximum in the longitudinal resistance.This behavior is been explained by a charge transfer model between regions of different doping in graphene. Nevertheless, the microscopic origin of these regions is poorly known. Finally, nonlocal measurements carried out on these samples showed the appearance of important nonlocal resistances which in some cases exceed the corresponding longitudinal resistances. The analysis of these results shows that the contribution of spin current and thermal effects on the occurrence of these nonlocal voltages is neglegible. In contrast, the experimental data are reproduced quite well by a model based on counter-propagating edge states backscattered by the bulk.