Novel two dimensional nanoscale materials like graphene and metal dichalcogenides (MX2) have attracted the attention of the scientific community, due to their rich physics and wide range of potential applications. It has been shown that novel graphene based transparent conductors and radiofrequency transistors are competitive with the existing technologies. Graphene’s properties are influenced sensitively by adsorbates and substrates. As such not surprisingly, physical properties of graphene are found to have a large variability, which cannot be controlled at the synthesis level, reducing the utility of graphene. As a part of my doctorate dissertation, I have developed atomic hydrogen as a novel technique to count the scatterers responsible for limiting the carrier mobility of graphene field effect transistors on silicon oxide (SiO2) and identified that charged impurities to be the most dominant scatterer. This result enables systematic reduction of the detrimental variability in device performance of graphene. Such sensitivity to substrates also gives an opportunity for engineering device properties of graphene using substrate interaction and atomic scale vacancies. Stacking graphene on hexagonal boron-nitride (h-BN) gives rise to nanoscale periodic potential, which influences its electronic graphene. Using state-of-the-art atomic-resolution scanning probe microscope, I correlated the observed transport properties to the substrate induced extrinsic potentials. Finally in efforts to exploit graphene’s sensitivity to discover new sensor technologies, I have explored noncovalent functionalization of graphene using peptides. Molybdenum disulfide (MoS2) exhibits thickness dependent bandgap. Transistors fabricated from single layer MoS2 have shown a high on/off ratio. It is expected that ad-atom engineering can be used to induce on demand a metal-semiconductor transition in MoS2. In this direction, I have iii explored controlled/reversible fluorination and hydrogenation of monolayer MoS2 to potentially derive a full range of integrated circuit technology. The in-depth characterization of the samples is carried out by Raman/photoluminescence spectroscopy and scanning tunneling microscopy

The thesis aims to illustrate and highlight the potential of graphene-based fillers in reinforcing organic coatings deposited by cataphoresis. Thanks to particular surface modification processes of the graphene flakes, these have been properly distributed within the polymer matrix, providing the composite coating with remarkable protective performance. The optimization of the deposition process parameters, as well as the amount of filler, also allowed to improve the mechanical and conductivity properties of the cataphoretic matrix, suggesting the possibility of realizing multifunctional coatings. Finally, these ’smart’ coatings were made by combining two deposition techniques, creating two layers with distinct purposes, containing different types of graphene-based fillers. The cataphoretic primer provided the substrate with high corrosion protection, while the spray top coat possessed high properties of electrical conductivity and resistance to abrasion phenomena. Ultimately, graphene has proven to be an excellent resource as a reinforcing filler in multifunctional organic coatings.

Graphene is a single sheet of graphite. While bulk graphite is semimetal, graphene is a zero bandgap semiconductor. Band structure calculations show graphene has a linear energy dispersion relation in the low energy region close to the Dirac points where the conduction band and the valence band touch. Carriers in graphene are described as massless Dirac fermions in contrast to massive carriers in normal metals and semiconductors that obey a parabolic energy dispersion relation. The uniqueness of graphene band structure indicates its peculiar electronic transport properties. In this thesis work, single- and multi-layer graphene films epitaxially grow on either the Si face or the C face of SiC substrates in a homemade induction vacuum chamber by thermal decomposition of SiC at high temperatures. The surface morphology and crystal structure of epitaxial graphene are studied with surface analysis tools. The transport properties of epitaxial graphene are studied by magnetotransport experiments. An epitaxial graphene film turns out to be a multilayered graphene because carriers in epitaxial graphene act as those in single layer graphene. Top gated and side gated epitaxial graphene field effect transistors (FETs) have also been successfully fabricated. These systematic studies unambiguously demonstrate the high quality of epitaxial graphene and the great potential of epitaxial graphene for electronic applications

No description available.

In this thesis, we have explored the possibilities to realize a Graphene Based Hybrid structures (GBHs) by the functionalization of a graphene layer on both sides. The first chapter gives a general introduction about graphene and a literature review of different metal intercalations on graphene. The second chapter explains the experimental techniques used in this work. In chapter 3, we studied the functionalization of epitaxial graphene on SiC(0001) by gold intercalation. With the help of Scanning Tunneling Microscopy, we have evidenced and characterized different intercalation modes such as the formation of aggregates of individual gold atoms and the formation of a continuous gold layer between the top graphene and the buffer layer. The free standing nature of the intercalated gold atoms was examined by differential charge density plot, projected density of states calculations and further by X-ray photoelectron spectroscopy. The band structure modification of graphene due to these intercalated gold atoms was evidenced by Angle-resolved photoemission spectroscopy, which reveals a strong Van Hove extension and an increase of the Fermi velocity. Extend to this research, to obtain an extended Van Hove singularity usually observed in highly doped graphene; we studied highly electron donor molecules, TetraThioFullvalene (TTF) on pristine and gold intercalated graphene and on graphite (chapter 4). The dependence of charge transfer of these molecules with their conformation and the reactivity of photochromic with conjugated molecules on graphene were also discussed. To understand the structural properties of these molecules photophysical measurements were performed in chapter 5.

O objetivo deste trabalho é investigar a aplicação ressoadores WGM (Whispering-Gallery Mode) em plataforma SOI (silicon-on-insulator) baseados em grafeno como candidatas potenciais para aplicações como moduladores de eletro-absorção. O grafeno apresenta variação de condutividade considerável quando submetido a uma aplicação de tensão, o que reflete na parte imaginária de seu índice de refração (relacionada às perdas de propagação). Com isso, é possível atribuir estados ligado e desligado (on-off) que conferem ao dispositivo sua característica de modulação óptica. A geometria utilizada é do tipo anel, o que permite uma elevada seletividade em frequência possibilitando, assim, uma grande profundidade de modulação. As simulações foram realizadas no software de elementos finitos COMSOL Multiphysics, o qual é bastante apropriado para a definição das diferentes figuras de mérito a serem utilizadas para a caracterização do desempenho do modulador. / The goal of this work is to investigate SOI (silicon on insulator) WGM (Whispering-Gallery Modes) resonators based on graphene as potential candidates for electro-absorption modulator applications. Graphene conductivity varies substantially when submitted to an applied voltage, which reflects directly in the imaginary part of its refractive index (responsible for the propagation losses). Therefore, it is possible to assign on-off states that render the device its optical modulation characteristics. The geometry adopted for the design is the ring type, which allows high frequency selectivity and modulation depth. The simulations were carried out in the finite elements software COMSOL Multiphysics, which is quite appropriate for the definition of the different figure of merits to be used in the modulator characterization.

O objetivo deste trabalho é investigar a aplicação ressoadores WGM (Whispering-Gallery Mode) em plataforma SOI (silicon-on-insulator) baseados em grafeno como candidatas potenciais para aplicações como moduladores de eletro-absorção. O grafeno apresenta variação de condutividade considerável quando submetido a uma aplicação de tensão, o que reflete na parte imaginária de seu índice de refração (relacionada às perdas de propagação). Com isso, é possível atribuir estados ligado e desligado (on-off) que conferem ao dispositivo sua característica de modulação óptica. A geometria utilizada é do tipo anel, o que permite uma elevada seletividade em frequência possibilitando, assim, uma grande profundidade de modulação. As simulações foram realizadas no software de elementos finitos COMSOL Multiphysics, o qual é bastante apropriado para a definição das diferentes figuras de mérito a serem utilizadas para a caracterização do desempenho do modulador. / The goal of this work is to investigate SOI (silicon on insulator) WGM (Whispering-Gallery Modes) resonators based on graphene as potential candidates for electro-absorption modulator applications. Graphene conductivity varies substantially when submitted to an applied voltage, which reflects directly in the imaginary part of its refractive index (responsible for the propagation losses). Therefore, it is possible to assign on-off states that render the device its optical modulation characteristics. The geometry adopted for the design is the ring type, which allows high frequency selectivity and modulation depth. The simulations were carried out in the finite elements software COMSOL Multiphysics, which is quite appropriate for the definition of the different figure of merits to be used in the modulator characterization.

Dans cette thèse, nous avons exploré les possibilités de réaliser des structures hybrides à base graphène (GBHS) par la fonctionnalisation bilatérale du graphène. Le premier chapitre donne une introduction générale sur le graphène et sur la littérature concernant les différentes méthodes d’intercalations de métaux dans le graphène. Le chapitre 2 décrit les techniques expérimentales utilisées. Le chapitre 3 concerne la fonctionnalisation du graphène épitaxié sur SiC (0001) par intercalation d'atome d’or. Les différents modes d'intercalation de l’or ont été mis en évidence par microscopie tunnelle (formation d'agrégats d'atomes d'or individuels et formation d'une couche d'or continue). La nature de ces atomes d'or intercalées a été examinée par des calculs d’image de densité de charge, et par Spectroscopie de Photoélectrons X (XPS). La modification de la structure de bande du graphène a aussi été mise en évidence par Spectroscopie de Photoélectrons UV Résolue Angulairement (ARPES) par des expériences sur synchrotron. Ces études ont révélé une forte extension de la singularité de Van Hove et une augmentation de la vitesse de Fermi. Afin d’agir sur cette extension de la singularité de Van Hove, des molécules fortement donneuses d'électrons, telle que la molécule de TetraThioFulvalene (TTF) ont été déposée sur graphène intercalé or et sur graphite (chapitre 4). La dépendance du transfert de charge de ces molécules avec leur conformation et la réactivité photochromique de ces molécules conjuguées sur le graphène ont également été abordés. Pour comprendre les propriétés structurales de ces molécules, des mesures photophysiques ont été effectuées qui apparaissant dans le chapitre 5. / In this thesis, we have explored the possibilities to realize a Graphene Based Hybrid structures (GBHs) by the functionalization of a graphene layer on both sides. The first chapter gives a general introduction about graphene and a literature review of different metal intercalations on graphene. The second chapter explains the experimental techniques used in this work. In chapter 3, we studied the functionalization of epitaxial graphene on SiC(0001) by gold intercalation. With the help of Scanning Tunneling Microscopy, we have evidenced and characterized different intercalation modes such as the formation of aggregates of individual gold atoms and the formation of a continuous gold layer between the top graphene and the buffer layer. The free standing nature of the intercalated gold atoms was examined by differential charge density plot, projected density of states calculations and further by X-ray photoelectron spectroscopy. The band structure modification of graphene due to these intercalated gold atoms was evidenced by Angle-resolved photoemission spectroscopy, which reveals a strong Van Hove extension and an increase of the Fermi velocity. Extend to this research, to obtain an extended Van Hove singularity usually observed in highly doped graphene; we studied highly electron donor molecules, TetraThioFullvalene (TTF) on pristine and gold intercalated graphene and on graphite (chapter 4). The dependence of charge transfer of these molecules with their conformation and the reactivity of photochromic with conjugated molecules on graphene were also discussed. To understand the structural properties of these molecules photophysical measurements were performed in chapter 5.

No description available.

No description available.