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.

Graphene nanoribbons, graphene based optical sensors, and grating based plasmonics are explored experimentally. Graphene nanoribbons exhibit highly insulating states that may allow for graphene based digital applications. We investigate the sensitivity of these states to local charged impurities in ultra high vacuum. We look into the possibility of isolating two-dimensional films of H-BN and BSCCO, and test for any interesting phenomena. We also assess graphene*s applicability for optical sensing by implementing a new style of spectral detector. Utilizing surface plasmon excitations nearby a graphene field-effect transistor we are able to produce a detector with wavelength sensitivity and selectivity in the visible range. Finally, we study another plasmonic phenomenon, and observe the resonant enhancement of diffraction into a symmetry-prohibited order in silver gratings.

Taking into account the technological demand for the controlled preparation of atomically precise graphene nanoribbons (GNRs) with well-defined properties, the present thesis is focused on the investigation of the role of the underlying metal substrate in the process of building GNRs using bottom-up strategy and on the changes in the electronic structure of GNRs induced by the GNR-metal interaction. The combination of surface sensitive synchrotron-radiation-based spectroscopic techniques and scanning tunneling microscopy with in situ sample preparation allowed to trace evolution of the structural and electronic properties of the investigated systems. Significant impact of the substrate activity on the growth dynamics of armchair GNRs of width N = 7 (7-AGNRs) prepared on inert Au(111) and active Cu(111) was demonstrated. It was shown that unlike inert Au(111) substrate, the mechanism of GNRs formation on Ag(111) and Cu(111) includes the formation of organometallic intermediates based on the carbon-metal-carbon bonds. Experiments performed on Cu(111) and Cu(110), showed that a change of the balance between molecular diffusion and intermolecular interaction significantly affects the on-surface reaction mechanism making it impossible to grow GNRs on Cu(110). It was demonstrated that deposition of metals on spatially aligned GNRs prepared on stepped Au(788) substrate allows to investigate GNR-metal interaction using angle-resolved photoelectron spectroscopy. In particular intercalation of one monolayer of copper beneath 7-AGNRs leads to significant electron injection into the nanoribbons, indicating that charge doping by metal contacts must be taken into account when designing GNR/electrode systems. Alloying of intercalated copper with gold substrate upon post-annealing at 200°C leads to a recovery of the initial position of GNR-related bands with respect to the Fermi level, thus proving tunability of the induced n-doping. Contrary, changes in the electronic structure of 7-AGNRs induced by the deposition of Li are not reversible.  It is demonstrated that via lithium doping 7-AGNRs can be transformed from a semiconductor into a metal state due to the partial filling of the conduction band. The band gap of Li-doped GNRs is reduced and the effective mass of the conduction band carriers is increased.

Cette thèse démontre le potentiel d'utilisation du graphène pour la fabrication de transistors à effet de champ à couche mince. Celui-ci est préparé par exfoliation en phase liquide et co-déposé avec un polymère semiconducteur du type n. Cette stratégie montre que le graphène améliore le comportement ambipolaire du polymère et plus particulièrement le transport des trous ce qui renforce l'application des matériaux composites au graphène dans les circuits logiques.Par la même approche de mélange, de nouveaux nanorubans de graphène dispersés en solution, ont été utilisés pour améliorer la performance des dispositifs basés sur un polymère amorphe de type p. Ces nanorubans forment une voie de percolation pour les charges améliorant ainsi la performance des dispositifs dans l'obscurité ainsi que sous illumination. Finalement, les dispositifs photosensibles multifonctionnels ont été examinés par l'introduction de molécules photochromiques avec différents substituants au sein des films semi-conducteurs à base de polymère ou de molécules de petite taille qui ont été trouvés influer la photocommutation. / This thesis demonstrates that graphene produced by liquid-phase exfoliation can be co-deposited with a polymerie semiconductor for the fabrication of thin film field-effect transistors. The introduction of graphene to the n-type polymeric matrix enhances not only the electrical characteristics of the devices, but also the ambipolar behavior and the hole transport in particular. This provides a prospective pathway for the application of graphene composites for logic circuits.The same approach of blending was adopted to enhance the electrical characteristics of an amorphous p-type polymer semiconductor by addition of an unprecedented solution processable ultra-narrow graphene nanoribbon. GNRs form percolation pathway for the charges resulting in enhanced deviee performance in daras weil as under illumination therefore paving the way for applications in (opto)electronics.Finally, multifunctional photoresponsive devices were examined by introducing photochromic molecules exposing different substituents into small molecule or polymeric semiconductor films that were found to affect the photoswitching behavior.

This thesis demonstrates that graphene produced by liquid-phase exfoliation can be co-deposited with a polymerie semiconductor for the fabrication of thin film field-effect transistors. The introduction of graphene to the n-type polymeric matrix enhances not only the electrical characteristics of the devices, but also the ambipolar behavior and the hole transport in particular. This provides a prospective pathway for the application of graphene composites for logic circuits.The same approach of blending was adopted to enhance the electrical characteristics of an amorphous p-type polymer semiconductor by addition of an unprecedented solution processable ultra-narrow graphene nanoribbon. GNRs form percolation pathway for the charges resulting in enhanced deviee performance in daras weil as under illumination therefore paving the way for applications in (opto)electronics.Finally, multifunctional photoresponsive devices were examined by introducing photochromic molecules exposing different substituents into small molecule or polymeric semiconductor films that were found to affect the photoswitching behavior.

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / This work presents a study of properties of functionalized carbon nanotubes and graphene nanoribbons. We studied, by first principles simulations, the structural and electronic properties of functionalized nanotubes and interacting with molecules of biological interest. Furthermore, we analyzed the properties of these systems under the action of applied electric fields, noting changes on their behavior due the external perturbation. In the case of nanotubes interacting with anti-inflammatory nimesulide, the interaction is repulsive, resulting in energetically unstable systems, but which may have their behavior controlled by the external field. We noted that when the carbon nanotube interacts with the antimalarial primaquine, a strong bond between the systems occurs, where the presence of primaquine can modify the electronic properties of nanotubes. In the other hand, for the case of carbon nanostructures interacting with vitamins, the interaction is weak. We also evaluated the structural, electronic and magnetic properties of Ti and Mn doped carbon nanoribbons (or graphene nanoribbons) and properties of defective nanoribbons, by first principles simulations (code SIESTA), and analyzed the electronic transport properties of some of these systems, by tight-binding methods associated with Green s functions. We noted that there is an edge and sublattice effect in zigzag edged nanoribbons, where the properties of the systems can be modified depending on the defect location with respect to the edge. We demonstrate that carbon nanostructures can act as selective sensors of atoms or adsorbed molecules, besides representing a route to drug delivery. / Este trabalho apresenta um estudo de propriedades de nanotubos e nanofitas de carbono funcionalizados. Estudamos, via simulação de primeiros princípios, as propriedades estruturais e eletrônicas de nanotubos funcionalizados e interagindo com moléculas de interesse biológico. Analisamos, ainda, as propriedades destes sistemas sob a ação de campos elétricos aplicados, observando alterações em seus comportamentos devido à perturbação externa. No caso dos nanotubos interagindo com o anti-inflamatório nimesulida, a interação é repulsiva, resultando em sistemas energeticamente instáveis, mas que podem ter seu comportamento controlado pelo campo externo. Notamos que quando o nanotubo de carbono interage com o antimalárico primaquina, ocorre uma ligação forte entre estes sistemas, onde a presença da primaquina pode alterar as propriedades eletrônicas dos nanotubos. Já para o caso de nanoestruturas de carbono interagindo com vitaminas, a interação é fraca. Avaliamos também as propriedades estruturais, eletrônicas e magnéticas de nanofitas de carbono (ou nanofitas de grafeno) dopadas por átomos de Ti e Mn e propriedades de nanofitas defeituosas, por meio de simulações de primeiros princípios (código SIESTA), e avaliamos as propriedades de transporte eletrônico de alguns destes sistemas, por métodos tight-binding associados a funções de Green. Observamos que existe um efeito de borda e de sub-rede nas nanofitas de borda zigzag, onde as propriedades dos sistemas podem ser alteradas de acordo com a localização do defeito com relação à borda. Nós mostramos que as nanoestruturas de carbono podem agir como sensores seletivos de átomos ou moléculas adsorvidos, além de representarem uma rota de carreamento de fármacos.

abstract: In mesoscopic physics, conductance fluctuations are a quantum interference phenomenon that comes from the phase interference of electron wave functions scattered by the impurity disorder. During the past few decades, conductance fluctuations have been studied in various materials including metals, semiconductors and graphene. Since the patterns of conductance fluctuations is related to the distributions and configurations of the impurity scatterers, each sample has its unique pattern of fluctuations, which is considered as a sample fingerprint. Thus, research on conductance fluctuations attracts attention worldwide for its importance in both fundamental physics and potential technical applications. Since early experimental measurements of conductance fluctuations showed that the amplitudes of the fluctuations are on order of a universal value (e2/h), theorists proposed the hypothesis of ergodicity, e.g. the amplitudes of the conductance fluctuations by varying impurity configurations is the same as that from varying the Fermi energy or varying the magnetic field. They also proposed the principle of universality; e.g., that the observed fluctuations would appear the same in all materials. Recently, transport experiments in graphene reveal a deviation of fluctuation amplitudes from those expected from ergodicity. Thus, in my thesis work, I have carried out numerical research on the conductance fluctuations in GaAs nanowires and graphene nanoribbons in order to examine whether or not the theoretical principles of universality and ergodicity hold. Finite difference methods are employed to study the conductance fluctuations in GaAs nanowires, but an atomic basis tight-binding model is used in calculations of graphene nanoribbons. Both short-range disorder and long-range disorder are considered in the simulations of graphene. A stabilized recursive scattering matrix technique is used to calculate the conductance. In particular, the dependence of the observed fluctuations on the amplitude of the disorder has been investigated. Finally, the root-mean-square values of the amplitude of conductance fluctuations are calculated as a basis with which to draw the appropriate conclusions. The results for Fermi energy sweeps and magnetic field sweeps are compared and effects of magnetic fields on the conductance fluctuations of Fermi energy sweeps are discussed for both GaAs nanowires and graphene nanoribbons. / Dissertation/Thesis / Doctoral Dissertation Electrical Engineering 2015

The transport properties of zigzag graphene nanoribbons (zGNRs) were studied by density functional theory (DFT) in conjunction with Green’s function analysis. In particular, spin transport through a zGNR (12,0) device was investigated under the constraint of ferromagnetic coordination of the ribbon edges. Several configurations with two vacant sites in the edge and the bulk region of the zGNR device were derived from this system. For all structures, magnetocurrent ratios (MCRs) were recorded as a function of the bias as well as the amount of strain applied longitudinally to the devices. ZGNR devices with vacancies in the edge regime turn out to exhibit perfect spin-filter activity for well-defined choices of the strain and the bias, carrying completely polarized minority spin currents. In the alternative structure, characterized by vacancies in the bulk regime, spin currents with majority orientation prevail. With respect to both the sign and the size, the MCR is seen to depend sensitively on the device parameters, i.e., the vacancy locations, the bias, and the amount of strain. These results are interpreted in terms of density-of-states distributions, transmission spectra, and transmission operator eigenstates.

The spin filter capability of a (0,8) armchair graphene nanoribbon with Fe atoms at substitutional sites is investigated by density functional theory in combination with the non-equilibrium Green's function technique. For specific arrangements, a high degree of spin polarisation is achieved. These include a single substitution at an edge position or double substitution in the central sector of the transmission element. The possibility of switching between majority and minority spin polarisation by changing the double substitution geometry is predicted. Including the bias dependence of the transmission function proves to be essential for correct representation of the spin-resolved current-voltage profiles.

CMOS SRAM area scaling is slowing down due to several challenges faced by transistors at nanoscale such as increased leakage. This calls for new concepts and technologies to overcome CMOS scaling limitations. In this thesis, we propose a multi-state memory to store multiple bits in a single cell, enabled by graphene and graphene nanoribbon crossbar devices (xGNR). This could provide a new dimension for scaling. We present a new multi-state volatile memory fabric called Graphene Nanoribbon Tunneling Random Access Memory (GNTRAM) featuring a heterogeneous integration between graphene and CMOS. A latch based on the xGNR devices is used as the memory element which exhibits 3 stable states. We propose binary and ternary GNTRAM and compare them with respect to 16nm CMOS SRAM and 3T DRAM. Ternary GNTRAM (1.58 bits/cell) shows up to 1.77x density-per-bit benefit over CMOS SRAMs and 1.42x benefit over 3T DRAM in 16nm technology node. Ternary GNTRAM is also up to 1196x more power-efficient per bit against high-performance CMOS SRAMs during stand-by. To enable further scaling, we explore two approaches to increase the number of bits per cell. We propose quaternary GNTRAM (2 bits/cell) using these approaches and extensively benchmark these designs. The first uses additional xGNR devices in the latch to achieve 4 stable states and the quaternary memory shows up to 2.27x density benefit vs. 16nm CMOS SRAMs and 1.8x vs. 3T DRAM. It has comparable read performance in addition to being power-efficient, up to 1.32x during active period and 818x during stand-by against high performance SRAMs. However, the need for relatively high-voltage operation may ultimately limit this scaling approach. An alternative approach is also explored by increasing the stub length in the xGNR devices, which allows for storing 2 bits per cell without requiring an increased operating voltage. This approach for quaternary GNTRAM shows higher benefits in terms of power, specifically up to 4.67x in terms of active power and 3498x during stand-by against high-performance SRAMs. Multi-bit GNTRAM has the potential to realize high-density low-power nanoscale memories. Further improvements may be possible by using graphene more extensively, as graphene transistors become available in future.