Dissertations / Theses on the topic 'Graphene Nanoribbons'

The graphitic systems have attracted intensive attention recently due to the discovery of graphene, a single layer of graphite. The low-energy band structure of graphene exhibits an unusual linear dispersion relation which hosts massless Dirac fermions and leads to intriguing electronic and optical properties. In particular, due to the high mobility and tunability, graphene and graphitic materials have been recognized as promising candidates for future nanoelectronics and optoelectronics. Electron-phonon coupling (EPC) plays a significant role in electronic and optoelectronic devices. Therefore, it is crucial to understand EPC in graphitic materials and then manipulate it to achieve better device performance. In the first part of this thesis, we explore EPC between Dirac-like fermions and infrared active phonons in graphite via infrared magneto-spectroscopy. We demonstrate that the EPC can be tuned by varying the magnetic field. The second part of this thesis deals with magnetoplasmons in quasineutral graphene nanoribbons. Multilayer epitaxial graphene grown on the carbon terminated silicon carbide surface behaves like single layer graphene. Plasmons are excited in the nanoribbons of undoped multilayer epitaxial graphene. In a magnetic field, the cyclotron resonance can couple with the plasmon resonance forming the so-called upperhybrid mode. This mode exhibits a distinct dispersion relation, radically different from that expected for conventional two dimensional systems.

We study the magnetic structure of narrow graphene ribbons with patterned edges. Neglecting interactions, a broad class of edge terminations support zero-energy states localized at the edges of the ribbon. For the simplest (zigzag) ribbon supporting these edge states, electron-electron interactions have been shown to induce ferromagnetic ordering along the edges of the ribbon. We generalize this argument for such a magnetic edge state to carbon ribbons with more complex chiral edge terminations.
Science, Faculty of
Physics and Astronomy, Department of
Graduate

Le graphène, une monocouche d'atomes de carbone disposées en nid d'abeille, a été caractérisé pour la première fois en 2004 et a immédiatement attiré beaucoup d'attention. Il présente des propriétés électroniques uniques et porte le potentiel de jouer un rôle crucial dans une future génération d'électronique. Cependant, son spectre sans gap le rend impropre à l'application directe comme semi-conducteur. Une façon de contourner ce déficit consiste à concevoir des nanorubans de graphène (GNRs). Dans ces systèmes, un gap s'ouvre en fonction de la largeur et la configuration des bords. Dans cette thèse, nous présentons des enquêtes sur les GNRs basées sur la théorie de la fonctionnelle de la densité (DFT). D'abord nous discutons la stabilité thermodynamique d'une large gamme de configurations des bords et leurs structures électroniques. Ensuite, pour les plus importants, nous analysons des aspects géométriques et des images simulées de microscope à effet tunnel. Au cours de ces investigations, nous avons trouvé la théorie de Clar de l'hexagone aromatique très utile pour la discussion de nos calculs DFT. Par conséquent, nous proposons une classification des GNRs en fonction de leurs formules Clar. Cela cerne de nombreuses propriétés mieux qu'une classification basée sur l'orientation cristallographique. La dernière partie de cette thèse traite une petite extension du logiciel DFT du nom Quantum Espresso. En particulier, il s'agit de la mise en œuvre des corrections à la troisième dérivée de l'énergie électronique dépendantes du gradient de la densité. Cela permettra d'éteindre les enquêtes des phénomènes anharmoniques à l'approximation du gradient généralisé
Graphene, a single layer of carbon atoms arranged in a honey-comb lattice, was first characterized in 2004 and immediately attracted a lot of attention. It exhibits unique electronic and transport properties and bears the potential to play a crucial role in a future generation of electronic devices. However, its gapless spectrum makes graphene unsuitable for direct application as semiconductor. One way to bypass this shortcoming consists in designing graphene nanoribbons (GNRs). In these systems, an electronic bandgap opens up as a function of the width and the edge configuration. In this thesis we present investigations of GNRs based on density functional theory (DFT). First we discuss the thermodynamic stability of a broad range of possible edge configurations and their electronic structures. Then, for the most relevant among them, we perform in-depth analyses of geometric aspects and simulated scanning tunneling microscope images. Throughout these investigations, we found Clar's theory of the aromatic sextet very useful to rationalize our DFT calculations. It is simple and elegant but still sophisticated enough to account for a large number of phenomena. Hence, we propose a classification scheme for GNRs based on their Clar formulae. This captures many properties better than a classification based on the crystallographic orientation. The last part of this thesis deals with a small extension to the DFT-framework Quantum Espresso. In particular, we discuss the implementation of the gradient corrections to the third order derivative of the electronic energy. This opens the way to extend investigations of anharmonic phenomena to the generalized gradient approximation

Since the isolation of graphene in 2004, this novel material has become the major object of modern condensed matter physics. Despite of enormous research activity in this field, there are still a number of fundamental phenomena that remain unexplained and challenge researchers for further investigations. Moreover, due to its unique electronic properties, graphene is considered as a promising candidate for future nanoelectronics. Besides experimental and technological issues, utilizing graphene as a fundamental block of electronic devices requires development of new theoretical methods for going deep into understanding of current propagation in graphene constrictions. This thesis is devoted to the investigation of the effects of electron-electron interactions, spin and different types of disorder on electronic and transport properties of graphene and graphene nanoribbons. In paper I we develop an analytical theory for the gate electrostatics of graphene nanoribbons (GNRs). We calculate the classical and quantum capacitance of the GNRs and compare the results with the exact self-consistent numerical model which is based on the tight-binding p-orbital Hamiltonian within the Hartree approximation. It is shown that electron-electron interaction leads to significant modification of the band structure and accumulation of charges near the boundaries of the GNRs. It's well known that in two-dimensional (2D) bilayer graphene a band gap can be opened by applying a potential difference to its layers.  Calculations based on the one-electron model with the Dirac Hamiltonian predict a linear dependence of the energy gap on the potential difference. In paper II we calculate the energy gap in the gated bilayer graphene nanoribbons (bGNRs) taking into account the effect of electron-electron interaction. In contrast to the 2D bilayer systems the energy gap in the bGNRs depends non-linearly on the applied gate voltage. Moreover, at some intermediate gate voltages the energy gap can collapse which is explained by the strong modification of energy spectrum caused by the electron-electron interactions. Paper III reports on conductance quantization in grapehene nanoribbons subjected to a perpendicular magnetic field. We adopt the recursive Green's function technique to calculate the transmission coefficient which is then used to compute the conductance according to the Landauer approach. We find that the conductance quantization is suppressed in the magnetic field. This unexpected behavior results from the interaction-induced modification of the band structure which leads to formation of the compressible strips in the middle of GNRs. We show the existence of the counter-propagating states at the same half of the GNRs. The overlap between these states is significant and can lead to the enhancement of backscattering in realistic (i.e. disordered) GNRs. Magnetotransport in GNRs in the presence of different types of disorder is studied in paper IV. In the regime of the lowest Landau level there are spin polarized states at the Fermi level which propagate in different directions at the same edge. We show that electron interaction leads to the pinning of the Fermi level to the lowest Landau level and subsequent formation of the compressible strips in the middle of the nanoribbon. The states which populate the compressible strips are not spatially localized in contrast to the edge states. They are manifested through the increase of the conductance in the case of the ideal GNRs. However due to their spatial extension these states are very sensitive to different types of disorder and do not significantly contribute to conductance of realistic samples with disorder. In contrast, the edges states are found to be very robust to the disorder. Our calculations show that the edge states can not be easily suppressed and survive even in the case of strong spin-flip scattering. In paper V we study the effect of spatially correlated distribution of impurities on conductivity in 2D graphene sheets. Both short- and long-range impurities are considered. The bulk conductivity is calculated making use of the time-dependent real-space Kubo-Greenwood formalism which allows us to deal with systems consisting of several millions of carbon atoms. Our findings show that correlations in impurities distribution do not significantly influence the conductivity in contrast to the predictions based on the Boltzman equation within the first Born approximation. In paper VI we investigate spin-splitting in graphene in the presence of charged impurities in the substrate and calculate the effective g-factor. We perform self-consistent Thomas-Fermi calculations where the spin effects are included within the Hubbard approximation and show that the effective g-factor in graphene is enhanced in comparison to its one-electron (non-interacting) value. Our findings are in agreement to the recent experimental observations.

The objective of this research was to perform a systematic investigation of the unique structural and electrical properties of epitaxial graphene at the nanoscale. As the semiconductor industry faces increasing challenges in the production of integrated circuits, due to process complexity and scaling limitations, new materials research has come to the forefront of both science and engineering disciplines. Graphene, an atomically-thin sheet of carbon, was examined as a material which may replace or become integrated with silicon nanoelectronics. Specifically, this research was focused on epitaxial graphene produced on silicon carbide. This material system, as opposed to other types of graphene, holds great promise for large-scale manufacturing, and is therefore of wide interest to the academic and industrial community. In this work, high-quality epitaxial graphene production was optimized, followed by the process development necessary to fabricate epitaxial graphene nanoribbon transistors for electrical characterization. The structural and electrical transport properties of the nanoribbons were elucidated through a series of distinct experiments. First, the size-dependent conductivity of epitaxial graphene at the nanoscale was investigated. Next, the alleviation of the detrimental effects revealed during the size-dependent conductivity study was achieved through the selective functionalization of graphene with hydrogen. Finally, two techniques were developed to allow for the complementary doping of epitaxial graphene. All of the experiments presented herein reveal new and important aspects of epitaxial graphene at the nanoscale that must be considered if the material is to be adopted for use by the semiconductor industry.

The acoustoelectric effect in graphene and graphene nanoribbons (GNRs) on lithium niobate surface acoustic wave (SAW) devices was studied experimentally. Monolayer graphene produced by chemical vapour deposition was transferred to the SAW devices. The photoresponse of the acoustoelectric current (Iae) was characterised as a function of SAW frequency and intensity, and illumination wavelength (using 450 nm and 735 nm LEDs) and intensity. Under illumination, the measured Iae increased by more than the measured decrease in conductivity, while retaining a linear dependence on SAW intensity. The latter is consistent with the piezoelectric interaction between the graphene charge carriers and the SAWs being described by a relatively simple classical relaxation model. A larger increase in Iae under an illumination wavelength of 450 nm, compared to 735 nm at the same intensity, is consistent with the generation of a hot carrier distribution. The same classical relaxation model was found to describe Iae generated in arrays of 500 nm-wide GNRs. The measured acoustoelectric current decreases as the nanoribbon width increases, as studied for GNRs with widths in the range 200 – 600 nm. This reflects an increase in charge carrier mobility due to increased doping, arising from damage induced at the nanoribbon edges during fabrication. 2 Lastly, the acoustoelectric photoresponse was studied as a function of graphene nanoribbon width (350 – 600 nm) under an illumination wavelength of 450 nm. Under illumination, the nanoribbon conductivity decreased, with the largest percentage decrease seen in the widest GNRs. Iae also decreased under illumination, in contrast to the acoustoelectric photoresponse of continuous graphene. A possible explanation is that hot carrier effects under illumination lead to a greater decrease in charge carrier mobility than the increase in acoustoelectric attenuation coefficient. This causes the measured decrease in Iae.

Over the past decade there has been a great deal of interest in graphene, a 2-dimensional allotrope of carbon with exceptional mechanical and electrical properties. Its outstanding mobility, minimal size, and mechanical stability make it an appealing material for use in next generation electronic devices. Epitaxial graphene growth on silicon carbide is a reliable, scalable method for the production of high quality graphene films. Recent work has shown that the SiC can be patterned prior to graphitization, in order to selectively grow graphene nanostructures. Graphene nanoribbons grown using this technique do not suffer from the rough edges caused by lithographic patterning, and recent measurements have revealed extraordinary transport properties. In this thesis the magnetic properties of these nanoribbons are investigated through spin polarized current injection. The sensitivity of these nanoribbons to spin polarized current is interesting from a fundamental physics standpoint, and may find applications in future spintronic devices.

It has become increasingly apparent that the future of next generation of electronic devices can and will rely on graphene nanoribbons. Graphene nanoribbons and sister structures showcase several key properties that can address the emerging need of terahertz science and technology, and break through the many technological limits on conventional semiconductor electronics operating in the terahertz spectrum. In this thesis, we focus on the study of the terahertz nonlinear optical response of metallic armchair graphene nanoribbons and sister structures using a k.p model and time dependent perturbation theory. We find that these nanoribbons exhibit a stronger interband optical response, and a smaller critical field strength (of the order of 10 kV/m) than does 2D single layer graphene. We demonstrate that finite ribbon size, spatial profile of the applied terahertz radiation field, polarization of the applied terahertz radiation, a small band gap opening, and application of a superlattice potential are several ways to tune the strong terahertz nonlinear optical response of metallic armchair graphene nanoribbons. The major contributions of this thesis include: 1) developes of a simpler method compared to other sophisticated methods of the terahertz nonlinear optical interband response of metallic armchair graphene nanoribbons; 2) extends the method in the characterization of various quantum size effects, elliptically polarized radiation field, small gap opening and superlattice on the terahertz optical response of these nanoribbons; 3) The versatility of the tunability showed in the terahertz nonlinear response of metallic armchair nanoribbons and sister structures will help advance the development of the nonlinear terahertz armchair graphene nanoribbon opto-electronic and photonic technology.

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Bachelors
Sciences
Physics

Les nanorubans de graphène (NRGs) sont des matériaux prometteurs pour l'organique électronique, à mi chemin entre polymères conjugués et nanotubes de carbone. Deux approches différentes pour la synthèse de nanorubans aromatiques sont développées et évaluées. La première est fondée sur la formation de céramidonines par cyclisation d'arylamino-anthraquinones en milieu acide. Plusieurs tétraaza-arènes incorporant deux de ces unités sont obtenus, mais l'approche s'est uniquement avérée appropriée dans le cas de courts substrats. La seconde approche repose sur la condensation d'acides aryle-acétiques avec des formylarènes ou acides aryle-glyoxyliques, suivie soit de cyclo-deshydrogénations en présence de quinone, soit de deshydrodebromation catalysée par le palladium, pour donner des arenes carboxy-substitués allongés. La méthode impliquant la quinone s'avère limitée à des substrats suffisamment réactifs tels que des thiophènes et laisse envisager des poly(arènodithiophènes) en partie rigidifiés et carboxy-substitués. La catalyse au palladium s'avère plus générale, ouvrant des perspectives d'obtention d'une grande variété de rubans aux propriétés électroniques ajustables
Graphene nanoribbons (GNRs) are promising materials for organic electronics, as they bridge the gap betweensingle-stranded conjugated polymers and carbon nanotubes. Two different synthetic approaches to GNRs aredeveloped and evaluated. The first approach is based on the acid-promoted cyclisation of arylaminoanthraquinonesto ceramidonines. Tetraazaarenes with two ceramidonine units are obtained, but the approachis found to be appropriate only to such small systems. The second approach is based on the condensation ofarylacetic acids with arenecarboxaldehydes or arylglyoxylic acids, followed either by quinone-assistedoxidative cyclodehydrogenation or palladium-catalysed dehydrodebromination to yield carboxy-substitutedelongated arenes. The quinone-based variant is found to be limited to reactive substrates such as thiophenederivatives and offers the perspective of partially rigidified carboxy-substituted poly(arenodithiophenes). Thepalladium-based variant is found to be more general, opening the prospect of obtaining a variety of ribbontypestructures with tunable electronic properties

The strategic importance of microelectronics is reflected in its ubiquity in the global production network and in our daily lives. Above all, the microelectronics revolution has been enabled and driven by the scalability of the silicon transistor and the computational efficiency of its CMOS architecture. While the semiconductor industry has been incredibly adept at pushing the boundaries of scaling in the last few decades, many factors suggest that silicon technology is running into scientific and practical limitations to further scaling. Thus, the push for a beyond-silicon computing platform is imperative. Akin to the transition from bipolar to MOSFET technology or from NMOS to CMOS architecture, the industry is once again looking for the next disruptive technology to continue the exponential growth of computing power. In 2004, two research groups, one from the University of Manchester and the other from Georgia Tech, reported on the electrical properties of ultrathin graphite. Their findings demonstrated the stability of graphene, an atomic layer of graphite, as well as its superb carrier mobility, spurring the semiconductor industry to invest in the material as a candidate for a beyond-silicon computing platform. Within this framework, this thesis explores the promise of graphene as a material and technological platform for electronic devices. The objectives of the thesis are (i) to elucidate opportunities and challenges in the design and fabrication of graphene field-effect devices, and (ii) to advance a new device platform based on graphene p-n junctions.

Over the past decade, the bottom-up synthesis of structurally defined graphene nanoribbons (GNRs) with various topologies has attracted significant attention due to the extraordinary optical, electronic, and magnetic properties of GNRs, rendering them suitable for a wide range of potential applications (e.g., nanoelectronics, spintronics, photodetectors, and hydrothermal conversion). Remarkable achievements have been made in GNR synthesis with tunable widths, edge structures, and tailor-made functional substitutions. In particular, GNRs with liquid-phase dispersibility have been achieved through the decoration of various functional substituents at the edges, providing opportunities for revealing unknown GNR physiochemical properties. Because of the promise of liquid-phase dispersible GNRs, this mini-review highlights recent advances in their synthetic strategies, physiochemical properties, and potential applications. In particular, deep insights into the dvantages and challenges of their syntheses and chemical methodologies are provided to encourage future endeavors and developments.

We have used a tight-binding Hamiltonian of an ABA-stacked trilayer zigzag graphene nanoribbon with -alignment edges to study the edge magnetizations. Firstly, in the neutral system we analyzed a magnetic state in which both edge magnetizations reach their maximum value; and is characterized by an intralayer ferromagnetic coupling between the magnetizations at opposite edges. The band structure and the location of the edge-state bands are calculated in order to understand the origins of the edge magnetizations. We have also introduced an electron doping so that the number of electrons in the ribbon unit cell is higher than in neutral case. As a consequence, we have obtained magnetization steps and charge accumulation at the edges of the sample, which are caused by the edge-state flat bands..

Structural properties of pristine and defected graphene nanoribbons have been investigated by stretching them under 5 percent and 10 percent uniaxial strain until fragmentation. The stretching process has been carried out by performing molecular dynamics simulations (MDS) at 1 K and 300 K to determine the temperature effect on the structure of the graphene nanoribbons. Results of the simulations indicated that temperature, edge shape of graphene nanoribbons and stretching speed have a considerable effect on structural properties, however they have a slight effect on the strain value. The maximum strain at which fracture occurs is found to be 46.41 percent whereas minimum strain value is calculated as 21.00 percent. On the other hand, the defect formation energy is strongly affected from temperature and edge shape of graphene nanoribbons. Stone-Wales formation energy is calculated as -1.60 eV at 1 K whereas -30.13 eV at 300 K for armchair graphene nanoribbon.

Einzelschichten von Graphen-Nanobänders (GNRs) wurden auf SiC(0001)-Substraten mit zwei unterschiedlichen Fehlschnitten bei Temperaturen von 1410 bis 1460 °C synthetisiert. Das GNR-Wachstum lässt sich bei niedriger Stufenkantenhöhe am besten durch eine exponentielle Wachstumsrate, welche mit der Energiebarriere für die Ausdiffusion von Si korreliert ist. Anderseits wird bei Substraten mit höheren Stufenkanten eine nicht-exponentielle Rate beobachtet, was mit der Bildung von mehrlagigen Graphen an den Stufenkanten in Verbindung gebracht wird. Die Sauerstoffinterkalation von epitaktischen GNRs mittels Ausglühen an Luft von Bändern wird als nächstes untersucht, welche auf unterschiedlichen SiC-Substraten gewachsen wurden. Neben der Umwandlung von monolagigem zu zweilagigem Graphen in der Nähe der Stufenkanten von SiC, führt die Sauerstoffinterkalation zusätzlich zu der Bildung einer Oxidschicht auf den Terrassen des Substrats, was die zweilagigen GNRs elektrisch isoliert voneinander zurücklässt. Die elektrische Charakterisierung der zweilagigen GNRs zeigten dass die Bänder durch die Behandlung mit Sauerstoff elektrisch voneinander entkoppelt sind. Eine robuste Lochkonzentration von etwa 1x10¹³ cm-² und Mobilitäten von bis zu 700 cm²/(Vs) wurden für die GNRs mit einer typischen Breite von 100 nm bei Raumtemperatur gemessen. Wohl definierte Mesastrukturen gebildet mittels Elektronenstrahllithographie auf SiC-Substraten, wurde zuletzt untersucht. Die Charakterisierung des Ladungsträgertransports von GNRs die auf den Seitenwänden der strukturierten Terrassen gewachsen wurden, zeigt eine Mobilität im Bereich von 1000 bis 2000 cm²/(Vs), welche für verschiedene Strukturen auf der gesamten Probe homogen ist, was die Reproduzierbarkeit dieses Herstellungsverfahrens hervorhebt, sowie dessen Potential für die Implementierung in zukünftigen Technologien, welche auf epitaktischgewachsenene GNRs basieren.
Monolayer graphene nanoribbons (GNRs) were synthesized on SiC(0001) substrates with two different miscut angles at temperatures ranging from 1410 to 1460 °C. The GNR growth in lower step heights is best described by an exponential growth rate, which is correlated with the energy barrier for Si out-diffusion. On the other hand, a non-exponential rate is observed for substrates with higher steps, which is associated with the formation of few-layer graphene on the step edges. Oxygen intercalation of epitaxial GNRs is investigated next by air annealing ribbons grown in different SiC(0001) substrates. Besides the conversion of monolayer into bilayer graphene near the step edges of SiC, the oxygen intercalation also leads to the formation of an oxide layer on the terraces of the substrate, leaving the bilayer GNRs electronically isolated from each other. Electrical characterization of bilayer GNRs reveals that the ribbons are electrically decoupled from the substrate by the oxygen treatment. A robust hole concentration of around 1x10¹³ cm-² and mobilities up to 700 cm²/(Vs) at room temperature are measured for GNRs whose typical width is 100 nm. Well defined mesa structures patterned by electron beam lithography on the surface of SiC substrates is lastly researched. Transport characterization of GNRs grown on the sidewalls of the patterned terraces shows a mobility in the range of 1000 – 2000 cm²/(Vs), which is homogeneous for various structures throughout the sample, indicating the reproducibility of this fabrication method and its potential for implementation in future technologies based on epitaxially grown GNRs.

Graphene, a 2D honeycomb lattice of sp² hybridized carbons, has attracted the attention of the scientific community not only for its interesting theoretical properties but also for its myriad of possible applications. The discovery of graphene led to the Nobel Prize in physics for 2010 to be awarded to Andrei Geim and Konstantin Novoselov. Since its discovery, many methods have been developed for the synthesis of this material. Two of those methods stand out for the growth of high quality and large area graphene sheets, namely, epitaxial growth from silicon carbide (SiC) and chemical vapor deposition (CVD). As it stands today, both methods make use of high concentrations of hydrogen (10-20%) in N₂ or Ar, high temperatures, and a vacuum system. Epitaxial growth from SiC in addition requires very expensive single crystal SiC wafers. In the case of CVD, organic molecules are used as the carbon source to grow graphene on a metal substrate. Although graphene has been grown on many metal substrates, the experiments highlighted here make use of copper as the metal substrate of choice since it offers the advantage of availability, low price, and, most importantly, because this substrate is self-limiting in other words, it mostly grows single layer graphene. Because the CVD method provides with a choice as for the carbon source to use, the following question arises: can a molecule, either commercially available or synthesized, be used as a carbon source that would allow for the synthesis of graphene under low temperatures, low concentrations of hydrogen and at atmospheric pressure? This dissertation focuses on the synthesis of graphene at lower temperatures by using carbon sources with characteristics that might make this possible. It also focuses on the use of forming gas (3% H₂ and 97% N₂ or Ar) in order to make the overall process a lot safer and cost effective. This dissertation contains two chapters on the synthesis of organic molecules of interest, and observations about their reactivity are included. CVD experiments were performed at atmospheric pressure, and under vacuum. In both instances forming gas was used as the annealing and carrier gas. Results from CVD at atmospheric pressure (CVDAP), using organic solvents as carbon sources, show that at 1000℃, low quality graphene was obtained. On the other hand, CVD experiments using a vacuum in the range of 25 mTorr to 1 Torr successfully produced good quality graphene. For graphene growth under vacuum conditions, commercially available and synthesized compounds were used. Attempts at growing graphene at 600℃ from the same carbon sources only formed amorphous carbon. These results point to the fact that good quality graphene can basically be grown from any carbonaceous material as long as the growth temperature is 1000℃ and the system is under vacuum. In addition to the synthesis of graphene at low temperatures, there is a great amount of interest on the synthesis of graphene nanoribbons (GNR’s) and, as with graphene, several approaches to their synthesis have been developed. One such method is the synthesis of GNRs encapsulated in carbon nanotubes. Experiments were conducted in which aluminosilicate nanotubes were used. These nanotubes provided for an easier interpretation of the Raman spectrum since the signals from the nanotubes do not interfere with those of the GNR’s as in the case when carbon nanotubes are used. The use of aluminosilicate nanotubes also allowed for the successful synthesis of GNR’s at temperatures as low as 200℃ when perylene was used as the carbon source.

Graphene is important in the study of 2D systems and has a number of unique properties and advantages: High charge-carrier mobilities and ballistic transport at room temperature, high structural stability, relativistic properties and a relatively simple production method. The potential of a tunable band-gap in graphene nanoribbons suggests that it could become a leading electrical component. One method that has emerged for modelling nanogaphene systems is the extended tight-binding model with Hubbard-\emph{U}. Within a real-space formalism, this model can be easily and efficiently applied to increasingly more complicated systems, where any number of edge defects, impurities and even patterning can be included, giving a more realistic description. This thesis investigated methods of structurally perturbing the ideal graphene nanoribbon device and probed the spin-dependent properties that arose: Random-edge vacancies, asymmetrical notches, uniaxial strain, magnetic inhomogeneity, chevron ZGNRs and patterned AGNRs. Random edge-vacancies have been used to perturb the electronic conductance in order to introduce the conductance gap observed in experimental results. These studies use the non-interacting tight-binding model, ignoring coulomb interactions. Introducing coulomb interactions within ideal ZGNRs has been shown to intrinsically include a conductance gap without edge-vacancies. The work presented in this thesis investigated the effects of edge-vacancies on the interacting model and demonstrated that, in general, the non-interacting model is insufficient to describe the physics of disordered ZGNRs. Controllable, asymmetric perturbations (i.e., notches and magnetic inhomogeneity) were added to interacting ideal ZGNRs to determine if the spin-dependent properties can be controlled. Asymmetrical perturbations exhibited spin-dependent conductance. In particular, magnetic inhomogeneity showed a transition from semi-conductive to half-metallic, suggesting a possible avenue for spin-filtering in spintronic devices. Finally, bottom-up synthesised GNRs were investigated (chevron ZGNRs and patterned AGNRs) and demonstrated controllable conductance properties and further work involving these systems was presented.

Le graphène peut être considéré comme l'un des matériaux les plus prometteurs pour les composants électroniques pratiques en raison de ses excellentes propriétés de transport de charge, de sa surface spécifique très élevée, de sa conductivité thermique excellente et de sa grande résistance mécanique. Cependant, ce graphène bidimensionnel est un semiconducteur à bande interdite nulle, ce qui limite son application pratique dans les dispositifs électroniques. L'une des méthodes les plus prometteuses pour ouvrir une bande interdite est le confinement structurel du graphène en bandes étroites, définies comme des nanorubans de graphène (GNR). La bande interdite des GNR peut être contrôlée avec précision par la largeur et la configuration des bords, ce qui donne aux GNR des propriétés optiques et électroniques réglables. La synthèse ascendante en solution est l’une des stratégies les plus prometteuses pour préparer des GNR structurellement bien définis avec des propriétés optiques et électroniques ajustables. Contrairement aux méthodes descendantes, la stratégie ascendante permet un contrôle précis de la largeur et de la configuration des bords des GNR. Une stratégie couramment utilisée, la réaction de cyclodéshydrogénation catalysée par l'acide de Lewis, appelée réaction de Scholl, a été largement utilisée pour synthétiser une grande variété de GNR bien définis sur des précurseurs de polyphénylène. Cependant, la réaction de Scholl présente de sérieux inconvénients qui limitent la portée et la polyvalence de cette réaction. Le premier est sa faible régiosélectivité qui entraîne des défauts de structure et affecte les propriétés des GNR. Ensuite, les réarrangements indésirables et l'utilisation d'un catalyseur métallique peuvent conduire à la formation de sous-produits. De plus, l'introduction de groupes fonctionnels sensibles aux oxydants et d'hétérocycles riches en électrons est difficile à réaliser en raison des conditions de réaction difficiles, qui limitent la diversité des propriétés structurelles et électroniques des GNR. Notre groupe a récemment développé une synthèse de nanographènes et de GNR à l'aide de la réaction de cyclodéhydrochloration photochimique (CDHC) sur des précurseurs de polyphénylène polychlorés. La réaction CDHC possède une haute régiosélectivité et se déroule sans réarrangement ni formation de sous-produits. De plus, la réaction CDHC est conduite sans catalyseur métallique ni oxydant dans des conditions très douces, permettant ainsi l’introduction de différents groupes fonctionnels et hétérocycles sur le GNR afin de moduler leurs propriétés optoélectroniques. En comparant avec la réaction de Scholl, la réaction CDHC permet de mieux contrôler les configuration de bord des GNR. Cette thèse présente en détail l'utilisation de la réaction CDHC pour la préparation de GNR et étudie avec soin les propriétés structurelles et optoélectroniques des GNR produits. Tout d'abord, les GNR asymétriques et latéralement symétriques ont été préparés pour démontrer la régiosélectivité, le contrôle des configuration de bord et l'efficacité de la réaction photochimique CDHC. Ensuite, les GNR à bord thiophène ont été synthétisés pour montrer la polyvalence de la réaction CDHC et étudier l'influence de l'introduction de groupes fonctionnels riches en électrons sur les structures et les propriétés optoélectroniques des GNR. Ensuite, les polymères échelle conjugués (CLP) contenant des unités pyrrole riches en électrons ont été synthétisés pour montrer la compatibilité de la réaction du CDHC avec des groupes fonctionnels très riches en électrons et le rendement élevé de la réaction du CDHC. Enfin, divers dérivés d'ullazine fusionnés avec des hétérocycles riches en électrons ou pauvres en électrons ont été préparés et une série de polymères donneurs-accepteurs conjugués (D-A CP) ont été synthétisés et ces polymères ont été utilisés avec succès dans les cellules solaires à polymères et ont présenté des performances très prometteuse, indiquant l’efficacité, la polyvalence et le caractère pratique de la réaction photochimique CDHC
Le graphène peut être considéré comme l'un des matériaux les plus prometteurs pour les composants électroniques pratiques en raison de ses excellentes propriétés de transport de charge, de sa surface spécifique très élevée, de sa conductivité thermique excellente et de sa grande résistance mécanique. Cependant, ce graphène bidimensionnel est un semiconducteur à bande interdite nulle, ce qui limite son application pratique dans les dispositifs électroniques. L'une des méthodes les plus prometteuses pour ouvrir une bande interdite est le confinement structurel du graphène en bandes étroites, définies comme des nanorubans de graphène (GNR). La bande interdite des GNR peut être contrôlée avec précision par la largeur et la configuration des bords, ce qui donne aux GNR des propriétés optiques et électroniques réglables. La synthèse ascendante en solution est l’une des stratégies les plus prometteuses pour préparer des GNR structurellement bien définis avec des propriétés optiques et électroniques ajustables. Contrairement aux méthodes descendantes, la stratégie ascendante permet un contrôle précis de la largeur et de la configuration des bords des GNR. Une stratégie couramment utilisée, la réaction de cyclodéshydrogénation catalysée par l'acide de Lewis, appelée réaction de Scholl, a été largement utilisée pour synthétiser une grande variété de GNR bien définis sur des précurseurs de polyphénylène. Cependant, la réaction de Scholl présente de sérieux inconvénients qui limitent la portée et la polyvalence de cette réaction. Le premier est sa faible régiosélectivité qui entraîne des défauts de structure et affecte les propriétés des GNR. Ensuite, les réarrangements indésirables et l'utilisation d'un catalyseur métallique peuvent conduire à la formation de sous-produits. De plus, l'introduction de groupes fonctionnels sensibles aux oxydants et d'hétérocycles riches en électrons est difficile à réaliser en raison des conditions de réaction difficiles, qui limitent la diversité des propriétés structurelles et électroniques des GNR. Notre groupe a récemment développé une synthèse de nanographènes et de GNR à l'aide de la réaction de cyclodéhydrochloration photochimique (CDHC) sur des précurseurs de polyphénylène polychlorés. La réaction CDHC possède une haute régiosélectivité et se déroule sans réarrangement ni formation de sous-produits. De plus, la réaction CDHC est conduite sans catalyseur métallique ni oxydant dans des conditions très douces, permettant ainsi l’introduction de différents groupes fonctionnels et hétérocycles sur le GNR afin de moduler leurs propriétés optoélectroniques. En comparant avec la réaction de Scholl, la réaction CDHC permet de mieux contrôler les configuration de bord des GNR. Cette thèse présente en détail l'utilisation de la réaction CDHC pour la préparation de GNR et étudie avec soin les propriétés structurelles et optoélectroniques des GNR produits. Tout d'abord, les GNR asymétriques et latéralement symétriques ont été préparés pour démontrer la régiosélectivité, le contrôle des configuration de bord et l'efficacité de la réaction photochimique CDHC. Ensuite, les GNR à bord thiophène ont été synthétisés pour montrer la polyvalence de la réaction CDHC et étudier l'influence de l'introduction de groupes fonctionnels riches en électrons sur les structures et les propriétés optoélectroniques des GNR. Ensuite, les polymères échelle conjugués (CLP) contenant des unités pyrrole riches en électrons ont été synthétisés pour montrer la compatibilité de la réaction du CDHC avec des groupes fonctionnels très riches en électrons et le rendement élevé de la réaction du CDHC. Enfin, divers dérivés d'ullazine fusionnés avec des hétérocycles riches en électrons ou pauvres en électrons ont été préparés et une série de polymères donneurs-accepteurs conjugués (D-A CP) ont été synthétisés et ces polymères ont été utilisés avec succès dans les cellules solaires à polymères et ont présenté des performances très prometteuse, indiquant l’efficacité, la polyvalence et le caractère pratique de la réaction photochimique CDHC
Graphene is considered as one of the most promising materials for practical electronic components because of its outstanding charge transport properties, very high specific surface area, excellent thermal conductivity, and high mechanical strength. However, this two dimensional graphene is a zero band gap semiconductor, which limits its practical application in electronic devices. One of the most promising methods to open a band gap is the structural confinement of graphene into narrow strips, which is defined as graphene nanoribbons (GNRs). The band gap of GNRs can be precisely controlled by the width and edge configuration, providing GNRs with tunable optical and electronic properties. Bottom-up, solution-phase synthesis is one of the most promising strategies to prepare structurally well-defined GNRs with tunable optical and electronic properties. Unlike the top-down methods, the bottom-up strategy allows a precise control over the width and edge configuration of GNRs. One of the most commonly used strategy, the Lewis acid catalyzed cyclodehydrogenation reaction, known as the Scholl reaction, has been widely used to synthesize a large variety of well-defined GNRs on polyphenylene precursors. However, the Scholl reaction possesses some serious drawbacks that limit the scope and versatility of this reaction. First is its poor regioselectivity that results in structural defects to affect the properties of GNRs. Then the undesired rearrangements and the use of a metal catalyst can lead to the formation of by-products. Moreover, the introduction of oxidant-sensitive functional groups and electron-rich heterocycles is difficult to achieve due to the harsh reaction conditions, which limits the diversity of structural and electronic properties of GNRs. Recently, our group reported the synthesis of nanographenes and GNRs using the photochemical cyclodehydrochlorination (CDHC) reaction on polychlorinated polyphenylene precursors. The CDHC reaction possesses high regioselectivity and it proceeds without rearrangements or the formation of side-products. Furthermore, the CDHC reaction is conducted without metal catalyst and oxidant under very mild conditions, thus enabling the introduction of different functional groups and heterocycles onto the GNRS to modulate their optoelectronic properties. And comparing with the Scholl reaction, the CDHC reaction provides better cont rol over the edge configuration of the GNRs. This paper investigates in detail the usefulness of the CDHC reaction for the preparation of GNRs and carefully studies the structur al and optoelectronic properties of the GNRs produced. First the laterally symmetrical and unsymmetrical GNRs were prepared to demonstrate the regioselectivity edge configuration control, and efficiency of the photochemical CDHC reaction. Then the thiophene edged GNRs were synthesized to show the versatility of the CDHC reaction and study the i nfluence of the introduction of electron rich functional groups on the structures and optoelectronic properties of GNRs. Then, the conjugated ladder polymers (CLPs) containing electron rich pyrrole units were synthesized to show the compatibility of the CDHC reaction with very electron rich functional groups and the high efficiency of the CDHC reaction. Finally various  extended ullazine derivatives fused with electron rich or electron poor heterocycles were prepared and a series of conjugated donor acceptor polymers (D A CPs) were synthesized and these polymers were successfully employed in the polymer solar cells and exhibited very promising performances, indicating the efficiency, versatility and practicality of the photochemical CDHC reactio n
Graphene is considered as one of the most promising materials for practical electronic components because of its outstanding charge transport properties, very high specific surface area, excellent thermal conductivity, and high mechanical strength. However, this two dimensional graphene is a zero band gap semiconductor, which limits its practical application in electronic devices. One of the most promising methods to open a band gap is the structural confinement of graphene into narrow strips, which is defined as graphene nanoribbons (GNRs). The band gap of GNRs can be precisely controlled by the width and edge configuration, providing GNRs with tunable optical and electronic properties. Bottom-up, solution-phase synthesis is one of the most promising strategies to prepare structurally well-defined GNRs with tunable optical and electronic properties. Unlike the top-down methods, the bottom-up strategy allows a precise control over the width and edge configuration of GNRs. One of the most commonly used strategy, the Lewis acid catalyzed cyclodehydrogenation reaction, known as the Scholl reaction, has been widely used to synthesize a large variety of well-defined GNRs on polyphenylene precursors. However, the Scholl reaction possesses some serious drawbacks that limit the scope and versatility of this reaction. First is its poor regioselectivity that results in structural defects to affect the properties of GNRs. Then the undesired rearrangements and the use of a metal catalyst can lead to the formation of by-products. Moreover, the introduction of oxidant-sensitive functional groups and electron-rich heterocycles is difficult to achieve due to the harsh reaction conditions, which limits the diversity of structural and electronic properties of GNRs. Recently, our group reported the synthesis of nanographenes and GNRs using the photochemical cyclodehydrochlorination (CDHC) reaction on polychlorinated polyphenylene precursors. The CDHC reaction possesses high regioselectivity and it proceeds without rearrangements or the formation of side-products. Furthermore, the CDHC reaction is conducted without metal catalyst and oxidant under very mild conditions, thus enabling the introduction of different functional groups and heterocycles onto the GNRS to modulate their optoelectronic properties. And comparing with the Scholl reaction, the CDHC reaction provides better cont rol over the edge configuration of the GNRs. This paper investigates in detail the usefulness of the CDHC reaction for the preparation of GNRs and carefully studies the structur al and optoelectronic properties of the GNRs produced. First the laterally symmetrical and unsymmetrical GNRs were prepared to demonstrate the regioselectivity edge configuration control, and efficiency of the photochemical CDHC reaction. Then the thiophene edged GNRs were synthesized to show the versatility of the CDHC reaction and study the i nfluence of the introduction of electron rich functional groups on the structures and optoelectronic properties of GNRs. Then, the conjugated ladder polymers (CLPs) containing electron rich pyrrole units were synthesized to show the compatibility of the CDHC reaction with very electron rich functional groups and the high efficiency of the CDHC reaction. Finally various  extended ullazine derivatives fused with electron rich or electron poor heterocycles were prepared and a series of conjugated donor acceptor polymers (D A CPs) were synthesized and these polymers were successfully employed in the polymer solar cells and exhibited very promising performances, indicating the efficiency, versatility and practicality of the photochemical CDHC reactio n

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.

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.

Grafeno e nanofitas de grafeno vêm, cada vez mais, atraindo o interesse da comunidade científica devido as suas notáveis propriedades. Neste trabalho realizou-se um estudo sistemático da estabilidade de defeitos do tipo divacância, vacância e Stone-Wales em grafeno e nanofitas de grafeno com bordas zigzag. Para tal, fizeram-se cálculos de primeiros princípios, baseados em teoria do funcional da densidade (DFT) na aproximação GGA com o uso de pseudopotenciais ultrasoft e uma base de ondas planas. Também foram feitas simulações de imagens de STM para os defeitos nas nanofitas. Além disso, foram encontrados dois novos defeitos, não publicados em nenhum outro lugar (até onde vai o conhecimento do autor), com energia de formação muito baixa.
Graphene and graphene nanoribbons have been attracting a lot of interest from the scientific community because of their novel properties. In this work, a systematic research has been done on the stability and energetics of divacancy, vacancy and Stone-Wales defects in graphene and zigzag graphene nanoribbons. With this goal in mind, ab initio density functional calculations within the GGA approximation, using ultrasoft pseudopotentials and a plane wave basis were done. Also, STM images were simulated for some selected defects. Besides, two new defects, not published elsewhere (to the best knowledge of the author), with very low formation energy are reported.

In this work, we demonstrate the bottom-up on-surface synthesis of poly(para-dibenzo[bc,kl]-coronenylene) (PPDBC), a zigzag edge-encased analog of poly(para-phenylene) (PPP), and its lateral fusion into zigzag edge-extended graphene nanoribbons (zeeGNRs). Toward this end, we designed a dihalogenated di(meta-xylyl)anthracene monomer displaying strategic methyl groups at the substituted phenyl ring and investigated its applicability as precursor in the thermally induced surface-assisted polymerization and cyclodehydrogenation. The structure of the resulting zigzag edge-rich (70%) polymer PPDBC was unambiguously confirmed by scanning tunneling microscopy (STM) and non-contact atomic force microscopy (nc-AFM). Remarkably, by further thermal treatment at 450 °C two and three aligned PPDBC chains can be laterally fused into expanded zeeGNRs, with a ribbon width of nine (N = 9) up to 17 (N = 17) carbon atoms. Moreover, the resulting zeeGNRs exhibit a high ratio of zigzag (67%) vs armchair (25%) edge segments and feature electronic band gaps as low as 0.9 eV according to gaps quasiparticle calculations.

This thesis is devoted to the optical properties of low-dimensional structures based on such two-dimensional materials as graphene, silicene and phosphorene. We investigate optical properties of a variety of quasi-one dimensional and quasi-zero-dimensional structures, which are promising for future optoelectronics. Primarily we focus on their low-energy optical properties and how these properties are influenced by the structures’ geometry, external fields, intrinsic strain and edge disorder. As a consequence of this endeavor, we find several interesting effects such as correlation between the optical properties of tubes and ribbons whose periodic and ‘hard wall’ boundary conditions are matched and a universal value of matrix element in narrow-gap tubes and ribbons characterizing probability of transitions across the band gap opened up by intrinsic strain originating from the tube’s surface curvature or ribbon’s edge relaxation. The analytical study of the gapped 2D Dirac materials such as silicene and germanene, which have some similarity to the aforementioned quasi-one-dimensional systems in terms of physical description, reveals a valley- and polarization-dependent selection rules. It was also found that absorption coefficient should change in gapped materials with increasing frequency and become a half of its value for gap edge transitions when the spectrum is linear. Our analysis of the electronic properties of flat clusters of silicene and phosphorene relates the emergence and the number of the peculiar edge states localized at zero energy, so-called zero-energy states, which are know to be of topological origin, to the cluster’s structural characteristics such as shape and size. This allows to predict the presence and the number of such states avoiding complicated topological arguments and provides a recipes for design of metallic and dielectric clusters. We show that zero-energy states are optically active and can be efficiently manipulated by external electric field. However, the edge disorder is important to take into account. We present a new fractal-based methodology to study the effects of the edge disorder which can be applied also to modeling of composite materials. These finding should be useful in design of optoelectronic devices such as tunable emitters and detectors in a wide region of electromagnetic spectrum ranging form the mid-infrared and THz to the optical frequencies.

Recent advances in experimental and computational methods have opened up new directions in graphene fundamental studies. In addition to understanding the basic properties of this material and its quasi-one dimensional structures, significant efforts are devoted to describing their long ranged dispersive interactions. Other two-dimensional materials, such as silicene, germanene, and transition metal dichalcogenides, are also being investigated aiming at finding complementary to graphene systems with other "wonder" properties. The focus of this work is to utilize first principles simulations methods to build our basic knowledge of structure-interaction relations in two-dimensional materials and design their properties. In particular, mechanical folding and extended defects in zigzag and armchair graphene nanoribbons can be used to modulate their electronic and spin polarization characteristics and achieve different stacking patterns. Our simulations concerning zigzag silicene nanoribbons show width-dependent antiferromagnetic-ferromagnetic transitions unlike the case of zigzag graphene nanoribbons, which are always antiferromagnetic. Heterostructures, build by stacking graphene, silicene, and MoS$_2$, are also investigated. It is found that hybridization alters the electronic properties of the individual layers and new flexural and breathing phonon modes display unique behaviors in the heterostructure compositions. Anchored to SiC substrate graphene nanoribbons are also proposed as possible systems to be used in graphene electronics. Our findings are of importance not only for fundamental science, but they could also be used for future experimental developments.

SENA, Silvia Helena Roberto de. Propriedades eletrônicas de tricamada de grafeno e nanofitas de carbono tensionadas. 2012. 112 f. Tese (Doutorado em Física) - Programa de Pós-Graduação em Física, Departamento de Física, Centro de Ciências, Universidade Federal do Ceará, Fortaleza, 2012.
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Graphene is a truly two-dimensional crystal with a gapless linear electronic spectrum at low energies (E<1 eV) which, along with the chiral nature of its charge carriers, is responsible for a variety of unusual properties. As a result of its uniqueness, a great effort has been made in order to understand all its fundamental properties and try to generate a new technology of them. In this thesis we theoretically study two types of graphene-related systems: graphene nanoribbons and trilayer graphene (TLG). Concerning the former, a tight-binding model is used to study the energy band of graphene and graphene ribbon under simple shear strain. The ribbon consists of lines of carbon atoms in an armchair or zigzag orientation where a simple shear strain is applied in the $x$-direction keeping the atomic distances in the $y$-direction unchanged. Such modification in the lattice gives an energy band that differs in several aspects from the one without any shear and with pure shear. The changes in the spectrum depend on the line displacement of the ribbon, and also on the modified hopping parameter. It is also shown that this simple shear strain tunes the electronic properties of both graphene and graphene ribbon, opening and closing energy gaps for different displacements of the system. The modified density of states is also shown. On the latter subject, the continuum model is used in order to investigate the electronic spectrum of three coupled graphene layers (graphene trilayers) in the presence of an external magnetic field. We obtain analytical expressions for the Landau level (LL) spectrum for both the ABA and ABC types of stacking, which exhibit very different dependence on the magnetic field. While the LL spectrum of ABA TLG is found to be a superposition of a monolayer-like and bilayer-like spectra, the ABC TLG present a nearly B^{3/2} field dependence. We show that layer asymmetry and an external gate voltage can strongly influence the properties of the system. In addition, the cyclotron resonance energies, the corresponding oscillator strengths, and the cyclotron absorption spectrum for trilayer graphene are calculated for both ABA and ABC stacking. A gate potential across the stacked layers leads to (1) a reduction of the transition energies, (2) a lifting of the degeneracy of the zero Landau level, and (3) the removal of the electron-hole symmetry.
Grafeno é um cristal bidimensional cujo espectro eletrônico a baixas energias (E <1 eV) apresenta dispersão linear e ausência de gap que, juntamente com a natureza quiral dos portadores de carga, são responsáveis por uma variedade de propriedades incomuns. Como resultado da sua natureza singular, um grande esforço tem sido feito para entender todas as suas propriedades fundamentais e tentar gerar uma nova tecnologia baseada nesse material. Nesta tese, nós realizamos um estudo teórico de dois tipos de sistemas: nanofitas de grafeno e tricamadas grafeno (TCG). No que diz respeito ao primeiro sistema, um modelo de ligação forte (tight-binding) é utilizado para estudar as bandas de energia do grafeno e fitas de grafeno sujeitas a uma tensão de cisalhamento. A fita é constituída por linhas de átomos de carbono cujas bordas estão orientadas nas direções conhecidas como “armchair” ou “zigzag”. Uma tensão de cisalhamento simples é aplicada na direção x de forma que as distâncias interatômicas na direção y são mantidas inalteradas. Esta modificação na rede cristalina origina bandas de energia que diferem em vários aspectos do sistema original sem qualquer deformação. As mudanças no espectro dependem do deslocamento entre linhas adjacentes da fita, bem como do parâmetro de “hopping” modificado. Mostra-se também que este cisalhamento simples modifica as propriedades eletrônicas de ambos os sistemas, fitas de grafeno e grafeno, abrindo e fechando gaps de energia para diferentes deslocamentos do sistema. A densidade de estados modificada também é mostrada. Por fim, o modelo contínuo é utilizado a fim de investigar o espectro electrônico de três camadas de grafeno acopladas (tricamada de grafeno), na presença de um campo magnético externo. Nesse contexto, obtemos expressões analíticas para os nveis de Landau para ambos os tipos de empilhamento: Bernal (ABA) e romboédrico (ABC), verificando-se uma forte dependência dos níveis de energia com o tipo de empilhamento. Embora o espectro de Landau para tricamadas ABA seja uma sobreposição dos espectros de uma monocamada e de uma bicamada, tricamadas com empilhamento ABC apresentam uma dispersão do tipo B3/2 com o campo magnético. Foi mostrado que uma assimetria entre as camadas, que pode ser introduzida por um potencial externo, pode influenciar fortemente as propriedades do sistema. Além disso, as energias de ressonância cíclotron, assim como forças de oscilador correspondentes, e o espectro de absorção para tricamadas de grafeno são calculadas para ambos os tipos de empilhamento. Verificou-se que um potencial de porta aplicado através das camadas leva a (1) uma redução das energias de transição, (2) um levantamento da degenerescência do nível de Landau n=0, e (3) a quebra de simetria entre elétrons e buracos.

Modeling approaches are developed to optimize emerging on-chip and off-chip electrical interconnect technologies and benchmark them against conventional technologies. While transistor scaling results in an improvement in power and performance, interconnect scaling results in a degradation in performance and electromigration reliability. Although graphene potentially has superior transport properties compared to copper, it is shown that several technology improvements like smooth edges, edge doping, good contacts, and good substrates are essential for graphene to outperform copper in high performance on-chip interconnect applications. However, for low power applications, the low capacitance of graphene results in 31\% energy savings compared to copper interconnects, for a fixed performance. Further, for characterization of the circuit parameters of multi-layer graphene, multi-conductor transmission line models that account for an alignment margin and finite width of the contact are developed. Although it is essential to push for an improvement in chip performance by improving on-chip interconnects, devices, and architectures, the system level performance can get severely limited by the bandwidth of off-chip interconnects. As a result, three dimensional integration and airgap interconnects are studied as potential replacements for conventional off-chip interconnects. The key parameters that limit the performance of a 3D IC are identified as the Through Silicon Via (TSV) capacitance, driver resistance, and on-chip wire resistance on the driver side. Further, the impact of on-chip wires on the performance of 3D ICs is shown to be more pronounced at advanced technology nodes and when the TSV diameter is scaled down. Airgap interconnects are shown to improve aggregate bandwidth by 3x to 5x for backplane and Printed Circuit Board (PCB) links, and by 2x for silicon interposer links, at comparable energy consumption.

ARAÚJO, F. R. V. Transporte Eletrônico em Phased Arrays de Nanofitas de Grafeno. 2017. 70 f. Dissertação (Mestrado em Física) – Centro de Ciências, Universidade Federal do Ceará, Fortaleza, 2017.
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Graphene, a layer of carbon atoms arranged in a honeycomb crystal lattice, has remarkable physical properties. After its experimental obtaining in 2004 by A. K. Geim and K. S. Novoselov, several researches were carried out aiming to understand such physical properties and several possibilities of applications were proposed. At the low energy limit, there is a linearity relationship between energy and momentum for the electric charge carriers in this material and, therefore, they behave as relativistic particles of zero mass, described by the Dirac equation. One of the implications is that the electron-associated eigenfunctions that cross a potential barrier may not undergo damping under certain circumstances, a phenomenon known as Klein's paradox. Even without damping, these eigenfunctions acquire a phase factors that may depend only on the height and width values of the potential barrier. In this study, we investigate the properties transport in two electronic devices that use this phenomenon and that may be associated to phased arrays (electronic systems that have several emitters of waves, mechanically or electromagnetic, properly organized). We studied the electronic transport mechanisms in these physical systems and performed numerical simulations of electrical conductance as a function of energy and electrical conductance as a function of the electric potential and it was observed that the direction of propagation of the electrons can be controlled by varying the values of height and width of potential barriers.
O grafeno, uma camada de átomos de carbono arranjados em uma rede cristalina honeycomb (favo de mel), possui propriedades físicas notáveis. Após sua obtenção experimental em 2004 por A. K. Geim e K. S. Novoselov, várias pesquisas foram realizadas objetivando compreender tais propriedades físicas e diversas possibilidades de aplicações foram propostas. No limite de baixas energias, existe uma relação de linearidade entre a energia e o momento para os portadores de carga elétrica nesse material e, com isso, os mesmos comportam-se como partículas relativísticas de massa nula, descritas pela equação de Dirac. Uma das implicações disso é que as autofunções associadas aos elétrons que atravessam uma barreira de potencial podem não sofrer amortecimento em dadas circunstâncias, fenômeno esse conhecido como paradoxo de Klein. Mesmo sem sofrer amortecimento, essas autofunções adquirem fatores de fase que podem depender apenas dos valores de altura e largura da barreira de potencial. Nesse trabalho investigamos as propriedades de transporte em dois dispositivos eletrônicos que utilizam-se desse fenômeno e que podem ser associados a phased arrays (sistemas eletrônicos que possuem vários emissores de ondas, mecânicas ou eletromagnéticas, devidamente organizados). Estudamos os mecanismos de transporte eletrônico nesses sistemas físicos e realizamos simulações numéricas da condutância elétrica em função da energia e da condutância elétrica em função do potencial elétrico e observamos que a direção de propagação dos elétrons pode ser controlada através da variação dos valores de altura e largura das barreiras de potencial.

Doctor of Philosophy
Department of Chemical Engineering
Vikas Berry
Graphene - a single atom thick two dimensional sheet of sp[superscript]2 bonded carbon atoms arranged in a honeycomb lattice - has shown great promise for both fundamental research & applications because of its unique electrical, optical, thermal, mechanical and chemical properties. Derivatization of graphene unlocks a plethora of novel properties unavailable to their pristine parent “graphene”. In this dissertation we have synthesized various structural and chemical derivatives of graphene; characterized them in detail; and leveraged their exotic properties for diverse applications. We have synthesized protein/DNA/ethylenediamine functionalized derivatives of graphene via a HATU catalyzed amide reaction of primary-amine-containing moieties with graphene oxide (GO) – an oxyfunctional graphene derivative. In contrast to non-specificity of graphene, this functionalization of GO has enabled highly specific interactions with analytes. Devices fabricated from the protein (concanavalin – A) and DNA functionalized graphene derivatives were demonstrated to enable label-free, specific detection of bacteria and DNA molecules, respectively, with single quanta sensitivity. Room temperature electrical characterization of the sensors showed a generation of ~ 1400 charge carriers for single bacterium attachment and an increase of 5.6 X 10[superscript]12 charge carriers / cm[superscript]2 for attachment of a single complementary strand of DNA. This work has shown for the first time the viability of graphene for bio-electronics and sensing at single quanta level. Taking the bio-interfacing of graphene to the next level, we demonstrate the instantaneous swaddling of a single live bacterium (Bacillus subtilis) with several hundred sq. micron (~ 600 µm[superscript]2) areal protein-functionalized graphene sheets. The atomic impermeability and high yield strength of graphene resulted in hermetic compartmentalization of bacteria. This enabled preservation of the dimensional and topological characteristics of the bacterium against the degrading effects of harsh environments such as the ultrahigh vacuum (~ 10[superscript]-5 Torr) and high intensity electron beam (~ 150 A/cm[superscript]2) in a transmission electron microscope (TEM) column. While an unwrapped bacterium shrank by ~ 76 % and displayed significant charge buildup in the TEM column; a wrapped bacterium remained uncontracted and undamaged owing to the graphenic wraps. This work has shown for the first time an impermeable graphenic encasement of bacteria and its application in high vacuum TEM imaging without using any lengthy traditional biological TEM sample preparation techniques. In an inch-scale, we fabricated robust free-standing paper composed of TWEEN/Graphene composite which exhibited excellent chemical stability and mechanical strength. This paper displayed excellent biocompatibility towards three mammalian cell lines while inhibiting the non-specific binding of bacteria (Bacillus cereus). We predict this composite and its derivatives to have excellent applications in biomedical engineering for transplant devices, invasive instrument coatings and implants. We also demonstrate a novel, ultra-fast and high yield process for reducing GO to reduced graphene oxide (RGO) using a facile hydride-based chemistry. The RGO sheets thus-produced exhibited high carrier mobilities (~ 100-600 cm[superscript]2/V•s) and reinstatement of the ambipolar characteristic of graphene. Raman spectra and UV-Vis spectroscopy on the RGO sheets displayed a high degree of restoration of the crystalline sp2 lattice with relatively low defects. We fabricated graphene nanoribbons (GNRs) – 1D structural derivatives of graphene – using a nano-scale cutting process from highly oriented pyrolytic graphite (HOPG) blocks, with widths pre-determinable between 5 nm to 600 nm. The as-produced GNRs had very high aspect ratio in the longitudinal direction (~ 0.01); exhibited predominantly mono-layered structure (< 10 % bilayer); and smooth edges (Raman I[subscript]D/G ~ 0.25 -0.28). Low temperature electrical transport measurements on back-gated thin film GNR devices were performed and a carrier mobility of ~ 20 ± 4 cm[superscript]2/V•s with sheet resistances of 2.2-5.1 MΩ / □ was extracted. Despite the ~ 50 nm thicknesses of the films, a clear bandgap scaling was observed with transport via variable range hopping (VRH) in 2 and 3 dimensions. This work demonstrates the first fully functional narrow pristine GNR thin-film field effect transistors (FETs). In addition we fabricated graphene quantum dots (GQDs) – 0D derivatives of graphene with dimensions < 100 nm – using a slight variation of our nano-scale cutting strategy, where the cleavage process is carried out in two dimensions. A high degree of control on the dimensions (Std. Dev. of ~ 5 nm for 50 X 50 nm square GQDs) and shape (pre-determinable between square, rectangle, triangle and trapezoid) of the as-synthesized GQDs is demonstrated. The optical properties of the GQDs such as the UV-Vis absorbance and photoluminescence were studied and their facile tunability was demonstrated depending on their dimensions. This work demonstrates for the first time the high throughput fabrication of GQDs with tunable dimensions and shape.

Graphene is foreseen to be the basis of future electronics owing to its ultra thin structure, extremely high charge carrier mobility,  high thermal conductivity etc., which are expected to overcome the size limitation and heat dissipation problem in silicon based transistors. But these great prospects are hindered by the metallic nature of pristine graphene even at charge neutrality point, which allows to flow current even when a transistor is switched off. A part  of the thesis is dedicated to invoke electronic band gaps in graphene to overcome this problem. The concept of quantum confinement has been employed to tune the band gaps in graphene by  dimensional confinement along with the functionalization of the edges of these confined nanostructures. Thermodynamic stability of the functionalized zigzag edges with hydrogen, fluorine and reconstructed edges has been presented in the thesis. Keeping an eye towards the same goal of band gap opening,  a different route has been considered by admixing insulating hexagonal boron nitride (h-BN) with semimetal graphene. The idea has been implemented in two  dimensional h-BN-graphene composites and three dimensional stacked heterostructures. The study reveals the possibility of tuning band gaps by controlling the admixture. Occurrence of defects in graphene has significant effect on its electronic properties. By random insertion of defects, amorphous graphene is studied, revealing a semi-metal to a metal transition. The field of molecular electronics and spintronics aims towards device realization at the molecular scale. In this thesis, different aspects of magnetic bistability in organometallic molecules have been explored in order to design  practical spintronics devices. Manipulation of spin states in organometallic molecules, specifically metal porphyrin molecules, is achieved by controlling surface–molecule interaction. It has been shown that by strain engineering in defected graphene, the magnetic state of adsorbed molecules can be changed. The spin crossover between different spin states can also be achieved by chemisorption on magnetic surfaces. A significant part of the thesis demonstrates that the surface-molecule interaction not only changes the spin state of the molecule, but allows to manipulate magnetic anisotropies and spin dipole moments via modified ligand fields. Finally, in collaboration with experimentalists, a practical realization of switching surface–molecule magnetic interactions by external magnetic fields is demonstrated.