Dissertations / Theses on the topic 'Graphene NanoRibbon'

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.

Graphene is an atomically thin two-dimensional (2-D) crystal with unique thermal, mechanical, and electronic transport properties such as the high mobility of carriers, perfect 2- D confinement and linear dispersion, etc., has been attracted many interest as a promising candidate for nano-scale devices over the past decades. Multilayer stacks of graphene and other stable, atomically thin, 2-D materials offer the prospect of creating a new class of heterostructure materials. Hexagonal boron- nitride (hBN), is a great candidate to be stacked with graphene due to an atomically 2-D layered structure with a lattice constant very similar to graphene (1.8% mismatch), large electrical band gap (∼4.7eV), and excellent thermal and chemical stability. The graphene/hBN based tunneling transistors show the resonant tunneling and strong negative differential resistance (NDR). These devices which have potential for future high-frequency and logic applications such as high-speed IC circuits, signal generators, data storage, etc., has been studied both theoretically and experimentally recently. The aim in this dissertation has been to study the effect of the uniaxial strain on the graphene nanoribbon resonant tunneling transistors (RTTs). The uniaxial strain may be induced either by an external stress applied to the graphene in a particular direction or by a substrate due to deposition of graphene on top of the other materials. The strain modifies distances between carbon atoms which leading to different hopping amplitudes among neighboring sites. A resonant tunneling transistor consisting of armchair graphene nanoribbon (AGNR) electrodes with three layers of hBN tunnel barrier between them has been considered. By using the nearest-neighbor tight-bind (TB) method and the nonequilibrium Green function (NEGF) formalism, the electronic transport characteristics of a RTT is calculated. In this work, we focus on how the strain affects the current-voltage characteristics of AGNR/hBN RTT.

Graphene was the first 2 dimensional material discovered and rapidly received a lot of attention because of its astounding properties. It is still the highest conductivity material recorded and very robust despite its single atomic layer thickness. However a key issue with graphene has been that it is a semimetal and not a semiconductor, so it lacks a band gap. Originally a large amount of focus was on researching methods to overcome this issue for logic devices. At first the patterning into nanoribbons was seen as a method to achieve this, but the fabrication of a nanoribbon came at a cost of graphene’s high mobility electrons. From conducting this research an interesting property of graphene emerged. It was capable of acting intrinsically as a single electron transistor, enabling a different type of more than Moore device to be fabricated that can be used in future nanoelectronic applications. The aim of this project has been to investigate the transport properties of polycrystalline graphene grown using chemical vapour deposition. The use of polycrystalline graphene enables the fabrication of wafer scale devices that can be stacked on a large variety of surfaces. So far though there has been a lack of investigation into the scaling effects of polycrystalline graphene nanoribbons and the single electron tunnelling properties associated with them. This work presents the first detailed investigation into their properties and shows that polycrystalline graphene can be used for producing high quality single electron transistors. Nanoribbons are fabricated down to sub 20 nm widths with high aspect ratio transitions from wide to narrow segments. The single electron transistor has demonstrated a single quantum dot impacted by the effect of energy level spacing.

Graphene can be used to create circuits that are almost superconducting, potentially speeding electronic components by as much as 1000 times [1]. Such blazing speed might also help produce ever-tinier computing devices with more power than your clunky laptop [2]. Graphite is a polymorph of the element carbon [3]. Graphite is made up of tiny sheets of graphene. Graphene sheets stack to form graphite with an interplanar spacing of 0.335 nm, which means that a stack of 3 million sheets would be only one millimeter thick. [1] This nano scale 2 dimensional sheet is graphene. Novoselov and Geim's discovery is now the stuff of scientific legend, with the two men being awarded the Nobel Prize in 2010 [4]. In 2004, two Russian-born scientists at the University of Manchester stuck Scotch tape to a chunk of graphite, then repeatedly peeled it back until they had the tiniest layer possible [2]. Graphene has exploded on the scene over the past couple of years. "Six years ago, it didn't exist at all, and next year we know that Samsung is planning to release their first mobile-phone screens made of graphene." - Dr Kostya Novoselov [4]. It is a lattice of hexagons, each vertex tipped with a carbon atom. At the molecular level, it looks like chicken wire [4]. There are two common lattice formations of graphene, armchair and zigzag. The most studied edges, zigzag and armchair, have drastically different electronic properties. Zigzag edges can sustain edge surface states and resonances that are not present in the armchair case Rycerz et al., 2007 [5]. This research focused on the armchair graphene nanoribbon formation (acGNR). Graphene has several notable properties that make it worthy of research. The first of which is its remarkable strength. Graphene has a record breaking strength of 200 times greater than steel, with a tensile strength of 130GPa [1]. Graphene has a Young's modulus of 1000, compared to just that of 150 for silicon [1]. To put it into perspective, if you had a sheet of graphene as thick as a piece of cellophane, it would support the weight of a car. [2] If paper were as stiff as graphene, you could hold a 100-yard-long sheet of it at one end without its breaking or bending. [2] Another one of graphene's attractive properties is its electronic band gap, or rather, its lack thereof. Graphene is a Zero Gap Semiconductor. So it has high electron mobility at room temperature. It's a Superconductor. Electron transfer is 100 times faster than Silicon [1]. With zero a band gap, in the massless Dirac Fermion structure, the graphene ribbon is virtually lossless, making it a perfect semiconductor. Even in the massive Dirac Fermion structure, the band gap is 64meV [6]. This research began, as discussed in Chapter 2, with an armchair graphene nanoribbon unit cell of N=8. There were 16 electron approximation locations (ψ) provided per unit cell that spanned varying Fermi energy levels. Due to the atomic scales of the nanoribbon, the carbon atoms are separated by 1.42Å. The unit vector is given as, ~a = dbx, where d = 3αcc and αcc = 1.42°A is the carbon bond length [5]. Because of the close proximity of the carbon atoms, the 16 electron approximations could be combined or summed with their opposing lattice neighbors. Using single line approximation allowed us to reduce the 16 points down to 8. These approximations were then converted into charge densities (ρ). Poisson's equation, discussed in Chapter 3, was expanded into the 3 dimensional space, allowing us to convert ρ into voltage potentials (φ). Even though graphene is 2 dimensional; it can be used nicely in 3 dimensional computations without the presence of a substrate, due to the electric field lines and voltage potential characteristics produced being 3 dimensional. Subsequently it was found that small graphene sheets do not need to rest on substrates but can be freely suspended from a scaffolding; furthermore, bilayer and multilayer sheets can be prepared and characterized.

Graphene, that is a monolayer of carbon atoms arranged in a honeycomb lattice, has been isolated only recently from graphite. This material shows very attractive physical properties, like superior carrier mobility, current carrying capability and thermal conductivity. In consideration of that, graphene has been the object of large investigation as a promising candidate to be used in nanometer-scale devices for electronic applications. In this work, graphene nanoribbons (GNRs), that are narrow strips of graphene, for which a band-gap is induced by the quantum confinement of carriers in the transverse direction, have been studied. As experimental GNR-FETs are still far from being ideal, mainly due to the large width and edge roughness, an accurate description of the physical phenomena occurring in these devices is required to have valuable predictions about the performance of these novel structures. A code has been developed to this purpose and used to investigate the performance of 1 to 15-nm wide GNR-FETs. Due to the importance of an accurate description of the quantum effects in the operation of graphene devices, a full-quantum transport model has been adopted: the electron dynamics has been described by a tight-binding (TB) Hamiltonian model and transport has been solved within the formalism of the non-equilibrium Green's functions (NEGF). Both ballistic and dissipative transport are considered. The inclusion of the electron-phonon interaction has been taken into account in the self-consistent Born approximation. In consideration of their different energy band-gap, narrow GNRs are expected to be suitable for logic applications, while wider ones could be promising candidates as channel material for radio-frequency applications.

Graphene, that is a monolayer of carbon atoms arranged in a honeycomb lattice, has been isolated only recently from graphite. This material shows very attractive physical properties, like superior carrier mobility, current carrying capability and thermal conductivity. In consideration of that, graphene has been the object of large investigation as a promising candidate to be used in nanometer-scale devices for electronic applications. In this work, graphene nanoribbons (GNRs), that are narrow strips of graphene, for which a band-gap is induced by the quantum confinement of carriers in the transverse direction, have been studied. As experimental GNR-FETs are still far from being ideal, mainly due to the large width and edge roughness, an accurate description of the physical phenomena occurring in these devices is required to have valuable predictions about the performance of these novel structures. A code has been developed to this purpose and used to investigate the performance of 1 to 15-nm wide GNR-FETs. Due to the importance of an accurate description of the quantum effects in the operation of graphene devices, a full-quantum transport model has been adopted: the electron dynamics has been described by a tight-binding (TB) Hamiltonian model and transport has been solved within the formalism of the non-equilibrium Green's functions (NEGF). Both ballistic and dissipative transport are considered. The inclusion of the electron-phonon interaction has been taken into account in the self-consistent Born approximation. In consideration of their different energy band-gap, narrow GNRs are expected to be suitable for logic applications, while wider ones could be promising candidates as channel material for radio-frequency applications.

Various solution-processed methods have been employed in this work. For the synthesis of graphene, a chemical exfoliation method has been used to generate large graphene flakes in the solution phase. In addition, chemical or electro polymerization has been used for synthesizing polyanthracene, which tends to form graphene nanoribbon through cyclodehydrogenation. For the device fabrication, graphene oxide (GO) thin films were deposited from solution phase on the vapor-silanzed aminosilane surface to make semiconducting active layer or conducting electrodes. Gold nanoparticles (AuNPs) were selectively self-assembled from solution phase to pattern nanowires.

Graphene nanostructures directly grown on SiC are appealing for their potential application to nano-scale electronic devices. In particular, epitaxial sidewall graphene nanoribbons have been a promising candidate in ballistic transport and band gap engineering. In this thesis, we study graphene nanoribbons by utilizing both nano-lithography and natural step bunching to control the step morphology of the SiC(0001) surface in order to guide the growth of graphene which initiates at step edges, and study their respective characteristics. With scanning tunneling microscopy and spectroscopy (STM/STS), we explore the local atomic and electronic structures of the graphene nanoribbons down to atomic scale. It is found that nanoribbon formation depends critically on nanofacet orientation, nanofacet density, and growth conditions. Under some conditions, nanoribbons grow predominantly on the nanofacet. Significant electronic density-of-states features, resolved by STS, are found to depend strongly on proximity to strained graphene near the step edge. Experimental results are compared to Molecular Dynamics simulations to better understand the origin of the discrete electronic states.

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..

Metal-oxide semiconductor field-effect transistor (MOSFET) scaling throughout the years has enabled us to pack million of MOS transistors on a single chip to keep in pace with Moore’s Law. After forty years of advances in integrated circuit (IC) technology, the scaling of silicon (Si) MOSFET has entered the nanometer dimension with the introduction of 90 nm high volume manufacturing in 2004. The latest technological advancement has led to a low power, high-density and high-speed generation of processor. Nevertheless, the scaling of the Si MOSFET below 22 nm may soon meet its’ fundamental physical limitations. This threshold makes the possible use of novel devices and structures such as carbon nanotube field-effect transistors (CNTFETs) and graphene nanoribbon field-effect transistors (GNRFETs) for future nanoelectronics. The investigation explores the potential of these amazing carbon structures that exceed MOSFET capabilities in term of speed, scalability and power consumption. The research findings demonstrate the potential integration of carbon based technology into existing ICs. In particular, a simulation program with integrated circuit emphasis (SPICE) model for CNTFET and GNRFET in digital logic applications is presented. The device performance of these circuit models and their design layout are then compared to 45 nm and 90 nm MOSFET for benchmarking. It is revealed through the investigation that CNT and GNR channels can overcome the limitations imposed by Si channel length scaling and associated short channel effects while consuming smaller channel area at higher current density.

Phonon properties of AB (Bernal) stacked bilayer graphene (BLG) with various types of defects have been investigated theoretically. Forced Vibrational (FV) method has been employed to compute the phonon modes of disordered BLG. A downward linear shift of E2g mode frequencies has been observed with an increasing amount of defect concentration. Moreover, two identical E2g peaks have been observed in PDOS of the bilayer system where the individual layer contains 12C and 13C atoms respectively. From computed typical mode patterns of in-plane K-point optical mode phonons, it has been noticed that phonons become strongly localized around a few nanometers area at the presence of defects and localized modes increase with the increasing amount of defect concentration. The edge effect on the localized phonon modes has also been discussed for bilayer armchair graphene nanoribbons (BiAGNRs). The impact of defects on the phonon conduction properties has also been studied for BiAGNRs. My investigated results can be convenient to study the thermal conductivity and electron-phonon interaction of bilayer graphene-based nanodevices and to interpret the Raman and infrared spectra of disordered system.

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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

Parmi les nouveaux matériaux émergents à base de carbone, le graphène est rapidement devenu un candidat idéal pour plusieurs applications en nanoélectronique. Dans ce contexte, différentes méthodes ont été proposées pour transformer ses propriétés électriques, et notamment supprimer sont point de dégénérescence de Dirac. L’ouverture d’un gap d’énergie peut ainsi conduire à l’usage du graphène dans des nano-transistors. Dans cette thèse, nous appliquons un modèle compact semi-analytique pour étudier deux types de nanotransistors à base de graphène: les transistors à nanorouban et les transistors à nanomesh. Un modèle de type thight-binding est utilisé pour déterminer les expressions analytiques des bandes d'énergie d'un nanorouban de graphène. Des comparaisons sont montrées avec des approches ab initio, et avec des mesures effectuées sur des transistors du même type mais à plus grande échelle. Dans le contexte de l'électronique pour applications souples, les contraintes mécaniques sur les circuits et les déformations géométriques des composants à base de graphène peuvent constituer un problème important. Nous étudions ces effets sur les propriétés de conduction des transistors à nanorubans (dans les régimes balistique et partiellement balistique). En supposant la présence de petites déformations, une mise à l'échelle spectrale et un décalage spectral dû à la présence d'une déformation peuvent être pris en compte de manière analytique. Ce modèle conduit à définir sous forme analytique les quantités effectives (masses, densités d’états) utilisées pour calculer numériquement les potentiels et les courants dans le nano-dispositif. Les résultats numériques sont présentés à la fois sous un régime balistique et partiellement balistique, avec ou sans contacts de Schottky. Les résultats proposés dans le Chapitre 2 illustrent de manière très simple comment la déformation du nanoribbon de graphène influence les caractéristiques I-V du transistor. Une autre solution pour réaliser un nanotransistor de graphène est la gravure de nano-trous dans une feuille de graphène (réalisant ainsi un nanomesh). Si le graphène nanomesh est correctement formé, le rapport de courant On / Off du transistor devrait être amélioré. Dans le Chapitre 3, la méthode semi-analytique est utilisée pour évaluer les performances d'un nanomesh à transistors à nanorubans. Les résultats sont à nouveau comparés à une méthode ab-initio. Les caractéristiques I-V du graphène nanomesh transistor sont présentées et comparées aux résultats expérimentaux. Les résultats proposés montrent comment la taille des nanomesh de graphène influence les caractéristiques I-V du transistor. Compte tenu de la simplicité et du temps de calcul réduit de l'approche proposé, ces résultats peuvent permettre des analyses paramétriques, des optimisations et des caractérisations de nano-transistor à graphène dans des circuits à plus grande échelle
Among emerging carbon materials, graphene has rapidly become an ideal candidate for nano-electronics. In this context, different methods have been proposed to transform its electric properties and remove the Dirac degeneracy point, leading to application to nano-transistors. In this thesis we apply a semi-analytical compact model to study two kinds of graphene-based nanotransistors: nanoribbon graphene transistor and nanomesh transistor. A tight-binding model is used to determine analytical expressions for the energy bands of a graphene nanoribbon. Comparisons are shown with ab-initio approaches and with measurements done on larger-scale transistors of the same kind. In the context of flexible electronics, mechanical stresses on circuits and subsequent geometric deformations of graphene-based components is an important issue. We investigate these effects on the conduction properties of nanoribbon transistors (both in ballistic and partially ballistic regimes). By assuming the presence of small deformations, a spectral scaling and a spectral shift due to the presence of a deformation can be taken into account analytically. This model leads to define in closed form effective quantities (masses, densities of states) used to numerically calculate potentials and currents in the nano-device. Numerical results are shown both in a ballistic and partially-ballistic regime, with and without the presence of Schottky contacts. The proposed results in Chapter 2 illustrate in a very simple way how the deformation of graphene nanoribbon influences the I-V characteristics of transistor. Another solution to realize graphene nanotransistor is the etching of nanoholes in a graphene sheet (thus realizing a nanomesh). If graphene nanomesh is properly shaped, the On/Off current ratio of transistor is expected to be enhanced. In Chapter 3, the semi-analytic method is used to evaluate the performance of nanomesh transistor with nanoribbon ones. The results are again compared with an ab-initio method. I-V characteristics of graphene nanomesh transistor are presented and compared with experimental results. The proposed results show how graphene nanomesh size influences the I-V characteristics of transistor. Given the simplicity and the reduced computation time of the approach, these results can lead to perform parametric analyses, optimizations and characterization of graphene nano-transistor when applied in larger-scale circuits

As the thinnest material ever with high carrier mobility and saturation velocity, graphene is considered as a candidate for future high speed electronics. After pioneering research on graphene-based electronics at Georgia Tech, epitaxial graphene on SiC, along with other synthesized graphene, has been extensively investigated for possible applications in high frequency analog circuits. With a combined effort from academic and industrial research institutions, the best cut-off frequency of graphene radio-frequency (RF) transistors is already comparable to the best result of III-V material-based devices. However, the power gain performance of graphene transistors remained low, and the absence of a band gap inhibits the possibility of graphene in digital electronics. Aiming at solving these problems, this thesis will demonstrate the effort toward better high frequency power gain performance based on mono-layer epitaxial graphene on C-face SiC. Besides, a graphene/Si integration scheme will be proposed that utilizes the high speed potential of graphene electronics and logic functionality and maturity of Si-CMOS platform at the same time.

Cette thèse présente des mesures de transport électronique dans des systèmes bi-dimensionels et uni-dimensionels à base de graphène sous champ magnétique pulsé (60T). L'objectif de ces travaux consiste à sonder la dynamique des porteurs de charge en modifiant la densité d'états du système par l'application d'un champ magnétique. Une première partie est consacrée à l'étude de l'influence des îlots électrons-trous sur les propriétés de transport du graphène au voisinage du point de neutralité de charge. Nous avons constaté l'apparition de fluctuations de la magnéto-résistance liée à la transition progressive des îlots de taille finie dans le régime quantique lorsque le champ magnétique augmente. Nous avons aussi montré que la variation de l'énergie de Fermi, liée à l'augmentation de la dégénérescence orbitale des niveaux de Landau, est directement responsable d'une modification du ratio entre électrons et trous. Dans une deuxième partie consacrée à l'étude des nanorubans de graphène, nous avons exploré deux gammes de largeur différentes. Dans les rubans larges (W>60nm), la quantification de la résistance a été observée révélant ainsi une signature évidente de la quantification du spectre énergétique en niveaux de Landau. Le confinement magnétique des porteurs de charge sur les bords des nanorubans a permis de mettre en évidence, pour la première fois, la levée de dégénérescence de vallée liée à la configuration armchair du ruban. Pour des rubans plus étroits (W<30nm), en présence de défauts de bord et d'impuretés chargées, la formation progressive des états de bords chiraux donne lieu à une magnéto-conductance positive quelque soit la densité de porteurs. Enfin, la dernière partie traite du magnéto-transport dans le graphene multi-feuillet. En particulier, nous avons observé l'effet Hall quantique dans les systèmes tri-couche de graphène. Une étude comparative des résultats expérimentaux avec des simulations numériques a permis de déterminer l'empilement rhombohedral des trois couches de graphene constituant l'échantillon
This thesis presents transport measurements on two-dimensional and one-dimensional graphene-based systems under pulsed magnetic field (60T). The objective of this work is to probe the dynamics of charge carriers by changing the density of states of the system by applying a strong magnetic field. The first part is devoted to the study of the influence of electron-hole pockets on the transport properties of graphene near the charge neutrality point. We found the appearance of fluctuations in the magneto-resistance due to the progressive transition of the electron/hole puddles of finite size in the quantum regime as the magnetic field increases. We have also shown that the variation of the Fermi energy, due to the increase of orbital Landau level degeneracy, is directly responsible of a change in the electron and hole ratio. The second part is devoted to the study of graphene nano-ribbons, we explored two different ranges of width. In the broad nano-ribbons of width W larger than 60 nm, the quantification of the resistance is observed, revealing a clear signature of the quantization of the energy spectrum into Landau levels. We show for the first time the effect of valley degeneracy lifting induced by the magnetic confinement of charge carriers at the edges of the armchair nano-ribbons. For narrower nano-ribbons (W <30 nm) in presence of edge defects and charged impurities, the progressive formation of chiral edge states leads to a positive magneto-conductance whatever the carrier density. Finally, the last part of this thesis deals with magneto-transport fingerprints in multi-layer graphene as we observed the quantum Hall effect in tri-layer graphene. A comparative study of the experimental results with numerical simulations was used to determine the rhombohedral stacking of three layers of graphene in the sample

Like cell to the human body, transistors are the basic building blocks of any electronics circuits. Silicon has been the industries obvious choice for making transistors. Transistors with large size occupy large chip area, consume lots of power and the number of functionalities will be limited due to area constraints. Thus to make the devices smaller, smarter and faster, the transistors are aggressively scaled down in each generation. Moore's law states that the transistors count in any electronic circuits doubles every 18 months. Following this Moore's law, the transistor has already been scaled down to 14 nm. However there are limitations to how much further these transistors can be scaled down. Particularly below 10 nm, these silicon based transistors hit the fundamental limits like loss of gate control, high leakage and various other short channel effects. Thus it is not possible to favor the silicon transistors for future electronics applications. As a result, the research has shifted to new device concepts and device materials alternative to silicon. Carbon is the next abundant element found in the Earth and one of such carbon based nanomaterial is graphene. Graphene when extracted from Graphite, the same material used as the lid in pencil, have a tremendous potential to take future electronics devices to new heights in terms of size, cost and efficiency. Thus after its first experimental discovery of graphene in 2004, graphene has been the leading research area for both academics as well as industries. This dissertation is focused on the analysis and optimization of graphene based circuits for future electronics. The first part of this dissertation considers graphene based transistors for analog/radio frequency (RF) circuits. In this section, a dual gate Graphene Field Effect Transistor (GFET) is considered to build the case study circuits like voltage controlled oscillator (VCO) and low noise amplifier (LNA). The behavioral model of the transistor is modeled in different tools: well accepted EDA (electronic design automation) and a non-EDA based tool i.e. \simscape. This section of the dissertation addresses the application of non-EDA based concepts for the analysis of new device concepts, taking LC-VCO and LNA as a case study circuits. The non-EDA based approach is very handy for a new device material when the concept is not matured and the model files are not readily available from the fab. The results matches very well with that of the EDA tools. The second part of the section considers application of multiswarm optimization (MSO) in an EDA tool to explore the design space for the design of LC-VCO. The VCO provides an oscillation frequency at 2.85 GHz, with phase noise of less than -80 dBc/Hz and power dissipation less than 16 mW. The second part of this dissertation considers graphene nanotube field effect transistors (GNRFET) for the application of digital domain. As a case study, static random access memory (SRAM) hs been design and the results shows a very promising future for GNRFET based SRAM as compared to silicon based transistor SRAM. The power comparison between the two shows that GNRFET based SRAM are 93% more power efficient than the silicon transistor based SRAM at 45 nm. In summary, the dissertation is to expected to aid the state of the art in following ways: 1) A non-EDA based tool has been used to characterize the device and measure the circuit performance. The results well matches to that obtained from the EDA tools. This tool becomes very handy for new device concepts when the simulation needs to be fast and accuracy can be tradeoff with. 2)Since an analog domain lacks well-design design paradigm, as compared to digital domain, this dissertation considers case study circuits to design the circuits and apply optimization. 3) Performance comparison of GNRFET based SRAM to the conventional silicon based SRAM shows that with maturation of the fabrication technology, graphene can be very useful for digital circuits as well.

Conselho Nacional de Desenvolvimento Científico e Tecnológico
In this work we present the results of a systematic study of the stability, and the electronic, stuctural and magnetic properties of graphene nanoribbons doped with Ni (Ni/GNR) and Mn (Mn/GNR), through ab initio density functional theory (DFT) calculations. Further, we analyse the electronic transport properties through the non-equilibrium Greens functions formalism (NEGF) coupled with DFT. The electronics and energetics of Si graphene-like monolayers and nanoribbons have also been studied. We determined the possible configurations of a Ni atom both adsorbed and substitutional in GNRs with zigzag edges. We show that the Ni atoms adsorb on the edges of the GNRs. This configuration is seen to be 0.3 eV lower in energy that the adsorption at the midlle of the GNR. The magnetic moments at the carbon atoms change due to the presence of the Ni, decreasing rapidly as the distance of the Ni atom decrease, recovering the value of the ideal GNR at 9 °A from the Ni atom. We obtained Ni d-levels inside a 1.0 eV energy window around the Fermi energy, leading to spin-dependent charge transport in the Ni/GNR. For the case of two Ni atoms adsorbed at the different edges of the GNR s, the antiferromagnetic coupling between both Ni atoms is energetically favored. For the case of the substitucional Ni atom, the edge position is also the energeticaly favored. It gives place to a spin-dependent charge transport, and suggest the use of these materials for spintronic devices. For the Mn doping in zigzag and armchair nanoribbons, it is shown that the edge site are the energetically favorable for adsorbed and substitucional Mn atoms. For the adsorbed Mn dimers, our calculations show that the sites along the border of the GNRs are the most stables ones. The distance between two Mn atoms of the adsorbed Mn2 is shorter than that for the isolated Mn2 molecule. For the zigzag nanoribbons, the magnetic moment of the Mn2 is not affected by magnetic state of the substrate, with the ground state being antiferromagnetic. The dimer/GNR configurations, Mn2/ferro A and Mn2/ferro F, show different elecrtonic properties. The Mn2/ferro A is seen to be semiconductor, while the Mn2/ferro F is semi-mettalic. These properties point to two interesting consequences: (i) the use of these systems as nanomemories, with the reading process made by measure of the electronic current through the nanoribbons and (ii) a spin-polarized current through the Mn2/GNR, with the control of the magnetization of the dimers. Finally, are show that H-passivate diamond-like Si monolayer and nanoribbons are semiconducting with low formation energies. Similarly to graphene, the non-H passivated Si monolayers, both planar and buckled, present linear dispersion of the ¼/¼¤ levels that cross at the Fermi energy.
Apresentamos neste trabalho os resultados do estudo sistemático da estabilidade energética e das propriedades estruturais, magnéticas e eletrônicas de nanofitas de grafeno (GNR) dopadas com Ni (Ni/GNR) e Mn (Mn/GNR), utilizando cálculos ab initio, realizados por meio da teoria do funcional da densidade (DFT). Também avaliamos as propriedades de transporte eletrônico dos sistemas por meio da metodologia de funções de Green fora do equilíbrio (NEGF), associadas a DFT. As propriedades eletrônicas, energéticas e magnéticas de monocamadas de Si, assim como de nanofitas de Si saturadas com H foram também estudadas. Avaliamos as configurações do átomo de Ni adsorvido e substitucional nas GNRs com bordas em formato zigzag. Nós obtivemos que os átomos de Ni adsorvem sobre as bordas da GNR, com uma diferença energética de aproximadamente 0.3 eV, quando comparadas com a adsorção no meio da nanofita. Os momentos magnéticos sobre os átomos de carbono da borda da nanofita se alteram pela presença do átomo de Ni, decrescendo rapidamente á medida que se aproximam do síıtio do Ni e recuperando os valores da nanofita pura a 9°A do átomo de Ni. Nós obtivemos estados d do Ni dentro de uma janela de energia de 1 eV acima e abaixo da energia de Fermi, os quais dão origem a um transporte de carga dependente do spin. Quando dois átomos de Ni são adsorvidos em bordas diferentes, a configuração com acoplamento antiferromagnetico entre os átomos de Ni é mais estável. O Ni substitucional na borda da nanofita é previsto como o sétio energeticamente mais favorável. Neste caso também obtivemos um transporte de carga dependente do spin, o que sugere a possibilidade de construção de dispositivos de filtro de spin baseados em GNRs com átomos de Ni adsorvidos ou substitucionais. Estudamos ainda a dopagem da GNR com Mn, onde foram consideradas as nanofitas com bordas zigzag e armchair. Em todas as nanofitas avaliadas, o Mn atômico apresenta maior estabilidade energética nos sítios junto à borda destas nanofitas. O mesmo se dá para as configurações com o Mn substitucional na nanofita. Para os d´ımeros de Mn adsorvidos sobre as nanofitas de carbono, nossos resultados revelam que existe uma preferência energética para os dímeros sobre sítios ao longo da borda das nanofitas. Nas configurações mais estáveis, os dímeros de Mn apresentam uma redução na distância de equilíbrio quando comparados ao Mn2 isolado. Para as nanofitas zigzag o estado da agnetização do dímero de Mn não é afetada pelo estado ferro F ou ferro A do substrato. Para ambas as configurações, o dímero de Mn na configuração antiferromagnética (AF) é o mais estável. As configurações dímero/nanofita: Mn2/ferro A e as Mn2/ferro F, apresentam propriedades eletrônicas distintas, sendo a primeira semicondutora (mantendo a característica eletrônica da nanofita ferro A não dopada), enquanto a última resulta semi-metálica. Estas propriedades eletrônicas apontam para duas consequências interessantes (i) o uso destes sistemas como nanomemórias, com um processo de leitura por meio da medida da corrente eletrônica através das nanofitas, e (ii) a obtenção de uma corrente com polarização de spin ao longo dos sistemas Mn2/nanofitas, através do controle da magnetização dos dímeros de Mn. Mostramos ainda que a monocamada e as nanofitas de Si passivadas com H, tipo diamante, são semicondutoras e apresentam uma reduzida energia de formação. De modo semelhante ao grafeno, a monocamada de Si não passivada planar e corrugada, apresenta dispersão linear dos níveis ¼/¼¤ que cruzam a energia de Fermi. A nanofita zigzag é obtida com os mesmos estados magnéticos da nanofita de grafeno correspondente.

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

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.

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 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.

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.

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.

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.

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.

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.

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.

Molecular electronics pursues the use of molecules as fundamental electronic components. The inherent properties of molecules such as nano-size, low cost, scalability, and self-assembly are seen by many as a perfect complement to conventional silicon electronics. Molecule based electronics has captured the attention of a broad cross section of the scientific community. In molecular electronic devices, the possibility of having channels that are just one atomic layer thick, is perhaps the most attractive feature that takes the attention to graphene.The conductivity, stability, uniformity, composition, and 2D nature of graphene make it an excellent material for electronic devices. In this thesis we focused on Zigzag Graphene NanoRibbon(ZGNR) as a transmission channel. Due to the importance of an accurate description of the quantum effects in the operation of graphene devices, a full-quantum transport model has been adopted: the electron dynamics has been described by Density Functional Theory(DFT) and transport has been solved within the formalism of Non-Equilibrium Green’s Functions (NEGF). Using DFT and NEGF methods, the transport properties of ZGNR and ZGNR doped with Si are studied by systematically computing the transmission spectrum. It is observed that Si barrier destroyed the electronic transport properties of ZGNR, an energy gap appeared for ZGNR, and variations from conductor to semiconductor are displayed. Its followed by a ZGNR grown on a SiO2 crystal substrate, while substituting the Graphene electrodes with the Gold ones, and its effect on transmission properties have been studied. Improvement in transmission properties observed due to the formation of C-O bonds between ZGNR and substrate that make the ZGNR corrugated. Finally, we modeled a nano-scale Field Effect Transistor by implementing a gate under SiO2 substrate. A very good I-ON/I-OFF ratio has been observed although the device thickness.

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.

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.

We studied the effect of low energy (30 keV) ionic implantation of Ga+ in the direction parallel to the graphene planes (perpendicular to the c-axis) in oriented graphite ribbons with widths around 500 nm. Our experiments have reproducibly shown a reduction of electrical resistance upon implantation consistent with the occurrence of ionic channeling in our devices. Our results allow for new approaches in the modulation of the charge carrier concentration in mesoscopic graphite.

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.

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