Thèses : « Graphene - Physical Properties »

1

Hills, Romilly D. Y. « Physical properties of graphene nano-devices ». Thesis, Loughborough University, 2015. https://dspace.lboro.ac.uk/2134/17993.

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In this doctoral thesis the two dimensional material graphene has been studied in depth with particular respect to Zener tunnelling devices. From the hexagonal structure the Hamiltonian at a Dirac point was derived with the option of including an energy gap. This Hamiltonian was then used to obtain the tunnelling properties of various graphene nano-devices; the devices studied include Zener tunnelling potential barriers such as single and double graphene potential steps. A form of the Landauer formalism was obtained for graphene devices. Combined with the scattering properties of potential barriers the current and conductance was found for a wide range of graphene nano-devices. These results were then compared to recently obtained experimental results for graphene nano-ribbons, showing many similarities between nano-ribbons and infinite sheet graphene. The methods studied were then applied to materials which have been shown to possess three dimensional Dirac cones known as topological insulators. In the case of Cd3As2 the Dirac cone is asymmetrical with respect to the z-direction, the effect of this asymmetry has been discussed with comparison to the symmetrical case.

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Dimov, Dimitar. « Fundamental physical properties of graphene reinforced concrete ». Thesis, University of Exeter, 2018. http://hdl.handle.net/10871/34648.

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The global warming has increased with unprecedented levels during the last couple of decades and the trend is uprising. The construction industry is responsible for nearly 10% of all carbon emissions, mainly due to the increasing global population and the large demand for housing and civil infrastructure. Concrete, which is the most used construction material worldwide, is found in every type of building as it provides long term structural stability, support and its main constituent cement, is very cheap. Consequently, due to the raising concerns of high average temperatures, the research community started investigating new, innovative methods for substituting cement with 'greener' materials whilst at the same time improving the intrinsic properties of concrete. However, the manufacturing complications and logistics of these materials make them unfavourable for industrial applications. A novel and truly revolutionary method of enhancing the performance of concrete, thus allowing for decreased consumption of raw materials, lies in nanoengineering the cement crystals responsible for the development of all mechanical properties of concrete. Graphene, a two-dimensional sheet of carbon atoms arranged in a hexagonal lattice, is the most promising nanomaterial for composites' reinforcement to this date, due to it's exceptional strength, ability to retain original shape after strain, water impermeability properties and non-hazardous large scale manufacturing techniques. I chose to investigate the addition of liquid-phase exfoliated graphene suspensions for concrete reinforcement, aiming to improve the fundamental mechanical properties of the construction material and therefore allowing the industry to design buildings using less volume of base materials. First, the method of liquid exfoliation of graphene was developed and the resulting water suspensions were fully characterised by Raman spectroscopy. Then, concrete samples were prepared according to British standards for construction and tested for various properties such as compressive and flexural strength, cyclic loading, water impermeability and heat transport. A separate, in-depth, study was carried out to understand the formation and propagation of micro-structural cracks between the concrete's internal matrix planes, and graphene's impact on total fracture capacity and resistance of concrete. Lastly, multiple experiments were performed to investigate the microcrystallinity of cement hydration products using X-Ray diffraction. In general, all experimental results show a consistent improvement in concrete's performance when enhanced with graphene on the nanoscale level. The nanomaterial improves the mechanical interlocking of cement crystal, thus strengthening the internal bonds of the composite matrix. This cheap and highly scalable method for producing and mixing graphene with concrete turns it into the first truly applicable method for industrial applications, with a real potential to have positive impact on the global warming by decreasing the production of concrete.

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Alsharari, Abdulrhman. « Tailoring Physical Properties of Graphene by Proximity Effects ». Ohio University / OhioLINK, 2018. http://rave.ohiolink.edu/etdc/view?acc_num=ohiou1525857318688345.

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4

Li, Hu. « Covalent Graphene Functionalization for the Modification of Its Physical Properties ». Doctoral thesis, Uppsala universitet, Institutionen för teknikvetenskaper, 2017. http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-314176.

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Graphene, a two dimensional monolayer carbon sheet with the atoms tightly packed in a hexagonal lattice, has exhibited so many excellent properties, which enable graphene to break several material records with regard to carrier mobility, strength yield and thermal conductivity to name a few. Therefore, graphene has been placed as a potential candidate to allow truly next-generation material. Graphene is a zero band gap material, implying that an energy band gap around the Dirac point is supposed to be open to make graphene applicable as a semiconductor. Covalent bond graphene functionalization becomes an essential enabler to open the energy gap in graphene and extend graphene applications in electronics, while the densely packed hexagonal carbon atoms as well as the strong sp2 hybridization carbon-carbon bonds jointly result in a changeling topic of allowing graphene to be decorated with functional groups. Here in this thesis, different routes to realize graphene functionalizations are implemented by using physical and chemical ways. The physical functionalization methods are the ion/electron beam induced graphene fluorination as well as local defect insertion and the chemical ways correspond to the photochemistry techniques to approach hydrogenation and hydroxypropylation of graphene. Furthermore, to incorporate graphene into devices, the tuning of mechanical properties of graphene is desired. Towards this aim, the structure modification of graphene is employed to investigate the nanometer size-effect of crystalline size of graphene on the mechanical properties, namely Young’s modulus and surface energy. In the process of the graphene hydrogenation project, we discovered a high yield way to synthesis high quality graphene nanoscroll (GNS). Interestingly, the GNS shows superadhesion property through our atomic force microscopy measurements. This superadhesion is around 6-order stronger than van der Waals interaction and even higher than the hydrogen bonding enhanced and solid/liquid interfaces.

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Malec, Christopher Evan. « Transport in graphene tunnel junctions ». Diss., Georgia Institute of Technology, 2011. http://hdl.handle.net/1853/41140.

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It has been predicted that gold, aluminum, and copper do not fundamentally change the graphene band structure when they are in close proximity to graphene, but merely increase the doping. My data confirms this prediction, as well as explores other consequences of the metal/graphene interface. First, I present a technique to fabricate thin oxide barriers between graphene and aluminum and copper to create tunnel junctions and directly probe graphene in close proximity to a metal. I map the differential conductance of the junctions versus tunnel probe and back gate voltage, and observe mesoscopic fluctuations in the conductance that are directly related to the graphene density of states. I develop a simple theory of tunneling into graphene to extract experimental numbers, such as the doping level of the graphene, and take into account the electrostatic gating of graphene by the tunneling probe. Next, results of measurements in magnetic fields will also be discussed, including evidence for incompressible states in the Quantum Hall regime wherein an electron is forced to tunnel between a localized state and an extended state that is connected to the lead. The physics of this system is similar to that encountered in Single Electron Transistors, and some work in this area will be reviewed. Finally, another possible method of understanding the interface between a metal and graphene through transport is presented. By depositing disconnected gold islands on graphene, I am able to measure resonances in the bias dependent differential resistance, that I connect to interactions between the graphene and gold islands.

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Brogi, Lorenzo. « Effects of low-environmental impact graphene on paints : chemical and physical properties ». Master's thesis, Alma Mater Studiorum - Università di Bologna, 2021. http://amslaurea.unibo.it/24415/.

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Paints and varnishes industry is a well known sector of chemical industry and its importance is due to the need of surfaces colouring and protection (metals, wood, concrete) from many natural or artificial chemical or physical agents. This work is based on the formulation of new graphene-based paints and the analysis of their physical and chemical properties. Graphene is the bidimensional sp2 carbon nanomaterial with extraordinary properties as electron mobility, thermal conductivity, mechanical strength and large surface area [ (1), (2), (3), (4), (5) ]. Due to these properties, it’s used as additive in paints formulations to improve mechanical properties. Graphene-XT produces graphene by Liquid Phase Exfoliation (top-down approach) with a mechanical exfoliation in water environment, using only graphite and a non-toxic exfoliating agent/solvent, avoiding the common toxic solvents as 1-methyl-2pirrolidone (NMP) or cyclopentanone (CPO). Self-produced graphene was added to two types of paint to obtain products with different properties one from each other. In particular acrylic and teflon paints were used as bases with the addition of powder-graphene/graphene-ink. During this experimental thesis, three graphene-based paints were formulated: the first with the goal of improving hardness, adhesion, lubricant properties and mechanical resistance of the virgin teflon paint; the second was an adhesive and electrically acrylic conductive paint; the third was a formulation that increases some mechanical performances of a virgin acrylic paint. All the formulations were created and tested inside Graphene-XT laboratory, except for some test on the first formulation (third-part requested product).

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Baker, Taleb. « Molecular Computer Simulations of Graphene oxide intercalated with methanol : Swelling Properties and Interlayer Structure ». Thesis, Umeå universitet, Institutionen för fysik, 2017. http://urn.kb.se/resolve?urn=urn:nbn:se:umu:diva-135941.

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Robert, Pablo T. [Verfasser], et H. von [Akademischer Betreuer] Löhneysen. « Physical properties of carbon nanotube, graphene junctions / Pablo T. Robert. Betreuer : H. von Löhneysen ». Karlsruhe : KIT-Bibliothek, 2012. http://d-nb.info/1032243104/34.

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Orlando, Fabrizio. « Physical Properties and Functionalization of Low-Dimensional Materials ». Doctoral thesis, Università degli studi di Trieste, 2014. http://hdl.handle.net/10077/9968.

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2012/2013
Recent years have witnessed fast advancements in the research on graphene, which is one of the most active fields in condensed matter physics, chemistry and materials science. The rising interest of the scientific community in graphene, motivated by its fascinating properties and wide range of potential applications, has triggered substantial interest also on other two-dimensional (2D) atomic crystals, and particularly on hexagonal boron nitride (h-BN). In spite of much effort, a number of challenges still awaits the scientific community before the full potential of 2D atomic crystals can be exploited, such as the development of reliable methods for the growth of high-quality graphene and h-BN single layers or the possibility to tune the graphene electronic structure. The research activity I have been pursuing faces these requirements by focusing on the growth of graphene and h-BN on transition metal surfaces – which appears as the most direct route towards a scalable production of single layers with low concentration of defects – and the investigation of fundamental properties related to the presence of the metal support, but also tackles issues which have a direct link to the fabrication of carbonbased devices. In this regard, one of the first targets has been to shed light on the morphology and the electronic structure of h-BN on Ir(111), and to improve the growth strategy for the synthesis of high-quality h-BN layers. I have subsequently turned my attention to the fine tuning of graphene electronic properties by tailoring the graphene-substrate interaction through intercalation of foreign atoms at the metal interface. This was investigated in the extreme situations of weak (Ir) and strong (Ru) coupling of graphene with the metal support. I have also focused on an aspect which is related to a specific technological issue, that is, the development of an approach for the direct synthesis of graphene on insulating oxide layers. Lastly, the structural geometry of single layer graphene functionalized with nitrogen atoms, which is considered as one of the most promising approaches to manipulate graphene chemistry and induce n-doping, was also addressed. The combined use of several surface science experimental techniques has been proved to be of a powerful approach to achieve the targets of this project, having given access to the understanding of different properties of the systems under investigation.
XXVI Ciclo
1985

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Hocker, John-andrew Samuel. « Molecular and Performance Properties of Poly(Amides & ; Imides) and the Use of Graphene Oxide Nano-Particles for Improvement ». W&M ScholarWorks, 2016. https://scholarworks.wm.edu/etd/1477068376.

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Macroscopic properties of polymers, and in general all types of materials, are innately related to their molecular structure and intermolecular properties. This dissertation offers insight into how the molecular structure, chain properties, and inter-molecular chain interactions within PA11 can be used to explain and predict improved macro level performance properties; and how the addition of graphene oxide nanosheets enhances the performance properties of PA11 and polyimide(ODA-BTDA) through those same intermolecular interactions. Chapter 3 describes the unexpected result that weak small organic acids at low concentrations hydrolyze a polyamide at rates approximately twice that of a water HCl solution of the same pH at 100 ˚C and 120 ˚C under anaerobic conditions. Chapter 4 discusses how by varying the rate of Mm degradation with small organic acids the correlated effects of Mm and crystallinity upon the ultimate strain of PA11 were decoupled. The result demonstrates the need for both crystallinity and molecular weight knowledge to monitor and predict the ductile to brittle transition and when embrittlement occurs. Chapter 5 explores the elevated resistance to hydrolytic degradation and molecular properties that a graphene oxide (GO) loaded PA11 has compared with neat PA11 at 100 and 120 ˚C. The decreased rate of degradation and resulting 50 % increased equilibrium molecular weight of PA11 was attributed to the highly asymmetric planar GO nano-sheets that inhibited the molecular mobility of water and the polymer chain. The crystallinity of the polymer matrix was similarly affected by a reduction in chain mobility during annealing due to the GO nanoparticles’ chemistry and highly asymmetric nano-planar sheet structure. Chapter 6 extends the experimental work to effects on polyimides. GO and GO functionalized with 4-4’ oxydianiline (ODAGO) are incorporated at 0.01 to 0.10 wt% into a polyimide (PI) made from 3,3’-benzophenonetetracarboxylic dianhydride (BTDA) and 4-4’ oxydianiline (ODA). The performance properties of these two systems GOPI and ODAGO-PI at extremely low GO concentrations of 1 part per 10,000 are comparable to previous results citing concentrations 10 times higher and displayed significantly greater improvement than unfunctionalized GOPI films. The results suggest that the improved barrier properties are due a toruosity effect of the GO sheets and to a stabilizing effect of the flakes on the polymer matrix, where reduced mobility of the PI chain reduces diffusion through the polymer matrix. ATR-FTIR, WAXS, Raman and Tg results support this view.

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Herring, Natalie. « Formation Mechanisms and Photocatalytic Properties of ZnO-Based Nanomaterials ». VCU Scholars Compass, 2013. http://scholarscompass.vcu.edu/etd/494.

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Zinc Oxide (ZnO) is one of the most extensively studied semiconductors because of its unique properties, namely, its wide band gap (3.37 eV) and high excitation binding energy (60 meV). These properties make ZnO a promising material for uses in a broad range of applications including sensors, catalysis and optoelectronic devices. The presented research covers a broad spectrum of these interesting nanomaterials, from their synthesis and characterization to their use as photocatalyts. A new synthetic approach for producing morphology controlled ZnO nanostructures was developed using microwave irradiation (MWI). The rapid decomposition of zinc acetate in the presence of a mixture of oleic acid (OAC) and oleylamine (OAM) results in the formation of hexagonal ZnO nanopyramids and ZnO rods of varying aspect ratios. The factors that influence the morphology of these ZnO nanostructures were investigated. Using ligand exchange, the ZnO nanostructures can be dispersed in aqueous medium, thus allowing their use as photocatalysts for the degradation of malachite green dye in water. Photocatalytic activity is studied as a function of morphology; and, the ZnO nanorods show enhanced photocatalytic activity for the degradation of the dye compared to hexagonal ZnO nanopyramids. After demonstrating the catalytic activity of these ZnO nanostructures, various ways to enhance photocatalytic activity were studied by modification of this MWI method. Photocatalytic activity is enhanced through band gap modulation and the reduction of electron-hole recombination. Several approaches were studied, which included the incorporation of Au nanoparticles, N-doping of ZnO, supporting ZnO nanostructures on reduced graphene oxide (RGO), and supporting N-doped ZnO on N-doped RGO. ZnO-based nanostructures were studied systematically through the entire process from synthesis and characterization to their use as photocatalysis. This allows for a thorough understanding of the parameters that impact these processes and their unique photocatalytic properties.

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Colangelo, Francesco. « Strain engineering of two-dimensional materials ». Doctoral thesis, Scuola Normale Superiore, 2019. http://hdl.handle.net/11384/85909.

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Graphene displays a range of remarkable properties that have catalyzed an impressive interest in the scientific community . Its unique electronic behavior stems from the hexagonal honeycomb structure of the one-atom thick carbon lattice, which forces low-energy conducting electrons to assume linear dispersions mimicking the one of massless relativistic fermions . After graphene, the past few years have been marked by the discovery and characterization of plenty of other two-dimensional (2D) materials and most of them share with graphene the hexagonal atomic network and have as many peculiar electrical and optical properties. Among this wide family, many of transition metal dichalcogenides (TMDs) have a gapped electronic structure and efficiently emit light, even at room temperature. Tungsten disulfide (WS2) belongs to the TMD family and has gained a preferential attention due its interesting features, including a strong, wide-band exciton photoluminescence (PL) around 630 nm when a monolayer is considered. Mechanical characterization of 2D materials revealed that they display an unusual good elasticity and high mechanical strength; for example, graphene crystals can sustain strain well beyond 10% without undergoing plastic deformation . In addition to that, it has been predicted , and in part experimentally demonstrated , that mechanical deformations in a 2D hexagonal system can significantly impact its physical properties. Random deformations are typically detrimental, but intriguing effects can emerge if proper strain fields are implemented in these systems . Strain gradients can be engineered to mimic the presence of a external magnetic field[18,19] applied perpendicularly to the 2D material plane. Such strain-induced field has been defined pseudo-magnetic field (PMF) since, at odd of the real magnetic field, it conserves the time reversal symmetry and has a pseudo-spin dependent orientation. The fact that 2D materials can sustain large strain, implies that large PMFs can be induced and used to dramatically change the material local energy spectrum. Strain can be also used to modulate the optoelectronic properties of TMDs by controlling the excitons PL wavelength with strain intensity and the excitons diffusion dynamics with strain gradients . However, these intriguing possibilities demand a control of strain that, while validated by artificial-lattice studies, to date remains elusive in the case of atomic lattices . Preliminary and remarkable results were obtained on self-assembled graphene nanobubbles and nano-wrinkles , using scanning probes or nano-patterned substrate , and in twisted bilayer systems . Micro-scale strain devices were implemented exploiting deformable substrates , uniform external loads , thin-film shrinkage or complex micro-actuation technologies, including in particular inorganic microelectromechanical systems (MEMS) . Nevertheless, in order to master the physical properties through deformations, a more accurate control and investigation of the 2D material strain field is required; the existing techniques lack of flexibility, they can be exploited on one flake at a time, and are in practice poorly suited to obtain arbitrary and reconfigurable strain fields. In this Thesis, I will demonstrate an innovative technique to strain 2D materials which deals with these limitations. The technique is based on polymeric micrometric artificial muscles (MAMs) which contract upon a high-dose electron beam exposure and can be used to induce a local deformation in the 2D material crystal. I will show that MAMs are patternable, therefore they are perfectly suited to create arbitrary strain patterns and the method will be first of all demonstrated on a set of suspended graphene devices. Moreover, I will show that by employing a clean and atomically flat substrate as a “low-friction” platform, the same technique can be applied to strain 2D materials while avoiding the critical steps of crystal transfer and suspension. To this end, I employed WS2 grown by chemical vapor deposition (CVD) directly on top of graphene obtained by thermal decomposition of silicon carbide (SiC). I investigated the WS2/graphene heterostructures combining a simple and scalable fabrication protocol with all the benefits of the MAMs-based strain engineering technique. I will demonstrate a local strain-controlled tuning of the WS2 photoluminescence wavelength as a proof of concept.

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Chamsa-ard, Wisut. « Synthesis, characterisation and thermo-physical properties of highly stable graphene oxide-based aqueous nanofluids for low-temperature direct absorption solar collectors and solar still desalination ». Thesis, Chamsa-ard, Wisut (2019) Synthesis, characterisation and thermo-physical properties of highly stable graphene oxide-based aqueous nanofluids for low-temperature direct absorption solar collectors and solar still desalination. PhD thesis, Murdoch University, 2019. https://researchrepository.murdoch.edu.au/id/eprint/55043/.

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Humanities’ insatiable demand for energy and fresh water has never been satisfied and continues to increase with an ever-increasing global population. Because both demands currently rely heavily on fossil fuels, the resulting detrimental consequences of rising greenhouse gas emissions, global warming and environmental degradation present major challenges for every nation. Importantly, the efficient conversion of energy to perform useful work is a factor that directly contributes to the financial development and economic sustainability of a country. Furthermore, the current global water demand is already much higher than the Earth’s natural water cycle can sustain, and this shortfall is presently being made up by the use of high energy consuming desalination processes. Thus, there is currently extensive research into developing eco-friendly and viable renewable energy sources to meet the demands for energy and fresh water. Solar energy offers an alternative energy source with the potential to alleviate the problems associated with fossil fuel-based energy generation and desalination. The performance of conventional solar thermal collectors is limited by poor optical properties and low thermal properties. Traditional solar stills are not widely used today, due to their poor thermal properties and low productivity levels. The thesis focuses on developing new and novel nanofluids based on novel graphene based materials with enhanced optical and thermal properties to improve the performance of direct absorption solar collectors (DASCs) and solar desalination stills. The thesis is arranged in two parts. The first part extensively reviews current solar collector systems for converting solar energy to thermal energy and also reviews current progress towards developing high thermal efficiency solar desalination systems. The second part presents actual case studies that evaluate the performance of newly developed novel graphene oxide and reduced graphene oxide based nanofluids for use in DASCs and solar stills.

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Bonifacio, Agathe. « Electronic Properties of Graphene ». Thesis, Uppsala universitet, Energimaterialens fysik, 2021. http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-447514.

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Samarakoom, Duminda K. « Structural and electronic properties of Hydrogenated Graphene ». DigitalCommons@Robert W. Woodruff Library, Atlanta University Center, 2011. http://digitalcommons.auctr.edu/dissertations/202.

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Graphane is a two-dimensional system consisting of a single planar layer of fully saturated carbon atoms, which has recently been realized experimentally through hydrogenation of graphene membranes. We have studied the stability of chair, boat, and twist-boat graphane structures using first-principles density functional calculations. Our results indicate that locally stable twist-boat membranes significantly contribute to the experimentally observed lattice contraction. The band gaps of graphane nanoribbons decrease monotonically with the increase of the ribbon width and are insensitive to the edge structure. We also have studied the electronic structural characteristics in a hydrogenated bilayer graphene under a perpendicular electric bias. The bias voltage applied between the two hydrogenated graphene layers allows continuously tuning the band gap and leads a transition from semiconducting to metallic state. Desorption of hydrogen from one layer in the chair conformation yields a ferromagnetic semiconductor with tunable band gap.

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Sonde, Sushant. « Local transport properties in graphene for electronic applications ». Thesis, Universita' degli Studi di Catania, 2011. http://hdl.handle.net/10761/91.

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In view of possible applications in electrostatically tunable two-dimensional field-effect devices, this thesis is aimed at discussing electronic properties in substrate-supported graphene. Original methods based on various variants of Scanning Probe Microscopy techniques are utilized to analyze graphene exfoliated- and-deposited (DG) on SiO2 /Si, SiC(0001) and high-k dielectric substrate (Strontium Titanate) as well as graphene grown epitaxially (EG) on SiC(0001). Scanning Capacitance Spectroscopy is discussed as a probe to evaluate the electrostatic properties (quantum capacitance, local density of states) and transport properties (local electron mean free path) in graphene. Furthermore, based on this method two important issues adversely affecting room temperature charge transport in graphene are addressed to elucidate the role of: 1. Lattice defects in graphene introduced by ion irradiation and 2. Charged impurities and Surface Polar Phonon scattering at the graphene/substrate interface. Moreover, a comparative investigation of current transport across EG/SiC(0001) and DG/SiC(0001) interface by Scanning Current Spectroscopy and Torsion Resonance Conductive Atomic Force Microscopy is discussed to explain electrical properties of the so-called 'buffer layer' commonly observed at the interface of EG/SiC(0001). This study also clarifies the local workfunction variation in EG due to electrically active buffer layer.

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Nanayakkara, Tharanga Ranjan. « Electronic properties of nitrophenyl functionalized graphene and boron nanotubes ». DigitalCommons@Robert W. Woodruff Library, Atlanta University Center, 2015. http://digitalcommons.auctr.edu/dissertations/3105.

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We have studied the electronic characteristics of covalently functionalized graphene by nitrophenol groups using first-principles density-functional theory calculations. The nitrophenyl functionalization leads to a band gap opening in graphene and transition from a semi-metallic to semiconducting state. The induced gap is shown to be attributed to the modification of the π-conjugation that depends on the configuration for a pair of monovalent adsorption. A detailed analysis reveals that this intriguing magnetism modulation by strain stems from the redistribution of spin-polarized electrons induced by local lattice distortions. A detailed analysis suggests a sensitive and effective way to tailor properties of graphene for future applications in nanoscale devices. The quest for low-dimensional boron structures has been motivated by the potential applications of light-weight materials. Recently, a semi-metallic two-dimensional boron allotrope was predicted via ab initio evolutionary structure search, which is markedly lower in energy than the planar structures composed of triangular motifs and hexagonal holes. The emergence of a Dirac cone in the band structure demonstrates an intriguing perspective for quasiplanar counterpart of graphene. We studied the corresponding single walled boron nanotubes derived from the quasiplanar boron structure. In particular, our results are identified to have a Dirac cone, as well. The buckling and coupling between the two sublattices not only enhance the stability, but also are key factors to the emergence of the Dirac cone.

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Panapitiya, Gihan Uthpala. « Electronic Properties of Graphene and Boron Nitride Nanoribbon Junctions ». University of Akron / OhioLINK, 2013. http://rave.ohiolink.edu/etdc/view?acc_num=akron1382986572.

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Hewa-Bosthanthirige, Mihiri Shashikala. « Structural and electronics properties of noncovalently functionalized graphene ». DigitalCommons@Robert W. Woodruff Library, Atlanta University Center, 2013. http://digitalcommons.auctr.edu/dissertations/1286.

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Recent experimental work has demonstrated production of quasi free-standing graphene by methane intercalation. The intercalation weakens the coupling of adjacent graphene layers and yields Dirac fermion behaviour of monolayer graphene. We have investigated the electronic characteristics of methane intercepted graphene bilayer under a perpendicularly applied electric field. Evolution of the band structure of intercalated graphene as a function of the bias is studied by means of density-functional theory including interlayer van der Waals interactions. The implications of controllable band gap opening in methane-intercalated graphene for future device applications are discussed. Noncovalent functionalization provides an effective way to modulate the electronic properties of graphene. Recent experimental work has demonstrated that hybrids of dipolar phototransductive molecules tethered to graphene are reversibly tunable in doping. We have studied the electronic structure characteristics of chromophore/graphene hybrids using dispersion-corrected density functional theory. The Dirac point of noncovalently functionalized graphene shifts upward via cis-trans isomerism, which is attributed to a change in the chromophore's dipole moment. Our calculation results reveal that the experimentally observed reversible doping of graphene is attributed to the change in charge transfer between the light-switchable chromophore and graphene via isomerization. Furthermore, we show that by varying the electric field perpendicular to the supramolecular functionalized graphene, additional tailoring of graphene doping can be accomplished.

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Hargrove, Jasmine J. « Structural and electronic properties of grapheme-based materials ». DigitalCommons@Robert W. Woodruff Library, Atlanta University Center, 2014. http://digitalcommons.auctr.edu/dissertations/2273.

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This thesis includes work done on graphene-based materials, examining their unique electronic properties using first-principles density-functional calculations. Abinitio methods such as density functional theory (DFT) are widely accepted as computational methods in condensed matter and materials physics. We begin by studying the electronics properties of graphene intercalation compounds (GICs). In order for bilayer graphene to be used for field effect transistors, the GIC must decouple the adajent graphene layers and decrease interlayer interaction. We conducted a theoretical study in order to elucidate the electronic characteristics of methane intercepted bilayer graphene under a perpendicularly applied electric field. We show that methane intercalated graphene can make a promising material for implimentations of graphene based field effect transistors since it has a controllable band gap. Finally, we show the evolution of band structure of graphene treated with fluorinated olefins through covalent functionalization. The bonding of fluorine to the graphene surface results in the transformation of orbital hybridization from sp2 to sp3. We find that the modification of graphene's electronic properties by such a drastic change in hybridization can lead to the elimination of the bands near the Fermi level and the opening of a band gap. We hope this work will help bring to light the promising electronic properties of graphene based materials for future device applications.

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Rossi, Antonio. « 2D Vertical Heterostructures : growth, properties and applications ». Doctoral thesis, Scuola Normale Superiore, 2018. http://hdl.handle.net/11384/85906.

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Lampert, Lester Florian. « High-Quality Chemical Vapor Deposition Graphene-Based Spin Transport Channels ». PDXScholar, 2017. https://pdxscholar.library.pdx.edu/open_access_etds/3327.

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Spintronics reaches beyond typical charge-based information storage technologies by utilizing an addressable degree of freedom for electron manipulation, the electron spin polarization. With mounting experimental data and improved theoretical understanding of spin manipulation, spintronics has become a potential alternative to charge-based technologies. However, for a long time, spintronics was not thought to be feasible without the ability to electrostatically control spin conductance at room temperature. Only recently, graphene, a 2D honeycomb crystalline allotrope of carbon only one atom thick, was identified because of its predicted, long spin coherence length and experimentally realized electrostatic gate tunability. However, there exist several challenges with graphene spintronics implementation including weak spin-orbit coupling that provides excellent spin transfer yet prevents charge to spin current conversion, and a conductivity mismatch due to the large difference in carrier density between graphene and a ferromagnet (FM) that must be mitigated by use of a tunnel barrier contact. Additionally, the usage of graphene produced via CVD methods amenable to semiconductor industry in conjunction with graphene spin valve fabrication must be explored in order to promote implementation of graphene-based spintronics. Despite advances in the area of graphene-based spintronics, there is a lack of understanding regarding the coupling of industry-amenable techniques for both graphene synthesis and lateral spin valve fabrication. In order to make any impact on the application of graphene spintronics in industry, it is critical to demonstrate wafer-scale graphene spin devices enabled by wafer-scale graphene synthesis, which utilizes thin film, wafer-supported CVD growth methods. In this work, high-quality graphene was synthesized using a vertical cold-wall furnace and catalyst confinement on both SiO2/Si and C-plane sapphire wafers and the implementation of the as-grown graphene for fabrication of graphene-based non-local spin valves was examined. Optimized CVD graphene was demonstrated to have ID/G ≈ 0.04 and I2D/G ≈ 2.3 across a 2" diameter graphene film with excellent continuity and uniformity. Since high-quality, large-area, and continuous CVD graphene was grown, it enabled the fabrication of large device arrays with 40 individually addressable non-local spin valves exhibiting 83% yield. Using these arrays, the effects of channel width and length, ferromagnetic-tunnel barrier width, tunnel barrier thickness, and level of oxidation for Ti-based tunnel barrier contacts were elucidated. Non-local, in-plane magnetic sweeps resulted in high signal-to-noise ratios with measured ΔRNL across the as-fabricated arrays as high as 12 Ω with channel lengths up to 2 µm. In addition to in-plane magnetic field spin signal values, vertical magnetic field precession Hanle effect measurements were conducted. From this, spin transport properties were extracted including: spin polarization efficiency, coherence lifetime, and coherence distance. The evaluation of industry-amenable production methods of both high-quality graphene and lateral graphene non-local spin valves are the first steps toward promoting the feasibility of graphene as a lateral spin transport interconnect material in future spintronics applications. By addressing issues using a holistic approach, from graphene synthesis to spin transport implementation, it is possible to begin assessment of the challenges involved for graphene spintronics.

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Malekpour, Hoda. « Optothermal Raman Studies of Thermal Properties of Graphene Based Films ». Thesis, University of California, Riverside, 2017. http://pqdtopen.proquest.com/#viewpdf?dispub=10252873.

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Efficient thermal management is becoming a critical issue for development of the next generation of electronics. As the size of electronic devices shrinks, the dissipated power density increases, demanding a better heat removal. The discovery of graphene’s unique electrical and thermal properties stimulated interest of electronic industry to development of graphene based technologies. In this dissertation, I report the results of my investigation of thermal properties of graphene derivatives and their applications in thermal management. The dissertation consists of three parts. In the first part, I investigated thermal conductivity of graphene laminate films deposited on thermally insulating polyethylene terephthalate substrates. Graphene laminate is made of chemically derived graphene and few layer graphene flakes packed in overlapping structure. Two types of graphene laminate were studied: as deposited and compressed. The thermal conductivity of the laminate was found to be in the range from 40 W/mK to 90 W/mK at room temperature. It was established that the average size and the alignment of graphene flakes are parameters dominating the heat conduction. In the second part of this dissertation, I investigated thermal conductivity of chemically reduced freestanding graphene oxide films. It was found that the in-plane thermal conductivity of graphene oxide can be increased significantly using chemical reduction and temperature treatment. Finally, I studied the effect of defects on thermal conductivity of suspended graphene. The knowledge of the thermal conductivity dependence on the concentration of defects can shed light on the strength of the phonon - point defect scattering in two-dimensional materials. The defects were introduced to graphene in a controllable way using the low-energy electron beam irradiation. It was determined that as the defect density increases the thermal conductivity decreases down to about 400 W/mK, and then reveal saturation type behavior. The thermal conductivity dependence on the defect density was analyzed using the Boltzmann transport equation and molecular dynamics simulations. The obtained results are important for understanding phonon transport in two-dimensional systems and for practical applications of graphene in thermal management.

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Eppell, Steven Joseph. « Platinum on graphite (0001) : A model system for the study of physical and chemical properties of small metal islands ». Case Western Reserve University School of Graduate Studies / OhioLINK, 1991. http://rave.ohiolink.edu/etdc/view?acc_num=case1055523982.

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Farrokhi, M. Javad. « ELECTRONIC PROPERTIES OF ATOMICALLY THIN MATERIAL HETEROSTRUCTURES ». UKnowledge, 2019. https://uknowledge.uky.edu/physastron_etds/67.

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There is a movement in the electronic industry toward building electronic devices with dimensions smaller than is currently possible. Atomically thin 2D material, such as graphene, bilayer graphene, hBN and MoS2 are great candidate for this goal and they have a potential set of novel electronic properties compare to their bulk counterparts due to the exhibition of quantum conﬁnement eﬀects. To this goal, we have investigated the electric ﬁeld screening of multilayer 2D materials due to the presence of impurity charge in the interface and vertical electric ﬁfield from back gate. Our result shows a dramatic diﬀerence of screening behavior in high and low charging limit, which depends on the number of layers as well. We also have an extensive study on quantum tunneling eﬀect in graphene and bilayer graphene heterojunctions. The peculiar electronic properties of graphene lead to an unusual scattering eﬀect of electron in graphene n-p junction. We implement the cohesive tunneling eﬀect to explain the nonlinear electron transport in ultrashort channel graphene devices. This nonlinear behavior could make them tremendously useful for ultra-fast electronic applications.

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26

Benasutti, Patrick B. « Electronic and Structural Properties of Silicene and Graphene Layered Structures ». Wright State University / OhioLINK, 2012. http://rave.ohiolink.edu/etdc/view?acc_num=wright1348192958.

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Phan, Anh Duc. « Graphene Casimir Interactions and Some Possible Applications ». Scholar Commons, 2012. http://scholarcommons.usf.edu/etd/4386.

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Scientific development requires profound understandings of micromechanical and nanomechanical systems (MEMS/NEMS) due to their applications not only in the technological world, but also for scientific understanding. At the micro- or nano-scale, when two objects are brought close together, the existence of stiction or adhesion is inevitable and plays an important role in the behavior operation of these systems. Such effects are due to surface dispersion forces, such as the van der Waals or Casimir interactions. The scientific understanding of these forces is particularly important for low-dimensional materials. In addition, the discovery of materials, such as graphitic systems has provided opportunities for new classes of devices and challenging fundermental problems. Therefore, invesigations of the van der Waals or Caismir forces in graphene-based systems, in particular, and the solution generating non-touching systems are needed. In this study, the Casimir force involving 2D graphene is investigated under various conditions. The Casimir interaction is usually studied in the framework of the Lifshitz theory. According to this theory, it is essential to know the frequency-dependent reflection coefficients of materials. Here, it is found that the graphene reflection coefficients strongly depend on the optical conductivity of graphene, which is described by the Kubo formalism. When objects are placed in vacuum, the Casimir force is attractive and leads to adhesion on the surface. We find that the Casimir repulsion can be obtained by replacing vacuum with a suitable liquid. Our studies show that bromobenzene is the liquid providing this effect. We also find that this long-range force is temperature dependent and graphene/bromobenzene/metal substrate configuration can be used to demonstrate merely thermal Casimir interaction at room temperature and micrometer distances. These findings would provide good guidance and predictions for practical studies.

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McAllister, Kelly Denise. « Modification of the electronic properties of fluorinated epitaxial graphene with an electric bias ». DigitalCommons@Robert W. Woodruff Library, Atlanta University Center, 2014. http://digitalcommons.auctr.edu/dissertations/1598.

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Ultraviolet photoemission spectroscopy measurements reveal that there is notable variation of the electron density of states in valence bands near the Fermi level. Evolution of the electronic structure of fluorinated graphene as a function ofthe applied electric bias is investigated. The experimental results demonstrate that the tailoring of electronic band structure correlates with the interlayer coupling tuned by the applied bias. The change in the work function of fluorinated graphene demonstrates the ability of fluorination to modify electron emissions characteristics of graphene.

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Mudiyanselage, Asanga B. Arampath. « Structural and electronic properties of boron monolayer sheets and nitrogen-seeded epitaxial graphene ». DigitalCommons@Robert W. Woodruff Library, Atlanta University Center, 2014. http://digitalcommons.auctr.edu/dissertations/1597.

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We employ a global optimization method to predict two-dimensional (2D) nanostructures through the particle swarm optimization (PSO) algorithm. By performing PSO simulations, we predict new stable structures of boron monolayer sheets for a wide range of boron concentrations. We present a new class of boron sheets, composed of triangular and hexagonal motifs, which are more likely to be the precursors of boron nanotubes. We describe a simple and clear picture of electronic bonding in boron sheets and highlight the importance of three-center bonding and its competition with two-center bonding, which can also explain the stability of recently discovered boron monolayer sheets. The graphene-based electronics is an intriguing prospective for replacing silicon for next-generation of transistors. Practical application of graphene based materials requests a band gap. We present results from first-principles density-function calculation for semiconducting graphene based on nitrogen-seeded epitaxial graphene. A small fraction of covalently-bonded nitrogen leads to a band gap opening in epitaxial graphene and a transition from a semi metallic to a semiconducting state. The induced gap is shown to be attributed to the modification of the 1t conjugation of epitaxial graphene. The resulting nitrogen-seeded graphene opens a band gap about 0.8 eV, in good agreement with experimental observations.

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30

Wu, Di, et 吳迪. « Theoretical studies of electronic tunneling properties in monolayer and bilayer graphene lattices ». Thesis, The University of Hong Kong (Pokfulam, Hong Kong), 2008. http://hub.hku.hk/bib/B40887960.

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31

Wu, Di. « Theoretical studies of electronic tunneling properties in monolayer and bilayer graphene lattices ». Click to view the E-thesis via HKUTO, 2008. http://sunzi.lib.hku.hk/hkuto/record/B40887960.

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32

Sabki, Syarifah Norfaezah. « The growth of graphene on nickel thin films ». Thesis, University of Nottingham, 2012. http://eprints.nottingham.ac.uk/14545/.

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The growth of graphene on Ni thin films using several different methods is discussed. These methods include no intentional introduction of carbon, immersion in an organic solvent, exposure to carbon-containing gas and a solid state approach by decomposition of molecules. All the methods have produced single layer graphene over a large area. We suggest that the graphene formation without intentional introduction of carbon involves conversion of carbon-containing adsorbates on Si02. This process has been verified by our experiment of graphene growth by decomposition of C6O, in which C60 is deposited on top of Si02 and buried under Ni thin film. Single layer graphene has successfully formed which suggests that the carbon from C60 has diffused and segregated to the top of the Ni surface. So we investigate the effect of outgassing aimed to eliminate adsorbates on Si02. Graphene growth by immersion in an organic solvent was initially performed to investigate the effect of outgassing process, and single layer graphene is formed but is highly defective, as determined by the intensity of the Raman D band. We found that outgassing the Si02 is important to produce single layer graphene, but the defects in graphene are not significantly reduced. Graphene growth method using propylene is carried out to identify the factors that influence the amount of defects and to reduce through optimization of growth parameters. The graphene defects are reduced significantly by varying the annealing temperature and exposure time to propylene. We found that different Ni thickness do not affect the defect formation in graphene but do improve the Ni surface morphology. Graphene growth by decomposition of C60 on Ni thin film produced graphene layers with controlled thickness. This molecular carbon source provides a method of controlling the total dosage of carbon introduced into the film with a high degree of precision. We found that the C60 coverage, annealing temperature, and deposition sequence influence the properties of graphene layers. We also presented preliminary results of graphene enhanced Raman scattering (GERS) of adsorbed PTCDI. We demonstrate that single layer graphene is a very good substrate for Raman enhancement in which the adsorbed molecules can be detected at a small fraction of monolayer coverage. Using the same transfer method typically used for graphene, we managed to transfer PTCDI on graphene from Ni film to Si02. Here we demonstrate the effect of a substrate for graphene which can give rise to the enhancement of a Raman signal of adsorbed molecules.

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33

Zhu, Tiancong. « Tuning the Spin Transport and Magnetic Properties of 2D Materials at the Atomic Scale ». The Ohio State University, 2019. http://rave.ohiolink.edu/etdc/view?acc_num=osu1563385375225464.

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Suggs, Kelvin L. « Tunable Electronic Properties of Chemically Functionalized Graphene and Atomic-Scale Catalytics ». DigitalCommons@Robert W. Woodruff Library, Atlanta University Center, 2015. http://digitalcommons.auctr.edu/cauetds/17.

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In this dissertation we discuss the electronic properties, structural configurations, and reaction mechanisms of chemically functionalized graphene and charged atomic metals. In general, we analyze fundamental atomic scale and nanoscale systems with density functional theory in order to investigate chemical reaction energetics for peroxide synthesis as well as methanol production without carbon emission. These systems were found to be tunable via the addition of cationic and anionic charges, change in transition metal type, and modification through chemical functionalization. Furthermore, transition state theory was used to predict an optimal configuration for chemically functionalized graphene, efficient use of anionic atomic gold and palladium for synthesis of water to peroxide, and clean conversion of methane to methanol without carbon dioxide emission utilizing anionic gold.

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35

Torrance, David Britt. « Growth and electronic properties of nanostructured epitaxial graphene on silicon carbide ». Diss., Georgia Institute of Technology, 2013. http://hdl.handle.net/1853/50205.

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The two-dimensional phase of carbon known as graphene is actively being pursued as a primary material in future electronic devices. The goals of this thesis are to investigate the growth and electronic properties of epitaxial graphene on SiC, with a particular focus on nanostructured graphene. The first part of this thesis examines the kinetics of graphene growth on SiC(0001) and SiC(0001 ̅) by high-temperature sublimation of the substrate using a custom-built, ultra-high vacuum induction furnace. A first-principles kinetic theory of silicon sublimation and mass-transfer is developed to describe the functional dependence of the graphene growth rate on the furnace temperature and pressure. This theory can be used to calibrate other graphene growth furnaces which employ confinement controlled sublimation. The final chapter in this thesis involves a careful study of self-organized epitaxial graphene nanoribbons (GNRs) on SiC(0001). Scanning tunneling microscopy of the sidewall GNRs confirms that these self-organized nanostructures are susceptible to overgrowth onto nearby SiC terraces. Atomic-scale imaging of the overgrown sidewall GNRs detected local strained regions in the nanoribbon crystal lattice, with strain coefficients as high as 15%. Scanning tunneling spectroscopy (STS) of these strained regions demonstrate that the graphene electronic local density of states is strongly affected by distortions in the crystal lattice. Room temperature STS in regions with a large strain gradient found local energy gaps as high as 400 meV. Controllable, strain-induced quantum states in epitaxial graphene on SiC could be utilized in new electronic devices.
Per request of the author and the advisor, and with the approval of the graduate office, the Acknowledgements page was replaced with an errata.

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36

Gaskell, J. « High-frequency oscillations in graphene resonant tunnelling heterostructures ». Thesis, University of Nottingham, 2016. http://eprints.nottingham.ac.uk/33694/.

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In this thesis, the form of the current-voltage characteristics and the resulting current oscillations in graphene-hexagonal boron nitride heterostructures are explored by means of theoretical investigation and are supported by experimental observations. The conditions for resonant tunnelling and the effect of device and circuit parameters are examined through simulation of the charge dynamics using the Bardeen Transfer Hamiltonian method. Studies of the effect of induced moir\'e patterns between the crystallographically aligned graphene and the boron nitride lattices are also undertaken, with recommendations for future investigation. It is theoretically shown that samples containing two layers of graphene, separated by hexagonal boron nitride tunnel barriers, produced higher frequency oscillations when the graphene lattices are aligned. This was found to be due to the decrease in wavefunction overlap in the misaligned samples, which is not compensated by the higher density of states available for tunnelling. Chemical doping of the graphene layers are also found to increase the frequency, as it allows the Dirac cones to be brought into alignment for resonant tunnelling with a higher number of states available. It is known that the mismatch in lattice constant between the graphene lattice and the hexagonal boron nitride lattice creates a moir\'e pattern. This, in turn, induces additional Dirac points in the band structure and thus leads to new features in the current-voltage characteristics. The theoretical simulations presented in this thesis are substantiated by recently-published experimental results, and provide insight into possible future high-frequency, room-temperature solid state oscillators and amplifiers. In conclusion, the mechanisms for resonant tunnelling in multiple graphene heterostructures are identified and demonstrated in this work, and provide promising evidence for novel high frequency technologies and further research.

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37

Ryan, Shawn David. « Bifurcation and Boundary Layer Analysis for Graphene Sheets ». University of Akron / OhioLINK, 2009. http://rave.ohiolink.edu/etdc/view?acc_num=akron1239646272.

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38

Zhang, Lipeng. « Theoretical study of oxygen reduction reaction catalytic properties of defective graphene in fuel cells ». Thesis, The University of Akron, 2015. http://pqdtopen.proquest.com/#viewpdf?dispub=3718274.

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In this dissertation density functional theory (DFT) was applied to study the electronic structure and catalytic properties of graphene containing different types of defects. These defects includes hetero-atoms such as nitrogen, sulfur doped graphene, point defects such as Stone-Wales defects, single vacancy, double vacancies and substituting pentagon ring at zigzag edge, line defects such as pentagon-heptagon carbon ring chains, pentagon-pentagon-octagon carbon ring chains locating at the middle of graphene. The mechanisms of oxygen reduction reaction (ORR) were studied on these defective graphene, and electron transfer processes were simulated. Using DFT methods, we also explored the effect of strains to ORR electronic catalytic properties on pure and nitrogen doped graphene.

Our simulaltion results show that nitrogen, sulfur doped graphene, graphene containing point defects, substituting pentagon ring at zigzag edge, graphene containing line defects, pentagon-heptagon chain or pentagon-pentagon-octagon chains which have odd number of heptagon or octagon carbon ring perform high catalytic properties for ORR. Four electron transfer reactions could occur, and there are also two electrons transfer occuring on these defective graphene. The Stone-Wales defect itself cannot generate the catalytic activity on the graphene, but can facilitate the formation of hetero atom doping on graphene, which could show high catalytic activities to ORR. The catalytic active sites on defective graphene are atoms possessing high spin or charge density, where the spin density plays more important effect on the catalytic properties. For the N-doped graphene, the identified active sites are closely related to doping cluster size and dopant-defect interactions. Generally speaking, a large doping cluster size (number of N atoms >2) reduces the number of catalytic active sites per N atom. In combination with N clustering, Stone-Wales defects can strongly promote ORR. For four-electron transfer, the effective reversible potential ranges from 1.04 to 1.15 V/SHE, depending on the defects and cluster size. The catalytic properties of graphene could be optimized by introducing small N clusters in combination with material defects. For S-doped graphene, sulfur atoms could be adsorbed on the graphene surface, substitute carbon atoms at the graphene edges in the form of sulfur/sulfur oxide, or connect two graphene sheets by forming a sulfur cluster ring. Catalytic active sites distribute at the zigzag edge or the neighboring carbon atoms of doped sulfur oxide atoms, which possess large spin or charge density. For those being the active catalytic sites, sulfur atoms with the highest charge density take two-electron transfer pathway while the carbon atoms with high spin or charge density follow four-electron transfer pathway. Stone-Wales defects not only promote the formation of sulfur-doped graphenes, but also facilitate the catalytic activity of these graphenes. The ORR catalytic capabilities of the graphene containing point or line defects denpend on whether the defects could introduce spin density into the system or not. The axial strain field applied on the graphene could change its electronic properties. Neither the compressive nor the tensile strain along the zigzag or armchair direction could facinitate the catalytic activities of perfect graphene without any defects. Tensile strain along zigzag direction could change the electronic properties of nitrogen doped graphene, which are favorable to its ORR catalytic property.

Our simulation results explored the ORR on defective graphene in essence and provide the theoretical base for searching and fabricating new high efficient catalysts using the carbon based materials for fuel cells.

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39

Sokmen, Gokce. « Molecular Dynamics Investigation Of Moire Patterns In Double-layer Graphene ». Master's thesis, METU, 2012. http://etd.lib.metu.edu.tr/upload/12614751/index.pdf.

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Before Moire patterns are discovered in graphene, graphene was assumed to be found in only the rhombohedral form in nature. After transfer of graphene layer over another substrate was achieved by Andre Geim and Konstantin Novoselov, studies on graphene gained momentum. Following this, moire paterns were observed by scanning tunelling microscopy (STM) and high resolution transmision electron microscopy (HR-TEM). However, stability of these structures are still unknown. In this thesis, for the first time in literature, molecular dynamics of double layer graphene based Moire patterns are investigated as a result of the rotation of two graphene layers with LAMMPS (Large-scale Atomic/Molecular Massively Parallel Simulator) which has a GNU general public license. To model the two graphene layers, hexagonal graphene layers are generated by home writen Octave code. Then, periodicity condition for the Moire patterns are derived in chapter 2 according to rotation of graphene layers around their central axis, perpendicular to the layers. Then these layers with different angles or temperature or size are simulated by LAMMPS. There are 4 kind of molecular dynamics simulations studied according to modeled flakes. These are grouped under the name of &rsquo
Experiment #&rsquo
according to the modeling structure. Experiment-1 simulates double layer hexagonal flakes of graphene at a temperature of 0.1K. Experiment-2 simulates periodic moire patterns under periodic boundary conditions and represents the infinitely large graphene layers at 10K. Experiment 3 is dierent version of the experiment 1 but at higher temperature (10K). Finally, experiment 4 is modeled to show the behaviour of the graphene flake on a growth or attached region. The atoms around the flakes are modeled as a rigid body and constructs some stress on the graphene flakes.

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40

Hewageegana, Prabath. « Theory of Electronic and Optical Properties of Nanostructures ». Digital Archive @ GSU, 2008. http://digitalarchive.gsu.edu/phy_astr_diss/27.

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"There is plenty of room at the bottom." This bold and prophetic statement from Nobel laureate Richard Feynman back in 1950s at Cal Tech launched the Nano Age and predicted, quite accurately, the explosion in nanoscience and nanotechnology. Now this is a fast developing area in both science and technology. Many think this would bring the greatest technological revolution in the history of mankind. To understand electronic and optical properties of nanostructures, the following problems have been studied. In particular, intensity of mid-infrared light transmitted through a metallic diffraction grating has been theoretically studied. It has been shown that for s-polarized light the enhancement of the transmitted light is much stronger than for p-polarized light. By tuning the parameters of the diffraction grating enhancement can be increased by a few orders of magnitude. The spatial distribution of the transmitted light is highly nonuniform with very sharp peaks, which have the spatial widths about 10 nm. Furthermore, under the ultra fast response in nanostructures, the following two related goals have been proved: (a) the two-photon coherent control allows one to dynamically control electron emission from randomly rough surfaces, which is localized within a few nanometers. (b) the photoelectron emission from metal nanostructures in the strong-field (quasistationary) regime allows coherent control with extremely high contrast, suitable for nanoelectronics applications. To investigate the electron transport properties of two dimensional carbon called graphene, a localization of an electron in a graphene quantum dot with a sharp boundary has been considered. It has been found that if the parameters of the confinement potential satisfy a special condition then the electron can be strongly localized in such quantum dot. Also the energy spectra of an electron in a graphene quantum ring has been analyzed. Furthermore, it has been shown that in a double dot system some energy states becomes strongly localized with an infinite trapping time. Such states are achieved only at one value of the inter-dot separation. Also a periodic array of quantum dots in graphene have been considered. In this case the states with infinitely large trapping time are realized at all values of inter-dot separation smaller than some critical value.

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41

Cavallucci, Tommaso. « Atomic and electronic properties of graphene based systems grown on silicon carbide : a density functional theory study ». Doctoral thesis, Scuola Normale Superiore, 2018. http://hdl.handle.net/11384/85918.

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42

Hartman, R. M. « Pyrolitic graphite films : Their preparation, crystal size, orientation and other properties ». Thesis, Lancaster University, 1985. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.377389.

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43

Zhang, Lipeng. « Theoretical Study of Oxygen Reduction Reaction Catalytic Properties of Defective Graphene in Fuel Cells ». University of Akron / OhioLINK, 2013. http://rave.ohiolink.edu/etdc/view?acc_num=akron1374245184.

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44

Szulakowska, Ludmila. « Electron-electron Interactions and Optical Properties of Two-dimensional Nanocrystals ». Thesis, Université d'Ottawa / University of Ottawa, 2020. http://hdl.handle.net/10393/40983.

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This thesis presents a theory of electron-electron interaction effects and optical properties of nanostructures of two-dimensional (2D) honeycomb crystals - graphene and transition metal dichalcogenides (TMDC). Graphene, a semimetallic hexagonal lattice of carbon atoms can be described by a massless Dirac fermion model, with the conduction band (CB) and valence band (VB) touching in the corners of a hexagonal Brillouin zone, valleys K and -K. TMDC crystals sites host either a transition metal atom or a chalcogen dimer, which opens the energy gap and allows for describing their low-energy nature with massive Dirac fermion (mDf) model. The metal atom in TMDC crystals causes strong spin-orbit (SO) coupling, resulting in large SO splitting in bands at both valleys. For TMDCs it is possible to excite carriers in each valley with oppositely circularly polarised light, which offers promising prospects for devices based on electrons valley index, i.e. valleytronic devices. Additionally, the optical response of TMDCs is enhanced by the presence of secondary CB minima, at Q-points. The dimensionality of 2D crystals can be further reduced to form quantum dots (QDs) - nanostructures con ned in all dimensions. This thesis first discusses hexagonal graphene QDs, which exhibit energy gap oscillation as a function of size, due to the edge type: zigzag or armchair. These QDs are divided into concentric rings, analysed with tight-binding (TB) model. An armchair edged QD is built from a zigzag edged QD by adding a 1D Lieb lattice of carbon atoms on its edge. The energy gap is formed differently for both edges: from the outer ring states for zigzag edge and from the 1D Lieb lattice zero-energy states for armchair edge, which causes the energy gap. The remaining portion of the thesis focuses on TMDC materials. First a TB model is presented for a member of TMDC group, MoS2, using three d orbitals of Mo atom and three p orbitals of the S2 dimers. The tunneling matrix elements between nearest-neighbor and next-nearest-neighbour sites are explicitly derived at K and -K to form a six band TB Hamiltonian. Its solutions are fitted to the bands obtained from the density functional theory ab initio calculations to obtain the correct behaviour of bands around K and additional minima at Q-points, which explains the role of d orbitals in TMDCs. Close to K the TB model is reduced to mDf model, which is then studied in response to light, yielding the valley-dependent selection rules for absorption. The interaction of mDf with light is further studied in the presence of strong external magnetic eld, which leads to the formation of Landau levels (LLs), asymmetric between both valleys, and valley Zeeman splitting. These LLs are populated with electrons to form a Hartree-Fock ground state (GS), which can exhibit valley polarisation due to the LL asymmetry. Quasi-electron-hole excitations out of the GS are then formed and their self-energy, vertex corrections and scattering energy is calculated. The effect of electron-electron interactions on valley Zeeman splitting is demonstrated and the Bethe-Salpeter equation is numerically solved to give magnetoexciton spectrum for both valleys. The results include a valley-dependent absorption spectrum for mDf magnetoexcitons that vary with the valley polarisation. The final part of this thesis discusses the single particle and interacting effects in gated MoS2 QDs. First, I perform a single electron atomistic calculation for a million-atom computational box with periodic boundary conditions based on a TB model developed from ab initio methods for bulk MoS2. Electrons are then con ned with a parabolic electrostatic potential from top metallic gates. They exhibit twofold degenerate harmonic oscillator energy spectrum with shell spacing ω associated with valleys K as well as a sixfold degenerate energy spectrum derived from the Q-points. The degeneracy of electronic shells is broken due to valley contrasting Berry curvature,which acts as an effective magnetic eld splitting opposite angular momentum states in both valleys. I populate up to ve K-derived harmonic oscillator shells with up to six electrons and turn on the electron-electron interactions. The resulting GS phases form two regimes dependent on ω, which are dominated each by a broken-symmetry phase, i.e. valley and spin polarised GS for low ω and valley and spin unpolarised but spin intervalley antiferromagnetic GS for higher ω. This behaviour is explained as an effect of the strong SO splitting, weak intervalley exchange interaction and strong correlations. Means of detecting these effects in experiment based on the spin and valley blockade are proposed. These results advance the understanding of interaction-driven breaking of symmetry for valley systems, crucial for designing of valleytronic devices in the future.

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45

Steinbeck, John W. (John William). « Studies of the high temperature properties of graphite and liquid carbon using pulsed laser heating ». Thesis, Massachusetts Institute of Technology, 1987. http://hdl.handle.net/1721.1/14646.

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46

Berglund, Anders. « Characterization of factors interacting in CGI machining : machinability - material microstructure - material physical properties ». Licentiate thesis, KTH, Production Engineering, 2008. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-9258.

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The Swedish truck industry is forced to find new material solutions to achieve lighter engines with increased strength. Customers and new environmental regulations demand both higher specific power and more environmentally friendly trucks, and this places a rising pressure on the manufactures. This demand could be met by increasing the peak pressure in the cylinders. Consequently, a more efficient combustion is obtained and the exhaust lowered. This however exposes the engine to higher loads and material physical properties must therefore be enhanced.

Today, alloyed gray iron is the predominantly used engine material. This material cannot meet the requirements of tomorrow’s engines. Compacted Graphite Iron has good potential to be the replacement; it opens new design opportunities with its superior strength, which can lead to smaller, more efficient engines and additional power. The question is: how will manufacturing be affected?

The main goal of this thesis is to identify and investigate the main factors’ effect and their individual contributions on CGI machining. When the relationship between the fundamental features; machinability, material microstructure, and material physical properties, are revealed, then the CGI material can be optimized, both regarding the manufacturing process and design requirements. The basic understanding is developed mainly through experimental analysis. No attempt has been made to optimize the material to be used as engine material in this thesis.

The thesis demonstrates the importance of having good casting process control. It also illustrates the microstructural properties’ effects on CGI machinability, and what new aspects of machining must be taken into account, compared to gray iron.

OPTIMA CGI

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Guzman-Verri, Gian Giacomo. « Electronic Properties of Silicon-based Nanostructures ». Wright State University / OhioLINK, 2006. http://rave.ohiolink.edu/etdc/view?acc_num=wright1158515644.

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Nasseri, Mohsen. « NANOSCALE DEVICES CONSISTING OF HETEROSTRUCTURES OF CARBON NANOTUBES AND TWO-DIMENSIONAL LAYERED MATERIALS ». UKnowledge, 2018. https://uknowledge.uky.edu/physastron_etds/59.

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One dimensional carbon nanotubes (CNTs) and two-dimensional layered materials like graphene, MoS2, hexagonal boron nitride (hBN), etc. with different electrical and mechanical properties are great candidates for many applications in the future. In this study the synthesis and growth of carbon nanotubes on both conducting graphene and graphite substrates as well as insulating hBN substrate with precise crystallographic orientation is achieved. We show that the nanotubes have a clear preference to align to specific crystal directions of the underlying graphene or hBN substrate. On thicker flakes of graphite, the edges of these 2D materials can control the orientation of these carbon nanotubes. This integrated aligned growth of materials with similar lattices provides a promising route to achieving intricate nanoscale electrical circuits. Furthermore, short channel nanoscale devices consisting of the heterostructure of 1D and 2D materials are fabricated. In these nanoscale devices the nanogap is created due to etching of few layer graphene flake through hydrogenation and the channel is either carbon nanotubes or 2D materials like graphene and MoS2. Finally the transport properties of these nanoscale devices is studied.

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Van, Meveren Mayme Marie. « Graphene-Based ‘Hybrids’ as High-Performance Electrodes with Tailored Interfaces for Alternative Energy Applications : Synthesis, Structure and Electrochemical Properties ». TopSCHOLAR®, 2017. https://digitalcommons.wku.edu/theses/2048.

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Technological progress is determined to a great extent by developments of novel materials from new combinations of known substances with different dimensionality and functionality. We investigate the development of 3D ‘hybrid’ nanomaterials by utilizing graphene based systems coupled with transition metal oxides (e.g. manganese oxides MnO2 and Mn3O4). This lays the groundwork for high performance electrochemical electrodes for alternative energy owing to their higher specific capacitance, wide operational window and stability through charge-discharge cycling, environmental benignity, cost effective, easily processed, and reproducible in a larger scale. Thus far, very few people have investigated the potential of combining carbon sheets that can function as a supercapacitor in certain systems with transition metals that have faradaic properties to create electrochemical capacitors. Previous work by Wang et al. has focused on the structural combination of Mn3O4 and graphene based materials,1 and research by Jafta et al. studied the electrochemical properties of MnO2 with GO.2 We find that both physical and chemical attachment of manganese oxide on graphene allows for electrical interplay of the materials as indicated in electrochemical analysis and Raman spectroscopy. Attachment of the two materials is also characterized by scanning electron microscopy and X-ray diffraction.

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Zhao, Liang. « Optical properties of two-dimemsional Van der Waals crystals : from terahertz to visible ». Case Western Reserve University School of Graduate Studies / OhioLINK, 2015. http://rave.ohiolink.edu/etdc/view?acc_num=case1433378350.

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Thèses sur le sujet « Graphene - Physical Properties »

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