Dissertations / Theses: 'Electronic Transport Properties -Graphene'

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Poole, Christopher J. "Electronic and transport properties of graphene nanostructures." Thesis, Lancaster University, 2012. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.654742.

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Britnell, Liam Richard. "Electronic transport properties of graphene-based heterostructures." Thesis, University of Manchester, 2013. https://www.research.manchester.ac.uk/portal/en/theses/electronic-transport-properties-of-graphenebased-heterostructures(db9e8d20-c1a4-401c-85d9-c62ebd5c4d2c).html.

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Heterostructures fabricated from atomically thin crystalline layers are new materials which offer exciting possibilities for next-generation electronic and optoelectronic sensors and devices. The idea of heterostructures is not new and traditional semiconductor heterostructures have already played an important technological role in many modern electronic components. It is possible to fabricate new and exciting structures by stacking single atomic layers of different materials into heterostructures. This technology can be used to create materials and devices with a wide variety of properties. The stacking order, thickness, doping and crystal orientation play the major roles in determining the characteristics of these new materials. The experimental work for this thesis involves the electrical characterisation of several different heterostructures.i Investigation of boron nitride as an atomically thin tunnel barrier, including its homogeneity across micron sized areas. The area normalised conductance was found to depend on boron nitride thickness, changing by 1.5 decades per layer.ii Graphene-based tunnelling transistors which exhibit current modulation by external gate voltage. With boron nitride as the tunnel barrier an on-off ratio of up to 40 was acheived.iii Resonant tunnelling devices which show negative differential conductivity in their current-voltage characteristics.iv Photodetection and solar cell devices using semiconducting tungsten disulfide. The maximum external quantum efficiency observed was 0.1 A/W which was approximately constant across the visible spectrum. The enormous array of possibilities made available by this technology means that there is huge scope for further investigation with more exploratory research to make proof-of-principle functional devices for application in technology.

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Burgos, Atencia Rhonald. "Electronic transport properties of graphene sheets under strain." Niterói, 2017. https://app.uff.br/riuff/handle/1/2932.

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Conselho Nacional de Desenvolvimento Científico e Tecnológico
Nesta tese estudamos três problemas teóricos relacionados ao grafeno e um problema relacionado a um sistema bosônico interagente e desordenado em uma dimensão. Sobre o grafeno, estudamos alguns efeitos das deformações. Primeiro, calculamos o efeito de campos magnéticos aleatórios devido às deformações fora do plano em uma folha de grafeno na condutividade de Boltzmann. Encontramos que essas deformações são uma fonte importante de desordem para condutividade. Tamb em estudamos as oscilações de Weiss no grafeno devido a deformações unidimensionais. Usamos uma equação de Boltzmann quântica e teoria de perturbações até primeira ordem para resolver esse problema. Encontramos valores acessíveis experimentalmente para a condutividade. O efeito de localização fraca na conductividade é ainda um problema em andamento. Mesmo sabendo que o pseudo-campo magnético devido a deformações não quebra a simetria de inversão temporal quando considerados os dois valleys, acreditamos que a parte respons avel pelo espalhamento intra-valleys deve sentir o efeito desse pseudo-campo. O tempo de desfasagem devido a esse campo foi calculado. O problema de sistemas bosônicos tamb em está ainda em andamento. Identificamos algumas dificuldades na teoria de perturbações usada normalmente para sistemas fermiônicos e uma possivel forma de resolver esse problema.
In this thesis we address three theoretical problems related to electronic transport properties of graphene and one related to interacting Bosonic systems with disorder in one dimension. Concerning graphene, we have studied some efects of strain. First, we calculated the efect of random gauge fields due to out of plane deformation in the Boltzmann conductivity. We have found that strain plays an important role as a disorder source that limits the conductivity. We have also studied Weiss oscillation in graphene due to uniaxial strain. We have used a quantum Boltzmann approach and first order perturbution theory to this end. We found measurable values to the conductivity in this system. The efect of weak localization is still a work in progress. Although the pseudo magnetic field in graphene does not break time reversal symmetry in the two valleys, we believe that the channel responsable for intravalley scattering must be sensitive to dephasing due to strain. This dephasing time has been calculated. Concerning the Bosonic system, this is also a work in progress. We have identified some difculties in the standard procedure of perturbation theory when applied to this system and a possible way to face them.

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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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Plachinda, Pavel. "Electronic Properties and Structure of Functionalized Graphene." PDXScholar, 2012. https://pdxscholar.library.pdx.edu/open_access_etds/585.

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The trend over the last 50 years of down-scaling the silicon transistor to achieve faster computations has led to doubling of the number of transistors and computation speed over about every two years. However, this trend cannot be maintained due to the fundamental limitations of silicon as the main material for the semiconducting industry. Therefore, there is an active search for exploration of alternate materials. Among the possible candidates that can may [sic] be able to replace silicon is graphene which has recently gained the most attention. Unique properties of graphene include exceedingly high carrier mobility, tunable band gap, huge optical density of a monolayer, anomalous quantum Hall effect, and many others. To be suitable for microelectronic applications the material should be semiconductive, i.e. have a non-zero band gap. Pristine graphene is a semimetal, but by the virtue of doping the graphene surface with different molecules and radicals a band gap can be opened. Because the electronic properties of all materials are intimately related to their atomic structure, characterization of molecular and electronic structure of functionalizing groups is of high interest. The ab-inito (from the first principles) calculations provide a unique opportunity to study the influence of the dopants and thus allow exploration of the physical phenomena in functionalized graphene structures. This ability paves the road to probe the properties based on the intuitive structural information only. A great advantage of this approach lies in the opportunity for quick screening of various atomic structures. We conducted a series of ab-inito investigations of graphene functionalized with covalently and hapticly bound groups, and demonstrated possible practical usage of functionalized graphene for microelectronic and optical applications. This investigation showed that it is possible [to] produce band gaps in graphene (i.e., produce semiconducting graphene) of about 1 eV, without degrading the carrier mobility. This was archived by considering the influence of those adducts on electronic band structure and conductivity properties.

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6

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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Martins, Ernane de Freitas. "QM/MM simulations of electronic transport properties for DNA sensing devices based on graphene." Universidade Estadual Paulista (UNESP), 2018. http://hdl.handle.net/11449/154328.

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Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)
Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)
Nanotechnology is an important and very active area of research contributing to many different fields. The development of new devices applied to personalized medicine is one of its applications. When we desire to develop new devices many effort are done, including experimental and theoretical investigations. The theoretical/computational physics can enormously contribute to this area, since the simulations can reveal the working mechanism in these systems being possible to understand and propose new devices with improved performance. We present an extensive theoretical investigation of the electronic transport properties of graphene-based devices for DNA sensing. We have used a hybrid methodology which combines quantum mechanics and molecular mechanics, the so called QM/MM method, coupled to electronic transport calculations using non-equilibrium Green’s functions. First, we studied graphene in solution in order to understand the effects of polarization on the electronic and transport properties under different salt concentrations. We also stud- ied graphene with Stone-Wales defect in pure water. For these systems we tested a simple polarization model based on rigid rods. Our analysis were also done over different QM/MM partitions including explicit water molecules in the quantum part. Our results showed that the inclusion of the solvent in the electronic transport calculations for graphene decreases the total transmission, showing the important role played by the water. Our results also showed that the electronic transport properties of graphene do not suffer significant changes as we increase the salt concentration in the solution. The inclusion of polarization effects in graphene, despite changing the structuring of water molecules that make up the first solvation shell of graphene, do not significantly affect the electronic transport through graphene. We then studied DNA sequencing devices. First we focused on sequencing using a nanopore between topological line defects in graphene. Our results showed that sequencing DNA with high selectivity and sensitivity using these devices appears possible. We also address nanogap in graphene. For this we looked at the effects of water on electronic transport by using different setups for the QM/MM partition. We showed that the inclusion of water molecules in the quantum part increases the electronic transmission in several orders of magnitude, also showing the fundamental role played by water in tunneling devices. The electronic transport simulations showed that the proposed device has the potential to be used in DNA sequencing, presenting high selectivity and sensitivity. We propose an graphene-based biochip for sequence-specific detection of DNA strands. The main idea of this sort of device is to detect hybridization of single-stranded DNA, forming double-stranded DNA. We showed that the vertical DNA adsorption, either through an anchor molecule (pyrene) or using the nucleotide itself as anchor, do not present good results for detection, since the signals for the single and double strands are quite similar. For the case of horizontal DNA adsorption on graphene our results indicated that the two signals can be distinguishable, showing promising potential for sensitivity and selectivity.
Nanotecnologia é uma importante e muito ativa área de pesquisa contribuindo para muitos campos diferentes. O desenvolvimento de novos dispositivos aplicados à medicina personalizada é uma de suas aplicações. Quando desejamos desenvolver novos dispositivos muitos esforços são feitos, incluindo investigações experimentais e teóricas. A Física teórica/computacional pode contribuir enormemente com esta área, já que simulações podem revelar o mecanismo de funcionamento nesses sistemas tornando possível entender e propor novos dispositivos com desempenho melhorado. Nós apresentamos uma extensa investigação teórica das propriedades de transporte eletrônico de dispositivos baseados em grafeno para sensoriamento de DNA. Utilizamos uma metodologia híbrida que combina mecânica quântica e mecânica molecular, o chamado método QM/MM, acoplado a cálculos de transporte eletrônico utilizando funções de Green fora do equilíbrio. Primeiramente nós estudamos grafeno em solução de modo a entender os efeitos de polarização nas propriedades eletrônica e de transporte em diferentes concentrações de sal. Também estudamos grafeno com defeito Stone-Wales em água pura. Para esses sistemas, testamos um modelo de polarização simples baseado em bastões rígidos. Nossas análises também foram feitas em diferentes partições QM/MM incluindo moléculas de água explícitas na parte quântica. Nossos resultados mostraram que a inclusão do solvente nos cálculos de transporte eletrônico para o grafeno diminui a transmissão total, mostrando o papel fundamento desempenhado pelo água. Nossos resultados também mostraram que as propriedades de transporte eletrônico do grafeno não sofrem mudanças significativas na medida em que aumentamos a concentração de sal na solução. A inclusão de efeitos de polarização em grafeno, apesar de mudar a estruturação das moléculas de água que compõem a primeira camada de solvatação do grafeno, não afeta significativamente o transporte eletrônico através do grafeno. Nós, então, estudamos dispositivos para sequenciamento de DNA. Focamos primeira- mente no sequenciamento usando nanoporo entre defeitos de linha topológicos no grafeno. Nossos resultados mostraram que o sequenciamento de DNA com alta seletividade e sensitividade usando esses dispositivos se mostra possível. Nós também abordamos nanogap em grafeno. Para tal, avaliamos os efeitos da água no transporte eletrônico utilizando diferentes configurações para a partição QM/MM. Mostramos que a inclusão de moléculas de água na parte quântica aumenta a transmissão eletrônica em várias ordens de grandeza, também mostrando o papel fundamental desempenhado pela água em dispositivos de tunelamento. As simulações de transporte eletrônico mostraram que o dispositivo proposto tem o potencial de ser usado em sequenciamento de DNA, apresentando alta seletividade e sensitividade. Propusemos um biochip baseado em grafeno para detecção de sequências específicas de fitas de DNA. A ideia principal desta classe de dispositivos é detectar a hibridização da fita simples de DNA, formando a fita dupla de DNA. Mostramos que a adsorção vertical de DNA, seja utilizando uma molécula âncora (pireno) ou utilizando o próprio nucleotídio como âncora, não apresenta bons resultados para detecção, já que os sinais para as fitas simples e dupla são bem próximos. Para o caso da adsorção horizontal de DNA em grafeno nossos resultados indicaram que os dois sinais podem ser distinguíveis, mostrando potencial promissor para sensitividade e seletividade.

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Verastegui, Wudmir Yudy Rojas. "Electronic and transport properties of graphene nanoribbons with adsorbed transition metal impurities : spin-orbit interaction." reponame:Repositório Institucional da UFABC, 2013.

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9

Seifert, Christian. "Control of the Electrical Transport through Single Molecules and Graphene." Doctoral thesis, Humboldt-Universität zu Berlin, 2020. http://dx.doi.org/10.18452/21647.

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Der Erste dieser Arbeit befasst sich mit der STM Untersuchung einer Grenzschicht in umgebender Atmosphäre, welche sich durch die Adsorption von Graphen auf einer Glimmeroberfläche ausbildet. Durch die umgebene Luftfeuchtigkeit interkalieren Wassermoleküle in diese Grenzschicht. Durch die Variation der relativen Luftfeuchtigkeit gibt diese Wasser ab bzw. nimmt auf, und es manifestieren sich sternförmig wachsende Fraktale, in denen Graphen etwa um den Durchmesser eines Wassermoleküls an Höhe absinkt. Die STM Untersuchung, welche primär sensitiv auf die Zustandsdichte von Graphen reagiert, zeigte, dass sich anders als in den SFM Untersuchungen, zusätzliche signifikante Höhenänderungen von Graphen innerhalb der Fraktale bildeten. Dieses deutet auf eine Wasserschicht hin, welche Domänen mit signifikant unterscheidbaren Polarisationsrichtungen aufweisen, welche die Zustandsdichte von Graphen verändern kann. Dies ist aber gleichbedeutend mit der Annahme, dass sich in jener Grenzschicht mindestens zwei oder mehr lagen Wasser bilden müssen. Der zweiten Teil befasst sich mit der STM Untersuchung einer funktionalisierten Oberfläche die charakterisiert ist durch eine leitende Oberfläche (Graphen und HOPG) adsorbierten funktionalisierte Dyade an einer Fest-Flüssig Grenzfläche. Diese Dyade besteht im Wesentlichen aus einem Zink-Tetraphenylporphyrin (ZnTPP) und mit diesem über einem flexiblen Arm verbundenen Spiropyranderivat. Letztere ändert seine Konformation durch die Einstrahlung mit Licht geeigneter Wellenlänge, womit sich das Dipolmoment stark ändert. Es zeigte sich, dass das Schaltverhalten auf einen Graphen mit dem Schaltverhalten einer Dyade in Lösung vergleichbar ist. Dieses lässt den Schluss zu, dass das Schalteigenschaften einer einzelnen Dyade auf das adsorbierte Kollektiv übertragen werden kann, da es keine signifikanten beeinflussenden Wechselwirkungen durch die leitende Oberfläche und der benachbarten Dyaden auswirkte.
The first of this two-part work deals with the STM investigation of an interface in the surrounding natural atmosphere, which is formed by the adsorption of the conductive graphene onto the mica surface. In this interface, water molecules may intercalate by the surrounding humidity. By varying the relative humidity, the interface is rewetted, respectively, dewetted and it manifests itself in a star shape growing fractals, where the height of graphene is decreased by approximately the diameter of one water molecule. The STM investigation - which is primarily sensitive to the density of states of graphene - shows that additional significant changes in the height of graphene are formed within the fractal, unlike in the SFM investigations. This suggests that there is a water layer by which the density of graphene is differently affected by domains with significant distinguishable polarisation alignments. However, this is equivalent to the assumption that there are two or more water layers exist within the interface. The second part of this work deals with the STM investigation of a functionalized surface characterised by a functionalized dyad adsorbed onto a conductive surface (graphene and HOPG) at a solid-liquid interface. This dyad essentially comprises a zinc-tetraphenylporphyrin (ZnTPP) and is connected with a spiropyran derivative via a flexible linker. This changes its conformation through irradiation with light with a suitable wavelength, by which the dipole moment is also strongly changed. It was found that the switching behaviour of a graphene-based conductive surface is comparable with the switching behaviour of a dyad, which itself can move freely in solution. This leads to the conclusion that the switching properties of a single dyad can be transmitted to its collective because it affected no significant influence interactions by the conductive surface and the adjacent dyads.

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Khademi, Ali. "Tuning graphene’s electronic and transport properties via adatom deposition." Thesis, University of British Columbia, 2017. http://hdl.handle.net/2429/62588.

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This thesis investigates the effect of adatom deposition, especially alkali and heavy adatoms, on graphene’s electronic and transport properties. While there are many theoretical predictions for tuning graphene’s properties via adatom deposition, only a few of them have been observed. Solving this enigma of inconsistency between theory and experiment raises the need for deeper experimental investigation of this matter. To achieve this goal, an experimental set up was built which enables us to evaporate different metal adatoms on graphene samples while they are at cryogenic temperatures and ultra-high vacuum (UHV) conditions. The critical role of in situ high-temperature annealing in creating reliable interactions between adatoms and graphene is observed. This contradicts the commonly accepted assumption in the transport community that placing a graphene sample in UHV and performing in situ 400-500 K annealing is enough to provide a reliable adatom-graphene interaction. Even charge doping by alkali atoms (Li), which is arguably the simplest of all adatom effects, cannot be achieved completely without in situ 900 K annealing. This observation may explain the difficulty many groups have faced in inducing superconductivity, spin-orbit interaction, or similar electronic modifications to graphene by adatom deposition, and it points toward a straightforward, if experimentally challenging, solution. The first experimental evidence of short-range scattering due to alkali adatoms in graphene is presented in this thesis, a result that contradicts the naive expectation that alkali adatoms on graphene only cause long-range Coulomb scattering. The induced short-range scattering by Li caused decline of intervalley time and length (i.e., enhancement of intervalley scattering). No signatures of theoretically predicted superconductivity of Li doped graphene were observed down to 3 K. Cryogenic deposition of copper increased the dephasing rate of graphene. This increase in dephasing rate is either a sign of inducing spin-orbit interaction or magnetic moments by copper. No similar effect was observed for indium.
Science, Faculty of
Physics and Astronomy, Department of
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11

Woollacott, Claire. "Electronic and plasmonic properties of real and artificial Dirac materials." Thesis, University of Exeter, 2015. http://hdl.handle.net/10871/18227.

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Inspired by graphene, I investigate the properties of several different real and artificial Dirac materials. Firstly, I consider a two-dimensional honeycomb lattice of metallic nanoparticles, each supporting localised surface plasmons, and study the quantum properties of the collective plasmons resulting from the near field dipolar interaction between the nanoparticles. I analytically investigate the dispersion, the effective Hamiltonian and the eigenstates of the collective plasmons for an arbitrary orientation of the individual dipole moments. When the polarisation points close to normal to the plane the spectrum presents Dirac cones, similar to those present in the electronic band structure of graphene. I derive the effective Dirac Hamiltonian for the collective plasmons and show that the corresponding spinor eigenstates represent chiral Dirac-like massless bosonic excitations that present similar effects to those of electrons in graphene, such as a non-trivial Berry phase and the absence of backscattering from smooth inhomogeneities. I further discuss how one can manipulate the Dirac points in the Brillouin zone and open a gap in the collective plasmon dispersion by modifying the polarisation of the localized surface plasmons, paving the way for a fully tunable plasmonic analogue of graphene. I present a phase diagram of gapless and gapped phases in the collective plasmon dispersion depending on the dipole orientation. When the inversion symmetry of the honeycomb structure is broken, the collective plasmons become gapped chiral Dirac modes with an energy-dependent Berry phase. I show that this concept can be generalised to describe many real and artificial graphene-like systems, labeling them Dirac materials with a linear gapped spectrum. I also show that biased bilayer graphene is another Dirac material with an energy dependent Berry phase, but with a parabolic gapped spectrum. I analyse the relativistic phenomenon of Klein Tunneling in both types of system. The Klein paradox is one of the most counter-intuitive results from quantum electrodynamics but it has been seen experimentally to occur in both monolayer and bilayer graphene, due to the chiral nature of the Dirac quasiparticles in these materials. The non-trivial Berry phase of pi in monolayer graphene leads to remarkable effects in transmission through potential barriers, whereas there is always zero transmission at normal incidence in unbiased bilayer graphene in the npn regime. These, and many other 2D materials have attracted attention due to their possible usefulness for the next generation of nano-electronic devices, but some of their Klein tunneling results may be a hindrance to this application. I will highlight how breaking the inversion symmetry of the system allows for results that are not possible in these system's inversion symmetrical counterparts.

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Hartmann, Richard Rudolph. "Optoelectronic properties of carbon-based nanostructures : steering electrons in graphene by electromagnetic fields." Thesis, University of Exeter, 2010. http://hdl.handle.net/10036/113452.

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Graphene has recently become the focus of enormous attention for experimentalists and theorists alike mainly due to its unique electronic properties. However, the limited way in which one can control these properties is a major obstacle for device applications. The unifying theme of this thesis is to propose and thoroughly justify ways to control the electronic properties of graphene and carbon nanotubes by light or static electric and magnetic fields and to harness these properties for optoelectronic applications. A linearly polarized excitation is shown to create a strongly anisotropic distribution of photoexcited carriers in graphene, where the momenta of photoexcited carriers are aligned preferentially normal to the polarization plane. This effect offers an experimental tool to generate highly directional photoexcited carriers which could assist in the investigation of "direction-dependent phenomena" in graphene-based nanostructures. The depolarization of hot photoluminescence is used to study relaxation processes in graphene, both free standing and grown on silicon carbide. This analysis is extended to include the effect of a magnetic field, thereby allowing one to obtain the momentum relaxation times of hot electrons. The analysis of momentum alignment in the high frequency regime shows that a linearly polarized excitation allows the spatial separation of carriers belonging to different valleys. Quasi-metallic carbon nanotubes are considered for terahertz applications. They are shown to emit terahertz radiation when a potential diﬀerence is applied across their ends and their spontaneous emission spectra have a universal frequency and bias voltage dependence. It is shown that the same intrinsic curvature which opens the gap in the quasi-metallic carbon nanotube energy spectrum also allows optical transitions in the terahertz range. The exciton binding energy in narrow-gap carbon nanotubes is calculated and found to scale with the band gap and vanishes as the gap decreases, even in the case of strong electron-hole attraction. Therefore, excitonic effects should not dominate in narrow-gap nanotubes. Contrary to widespread belief, it is shown that full conﬁnement is possible for zero-energy states in pristine graphene. The exact analytical solutions for the zero-energy modes conﬁned within a smooth one-dimensional potential V = α/ cosh (βx) are presented. This potential provides a good fit for the potential proﬁles of top-gated graphene structures. It is shown that there is a threshold value of the characteristic potential strength α/β for which the first mode appears, in striking contrast to the non-relativistic case. A relationship between the characteristic strength and the number of modes within the potential is found. An experimental setup is proposed for the observation of these modes. The proposed geometry could be utilized in future graphene-based devices with high on/off current ratios.

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NOVELLI, Pietro. "Electron-electron interaction effects in the optical and transport properties of 2D materials beyond graphene." Doctoral thesis, Scuola Normale Superiore, 2021. http://hdl.handle.net/11384/105435.

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This Thesis studies optical, plasmonic, and transport phenomena in two-dimensional materials. In particular, optical and plasmonic properties in twisted bilayer graphene are analyzed in the first part of the Thesis, whereas transport phenomena in twodimensional topological insulators are left for the second part. A common element of the physical systems studied here are electron-electron interactions, whose presence is pivotal for many of the results presented. The first Chapter of the manuscript is devoted to a review and critical analysis of the experimental results that motivated our work. Among these results, I emphasize recent experimental work carried out at ICFO on MIT samples on the plasmonic properties of twisted bilayer graphene whose theoretical interpretation was accomplished mostly thanks to the original theory presented in this Thesis. The first Chapter is also devoted to present some of the necessary theoretical concepts and tools forming the basis of this manuscript. The second Chapter of the thesis presents a theory of twisted bilayer graphene, an atomically-thin heterostructure which in early 2018 was showed to host a plethora of exotic quantum phases of matter. This Chapter also includes a technical Section where the details of the numerical codes developed for our study of twisted bilayer graphene are thoroughly discussed. These numerical codes are planned to be fully released and openly available for the scientific community in the near future. The third Chapter contains original results on the optical and plasmonic properties of twisted bilayer graphene. These results are obtained for a large variety of different parameter configurations, in the spirit of giving as much information as possible for a material (twisted bilayer graphene) whose actual physical properties are to a large extent still unknown. This Chapter is concluded by the presentation of preliminary results on the density-density response function of twisted bilayer graphene, which is essential to understand its dielectric properties, and hence how and how much the electron-electron interactions are screened. The fourth Chapter is about the theory of two-dimensional topological insulators, a class of materials hosting very interesting transport phenomena that are related to to the topological nature of their non-interacting bands and eigenstates. A Section of this Chapter is also devoted to the theory of ballistic electron transport, which is essential to understand many properties of two-dimensional topological insulators. In the fifth and last Chapter of the Thesis, we present an original result on the interplay between electron-electron interactions and localized defects in two-dimensional topological insulators. The theory presented in this Chapter provides a straightforward conceptual framework to explain experimental results on the transport properties of two-dimensional topological insulators, especially those in atomically thin crystals, plagued by short-range edge disorder.

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Kurfürstová, Markéta. "Vodíkem modifikované grafenové struktury pro polem řízené tranzistory." Master's thesis, Vysoké učení technické v Brně. Fakulta strojního inženýrství, 2016. http://www.nusl.cz/ntk/nusl-254358.

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This master’s thesis is focused on the subject of graphene modified with atomic hydrogen and its electronic transport properties. Structural and electronic properties of graphene and hydrogenated graphene are compared in the theoretical part of the thesis. The Raman spectroscopy technique is described, including characterization of typical Raman spectra of both unmodified and modified graphene. Samples used during experimental part of the thesis are prepared via laser and electron lithography, and are set to be measured in a vacuum chamber. Subsequently, electronic transport properties are measured before and after hydrogen modification of graphene. Finally, hydrogenated graphene is irradiated using electron beam and changes in its structure are analyzed with Raman spectroscopy techniques.

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15

Sousa, Duarte José Pereira de. "Transporte eletrônico em anéis quânticos de grafeno." reponame:Repositório Institucional da UFC, 2015. http://www.repositorio.ufc.br/handle/riufc/14677.

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SOUSA, Duarte José Pereira de. Transporte eletrônico em anéis quânticos de grafeno. 2015. 83 f. Dissertação (Mestrado em Física) - Programa de Pós-Graduação em Física, Departamento de Física, Centro de Ciências, Universidade Federal do Ceará, Fortaleza, 2015.
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In this work, we propose a current switch device that exploits the phase acquired by a charge carrier as it tunnels through a potential barrier in graphene in the ballistic regime without the need of the presence of a gap in the spectrum. The system acts as an interferometer based on an armchair graphene quantum ring, where the phase difference between interfering electronic wave functions for each path can be controlled by tuning the height of a potential barrier in the ring arms. By varying the parameters of the potential barriers the interference can become completely destructive. We demonstrate how this interference effect can be used for developing a simple graphene-based logic gate.
Neste trabalho, é proposto um dispositivo de controle de corrente que explora a fase adquirida por um portador de carga quando este tunela através de uma barreira de potencial no grafeno no regime balístico sem a necessidade da presença de um gap no espectro de energias. O sistema atua como um interferômetro baseado em um anel quântico de grafeno com bordas armchair, onde a diferença de fase entre as funções de onda para elétrons que tomam diferentes caminhos pode ser controlada através da intensidade das barreiras de potencial nos braços do anel. Variando os parâmetros das barreiras a interferência pode tornar-se completamente destrutiva. É demonstrado como esse efeito de interferência pode ser utilizado para o desenvolvimento de portas lógicas simples baseadas em grafeno.

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16

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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Sousa, Duarte JosÃ Pereira de. "Transporte eletrÃnico em anÃis quÃnticos de grafeno." Universidade Federal do CearÃ, 2015. http://www.teses.ufc.br/tde_busca/arquivo.php?codArquivo=15743.

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Conselho Nacional de Desenvolvimento CientÃfico e TecnolÃgico
Neste trabalho, Ã proposto um dispositivo de controle de corrente que explora a fase adquirida por um portador de carga quando este tunela atravÃs de uma barreira de potencial no grafeno no regime balÃstico sem a necessidade da presenÃa de um gap no espectro de energias. O sistema atua como um interferÃmetro baseado em um anel quÃntico de grafeno com bordas armchair, onde a diferenÃa de fase entre as funÃÃes de onda para elÃtrons que tomam diferentes caminhos pode ser controlada atravÃs da intensidade das barreiras de potencial nos braÃos do anel. Variando os parÃmetros das barreiras a interferÃncia pode tornar-se completamente destrutiva. Ã demonstrado como esse efeito de interferÃncia pode ser utilizado para o desenvolvimento de portas lÃgicas simples baseadas em grafeno.
In this work, we propose a current switch device that exploits the phase acquired by a charge carrier as it tunnels through a potential barrier in graphene in the ballistic regime without the need of the presence of a gap in the spectrum. The system acts as an interferometer based on an armchair graphene quantum ring, where the phase difference between interfering electronic wave functions for each path can be controlled by tuning the height of a potential barrier in the ring arms. By varying the parameters of the potential barriers the interference can become completely destructive. We demonstrate how this interference effect can be used for developing a simple graphene-based logic gate.

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18

Vysocký, Filip. "Interakce pomalých elektronů s grafenovými polem řízenými tranzistory." Master's thesis, Vysoké učení technické v Brně. Fakulta strojního inženýrství, 2019. http://www.nusl.cz/ntk/nusl-402578.

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This diploma thesis is focused on fabrication of graphene field-effect transistors, characterisation of their transport properties and investigation of low-energy electron beam influence on the devices' properties under UHV conditions. The theoretical part of this work describes graphene fabrication methods, options of graphene transfer onto the substrates for graphene field-effect transistor manufacture. Furthermore, model of graphene doping via electrostatic interaction or photon, resp. electron beam exposition is explained. The experimental part of this work consist of manufacture of the graphene field-effect transistor in order to examine the change of its transport properties induced by doping of the graphene via low-energy electron beam exposition.

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Herrmann, Oliver [Verfasser], Laurens W. [Gutachter] Molenkamp, and Matthias [Gutachter] Bode. "Graphene-based single-electron and hybrid devices, their lithography, and their transport properties / Oliver Herrmann ; Gutachter: Laurens W. Molenkamp, Matthias Bode." Würzburg : Universität Würzburg, 2017. http://d-nb.info/1130587924/34.

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20

Costa, Diego Rabelo da. "Transportes e confinamento em monocamada e bicamada de nanoestruturas de grafeno com diferentes bordas, interfaces e potenciais." reponame:Repositório Institucional da UFC, 2014. http://www.repositorio.ufc.br/handle/riufc/12555.

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COSTA, Diego Rabelo da. Transportes e confinamento em monocamada e bicamada de nanoestruturas de grafeno com diferentes bordas, interfaces e potenciais. 2014. 201 f. Tese (Doutorado em Física) - Programa de Pós-Graduação em Física, Departamento de Física, Centro de Ciências, Universidade Federal do Ceará, Fortaleza, 2014.
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Graphene, a two-dimensional lattice of carbon atoms, has been widely studied during the past few years. The interest in this material is not only due to its possible future technological applications, but also because it provides the possibility to probe interesting phenomena predicted by quantum field theories, ranging from Klein tunneling and other quasi-relativistic effects to the existence of new types of electron degrees of freedom, namely, the pseudo-spin, and the existence of two inequivalent electronic valleys in the vicinity of the gapless points of its energy spectrum. Several of the exotic properties observed in graphene originate from the fact that within the low energy approximation for the tight-binding Hamiltonian of graphene, electrons behave as massless Dirac fermions, with a linear energy dispersion. Just like in single layer graphene, the low-energy eletronic spectrum in bilayer graphene is gapless, but in this case it is dominated by the parabolic dispersion. Nevertheless, one interesting feature is shared by both monolayer and bilayer graphene: the valley degree of freedom. In this thesis, we theoretically investigate: (i) the dynamic properties in mono and bilayer graphene, performing a systematic study of wave packet scattering in different interface shapes, edges and potentials; and furthermore (ii) the energy levels of confined systems in graphene in the presence or absence of external magnetic and electric fields. In the first part of the work, we use the tight-binding approach to study the scattering of a Gaussian wave packet on monolayer graphene edges (armchair and zigzag) in the presence of real and pseudo (strain induced) magnetic fields and also calculate the transmission probabilities of a Gaussian wave packet through a quantum point contact defined by electrostatic gates in bilayer graphene. These numerical calculations are based on the solution of the time-dependent Schrödinger equation for the tight-binding model Hamiltonian, using the Split-operator technique. Our theory allows us to investigate scattering in reciprocal space, and depending on the type of graphene edge we observe scattering within the same valley, or between different valleys. In the presence of an external magnetic field, the well known skipping orbits are observed. However, our results demonstrate that in the case of a pseudo-magnetic field, induced by non-uniform strain, the scattering by an armchair edge results in a non-propagating edge state. We propose also a very efficient valley filtering through a quantum point contact system defined by electrostatic gates in bilayer graphene. For the suggested bilayer system, we investigate how to improve the efficiency of the system as a valley filter by varying parameters, such as length, width and amplitude of the applied potential. In the second part of the thesis, we present a systematic study of the energy spectra of graphene quantum rings having different geometries and edge types, in the presence of a perpendicular magnetic field. We discuss which features obtained through a simplified Dirac model can be recovered when the eigenstates of graphene quantum rings are compared with the tight-binding results. Furthermore, we also investigate the confined states in two different hybrid monolayer - bilayer systems, identifying dot-localized states and edge states for the suggested bilayer confinement structures, as well as we will study the behavior of the energy levels as a function of dot size and under an applied external magnetic field. Finally, using the four-band continuum Dirac model, we also derive a general expression for the infinite-mass boundary condition in bilayer graphene in order to apply this boundary condition to calculate analytically the confined states and the corresponding wave functions in a bilayer graphene quantum dot in the absence and presence of a perpendicular magnetic field. Our analytic results exhibit good agreement when compared with the tight-binding ones.
Grafeno, uma rede bidimensional de átomos de carbono, tem sido amplamente estudado durante os últimos anos. O interesse por este material não é apenas devido às suas possíveis aplicações tecnológicas futuras, mas também porque oferece a possibilidade de investigar fenômenos interessantes previstos pelas teorias quânticas de campo, que vão desde o tunelamento de Klein e outros efeitos quasi-relativísticos à existência de novos tipos de graus de liberdade do elétron, ou seja, o pseudo-spin, e a existência de dois vales eletrônicos não-equivalentes na vizinhança dos pontos sem gap do seu espectro de energia. Várias das propriedades exóticas observadas no grafeno originam-se do facto de que dentro da aproximação de baixas energias para o Hamiltoniano tight-binding do grafeno, elétrons se comportam como férmions de Dirac sem massa, com uma dispersão de energia linear. Assim como no caso de uma monocamada de grafeno, o espectro eletrônico de baixas energias para uma bicamada de grafeno é sem gap, mas, neste caso, é dominado pela dispersão parabólica. No entanto, uma característica interessante é compartilhada por ambas monocamada e bicamada de grafeno: o grau de liberdade de vale. Nesta tese, nós investigamos teoricamente: (i) as propriedades dinâmicas em mono e bicamadas de grafeno, realizando um estudo sistemático do espalhamento de pacotes de onda em diferentes formas de interfaces, bordas e potenciais; e, além disso, (ii) os níveis de energia de sistemas confinados no grafeno na presença ou ausência de campos magnéticos e elétricos externos. Na primeira parte do trabalho, nós utilizamos a abordagem tight-binding para estudar o espalhamento de um pacote de onda Gaussiano nas bordas de uma monocamada de grafeno (armchair e zigzag) na presença de campos magnéticos reais e pseudo-magnéticos (induzidos por tensão) e também calculamos as probabilidades de transmissão de um pacote de onda Gaussiano através de um contato de ponto quântico definido por potenciais eletrostáticos em bicamadas de grafeno. Estes cálculos numéricos são baseados na solução da equação de Schrödinger dependente do tempo para o Hamiltoniano do modelo tight-binding, usando a técnica Split-operator. Nossa teoria permite investigar espalhamento no espaço recíproco, e dependendo do tipo de borda do grafeno, nós observamos espalhamento dentro do mesmo vale, ou entre diferentes vales. Na presença de um campo magnético externo, as bem conhecidas órbitas skipping orbits são observadas. No entanto, nossos resultados demonstram que, no caso de um campo pseudo-magnético induzido por uma tensão não-uniforme, o espalhamento por uma borba armchair resulta em um estado de borda não-propagante. Nós também propomos um sistema de filtragem de vales muito eficiente através de um sistema de contato de ponto quântico definido por portas eletrostáticas em uma bicamada de grafeno. Para o sistema de bicamadas sugerido, nós investigamos a forma de melhorar a eficiência do sistema como um filtro de vales por diferentes parâmetros, como comprimento, largura e amplitude do potencial aplicado. Na segunda parte da tese, nós apresentamos um estudo sistemático dos espectros de energia de anéis quânticos de grafeno com diferentes geometrias e tipos de borda, na presença de um campo magnético perpendicular. Nós discutimos quais características obtidas por meio de um modelo simplificado de Dirac podem ser recuperadas quando os auto-estados de anéis quânticos de grafeno são comparados com os resultados do modelo tight-binding. Além disso, nós também investigamos os estados confinados em dois sistemas híbridos diferentes de monocamada - bicamada, identificando estados localizados dentro do ponto e estados de borda para as estruturas de confinamento em bicamadas sugeridas, assim como vamos estudar o comportamento dos níveis de energia em função do tamanho do ponto e sob um campo magnético externo aplicado. Finalmente, usando o modelo contínuo de Dirac de quatro bandas, nós também derivamos uma expressão geral para a condição de contorno de massa infinita em bicamada de grafeno, a fim de aplicar essa condição de contorno para calcular analiticamente os estados confinados e as correspondentes funções de onda em um ponto quântico em uma bicamada de grafeno na ausência e na presença de um campo magnético perpendicular. Nossos resultados analíticos apresentam boa concordância quando comparados com os resultados tight-binding.

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Kormoš, Lukáš. "Aplikace grafénové membrány v nanoelektronických zařízeních." Master's thesis, Vysoké učení technické v Brně. Fakulta strojního inženýrství, 2015. http://www.nusl.cz/ntk/nusl-231954.

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This diploma thesis is focused on the applications and fabrication of graphene membrane from graphene prepared by the chemical vapor deposition. Theoretical part deals with transport properties of the graphene and multiple scattering processes limiting the charge carrier mobility in this material. Included is short review of graphene membrane applications. Experimental part provides fabrication process for achieving suspended graphene device by utilizing electron beam lithography, focused ion beam, chemical etching and patterning of graphene. Graphene membrane is characterized by transport properties measurement and compared to non-suspended graphene.

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22

Carrega, Matteo. "Coulomb drag and Dirac plasmons in novel 2D electron systems." Doctoral thesis, Scuola Normale Superiore, 2014. http://hdl.handle.net/11384/85870.

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[from the introduction]: This Thesis focusses on the physics of e-e interactions in single-layer graphene and on the role of interlayer e-e interactions in vertical heterostructures comprised of two closely spaced graphene sheets.

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23

Mareček, David. "Vliv elektronového svazku na grafenové polem řízené tranzistory." Master's thesis, Vysoké učení technické v Brně. Fakulta strojního inženýrství, 2017. http://www.nusl.cz/ntk/nusl-320004.

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This diploma thesis deals with electrical conductivity of a graphene sample, preparation of a graphene field-effect transistor and his irradiation by electron beam. In the theoretical part of the thesis, we describe electronic properties of graphene, preparation of graphene by CVD and its transfer to Si substrate with SiO_2 layer. Experimental part of this thesis is focused on the preparation of a graphene field-effect transistor for use in UHV conditions. Futher describes electron beam scanning over the transistor and creation of current maps of tranzistor. In the last part, the thesis deals with influence of electron beam on transport properties of graphene layer and doping of graphene layer by electron beam.

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24

Guillemette, Jonathan. "Electronic transport in hydrogenated graphene." Thesis, McGill University, 2014. http://digitool.Library.McGill.CA:80/R/?func=dbin-jump-full&object_id=123041.

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The overarching goal of this thesis is to make use of adatoms to tune the electronic properties of a 2D atomic crystal. Specifically, covalently bonding hydrogen adatoms on a graphene sheet by creating CH_x with the aim of opening a band gap. This thesis reports the modification of the Raman spectra, the temperature dependent transport and the magnetoresistance as a result of hydrogenating graphene.Presented in this thesis is a method by which graphene is hydrogenated, the spatial map and density of defects from Raman spectroscopy and the temperature dependent resistance of a hydrogenated graphene sheet. The hydrogenation time was found to correlate poorly with the mean D/G ratio over an area of 200 by 200 um^2. Samples are observed to hydrogenate at different rates. Also presented are two potential technological applications of hydrogenated graphene: thermometers and bolometers. Thermometers with sensitivities up to 10^7 Omega/K at 10 K were measured as well as bolometers with responsivities up to 10^5 V/W at 10 K and a thermal resistance of R_th = 3 K/nW at 10 K. Measurements of the magnetic field dependent transport in both perpendicular and parallel field configurations have been shown to yield opposite magnetoresistance signs. The colossal negative magnetoresistance found in the perpendicular configuration led to the emergence of the nu = -2 quantum Hall state from an insulating state in the most disordered 2D system to date as measured by a Ioffe-Regel parameter of 250.
Le but sous-jacent de cette thèse est d'utiliser des adatomes sur un crystal atomique bi-dimensionnel afin d'en ajuster les propriétés électriques. Plus précisément, un lien covalent s'établira entre les atomes d'hydrogène et la feuille de graphène en créant CH_x afin d'ouvrir un interstice entre les bandes de valence et de conduction. En attachant les atomes d'hydrogène sur la feuille de graphène, son spectre Raman, son transport en fonction de la température et sa magnétorésistance ont tous étés modifiés. L'investigation de ces modifications est le thème central de cette thèse.Cette thèse contient: la méthode par laquelle le graphène est hydrogéné, l'analyse de l'hydrogénation en obtenant une cartographie Raman de l'ampleur des défauts et de leur emplacement sur l'échantillon. Également présent est l'analyse de l'hydrogénation du point de vue du transport électrique en fonction de la température. La durée de l'hydrogénation ne démontre pas une bonne corrélation avec le ratio D/G moyen sur une surface de 200 par 200 um^2 obtenu par spectroscopie Raman. De plus, il est démontré que les échantillons s'hydrogènent à des rythmes différents. Également présentés sont deux applications technologiques qui pourraient être potentiellement bonifiées en utilisant le graphène hydrogéné: les thermomètres et les bolomètres. Les thermomètres démontrent une sensibilité allant jusqu'à 10^7 Omega/K à 10 K et les bolomètres démontrent une responsivité allant jusqu'à 10^5 V/W à 10 K et une résistance thermique allant jusqu'à R_th = 3 K/nW, à 10 K. La magnétorésistance colossale négative mesurée dans la configuration perpendiculaire a mené à la mesure de l'effet Hall quantique dans le système le plus désordonné à ce jour en termes du paramètre Ioffe-Regel qui est de 250. La magnétorésistance colossale négative démontre la transition d'un état isolant à l'état de l'effet Hall quantique nu = -2.

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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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26

Rainis, Diego. "Electronic Transport in Graphene Hybrid Structures." Doctoral thesis, Scuola Normale Superiore, 2013. http://hdl.handle.net/11384/85932.

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Yang, Rui. "A study of electronic transport in graphene." Thesis, University of Manchester, 2009. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.506236.

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There has been a rapid growth of interest in graphene, a strictly two-dimensional material, since its discovery. The exceptionally high crystal and electronic quality of graphene makes it not only an excellent platform to study fundamental physics but also a promising material for future nanoelectronics in post-silicon era. The work presented In this thesis aims to probe into the electronic transport in graphene.

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Allen, Monica Theresa. "Quantum Electronic Transport in Mesoscopic Graphene Devices." Thesis, Harvard University, 2016. http://nrs.harvard.edu/urn-3:HUL.InstRepos:33493258.

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Graphene provides a rich platform for the study of interaction-induced broken symmetry states due to the presence of spin and sublattice symmetries that can be controllably broken with external electric and magnetic fields. At high magnetic fields and low temperatures, where quantum effects dominate, we map out the phase diagram of broken symmetry quantum Hall states in suspended bilayer graphene. Application of a perpendicular electric field breaks the sublattice (or layer) symmetry, allowing identification of distinct layer-polarized and canted antiferromagnetic v=0 states. At low fields, a new spontaneous broken-symmetry state emerges, which we explore using transport measurements. The large energy gaps associated with the v=0 state and electric field induced insulating states in bilayer graphene offer an opportunity for tunable bandgap engineering. We use local electrostatic gating to create quantum confined devices in graphene, including quantum point contacts and gate-defined quantum dots. The final part of this thesis focuses on proximity induced superconductivity in graphene Josephson junctions. We directly visualize current flow in a graphene Josephson junction using superconducting interferometry. The key to our approach involves reconstruction of the real-space current density from magnetic interference using Fourier methods. We observe that current is confined to the crystal boundaries near the Dirac point and that edge and bulk currents coexist at higher Fermi energies. These results are consistent with the existence of "fiber-optic" edge modes at the Dirac point, which we model theoretically. Our techniques also open the door to fast spatial imaging of current distributions along more complicated networks of domains in larger crystals.
Physics

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CASTILLO, E. DEL. "ELECTRONIC TRANSPORT IN GRAPHENE-BASED NANO JUNCTIONS." Doctoral thesis, Università degli Studi di Milano, 2016. http://hdl.handle.net/2434/372370.

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Aim of this thesis is the theoretical investigation of these aspects. We start addressing the band gap opening in graphene upon adsorption of transition metal atoms. Depending on the TM considered, we observe the opening of gaps with different widths and energies in the two spin components. Furthermore, the fact that some of these gaps comprise the Fermi level allows to speculate that the electron transport through these systems should display spin-dependent behavior. We then tackle the more complex topic of electron transport with particular attention to the spin dependent properties. In this case we have to deal with open, non periodic systems whose electronic properties cannot be easily obtained with standard methods. For the calculation of the charge transport we make use of the Non Equilibrium Green's Function approach, that provides a rigorous description of quantum transport allowing the self-consistent calculation of the charge density under a bias voltage. Within this framework, we investigate two different types of graphene junctions. The first type is made by a graphene sheet with transition metal atoms adsorbed in a regular array on a finite region. Our results show that with Fe adatoms the currents of the two spin components are dramatically different displaying a 100% polarization for all the biases considered because of a complete quenching of the minority current. Also for Ti we find a similar behavior but with an opposite polarization: in this case in fact we observe a damping of the majority current. Co adsorption induces a polarization analogous to that of Fe but less intense. The second system analyzed is a molecular magnetic junction where a Fe porphyrin molecule is connected with graphene electrodes. While in the pristine case no relevant effects can be pointed out, when the electrodes are doped with boron atoms a non negligible current polarization is observed. The doping with nitrogen atoms gives instead rise to a different effect, namely Negative Differential Resistance, with a reduction of the current for increasing voltages. Finally, we address the problem of the possible employment of this molecular junction in a gas sensor device investigating the changes induced in the charge transport by the adsorption of two gas molecules, O2 and CO. The only relevant effect observed is the quenching of the polarization in the B-doped system.

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Neumann, Ingmar. "Electronic spin transport and thermoelectric effects in graphene." Doctoral thesis, Universitat Autònoma de Barcelona, 2014. http://hdl.handle.net/10803/145396.

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La espintrónica y la espín caloritronica en grafeno son campos de investigación muy activos, y esta tesis es una contribución a ambos campos. El tema principal es el estudio de la corriente de espín a través de métodos de inyección y detección eléctrica en válvulas de espín no locales de grafeno. Preliminarmente, estudiamos analíticamente el efecto túnel de electrones de conducción entre materiales ferromagnéticos y no magnéticos. En la parte experimental, se investiga la precesión de espín en las válvulas de espín de grafeno en suspensión. En este contexto, hemos desarrollado un nuevo método para la fabricación de dispositivos de grafeno suspendido que, además, proporciona una inyección y detección de espín más eficiente. Con el fin de investigar estas corrientes de espín más eficientes, hemos realizado medidas en función de la corriente inyectada. Estos experimentos han dado lugar a la demonstración experimental de un termopar de espín en grafeno. La predicción teórica de distancias de relajación de espín de varias decenas de micrómetros en grafeno nos ha motivado a estudiar las propiedades intrínsecas de grafeno. Para ello el grafeno es suspendido libremente, eliminando así las influencia de substrato y permitiendo a posteriori emplear métodos de limpieza. Con el fin de lograr este objetivo, hemos desarrollado un método de fabricación de válvulas de espín no locales de grafeno suspendido libremente, que implica la utilización en el proceso de un número mínimo de pasos y productos químicos. Dado que en este método no se emplean ácidos, el rendimiento de estos dispositivos de alta calidad se mejora notablemente comparado con dispositivos elaborados con un proceso de fabricación estándar. Por lo tanto, nuestros dispositivos presentan una excelente movilidad, alcanzando valores de 20.000cm^2/(Vs) a temperatura ambiente. La detección eléctrica de la precesión de espín nos permite extraer la longitud de relajación en estos dispositivos, encontrando valores de pocos micrómetros. Hemos observado además una alta eficiencia tanto de inyección como en la detección de espín en nuestros dispositivos. Esta mejora es atribuida a la formación de una barrera de carbono amorfo inducida por haz de electrones en la interfaz grafeno/electrodo ferromagnético. Estas interfaces son estables incluso a altas densidades de corriente. Obtenemos una mejora 10000x de la señal de espín con respecto a los contactos óhmicos. La resistencia de contacto y acumulación de espín aumentada sugiere que la interfaz es una combinación de contacto óhmico y barrera túnel. La simplicidad y capacidad de transferencia del proceso de fabricación contrasta con la complejidad de obtener barreras aislantes convencionales utilizadas hoy en día en dispositivos espintrónicos. Por tanto, esperamos que las barreras de carbono amorfo sean una alternativa viable para mejorar tanto la eficiencia en la inyección como en la detección de corrientes de espín en otros materiales distintos a grafeno. Por último, hemos realizado medidas en estos dispositivos en función de la corriente inyectada, observando un fenómeno nuevo gracias a las propiedades particulares de grafeno como su movilidad dependiente de la energía. Se demuestra que un aumento anómalo de la acumulación de espín en el punto Dirac, que es causado por calentamiento en los inyectores. Debido a este contribución de grados mayores para la acumulación de espín , los potenciales electroquímicos de los sub bandas de espín presentan un comportamiento supra lineal en función de la corriente de bias. El desdoblamiento de espín se hace tan grande que en el punto Dirac se observa una gran cantidad de portadores de espín y carga opuestos. Demuestramos que este constituye un termopar de espín, donde el voltaje termoeléctrico entre el espín hacia arriba y hacia abajo aumenta la acumulación total de espín
Spintronics and spin caloritronics in graphene are recently very active fields of research, and this thesis is a contribution to both. The main topic is the study of spin currents in graphene non local spin valves via means of electrical spin injection and detection. In a preliminary work, we analytically investigate the tunneling process of conduction electrons between ferro- and non magnetic materials. On the experimental side, we report on spin precession in freely suspended graphene spin valves. In this context, we have developed a novel method for the fabrication of freely suspended graphene devices, which additionally is beneficial for the spin injection/detection efficiency of the devices. In order to investigate these enhanced spin signals, we have performed bias dependent measurements, which lead to the experimental demonstration of a spin thermocouple in graphene. In order to investigate tunneling of conduction electrons between ferro- and non magnetic electrodes, we have developed a theoretical model based on the analytical solution of the one-dimensional, time-independent Schrˆdinger equation. The model shows that a complex behavior of the polarization is intrinsic to the tunneling process of electrons between ferro- and non magnetic materials. Spin relaxations times of several tens of micrometers in graphene have been predicted. A promising approach to studying the intrinsic properties of graphene is to suspend the flakes, thus eliminating the influence of the substrate and enabling cleaning methods. In order to achieve this, we have developed a method to fabricate freely suspended graphene non local spin valves that involves a minimal number of steps and chemicals. Since the method is acid free, the yield of high quality, as-processed devices is notably improved when comparing to the standard fabrication process. Therefore, our as-processed devices exhibit excellent mobility, as high as 20000 cm^2/(Vs) at room temperature. We demonstrate electrical detection of spin precession, allowing us to extract the spin relaxation length in these devices, finding values of a few micrometers. We expect that by applying cleaning methods to freely suspended spin valves, it will be possible to investigate the origins of spin relaxation in intrinsic graphene. We have further observed enhanced spin injection/detection efficiency in our devices. We attribute the enhancement to the formation of an amorphous carbon layer at the interface between graphene and ferromagnet due to electron beam induced deposition. The interfaces are stable even for large applied bias current densities. We obtain a 10000x enhancement of the spin signal as compared to Ohmic contacts, but expect further increase after optimizing the deposition method. The increased contact resistance and spin accumulation suggests that the interface has a combination of Ohmic and tunneling properties. The simplicity and transferability of the fabrication process is in contrast to those of the conventional insulators used in spintronics. Therefore, we expect that amorphous carbon barriers are a viable alternative, which might improve the spin injection/detection efficiency in other materials as well. Finally, we have performed bias dependent measurements in our samples, observing a novel phenomenon which is due to the particular properties of graphene such as its energy dependent mobility. We demonstrate an anomalous enhancement of the spin accumulation at the Dirac point, which is caused by heating in the injector contacts. Because of this higher order contribution to the spin accumulation, the electrochemical potentials of the spin sub bands exhibit supralinear behavior as a function of the bias current. The spin splitting becomes so large that at the Dirac point we observe a huge quantity of carriers of opposite spin and charge. We show that this constitutes a spin thermocouple, where the thermoelectric voltage between spin up and spin down enhances the total spin accumulation.

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31

Wang, Zi. "Electronic structure and quantum transport in disordered graphene." Thesis, McGill University, 2011. http://digitool.Library.McGill.CA:80/R/?func=dbin-jump-full&object_id=104783.

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Graphene, a single sheet of graphite, has many interestingelectronic and mechanical properties, making it a viable candidate fortomorrow's electronics. It remains the most widely studied material in condensed matter physics as of2011. Due to various disorder effects, manyuseful properties of pristine graphene predicted by theory may notshow up in real world systems, and the exact effects of disorder on graphenenanoelectronics have not been investigated to any satisfaction.The research goal of this thesis is to provide first principles calculations to study disorder scattering in graphene nanostructures.We shall briefly review the basic concepts of electronicstructure theory of condensed matter physics, followed by a moredetailed discussion on density functional theory (DFT) which is themost widely applied atomistic theory of materials physics. We thenpresent the LMTO implementation of DFT specialized in calculatingsolid crystals. LMTO is computationally very efficient and isable to handle more than a few thousand atoms, while remaining reasonablyaccurate. These qualities make LMTO very useful for analysingquantum transport. We shall then discuss applying DFT within the Keldysh non-equilibrium Green's function formalism(NEGF) to handle non-equilibrium situations such as current flow. Finally, within NEGF-DFT, we shall use the coherentpotential approximation (CPA) and the non-equilibriumvertex correction (NVC) theory to carry out configurational disorder averaging. This theoretical framework is thenapplied to study quantum transport in graphene with atomisticdisorder. We shall investigate effects of substitutional boron (B)and nitrogen (N) doping in a graphene device connected to intrinsicgraphene electrodes. We have calculated quantum transport oftwo-probe graphene devices versus disorder concentration x, device length L, electron electron energy E, and our results suggest that doping greatlyaffects quantum transport properties by inducing significantdiffusive scattering.In particular, it is the first time inliterature that conductance versus doping concentration x isobtained from atomic first principles. Importantly, the NVC theoryallows us to directly determine the diffusive scatteringcontribution to the total conductance. Since B and Natoms are located on either side of carbon in the periodic table, avery interesting finding is that disorder scattering due to theseimpurities are mirrored almost perfectly on either side of the graphene Fermilevel. Such a behavior can be understood from the point of view ofcharge doping.
Le graphène, une seule feuille de graphite, a de nombreuse propriétés électroniques et mécaniques intéressantes, et ce qui en fait une solution viable pour l'électronique de demain. Il reste le matériau le plus largement étudié en physique de la matière condensée en 2011. En raison des effets du désordre, de nombreux propriétés utiles du graphène prédite par la théorie n'apparaissent pas dans les systèmes du monde réel, et les effets exacts du désordre dans le graphène n'ont pas été étudiées à toute satisfaction. L'objectif de cette thèse est de fournir une étude premiers principes de l'effet du désordre introduit dans des nanostructures de graphène. Nous allons passer brièvement en revue les concepts de base de la théorie électronique de la matière condensée, suivie par une discussion plus détaillée sur la théorie de la fonctionnelle de la densité (DFT) qui est la théorie atomique la plus couramment appliquée pour la physique matériaux. Nous allons ensuite présenter la méthode LMTO, des de la DFT, qui est spécialisée dans le calcul des cristaux solides. LMTO est mathématiquement très efficace et est en mesure de traiter plus de quelques milliers d'atomes, tout en restant raisonnablement précise. Ces qualités font que la méthode LMTO est très utile pour l'analyse du transport quantique. Nous discuterons ensuite l'application du DFT est dans le formalisme de la fonction non-équilibre de Green de Keldysh (NEGF) pour traiter les systèmes non-équilibre, tels que le courant de charge. Enfin, dans NEGF-DFT, nous allons utiliser l'approximation du potentiel cohérent (CPA) et la correction non-équilibre de vertex (NVC) afin d'appliquer la théorie de la moyenne du désordre de configuration. Ce cadre théorique est ensuite appliquée à l'étude du transport quantique dans le graphène avec du désordre atomique. Nous allons étudier les effets de la substitution du bore (B) et de l'azote (N) dans le graphène connecté aux électrodes de graphène pure. Nous avons calculé le transport quantique des dispositifs de graphène en fonction de la concentration du désordre x, longueur du dispositif L, l'énergie E, et nos résultats suggèrent que le dopage affecte grandement les propriétés de transport quantique en induisant diffusion de maniere significante. En particulier, ceci est la première fois que la conductance en fonction de la concentration du dopage x est obtenue à partir de théorie premiers principes atomiques. Il est important de noter que la théorie de la NVC nous permet de déterminer directement la contribution de la diffusion à la conductance totale. étant donné que les atomes B et N les atomes sont situés de chaque côté du carbone dans le tableau périodique, il est intéressant de constater que la diffusion du désordre due à ces impuretés apparait presque parfaitement de chaque côté du niveau de Fermi dans le graphène. Un tel comportement peut être compris du point de vue de la charge des dopants.

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32

Kaverzin, Alexey. "Electronic transport and flicker noise in graphene structures." Thesis, University of Exeter, 2011. http://hdl.handle.net/10036/3373.

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In this thesis the properties of graphene are studied via the various aspects of the quantum transport: doping of the graphene surface with organic molecules, flicker noise and transport in the quantum Hall regime. First, it was shown that certain molecules (toluene, aniline and water), which possess such common properties as non zero dipole moment and ability to undergo the electrochemical reaction, have a peculiar doping effect on graphene. The effect of toluene doping was studied in detail and is explained by the electrochemical reaction, which takes place in the vicinity of the graphene and results in a gate voltage dependent doping. Second, the flicker noise in graphene and its relation to the scattering mechanisms were studied. The flicker noise as a function of the carrier concentration was demonstrated to be sensitive to the scattering potential determining the resistance of the graphene. Therefore, as it was suggested, the flicker noise can be used as a tool for determining the dominant scattering mechanism in graphene, although it was found that the resistance and noise can originate from different scattering potentials. Also, the flicker noise spectrum was shown to decompose into individual lorentzians at low temperatures (below ∼ 25 K), where the fluctuations of the resistance is supposedly coming from the random jumps of electrons between the conductive channel in the graphene flake and the nearby impurity states. Third, the transport properties of the bilayer/trilayer graphene structure were studied at different temperatures and different magnetic fields including the quantum Hall regime. Bilayer and trilayer parts of the sample revealed the signatures of the quantum Hall effect predicted theoretically. The transport through the interface between bilayer and trilayer parts was also investigated. Signatures of the interface resistance were seen, although the observed behaviour is not explained. Under high magnetic fields the properties of the interface longitudinal resistance were described qualitatively by the classic transport equations.

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Cao, Yuan Ph D. Massachusetts Institute of Technology. "Electronic transport in low-angle twisted bilayer graphene." Thesis, Massachusetts Institute of Technology, 2016. http://hdl.handle.net/1721.1/105685.

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Thesis: S.M., Massachusetts Institute of Technology, Department of Electrical Engineering and Computer Science, 2016.
Cataloged from PDF version of thesis.
Includes bibliographical references (pages 47-48).
Graphene is a two-dimensional material with exotic electronic, optical and mechanical properties. By stacking two layers of graphene together with a small rotation angle between them, a superlattice of arbitrarily large size can be formed. The hybridization of the electronic states in the two layers can result in reduced Fermi velocity, van Hove singularities and a gapped band structure. In this work, a novel tear-and-stack technique is developed to reliably produce twisted bilayer graphene with controlled angle, and electronic transport measurements of the resulting high-quality samples are performed and discussed. We discover novel insulating states that purely results from the moiŕe superlattice band structure. The magnetotransport properties of these insulating states are studied and indicate that these states have different structure with those in either graphene or AB-stacked bilayer graphene; it shows a non-monotonous change of Fermi surface area which agrees with theoretical calculations. The results point toward a new pathway for graphene-related physics and material research.
by Yuan Cao.
S.M.

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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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Xu, Shu [Verfasser]. "Graphene Electronics : Device Fabrication and Electronic Transport / Shu Xu." Kiel : Universitätsbibliothek Kiel, 2012. http://d-nb.info/1020496436/34.

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Gorbachev, Roman. "Fabrication and transport properties of graphene-based nanostructures." Thesis, University of Exeter, 2009. http://hdl.handle.net/10036/89275.

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In this work fabrication and studies of transistor structures based on an atomic sheet of graphite, graphene, are described. Since graphene technology is in its early stages, the development and optimisation of the fabrication process are very important. In this work the impact of various fabrication conditions on the quality of graphene devices is investigated, in particular the effects on the carrier mobility of the details of the mechanical exfoliation procedure, such as environmental conditions and humidity, source of graphite and wafer cleaning procedure. In addition, a comparison is made between the conventional e-beam lithorgaphy and lithography-free fabrication of samples. It was also demonstrated that water and other environmental species play an important role in graphene-to-substrate adhesion and can also contribute to the carrier scattering in graphene. A technique for creating suspended metal gates was developed for the fabrication of graphene p-n-p structures, and charge transport has been studied in such top-gated graphene devices. Depending on the relation between the carrier mean free path and the length of the top-gate we have realized three distinct transport regimes through the p-n-p structure: a) diffusive across the structure; b) ballistic in the regions of p-n junctions but diffusive in the n-region; c) ballistic across the whole p-n-p structure. The second regime has revealed the chiral nature of carriers in graphene. This was demonstrated by comparing the experimental resistance of a single p-n junction with results of electrostatic modeling in the diffusive model. In the third regime we have observed oscillations of the device resistance as a function of carrier concentration in the n-region, which are also dependent on magnetic field. These oscillations have been demonstrated to be a direct consequence of a Fabri-Perot-like interference effect in the graphene p-n-p structures.

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Huang, Shengqiang, and Shengqiang Huang. "Electronic and Optical Properties of Twisted Bilayer Graphene." Diss., The University of Arizona, 2018. http://hdl.handle.net/10150/626686.

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The ability to isolate single atomic layers of van der Waals materials has led to renewed interest in the electronic and optical properties of these materials as they can be fundamentally different at the monolayer limit. Moreover, these 2D crystals can be assembled together layer by layer, with controllable sequence and orientation, to form artificial materials that exhibit new features that are not found in monolayers nor bulk. Twisted bilayer graphene is one such prototype system formed by two monolayer graphene layers placed on top of each other with a twist angle between their lattices, whose electronic band structure depends on the twist angle. This thesis presents the efforts to explore the electronic and optical properties of twisted bilayer graphene by Raman spectroscopy and scanning tunneling microscopy measurements. We first synthesize twisted bilayer graphene with various twist angles via chemical vapor deposition. Using a combination of scanning tunneling microscopy and Raman spectroscopy, the twist angles are determined. The strength of the Raman G peak is sensitive to the electronic band structure of twisted bilayer graphene and therefore we use this peak to monitor changes upon doping. Our results demonstrate the ability to modify the electronic and optical properties of twisted bilayer graphene with doping. We also fabricate twisted bilayer graphene by controllable stacking of two graphene monolayers with a dry transfer technique. For twist angles smaller than one degree, many body interactions play an important role. It requires eight electrons per moire unit cell to fill up each band instead of four electrons in the case of a larger twist angle. For twist angles smaller than 0.4 degree, a network of domain walls separating AB and BA stacking regions forms, which are predicted to host topologically protected helical states. Using scanning tunneling microscopy and spectroscopy, these states are confirmed to appear on the domain walls when inversion symmetry is broken with an external electric field. We observe a double-line profile of these states on the domain walls, only occurring when the AB and BA regions are gaped. These states give rise to channels that could transport charge in a dissipationless manner making twisted bilayer graphene a promising platform to realize controllable topological networks for future applications.

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Newman, Matthew. "Optimisation of the electronic properties of graphene devices." Thesis, University of Leeds, 2012. http://etheses.whiterose.ac.uk/3411/.

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The isolation of graphene has generated a great deal of excitement because of its unique properties. From a fundamental physics standpoint the most exciting aspect of the material is its electronic properties. One interesting method available to explore this electronic system is to investigate how the material interacts with superconductors. This interaction has been investigated by several groups via the production of superconductor-graphene superconductor devices, although their observed transport properties have been less than optimal. This thesis explores the factors which can limit the performance of these graphene devices. Suggestions are made regarding possible methods of improving device performance through the optimisation of the fabrication procedures. Graphene field effect transistors are produced using a combination of mechanical exfoliation, lithography and sputtering techniques. These devices are then characterised using a combination of transport and optical measurements. Two annealing methods are explored to reduce the concentration of charged impurities on the samples, using both an existing current annealing technique and a novel annealing technique using an on-chip platinum heater. Quantum Hall effect measurements are performed confirming the high quality of our graphene. Making poor contact to graphene is a possible performance limiter. The transfer length method is used to measure the contact resistance in our devices directly. A large contact resistance is observed, attributed to amorphisation of the underlying graphene by the sputtered material. This is confirmed using Raman spectroscopy. Asymmetry in the electric field measurements are also explained using an existing contact induced doping model. Extension of this model to include alternative doping profiles is shown to improve the fit to data. Measurements of the opto-electronic response of our graphene devices using scanning photocurrent microscopy supports the observation of contact induced doping and carrier density inhomogeneity in graphene devices which can limit device performance.

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RAVIKUMAR, ABHILASH. "Electronic, spin dependent conductive properties of modified graphene." Doctoral thesis, Università degli Studi di Milano-Bicocca, 2017. http://hdl.handle.net/10281/170813.

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Nella prima parte della ricerca che presentiamo abbiamo considerato l’eccitazione di stati elettronici profondi di molecole organiche adsorbite sul grafene. Per tali sistemi abbiamo dedotto l’induzione o la soppressione di un momento di dipolo magnetico relativo alla banda di valenza di molecole sulla scala temporale del femtosecondo. Abbiamo considerato tre molecole organiche, prototipi di diversi tipi di legame con la superficie: la Piridina, la cui interazione con il substrato di grafene è dovuta principalmente a forze di van der Waals, il radicale di Piridina che viceversa si lega alla superficie in maniera covalente e il radicale di Picolina, che rappresenta una situazione intermedia. In tutti e tre i sistemi abbiamo studiato le proprietà elettroniche sia dello stato fondamentale che di quello ottenuto eccitando lo stato 1s dell’atomo di azoto. Nel primo caso, mentre la molecola fisisorbita mostra uno stato fondamentale non-magnetico le simulazioni numeriche indicano che dopo l’eccitazione di un elettrone proveniente da un stato profondo i restanti elettroni di valenza rilassano in una configurazione polarizzata in spin. Il magnetismo indotto dipende dall’efficienza del trasferimento di carica dal grafene, sulla scala temporale del femtosecondo. Nel caso invece di una molecola chemisorbita, lo stato fondamentale del sistema è magnetico, in cui sono presenti due stati dipendenti dallo spin all’interno del gap di energia e localizzati sul sito di adsorbimento. Al contrario del caso precedente, l’eccitazione elettronica permette l’ibridazione del LUMO della molecola con gli stati del grafene all’interno del gap, risultando in una configurazione non-magnetica. Il passo successivo nella nostra analisi riguarda il legame tra il tempo di vita del trasferimento di carica in uno stato eccitato, creato a partire da livelli elettronici profondi di molecole adsorbite su grafene, e la modifica della struttura elettronica di tale interfaccia dovuta all’accoppiamento, di intensità variabile, con un substrato metallico. Abbiamo considerato la fotoemissione di un elettrone dallo stato 1s dell’azoto della molecola 1,10-bipiridina (C5H4N)2 adsorbita su un bilayer grafene/nickel(111) (BP/BLG/Ni) e su un substrato cresciuto per epitassia grafene/Ni(111) (BP/EG/Ni). Tramite simulazioni ab initio abbiamo osservato che il tempo caratteristico del trasferimento di carica durante il processo di eccitazione dipende fortemente dal tipo di interazione che si sviluppa tra il grafene ed il substrato di Ni sottostante. In entrambi i sistemi che abbiamo considerato, nello stato fondamentale il LUMO della molecola è fortemente accoppiato con la superficie. Nel caso del sistema BP/BLG/Ni, lo strato di grafene in contatto con il nickel è fortemente ibridizzato con il metallo, mentre lo strato superiore di grafene rimane sostanzialmente disaccoppiato. Il livello eccitato LUMO* della molecola ha la possibilità di ibridizzarsi con pochi livelli di grafene, intorno al punto di Dirac all’energia di Fermi. Per questo motivo il tempo di vita dello stato eccitato cresce significativamente (∼ 116 fs). Invece nel caso del sistema BP/EG/Ni la forte ibridizzazione del grafene con il sottostante substrato di nickel ne distorce significativamente la struttura elettronica, creando degli stati in prossimità del livello di Fermi. Questi livelli si possono accoppiare con il LUMO* della molecola, risultando in un tempo di vita sostanzialmente ridotto (∼ 33 fs). Abbiamo cercato delle conferme ai nostri risultati tramite misure sperimentali basate sul metodo della spettroscopia core-hole-clock. Il tempo caratteristico del trasferimento di carica che abbiamo ricavato è di ∼ 30 fs±5 fs per il sistema BP/BLG/Ni e ∼ 4 fs±1 fs per quello BP/EG/Ni. Questi risultati verificano le nostre previsioni teoriche, dimostrando l’effetto del substrato sulla dinamica del trasferimento di carica.
The first part of research we present is the adsorption of core-excited organic molecules on graphene. We predict the induction or suppression of magnetism in the valence shell of physisorbed and chemisorbed organic molecules on graphene occurring on the femtosecond time scale as a result of core level excitations. We consider three organic molecules: Pyridine - whose interaction with graphene is mainly facilitated by van der Waals forces, Picoline radical - an intermediate case where there is a strong van der Waals interaction of the pyridine π ring with graphene but a covalent bonding of the molecule and pyri-dine radical - where the interaction is mainly by covalent bonding, and study the ground state and N 1s core excited state electronic properties for these systems. For physisorbed molecules, where the interaction with graphene is dominated by van der Waals forces and the system is non-magnetic in the ground state, numeri- cal simulations based on density functional theory show that the valence electrons relax towards a spin polarized configuration upon excitation of a core-level electron. The magnetism depends on efficient electron transfer from graphene on the femtosecond time scale. On the other hand, when graphene is covalently functionalized, the system is magnetic in the ground state showing two spin dependent midgap states localized around the adsorption site. At variance with the physisorbed case upon core-level excitation, the LUMO of the molecule and the mid gap states of graphene hybridize and the relaxed valence shell is not magnetic anymore. Next we discuss the interplay between the charge transfer lifetime of core excited organic molecules adsorbed on graphene and the modification of its electronic structure by a variable coupling with a metal substrate. Nitrogen 1s core electron of 1,10- bipyridine (C5H4N)2 is photoexcited and adsorbed on bilayer graphene/nickel(111) (BP/BLG/Ni) and epitaxially grown graphene/Ni(111) (BP/EG/ Ni). We predict from first principle calculations that the charge transfer time of core excited molecules depend strongly on the coupling of graphene to the underlying Ni substrate. In the ground state, the LUMO of the molecule is quite strongly coupled with the substrate in both the cases (BP/BLG/Ni and BP/EG/Ni). In the case of BP/BLG/Ni, the layer of graphene in contact with nickel substrate strongly hybridizes but the upper layer of graphene remains fairly decoupled. The excited molecular LUMO* finds very few states of graphene close to the Dirac point at the Fermi level to hybridize with. This leads to a decoupled molecular LUMO* and the lifetime increases significantly (∼ 116 fs). But in the case of BP/EG/Ni, the strong hybridization of graphene with the underlying nickel substrate significantly distorts the electronic structure of graphene generating states close to the Fermi level. The LUMO* of the molecule strongly couples with these states resulting in a substantially smaller lifetime (∼ 33 fs). We also find experimental evidence to confirm this trend by performing core-hole-clock spectroscopy. The resonant charge transfer lifetime we find is ∼ 30 fs±5 fs for the BP/BLG/Ni and ∼ 4 fs±1 fs for the BP/EG/Ni, thus clearly demonstrating the effect of substrate on the charge transfer dynamics of organic molecules on graphene.

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Benjamin, Daniel. "Thermal transport and photo-induced charge transport in graphene." Diss., Georgia Institute of Technology, 2011. http://hdl.handle.net/1853/42746.

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The electronic material graphene has attracted much attention for its unique physical properties such as, linear band structure, high electron mobility, and room temperature ballistic conduction. The possibilities for device applications utilizing graphene show great variety, from transistors for computing to chemical sensors. Yet, there are still several basic physical properties such as thermal conductivity that need to be determined accurately. This work examines the thermal properties of graphene grown by the chemical vapor deposition technique. The thermoelectric power of graphene is studied in ambient and vacuum environments and is shown to be highly sensitive to surface charge doping. Exploiting this effect, we study the change in thermoelectric power due to introduction of gaseous species. The temperature dependent thermal conductivity of graphene is measured using a comparison method. We show that the major contribution to the thermal conductivity is the scattering of in-plane phonons. Graphene also shows promise as an optoelectronic material. We probe the Landau level structure of graphene in high magnetic fields using a differential photoconductivity technique. Using this method we observed the lifting of spin and valley degeneracies of the lowest Landau level in graphene.

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41

Khodkov, Tymofiy. "Probing the electrical properties of multilayer graphene." Thesis, University of Exeter, 2012. http://hdl.handle.net/10036/4352.

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Graphene is a new two-dimensional (2D) material with unique electrical transport, optical and mechanical properties. However, monolayer graphene (MLG) is a gapless semiconductor, which limits its relevance for transistor applications where a large on/off ratio of the current is required. In this work the investigation of transport properties of few-layer graphene (FLG) is presented. These 2D electronic systems offer a novel solution to the problem concerned the absence of an energy gap in single layer graphene, since they exhibit an electric field and stacking-dependent band gap in the energy dispersion. Thus far, a clear observation of a band-gap in multilayer graphene (e.g. Bernal-stacked bilayers) in transport measurements was hindered by the presence of disorder. Here we develop a reliable and effective method of fabrication of high-quality suspended double-gated graphene devices, which are of crucial importance for probing the low energy dispersion of few-layer graphene. The current annealing technique, described in details, improves transport characteristics like carrier mobility, which is typically higher than ∼ 104 cm2/Vs for our multilayer devices. Electrical transport experiments on suspended dual-gated ABC-stacked trilayer are performed. We report the direct evidence of the opening of a tunable band-gap with an external perpendicular electric field, ranging from 0 meV up to 5.2 meV for an electric field of 117 mV/nm. Thermally activated transport is observed in these samples over the temperature range 0.5 - 80 K. The values of energy gap extracted from both temperature dependence of minimum conductivity measurements and non-linear I –V characteristics correlate well. Our experimental results are in a good agreement with theoretical approximation, based on self-consistent tight-binding calculations. The high quality of our ABC trilayer samples is also demonstrated by a particularly high on/off ratio of the current (250 at applied electrical displacement as low as 80 mV/nm), which makes these devices promising for future semiconductor electronics. FLG samples with reduced disorder allow us to observe quantum Hall effect (QHE) at magnetic field as low as 500 mT. We present the first study of electric field- induced new QH states in ABC trilayer graphene (TLG). The transitions between spin-polarized and valley polarized phases of the sample at the charge neutrality point are investigated. Resolved novel broken symmetry states along with observed Lifshitz transition in rhombohedral TLG display exciting phenomena attributed to rich physics in these interactive electronic systems.

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42

Ancum, Gerard Klaas van. "Electronic transport properties of PrBa2Cu3O7-d." Enschede : University of Twente [Host], 1996. http://doc.utwente.nl/58674.

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43

Taychatanapat, Thiti. "From Hopping to Ballistic Transport in Graphene-Based Electronic Devices." Thesis, Harvard University, 2013. http://dissertations.umi.com/gsas.harvard:10815.

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This thesis describes electronic transport experiments in graphene from the hopping to the ballistic regime. The first experiment studies dual-gated bilayer graphene devices. By applying an electric field with these dual gates, we can open a band gap in bilayer graphene and observe an increase in resistance of over six orders of magnitude as well as a strongly non-linear behavior in the transport characteristics. A temperature-dependence study of resistance at large electric field at the charge neutrality point shows the change in the transport mechanism from a hopping dominated regime at low temperature to a diffusive regime at high temperature.
Physics

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44

Joung, Daeha. "Electronic Transport Investigation of Chemically Derived Reduced Graphene Oxide Sheets." Doctoral diss., University of Central Florida, 2012. http://digital.library.ucf.edu/cdm/ref/collection/ETD/id/5332.

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Reduced graphene oxide (RGO) sheet, a chemically functionalized atomically thin carbon sheet, provides a convenient pathway for producing large quantities of graphene via solution processing. The easy processibility of RGO sheet and its composites offer interesting electronic, chemical and mechanical properties that are currently being explored for advanced electronics and energy based materials. However, a clear understanding of electron transport properties of RGO sheet is lacking which is of great significance for determining its potential application. In this dissertation, I demonstrate fabrication of high-yield solution based graphene field effects transistor (FET) using AC dielectrophoresis (DEP) and investigate the detailed electronic transport properties of the fabricated devices. The majority of the devices show ambipolar FET properties at room temperature. However, the mobility values are found to be lower than pristine graphene due to a large amount of residual defects in RGO sheets. I calculate the density of these defects by analyzing the low temperature (295 to 77K) charge transport data using space charge limited conduction (SCLC) with exponential trap distribution. At very low temperature (down to 4.2 K), I observe Coulomb blockade (CB) and Efros-Shklovskii variable range hopping (ES VRH) conduction in RGO implying that RGO can be considered as a graphene quantum dots (GQD) array, where graphene domains act like QDs while oxidized domains behave like tunnel barriers between QDs. This was further confirmed by studying RGO sheets of varying carbon sp2 fraction from 55 – 80 % and found that both the localization length and CB can be tuned. From the localization length and using confinement effect, we estimate tunable band gap of RGO sheets with varying carbon sp2 fraction. I then studied one dimensional RGO nanoribbon (RGONR) and found ES VRH and CB models are also applicable to the RGONR. However, in contrast to linear behavior of decrease in threshold voltage (Vt) with increasing temperature (T) in the RGO, sub linear dependence of Vt on T was observed in RGONR due to reduced transport pathways. Finally, I demonstrate synthesis and transport studies of RGO/nanoparticles (CdS and CeO2) composite and show that the properties of RGO can be further tuned by attaching the nanoparticles.
Ph.D.
Doctorate
Physics
Sciences
Physics

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45

Yu, Geliang. "Transport properties of graphene based van der Waals heterostructures." Thesis, University of Manchester, 2015. https://www.research.manchester.ac.uk/portal/en/theses/transport-properties-of-graphene-based-van-der-waals-heterostructures(5cbb782f-4d49-42da-a05e-15b26606e263).html.

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In the past few years, led by graphene, a large variety of two dimensional (2D) materials have been discovered to exhibit astonishing properties. By assembling 2D materials with different designs, we are able to construct novel artificial van der Waals (vdW) heterostructures to explore new fundamental physics and potential applications for future technology. This thesis describes several novel vdW heterostructures and their fundamental properties. At the beginning, the basic properties of some 2D materials and assembled vdW heterostructures are introduced, together with the fabrication procedure and transport measurement setups. Then the graphene based capacitors on hBN (hexagonal Boron Nitride) substrate are studied, where quantum capacitance measurements are applied to determine the density of states and many body effects. Meanwhile, quantum capacitance measurement is also used to search for alternative substrates to hBN which allow graphene to exhibit micrometer-scale ballistic transport. We found that graphene placed on top of MoS2 and TaS2 show comparable mobilities up to 60,000cm2/Vs. After that, the graphene/hBN superlattices are studied. With a Hall bar structure based on the superlattices, we find that new Dirac minibands appear away from the main Dirac cone with pronounced peaks in the resistivity and are accompanied by reversal of the Hall effects. With the capacitive structure based on the superlattices, quantum capacitance measurement is used to directly probe the density states in the graphene/hBN superlattices, and we observe a clear replica spectrum, the Hofstadter-butterfly fan diagram, together with the suppression of quantum Hall Ferromagnetism. In the final part, we report on the existence of the valley current in the graphene/hBN superlattice structure. The topological current originating from graphene’s two valleys flows in opposite directions due to the broken inversion symmetry in the graphene/hBN superlattice, meaning an open band gap in graphene.

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46

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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47

Haberer-Gehrmann, Danny. "Electronic Properties of Functionalized Graphene Studied With Photoemission Spectroscopy." Doctoral thesis, Saechsische Landesbibliothek- Staats- und Universitaetsbibliothek Dresden, 2012. http://nbn-resolving.de/urn:nbn:de:bsz:14-qucosa-97417.

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Graphene, a two dimensional single layer of graphite, attracts a lot of attention of researchers around the globe due to its remarkable physical properties and application potential. The origin can thereby be found in the peculiar electronic structure since graphene is a zero gap semi-conductor with a linear energy dispersion in the vicinity of the Fermi level. Consequently, the charge carriers in graphene mimic massless Dirac Fermions which brings principles of quantum electrodynamics and exotic effects like Klein tunneling into a bench-top experiment. Modifying the electronic and/or crystal structure structure by functionalization might therefore as well lead to new tantalizing physical properties, novel compound materials based on graphene like graphane (fully hydrogenated graphene) or flourographene (fluorinated graphene), and ultimately new applications. In this work, the influences on the electronic structure of graphene are investigated with photoemission spectroscopies after covalent functionalization by atomic hydrogen and ionic functionalization with potassium. Regarding hydrogenation, the formation of tunable bandgap is observed along with a full recovery of the electronic properties of graphene upon removing the hydrogen by thermal annealing. Using high resolution x-ray photoemission and molecular dynamics simulations, the formation of a C4H structure is predicted for substrate supported graphene at a saturation H-coverage of 25%, due to a preferential para- arrangement of hydrogen atoms. In fully electron doped, hydrogenated graphene the formation of dispersionless hydrogen impurity state is observed with angle-resolved photoemission spectroscopy. This flat state is extended over the whole Brillouin zone and according to calculations not localized. Potassium-doped graphene shows a similar doping level as its 3D parent component, the graphite intercalation compound KC8. Investigating the electron-phonon coupling in doped graphene, by direct derivation of the Eliashberg-function, shows an asymmetric coupling strength along the high-symmetry directions in the Brillouin Zone of graphene. In the K-M direction additional low energetic contributions could be identified which may originate from out-of-plane phonon modes. Regarding the electron-phonon-coupling strength of the high energy in-plane phonon modes a reasonable agreement with theoretical predictions is found.

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48

Samarakoon, Duminda K. "Structural, electronic, and magnetic properties of graphene-based nanomaterials." DigitalCommons@Robert W. Woodruff Library, Atlanta University Center, 2013. http://digitalcommons.auctr.edu/dissertations/708.

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The binding of radical groups such as hydrogen, hydroxyl, epoxide, or fluorine to the graphene surface, forms covalent bonds and transforms the trigonal sp2 orbital to the tetragonal sp3 orbital. Such a transformation drastically modifies electronic properties, which leads to the opening of a bandgap through the removal of the bands near the Fermi level of the pristine graphene. We have investigated the structural, electronic, magnetic, and vibrational properties of functionalized graphene based on first-principles densityfunctional calculations. A twist-boat conformation is identified as the energetically most favorable nonmetallic configuration for fully oxidized graphene. The calculated Raman G-band blue shift is in very good agreement with experimental observations. A detailed analysis of fluorographene membranes indicates that there exist prominent chair and stirrup conformations, which correlate with the experimentally observed in-plane lattice expansion contrary to a contraction in graphane. The optical response of fluorographene is investigated using the GW-Bethe-Salpeter equation approach. The results are in good conformity with the experimentally observed optical gap and reveal predominant chargetransfer excitations arising from strong electron-hole interactions. The appearance of bounded excitons in the ultraviolet region can result in an excitonic Bose-Einstein condensate in fluorographene. Hydrogenated epitaxial graphene has distinctive electronic properties compared to the two-sided hydrogenated graphene. The stability of a given hydrogenation pattern is strongly influenced by the amount of sp2-hybridized bonding in the structure. A trigonal planar networked hydrogenation pattern is identified as an intrinsic ferromagnetic semiconductor, which is in good conformity with experimental observations. The electronic structure of graphite and rotational-stacked multilayer epitaxial graphene as a function of the applied electric bias is investigated using dispersion-corrected density-functional theory. The tailoring of electronic band structure correlates with the interlayer coupling tuned by the applied bias. The implications of controllable electronic structure of rotationally fault-stacked epitaxial graphene grown on the C-face of SiC for future device applications are discussed. We have also investigated the electronic properties of fully hydrogenated boron-nitride (BN) layer and zigzag-edged nanoribbons using dispersion-corrected density-functional calculations. Among various low-energy hydrogenated membranes referred to as chair, boat, twist-boat, and stirrup, the stirrup conformation is the most energetically favorable one. The zigzag-edged BN nanoribbon, prominently fabricated in experiments, possesses intrinsic half-metallicity with full hydrogenation. The half-metallicity can be tuned by applying a transverse electric bias, thereby providing a promising route for spintronics device applications.

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49

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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50

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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Dissertations / Theses on the topic 'Electronic Transport Properties -Graphene'

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