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Nair, Rahul Raveendran. "Atomic structure and properties of graphene and novel graphene derivatives." Thesis, University of Manchester, 2010. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.527419.
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Pierce, James Kevin. "Magnetic structure of chiral graphene nanoribbons." Thesis, University of British Columbia, 2016. http://hdl.handle.net/2429/57782.
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We study the magnetic structure of narrow graphene ribbons with patterned edges. Neglecting interactions, a broad class of edge terminations support zero-energy states localized at the edges of the ribbon. For the simplest (zigzag) ribbon supporting these edge states, electron-electron interactions have been shown to induce ferromagnetic ordering along the edges of the ribbon. We generalize this argument for such a magnetic edge state to carbon ribbons with more complex chiral edge terminations.Science, Faculty of
Physics and Astronomy, Department of
Graduate
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Pradhan, Siddharth. "Quantification of Graphene Oxide Structure Using an Improved Model." University of Cincinnati / OhioLINK, 2012. http://rave.ohiolink.edu/etdc/view?acc_num=ucin1342730902.
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Wang, Jun, and 王俊. "Optical properties of graphene/GaN hybrid structure." Thesis, The University of Hong Kong (Pokfulam, Hong Kong), 2014. http://hdl.handle.net/10722/206660.
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Optical properties of graphene/GaN hybrid structure were investigated by using a variety of optical spectroscopy techniques including low-temperature photoluminescence (PL) spectroscopy, time-resolved PL (TRPL) spectroscopy, confocal scanning micro-Raman spectroscopy.
Single-layer graphene grown by chemical vapor deposition was transferred to GaN epilayer surface, which is verified by the Raman spectrum with a sharp characteristic peak at ~2690 cm-1and a homogeneous Raman image. Three main band-edge emissions including the free exciton A transition (denoted as FXA), the donor bound exciton transition (denoted as DX) and the third peak (denoted as Ix) were well resolved in the PL spectra of the hybrid structure as well as the as-grown GaN epilayer at low temperatures. Interestingly, the FXA transition and Ix line of the GaN epilayer were found to be dramatically altered by the top graphene layer while the DX is almost unaffected. The intensity of Ix line substantially drops after the transfer of graphene layer on GaN, indicating surface defect nature of the Ix line. More interestingly, an unpredictable dip structure develops in the FXA peak when the temperature is beyond 50 K. Similar spectral structure change also occurred in the emission of free exciton B (referred as FXB)with higher transition energy .A free exciton dissociation and electron transfer model was proposed to explain the “dip effect”. More supporting evidence to the model was found in the time-resolved PL spectra of the hybrid structure and the control sample. The results showed the significant influence of graphene monolayer on the fundamental optical properties of GaN.published_or_final_version
Physics
Master
Master of Philosophy
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Pandey, Priyanka A. "Structure and applications of chemically modified graphene." Thesis, University of Warwick, 2012. http://wrap.warwick.ac.uk/55111/.
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Owing to its extraordinary electrical, optical, and mechanical properties, graphene has emerged as a promising material for a variety of applications in the future. However, not all these applications will be able to employ or require pristine graphene; hence several alternative methods have developed for the mass production of graphene and related materials. Graphene oxide (GO), a material closely related to graphene, allows engineering of its chemical composition by means of chemical, thermal, and electrochemical methods. This provides an opportunity to tune physical and chemical properties of graphene. This work reports on investigations of the structure of chemically modified graphenes (CMGs) derived from GO, interactions of metals and organic thin films with CMG, and application of metal-CMG as a hydrogen gas sensor. GO was fabricated by a modified Hummers method. GO, being insulating, was reduced by hydrazine and thermal annealing to produce reduced graphene oxide (rGO). The CMG sheets were deposited on TEM grids and on Si/SiO2 substrates for characterization by atomic force microscopy, transmission electron microscopy (TEM), xray photoelectron spectroscopy, and Raman spectroscopy. The structural analysis of GO performed by TEM revealed that in GO, on average, the underlying carbon lattice maintains the symmetry and lattice-spacings of graphene. Compositional analysis disclosed that the as-produced GO is actually made of oxidized graphene like sheets strongly attached with oxidative debris that make the as produced GO hydrophilic and insulating. In the TEM, both GO and reduced GO (rGO) were nearly transparent and stable under the electron beam and hence they made excellent supports to study the growth of thin organic and metal films deposited by physical vapour deposition. The study revealed the interactions of organic molecules, fluorinated copper phthalocyanine, with CMG and packing of the molecules in the crystal structure. Film-thicknesses from sub-monolayer to tens of monolayers were analysed. In the study of metal thin film growth, the factors determining the growth and morphology of different metals-on-CMG were studied. Fine control over the size and coverage of nanoparticles were achieved. This control was used to combine Pd nanoparticles and rGO to design selective, highly sensitive, and practical hydrogen gas sensor.
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Thomas, Helen R. "The structure and reactivity of graphene oxide." Thesis, University of Warwick, 2015. http://wrap.warwick.ac.uk/74090/.
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Graphene oxide (GO) can provide a cost-effective route to a graphene-like material on an industrial scale, but produces an imperfect product. In order to improve the quality of the resultant graphene, unanswered questions regarding the structure and chemical reactivity of GO need to be addressed. In this thesis, chapters 1 and 2 serve to introduce the field of graphene and graphene oxide research, as well as standard characterisation techniques. Chapter 3 is concerned with investigating the validity and general applicability of a recently proposed two-component model of GO – the formation of the two components was shown to be largely independent of the oxidation protocol used in the synthesis, and additional characterisation data was presented for both base-washed graphene oxide (bwGO) and oxidation debris (OD). The removal of the OD cleans the GO, revealing its true mono-layer nature and in the process increases the C:O ratio, i.e. a deoxygenation. By contrast, treating GO with hydrazine was found to both remove the debris and reduce (cleaning and deoxygenation) the graphene-like sheets. In chapter 4, different nucleophiles were used to explore bwGO functionalisation via epoxy ring-opening reactions. Treatment of bwGO with potassium thioacetate, followed by an aqueous work-up, was shown to yield a new thiol functionalised material (GO-SH). As far as is known, this was the first reported example of using a sulfur nucleophile to ring open epoxy groups on GO. The incorporation of malononitrile groups, and the direct grafting of polymer chains to the graphene-like sheets was also demonstrated. The thiol groups on GO-SH are amendable to further chemistry and in chapter 5 this reactivity is exploited with alkylation, thiol-ene click and sultone ring-opening reactions. Au(I) and Pd(II) metallo-organic complexes were also prepared, and gold deposition experiments were carried out, demonstrating that GO-SH has a strong affinity for AuNPs. These CMGs have varying solubility and improved thermal stability. Chapter 6 concludes the work covered in this thesis, and full experimental details can be found in chapter 7.
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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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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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Mohd, Halit Muhammad Khairulanwar Bin. "Processing, structure and properties of polyamide 6/graphene nanoplatelets nanocomposites." Thesis, University of Manchester, 2018. https://www.research.manchester.ac.uk/portal/en/theses/processing-structure-and-properties-of-polyamide-6graphene-nanoplatelets-nanocomposites(e879fdef-d5d4-4797-a865-58b61cb257d1).html.
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Graphene Nanoplatelets (GNP) was incorporated into polyamide 6 (PA6) matrix by melt compounding method and the enhancements in the properties of the nanocomposites were studied. Response Surface Methodology (RSM) was employed to assist in the study of processing conditions in melt compounding. RSM analysis revealed that the GNP concentrations to be the most significant term to affect the tensile modulus and crystallinity followed by the screw speed whereas the residence time was found to be non-significant. GNP with 5 Î1⁄4m (G5) and 25 Î1⁄4m (G25) were used in the GNP aspect ratio study. The average flake size of G5 and G25 to was measured to be 5.07 Î1⁄4m and 22.0 Î1⁄4m, respectively with the G5 distributed narrowly whereas the G25 exhibit broad distribution. TGA analysis shown that HT25 is more thermally stable compared to G25 due to some remnants lost during thermal treatment and this was confirmed by EDX and CHNS analysis. XRD profiles of the PA6-G-NC illustrate typical peaks of PA6 crystals phase as well as pure graphite characteristic peak. PA6-G25-NC observed to exhibit slightly higher peak intensity compared to PA6-G5-NC suggesting more formation of PA6 crystals. Similar improvement was observed on PA6-HT25-NC compared to PA6-G25-NC indicating more formation of PA6 crystals due improved dispersion of HT25. DSC on PA6-G25-NC showed higher cooling temperature and crystallinity compared to PA6-G5-NC due to larger surface area of the G25. Similarly, PA6-HT25 showed better improvement in crystallinity over PA6-G25-NC due to increase nucleation sites by the HT25. The thermal conductivity of PA6-G25-NC is slightly higher than the thermal conductivity of PA6-G5-NC but not significant considering the G25 is 5 times larger than G5. Instead, no significant difference was observed between PA6-HT25-NC and PA6-G25-NC. Addition of GNP increased the thermal stability of the PA6-G-NC systems under both nitrogen and air atmospheres regardless of the GNP aspect ratio. The viscoelastic properties showed insignificant difference between PA6-G5-NC and PA6-G25-NC. The inefficient improvement by G25 might be due to agglomeration formed during processing. The storage modulus and tan Î ́ of PA6-HT25-NC decreased but the Tg significantly improved compared to PA6-G25-NC. This was assumed to be because of improved dispersion of HT25 but reduced interfacial interaction after the heat treatment. The shear storage modulus, Gâ and complex viscosity, |η*| were observed to increase with increasing GNP content with more pronounced improvement seen on PA6-G25-NC compared to PA6-G5-NC. However, no network percolation threshold was observed until 20 wt.% of GNP. The poor interfacial interaction of HT25 resulted in lower Gâ and |η*| compared to G25. Tensile test results showed typical improvement with PA6-G25-NC having higher tensile modulus compared to PA6-G5-NC. Further enhancement was obtained with PA6-HT25-NC suggesting improved dispersion and volume of constrained chains mobility despite the poor surface interaction. Comparison with Halphin-Tsai modulus revealed that the effective modulus to be 150 GPa for G5 and 200 GPa for G25. The water uptake measurement results showed that GNP reduced the water uptake percentage and diffusion coefficient especially with G25. The test conducted on saturated PA6-G-NC results in improved thermal conductivity due to the high thermal conductivity of water but the viscoelastic and tensile properties severely reduced due to plasticisation effect.
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Xian, Lede. "Electronic structure and interlayer coupling in twisted multilayer graphene." Diss., Georgia Institute of Technology, 2014. http://hdl.handle.net/1853/51811.
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It has been shown recently that high-quality epitaxial graphene (EPG) can be grown on the SiC substrate that exhibits interesting physical properties and has great advantages for varies device applications. In particular, the multilayer graphene films grown on the C-face show rotational disorder. It is expected that the twisted layers exhibit unique new physics that is distinct from that of either single layer graphene or graphite. In this work, by combining density functional and tight-binding model calculations, we investigate the electric field and doping effects on twisted bilayer graphene (TBG), multiple layer effects on twisted triple-layer graphene, and wave packet propagation properties of TBG. Though these studies, we obtain a comprehensive description of the interesting interlayer interaction in this twisted multilayer graphene system.
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FIORI, SARA. "N-doped Graphene on Ni: growth, structure and reactivity." Doctoral thesis, Università degli Studi di Trieste, 2021. http://hdl.handle.net/11368/2982138.
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The experimental work presented in this PhD thesis fits into the fields of gas sensors, gas storage and catalysis under-cover. In particular, this study focuses on the characterization of the growth, structure and reactivity upon carbon monoxide exposure of pristine and nitrogen-doped graphene (Gr) on Ni(111), by means of Scanning Tunneling Microscopy (STM), X-ray Photoemission Spectroscopy (XPS), supported by Low-Energy Electron Diffraction (LEED). The experimental results were corroborated by theoretical calculations in collaboration with the University of Milano-Bicocca and the University of Trieste.
Gr is widely studied for a variety of applications and its already fascinating properties can be tuned by introducing doping centers in the honeycomb network. For this aim, nitrogen is one of the most promising dopants, being predicted to improve Gr performances.
However, the production of nitrogen-doped graphene (N-Gr) is not trivial. Several approaches were reported in literature which, however, many times do not result in reproducible and high-quality Gr layers.
In the first part of this thesis, we present an alternative and reproducible growth method, which ensures the formation of N-Gr layers of high morphological quality.
For this purpose, we exploited a Ni substrate, a low-cost and widely available metal. Its high catalytic activity is particularly suitable for production of high-quality Gr layers via standard chemical vapor deposition (CVD) which, combined with its capability to dissolve/segregate N into the bulk/surface, allows to easily obtain N-Gr sheets. Morphological and chemical characterization by STM and XPS shows that the process yields a flat and continuous N-Gr layer. Experimental results are complemented by a Density Functional Theory (DFT) investigation of possible structural models, to identify at the atomic scale the various N configurations in the Gr mesh. This joint approach allowed unveiling the structural, morphological and chemical properties of N dopants trapped in the network, found mainly in two configurations: graphitic N defects, where an N atom substitutes a C atom in the mesh and bonds to three neighboring C atoms, and 3N pyridinic defects, where three N are placed at the edge of a C vacancy and each of them bonds to two C atoms as part of a six-membered ring.
The second part of this PhD thesis is dedicated to the investigation of the reactivity of N-Gr on Ni in comparison to pristine Gr. In particular, we focus on the reactivity towards carbon monoxide (CO), one of the simplest molecules in nature, but also potentially dangerous and lethal.
We confirmed that, in the near-ambient pressure regime and at room temperature, CO intercalates at the interface between Gr and Ni, detaching the Gr layer from its substrate.
We demonstrated that the same behavior occurs for N-Gr exposed to CO, but at a pressure one order of magnitude lower with respect to the pristine case, thus pointing out an enhancement of the reactivity of the layer in presence of N dopants. By means of LEED, XPS and STM measurements, we present a full chemical and morphological characterization of the pristine and N doped Gr surfaces after CO exposure.
We finally rationalize the intercalation mechanism itself, not trivial for impermeable materials like Gr. Combining an experimental (STM, XPS, LEED) and theoretical (DFT) study, we described how CO molecules permeate the Gr layer, getting into the confined zone between Gr and Ni. Suitably large defects allow CO reaching the interface and the presence of N pyridinic dopants at their edges is found to stabilize the multiatomic vacancy and facilitate the permeation process, reducing the CO threshold pressure by more than one order of magnitude.
Summarizing, the alternative N-Gr growth method we developed is potentially scalable and suitable for the production of high-performance nano-devices, with crucial implications for Gr-based gas sensors and storage devices.
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Huder, Loïc. "Lien entre structure et propriétés électroniques des moirés de graphène étudié par microscopie à effet tunnel." Thesis, Université Grenoble Alpes (ComUE), 2017. http://www.theses.fr/2017GREAY083/document.
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Les dernières années ont vu l'avènement des couches cristallines bidimensionnelles, appelées matériaux 2D. L'exemple le plus connu est le graphène, d'autres étant le nitrure de bore hexagonal isolant et le diséléniure de niobium supraconducteur. Ces matériaux 2D peuvent être empilés de manière contrôlée sous la forme d'hétérostructures de van der Waals pour obtenir les propriétés électroniques désirées. L’une des plus simples hétérostructures de van der Waals est l'empilement de deux couches de graphène tournées. Cet empilement donne naissance à un moiré qui peut être vu comme un potentiel superpériodique dépendant de l'angle entre les deux couches. Les propriétés électroniques des couches tournées de graphène sont intimement liées à ce moiré.Le sujet de cette thèse est l'étude expérimentale du lien entre la structure et les propriétés électroniques des couches tournées de graphène par Microscopie et Spectroscopie à effet tunnel à basse température.Alors que l'effet de l'angle entre les couches sur les propriétés électroniques a déjà été étudié en détail, la modification de celles-ci par une déformation des couches n'a été envisagée que récemment. La première partie de ce travail expérimental étudie la modification par la déformation des propriétés électroniques de couches de graphène tournées d'un angle de 1.26° crûes sur carbure de silicium. La déformation en question est différente dans les deux couches et son effet apparait clairement dans la densité locale d'états électroniques du moiré. Contrairement à une déformation appliquée identiquement aux deux couches, une différence de déformations entre les couches (déformation relative) modifie fortement la structure de bandes même à faibles valeurs de déformations. Alors que la déformation relative était spontanément présente, la deuxième partie de cette thèse s'intéresse à l'effet d'une déformation appliquée directement aux couches de graphène. Cette déformation vient d'une interaction induite par l'approche de la pointe STM vers la surface de graphène. La modification active de la densité d'états qui en résulte dépend de la position de la pointe dans le moiré avec l'apparition d'instabilités périodiques lorsque la distance entre la pointe et l'échantillon est très faible.La troisième partie de cette thèse concerne l'étude d'un autre type de modification des propriétés électroniques consistant en l'induction de supraconductivité dans les couches de graphène. Cette modification est effectuée par une croissance du graphène en une seule étape sur du carbure de tantale supraconducteur. Les résultats montrent la formation d'une couche de carbure de tantale de grande qualité sur laquelle les couches de graphène forment des moirés. La mesure à basse température de la densité d'états de ces moirés montre la présence d'un effet de proximité supraconducteur induit par le carbure de tantaleRecent years have seen the emergence of two-dimensional crystalline layers, called 2D materials. Examples include the well-known graphene, insulating hexagonal boron nitride and superconducting niobium diselenide. The stacking of these 2D materials can be controlled to achieve desirable electronic properties under the form of van der Waals heterostructures. One of the simplest van der Waals heterostructures is the misaligned stacking of two graphene layers. Twisted graphene layers show a moiré pattern which can be viewed as a superperiodic potential that depends on the twist angle. The electronic properties of the twisted graphene layers are strongly linked to this moiré pattern.The subject of the present thesis is the experimental study of the link between the structural and the electronic properties of twisted graphene layers by means of low-temperature Scanning Tunneling Microscopy and Spectroscopy (STM/STS).While the effect of the twist angle has already been studied in great details, the modulation of the electronic properties by the deformation of the layers has been explored only recently. In the first part of this experimental work, a strain-driven modification of the electronic properties is probed in graphene layers with a twist angle of 1.26° grown on silicon carbide. The determined strain is found to be different in the two layers leading to a clear signature in the local electronic density of states of the moiré even at low strain magnitudes. Contrary to a strain applied in the two layers, this difference of strain between the layers (relative strain) modifies strongly the electronic band structure even at low strain magnitudes. While this relative strain is natively present, the second part of the work explores the effect of an applied strain in the layers. This is realized by approaching the STM tip to the graphene surface to trigger an interaction between the two. The resulting active modification of the density of states is shown to depend on the position on the moiré, leading to periodic instabilities at very low tip-sample distances.In the third part of the work, another type of modification of the electronic properties is studied when superconductivity was induced in the graphene layers. This is done by growing graphene on superconducting tantalum carbide in a single-step annealing. The results show the formation of a high-quality tantalum carbide layer on which graphene layers form moiré patterns. The low-temperature density of states of these moirés show evidence of a superconducting proximity effect induced by the tantalum carbide
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Jean, Fabien. "Growth and structure of graphene on metal and growth of organized nanostructures on top." Thesis, Université Grenoble Alpes (ComUE), 2015. http://www.theses.fr/2015GREAY097/document.
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Le graphène, une monocouche de graphite, est composé d'atomes de carbone avec une structure en nid d'abeilles. Ses propriétés exceptionnelles ont attiré un intérêt mondial, dont le Prix Nobel de Physique en 2010. Le graphène épitaxié sur métal à rapidement été identifié comme un moyen de production de graphène de haute qualité de taille métrique, et est le sujet d'intenses activités de recherche en sciences de surface pour caractériser ses propriétés. En outre, ces études concernent aussi des systèmes plus complexes avec pour base le graphène, par exemple les réseaux ordonnés de nanoparticules à sa surface. Tout cela a mené à l'étude de la croissance, de la structure et des défauts du graphène épitaxié avec un grande variété de techniques expériementales, tel que la microscopie par effet tunnel, spectroscopie par photo-émission résolue en angle ou encore la microscopie électronique à basse énergie. Ce travail de recherche se concentre sur le graphène obtenu par croissance sur la surface (111) d'un monocristal d'iridium dans des conditions d'ultra vide et étudié avec plusieurs techniques de mesure par diffraction (diffraction de surface des rayons X, diffraction des rayons X en incidence rasante, réflectivité des rayons X et diffraction des électrons à haute énergie en réflexion). Ces expériences ont été faites au synchrotron européen ESRF à Grenoble, en France. La première partie de cette étude a été de déterminer la structure du graphène à l'échelle atomique. Le système montre une tendance à la commensurabilité, mais sa structure précise dépend fortement des conditions de préparation et de la température appliqué au système. En outre, en combinant des techniques de diffraction à haute résolution, une caractérisation précise de la structure, qui fait débat dans la littérature, est dévoilée. Le système étudié présente aussi une surperstructure, typique du graphène épitaxié, nommé moiré pour ses similarités avec l'effet optique du même nom. Celle-ci est utilisée comme gabarit pour faire croître des nanoparticules monodisperses à la surface en réseau auto-organisé. Durant cette étude, trois types de nanoparticules ont été examinés, des particules de platine de deux tailles différentes et des particules composées de platine et de cobalt. Ces systèmes hybrides présentent un fort degré d'organisation, partiellement hérité de la superstructure du moiré. Les nanoparticules forme une interaction forte avec leur support et elles subissent des contraintes de surface causées par leurs petites tailles. Par ailleurs, les nanoparticules de platine-cobalt, dont la croissance est en deux étapes, gardent une structure en couche et non une structure d'alliage métalliqueGraphene, a monolayer of graphite, is composed of carbon atoms arranged in a honeycomb lattice. Its exceptional properties have attracted a worldwide interest, including the Novel Prize in Physics in 2010. Epitaxial graphene on a metal was rapidly identified as an efficient method for large-area production of high quality graphene, and also was the matter of intense activities exploiting surface science approaches to address the various properties of graphene and of advanced systems based on graphene, for instance ordered lattice of metal nanoparticles on graphene. This resulted in the study of growth, structure and defects of epitaxial graphene on a wide variety of substrates with various techniques such as scanning tunneling microscopy, angle-resolved photoemission spectroscopy or low-energy electron microscopy. This work focuses on graphene grown on the (111) surface of iridium in ultra-high vacuum conditions and studied with several diffraction techniques (surface X-ray diffraction, grazing incidence X-ray diffraction, X-ray reflectivity, and reflection-high energy electron diffraction). These experiments were performed at the European Synchrotron Radiation Facility in Grenoble, France. The first step in our study was to determine the structure of graphene at the atomic scale. The system was found to have a tendency to commensurability, but that the precise structure depends on temperature and on preparation conditions. Moreover, with the combination of high resolution diffraction techniques, a precise characterization about the debated structure of graphene perpendicular to the surface was unveiled. The system, exhibits a superstructure, typical of epitaxial graphene, called a moiré, as an equivalent of the moiré effect in optics. This is used as a template to grown nanoparticles on top of the system to achieve the self-organisation of monodisperse nanoparticles. In this study, three type of nanoparticles were investigated, two different size of pure platinum ones and bimetallic ones, platinum and cobalt. These hybrid systems show very high degree of order, partly inherited by the superstructure lattice. The nanoparticles were found to strongly bond to their support, experience substantial surface strain related to their small size, and that bimetallic ones grown in a sequential manner retain a chemically layered structure
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Zan, Recep. "Microscopy and spectroscopy of graphene : atomic scale structure and interaction with foreign atom species." Thesis, University of Manchester, 2013. https://www.research.manchester.ac.uk/portal/en/theses/microscopy-and-spectroscopy-of-graphene-atomic-scale-structure-and-interaction-with-foreign-atom-species(7ff75244-c81a-414f-88d2-e614297f0390).html.
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Since its discovery, the one atom thick material graphene has been at the centre of growing interest in two-dimensional materials. Due to its exceptional properties, graphene is a rich topic to explore by physicists, chemists, engineers and materials scientists. In addition to its use in the fundamental research, graphene is also a promising candidate for future electronics, photonics and energy storage devices.The project presented in this thesis was carried out to explore the structure of suspended graphene in particular in order to probe the metal-graphene interaction via Transmission Electron Microscopy, as most graphene applications require interfacing with metals. As the work was based on free standing graphene, graphene layers obtained by mechanical cleavage or growth on a substrate were transferred onto TEM-grids. Therefore, fabrication, suspended sample preparation and identification of graphene layers were first discussed for a better understanding of how to obtain high quality graphene, as this was essential for the rest of the project.Structural, topographic and chemical analysis of pristine suspended graphene layers were investigated in detail via Transmission Electron Microscopy and Scanning Tunnelling Microscopy. The latter technique was also employed for graphene on a substrate along with establishing annealing conditions for residue free graphene.Metal deposited suspended graphene layers were then investigated in the electron microscopes. Different metal behaviours were observed on the graphene surfaces for the same amount of metal evaporation. Generally, metals interact only weakly with graphene as they are not observed on clean (residue free) parts and are mainly clustered. On the other hand, graphene etching has been observed in the presence of metals. The etching was initiated with graphene vacancy formation as a result of the interaction between metal and carbon atoms on clean graphene. Once a vacancy was created, a hole quickly formed and eventually the graphene layers were destroyed. However, those holes created by metals were healed spontaneously either by non-hexagonal or perfect hexagonal rings. The possible etching and healing mechanisms of the suspended graphene were also discussed.
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Faizy, Namarvar Omid. "Structure électronique et transport quantique dans les nanostructures de Graphène." Phd thesis, Université de Grenoble, 2012. http://tel.archives-ouvertes.fr/tel-00870405.
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Le graphène est un matériau constitué d'une seule couche atomique de carbone et représente un sujet majeur de la physique de la matière condensée. Le graphène possède de nombreuses propriétés remarquables : structure électronique décrite par une equation de Dirac sans masse, forte mobilité électronique, effet Hall quantique anormal, résistance ,rigidité et conductivité thermique élevée. Cette these concerne la structure électronique et le transport dans le graphène. Nous considérons en particulier le cas des bicouches tournées de graphène. Ces systèmes ont été découverts en particulier dans le graphène produit sur le carbure de silicium et présentent des propriétés originales par rapport aux bicouches dans l' empilement AB qui existe par exemple dans le graphite. Nous analysons au moyen d'une théorie perturbative et aussi par des approches numériques la densité d'états dans ces systèmes.Nous montrons que la densité d'états présente des oscillations avec la même période que celle du Moiré produit par ces bicouches. Nous analysons aussi le rôle des défauts sur les propriétés de transport en particulier dans le cas ou les défauts sont répartis uniquement sur une des deux couches. Ici aussi notre approche combine théorie perturbative du couplage interplans et approches purement numérique en liaisons fortes. Nous considérons aussi le role joué par les adatomes comme l'hydrogène par exemple. Nous analysons la modification de la densité d'états induite autour de l'adatome et les variations correspondantes de densité de charge et de potentiel électrostatique. Ces systèmes tendent à produire des états resonants près de l'énergie de Dirac qui dependent beaucoup aussi de la position top ou hollow de l' adsorbat. Pour des orbitales de type "s" la resonance est plus marquée si l'adatome est en position hollow. Nous montrons que l'image par experience STM (microscopie à effet tunnel) depend beaucoup de la distance entre l'adsorbat et la pointe du STM. Dans un régime de champ proche la résonance de l'adsorbat peut même apparaître comme un creux dans le signal dI/dV du STM.
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Wang, S. Q. "Car-Parrinello Molecular Dynamics of Nanosized Graphene Sheets." Thesis, Sumy State University, 2013. http://essuir.sumdu.edu.ua/handle/123456789/35242.
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Car-Parronello molecular dynamics simulations of twelve nanosized graphene sheets with a dozen to a hundred carbon atoms are performed using a mixed Gaussian and planewave approach within the frame-work of the density-functional theory. Two different origins for the rippled structure of graphene are found: the thermodynamic vibration of atoms and the local lattice defect. We suggest that the lattice defect, which changes the local atomic bonding state, should be responsible for the intrinsic ripples in graphene sheet.
When you are citing the document, use the following link http://essuir.sumdu.edu.ua/handle/123456789/35242
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Srivastava, Nishtha. "Interface Structure of Graphene on SiC for Various Preparation Conditions." Research Showcase @ CMU, 2012. http://repository.cmu.edu/dissertations/90.
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In this thesis we study the preparation dependence of the interface structure of graphene on SiC. We compare epitaxial graphene grown in ultra-high vacuum (UHV), in an atmosphere of argon, and in a background of 10-4 Torr of disilane. Graphene growth is studied on both the polar faces of SiC – the SiC(0001) surface, also known as the Si-face and the SiC(0001 ) surface, also known as the C-face. We find that the quality of graphene and the interface of graphene on the substrate depend on which face of SiC is used, and also what environment it was prepared in. Characterization using atomic force microscopy (AFM), low energy diffraction (LEED) and low energy electron microscopy (LEEM) reveals that on the C-face the interface structure prior to graphitization sensitively depends on the preparation conditions. On the Si-face the interface structure prior to graphitization does not change for any of the three environments, however the quality of the graphene formed shows an improvement when prepared in disilane or argon compared to UHV.
When SiC is heated in vacuum the Si atoms preferentially sublimate leaving behind C atoms that rearrange to form graphene. In our prior work we found that compared to the Si-face graphitization of the C-face in vacuum results in thicker graphene films with a larger distribution of thicknesses. A reason for this could be the different nature of the graphene-substrate interface in the two cases. On the Si-face graphene growth is mediated through an interface layer that displays a 6√3×6√3-R30 LEED pattern (this interface layer is also known as the "buffer layer") which acts as a template for graphene growth, while on the C-face no such buffer layer is formed. This buffer layer on the Si-face is known to consist of basically a graphene monolayer, but with some of the carbon atoms bonded to the underlying SiC.
Graphene produced on SiC in vacuum conditions is quite inhomogeneous with small domain sizes and (we refer to an area with a constant thickness of multilayer graphene as a "domain") and numerous pits. On the Si-face it has been found by many research groups including our own that graphitization in an atmosphere of argon results in large monolayer domains with an elimination of pits. The argon decreases the Si sublimation rate, thus increasing the temperature required for graphene formation. The higher graphitization temperature results in an improved morphology of the graphene film. Some researchers use a background of disilane instead of an atmosphere of argon, and in this thesis we report our results for graphitization of SiC in a disilane environment. On the Si-face we find an improvement in the morphology of disilane prepared graphene films compared to those prepared in vacuum, consistent with other researchers. In terms of the interface structure prior to graphitization no difference was found for graphene produced in UHV, argon or disilane. For all three environments the LEED pattern from the interface prior to graphitization displayed 6√3 x 6√3-R30 (6√3 for short) symmetry.
In an attempt to controllably form thin layers of graphene on the C-face we previously tried graphitizing in an atmosphere of argon, however that led to inhomogeneous islands of thick graphene forming over the surface. It was found that due to an unintentional oxidation of the surface during graphitization, the surface became resistant to graphitization. In this thesis we present results for graphitization of the C-face in a background of disilane, which to our knowledge has not been attempted before. We are able to form graphene films that are thin and uniform relative to those prepared in vacuum or argon. We demonstrate that by graphitizing in a background of disilane we avoid the unintentional oxidation that inhibits graphene formation on the C-face in argon.
For C-face samples prepared in disilane, prior to graphitization we observe a √43×√43±7.6° (√43 for short) in situ LEED pattern that has never been observed in vacuum prepared samples. This √43 pattern is found to disappear after air exposure. The ex situ LEEM reflectivity curves of such a disilane prepared sample show unique features not seen in any vacuum prepared sample. By analyzing the LEED pattern and the LEEM reflectivity curves we associate the unusual reflectivity curves we observe on the C-face with a buffer layer, analogous to the 63 layer that form on the Si-face. This buffer layer has the 43 symmetry due to bonding to the underlying SiC, but upon air exposure these bonds are broken (due to oxidation of the SiC) and the layer becomes "decoupled" from the SiC. This decoupling of the buffer layer on the C-face is analogous to what occurs upon oxidation or hydrogenation of the 63 layer on the Si-face. We believe that the √43 layer does not form in vacuum prepared samples due to kinetic limitations but is able to form in Si-rich environments (such as disilane or furnace grown graphene) as the graphitization takes place with the Si sublimation rate closer to equilibrium. The schematic shown below summarizes the differences between Si-face and C-face SiC/graphene interface structures, depending on preparation conditions (vacuum or Si-rich). The right most figure shows the main result of this work, which puts graphene formation on the C-face on a similar footing as for the Si-face since the "buffer" layer provides a template for the graphene growth.
A separate project discussed in this thesis is scanning tunneling microscopy/spectroscopy (STM/S) on epitaxial graphene on the Si-face. Two different studies were performed. In the first study we performed STM/S on a Si-face graphene sample in which a large fraction of the area was covered by a secondary disordered phase. The disordered phase showed a graphene-like spectrum with additional features that could arise from dangling bonds or defects. On the basis of additional data from AFM and Auger electron spectroscopy we argue that this secondary phase is similar to the nanocrystalline graphite (NCG) phase that we observe on C-face samples. In the second study we performed STS on a graphene sample that was functionalized by hydrogen. Functionalizing graphene changes the nature of its bonding and can open up a band gap in it. Our STS results indicate that no band gap opened up in the graphene, however we found the presence of additional states in the spectra that indicate the nature of the bonds in graphene had changed due to the hydrogen functionalization.
In the last study of this thesis we perform LEEM on graphene samples prepared on Cu foil. The samples were made in a chemical vapor deposition (CVD) chamber under different growth conditions. LEEM was used to measure the reflectivity curves and perform selected area diffraction on the samples. The reflectivity curves allowed us to determine the graphene thickness, and the selected area diffraction allowed us to determine the orientation of the graphene and whether it was single-crystal or not.
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Mazaleyrat, Estelle. "Croissance, structure et propriétés électroniques du graphène épitaxié sur rhénium, vers une plateforme bidimensionnelle et supraconductrice." Thesis, Université Grenoble Alpes (ComUE), 2019. http://www.theses.fr/2019GREAY079.
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La réalisation de structures hybrides à base de graphène, dans lesquelles le graphène est associé à d'autres matériaux, constitue une piste prometteuse pour l'étude de nombreux phénomènes. En particulier, il est possible de cette façon d'induire des propriétés dans le graphène via des effets de proximité. Ici, le système cible que nous avons considéré consiste en une plateforme de graphène quasi-flottant, au caractère supraconducteur induit, et qui est placée à proximité d'impuretés magnétiques. A la lecture d'articles théoriques parus récemment, il semble qu'un tel échantillon pourrait présenter des états de Yu-Shiba-Rusinov (YSR) non conventionnels.Bien que le système cible n'ait pas encore été fabriqué, les trois ingrédients nécessaires à sa réalisation (graphène quasi-flottant, caractère supraconducteur induit et proximité à des impuretés magnétiques) ont été abordés, et ce à l'aide d'outils de la science des surfaces.Comme cela a pu être démontré précédemment, le graphène peut être rendu supraconducteur lorsqu'il est crû directement sur un matériau supraconducteur tel que le rhénium. Des aspects structuraux liés au graphène crû sur Re(0001) ont été explorés. En particulier, nous avons montré qu'augmenter le nombre de cycles de recuit contribue positivement à la croissance de domaines de graphène étendus et de bonne qualité. La structure d'un carbure de surface du rhénium, habituellement mal comprise, a également fait l'objet d'une étude.De plus, nous avons examiné un défaut présent dans le graphène crû sur des métaux interagissant fortement, tels que le Re(0001) et le Ru(0001). Dans la structure ondulée à l'échelle nanométrique du graphène, ce défaut apparaît sous la forme d'une dépression. Sa présence a été attribuée à des défauts d'empilement se trouvant soit dans le graphène, soit dans le substrat métallique.En prenant le graphène supraconducteur crû sur Re(0001) comme point de départ pour la fabrication de notre système cible, nous avons recouvré le caractère quasi-flottant du graphène (perdu à cause de sa forte interaction avec le substrat de rhénium) via l'intercalation d'une sub-monocouche ou de quelques couches d'atomes d'or. La présence d'une forte densité de défauts, observée dans le graphène sur Re(0001) intercalé à l'or, a été attribuée au processus d'intercalation lui-même. Par ailleurs, nous avons démontré que le caractère supraconducteur du graphène, induit par le rhénium, n'est pas affecté par l'intercalation d'or. A ce stade, deux des trois conditions prévues pour la réalisation du système cible étaient remplies.A condition d'amener des impuretés magnétiques à proximité immédiate d'un tel échantillon, des états de YSR étendus sur plusieurs nanomètres devraient être observables. Des résultats préliminaires impliquant deux composés magnétiques de type verdazyl ont été présentés. L'un de ces deux composés fut déposé sur un système modèle : le Cu(111). Avant de considérer l'usage du graphène quasi-flottant et supraconducteur comme substrat-hôte de ces composés magnétiques, des études complémentaires sur des systèmes modèles sont nécessaires. Et pour cause, nous n'avons pas encore réussi à résoudre la structure exacte des assemblées moléculaires observées sur la surface de Cu(111) ; la stabilité thermique de ces composés a été mise en causeThe realization of graphene-based hybrid structures, where graphene is associated with other materials, offers a promising avenue for testing a variety of phenomena. In particular, one can induce properties in graphene by proximity effects. Here, the targeted graphene-based system consists of a quasi free-standing graphene platform with induced superconducting character and in close vicinity to magnetic impurities. According to recent theoretical articles, such a sample could exhibit unconventional Yu-Shiba-Rusinov (YSR) states.Although the targeted graphene-based system was not fabricated yet, we have addressed, with the help of surface science tools, all three ingredients required for its realization (quasi-free standing graphene, induced superconducting character and proximity to magnetic impurities).As previously demonstrated, graphene can be rendered superconducting by growing it directly on top of a superconducting material such as rhenium. Structural aspects related to graphene grown on Re(0001) were investigated. In particular, we showed that increasing the number of annealing cycles positively contributes to growing high-quality extended graphene domains. The structure of a surface rhenium carbide, which constitutes a usually ill-characterized object, was studied as well.Additionnally, a defect appearing as a depression in the nanorippled structure of graphene on strongly interacting metals such as Re(0001) and Ru(0001) was investigated and ascribed to stacking faults either in graphene or in the metal substrate.Using superconducting graphene grown on Re(0001) as a starting point for the fabrication of the targeted graphene-based system, we recovered the quasi free-standing character of graphene (lost due to its strong interaction with the rhenium substrate) via intercalation of sub-monolayer to few layers of gold atoms. A high density of defects observed in gold-intercalated graphene on Re(0001) was attributed to the intercalation process itself. Besides, we demonstrated that the rhenium-induced superconducting character in graphene was not affected by gold intercalation. At this point, two of the three requirements for realizing the targeted graphene-based system were fulfilled.Provided that we bring magnetic impurities in close proximity to such a sample, few-nanometers extended YSR states could be observed. Preliminary results involving two original magnetic verdazyl compounds were presented, one of which was deposited on a model system, namely Cu(111). Before turning to quasi-free standing superconducting graphene as a hosting material for these magnetic compounds, further investigations on model systems are needed. Indeed, we could not resolve the precise structure of the molecular assemblies covering the Cu(111) surface yet, and the thermal stability of the compounds was discussed
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Halbig, Christian Eberhard [Verfasser]. "Fundamental Aspects on the Formation, Structure and Functionalisation of oxo-functionalised Graphene and thereout derived Graphene / Christian Eberhard Halbig." Berlin : Freie Universität Berlin, 2019. http://d-nb.info/1181788684/34.
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Gillen, Roland. "Band structure and defect calculations within a screened-exchange hybrid functional scheme." Thesis, University of Cambridge, 2013. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.608268.
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Le, Nam B. "Structure-Interaction Effects In Novel Nanostructured Materials." Scholar Commons, 2016. http://scholarcommons.usf.edu/etd/6296.
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Recent advances in experimental and computational methods have opened up new directions in graphene fundamental studies. In addition to understanding the basic properties of this material and its quasi-one dimensional structures, significant efforts are devoted to describing their long ranged dispersive interactions. Other two-dimensional materials, such as silicene, germanene, and transition metal dichalcogenides, are also being investigated aiming at finding complementary to graphene systems with other "wonder" properties. The focus of this work is to utilize first principles simulations methods to build our basic knowledge of structure-interaction relations in two-dimensional materials and design their properties. In particular, mechanical folding and extended defects in zigzag and armchair graphene nanoribbons can be used to modulate their electronic and spin polarization characteristics and achieve different stacking patterns. Our simulations concerning zigzag silicene nanoribbons show width-dependent antiferromagnetic-ferromagnetic transitions unlike the case of zigzag graphene nanoribbons, which are always antiferromagnetic. Heterostructures, build by stacking graphene, silicene, and MoS$_2$, are also investigated. It is found that hybridization alters the electronic properties of the individual layers and new flexural and breathing phonon modes display unique behaviors in the heterostructure compositions. Anchored to SiC substrate graphene nanoribbons are also proposed as possible systems to be used in graphene electronics. Our findings are of importance not only for fundamental science, but they could also be used for future experimental developments.
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Gong, Chuncheng. "Atomic structure and dynamics study of defects in graphene by aberration-corrected transmission electron microscope." Thesis, University of Oxford, 2016. https://ora.ox.ac.uk/objects/uuid:53bd9a04-71ad-4da8-b982-cb45a005e791.
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Graphene has grabbed enormous research attention due to its multiple unique properties. These properties, however, can be strongly influenced by lattice imperfections. Aberration corrected transmission electron microscopy (AC-TEM) is one of the leading methods to image two-dimensional materials at the atomic level. This thesis addresses the issue of structure and dynamics characterization of dislocations and grain boundaries (GBs) in graphene with single atom sensitivity using the state-of-the-art AC-TEM in Department of Materials, University of Oxford. My first goal is to understand the interaction between dislocation and the edge of graphene. When a dislocation is located near an edge, a decrease in the rippling and increase of the in-plane rotation occurs relative to the dislocations in the bulk. The increased in-plane rotation near the edge causes bond rotations at the edge of graphene to reduce the overall strain in the system. Dislocations are highly stable and remain fixed in their position even when located within a few lattice spacings from the graphene edge. With the aid of an in situ heating holder, the high temperature behavior of dislocations is then investigated. Control of temperature enables the differentiation of electron beam induced effects and thermally driven processes. An analysis of the dislocation movement shows both climb and glide processes, including new complex pathways for migration and large nanoscale rapid jumps between fixed positions in the lattice. The improved understanding of the high temperature dislocation movement provides insights into annealing processes in graphene and the behavior of defects with increased heat. The in situ heterogeneous nucleation and growth of graphene are also studied within the AC-TEM. The growth mechanism consists of alternating carbon cluster attachment and indentation filling to maintain a uniform growth front of lowest energy. The highly polycrystalline graphene seed is found to evolve with time into a higher order crystalline structure. The motion of GBs is discontinuous and mediated by both bond rotation and atom evaporation. These results provide insights into the formation of crystalline seed domains that are generated during bottom-up graphene synthesis. Finally, the formation, reconfiguration and annihilation of GB loops are demonstrated. It is shown that the GB loop cannot fully relaxed under electron beam irradiation with its terminal state being isolated dislocations far apart from each other. Line defects composed of several adjacent excess-atom defects can be found during the reconfiguration process. This work gives detailed information about the stability and behavior of large GB loops in two dimensional materials.
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Sharma, Nikhil. "Microscopic and spectroscopic studies of growth and electronic structure of epitaxial graphene." Diss., Georgia Institute of Technology, 2009. http://hdl.handle.net/1853/33844.
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It is generally believed that the Si technology is going to hit a road block soon. Amongst all the potential candidates, graphene shows the most promise as replacement material for the aging Si technology. This has caused a tremendous stir in the scientific community. This excitement stems from the fact that graphene exhibits unique electronic properties. Physically, it is a two-dimensional network of sp₂bonded carbon atoms. The unique symmetry of two equivalent sublattices gives rise to a linear energy dispersion for the charge carriers. As a consequence, the charge carriers behave like massless Dirac particles with a constant speed of c/300, where c is the speed of light. The sublattice symmetry gives rise to unique half-integer quantum hall effect, Klein's paradox, and weak antilocalization.
In this research work, I was able to successfully study the growth and electronic structure of EG on SiC(0001), in ultra-high vacuum and low-vacuum furnace environment. I used STM to study the growth at an atomic scale and macroscopic scale. With STM imaging, I studied the distinct properties of commonly observed interface region (layer 0), first graphene layer, and the second graphene layer. I was able to clearly resolve graphene lattice in both layer 1 and 2. High resolution imaging of the defects showed a unique scattering pattern. Raman spectroscopy measurements were done to resolve the layer dependent signatures of EG. The characteristic Raman 2D peak was found to be suppressed in layer 1, and a single Lorentzian was seen in layer 2. Ni metal islands were grown on EG by e-beam deposition. STM/ STS measurements were done to study the changes in doping and the electronic structure of EG with distance from the metal islands.
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Wang, Zegao, Pingjian Li, Yuanfu Chen, Jiarui He, Wanli Zhang, Oliver G. Schmidt, and Yanrong Li. "Pure thiophene–sulfur doped reduced graphene oxide: synthesis, structure, and electrical properties." Royal Society of Chemistry, 2014. https://tud.qucosa.de/id/qucosa%3A36294.
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Here we propose, for the first time, a new and green ethanol-thermal reaction method to synthesize highquality and pure thiophene–sulfur doped reduced graphene oxide (rGO), which establishes an excellent platform for studying sulfur (S) doping effects on the physical/chemical properties of this material. We have quantitatively demonstrated that the conductivity enhancement of thiophene–S doped rGO is not only caused by the more effective reduction induced by S doping, but also by the doped S atoms, themselves. Furthermore, we demonstrate that the S doping is more effective in enhancing conductivity of rGO than nitrogen (N) doping due to its stronger electron donor ability. Finally, the dye-sensitized solar cell (DSCC) employing the S-doped rGO/TiO₂ photoanode exhibits much better performance than undoped rGO/TiO₂, N-doped rGO/TiO₂ and TiO₂ photoanodes. It therefore seems promising for thiophene–S doped rGO to be widely used in electronic and optoelectronic devices.
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Fedorov, Alexander. "Electronic structure of doped 2D materials." Doctoral thesis, Saechsische Landesbibliothek- Staats- und Universitaetsbibliothek Dresden, 2016. http://nbn-resolving.de/urn:nbn:de:bsz:14-qucosa-203500.
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Electronic systems are an indivisible part of modern life. Every day, new materials, devices, passive components, antennas for wireless communication are needed to be designed and developed. In particular, flexible and biocompatible wearable devices are urgent required for medical and industrial applications. The great hope lies in the materials with high crystalline quality and flexibility such as graphene and other 2D semiconductors and insulators. Doping is a conventional tool for tailoring of the electronic properties of the functional materials.
Here we examine application of the widely used the electron donor species to the graphene and hexagonal boron nitride monolayer (h-BN). For each we determine surface-interface properties and the full electronic band structure using the combination of the surface science methods such as angle-integrated and angle resolved photoemission (XPS, ARPES), electron diffraction (LEED) and photo absorption (XAS).
As the result we provided insight into mechanisms underlying the doping gating of the graphene h-BN monolayer by the alkali metals. We fully characterized their surface and interface structure. Finally we studied the interplay between electrons and phonons in the doped graphene and we demonstrated that Ca-doped graphene is the promising candidate for realizing superconductivity in graphene.
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Narayanan, Nair Maya. "Functionalization of epitaxial graphene by metal intercalation and molecules." Phd thesis, Université de Haute Alsace - Mulhouse, 2013. http://tel.archives-ouvertes.fr/tel-01064523.
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In this thesis, we have explored the possibilities to realize a Graphene Based Hybrid structures (GBHs) by the functionalization of a graphene layer on both sides. The first chapter gives a general introduction about graphene and a literature review of different metal intercalations on graphene. The second chapter explains the experimental techniques used in this work. In chapter 3, we studied the functionalization of epitaxial graphene on SiC(0001) by gold intercalation. With the help of Scanning Tunneling Microscopy, we have evidenced and characterized different intercalation modes such as the formation of aggregates of individual gold atoms and the formation of a continuous gold layer between the top graphene and the buffer layer. The free standing nature of the intercalated gold atoms was examined by differential charge density plot, projected density of states calculations and further by X-ray photoelectron spectroscopy. The band structure modification of graphene due to these intercalated gold atoms was evidenced by Angle-resolved photoemission spectroscopy, which reveals a strong Van Hove extension and an increase of the Fermi velocity. Extend to this research, to obtain an extended Van Hove singularity usually observed in highly doped graphene; we studied highly electron donor molecules, TetraThioFullvalene (TTF) on pristine and gold intercalated graphene and on graphite (chapter 4). The dependence of charge transfer of these molecules with their conformation and the reactivity of photochromic with conjugated molecules on graphene were also discussed. To understand the structural properties of these molecules photophysical measurements were performed in chapter 5.
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Huber, Robin Tobias [Verfasser], and Jonathan [Akademischer Betreuer] Eroms. "Superlattice Band Structure Engineering of Graphene / Robin Tobias Huber ; Betreuer: Jonathan Eroms." Regensburg : Universitätsbibliothek Regensburg, 2021. http://d-nb.info/1237105838/34.
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Huang, Shengqiang, Matthew Yankowitz, Kanokporn Chattrakun, Arvinder Sandhu, and Brian J. LeRoy. "Evolution of the electronic band structure of twisted bilayer graphene upon doping." NATURE PUBLISHING GROUP, 2017. http://hdl.handle.net/10150/625515.
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The electronic band structure of twisted bilayer graphene develops van Hove singularities whose energy depends on the twist angle between the two layers. Using Raman spectroscopy, we monitor the evolution of the electronic band structure upon doping using the G peak area which is enhanced when the laser photon energy is resonant with the energy separation of the van Hove singularities. Upon charge doping, the Raman G peak area initially increases for twist angles larger than a critical angle and decreases for smaller angles. To explain this behavior with twist angle, the energy separation of the van Hove singularities must decrease with increasing charge density demonstrating the ability to modify the electronic and optical properties of twisted bilayer graphene with doping.
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KANDYBA, VIKTOR. "Electronic structure of single and few layered graphene studied by angle resolved photoemission spectro-microscopy." Doctoral thesis, Università degli Studi di Trieste, 2018. http://hdl.handle.net/11368/2929830.
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This thesis reports the study of electronic band structure of single and few layered graphene grown by thermal decomposition of SiC at the surface and by C-sublimation on Ru single crystals. Growth conditions were optimized in order to obtain big few micrometer sized graphene domains. For the first system twisted multilayer graphene domains were found and chosen for study. On ruthenium only single layer graphene domains and also the domains with incorporated bilayer patches were obtained and their electronic properties were investigated after oxidation-reduction reactions at graphene/Ru interface. The electronic band structure was analyzed using high resolution angle resolved photoelectron spectroscopy. In order to obtain spectra from individual domains novel spectromicroscopy end station was used for focusing synchrotron radiation beam to sub-micrometer spot on the sample surface. Experimental results on twisted graphene confirmed interlayer coupling and resulting van Hove singularities, graphene Dirac fermions velocity renormalization and other exotic phenomena predicted by theoretical calculations and partially observed by scanning tunneling spectroscopy technique. Particular attention has been paid to poorly studied interlayer coupling in trilayer systems where middle layer has two different couplings being sandwiched between differently twisted layers. These multilayer graphene domains were also investigated in detail upon alkali metal intercalation and unexpected splitting of upper part of Dirac cone, related to graphene sublattice symmetry breaking in the middle graphene layer was found. In graphene on Ru it was first confirmed that oxidation of Ru under graphene decouples its strongly hybridized π orbitals making graphene p-doped. Our observations indicate that bilayer patches incorporated into single layer background remain n-doped and decorated by intercalated oxygen, thereby forming lateral p-n junctions in the same graphene layer. It was found that hydrogen atmosphere helps to reduce RuOx without the formation of carbon vacancy defects. However, structural wrinkle patterns appeared due to loss of original graphene/Ru epitaxial order remain, and in big graphene domains they can trap H2+RuOx reaction products, making graphene fully decoupled and undoped.
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Landgraf, Wolfgang [Verfasser], and Oleg [Akademischer Betreuer] Pankratov. "Electronic structure of twisted graphene nanoflakes and nanoribbons / Wolfgang Landgraf. Gutachter: Oleg Pankratov." Erlangen : Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU), 2014. http://d-nb.info/1075478111/34.
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Memarian, Fereshteh Memarian. "EFFECT OF ELECTRON-ELECTRON SCATTERING ON LINEAR CONDUCTIVITY FOR GRAPHENE-LIKE BAND STRUCTURE." University of Akron / OhioLINK, 2018. http://rave.ohiolink.edu/etdc/view?acc_num=akron1532977274517365.
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Farbos, Baptiste. "Structure et propriétés de carbones anisotropes par une approche couplant analyse d’image et simulation atomistique." Thesis, Bordeaux, 2014. http://www.theses.fr/2014BORD0331/document.
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Des techniques combinées d'analyse/synthèse d'images et de simulation atomistique ont permis d’étudier la nanostructure/-texture de matériaux carbonés anisotropes et denses de type pyrocarbone (PyC) laminaire hautement texturé. Des représentations atomiques d’un PyC laminaire rugueux tel que préparé (AP) ainsi que d’un PyC laminaire régénéré AP et après plusieurs traitements thermiques (HT) ont été reconstruites pour mieux caractériser ces matériaux. Ces modèles comportent des domaines graphéniques de quelques nanomètres, joints entre eux par des lignes de défauts formées de paires de cycles à 5 et 7 carbones dans le plan et par des dislocations vis et des atomes tétravalents entre les plans. Les modèles les plus ordonnés ont des domaines plus étendus et un plus faible taux de connexions inter-plan. Les propriétés mécaniques et thermiques prédites à partir de ces modèles sont proches de celles du graphite et augmentent avec la cohérence intra-plan et la densité de connexions inter-plans. Des modèles de graphène polycristallins ont aussi été générés. Ils sont apparus, du point de vue structural et des propriétés mécaniques, très proches des feuillets de carbones des PyCs. Ils ont permis d'étudier la réorganisation structurale se produisant au cours du HT : formation de lignes de défauts, réparation de lacunes, … Il s'agit d'un premier pas vers l'étude de la graphitation des PyCs. La méthode de reconstruction a enfin été adaptée à l'étude de l'évolution structurale d'un graphite au cours de son irradiation par les électrons. Cela a permis d'observer à l'échelle atomique la création et la propagation des défauts au cours de l'irradiationCombined images analysis/synthesis techniques and atomistic simulation methods have allowed studying the nanostructure/-texture of anisotropic dense carbons of the highly textured laminar pyrocarbon (PyC) type.Atomic representations of an as-prepared (AP) rough laminar PyC as well as a regenerative laminar PyC AP and after several heat treatments (HT) were reconstructed to better characterize these materials.The models contain nanosized graphene domains connected between them by line defects formed by pairs of rings with 5 and 7 carbons inside layers and by screw dislocations and fourfold atoms between layers. The most ordered models have larger domains and a lower percentage of connections between the layers.Mechanical and thermal properties predicted from these models are close to those of graphite and increase with the coherence inside layers and the density of connections between layers.Models of polycrystalline graphene were also generated, showing structure and mechanical properties very close to those of the carbon layers extracted from PyCs. The structural reorganization occurring during the HT of such materials was studied: thinning of line defects and vacancy healing were observed. This represents a first step towards the study of the graphitization of PyCs.The reconstruction method was eventually adapted to study the structural evolution of a nuclear-grade graphite during its irradiation by electrons, allowing us to observe how defects are created and propagate during irradiation
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Chuang, Kai-Chieh. "Electronic band structure of carbon nanomaterials." Thesis, University of Oxford, 2009. http://ora.ox.ac.uk/objects/uuid:590ccfa7-2737-40c4-9a9c-ddb1b710cfef.
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This thesis reports the study of electronic structures for single-walled carbon nanotubes, single layer graphene and thin graphite. A brief introduction is given in Chapter 1 for the geometric and electronic structures of the materials studied while a review for the theory and experimental results relevant to this thesis is given in Chapter 2. The effects of hydrostatic pressure on surfactant-wrapped-single walled carbon nanotubes are studied in Chapter 3 by using photoluminscence and photoluminscence excitation mapping. It is found that the changes to the optical properties can be explained by the compression in carbon-carbon bonds, an effective uniaxial strain exerted on the nanotubes and changes in the surrounding environment leading to changes in the many-body interactions experienced by the nanotubes. Chapter 4 reports the study of cross-polarized photoluminescence of nanotubes isolated by conjugated polymers dispersed in solvents. The effects of Coulomb interactions on the optical bandgaps of the nanotubes are discussed here. Chapter 5 reports Cyclotron resonances studies of graphene monolayers. It is found that a significant asymmetry exists between the electron and hole band structures near the Dirac point, and the asymmetry is bigger than that is expected in a simple tight-binding model. Chapter 6 reports a magnetoabsorption study of the electronic structures near the K- and H- points. It is found that the transitions are not describe well by the conventional Slonczewski-Weiss-McClure model, but can be described instead with a simplified asymmetric effective bilayer model.
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Vincent, Thomas. "Ingénierie de bande du graphène par intercalation de monocouche atomique." Thesis, Paris Sciences et Lettres (ComUE), 2019. http://www.theses.fr/2019PSLET054.
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De nos jours, les systèmes basés sur le graphène ont aucune utilité particulière dans les applications de stockage d'informations magnétiques ou dans la spintronique, principalement en raison de l'absence de moments magnétiques dans le carbone et la négligeable interaction entre le spin des électrons avec le mouvement des charges électroniques (couplage spin-orbite). Cette situation change radicalement lorsque le graphène est interfacé avec d'autres matériaux. En particulier le couplage spin-orbite peut être induite dans le graphène par ses interactions avec les métaux lourds (HM) et les atomes de carbone peuvent acquérir moment magnétique quand le graphène est interfacé avec des éléments ferromagnétiques (FM). Ce projet vise à la fabrication et la caractérisation du premier système hybride à haute cristallinité graphène/HM/FM. Ce sera un système modèle possible pour des prototypes de stockage magnétique de l'information et l'étude de ses propriétés électroniques va élucider la possibilité de concevoir la texture de spin du graphene allant vers son utilisation comme matériau de base pour des applications spintroniquesNowadays graphene based systems have no particular use in spintronic or magnetic information storage applications mainly due to the lack of magnetic moments in the carbon and the negligible electron spin interaction with the electron motion (spin-orbit coupling). This situation changes dramatically when graphene is interfaced with other materials. Spin-orbit coupling can be induced in graphene by its interactions with heavy metals (HM) and carbon atoms can acquire magnetic moment when interfaced with ferromagnetic elements (FM). This project aims to the fabrication and the characterization of the first highly crystalline graphene/HM/FM hybrid system. This will serve as a possible model system for novel magnetic information storage prototypes, whereas the study of the hybrid electronic properties will elucidate the possibility to engineer the graphene spin texture going towards its use as base material for spintronic applications
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Maneshian, Mohammad Hassan. "The Influence of Ohmic Metals and Oxide Deposition on the Structure and Electrical Properties of Multilayer Epitaxial Graphene on Silicon Carbide Substrates." Thesis, University of North Texas, 2011. https://digital.library.unt.edu/ark:/67531/metadc68009/.
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Graphene has attracted significant research attention for next generation of semiconductor devices due to its high electron mobility and compatibility with planar semiconductor processing. In this dissertation, the influences of Ohmic metals and high dielectric (high-k) constant aluminum oxide (Al2O3) deposition on the structural and electrical properties of multi-layer epitaxial graphene (MLG) grown by graphitization of silicon carbide (SiC) substrates have been investigated. Uniform MLG was successfully grown by sublimation of silicon from epitaxy-ready, Si and C terminated, 6H-SiC wafers in high-vacuum and argon atmosphere. The graphene formation was accompanied by a significant enhancement of Ohmic behavior, and, was found to be sensitive to the temperature ramp-up rate and annealing time. High-resolution transmission electron microscopy (HRTEM) showed that the interface between the metal and SiC remained sharp and free of macroscopic defects even after 30 min, 1430 °C anneals. The impact of high dielectric constant Al2O3 and its deposition by radio frequency (RF) magnetron sputtering on the structural and electrical properties of MLG is discussed. HRTEM analysis confirms that the Al2O3/MLG interface is relatively sharp and that thickness approximation of the MLG using angle resolved X-ray photoelectron spectroscopy (ARXPS) as well as variable-angle spectroscopic ellipsometry (VASE) is accurate. The totality of results indicate that ARXPS can be used as a nondestructive tool to measure the thickness of MLG, and that RF sputtered Al2O3 can be used as a (high-k) constant gate oxide in multilayer grapheme based transistor applications.
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Baker, Taleb. "Molecular Computer Simulations of Graphene oxide intercalated with methanol: Swelling Properties and Interlayer Structure." Thesis, Umeå universitet, Institutionen för fysik, 2017. http://urn.kb.se/resolve?urn=urn:nbn:se:umu:diva-135941.
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Worth, Nicholas Gower. "Theoretical studies of compressed xenon oxides, tin selenide thermoelectrics, and defects in graphene." Thesis, University of Cambridge, 2018. https://www.repository.cam.ac.uk/handle/1810/274462.
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Enormous advances in computing power in recent decades have made it possible to perform accurate numerical simulations of a wide range of systems in condensed matter physics. At the forefront of this progress has been density functional theory (DFT), a very popular approach to tackling the complexity of quantum-mechanical systems that very often strikes a good balance between accuracy and tractability in light of the finite computational resources available to researchers. This thesis describes work utilising DFT methods to tackle two distinct problems. Firstly, the theoretical prediction of stable and metastable periodic structures under specified conditions using the ab initio random structure searching (AIRSS) method, which involves a large scale exploration of the Born-Oppenheimer energy surface, and secondly the use of a vibrational self-consistent field (VSCF) approach to investigate the effects of nuclear motion and anharmonicity in crystal systems, which involves a local exploration of the Born-Oppenheimer energy surface. The AIRSS crystal structure prediction method is here applied to a study of defect structures in graphene. It is also applied to a study of the xenon-oxygen binary system under a range of geological pressures (83–200 GPa). Novel xenon oxide structures are predicted and characterised theoretically. This work was carried out in collaboration with an experimental study of the system at the lower end of the pressure range. The VSCF approach to investigating anharmonicity is here applied to the study of tin selenide (SnSe), a material that has recently been shown to demonstrate consider- able promise as a thermoelectric material. In this thesis, the effects of the anharmonic nuclear motion on the vibrational and electronic properties of SnSe are investigated quantitatively.
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Mishra, Siddharth. "Plasma Enhanced Synthesis of Novel N Doped Vertically Aligned Carbon Nanofibers-3D Graphene hybrid structure." University of Cincinnati / OhioLINK, 2019. http://rave.ohiolink.edu/etdc/view?acc_num=ucin1552380299631335.
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Ilkiv, B. I., S. Petrovska, R. Sergiienko, and Ya V. Zaulychnyy. "X-ray Spectral Investigation of Carbon Nanoshells." Thesis, Sumy State University, 2012. http://essuir.sumdu.edu.ua/handle/123456789/35301.
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Carbon nanocapsules synthesized by plasma method in hexane were investigated using the ultra-soft
X-ray emission spectroscopy method. It has been revealed that additional mixed π+σ-overlapping form in
nanocapsules in a result of folding of graphene sheets. It has been found that in nanocapsules sp-hybrid
bonds between carbon and residual iron atoms form when overlapping high-energy 3d+4s-states with spnhybrid
orbitals (2
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Runte, Sven [Verfasser], CARSTEN [Akademischer Betreuer] BUSSE, and Stephan [Akademischer Betreuer] Schlemmer. "Atomic and Electronic Structure of Graphene and Graphene Intercalation Compounds. X-Ray Standing Wave and Scanning Tunnelling Microscopy Studies / Sven Runte. Gutachter: Carsten Busse ; Stephan Schlemmer." Köln : Universitäts- und Stadtbibliothek Köln, 2013. http://d-nb.info/1046175831/34.
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Peng, Han. "Spatial resolved electronic structure of low dimensional materials and data analysis." Thesis, University of Oxford, 2018. http://ora.ox.ac.uk/objects/uuid:2f3503eb-93bf-48d6-b6fb-13409b925748.
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Two dimensional (2D) materials with interesting fundamental physics and potential applications attract tremendous efforts to study. The versatile properties of 2D materials can be further tailored by tuning the electronic structure with the layer-stacking arrangement, of which the main adjustable parameters include the thickness and the in-plane twist angle between layers. The Angle-Resolved Photoemission Spectroscopy (ARPES) has become a canonical tool to study the electronic structure of crystalline materials. The recent development of ARPES with sub-micrometre spatial resolution (micro-ARPES) has made it possible to study the electronic structure of materials with mesoscopic domains. In this thesis, we use micro-ARPES to investigate the spatially-resolved electronic structure of a series of few-layer materials: 1. We explore the electronic structure of the domains with different number of layers in few-layer graphene on copper substrate. We observe a layer- dependent substrate doping effect in which the Fermi surface of graphene shifts with the increase of number of layers, which is then explained by a multilayer effective capacitor model. 2. We systematically study the twist angle evolution of the energy band of twisted few-layer graphene over a wide range of twist angles (from 5° to 31°). We directly observe van Hove Singularities (vHSs) in twisted bilayer graphene with wide tunable energy range over 2 eV. In addition, the formation of multiple vHSs (at different binding energies) is observed in trilayer graphene. The large tuning range of vHS binding energy in twisted few-layer graphene provides a promising material base for optoelectrical applications with broad-band wavelength selectivity. 3. To better extract the energy band features from ARPES data, we propose a new method with a convolutional neural network (CNN) that achieves comparable or better results than traditional derivative based methods. Besides ARPES study, this thesis also includes the study of surface reconstruction for the layered material Bi2O2Se with the analysis of Scanning Tunnelling Microscopy (STM) images. To explain the origin of the pattern, we propose a tile model that produces the identical statistics with the experiment.
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Karimi, Hamed. "Structure factors of s=1/2 spin chains and magnetism at the edges of graphene ribbons." Thesis, University of British Columbia, 2014. http://hdl.handle.net/2429/50310.
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In this thesis we study two different one dimensional systems. The first project is on the transverse dynamical structure factors of the XXZ spin chain and the second project is on magnetism of zigzag edges of graphene
nano-ribbons.
In chapter 2, we apply field theory methods, first developed to study x-ray edge singularities, to interacting one-dimensional systems in order to
include band curvature effects and study edge singularities at arbitrary momentum. We point out that spin chains with uniform Dzyaloshinskii-Moriya
interactions provide an opportunity to test these theories since these interactions may be exactly eliminated by a gauge transformation that
shifts the momentum. However, this requires an extension of these x-ray edge methods to the transverse spectral function of the XXZ spin chain in
a magnetic field.
In chapter 3, by considering the Hubbard model in the weak coupling limit, U << t, for bearded as well as zigzag edges, we argue for existence of magnetic edges. We first present an argument based on Lieb's theorem.
Then, projecting the Hubbard interactions onto the flat edge band, we prove that the resulting one-dimensional model has a fully polarized ferromagnetic ground state. We also study excitons and the effects of second neighbor hopping as well as a potential energy term acting on the
edge only, proposing a simple and possibly exact phase diagram with the magnetic moment varying smoothly to zero. Finally, we consider corrections
of second order in U, arising from integrating out the gapless bulk Dirac excitations.Science, Faculty of
Physics and Astronomy, Department of
Graduate
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Sharma, Surbhi. "Synthesis and electronic structure of graphene oxide for applications as suppport and catalyst for fuel cells." Thesis, University of Ulster, 2010. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.526957.
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Kumar, Ajay. "An Investigation of Functionalization, Electronic structure of Multilayer Graphene Nanoflake Films (MGNFs)and their electrochemical properties." Thesis, University of Ulster, 2009. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.516459.
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Robertson, Alexander William. "Synthesis and characterisation of large area graphene." Thesis, University of Oxford, 2013. http://ora.ox.ac.uk/objects/uuid:aee750dd-41b8-4462-9efa-4e89e06e0ed7.
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The pursuit of high quality, large area graphene has been a major research focus of contemporary materials science research, in the wake of the discovery of the multitude of exceptional properties exhibited by the material. The DPhil project was undertaken with the objective of developing an understanding of the growth of large graphene sheets by chemical vapour deposition (CVD), and also in the subsequent characterisation of their material properties. By conducting atmospheric pressure CVD growth at high methane flow rates, it was found that few-layered graphene (FLG) could be deposited on a copper catalyst. It is demonstrated that the self-limiting property of a copper catalyst is not universal to all deposition conditions, and shown that FLG grows in a terrace-like configuration. In depth transmission electron microscopy (TEM) studies were carried out on FLG. By selective image reconstruction from the inverse power spectrum of the TEM images, it was possible to elucidate the inter-grain connectivity of few-layer graphenes. It was determined that there were two possible inter-grain configurations possible; specifically an overlap of graphene layers or a discrete atomic bonding edge. The perturbation of the few-layer structure when subject to an out of plane distortion was found to incur a shift in the conventional AB-Bernal stacking of FLG. By utilising the aberration corrected TEM (AC-TEM) at Oxford it was possible to resolve atomic detail in CVD synthesised monolayer films, including atomic bond rotations and vacancies. The use of a high current density at low accelerating voltage (80 kV) was demonstrated to allow for the controlled defect creation in graphene sheets. This permitted the creation of monovacancies and iron doped vacancy complexes suitable for further study. The behaviour of these two defect types under electron beam irradiation was subsequently studied.
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Di, Felice Daniela. "Electronic structure and transport in the graphene/MoS₂ heterostructure for the conception of a field effect transistor." Thesis, Université Paris-Saclay (ComUE), 2018. http://www.theses.fr/2018SACLS267/document.
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L'isolement du graphène, une monocouche de graphite composée d'un plan d’atomes de carbone, a démontré qu'il est possible de séparer un seul plan d'épaisseur atomique, que l'on appelle matériau bidimensionnel (2D), à partir des solides de Van de Waals (vdW). Grâce à leur stabilité, différents matériaux 2D peuvent être empilés pour former les hétérostructures de vdW. L'interaction vdW à l'interface étant suffisamment faible, les propriétés spécifiques de chaque matériau demeurent globalement inchangées dans l’empilement. En utilisant une démarche théorique et computationnelle basée sur la théorie de la fonctionnelle de la densité (DFT) et le formalisme de Keldysh-Green, nous avons étudié l'hétérostructure graphène/MoS₂ . Le principal intérêt des propriétés spécifiques du graphène et du MoS₂ pour la conception d'un transistor à effet de champ réside dans la mobilité du graphène, à la base d'un transistor haute performance et dans le gap électronique du MoS₂, à la base de la commutation du dispositif. Tout d'abord, nous avons étudié les effets de la rotation entre les deux couches sur les propriétés électroniques à l'interface, en démontrant que les propriétés électroniques globales ne sont pas affectées par l'orientation. En revanche, les images STM (microscope à effet tunnel) sont différentes pour chaque orientation, en raison d'un changement de densité de charge locale. Dans un deuxième temps, nous avons utilisé l’interface graphène/MoS₂ en tant que modèle très simple de Transistor à Effet de Champ. Nous avons analysé le rôle des hétérostructures de vdW sur la performance du transistor, en ajoutant des couches alternées de graphène et MoS₂ sur l'interface graphène/MoS₂. Il a ainsi été démontré que la forme de la DOS au bord du gap est le paramètre le plus important pour la vitesse de commutation du transistor, alors que si l’on ajoute des couches, il n’y aura pas d’amélioration du comportement du transistor, en raison de l'indépendance des interfaces dans les hétérostructures de vdW. Cependant, cela démontre que, dans le cadre de la DFT, on peut étudier les propriétés de transport des hétérostructures de vdW plus complexes en séparant chaque interface et en réduisant le temps de calcul. Les matériaux 2D sont également étudiés ici en tant que pointe pour STM et AFM (microscope à force atomique) : une pointe de graphène testée sur MoS₂ avec défauts a été comparée aux résultats correspondants pour une pointe en cuivre. La résolution atomique a été obtenue et grâce à l'interaction de vdW entre la pointe et l’échantillon, il est possible d’éviter les effets de contact responsables du transfert d'atomes entre la pointe et l'échantillon. En outre, l'analyse des défauts est très utile du fait de la présence de nouveaux pics dans le gap du MoS₂ : ils peuvent ainsi être utilisés pour récupérer un pic de courant et donner des perspectives pour améliorer la performance des transistorsThe isolation of graphene, a single stable layer of graphite, composed by a plane of carbon atoms, demonstrated the possibility to separate a single layer of atomic thickness, called bidimensional (2D) material, from the van der Waals (vdW) solids. Thanks to their stability, 2D materials can be used to form vdW heterostructures, a vertical stack of different 2D crystals maintained together by the vdW forces. In principle, due to the weakness of the vdW interaction, each layer keeps its own global electronic properties. Using a theoretical and computational approach based on the Density Functional Theory (DFT) and Keldish-Green formalism, we have studied graphene/MoS₂ heterostructure. In this work, we are interested in the specific electronic properties of graphene and MoS₂ for the conception of field effect transistor: the high mobility of graphene as a basis for high performance transistor and the gap of MoS₂ able to switch the device. First, the graphene/MoS₂ interface is electronically characterized by analyzing the effects of different orientations between the layers on the electronic properties. We demonstrated that the global electronic properties as bandstructure and Density of State (DOS) are not affected by the orientation, whereas, by mean of Scanning Tunneling Microscope (STM) images, we found that different orientations leads to different local DOS. In the second part, graphene/MoS₂ is used as a very simple and efficient model for Field Effect Transistor. The role of the vdW heterostructure in the transistor operation is analyzed by stacking additional and alternate graphene and MoS₂ layers on the simple graphene/MoS₂ interface. We demonstrated that the shape of the DOS at the gap band edge is the fundamental parameter in the switch velocity of the transistor, whereas the additional layers do not improve the transistor behavior, because of the independence of the interfaces in the vdW heterostructures. However, this demonstrates the possibility to study, in the framework of DFT, the transport properties of more complex vdW heterostructures, separating the single interfaces and reducing drastically the calculation time. The 2D materials are also studied in the role of a tip for STM and Atomic Force Microscopy (AFM). A graphene-like tip, tested on defected MoS₂, is compared with a standard copper tip, and it is found to provide atomic resolution in STM images. In addition, due to vdW interaction with the sample, this tip avoids the contact effect responsible for the transfer of atoms between the tip and the sample. Furthermore, the analysis of defects can be very useful since they induce new peaks in the gap of MoS₂: hence, they can be used to get a peak of current representing an interesting perspective to improve the transistor operation
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Almahmoud, Khaled Hasan Musa. "Thermal Transport Modeling in Three-Dimensional Pillared Graphene Structures for Efficient Heat Removal." Thesis, University of North Texas, 2020. https://digital.library.unt.edu/ark:/67531/metadc1752407/.
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Pillared-graphene structure (PGS) is a novel three-dimensional structure consists of parallel graphene sheets that are separated by carbon nanotube (CNT) pillars that is proposed for efficient thermal management of electronics. For microscale simulations, finite element analyses were carried out by imposing a heat flux on several PGS configurations using a Gaussian pulse. The temperature gradient and distribution in the structures was evaluated to determine the optimum design for heat transfer. The microscale simulations also included conducting a mesh-independent study to determine the optimal mesh element size and shape. For nanoscale simulations, Scienomics MAPS software (Materials And Processes Simulator) along with LAMMPS (Large-scale Atomic/ Molecular Massively Parallel Simulator) were used to calculate the thermal conductivity of different configurations and sizes of PGS. The first part of this research included investigating PGS when purely made of carbon atoms using non-equilibrium molecular dynamics (NEMD). The second part included investigating the structure when supported by a copper foil (or substrate); mimicking production of PGS on copper. The micro- and nano-scale simulations show that PGS has a great potential to manage heat in micro and nanoelectronics. The fact that PGS is highly tunable makes it a great candidate for thermal management applications. The simulations were successfully conducted and the thermal behavior of PGS at the nanoscale was characterized while accounting for phonon scattering the graphene/CNT junction as well as when PGS is supported by a copper substrate.
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Simonov, Konstantin. "Effect of Substrate on Bottom-Up Fabrication and Electronic Properties of Graphene Nanoribbons." Doctoral thesis, Uppsala universitet, Molekyl- och kondenserade materiens fysik, 2016. http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-295884.
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Taking into account the technological demand for the controlled preparation of atomically precise graphene nanoribbons (GNRs) with well-defined properties, the present thesis is focused on the investigation of the role of the underlying metal substrate in the process of building GNRs using bottom-up strategy and on the changes in the electronic structure of GNRs induced by the GNR-metal interaction. The combination of surface sensitive synchrotron-radiation-based spectroscopic techniques and scanning tunneling microscopy with in situ sample preparation allowed to trace evolution of the structural and electronic properties of the investigated systems. Significant impact of the substrate activity on the growth dynamics of armchair GNRs of width N = 7 (7-AGNRs) prepared on inert Au(111) and active Cu(111) was demonstrated. It was shown that unlike inert Au(111) substrate, the mechanism of GNRs formation on Ag(111) and Cu(111) includes the formation of organometallic intermediates based on the carbon-metal-carbon bonds. Experiments performed on Cu(111) and Cu(110), showed that a change of the balance between molecular diffusion and intermolecular interaction significantly affects the on-surface reaction mechanism making it impossible to grow GNRs on Cu(110). It was demonstrated that deposition of metals on spatially aligned GNRs prepared on stepped Au(788) substrate allows to investigate GNR-metal interaction using angle-resolved photoelectron spectroscopy. In particular intercalation of one monolayer of copper beneath 7-AGNRs leads to significant electron injection into the nanoribbons, indicating that charge doping by metal contacts must be taken into account when designing GNR/electrode systems. Alloying of intercalated copper with gold substrate upon post-annealing at 200°C leads to a recovery of the initial position of GNR-related bands with respect to the Fermi level, thus proving tunability of the induced n-doping. Contrary, changes in the electronic structure of 7-AGNRs induced by the deposition of Li are not reversible. It is demonstrated that via lithium doping 7-AGNRs can be transformed from a semiconductor into a metal state due to the partial filling of the conduction band. The band gap of Li-doped GNRs is reduced and the effective mass of the conduction band carriers is increased.
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Liu, Kewei. "Macro Porous Graphene from Hollow Ni Templates via Polymer Templates with Bi-Continuous." University of Akron / OhioLINK, 2014. http://rave.ohiolink.edu/etdc/view?acc_num=akron1397135127.
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Mathur, Shashank. "Croissance et structure à l'échelle atomique d'un nouveau matériau cristallin bidimensionnel à base de silicium et d'oxygène." Thesis, Université Grenoble Alpes (ComUE), 2016. http://www.theses.fr/2016GREAY019/document.
Full textAbstract:
L'oxyde de silicium est un composé très largement abondant qui existe sous différentes phases, cristallines ou amorphes, qui se présentent sous la forme de structures poreuses ou de films minces. Il s'agit d'un diélectrique traditionnel pour la microélectronique et d'un support de choix pour des nanoparticules dans des systèmes catalytiques. Sa structure, amorphe ou tridimensionnelle et complexe, rend difficile la compréhension des propriétés jusqu'aux échelles les plus élémentaires. Les films utra-minces épitaxiés, parfois nommés « silice bidimensionelle » se prêtent au contraire à des caractérisation fines de la structure et des propriétés.Cette thèse avait pour objectif de préparer une telle phase d'oxide de silicium. A l'aide de sondes de sciences des surfaces, la microscopie à effet tunnel (STM), la diffraction d'électrons rapides en réflexion (RHEED), dont les analyses ont été confrontées aux résultats de calculs en théorie de la fonctionnelle de la densité (DFT), la structure de cette phase à pu être résolue jusqu'à l'échelle atomique. Nous avons mis en évidence l'arrangement hexagonal de tétraèdres de [SiO4], chimisorbés sur la surface (0001) du ruthenium en des sites spécifiques. Une phase d'oxygène diluée, reconstruite sur le Ru(0001), a été observée, qui coexiste avec l'oxide de silicium.La croissance de l'oxyde de silicium, a également été étudiée, par un suivi in operando, en temps réel pendant la croissance, par RHEED. Une évolution marquée de taille de domaines et/ou de l'accumulation et de la relaxation de déformations a été observée alors que l'oxyde de silicium crystallise. Un mécanisme de croissance a été proposé sur la base de ces observations, selon lequel les espèces chimiques à la surface se réorganisent par des déplacements latéraux élémentaires. Ce mécanisme s'accompagne de la formation, inévitable, de lignes de défauts uni-dimensionnelles, dont la structure a été déterminée à l'échelle atomique par STMSilicon oxide is a widely abundant compound existing in various forms from amorphous to crystalline, bulk to porous and thin films. It is a common dielectric in microelectronics and widely used host for nanoparticles in heterogenous catalysis. Its amorphous nature and the ill-defined complex three dimensional structure is a hurdle to the understanding of its properties down to the smallest scales. Resorting to epitaxially grown ultra-thin phase (also called a two-dimensional material) can help overcome these issues and provide clear-cut information regarding the structure and properties of the material.In this thesis, studies were aimed at growing this promising novel phase of silicon oxide. Using surface science tools, including scanning tunelling microscopy (STM) and reflection high energy electron diffraction (RHEED) supported by density functional theory calculations, the atomic structure was resolved to high resolution. The monolayer was found to have a hexagonal arrangement of the [SiO4] tetrahedra chemisorbed on the surface of Ru(0001) into specific sites. This lattice of monolayer silicon oxide was also found to coexist with an oxygen reconstruction of the bare Ru(0001) inside each silicon oxide cell.The growth of this monolayer was monitored in real-time by in operando RHEED studies. These experiments provided with insights the domain size evolution and the build up/release of strain field during the growth that. Based on the experimental observations, a growth mechanism leading to the formation of monolayer silicon oxide could be proposed in terms of geometrical translations of the atomic species on the surface of Ru(0001) support. This mechanism results in unavoidable formation of one-dimensional line-defects that were precisely resolved by the STM