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Dissertations / Theses on the topic 'Graphene'
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1
Geng, Yan. "Preparation and characterization of graphite nanoplatelet, graphene and graphene-polymer nanocomposites /." View abstract or full-text, 2009. http://library.ust.hk/cgi/db/thesis.pl?MECH%202009%20GENG.
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Wang, Yu. "Graphenide solutions and graphene films." Thesis, Bordeaux, 2014. http://www.theses.fr/2014BORD0161/document.
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Les travaux de recherche effectués lors de cette thèse s'articulent autour de matériaux graphène. Une méthode est développée pour produire graphène en masse avec solution de graphenure. Les études effectuées les solutions de graphenure sont basées sur les composés d'intercalation du grpahite (GICs) synthétisé avec du potassium et l'exfoliation de GIC dans un solvant organique. Différentes techniques d'analyse ont été employées pour caractériser les graphène produits. Afin de tirer parti des propriétés électriques du graphène, les solutions de graphenure ont ensuite été utilisées pour produire des films transparents conducteurs. Des traitements de recuit à sous atmosphère d'argon ont été effectués pour améliorer les propriétés électriques du film. Les résultats de caractérisation montrent que l'élimination des groupes fonctionnels contenant des atomes d'oxygène et l'amélioration structurale peuvent largement améliorer les propriétés électriques des films de graphène avec ce traitement de recuitThe graphene is promising materials in future industrial applications due to its excellent properties. In recent years, different production methods have been developed in order to pave the way for applications. One topic of this thesis focuses on graphenidesolutions, which provide an efficient route to produce graphene. Using this method, graphite intercalation compounds(GICs)can be exfoliated into negativelz charged grapheme organic solvent under inert atmosphere. Withits high conductivity and bendable feature, one of the promising applications of graphene is flexible transparent conductive films. The second main topic of this thesis consists in applying produced graphene to produce transparent conductive films.With mild thermal treatments, the electrical properties of graphene film can be largely improved
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Qiu, Xiaoyu. "Procédé d'exfoliation du graphite en phase liquide dans des laboratoires sur puce." Thesis, Université Grenoble Alpes (ComUE), 2018. http://www.theses.fr/2018GREAI056/document.
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L’exfoliation en phase liquide du graphite est un procédé simple susceptible de produire du graphène à faible coût. Ces dernières années, de nombreuses équipes ont exploité la cavitation acoustique et la cavitation hydrodynamique comme moyen d’exfoliation. La cavitation acoustique ne peut traiter qu’une quantité limitée de fluide et génère des défauts sur la structure du graphène,tandis que la cavitation hydrodynamique dans une solution en écoulement n’agit que localement pendant une durée très brève. Les équipes de recherche utilisant ce dernier procédé compensent cette brièveté en imposant à la solution chargée en graphite des différences de pression très fortes, et utilisent alors des infrastructures macroscopiques lourdes pour lesquelles il est difficile de distinguer le rôle du cisaillement de celui de la cavitation. Nous avons cherché à développer un nouveau procédé d’exfoliation basé sur l’utilisation de microsystèmes fluidiques capables de générer un écoulementcavitant avec un débit supérieur à 10 L/h pour une différence de pression modérée n’excédant pas 10 bar. Une nouvelle génération de laboratoires ‘sur puce’ a ainsi été imaginée et réalisée, permettant de traiter des solutions surfactées chargées en microparticules de graphite. Il est apparu que laconcentration solide et la durée de traitement sont des paramètres cruciaux pour l’efficacité du procédé. Par rapport à un écoulement monophasique laminaire microfluidique, l’écoulement cavitant produit plus de produits exfoliés et de graphène, avec un rendement de l’ordre de 6%. Ceci indique que l’implosion des bulles et la turbulence favorisent également les interactions entre particules. Ce procédé d’exfoliation microfluidique, qui ne nécessite une puissance que de quelques Watts, permet d’envisager à terme une production économe et écologique de graphène en suspensionLiquid phase exfoliation of graphite is a simple and low-cost process, that is likely to produce graphene. The last few years, many researchers have used acoustic or hydrodynamic cavitation as an exfoliating tool. Acoustic cavitation is limited to low volumes and defects are present on the graphenesheets ; hydrodynamic cavitation inside a flowing solution acts briefly. So, people are using big reactors running with high pressure drops, and it is difficult from a fundamental point of view to know the physical role of shear rate versus cavitation, in the exfoliation process. We have tried to develop a new process funded on hydrodynamic cavitation ’on a chip’, with flow rates above 10 L/h and pressure drop below 10 bar. A new generation of ’labs on a chip’ has been designed and performed, processing with aqueous surfactant graphite solutions. The solid concentration and the duration of the process have proved to be key parameters. Cavitating microflows have exhibited a better efficiency (up to ~6%) than laminar liquid microflows, for the production of graphene flakes. Collapsing bubbles and turbulence are also likely to enhance particles interactions. Such a microfluidic process, which requires an hydraulic power of a few Watt, makes possible a further low-cost and green production of graphene sheets
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Melios, Christos. "Graphene metrology : substrate and environmental effects on grapheme." Thesis, University of Surrey, 2017. http://epubs.surrey.ac.uk/845201/.
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Graphene, a single layer of sp2-bonded carbon atoms, has received significant attention due to its exceptional opto-electronic properties and potentially scalable production processes. However, scalable graphene requires an underlying substrate, which is often a source of strain, doping and carrier scattering, limiting the mobility and quality of graphene. It was shown that by intercalating graphene on SiC by hydrogen, the interfacial layer, associated with n-doping and mobility degradation, is de-coupled from the substrate. The transformations of the H2-intercalation were demonstrated using Raman spectroscopy, while the SiC/interface changes were probed using surface enhanced Raman scattering. The H2-intercalation resulted in carrier type inversion, where the decoupled graphene change from n- to p-type, as well as showing mobility enhancement, up to more than four times, compared to as-grown graphene. Using calibrated Kelvin probe force microscopy, local work function maps were generated, demonstrating the changes in local electronic properties with nanoscale resolution. Furthermore, the layer structure, doping and strain induced by the underlying substrate are compared to CVD grown graphene transferred onto Si/SiO2. In addition to the substrate effects, the electronic properties of graphene are also significantly affected due to the direct exposure of π electrons to the environment. For the investigation of the environmental effects on graphene (i.e. H2O and NO2), a custom-built environmental transport properties measurement system was designed and developed, allowing magneto-transport measurements to be conducted in highly controlled environments. Using this system and calibrated local work function mapping, it is demonstrated that water withdraws electrons from graphene on SiC and SiO2 substrates, as well as acting as a source of impurity scattering. However, the sensitivity of graphene to water depends highly on the underlying substrate and substrate-induced doping. Moreover, it is shown that epitaxial graphene can successfully be used as the sensing material with detection down to 10 parts-per-billion molecules. Considering the environmental effects on the electronic properties of graphene, the importance of clearly reporting the measurement environmental conditions is high-lighted, whenever a routine characterisation for carrier concentration and mobility is reported.
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Nyangiwe, Nangamso Nathaniel. "Graphene based nano-coatings: synthesis and physical-chemical investigations." Thesis, UWC, 2012. http://hdl.handle.net/11394/3237.
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Magister Scientiae - MScIt is well known that a lead pencil is made of graphite, a naturally form of carbon, this is important but not very exciting. The exciting part is that graphite contains stacked layers of graphene and each and every layer is one atom thick. Scientists believed that these graphene layers could not be isolated from graphite because they were thought to be thermodynamically unstable on their own and taking them out from the parent graphite crystal will lead them to collapse and not forming a layer. The question arose, how thin one could make graphite. Two scientists from University of Manchester answered this question by peeling layers from a graphite crystal by using sticky tape and then rubbing them onto a silicon dioxide surface. They managed to isolate just one atom thick layer from graphite for the first time using a method called micromechanical cleavage or scotch tape. In this thesis chemical method also known as Hummers method has been used to fabricate graphene oxide (GO) and reduced graphene oxide. GO was synthesized through the oxidation of graphite to graphene oxide in the presence of concentrated sulphuric acid, hydrochloric acid and potassium permanganate. A strong reducing agent known as hydrazine hydrate has also been used to reduce GO to rGO by removing oxygen functional groups, but unfortunately not all oxygen functional groups have been removed, that is why the final product is named rGO. GO and rGO solutions were then deposited on silicon substrates separately. Several characterization techniques in this work have been used to investigate the optical properties, the morphology, crystallography and vibrational properties of GO and rGO.
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Yu, Wenlong. "Infrared magneto-spectroscopy of graphite and graphene nanoribbons." Diss., Georgia Institute of Technology, 2014. http://hdl.handle.net/1853/54244.
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The graphitic systems have attracted intensive attention recently due to the discovery of graphene, a single layer of graphite. The low-energy band structure
of graphene exhibits an unusual linear dispersion relation which hosts massless Dirac fermions and leads to intriguing electronic and optical properties. In particular, due to the high mobility and tunability, graphene and graphitic materials have been recognized as promising candidates for future nanoelectronics and optoelectronics. Electron-phonon coupling (EPC) plays a significant role in electronic and optoelectronic devices. Therefore, it is crucial to understand EPC in graphitic materials and then manipulate it to achieve better device performance. In the first part of this thesis, we explore EPC between Dirac-like fermions and infrared active phonons in graphite via infrared magneto-spectroscopy. We demonstrate that the EPC can be tuned by varying the magnetic field. The second part of this thesis deals with magnetoplasmons in quasineutral graphene nanoribbons. Multilayer epitaxial graphene grown on the carbon terminated silicon carbide surface behaves like single layer graphene. Plasmons are excited in the nanoribbons of undoped multilayer epitaxial graphene. In a magnetic field, the cyclotron resonance can couple with the plasmon resonance forming the so-called upperhybrid mode. This mode exhibits a distinct dispersion relation, radically different from that expected for conventional two dimensional systems.
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Bleu, Yannick. "Graphene and doped graphene elaborated by pulsed laser deposition." Thesis, Lyon, 2020. http://www.theses.fr/2020LYSES033.
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Le graphène est, par définition, un matériau bidimensionnel, cristallin, constitué d’un réseau d’atomes de carbone en nid d’abeilles répartis sur une monocouche atomique. Le graphène a suscité un grand intérêt dans les communautés scientifiques au cours des 15 dernières années, en raison de propriétés remarquables, en particulier la conductivité électrique, la transparence optique, la résistance et la conductivité thermique, avec de nombreuses applications technologiques potentielles, comme les électrodes transparentes, l’émission de champs, les biocapteurs, les futures générations de batteries, les matériaux composites, etc. L’un des plus grands défis avec le graphène, demeure le contrôle et la reproductibilité de la synthèse sur de grandes surfaces, ainsi que l'étude analytique, à l’échelle nanométrique, de films si particuliers à une échelle très réduite, films constitués de l’élément carbone formant une ou plusieurs couches déposées sur des substrats adéquats en fonction des applications visées. Dans cette thèse, nous avons proposé une méthode de synthèse alternative basée sur un procédé physique (et non chimique), combinant le dépôt par laser pulsé (PLD) avec un recuit thermique rapide (Rapid Thermal Annealing). Cette approche particulière permet en particulier le dopage des couches de graphène par des atomes choisis, de manière contrôlée et reproductible. Nos travaux ont contribué à élargir les champs d'études de la PLD dans le domaine de la synthèse des couches minces. Aussi, Ils contribuent à une avancée des connaissances fondamentales sur la synthèse du graphène et du graphène dopé au bore, au cœur des efforts actuels de la recherche pour intégrer ces matériaux dans des applications technologiques exigeants des performances toujours plus élevéesGraphene is, by definition, a one-atom-thick pure carbon crystal with a honeycomb-like structure. Graphene has become of great interest in both scientific and engineering communities from the past 15 years, owing to its range of unique properties including high conductivity, transparency, strength, and thermal conductivity, with many potential applications in research and industry, as transparent electrodes, field emitters, biosensors, batteries, composites, and so on. One of the greatest challenges with graphene remains the control and reproducibility of the synthesis on large surfaces, as well as the analytical study, at the nanometric scale. In this thesis, we have proposed an alternative synthesis method based on a physical (and not chemical) process, combining pulsed laser deposition (PLD) with rapid thermal annealing (Rapid Thermal Annealing). This particular approach allows in particular the doping of the graphene layers with selected atoms, in a controlled and reproducible manner. Our work has contributed to broadening the fields of study of PLD in the field of thin-film synthesis. It also contribute to an advance in fundamental knowledge on the synthesis of graphene and boron-doped graphene, at the heart of current research efforts to integrate these materials into technological applications requiring ever-higher performance
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Li, Yuan. "New functionalized graphene nanocomposites for applications in energy storage and catalysis." Thesis, Université Paris-Saclay (ComUE), 2016. http://www.theses.fr/2016SACLN025.
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Matériaux à base de graphène et d’oxyde de graphène ont attiré une grande attention depuis sa découverte. Cependant, comme la feuille de graphène a une surface spécifique élevée, il tend à former un agglomérat irréversible ou même empiler pour former le graphite par π-π empilage et Van-der Waals interactions. Les modifications doivent être faites pour séparer les feuilles de graphène sans apporter trop de dégâts dans sa structure aromatique. Dans cette thèse, nous avons lancé deux méthodes pour faire la modification du graphène, réaction de substitution nucléophile pour l’oxyde de graphène avec un C/O ~ 2 (FGS2), tandis que la demande électronique inverse réaction de Diels-Alder pour l’oxyde de graphène avec un très faible teneur en oxygène C/O ~ 20 (FGS20). Comme dans le second cas, FGS20 fonctionnalisés par tetrazine possède une excellente conductivité, il a été en outre combiné avec un polypyrrole pour fabriquer un matériau de supercondensateur.Dans le chapitre 2, nous avons greffé de manière covalente des dérivés de tétrazine à l'oxyde de graphène par substitution nucléophile. Comme l'unité de tétrazine est électroactif et riche en azote, avec un potentiel de réduction sensible du type de substituant et degré de substitution, nous avons utilisé l'électrochimie et la spectroscopie de photoélectrons X pour démontrer des preuves claires pour le greffage par liaison covalente. La modification chimique a été soutenue par spectroscopie infrarouge à transformée de Fourier et analyse thermique. Tétrazines greffé sur l'oxyde de graphène affichent différentes pertes de masse par rapport à graphène non modifiée et sont plus stables que les précurseurs moléculaires. Enfin, un dérivé de pontage tétrazine a été greffée entre des feuilles d'oxyde de graphène pour démontrer que la distance de séparation entre les feuilles peut être maintenue lors de la conception de nouveaux matériaux à base de graphène, y compris les structures d'oxydo-réduction chimiquement liés, les structures d'oxydoréduction.Dans le chapitre 3, des molécules modèles de graphène ont été sélectionnés afin de déterminer les conditions optimales de réaction entre graphène et tétrazine dérivés. Toutes les molécules de tétrazine ont d'abord été étudiés par électrochimie et ensuite mis à réagir avec le graphène par la demande électronique inverse Diels-Alder (DAinv) réaction dans un réacteur à micro-ondes, la XPS a été réalisée pour étudier sa composition chimique et de prouver la modification avec succès du graphène. Ensuite, le matériau de graphène tétrazine fonctionnalisé a été appliqué sur une électrode en acier inoxydable et ses performances électrochimiques ont été évaluées par voltamétrie cyclique et les tests de charge-décharge. La plupart des tétrazine modifié matériaux de graphène a montré de très bonnes performances électrochimiques et une faible résistance due à une bonne accessibilité des ions, ce qui en fait l'un des matériaux d'électrodes les plus prometteuses pour les supercondensateurs jusqu'à présent. Dans le chapitre 4, polypyrrole (PPy)-graphène nanocomposites ont été synthétisés par polymérisation de PPy sur les feuilles de graphène fonctionnalisés par tétrazine. Le matériau de graphène modifié contient des unités pyridazine tel que démontré par XPS. Puis PPy a été déposé sur ce matériau de graphène fonctionnalisé soit par polymérisation chimique ou électrochimique. Cellules de pièces symétriques ont été faites pour mesurer la capacité dans une configuration à deux électrodes. Les nanocomposites de polypyrrole-graphène avec 40% PPy présentent les meilleures performances électrochimiques et une faible résistance en raison d'une bonne accessibilité des ions, ce qui en fait l'un des meilleurs matériaux d'électrodes pour supercapacitor jusqu'à présentGraphene and graphene oxide based materials have attracted great attention since its discovery. However, as graphene sheet has a high specific surface area, it tends to form an irreversible agglomerates or even restack to form graphite through π–π stacking and van-der Waals interactions. Modifications need to be done to separate graphene sheets without bringing too much damage in its aromatic structure.In this thesis, two methods have been introduced to do the modification of graphene, nucleophilic substitution reaction for graphene oxide with a C/O~2 (FGS2), while inverse electron demand Diels-Alder reaction for graphene oxide with a very low oxygen content C/O~20 (FGS20). As in the latter case, tetrazine functionalized FGS20 has excellent conductivity, it has been further combined with polypyrrole to fabricate supercapacitor material.In chapter 2, we have covalently grafted tetrazine derivatives to graphene oxide through nucleophilic substitution. Since the tetrazine unit is electroactive and nitrogen-rich, with a reduction potential sensitive to the type of substituent and degree of substitution, we used electrochemistry and X-ray photoelectron spectroscopy to demonstrate clear evidence for grafting through covalent bonding. Chemical modification was supported by Fourier transform infrared spectroscopy and thermal analysis. Tetrazines grafted onto graphene oxide displayed different mass losses compared to unmodified graphene and were more stable than the molecular precursors. Finally, a bridging tetrazine derivative was grafted between sheets of graphene oxide to demonstrate that the separation distance between sheets can be maintained while designing new graphene-based materials, including chemically bound, redox structures.In chapter 3, model molecules of graphene were selected to determine the optimal reaction conditions between graphene and tetrazine derivatives. All tetrazine molecules were firstly studied by electrochemistry and then reacted with graphene through inverse electron demand Diels-Alder (DAinv) reaction in microwave reactor, X-ray photoelectron spectroscopy was carried out to study its chemical composition and prove the successfully modification of graphene. Then the tetrazine functionalized graphene material was coated on a Stainless Steel electrode and its electrochemical performances were assessed by cyclic voltammetry and charge-discharge experiments. Most of the tetrazine modified graphene materials showed very good electrochemical performance and a small resistance due to a good ion accessibility, which makes it one of the most promising electrode materials for supercapacitors so far.In chapter 4, polypyrrole (PPy)-graphene sheet nanocomposites have been synthesized by both chemical and in situ electrochemical polymerization of PPy on tetrazine derivatives functionalized graphene sheets. The modified graphene material contains pyridazine units as demonstrated by XPS. Then PPy was deposited on this functionalized graphene material either by chemical or electrochemical polymerization. Symmetrical coin cells were made to measure the capacitance in a two-electrode configuration. Polypyrrole-graphene nanocomposites with 40% PPy show the best electrochemical performances, with a very large capacitance per weight (326 F g-1 at 0.5 A g-1 and 250 F g-1 at 2 A g-1) and a small resistance due to a good ion accessibility, which makes it one of the best electrode materials for supercapacitors so far
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Poole, Timothy. "Acoustoelectric properties of graphene and graphene nanostructures." Thesis, University of Exeter, 2017. http://hdl.handle.net/10871/29838.
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The acoustoelectric effect in graphene and graphene nanoribbons (GNRs) on lithium niobate surface acoustic wave (SAW) devices was studied experimentally. Monolayer graphene produced by chemical vapour deposition was transferred to the SAW devices. The photoresponse of the acoustoelectric current (Iae) was characterised as a function of SAW frequency and intensity, and illumination wavelength (using 450 nm and 735 nm LEDs) and intensity. Under illumination, the measured Iae increased by more than the measured decrease in conductivity, while retaining a linear dependence on SAW intensity. The latter is consistent with the piezoelectric interaction between the graphene charge carriers and the SAWs being described by a relatively simple classical relaxation model. A larger increase in Iae under an illumination wavelength of 450 nm, compared to 735 nm at the same intensity, is consistent with the generation of a hot carrier distribution. The same classical relaxation model was found to describe Iae generated in arrays of 500 nm-wide GNRs. The measured acoustoelectric current decreases as the nanoribbon width increases, as studied for GNRs with widths in the range 200 – 600 nm. This reflects an increase in charge carrier mobility due to increased doping, arising from damage induced at the nanoribbon edges during fabrication. 2 Lastly, the acoustoelectric photoresponse was studied as a function of graphene nanoribbon width (350 – 600 nm) under an illumination wavelength of 450 nm. Under illumination, the nanoribbon conductivity decreased, with the largest percentage decrease seen in the widest GNRs. Iae also decreased under illumination, in contrast to the acoustoelectric photoresponse of continuous graphene. A possible explanation is that hot carrier effects under illumination lead to a greater decrease in charge carrier mobility than the increase in acoustoelectric attenuation coefficient. This causes the measured decrease in Iae.
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Huang, Xianjun. "Electromagnetic applications of graphene and graphene oxide." Thesis, University of Manchester, 2016. https://www.research.manchester.ac.uk/portal/en/theses/electromagnetic-applications-of-graphene-and-graphene-oxide(873c9618-19a3-4818-b47a-9afbca39857c).html.
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Since the isolation of graphene in 2004, a large amount of research has been directed at 2D materials and their applications due to their unique characteristics. This thesis delivers pioneering developments on the applications of graphene and graphene oxide (GO) on electromagnetic ranges such as radio frequency, microwave frequency and THz bands, and specifically 2D materials based antennas, absorbers, sensors and etc. This thesis focuses on exploring electromagnetic applications of monolayer graphene, printed graphene and graphene oxide. In study of monolayer graphene applications, the theoretical and simulation studies are carried out to design tunable terahertz (THz) absorbers, tunable microwave wideband absorbers, and reconfigurable antennas, etc. These studies on the applications of monolayer graphene have proved prospective potentials of graphene in THz sensing, RCS reduction, and reconfigurable antennas. This thesis also presents pioneering advances on electromagnetic applications of printed graphene. Among these works, low-cost highly conductive and mechanically flexible printed graphene is developed for radio frequency (RF) applications. For the first time, effective RF radiation of printed graphene is experimentally demonstrated. Based on these results, applications of printed graphene including RFID (radio frequency identification) tags, anti-tampering RFID, EMI shielding, flexible microwave components such as transmission lines, resonators and antennas, conformable wideband radar absorbers, graphene oxide based wireless sensors, etc. are developed and experimentally demonstrated. This work significantly expands applications of graphene in electromagnetic areas.
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Ghosh, Suchismita. "Thermal conduction in graphene and graphene multilayers." Diss., [Riverside, Calif.] : University of California, Riverside, 2009. http://proquest.umi.com/pqdweb?index=0&did=1957308711&SrchMode=2&sid=2&Fmt=2&VInst=PROD&VType=PQD&RQT=309&VName=PQD&TS=1268427434&clientId=48051.
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Thesis (Ph. D.)--University of California, Riverside, 2009.Includes abstract. Available via ProQuest Digital Dissertations. Title from first page of PDF file (viewed March 12, 2010). Includes bibliographical references (p. 96-107). Also issued in print.
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Risley, Mason J. "Surfactant-assisted exfoliation and processing of graphite and graphene." Thesis, Georgia Institute of Technology, 2013. http://hdl.handle.net/1853/48980.
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Surfactant assisted solution exfoliation of expanded graphite by means of sonication was carried out in an attempt to produce non-covalent charge functionality on the surface of graphene for the directed self assembly of graphene films on patterned substrates via electrostatic interactions. This thesis includes the results of experimental research associated with: 1) quantifying the effectiveness of various di-functionalized dithienothiophene surfactant small molecules, 2) further understanding the surface affinity and interaction mechanism between these surfactant molecules and the surface of expanded graphite and graphene and 3) experimentally testing the feasibility of the directed self-assembly of graphene films by means of charge functionalization of graphene by the surfactant molecules adsorbed onto the surface of exfoliated graphene.
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Abro, Mehwish. "Modelling the exfoliation of graphite for production of graphene." Thesis, Uppsala universitet, Fasta tillståndets elektronik, 2015. http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-272339.
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The aim of my thesis is to make a theoretical model of data obtained from liquid-phase exfoliation of graphene. The production of graphene in the liquid phase exfoliation is a cost efficient method One part of this work is devotedto learn the method of production of graphene by the shear mixing technique from the graphite and to estimate some important parameters which are crucial for the process. Other part of my work is based on studying the liquid-phase exfoliation mechanism of graphene through ultrasonication technique. This method is time consuming as compared to shearmixing.
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Alofi, Ayman Salman Shadid. "Theory of phonon thermal transport in graphene and graphite." Thesis, University of Exeter, 2014. http://hdl.handle.net/10871/15687.
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Thermal properties of graphene and graphite have been investigated by employing the analytical expressions for the phonon dispersion relations and the vibrational density of states derived by Nihira and Iwata, which are based on the semicontinuum model proposed by Komatsu and Nagamiya. The thermal conductivities of graphene and graphite are computed within the framework of Callaway’s effective relaxation time theory. The Normal-drift contribution (the correction term in Callaway’s theory) produces a significant addition to the result obtained from the single-mode relaxation time theory, clearly suggesting that the single-mode relaxation time approach alone is inadequate for describing the phonon conductivity of graphene. Its contribution to the thermal conductivity arises from the consideration of the momentum conserving nature of three-phonon Normal processes and is found to be very important for explaining the magnitude as well as the temperature dependence of the experimentally measured results for graphene and graphite. This model has not been implemented before for studying the thermal conductivity of graphene. Also the model has been applied to compute the thermal conductivity of graphene, graphite basal planes, and graphite c-axis. This has further been used to investigate the evolution of thermal properties from graphene to graphite as a function of layer thickness and temperature. The effects of isotopes and tensile strain on the graphene thermal properties have been examined within this model and compared with other available studies.
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Avril, Florian. "Contribution à l'élaboration d'un supercondensateur à basse de graphène." Thesis, Montpellier, 2018. http://www.theses.fr/2018MONTS034/document.
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L'utilisation de l'énergie des micro-sources de production d'électricité est un concept prometteur qui consiste à récolter des sources d'énergie faible et diffuse présent dans notre environnement pour l’alimentation de systèmes autonomes. Le nombre en croissance de nouveaux appareils miniaturisés et communicants dans les domaines civils et militaires devrait accentuer le phénomène de dépendance énergétique et ouvre de nouveaux marché.Parmi les éventuelles sources d’énergies renouvelables, l’énergie solaire est la source la plus prometteuse car elle est potentiellement la plus puissante et la mieux répartie. Le développement de ces systèmes de récupération des micro-sources d’énergie passe par de faibles coûts avec substrat souple (papier,polymère) et des matériaux facilement exploitables. Après la récupération de l’énergie, il est nécessaire pour les systèmes autonomes de stocker l'électricité.Dans cet objectif, les supercondensateurs sont les candidats idéaux. En effet, Le principal avantage des supercondensateurs par rapport aux batteries est leur haute densité de puissance (la collecte rapide de l’énergie) ainsi qu'une longue durée de vie. La thèse concerne donc la fabrication d’un supercondensateur et in fine le couplage avec une cellule solaire. Les travaux concernent spécifiquement l’étude de l'oxyde de graphène (GO) synthétisé par la méthode Hummers et Marcano, de sa réduction en oxyde de graphène réduit (RGO) par les voies chimique et électrochimique et de réalisation du supercondensateur. Dans ce projet, les propriétés de l'oxyde de graphène réduit (RGO) seront optimisées lors de l'étape de réduction et le matériau sera mis en forme dans une structure sandwich (RGO/ électrolyte /RGO) ou interdigité Mots clés: Graphène,supercondensateur, oxyde de graphène, micro-source d'énergieThe use of micro-power generation energy is a promising concept that consists in harvesting low and diffuse energy sources present in our environment for the supply of autonomous systems. The growing number of new miniaturized and communicating devices in civil and military fields should accentuate the phenomenon of energy dependence and open up new markets.Among possible sources of renewable energy, solar energy is the most promising source because it is potentially the most powerful and best distributed. The development of these micro-energy recovery systems involves low costs with flexible substrate (paper, polymer) and easily exploitable materials. After energy recovery, it is necessary for the autonomous systems to store electricity.For this purpose, supercapacitors are ideal candidates. Indeed, the main advantage of supercapacitors over batteries is their high power density (fast energy collection) as well as a long cycle life. The thesis concerns the manufacture of a supercapacitor and ultimately coupling with a solar cell. The work specifically concerns the study of graphene oxide (GO) synthesized by the Hummers and Marcano methods, its reduction in reduced graphene oxide (RGO) by chemical and electrochemical routes and the realization of supercapacitor. In this project, the properties of reduced graphene oxide (RGO) will be optimized during the reduction step and the material will be shaped into a sandwich structure (RGO / electrolyte / RGO) or interdigitated.Keywords: Graphene,supercapacitor, graphene oxide,energy micro-source
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Chu, Hua-Wei. "Development of solution-processed methods for graphene synthesis and device fabrication." Diss., Georgia Institute of Technology, 2011. http://hdl.handle.net/1853/44738.
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Various solution-processed methods have been employed in this work. For the synthesis of graphene, a chemical exfoliation method has been used to generate large graphene flakes in the solution phase. In addition, chemical or electro polymerization has been used for synthesizing polyanthracene, which tends to form graphene nanoribbon through cyclodehydrogenation. For the device fabrication, graphene oxide (GO) thin films were deposited from solution phase on the vapor-silanzed aminosilane surface to make semiconducting active layer or conducting electrodes. Gold nanoparticles (AuNPs) were selectively self-assembled from solution phase to pattern nanowires.
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Shokri, Roozbeh [Verfasser], and Günter [Akademischer Betreuer] Reiter. "Self-Assembly of supra-molecular systems on graphene or graphite = Selbstorganisation von Supramolekularen Systemen auf Graphen oder Graphit." Freiburg : Universität, 2013. http://d-nb.info/1123475415/34.
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El, Gemayel Mirella. "Graphene based supramolecular architectures and devices." Phd thesis, Université de Strasbourg, 2014. http://tel.archives-ouvertes.fr/tel-01070648.
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This thesis demonstrates that graphene produced by liquid-phase exfoliation can be co-deposited with a polymerie semiconductor for the fabrication of thin film field-effect transistors. The introduction of graphene to the n-type polymeric matrix enhances not only the electrical characteristics of the devices, but also the ambipolar behavior and the hole transport in particular. This provides a prospective pathway for the application of graphene composites for logic circuits.The same approach of blending was adopted to enhance the electrical characteristics of an amorphous p-type polymer semiconductor by addition of an unprecedented solution processable ultra-narrow graphene nanoribbon. GNRs form percolation pathway for the charges resulting in enhanced deviee performance in daras weil as under illumination therefore paving the way for applications in (opto)electronics.Finally, multifunctional photoresponsive devices were examined by introducing photochromic molecules exposing different substituents into small molecule or polymeric semiconductor films that were found to affect the photoswitching behavior.
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Pakulski, Dawid. "Graphene based materials and their potential applications." Thesis, Strasbourg, 2019. http://www.theses.fr/2019STRAF060.
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Cette thèse de doctorat a pour objectif scientifique la synthèse de matériaux bidimensionnels fonctionnalisés (graphène et oxyde de graphène) et leur caractérisation physicochimique complète, avec un accent particulier apporté sur les propriétés d'adsorption et de stockage d'énergie. Nous avons démontré que la modification covalente de l'oxyde de graphène (GO) avec un polymère organique (BPEI) affecte très favorablement l'efficacité du processus d'adsorption. Les valeurs de la capacité maximale d'adsorption (qmax) des ions de métaux lourds favorisent de manière significative ce matériau par rapport à la majorité des adsorbants connus à base de carbone. En outre la fonctionnalisation de GO avec l'aminosilicate mésoporeuse (SiO2NH2) conduit à l'obtention d'un adsorbant efficace et rapide des colorants organiques cationiques (MB, RhB, MV). En plus nous avons prouvé que la fonctionnalisation du graphène (EEG), en utilisant les sous-unités de surfactant POM, a montré que ce type de matériau hybride organique-inorganique est très stable et présente des propriétés électriques intéressantes pouvant être utilisées dans la production de supercondensateursScientific purpose of this doctoral dissertation is synthesis of functionalized two-dimensional materials (graphene and graphene oxide) and their comprehensive physicochemical characterization, with particular emphasis on adsorption and energy storage properties. We could demonstrate that covalent modification of graphene oxide (GO) with an organic polymer (BPEI) very favorably affects the efficiency of the adsorption process. The maximum adsorption capacity (qmax) values for heavy metal ions significantly favour this material in comparison to the majority of known carbon adsorbents. Moreover, functionalization of GO with mesoporous aminosilica (SiO2NH2) leads to obtaining an efficient and rapid adsorbent of organic cationic dyes (MB, RhB, MV). ln addition we proved that the functionalization of graphene (EEG) using the POM-surfactant su bu nits proved that this type of organic-inorganic hybrids material is very stable and have interesting electrical properties with potential application in the production of supercapacitors
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Schlierf, Andrea. "Graphene organic hybrid materials." Thesis, Strasbourg, 2014. http://www.theses.fr/2014STRAF050/document.
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En 2004, le carbone, la base de toute vie connue sur Terre, a marqué les esprits une fois de plus: Les scientifiques de l’Université de Manchester au Royaume Uni ont pu extraire une matière carbonée complètement nouvelle, le graphène à partir d’un morceau de graphite comme celui qui compose les crayons. À l’aide d’un ruban adhésif, ils ont obtenu une paillette de carbone de l’épaisseur d’un atome seulement, à une époque où beaucoup pensaient qu’un matériaux cristallin aussi fin ne pouvait pas être stable. Le graphène parfait est une couche monoatomique composée d’atomes de carbone hybridés sp2, arrangés en structure alvéolaire; sa structure chimique particulière lui donne des propriétés physiques et chimique remarquable. Le graphène est devenu rapidement la matière carbonée la plus intensivement étudiée parmi celles «possiblement révolutionnaires», avec ses applications potentielles s’étendant de la microélectronique aux composites, des énergies renouvelables à la médecine. En 2010, Geim et Novoselov ont été récompensés par le prix Nobel de physique pour leurs «expériences révolutionnaires sur les matériaux bi-dimensionnels en graphène» qui a ouvert une nouvelle ère dans la science des matières carbonées.La chimie non-covalente du graphène est exploitée et étudiée dans cette thèse dans le but de concevoir, produire, transformer et caractériser les nouveaux matériaux hybrides graphène-organique. L’étendue de ce travail couvre les aspects mécanistiques de l’exfoliation en phase liquide du graphène avec des colorants, les aspects fondamentaux des interactions entre le graphène et le chromophore, en phase liquide et solide, ainsi que l’élaboration de suspensions hybrides de graphène dans le but d‘applications en électronique organique et dans les matériaux composites polymères fonctionnelsIn 2004, carbon, the basis of all known life on earth, has surprised once again: Researchers from University of Manchester, UK, extracted a completely new carbon material, graphene, from a piece of graphite such as is found in pencils. Using adhesive tape, they obtained a flake of carbon with a thickness of just one single atom, at a time when many believed it impossible for such thin crystalline materials to be stable. Pristine graphene is a mono-atomic sheet of, sp2 hybridized carbon atoms arranged in a honeycomb network; this particular chemical structure gives rise to its outstanding physical and chemical properties. Graphene rapidly became the most intensively studied among the ‘possibly revolutionary’carbon materials, with its potential applications reaching from microelectronics to composites, from renewable energy to medicine. In 2010, Geim and Novoselov were honored with the Nobel Prize in Physics for their “ground breaking experiments regarding the two-dimensional material graphene” that started a new era in the science of carbon materials.In this thesis we exploit and study the non-covalent chemistry of graphene to design, produce, process and characterize novel graphene organic hybrid materials. The scope of this work covers mechanistic aspects of graphene liquid phase exfoliation with dyes, fundamental aspects of graphene chromophore interactions in liquid and solid phase and the formulation of graphene hybrid suspensions towards application in organic electronics and functional polymer composite materials
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Barabanova, Liudmyla. "Frictional Anisotropy of Graphene and Graphene Based Materials." University of Akron / OhioLINK, 2016. http://rave.ohiolink.edu/etdc/view?acc_num=akron1461941753.
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Wang, Mingchao. "Numerical investigation of graphene and graphene-polymer nanocomposites." Thesis, Queensland University of Technology, 2014. https://eprints.qut.edu.au/76082/1/Mingchao_Wang_Thesis.pdf.
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This thesis is a comprehensive and deep investigation on graphene and graphene-polymer nanocomposites. It explores the strong structure-property relationships in both graphene and graphene-based polymeric nanocomposites. A number of significant conclusions, including failure mechanism in graphene, interfacial load transfer and thermal transport mechanisms in graphene-polymer nanocomposites, have been drawn through both atomistic simulations and theoretical analysis. These results can provide direct guidelines for development of new graphene-based materials and devices.
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Zhao, Shen. "Propriétés optiques de nanorubans et boites quantiques de graphène." Thesis, Université Paris-Saclay (ComUE), 2018. http://www.theses.fr/2018SACLN032/document.
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Ce manuscrit présente une étude expérimentale sur les propriétés optiques des nanorubans de graphène (acronyme anglais : GNRs) et des boites quantiques de graphène (acronyme anglais : GQDs) synthétisés par la chimie ascendante.Pour la partie sur les GNRs, les spectres d'absorption et de photoluminescence ainsi que les mesures de la durée de vie sur la dispersion impliquent la formation d'états excimères résultant de l'agrégation des GNRs. Au moyen de la microscopie confocale et de la microscopie à force atomique, nous observons l'émission de petits agrégats de GNRs confirmant leur capacité à émettre de la lumière à l'état solide. D'autre part, les caractérisations optiques des GNRs synthétisés sur une surface d’or présentent des caractéristiques de Raman remarquables, impliquant les propriétés vibrationnelles spécifiques des GNRs par rapport au graphène et aux nanotubes de carbone. La PL observée est spectralement large avec une énergie plus élevée que celle de la bande interdite des GNRs. Cela pourrait être lié aux défauts créés lors de la préparation de l'échantillon.Pour la partie sur les GQDs, les résultats de spectroscopie optique indiquent que les GQDs sont individualisées en dispersion plutôt que sous la forme d’agrégats. Ensuite, grâce à la microphotoluminescence, nous abordons directement les propriétés intrinsèques des GQDs uniques. Des mesures de corrélation de photons de second ordre révèlent que les GQDs présentent une émission de photons uniques avec une grande pureté. De plus, l'émission de GQD présente une bonne photo-stabilité avec une brillance élevée. Comme premier exemple de l'accordabilité optique des GQDs via le contrôle de la structure, nous observons que l'émission de GQDs fonctionalisés avec des atomes de chlore est décalée de près de 100 nm tout en maintenant une émission de photons uniquesThis manuscript presents an experimental study on the optical properties of graphene nanoribbons (GNRs) and graphene quantum dots (GQDs) synthesized by bottom-up chemistry.For the part on GNRs, the optical absorption and photoluminescence spectra as well as the life-time measurements on the dispersion of solution-mediated synthesized GNRs implies the formation of excimer states as a result of aggregation of GNRs. By means of confocal fluorescence microscopy and atomic force microscopy, we observe the emission of small GNR aggregates confirming the ability of GNRs to emit light in the solid state. On the other hand, the optical characterizations of on-surface synthesized GNRs shows remarkable Raman features, implying the distinct vibrational properties of GNRs compared to graphene and carbon nanotubes. The observed PL is spectrally broad with higher energy instead of a bright bandgap emission, which might be related to the defects created during the sample preparation.For the part on GQDs, the optical spectroscopy results indicate that GQDs are individualized in dispersions rather than in the form of aggregates. Then by means of microphotoluminescence, we directly address the intrinsic properties of single GQDs. Second-order photon correlation measurements reveal that GQDs exhibit single-photon emission with a high purity. Notably, the emission of GQDs has good photo-stability with high brightness. As a first example of the optical tunability of GQDs through the control of their structure, we observe that the emission of single edge-chlorinated GQDs is redshifted by almost 100 nm while maintaining the single-photon emission
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Henni, Younes. "Etudes magnéto-Raman de systèmes - graphène multicouches et hétérostructures de graphène-nitrure de bore." Thesis, Université Grenoble Alpes (ComUE), 2016. http://www.theses.fr/2016GREAY060/document.
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Comme le quatrième élément le plus abondant dans l’univers, le carbone joue un rôle important dans l’émergence de la vie sur la terre comme nous la connaissons aujourd’hui. L’ère industrielle a vu cet élément au cœur des applications technologiques en raison des différentes façons dont les atomes forment les liaisons chimiques, ce qui donne lieu à une série d’allotropies chacun ayant des propriétés physiques extraordinaires. Par exemple, l’allotrope le plus thermodynamiquement stable du carbone, le cristal de graphite, est connu pour être un très bon conducteur électrique, tandis que le diamant, très apprécié pour sa dureté et sa conductivité thermique, est néanmoins considéré comme un isolant électrique en raison de sa structure cristallographique différente par rapport au graphite. Les progrès de la recherche scientifique ont montré que les considérations cristallographiques ne sont pas le seul facteur déterminant pour une telle variété dans les propriétés physiques des structures à base de carbone. Ces dernières années ont vu l’émergence de nouvelles formes allotropiques de structures de carbone qui sont stables dans les conditions ambiantes, mais avec dimensionnalité réduite, ce qui entraîne des propriétés largement différentes par rapport aux structures en trois dimensions. Parmi ces nouvelles classes d’allotropes il y a le graphene, qui est le premier matériau à deux dimensions. L’isolation réussi de monocouches de graphène a contesté une croyance établie depuis longtemps en physique : le fait que les matériaux purement 2D ne peuvent pas exister dans les conditions ambiantes parce qu'ils sont instables en raison de l’augmentation des fluctuations thermiques lorsqu’ils se prolongent dans les 2D. Afin de minimiser son énergie, un matériau se brisera en îlots coagulées. Le graphène arrive cependant à surmonter cette barrière en formant des ondulations continues sur la surface du substrat et est stable même à température ambiante et pression atmosphérique. Une grande intention dans la communauté scientifique a été donnée au graphène, après les premiers résultats publiés sur les propriétés électroniques de ce matériau. Les propriétés fondamentales et mécaniques du graphène sont fascinants. Grace aux atomes de carbone qui sont emballés dans un mode sp2 hybridé, formant ainsi une structure de réseau hexagonal, le graphène possède le plus grand module de Young et la plus grande capacité d’étirement, en même temps des centaines de fois plus dur que l’acier. Il conduit la chaleur et l’électricité de manière très efficace. L’aspect le plus fascinant à propos du graphène est surement la nature de ses porteurs de charge à basse énergie. En effet, le graphène présente des bandes d’énergie linéaires au point de neutralité de charge, donnant aux porteurs de charge une nature relativiste. De nombreux phénomènes observés dans ce matériau sont des conséquences de la nature relativiste de ses porteurs. Transport balistique, conductivité optique universelle, absence de rétrodiffusion, et une nouvelle classe d’effet Hall quantique sont de bons exemples de phénomènes nouvellement découverts dans ce matériau. Il est cependant encore trop tôt pour affirmer que toutes les propriétés physiques du graphene sont bien comprises. Dans cette thèse, nous avons mené des expériences de spectroscopie magnéto-Raman pour répondre à certaines des questions ouvertes dans la physique du graphène, notamment l’effet de couplage de Coulomb sur le spectre d’énergie du graphène, et le changement dans les propriétés physiques du graphène multicouche en fonction de sa cristallographie. Nos echantillions ont été soumis à de forts champs magnétiques, appliqués perpendiculairement aux plans atomiques. Le spectre d’excitation sous champ magnétique montre un couplage entre ces excitations et les modes de vibratoires. Cette approche expérimentale permet de remonter à la structure de bande du graphene en champs nul, ainsi que de nombreuses autres propriétés du matériauAs the fourth most abundant element in the universe, Carbon plays an important rolein the emerging of life in earth as we know it today. The industrial era has seen this element at the heart of technological applications due to the different ways in which carbon forms chemical bonds, giving rise to a series of allotropes each with extraordinary physical properties. For instance, the most thermodynamically stable allotrope of carbon, graphite crystal, is known to be a very good electrical conductor, while diamond very appreciated for its hardness and thermal conductivity is nevertheless considered as an electrical insulator due to different crystallographic structure compared to graphite. The advances in scientific research have shown that crystallographic considerations are not the only determining factor for such a variety in the physical properties of carbon based structures. Recent years have seen the emergence of new allotropes of carbon structures that are stable at ambient conditions but with reduced dimensionality, resulting in largely different properties compared to the three dimensional structures. Among these new classes of carbon allotropes is the first two-dimensional material: graphene.The successful isolation of monolayers of graphene challenged a long established belief in the scientific community: the fact that purely 2D materials cannot exist at ambient conditions. The Landau-Peierls instability theorem states that purely 2D materials are very unstable due to increasing thermal fluctuations when the material in question extends in both dimensions. To minimize its energy, the material will break into coagulated islands, an effect known as island growth. Graphene happens to overcome such barrier by forming continuous ripples on the surface of its substrate and thus is stable even at room temperature and atmospheric pressure.A great intention from the scientific community has been given to graphene, since 2004. Both fundamental and mechanical properties of graphene are fascinating. Thanks to its carbon atoms that are packed in a sp2 hybridized fashion, thus forming a hexagonal lattice structure, graphene has the largest young modulus and stretching power, yet it is hundreds of times stronger than steel. It conducts heat and electricity very efficiently, achieving an electron mobility as high as 107 cm−2V−1 s−1 when suspended over the substrate. The most fascinating aspect about graphene is the nature of its low energy charge carriers. Indeed, graphene has a linear energy dispersion at the charge neutrality, giving the charge carriers in graphene a relativistic nature. Many phenomena observed in this material are consequences of this relativistic nature of its carriers. Ballistic transport, universal optical conductivity, absence of back-scattering, and a new class of room temperaturequantum Hall effect are good examples of newly discovered phenomena in thismaterial. Graphene has become an active research area in condensed matter physics since 2004. It is however still early to state that all the physical properties of this material are well understood. In this thesis we conducted magneto-Raman spectroscopy experiments to address some of the open questions in the physics of graphene, such as the effect of electron-electron coupling on the energy spectrum of monolayer graphene, and the change in the physical properties of multilayer graphene as a function of the crystallographic stacking order. In all our experiments, the graphene-based systems have been subject to strong continuous magnetic fields, applied normal to the graphene layers. We study the evolution of its energy excitation spectra in the presence of the magnetic field, and also the coupling between these excitations and specific vibrational modes that are already in the system. This experimental approach allows us to deduce the band structure of the studied system at zero field, as well as many other lowenergy properties
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Nemec, Lydia. "Graphene engineering." Doctoral thesis, Humboldt-Universität zu Berlin, Mathematisch-Naturwissenschaftliche Fakultät, 2015. http://dx.doi.org/10.18452/17262.
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Die besonderen Eigenschaften von Graphen ermöglichen das Design von elektronischen Bauteilen im Nanometerbereich. Graphen kann auf der Oberfläche von Siliziumkarbonat (SiC) durch das Ausdampfen von Si epitaktisch gewachsen werden. Ein detailliertes Verständnis der atomaren und elektronischen Struktur der Grenzschicht zwischen Graphen und SiC ist ein wichtiger Schritt um die Wachstumsqualität zu verbessern. Wir nutzen Dichtefunktionaltheorie um das Hybridsystem Graphen-SiC auf atomarer Ebene zu beschreiben. Experimentelle Arbeiten auf der Si Seite von SiC haben gezeigt, dass die Grenzschicht (ZLG) durch eine teilweise kovalent gebundene Kohlenstofflage wächst; darüber bildet sich die erste Graphenlage (MLG). Durch das Konstruieren eines ab initio Oberflächenphasendiagrams zeigen wir, dass sowohl ZLG als auch MLG Gleichgewichtsphasen sind. Unsere Ergebnisse implizieren, dass Temperatur- und Druckbedingungen für den selbstbegrenzenden Graphenwachstum existieren. Wir zeigen, dass sich das Doping und die Riffellung von epitaktischem Graphene durch H-Interkalation reduzieren. Im Experiment unterscheidet sich das Graphenwachstum auf der C Seite qualitativ von der Si Seite. Zu Beginn des Graphenwachstums wird eine Mischung verschiedener Oberflächenphasen beobachtet. Wir diskutieren die Stabilität dieser konkurierenden Phasen. Die atomaren Strukturen von einigen dieser Phasen, inklusive der Graphen-SiC Grenzschicht, sind nicht bekannt wodurch die theoretische Beschreibung erschwert wird. Wir präsentieren ein neues Model für die bisher unbekannte (3x3) Rekonstruktion, das Si Twist Model. Die Oberflächenenergie vom Si Twist Model und von der bekannten (2x2)c Phase schneiden sich direkt an der Grenze zur Graphitbildung. Dies erklärt die experimentell beobachtete Phasenkoexistenz zu Beginn des Graphenwachstums. Wir schlussfolgern, dass auf der C Seite der kontrollierte Graphenewachstum durch Si-reiche Oberflächenphasen blockiert wird.Graphene with its unique properties spurred the design of nanoscale electronic devices. Graphene films grown by Si sublimation on SiC surfaces are promising material combinations for graphene applications. Understanding the atomic and electronic structure of the SiC-graphene interface, is an important step to refine the growth quality. In this work, density-functional theory is used to simulate the SiC-graphene interface on an atomistic level without empirical parameters. Experimental work has shown that on the Si face of SiC, a partially covalently bonded carbon layer, the zero-layer graphene (ZLG), grows. On top of the ZLG layer forms mono-layer graphene (MLG) as large ordered areas and then few-layer graphene. By constructing an ab initio surface phase diagram, we show that ZLG and MLG are at least near equilibrium phases. Our results imply the existence of temperature and pressure conditions for self-limiting growth of MLG key to the large-scale graphene production. H intercalation significantly reduces both the corrugation and the graphene doping. Our calculations demonstrate that unsaturated Si atoms in the ZLG influence the electronic structure of graphene. The situation on the C face of SiC is very different. The experimental growth of large areas of graphene with well defined layer thickness is difficult. At the onset of graphene formation a phase mixture of different surface phases is observed. We will address the stability of the different occuring surface phases. However, the atomic structure of some of the competing surface phases, as well as of the SiC-graphene interface, is unknown. We present a new model for the (3x3) reconstruction, the Si twist model. The surface energies of this Si twist model, the known (2x2)c adatom phase, and a graphene covered (2x2)c phase cross at the chemical potential limit of graphite, which explains the observed phase mixture. We argue that well-controlled graphene formation is hindered by Si-rich surface phases.
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Leve, Zandile Dennis. "Determination of paracetamol at the electrochemically reduced graphene oxide-metal nanocomposite modified pencil graphite (ERGO-MC-PGE) electrode using adsorptive stripping differential pulse voltammetry." University of Western Cape, 2020. http://hdl.handle.net/11394/7350.
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>Magister Scientiae - MScThis project focuses on the development of simple, highly sensitive, accurate, and low cost electrochemical sensors based on the modification of pencil graphite electrodes by the electrochemical reduction of graphene oxide-metal salts as nanocomposites (ERGO-MC-PGE; MC = Sb or Au nanocomposite). The electrochemical sensors ERGO-Sb-PGE and ERGO-Au-PGE were used in the determination of paracetamol (PC) in pharmaceutical formulations using adsorptive stripping differential pulse voltammetry. The GO was prepared from graphite via a modified Hummers’ method and characterized by FTIR and Raman spectroscopy to confirm the presence of oxygen functional groups in the conjugated carbon-based structure whilst, changes in crystalline structure was observed after XRD analysis of graphite and GO.
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Sokolov, Denis A. "Investigation of Graphene Formation from Graphite Oxide and Silicon Carbide." Diss., Georgia Institute of Technology, 2013. http://hdl.handle.net/1853/53642.
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Graphene is a novel two dimensional material that is revolutionizing many areas of science and it is no surprise that a significant amount of effort is dedicated to its investigation. One of the major areas of graphene research is the development of procedures for large scale production. Among many recently developed methodologies, graphene oxide reduction stands out as a straightforward and scalable procedure for producing final material with properties similar to those of graphene. Laser reduction of graphite oxide is one of the novel approaches for producing multilayer graphene, and this work describes a viable approach in detail. It is determined that a material which is comprised of a combination of laser reduced graphite oxide-coupled to an unreduced graphite oxide layers beneath it, produces a broadband photosensitive material. The efficiency of light conversion into electrical current is greatly dependent upon the oxygen content of the underlying graphite oxide. Developing novel ways for reducing graphite oxide is an ongoing effort. This work also presents a new method for achieving complete reduction of graphite oxide for producing predominantly sp2 hybridized material. This approach is based on the irradiation of graphite oxide with a high flux 3 keV Ar ion beam in vacuum. It is determined that the angle of irradiation greatly influences the final surface morphology of reduced graphite oxide. Also, multilayer epitaxial graphene growth on silicon carbide in ultra-high vacuum was investigated with quadrupole mass spectrometry (QMS). Subliming molecular and atomic species were monitored as a function of temperature and heating time. The grown films were characterized with X-ray photoelectron spectroscopy coupled with Ar ion depth profiling.
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Cuharuc, Anatolii S. "Electron transfer on graphene and graphite : theoretical and experimental study." Thesis, University of Warwick, 2015. http://wrap.warwick.ac.uk/78764/.
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The redox behaviour of several couples on highly oriented pyrolytic graphite (HOPG) and graphene has been studied using both micro- and macroscopic measurements supported by detailed analysis. The ET kinetics of Ru(NH3)6 3+/2+ , (ferrocenylmethyl)trimethylammonium (FcTMA2+/+ ), and ferrocenylcarboxylic acid (FcCOO-/0 ) was found fast, on the time-scale of voltammetric measurements, on a freshly cleaved HOPG surface, but on “aged” one, Ru(NH3)6 3+/2+ exhibited sluggish ET, showing quite unusual macroscopic cyclic voltammograms. The other two couples retained their fast response on an “aged” surface. The surface of “aged” HOPG is proposed to consist of graphene layers of different thicknesses, which, from graphite to monolayer graphene, have progressively diminishing capacity to support fast ET specifically for Ru(NH3)6 3+/2+ . Such a redox-selectivity correlates with the position of formal potentials of the three redox couples relative to the band structure of graphene, with Ru(NH3)6 3+/2+ being most close to the minimum in density of states (DOS) of undoped monolayer graphene. Based on macroscopic voltammetry, low grade HOPG, whose surface is abundant with step edges, was found not to be redox-selective in the sense described above, meaning that its “aged” surface was as good as fresh one. It is highly likely that step edges being a type of crystal lattice defects, retain their capacity for fast ET, which correlates with their elevated DOS, and, thus, secure fast voltammetric response of low grade HOPG in macroscopic experiments on these complex “aged” surfaces. IrCl6 2-/3- , Fe(CN)6 3-/4- and the three couples discussed above were all found to have fast electrochemistry on freshly cleaved surfaces of high grade HOPG. Estimated heterogeneous rate constants were > 0.1 cm s- 1 for Ru(NH3)6 3+/2+ and > 1.7 cm s- 1 for IrCl6 2-/3-and Fe(CN)6 3-/4 . This suggests that basal planes of graphite have, though low, but sufficient DOS to perform ET at a rate comparable with that on some metals. Furthermore, these results unarguably defy those opinions, that have been long circulating in the published literature, stating that basal plane of HOPG (and also sidewalls of carbon nanotubes) are nearly inert towards ET and that the defects were solely responsible for the observed electrochemical activity of this material. The study of adsorption of three ferrocene derivatives on HOPG revealed FcTMA+ and FcCOO- adsorb weakly and approximately equally whereas zero-charged FcCH2OH adsorbs notably more strongly. The adsorption was studied with cyclic voltammetry and quantitative information was extracted from the experimental data basing on a simple dedicated theory developed in this thesis. The “aged” surface of high grade HOPG exhibited enhanced adsorption as compared to fresh one, but low grade HOPG did not show the difference. It is suggested that the highly flat surface of high grade HOPG (large terraces widths) facilitates formation of airborne contaminating film made of hydrocarbons on, which serves as a “trap” for ferrocene derivatives owing to their largely non-polar ferrocene moieties.
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Arbuzov, A. A., V. E. Muradyan, and B. P. Tarasov. "Synthesis of Few-layer Graphene Sheets via Chemical and Thermal Reduction of Graphite Oxide." Thesis, Sumy State University, 2012. http://essuir.sumdu.edu.ua/handle/123456789/35063.
Full textAbstract:
Few-layer graphene sheets were produced from graphite oxide (GO) chemical and thermal reduction.
For the chemical reduction of GO as reducing agents were used hydrazine hydrate, hydroxylammonium
chloride, sodium borohydride and sodium sulfite. The reduced material was characterized by elemental
analysis, thermo-gravimetric analysis, scanning electron microscopy, X-ray diffraction, Fourier transform
infrared and Raman spectroscopy. A comparison of the deoxygenation efficiency of graphene oxide suspension by different method or reductants has been made, revealing that the highest degree of reduction was
achieved by thermal reduction and using hydrazine hydrate and hydroxylammonium chloride as a reducing agents.
When you are citing the document, use the following link http://essuir.sumdu.edu.ua/handle/123456789/35063
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Mittendorff, Martin. "Carrier Relaxation Dynamics in Graphene." Forschungszentrum Dresden, 2015. http://nbn-resolving.de/urn:nbn:de:bsz:d120-qucosa-162161.
Full textAbstract:
Graphene, the two-dimensional lattice of sp2-hybridized carbon atoms, has a great potential for future electronics, in particular for opto-electronic devices. The carrier relaxation dynamics, which is of key importance for such applications, is in the main focus of this thesis. Besides a short introduction into the most prominent material properties of graphene and the experimental techniques, this thesis is divided into three main parts.
The investigation of the carrier relaxation dynamics in the absence of a magnetic field is presented in Chapter 3. In the first experiment, the anisotropy of the carrier excitation and relaxation in momentum space was investigated by pump-probe measurements in the near-infrared range.
While this anisotropy was not considered in all previous experiments, our measurements with a temporal resolution of less than 50 fs revealed the polarization dependence of the carrier excitation and the subsequent relaxation. About 150 fs after the electrons are excited, the carrier distribution in momentum space gets isotropic, caused by electron-phonon scattering. In a second set of two-color pump-probe experiments, the temperature of the hot carrier distribution, which was obtained within the duration of the pump pulse (about 200 fs), could be estimated. Furthermore, a change in sign of the pump-probe signal can be used as an indicator for the Fermi energy of different graphene layers.
Pump-probe experiments in the far-infrared range in reflection and transmission geometry were performed at high pump power. A strong saturation of the pump-induced transmission was found in previous experiments, which was attributed to the pump-induced change in absorption. Our investigation shows the strong influence of pump-induced reflection at long wavelengths, as well as a lot smaller influence of the saturation of the pump-induced change in absorption. At a high pump power, the increase of the reflection exceeds the change in absorption strongly, which leads to negative pump-probe signals in transmission geometry.
In Chapter 4, investigations of the carrier dynamics of graphene in magnetic fields of up to 7T are presented. Even though the optical properties of Landau-quantized graphene are very interesting, the carrier dynamics were nearly unexplored. A low photon energy of 14meV allows the investigation of the intraband Landau-level (LL) transitions. These experiments revealed two main findings: Firstly, the Landau quantization strongly suppresses the carrier relaxation via optical-phonon scattering, resulting in an increased relaxation time.
Secondly, a change in sign of the pump-probe signal can be observed when the magnetic field is varied. This change in sign indicates a hot carrier distribution shortly after the pump pulse, which means that carrier-carrier scattering remains very strong in magnetic fields. In a second set of pump-probe measurements, carried out at a photon energy of 75meV, the relaxation dynamics of interband LL transitions was investigated. In particular, experiments on the two energetically degenerate LL transitions LL(−1)->LL(0) and LL(0)->LL(1) showed the influence of extremely strong Auger processes.
An ultrafast and extremely broadband terahertz detector, based on a graphene flake, is presented in the last chapter of this thesis. To couple the radiation efficiently to the small flake, the inner part of a logarithmic periodic antenna is connected to it. With a rise time of about 50 ps in a wavelength range of 9 μm to 500 μm, this detector is very interesting to obtain the temporal overlap in two-color pump-probe experiments with the free-electron laser FELBE. Furthermore, the importance of the substrate material, in particular for the high-speed performance, is discussed.
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Dlubak, Bruno. "Transport dépendant du spin dans le graphène." Thesis, Paris 11, 2011. http://www.theses.fr/2011PA112115.
Full textAbstract:
Par delà ses applications largement distribuées pour le stockage de l’information binaire, la spintronique vise le traitement Booléen de l’information. Des dispositifs de logique de spin (spin-FETs et portes logiques de spin) sont envisagés en se basant sur la propagation et la manipulation de porteurs polarisés en spin injectés dans un canal latéral depuis un contact magnétique. En dépit de deux décennies de recherche active, l’efficacité des dispositifs (notamment en termes de longueur de propagation du spin et d’amplitude du signal de spin) est toujours limitée quand le canal latéral est un métal ou un semi-conducteur conventionnel : la mise en évidence d’un medium adapté est nécessaire.Le transport dépendant du spin dans le graphène a été étudié dans le cadre de cette thèse. Dans un premier temps, l'impact et la structure de barrières tunnel de haute qualité déposées sur le graphène, nécessaires pour l'injection efficace de l'information de spin, ont été étudiés. A partir de ces résultats, des dispositifs complets d'injection, transport et détection de spin basés sur un feuillet de graphène épitaxié ainsi que sur une bicouche de graphène exfoliée ont alors été construits. Enfin, des mesures de transport du spin dans ces dispositifs ont été effectuées, puis analysées via les modèles classiques de drift/diffusion. Les forts signaux obtenus (gamme du MΩ), en validant l'existence d'un support capable de transporter le spin avec une très forte efficacité sur des distances macroscopiques (jusqu’à 200 µm), ouvrent la voie à une implémentation des concepts de traitement de l'information de spinBeyond its widely distributed applications for binary data storage, spintronics aims Boolean information processing. Spin logic devices (spin-FETs and spin logic gates) are envisioned through the propagation and the manipulation of a spin-polarized carriers injected into a lateral channel from a magnetic contact. In spite of two decades of active research, the devices efficiency (in particular in terms of spin propagation length and spin signal amplitude) is still limited when the lateral channel is made of conventional metals or semiconductors: a suitable host still lacks. Spin-dependant transport in graphene is investigated in this thesis. At first, the impact of the growth and on the structure of high quality tunnel barriers deposited on graphene, required to obtain an efficient injection of the spin information, has been studied. From these results, complete devices for spins injection, transport and detection based on an epitaxial graphene sheet and also on an exfoliated bilayer graphene flake were built. Finally, measurements of spin transport in these devices were carried, and then analyzed through classical drift/diffusion models. The strong signals obtained (MΩ range), by validating the existence of a substrate able to carry the spin information with a very large efficiency on macroscopic distances (up to 200 µm), open the way for the implementation of spin processing concepts
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Haar, Sébastien. "Supramolecular approaches to graphene : generation of functional hybrid assemblies." Thesis, Strasbourg, 2015. http://www.theses.fr/2015STRAF040/document.
Full textAbstract:
Cette thèse démontre le potentiel dont dispose l’exfoliation en phase liquide du graphite dans le but d’obtenir des feuillets de graphène dispersés dans un solvant organique. Ainsi le mécanisme d’exfoliation a été étudié en profondeur, en particulier, l’influence de plusieurs paramètres (température, puissance et solvants). Le choix de ses paramètres se montre crucial dans le contrôle du procédé, et pour l’obtention des feuillets de graphène ayant une taille ciblée. Il est donc possible de fabriquer des nano-feuillets de quelques dizaines de nanomètre qui en plus possèdent des propriétés de photoluminescence.Dans le but de comprendre le mécanisme d’exfoliation en phase liquide assistée par des molécules, une nouvelle approche a été mise au point : l’approche supramoléculaire. Cette approche se base sur l’utilisation de surfactants d’un nouveau type. En effet, les molécules sélectionnées possèdent une longue chaine alkyle. Cette chaine s’adsorbe sur la surface du graphène et permet de stabiliser les feuillets lors de l’exfoliation. L’influence de la taille de la chaine alkyle de ces molécules lors de l’exfoliation a été vérifiée. De plus, ces molécules ont été équipées de différentes fonctions supramoléculaires afin qu’elles puissent former des dimères sur la surface du graphène. L’ajout de ces molécules augmente non seulement le rendement d’exfoliation mais aussi le nombre de mono-feuillets présents dans ces dispersions. Ces dispersions présentent des propriétés conductrices lorsqu’elles sont déposées sur des substrats. Une nouvelles méthode de déposition a été mise au point afin d’améliorer et d’augmenter la conductivité mais aussi le pourcentage de transparenceThis thesis demonstrates the potential of exfoliation of the graphite in the liquid phase in order to obtain graphene sheets dispersed in an organic solvent. Thus the exfoliation mechanism has been studied, in particular, the influence of several parameters (temperature, power and solvents). The choice of parameters is actually crucial for the control of the process, and to obtain graphene sheets having a targeted size. It is therefore possible to manufacture nanosheets of several tens of nanometers, which in addition exhibit photoluminescence properties.In order to understand the exfoliation mechanism in liquid phase assisted by molecules, a new approach has been developed: the supramolecular approach. This approach is based on using a new type of surfactant. Indeed, the selected molecules carry a long alkyl chain. This chain is adsorbed on the surface of graphene and can stabilize the sheets during exfoliation. The influence of the size of the alkyl chain of these molecules during exfoliation was verified. Furthermore, these molecules have been equipped with various supramolecular functions, which can form dimers on the surface of graphene. The addition of these molecules not only increases exfoliation performance but also the number of mono-layers present in these dispersions. These dispersions have conductive properties when deposited on substrates. A new deposition method was developed to enhance and increase conductivity but also the percentage of transparency
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Nair, Nityan. "Photon-induced tunneling in graphene-boron nitride-graphene heterostructures." Thesis, Massachusetts Institute of Technology, 2013. http://hdl.handle.net/1721.1/83803.
Full textAbstract:
Thesis (S.B.)--Massachusetts Institute of Technology, Dept. of Physics, 2013.Cataloged from PDF version of thesis.
Includes bibliographical references (pages 55-56).
Graphene is a material that has generated much interest due to its many unique electronic and optical properties. In this work, we present optoelectronic measurements performed on ultrathin graphene-boron nitride-graphene heterostructures. Scanning photocurrent spectroscopy allows us to explore the tunneling behavior of these devices as a function of both photon energy and bias voltage. Tunneling through the boron nitride insulator is found to be dramatically enhanced by the presence of light, showing a high-bias behavior that can be well described using Fowler-Nordheim tunneling. These measurements indicate that tunneling is dominated by photoexcited positive charge carriers (holes) with an intrinsic barrier height and effective mass of 1.33eV and 1.1 9me, respectively. These numbers agree well with theoretical calculations of the offset between the top of the valence band in boron nitride and the charge neutrality point in graphene, and the effective mass of holes in boron nitride. Moreover, a peak in the conductance was observed at zero bias voltage, indicating the presence of thermionic emission near the charge neutrality point..
by Nityan Nair.
S.B.
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Bointon, Thomas H. "Graphene and functionalised graphene for flexible and optoelectric applications." Thesis, University of Exeter, 2015. http://hdl.handle.net/10871/17620.
Full textAbstract:
The landscape of consumer electronics has drastically changed over the last decade. Technological advances have led to the development of portable media devices, such as the iPod, smart phones and laptops. This has been achieved primarily through miniaturisation and using materials such as Lithium and Indium Tin Oxide (ITO) to increase energy density in batteries and as transparent electrodes for light emitting displays respectively. However, ten years on there are now new consumer demands, which are dictating the direction of research and new products are under constant development. Graphene is a promising next-generation material that was discovered in 2004. It is composed of a two-dimensional lattice made only from carbon. The atoms are arranged in a two atom basis hexagonal crystal structure which forms a fundamental building block of all sp2 hybrid forms of carbon. The production of large area graphene has a high cost, due to the long growth times and the high temperatures required. This is relevant as graphene is not viable compared to other transparent conductors which are produced on industrial scales for a fraction of the cost of graphene growth. Furthermore, graphene has a high intrinsic resistivity (2KW/_) which is three orders of magnitude greater than the current industry standard ITO. This limits the size of the electrodes as there is dissipation of energy across the electrode leading to inefficiency. Furthermore a potential drop occurs across the electrode leading to a non-uniform light emission when the electrode is used in a light emitting display. I investigate alternative methods of large area graphene growth with the aim of reducing the manufacturing costs, while maintaining the quality required for graphene human interface devices. Building on this I develop new fabrication methods for the production of large-area graphene devices which are flexible and transparent and show the first all graphene touch sensor. Focusing on the reducing the high resistivity of graphene using FeCl3 intercalation, while maintaining high optical transmission, I show low resistivity achieved using this process for microscopic graphene flakes, large-area graphene grown on silicon carbide and large-area graphene grown by CVD. Furthermore, I explore the stability of FeCl3 intercalated graphene and a process to transfer a material to arbitrary flexible substrates.
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van, der Laan Timothy Anthony. "Plasma-based growth of graphene and graphene-related structures." Thesis, The University of Sydney, 2016. http://hdl.handle.net/2123/16143.
Full textAbstract:
The desire for smaller, faster, stronger, more efficient, more selective and more sensitive devices provides the impetus for research into materials at the nanoscale. Atomically thin carbon, arranged in a flat honeycomb lattice, where each carbon is bonded to 3 others by sp2 carbon bonds forms graphene. This material is of particular interest to researchers as it has the potential to enable new functionalities as well as improve existing devices in a number of the desired ways. Graphene is a material particularly relevant to electronics, energy, sensing and bio-medical applications however limitations in synthesising and incorporating it into devices present a major hurdle in its use. These two key issues need to be resolved to allow a new generation of graphene-enabled devices to be produced. Techniques to synthesise graphene are varied. They are predominantly made up of wet chemistry, mechanical cleaving and chemical vapour deposition (CVD) techniques. These techniques, however, have yet to resolve the problems related to making graphene in a device compatible manner nor allowing its transfer onto a device. One of the most promising techniques is CVD on a copper catalyst at elevated temperatures. This technique though, requires high temperatures of ~900°C, lacks appreciable control and requires the etching or chemical treatment of the catalyst to separate it from the graphene grown. It does however allow scalability and produces high quality and large crystal grain graphene. The work outlined in this thesis targets graphene growth on copper, however, instead of using elevated temperatures a plasma is implemented to allow growth. Plasma-based approaches to graphene growth have been limited. This work utilises a custom built system featuring an inductively coupled plasma generated using a radio frequency power supply operating at 13.56 MHz. The set-up allows the generation of a dense, low temperature plasma which has beneficial properties which can be harnessed for graphene growth. Through plasma heating, selective etching, ionisation, and plasma enhancement of surface processes, single layer graphene can be grown without the need for an additional heating source. Graphene can be grown at temperatures as low as 220 °C. This process features many of the same beneficial features of the copper CVD process however trades crystal size and quality for significantly reduced temperatures. Furthermore, this process has provided a new method for transferring graphene from the catalyst to a device. The transfer method presented here uses no etchants or chemicals at all, just deionised water. This process therefore offers solutions to two of the major problems for incorporating graphene into new devices: reduced growth temperature and non-destructive, simple graphene transfer. This novel growth process was investigated further to elucidate some of the interesting phenomena occurring allowing graphene growth. Control over the plasma process was demonstrated, with different graphene films produced. These films differ by quality and morphology, both horizontal and vertically aligned graphene materials can be grown. It was discovered that by controlling the carbon concentration in the plasma growth of vertical or horizontal structures can be controlled. Considerable changes are also made to the catalyst during plasma exposure. Crystallography and surface energy can be tuned and modified through plasma exposure. There are real benefits to modifying the crystallography of the catalyst for graphene growth. Modification of the catalyst and the graphene film was also found to be crucial for allowing the film to be removed from the catalyst. By modifying the plasma before and after graphene growth we are able to determine whether or not the film can decouple from the catalyst. The properties of the films were found to be dependent on both the growth conditions and the transfer of the films. Whilst control exists in the plasma environment in modifying crystal properties it was found that the films electronic properties was more influenced by the transfer. The plasma based process developed in this thesis offers benefits of catalyst reuse, energy efficiency, control, versatility and simplicity and is presented as a viable alternative to other approaches to synthesise graphene for new generation graphene-enabled devices.
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Liu, Shizhen. "Graphene oxide and graphene based catalysts in photochemical ractions." Thesis, Curtin University, 2013. http://hdl.handle.net/20.500.11937/882.
Full textAbstract:
Graphene has impressible absorbing ability and its electron transmission capacity makes it a great prosperity in many science horizons. In this study graphene or graphite nitride has been employed as a carrier in order to modify TiO2, ZnO and Ta2O5 photocatalysts.Graphene modified TiO2 particles were obtained by a sol-gel method from titanium isoproproxide (or P25) and reduced graphene oxide (RGO). The X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), field emission scanning electron microscopy (FE-SEM), UV-vis diffuse reflectance (UV-vis DRS) and thermo gravimetric differential thermal analysis (TG-DTA) were investigated over the samples. The diffuse reflectance spectra (DRS) studies indicate that G-TiO2 has a significant light absorption increasing and red shift of absorption peak. G-TiO2 photocatalyst could decompose methylene blue under visible light (> 430 nm). G-TiO2 synthesised from titanium isoproproxide presented better activity than G-TiO2 (P25). The catalysts could also produce ●OH and [O2]- radicals via electron scavengers (peroxymonosulphate, peroxydisulphate and hydrogen peroxide) to enhance degradation process with visible illumination.ZnO loaded RGO photocatalysts were synthesized through Zn powder and graphite oxide. The structural, morphological, and physicochemical properties of the samples were thoroughly investigated by XRD, FT-IR, FE-SEM, UV-visible DRS, TG-DTA, and Raman spectroscopy. Zn powder could successfully reduce GO and ZnO was obtained simultaneously by one-step hydrothermal method. RGO-ZnO photocatalysts could bleach MB under UV-vis illumination.Three different compounds: ammonia, graphene and C3N4 were utilized to dope tantalum pentoxide photocatalyst. Catalysts were analyzed by X-ray diffraction, UV–vis diffuse reflectance spectra and FTIR spectroscopy. The photocatalytic behavior was thorough investigated in bleaching methylene blue under UV-visible illuminations; the modified catalysts could decompose methylene blue, showing better activity than undoped Ta2O5. However, only N-doped Ta2O5 will show activity under visible light.
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BARBARINO, GIULIANA. "Thermal properties of graphene and graphene-based thermal diodes." Doctoral thesis, Università degli Studi di Cagliari, 2016. http://hdl.handle.net/11584/266670.
Full textAbstract:
In the perspective of manipulating and controlling heat fluxes, graphene
represents a promising material revealing an unusually high thermal
conductivity �. However, both experimental and theoretical previous
works lack of a strict thermal conductivity value, estimating results
in the range 89-5000 W m-1 K-1. In this scenario, I address graphene
thermal transport properties by means of molecular dynamics simulations
using the novel "approach to equilibrium molecular dynamics"
(AEMD) technique.
The first issue is to offer some insight on the active debate about
graphene thermal conductivity extrapolation for infinite sample. To
this aim, I perform unbiased (i.e. with no a priori guess) direct atomistic
simulations aimed at estimating thermal conductivity in samples
with increasing size up to the unprecedented value of 0.1 mm. The
results provide evidence that thermal conductivity in graphene is definitely
upper limited, in samples long enough to allow a diffusive
transport regime for both single and collective phonon excitations.
Another important issue is to characterize at atomistic level the experimental
techniques used to estimate graphene thermal conductivity.
Some of these use laser source to provide heat. For these reasons,
I deal with the characterization of the transient response to a
pulsed laser focused on a circular graphene sample. In order to reproduce
the laser effect on the sample, the K - A01
and
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Zheng, Yi. "Compact multiscale modeling of carbon-based nano-transistors." Thesis, Sorbonne université, 2018. http://www.theses.fr/2018SORUS518.
Full textAbstract:
Parmi les nouveaux matériaux émergents à base de carbone, le graphène est rapidement devenu un candidat idéal pour plusieurs applications en nanoélectronique. Dans ce contexte, différentes méthodes ont été proposées pour transformer ses propriétés électriques, et notamment supprimer sont point de dégénérescence de Dirac. L’ouverture d’un gap d’énergie peut ainsi conduire à l’usage du graphène dans des nano-transistors. Dans cette thèse, nous appliquons un modèle compact semi-analytique pour étudier deux types de nanotransistors à base de graphène: les transistors à nanorouban et les transistors à nanomesh. Un modèle de type thight-binding est utilisé pour déterminer les expressions analytiques des bandes d'énergie d'un nanorouban de graphène. Des comparaisons sont montrées avec des approches ab initio, et avec des mesures effectuées sur des transistors du même type mais à plus grande échelle. Dans le contexte de l'électronique pour applications souples, les contraintes mécaniques sur les circuits et les déformations géométriques des composants à base de graphène peuvent constituer un problème important. Nous étudions ces effets sur les propriétés de conduction des transistors à nanorubans (dans les régimes balistique et partiellement balistique). En supposant la présence de petites déformations, une mise à l'échelle spectrale et un décalage spectral dû à la présence d'une déformation peuvent être pris en compte de manière analytique. Ce modèle conduit à définir sous forme analytique les quantités effectives (masses, densités d’états) utilisées pour calculer numériquement les potentiels et les courants dans le nano-dispositif. Les résultats numériques sont présentés à la fois sous un régime balistique et partiellement balistique, avec ou sans contacts de Schottky. Les résultats proposés dans le Chapitre 2 illustrent de manière très simple comment la déformation du nanoribbon de graphène influence les caractéristiques I-V du transistor. Une autre solution pour réaliser un nanotransistor de graphène est la gravure de nano-trous dans une feuille de graphène (réalisant ainsi un nanomesh). Si le graphène nanomesh est correctement formé, le rapport de courant On / Off du transistor devrait être amélioré. Dans le Chapitre 3, la méthode semi-analytique est utilisée pour évaluer les performances d'un nanomesh à transistors à nanorubans. Les résultats sont à nouveau comparés à une méthode ab-initio. Les caractéristiques I-V du graphène nanomesh transistor sont présentées et comparées aux résultats expérimentaux. Les résultats proposés montrent comment la taille des nanomesh de graphène influence les caractéristiques I-V du transistor. Compte tenu de la simplicité et du temps de calcul réduit de l'approche proposé, ces résultats peuvent permettre des analyses paramétriques, des optimisations et des caractérisations de nano-transistor à graphène dans des circuits à plus grande échelleAmong emerging carbon materials, graphene has rapidly become an ideal candidate for nano-electronics. In this context, different methods have been proposed to transform its electric properties and remove the Dirac degeneracy point, leading to application to nano-transistors. In this thesis we apply a semi-analytical compact model to study two kinds of graphene-based nanotransistors: nanoribbon graphene transistor and nanomesh transistor. A tight-binding model is used to determine analytical expressions for the energy bands of a graphene nanoribbon. Comparisons are shown with ab-initio approaches and with measurements done on larger-scale transistors of the same kind. In the context of flexible electronics, mechanical stresses on circuits and subsequent geometric deformations of graphene-based components is an important issue. We investigate these effects on the conduction properties of nanoribbon transistors (both in ballistic and partially ballistic regimes). By assuming the presence of small deformations, a spectral scaling and a spectral shift due to the presence of a deformation can be taken into account analytically. This model leads to define in closed form effective quantities (masses, densities of states) used to numerically calculate potentials and currents in the nano-device. Numerical results are shown both in a ballistic and partially-ballistic regime, with and without the presence of Schottky contacts. The proposed results in Chapter 2 illustrate in a very simple way how the deformation of graphene nanoribbon influences the I-V characteristics of transistor. Another solution to realize graphene nanotransistor is the etching of nanoholes in a graphene sheet (thus realizing a nanomesh). If graphene nanomesh is properly shaped, the On/Off current ratio of transistor is expected to be enhanced. In Chapter 3, the semi-analytic method is used to evaluate the performance of nanomesh transistor with nanoribbon ones. The results are again compared with an ab-initio method. I-V characteristics of graphene nanomesh transistor are presented and compared with experimental results. The proposed results show how graphene nanomesh size influences the I-V characteristics of transistor. Given the simplicity and the reduced computation time of the approach, these results can lead to perform parametric analyses, optimizations and characterization of graphene nano-transistor when applied in larger-scale circuits
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Woo, Heechul. "The selective low cost gas sensor based on functionalized graphene." Thesis, Université Paris-Saclay (ComUE), 2016. http://www.theses.fr/2016SACLX050/document.
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Les progrès récents dans les nanomatériaux présentent un fort potentiel pour la réalisation de capteurs de gaz avec de nombreux avantages tels que : la grande sensibilité de détection de molécule unique, le faible coût et la faible consommation d'énergie. Le graphène, isolé en 2004, est l'un des meilleurs candidats prometteurs pour le développement de futurs nanocapteurs en raison de sa structure à deux dimensions, sa conductivité élevée et sa grande surface spécifique. Chaque atome de la monocouche de graphène peut être considéré comme un atome de surface, capable d'interagir même avec une seule molécule de l'espèce gazeuse ou de vapeur cible, ce qui conduit finalement à un capteur ultrasensible.Dans cette thèse, des composants à base de graphène ont été fabriqués et caractérisés. Les films de graphène ont été synthétisés par dépôt chimique à phase vapeur (CVD) sur des substrats de verre. La spectroscopie Raman a été utilisée pour analyser la qualité et le nombre de couches de graphène. La microscope à force atomique (AFM) et la microscopie électronique à balayage (MEB) ont été également réalisées pour analyser la qualité du graphène. Après la caractérisation de couches de graphène, des dispositifs résistifs à base de graphène ont été fabriquées : quatre électrodes identiques ont été évaporées thermiquement et directement sur le film de graphène comme des électrodes métalliques. La caractérisation électrique a été réalisée à l'aide de Keithley-4200.La réponse de dispositif Intrinsèque a été étudiée sous différents conditions (pression, humidité, exposition à la lumière). Le dispositif a été fonctionnalisé de manière non covalente avec le complexe organométallique (Ru (II) trisbipyridine) et son effet sous exposition à la lumière a été étudié. La réponse de dispositif était reproductible même après de nombreux cycles en présence et en absence de la lumière. Les approches théoriques et expérimentales ainsi que les résultats obtenus au cours de cette thèse ouvrent un moyen de comprendre et de fabriquer des futurs dispositifs de détection de gaz à base du graphène fonctionnalisé de manière non covalenteRecent advances in nanomaterials provided a strong potential to create a gas sensor with many advantages such as high sensitivity of single molecule detection, low cost, and low power consumption. Graphene, isolated in 2004, is one of the best promising candidate for the future development of nanosensors applications because of its atom-thick, two-dimensional structures, high conductivity, and large specific surface areas. Every atom of a monolayer graphene can be considered as a surface atom, capable of interacting even with a single molecule of the target gas or vapor species, which eventually results in the ultrasensitive sensor response.In this thesis work, graphene films were synthesized by Chemical Vapor Deposition (CVD) on the glass substrate. Raman spectroscopy was used to analyze the quality and number of layers of graphene. Atomic Force Microscope (AFM) and Scanning Electron Microscopy (SEM) were also performed to analyze the quality of graphene. After the characterization of graphene films, graphene based resistive devices (four identical electrodes are thermally evaporated directly onto the graphene film as metal electrodes) were fabricated. The electrical characterization has been carried out using Keithley-4200.Intrinsic device response was studied with different external condition changes (pressure, humidity, light illumination). The device was non-covalently functionalized with organometallic complex (Ru(II) trisbipyridine) and the its light exposure response was studied. The observed device response was reproducible and similar after many cycles of on and off operations. The theoretical and experimental approaches and the results obtained during the thesis are opening up a way to understand and fabricate future gas sensing devices based on the non-covalentely functionalized graphene
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Li, Yangdi. "Innovative synthesis and characterization of large h-BN single crystals : From bulk to nanosheets." Thesis, Lyon, 2019. http://www.theses.fr/2019LYSEI025/document.
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Au cours des dernières décennies, en raison de sa stabilité́ chimique et thermique exceptionnelle associée à son caractère isolant, le nitrure de bore hexagonal sous forme de nanofeuillets (BNNSs) trouve un grand intérêt dans de nombreuses applications. En effet, il est sérieusement envisagé l’utilisation de ces nanomatériaux comme support de graphene ou pour la fabrication d’hétérostructures horizontales utilisables dans le domaine de la microélectronique pour des applications de nouvelle génération. Il existe deux grandes voies de synthèse de ces nanostructures 2D de h-BN, par dépôt chimique en phase vapeur (CVD), ou par exfoliation d’un monocristal. Dans le but de préparer des BNNS de grande qualité́ chimique et cristalline, notre groupe propose une nouvelle stratégie de synthèse en associant la voie polymère précéramique (PDC) à des techniques de frittage, par Spark Plasma Sintering (SPS) et Hot Isostatic Pressing (HIP). Premièrement, le comportement thermique du précurseur précéramique, le polyborazilène (PBN) a été étudié en conditions dynamiques in-situ. Il a ainsi été mis en évidence, le rôle bénéfique du promoteur de cristallisation (Li3N) sur la qualité cristalline du matériau final. Cependant, une étape de frittage complémentaire reste obligatoire pour parfaire la structuration cristalline du h-BN. Premièrement, un procédé́ de frittage par SPS a été mis en œuvre. Dans cette étude, ont été particulièrement étudiés l’influence de la température de frittage (1200-1950°C) ainsi que la teneur en promoteur de cristallisation (0-10% mass.) sur la qualité cristalline du matériau final. Après optimisation des conditions de synthèse, des pastilles de h-BN composées d’une grande quantité de plaquettes monocristallines de taille d’environ 200 μm2 ont été obtenues. Les caractérisations de ces monocristaux attestent d'une haute qualité́ chimique et cristalline, même si des impuretés, sans doutes dues à l’environnement en graphite dans le SPS, sont détectées par cathodoluminescence. Enfin, des mesures physiques montrent que les BNNSs préparés présentent une constante diélectrique intéressante de 3,9, associée à une résistance diélectrique correcte de 0,53 V/nm. Afin d’augmenter encore la taille des monocristaux préparés, un second procédé́ de frittage, par HIP, a été étudié́. Cette autre combinaison originale conduit alors à des monocristaux de h-BN significativement plus gros (jusqu’à 2000 μm de taille latérale), transparents, incolores et très faciles à exfolier. Ainsi cette nouvelle association de la synthèse de PBN par voie PDCs et du procédé́ de céramisation par HIP nous semble une voie des plus prometteuses pour générer de grands monocristaux de h-BN et des nanofeuillets susceptibles de supporter des hétérostructures à base de graphèneIn the past decades, due to their exceptional chemical and thermal stabilities together with their electrical insulation properties, hexagonal boron nitride nanosheets (BNNSs) have become a promising support substrate for graphene and promoted the incentive of various van der Waals heterostructures. For such applications, BNNSs are generally obtained by Chemical Vapor Deposition (CVD) or exfoliation. In order to achieve high quality and large BNNSs, our group has proposed a novel synthesis strategy based on the Polymer Derived Ceramics (PDCs) route combined with sintering techniques: Spark Plasma Sintering (SPS) or Hot Isostatic Pressing (HIP). Since hexagonal boron nitride (h-BN) crystallization is a key point in the synthesis of high quality BNNSs, efforts have been led to understand the beneficial role of a promotor of crystallization (Li3N), adopting a suitable in situ dynamic approach. It has been established that Li3N does improve the crystallization level of the product, and lower the transformation temperatures from polyborazylene to h-BN. Then, we have further investigate the influence of the SPS sintering temperature (1200-1950°C) and of the crystal promoter content (Li3N, 0-10 wt.%) on BN growth. The tested SPS parameters strongly modify the size of the resulting h-BN flakes. For an optimal Li3N concentration of 5 wt.%, h-BN flakes larger than 200 μm2 (average flake area) have been obtained. A high degree of crystallinity and purity have been achieved, even if the very-sensitive cathodoluminescence technic indicated traces of impurities, probably due to surrounding graphite parts of the SPS. Few-layered BNNSs have been successfully isolated, through exfoliation process. As a final application purpose, further physical measurements have confirmed that SPS derived h-BN exhibits an interesting dielectric constant of 3.9 associated with a dielectric strength of 0.53 V/nm. Due to a very high compact character of SPS-derivative h-BN crystals, the post-exfoliation step is made very difficult, resulting in BNNSs of tens of microns lateral size. Therefore, we have studied another sintering procedure by HIP for the ceramization process. Through this combination, we aim to promote the size of h-BN single crystals, leading to larger size exfoliated BNNSs. Characterizations from bulk crystals to BNNSs have been carried out in three aspects: morphology, lattice structure and chemical composition. This novel attempt has provided us transparent and colorless h-BN single crystals with large lateral size, up to 2000 μm. Besides, BNNSs with high purity have also been confirmed. HIP, as a new ceramization process of PDCs, has to be considered as a promising way to obtain large h-BN single crystals and nanosheets for supporting graphene and 2D heterostructures
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Rus, Yahdi Bin. "Nanocomposites à base de graphène fonctionnalisé pour le stockage de l'énergie et la catalyse." Thesis, Université Paris-Saclay (ComUE), 2019. http://www.theses.fr/2019SACLN068.
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Résumé : Des feuillets de graphène fonction-nalisés FGSx avec un rapport C/O~x ont été post-fonctionnalisés avec des dérivés de tétrazine (3,6-bis(2-pyridyl)-1,2,4,5-tétrazine) par cycloa-ddition de Diels-Alder à demande inverse. Des applications potentielles de ces graphènes fonctionnalisés ont été explorées sur le stockage d'énergie (supercondensateur) et l’électrocatalyse (réaction de réduction de l'oxygène).Pour les applications de supercondensateurs, des nanocomposites constitués de FGS20 fonction-nalisé et de polypyrrole ont été synthétisés en deux étapes en incorporant d’abord les fonctions pyridine-pyridazine à la surface du FGS20 par cycloaddition suivie de l’électropolymérisation du pyrrole dans l’acétonitrile. La capacité spéci-fique du matériau a été mesurée par des cycles de charge-décharge galvanique et la stabilité au cyclage a été étudiée dans divers milieux électro-lytiques (acétonitrile, liquide ionique, eau acide et eau neutre) et les résultats comparés par rapport au FGS20 non fonctionnalisé avec ou sans polypyrrole. Alors que l'acétonitrile révèle un comportement capacitif pur pour tous les matériaux étudiés, l'eau acide est le milieu où les valeurs de capacité sont les plus élevées et, de manière surprenante, où les nanocomposites contenant du polypyrrole présentent une meilleure rétention de capacité lors du cyclage que le graphène seul. Un impact positif de la fonctionnalisation du graphène avant l'électropoplymérisation a été mis en évidence dans tous les milieux électrolytiques (pertes de capacité limitées à moins de 8% après 1500 cycles dans tous les milieux sauf l'eau neutre), soulignant l'intérêt du contrôle d'interface dans ce type de nanocompositesAbstract: Functionalized graphene sheets with a ratio C/O~x (FGSx) was further functionalized with a tetrazine derivatives (3,6-bis(2-pyridyl)-1,2,4,5-tetrazine) by inverse demand Diels-Alder cycloaddition reactions. Functionalized FGS20 and FGS13 potential applications were explored on energy storage (super-capacitor) and catalysis (oxygen reduction reaction).In supercapacitor applications, nanocomposites made of functionalized FGS20 with polypyrrole were synthesized in two steps by first incorporating pyridine-pyridazine functions on FGS20 surface through cycloaddition followed by electropolymerization of pyrrole in acetonitrile. The specific capacitance of the material was measured by galvanic charge-discharge cycles and the stability upon cycling investigated in various electrolytic media (acetonitrile, ionic liquid, acidic, and neutral water) in comparison with non-functionalized FGS20 with or without polypyrrole. While acetonitrile reveals pure capacitive behaviour for all investigated mate-rials, acidic water is the medium where the capacitance values are the highest and surprisingly where nano-composites with polypyrrole show better capacitance retention upon cycling than graphene alone. A positive impact of graphene functionalization prior to electropolymerization was demonstrated in all electrolytic media (capacitance losses limited to less than 8% after 1500 cycles in all media but neutral water), highlighting the interest of interface control in this kind of nanocomposites
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Zhang, Guohui. "Electrochemistry and applications of sp2 carbon materials : from graphite to graphene." Thesis, University of Warwick, 2016. http://wrap.warwick.ac.uk/89303/.
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This thesis can be divided into three themes: (i) the electrochemistry of sp2 carbon materials, with a focus on graphite and graphene, where electron transfer (ET) kinetics and surface functionalisation were considered; (ii) methodology development for graphene transfer, to facilitate the fabrication of versatile tools for microscopy research and allow the properties of supported and suspended graphene to be readily assessed and compared; (iii) the electrowetting of graphite, providing a new mechanism for droplet actuation on a conducting surface with an applied electric field. There is a large body of literature that the basal plane of highly oriented pyrolytic graphite (HOPG) is inert or has little electroactivity for outer-sphere redox couples and adsorbed species. Here, the model is revisited with the macroscopic ET kinetics studies of three classical (outer-sphere) redox couples on different grades of HOPG using a droplet-cell setup. It is shown that the ET kinetics for all of the redox species studied is fast on all grades of HOPG (comparable to metal electrodes), despite the low density of electronic states (DOS) on graphite. This is in line with the results where the ‘special’ redox couple, Fe3+/2+, associated with a slow kinetics, is tested. Moreover, localised surface mapping measurements of HOPG using scanning electrochemical cell microscopy (SECCM), reveal a relatively uniform activity on basal plane and step edges of HOPG towards Fe3+/2+, highlighting that the basal plane is electroactive and the major site for the ET kinetics of Fe3+/2+. The next goal is to elucidate whether adsorbed electroactive anthraquinone-2,6-disulfonate (AQDS) can be used as a marker of step edges, previously regarded as the main electroactive sites of graphite. Step edges are shown to have little effect on the extent of adsorbed electroactive AQDS in macroscopic studies. The amount of adsorbed electroactive AQDS and the ET kinetics are independent of the step edge coverage, as determined by fast scan cyclic voltammetry-SECCM. Further, SECCM reactive patterning shows essentially uniform and high activity across the basal surface of HOPG, indicative of the dominance of basal plane in HOPG electroactivity. Regarding the close relation between graphene and graphite, effort is put to introduce a polymer-free method for transferring chemical vapour deposition (CVD)-grown graphene, to arbitrary substrates, using an organic/aqueous biphasic configuration. Avoiding any polymeric contamination, graphene is coated on arbitrary substrates, such as atomic force microscopy (AFM) tips and transmission electron microscopy (TEM) grids, generating tools for conductive AFM and high resolution TEM imaging. Furthermore, electrochemical and wetting measurements at either a freestanding graphene film or a copper-supported graphene area, are readily made and compared. As an example of the myriad potential applications of graphite, electrowetting is demonstrated at HOPG, using cyclic voltammetry, with significant changes in contact angle and relative contact diameter seen. These are comparable to the widely studied electrowetting-on-dielectric (EWOD) system, but over a much lower voltage range. Electrowetting is found to be due to the intercalation/de-intercalation of anions between the graphene layers of graphite, driven by the applied potential, providing a new mechanism for electrowetting and diversifying the means by which electrowetting can be controlled and applied.
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Shylau, Artsem. "Electron transport, interaction and spin in graphene and graphene nanoribbons." Doctoral thesis, Linköpings universitet, Fysik och elektroteknik, 2012. http://urn.kb.se/resolve?urn=urn:nbn:se:liu:diva-80621.
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Since the isolation of graphene in 2004, this novel material has become the major object of modern condensed matter physics. Despite of enormous research activity in this field, there are still a number of fundamental phenomena that remain unexplained and challenge researchers for further investigations. Moreover, due to its unique electronic properties, graphene is considered as a promising candidate for future nanoelectronics. Besides experimental and technological issues, utilizing graphene as a fundamental block of electronic devices requires development of new theoretical methods for going deep into understanding of current propagation in graphene constrictions. This thesis is devoted to the investigation of the effects of electron-electron interactions, spin and different types of disorder on electronic and transport properties of graphene and graphene nanoribbons. In paper I we develop an analytical theory for the gate electrostatics of graphene nanoribbons (GNRs). We calculate the classical and quantum capacitance of the GNRs and compare the results with the exact self-consistent numerical model which is based on the tight-binding p-orbital Hamiltonian within the Hartree approximation. It is shown that electron-electron interaction leads to significant modification of the band structure and accumulation of charges near the boundaries of the GNRs. It's well known that in two-dimensional (2D) bilayer graphene a band gap can be opened by applying a potential difference to its layers. Calculations based on the one-electron model with the Dirac Hamiltonian predict a linear dependence of the energy gap on the potential difference. In paper II we calculate the energy gap in the gated bilayer graphene nanoribbons (bGNRs) taking into account the effect of electron-electron interaction. In contrast to the 2D bilayer systems the energy gap in the bGNRs depends non-linearly on the applied gate voltage. Moreover, at some intermediate gate voltages the energy gap can collapse which is explained by the strong modification of energy spectrum caused by the electron-electron interactions. Paper III reports on conductance quantization in grapehene nanoribbons subjected to a perpendicular magnetic field. We adopt the recursive Green's function technique to calculate the transmission coefficient which is then used to compute the conductance according to the Landauer approach. We find that the conductance quantization is suppressed in the magnetic field. This unexpected behavior results from the interaction-induced modification of the band structure which leads to formation of the compressible strips in the middle of GNRs. We show the existence of the counter-propagating states at the same half of the GNRs. The overlap between these states is significant and can lead to the enhancement of backscattering in realistic (i.e. disordered) GNRs. Magnetotransport in GNRs in the presence of different types of disorder is studied in paper IV. In the regime of the lowest Landau level there are spin polarized states at the Fermi level which propagate in different directions at the same edge. We show that electron interaction leads to the pinning of the Fermi level to the lowest Landau level and subsequent formation of the compressible strips in the middle of the nanoribbon. The states which populate the compressible strips are not spatially localized in contrast to the edge states. They are manifested through the increase of the conductance in the case of the ideal GNRs. However due to their spatial extension these states are very sensitive to different types of disorder and do not significantly contribute to conductance of realistic samples with disorder. In contrast, the edges states are found to be very robust to the disorder. Our calculations show that the edge states can not be easily suppressed and survive even in the case of strong spin-flip scattering. In paper V we study the effect of spatially correlated distribution of impurities on conductivity in 2D graphene sheets. Both short- and long-range impurities are considered. The bulk conductivity is calculated making use of the time-dependent real-space Kubo-Greenwood formalism which allows us to deal with systems consisting of several millions of carbon atoms. Our findings show that correlations in impurities distribution do not significantly influence the conductivity in contrast to the predictions based on the Boltzman equation within the first Born approximation. In paper VI we investigate spin-splitting in graphene in the presence of charged impurities in the substrate and calculate the effective g-factor. We perform self-consistent Thomas-Fermi calculations where the spin effects are included within the Hubbard approximation and show that the effective g-factor in graphene is enhanced in comparison to its one-electron (non-interacting) value. Our findings are in agreement to the recent experimental observations.
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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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Yuan, Jie, and 袁杰. "Theoretical studies of correlation effects in graphene and graphene layers." Thesis, The University of Hong Kong (Pokfulam, Hong Kong), 2013. http://hub.hku.hk/bib/B50899697.
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This thesis discusses correlation effects in graphene and bilayer graphene. The discovery of graphene was awarded 2010 Nobel Prize in Physics. Graphene is one of the most intriguing topics around the world. Its flexibilities make it a very promising material in device physics. From theoretical point of view, graphene connects condensed matter physics to quantum field theory, and is an excellent candidate for model studies. Furthermore, it stimulates researches in low-dimensional electron systems. Bilayer graphene is an interesting variant of graphene, and is one of the new directions in developing low-dimensional systems.
Due to honeycomb lattice symmetry, the low-energy effective Hamiltonian of a graphene is described by gapless Dirac equation a(_σ^→).p round K(K^1) point. In this thesis, symmetry of Dirac equation is reviewed. In graphene, there are four copies of gapless Dirac equations. In addition, spin-orbit couplings are also discussed by using point-group techniques. We calculate screening and collective modes by using lattice Green's functions within random phase approximations. Some typical models on honeycomb lattice are reviewed, including Haldane model and Kane-Mele model. Interaction effects are further discussed within the Hubbard and extended models. It is reported there are some interesting phases both in doped and undoped cases. Graphene ribbons are also discussed in this thesis: zigzag ribbons and armchair ones. We investigate the attractive-U Kane-Mele-Hubbard model by using a mean-field theory, and find strong superconducting correlations along the edge, analogous to edge magnetism in positive U case. We investigate mesoscopic spin Hall effect on the surface of a three-dimensional topological insulator using McMillan Green's function techniques, and discuss the robustness of edge states and stabilities against interactions in topological insulator.
Bilayer graphene is also investigated. Our study follows the recent experiments and theoretical proposals. As suggested by previous works, quantum spin Hall state and layer antiferromagnetic state are two most possible candidates of the ground state. We propose by tiny doping, a half-metallic state can be realized based on layer antiferromagnetic state. The responses to in-plane and perpendicular magnetic fields are also reviewed.published_or_final_version
Physics
Doctoral
Doctor of Philosophy
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Eckmann, Axel. "Raman spectroscopy of graphene, its derivatives and graphene-based heterostructures." Thesis, University of Manchester, 2013. https://www.research.manchester.ac.uk/portal/en/theses/raman-spectroscopy-of-graphene-its-derivatives-and-graphenebased-heterostructures(fbb9d645-4fb3-4a75-b5c9-9a8483d6e9ac).html.
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In less than a decade of research, graphene has earned a long list of superlatives to its name and is expected to have applications in various fields such as electronics, photonics, optoelectronics, materials, biology and chemistry. Graphene has also attracted a lot of attention because its properties can be engineered either via intrinsic changes or by modification of its environment. Raman spectroscopy has become an ideal characterization method to obtain qualitative and quantitative information on these changes. This thesis investigates the possibility to change, supplement and monitor the electonic and optical properties as well as the chemical reactivity of graphene. It is achieved by i) substrate effect, ii) introduction of defects in the structure of graphene and iii) the combination of graphene with other two- dimensional crystals such as hexagonal boron nitride (h-BN) and transition metal dichacolgenides. In particular, the experimental work presented here describes: I - The influence of the type of substrate on the Raman intensity of graphene. This work leads to the calculation of the Raman scattering efficiency of graphene after CaF2 is found to be a suitable substrate for this kind of study in contrast to Si/SiOx that strongly modulates the Raman intensities. The G peak scattering efficiency is found to be about 200 x 10-5 m-1 Sr-1 at 2.4 eV while that of the 2D peak is one order of magnitude higher, confirming the resonant nature of the 2D peak Raman scattering process. II - An attractive method to produce large (up to several hundreds of microns across) and high quality graphene by anodic bonding. This cheap, fast and solvent-free method also allows introduction of vacancy like defects in the samples in a relatively controllable way. III - The Raman signatures of several types of defect such as sp3 sites, vacancies and substitutional atoms. For low defect concentration (stage 1) the intensitiy ratio I(D)/I(D') is constant and is 13 for sp3 sites, 9 for substitutional atoms and 7 for vacancies. This signature is explained using the local activation model recently proposed to model the amorphization trajectory of graphene with containing vacancy-like defects. IV - Controlled modification of graphene through mild oxygen plasma. The influence of sp3 sites on monolayer and bilayer graphene's electrical properties are discussed. In the case of bilayer under controlled conditions, it is possible to modify only the top layer. This may lead to decoupling between the two layers, which could explain the good mobility measured for this system. The possiblity to use such system as a sensor is discussed. V - The characteristic Raman signature of aligned graphene/h-BN superlattices. The Raman spectrum shows strong changes in perfectly aligned superlattices, which could be attributed to the reconstruction of the Dirac spectrum. VI - A prototype photovoltaic cell made of a graphene and tungsten disulphide (WS2) heterostructure with an external quantum efficiency of about 30%. The beneficial combination of an excellent absorption in WS2 atomically thin films due to the presence of van Hove singularities and graphene used as a transparent, flexible and conductive electrode is demonstrated.
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Andersson, Fredrik. "Graphene and graphene oxide as new lubricants in industrial applications." Thesis, Uppsala universitet, Tillämpad materialvetenskap, 2015. http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-264853.
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This master thesis report evaluatesthe lubricating effect of graphene (G)and graphene oxide (GO). Thesematerials have been added, in particlecondition, in Ag-based slidingcontacts and lubricating greases. Thework focuses on the tribologicalevaluation of these materials,especially friction, wear and contactresistance analyses. The friction andwear behaviors of Ag-based contactscontaining of a wide concentrationrange of graphene and graphene oxideare tested against pure silver using atest load of 2 and 10 N at a constantspeed of 5 cm/s. It was revealed thatsmall amounts of G and GO are able tosignificantly reduce the frictioncoefficient and wear rate. Contactresistance measurements revealed thatresults similar to pure Ag can beachieved with G content up to 10vol%.Possible mechanisms, which maycontribute to this tribologicalbehavior are the Ag-C interactions andthe lubricating nature of graphene.Friction tests with G and GOcontaining lubricating greases showinconsistent results, and both greasesand corresponding test methods forevaluation require furtheroptimization. The overall, promising,tribological behavior of G and GOholds for the implementation invarious industrial applications. Thereis no doubt that these kinds ofmaterials can play an important rolein ABBs future work. This masterthesis report shows yet anotherapplication area for theseextraordinary materials.
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Hick, Ralph. "Hierarchical graphene supercapacitors." Thesis, University of Manchester, 2018. https://www.research.manchester.ac.uk/portal/en/theses/hierarchical-graphene-supercapacitors(c3583283-c6a7-439f-9459-217c3ff2c44f).html.
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Electrochemical supercapacitors are promising devices for energy storage applications. However, their uptake is currently limited by their relatively low energy density. The recent discovery of graphene has strengthened supercapacitor research, due to graphene's high surface area, conductivity, strength, and flexibility. However, the synthesis of large quantities of defect-free graphene and its subsequent incorporation into supercapacitors has proved difficult due to aggregation and restacking of the graphene. Hence, in order to retain the high surface area of graphene, it needs to be incorporated into hierarchical structures. Given these issues, this thesis aimed to produce high quality graphene flakes via electrochemical exfoliation. These flakes were then processed into hierarchical structures (foams and fibres) for supercapacitor devices. The graphene was exfoliated using a reductive process, with two different cells designs explored. The influence of the microstructure of the initial graphite on the exfoliation process was also studied. The hierarchical foams were produced by depositing the graphene onto nickel foam. It was found that the degree of exfoliation has a marginal effect on the capacitance of the device. This electrochemically exfoliated graphite was also wet-spun with polyacrylonitrile (PAN) and carbonised to produce carbon fibre-graphene composites. It was found that the carbonised materials had a higher capacitance than the precursor material (33 F g-1 and 47 F g-1 respectively). As a comparison, wet-spun graphene oxide fibres were synthesised with polyvinyl alcohol and were subsequently carbonised and reduced. These fibres gave comparable capacitance results to the carbonised polyacrylonitrile fibres (47 F g-1 and 40 F g-1 respectively).
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Jeon, Intak. "Graphene at interfaces." Thesis, Massachusetts Institute of Technology, 2018. http://hdl.handle.net/1721.1/117790.
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Thesis: Ph. D., Massachusetts Institute of Technology, Department of Materials Science and Engineering, 2018.This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.
Cataloged student-submitted from PDF version of thesis.
Includes bibliographical references (pages 137-146).
Strong attractive interactions between extended π-systems of graphene in various graphites have been studied for several decades. However, to date, no effective scalable exfoliation mechanism has been reported to exclusively synthesize dispersible single layer graphene in liquid environments. The first feature of this thesis is to use the repulsive electrostatic effect to modify the geometry of [pi]-[pi] interaction of graphenes against attractive van der Waals interactions. We explored an electrochemical method, inducing the large offset from ideal tight stacked [pi]-[pi] geometry, to generate Hyperstage-1 graphite intercalation compounds. Subsequently, highly functionalized graphenes are spontaneously exfoliated from Hyperstage-1 graphite intercalation compounds by reaction with an aryl diazonium salt solution under electrochemical reducing conditions. The covalent functionalization of graphene successfully provided the appropriate chemical activity/reactivity for the desired applications. Through the reactivity of functional graphene, we developed the fluid-like graphene matrix to attain a superlubric state. In addition, Janus graphenes are simply synthesized by Meisenheimer complexes reactions of hydrocarbon, perfluorocarbon and water-soluble alkyl amines on the surface of graphene at room temperature. This approach opens the door to a rich variety of functional single layer graphenes for applications.
by Intak Jeon.
Ph. D.
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Zhao, F. "Graphene-diamond heterostructures." Thesis, University College London (University of London), 2015. http://discovery.ucl.ac.uk/1462910/.
Full textAbstract:
Graphene obtained by mechanical exfoliation of graphite displays unique electronic properties with high mobility and saturation velocity. However, this is not a scalable technique, the film being limited to small area. Large area synthesis of good quality graphene has been achieved by CVD. The choice of substrate apparently influences the electronic properties of graphene. Most of reports have used SiO2-Si due to the widespread availability, but it is a poor choice of the material to degrade the graphene performace. In this thesis, more ideal platforms are introduced, including single crystal diamond (SCD), nanodiamond (ND), and diamond-like-carbon (DLC). It was found that different terminations for substrates caused strong effects for graphene properties. For H-terminated diamond, it was found that a p-type layer with good mobility and a small bang gap, whilst when N/F-terminations are introduced it was found that a layer with more metallic-like characters arises. Furthermore, different orientations of H-terminated SCD(100)/(111) were found to induce different band-gap of graphene. Simulation analysis proves the difference. However, the mobility results of graphene-H-terminated ND herostructure are better than graphene supported by SCD, which offers the prospect of low cost sp2 on sp3 technology. Raman and XPS results reveal the influence from the C-H band of ND surface. Impedance measurements show two conductive paths in the graphene-HND heterostructure. Graphene FET was built on this heterostructure, which exhibited n-type and high mobility. The family of amorphous carbon films, DLC, appeal to a preferable choice of graphene supporting substrate since IBM built the high-frequency graphene FET on DLC. For N-termination it was found that the optical band gap of DLC shrinked, whilst for F-terminated DLC it was found that fluorine groups reduce the DLC’s surface energy. Owing to different phonon energies and surface trap densities, graphene-DLC heterostructures give different electronic properties and offer the prospect for 2D lateral control applications.