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Dissertations / Theses on the topic 'Few-layer graphene'
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1
Yan, Wenjing. "Spin transport in few-layer graphene." Thesis, University of Cambridge, 2014. https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.708038.
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Che, Shi. "Quantum Transport in Few-layer Graphene." The Ohio State University, 2019. http://rave.ohiolink.edu/etdc/view?acc_num=osu1574864398913631.
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Pavitt, David. "Few-layer transition metal chloride graphene intercalation compounds." Thesis, University of London, 2013. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.603531.
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Few-layer transition metal chloride graphene intercalation compounds have becJI fabricated by the mechanical exfoliation of graphite intercalation compounds (GTCs) containing CoC12 , NiCh, CuCl2 , MnCh and FeC13 . The number of graphene layers and the distribution of the intercalate in the few-layer graphene intercalation compounds (FLGICs) have been characterised using the optical contrast of the FLGICs against the Si02 substrate and the G-peak of the Raman spectrum. FLGICs containing CL single intercalate layer surrounded by two graphene layers have been fabricated and characterised, which are an ideal system to study 2D magnetic phase transitions.
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YASIR, MUHAMMAD. "Tunable Microwave Components based on Few Layer Graphene." Doctoral thesis, Università degli studi di Pavia, 2019. http://hdl.handle.net/11571/1245811.
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This doctoral thesis is about the use of graphene for microwave tunable passive components. It opens a new paradigm in the use of innovative and cost-effective methods for producing tunable microwave components based on graphene. Specifically, it paves the way for future key components of microwave and wireless communication systems such as attenuators, phase shifters and antennas. A review of the state-of-the-art microwave passive components based on graphene in terahertz and microwaves is also provided.
The integration of a number of components on a transmitter receiver system requires functional materials of nanometric scale. The use of innovative nanomaterials for designing state of the art microwave components is not new. The signature property of monolayer graphene that can be exploited for tunable microwave components is its electronically tunable resistance. This property is valid for dimensions as large as mm/cm to as small as micro and nanometers keeping a constant aspect ratio. The big challenge in research on future communication systems is to cost effectively design, implement and measure such proposed components. To this aim, in this thesis few layer graphene is deployed in the design of tunable attenuators, phase shifter and antenna. The advantage of using FLG is its cost effectiveness, technological simplicity and eco friendliness unlike most nanomaterials.
A new design of tunable graphene attenuator was proposed based on shunt grounded vias connected to FLG flakes and a microstrip line. The grounded vias were symmetrically placed on each side of the microstrip line with two ports. The design, even though of not very high structural complexity resulted in superior functionality both in terms of dynamic range of insertion loss and the reflective insertion loss. The number of vias were then increased for improved functionality. With the increase in the number of vias, emerging structural parameters were optimized for higher insertion loss and improved mismatch. Simulations were performed for the optimization while fabrication of prototype and measurements were performed which were in good agreement to the simulated results. For the final case of eight vias connected to FLG, a total of more than 65 dB insertion loss was measured with reflective insertion loss as low as 2dB.
Phase Shifter being a vital component of a communication system was also made incorporating FLG flakes. The tunable FLG resistance was converted to tunable reactance by the help of a stub composed of tapered line connected to FLG and a shorted stub. The various lengths and widths of the line were optimized so as to provide maximum shift in reactance when the change in FLG resistance would occur by an applied DC bias voltage. Subsequently, the optimized stub with variable reactance was connected to a two-port 50 Ω transmission line, the transmission on which would cause a phase shift by an applied DC voltage across the FLG. The maximum phase shift obtained was 43 degree with an additional insertion loss of 3 dB. The concept can be applied to a number of such units connected in cascade since the insertion loss is not very high. A combination of the phase shifter and attenuator can be used in the design of a tunable modulator based on a combination of amplitude and phase variation.
The concept of the phase shifter was applied to a frequency reconfigurable patch antenna. FLG accompanied by a shorted stub optimized for maximum reactance change were deployed in a microstrip antenna. A total shift in the radiating frequency of 450 MHz was measured at an applied DC bias voltage of 5V with limited gain degradation.This doctoral thesis is about the use of graphene for microwave tunable passive components. It opens a new paradigm in the use of innovative and cost-effective methods for producing tunable microwave components based on graphene. Specifically, it paves the way for future key components of microwave and wireless communication systems such as attenuators, phase shifters and antennas. A review of the state-of-the-art microwave passive components based on graphene in terahertz and microwaves is also provided. The integration of a number of components on a transmitter receiver system requires functional materials of nanometric scale. The use of innovative nanomaterials for designing state of the art microwave components is not new. The signature property of monolayer graphene that can be exploited for tunable microwave components is its electronically tunable resistance. This property is valid for dimensions as large as mm/cm to as small as micro and nanometers keeping a constant aspect ratio. The big challenge in research on future communication systems is to cost effectively design, implement and measure such proposed components. To this aim, in this thesis few layer graphene is deployed in the design of tunable attenuators, phase shifter and antenna. The advantage of using FLG is its cost effectiveness, technological simplicity and eco friendliness unlike most nanomaterials. A new design of tunable graphene attenuator was proposed based on shunt grounded vias connected to FLG flakes and a microstrip line. The grounded vias were symmetrically placed on each side of the microstrip line with two ports. The design, even though of not very high structural complexity resulted in superior functionality both in terms of dynamic range of insertion loss and the reflective insertion loss. The number of vias were then increased for improved functionality. With the increase in the number of vias, emerging structural parameters were optimized for higher insertion loss and improved mismatch. Simulations were performed for the optimization while fabrication of prototype and measurements were performed which were in good agreement to the simulated results. For the final case of eight vias connected to FLG, a total of more than 65 dB insertion loss was measured with reflective insertion loss as low as 2dB. Phase Shifter being a vital component of a communication system was also made incorporating FLG flakes. The tunable FLG resistance was converted to tunable reactance by the help of a stub composed of tapered line connected to FLG and a shorted stub. The various lengths and widths of the line were optimized so as to provide maximum shift in reactance when the change in FLG resistance would occur by an applied DC bias voltage. Subsequently, the optimized stub with variable reactance was connected to a two-port 50 Ω transmission line, the transmission on which would cause a phase shift by an applied DC voltage across the FLG. The maximum phase shift obtained was 43 degree with an additional insertion loss of 3 dB. The concept can be applied to a number of such units connected in cascade since the insertion loss is not very high. A combination of the phase shifter and attenuator can be used in the design of a tunable modulator based on a combination of amplitude and phase variation. The concept of the phase shifter was applied to a frequency reconfigurable patch antenna. FLG accompanied by a shorted stub optimized for maximum reactance change were deployed in a microstrip antenna. A total shift in the radiating frequency of 450 MHz was measured at an applied DC bias voltage of 5V with limited gain degradation.
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KUMAR, VINEET. "Few layer graphene reinforced rubber compounds for tires." Doctoral thesis, Università degli Studi di Milano-Bicocca, 2014. http://hdl.handle.net/10281/83643.
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In last decade, “Nanofillers” have been explored extensively in rubber compounds to improve dynamic-mechanical properties. Three classes of nanofillers: Clay minerals, Carbon nanoTubes and Graphitic nanofillers have been often used. Most recently, an attention towards “graphene” as nanofiller was reported due to its exceptional mechanical, thermal and electrical properties. In present Ph.D. thesis, different types of commercially available “few layer graphene” were explored in both apolar and polar diene rubbers. These nanofillers were dispersed with melt mixing technique which is most suitable technology for industrial applications, such as for tires. Structural-morphological characteristics of the nanofillers were made with SEM, TEM, XRD and static adsorption isotherms. Features such as shape anisotropy, number of graphene layers in a stack, BET surface area, surface activity and porosity of nanofillers were obtained. Optical microscopy was employed to obtain filler dispersion index and estimation of filler’s aggregates, agglomerates. Dynamic mechanical properties of the rubber compounds were made with rheometric curves for scorch and curing time, rheological properties through RPA (strain sweep and frequency sweep) for viscoelastic properties and filler networking, stress-strain for tensile strength and multi-hysteresis cycles for energy dissipation, dynamic mechanical thermal analysis for high and low temperature properties, hardness of compound for processing features and tear strength tests for compound durability. The electrical properties of rubber compounds were investigated via dielectric AC conductivity and permittivity tests. Epoxidation of diene rubbers (low rate, <10%) was obtained to investigate the effects of presence of epoxy functional groups along polymer chains on filler networking, polymer-filler interactions, filler dispersion and dynamic mechanical properties of rubber compounds. Quantitative analysis of epoxidation, rate of epoxidation and its influence on rubber matrix (such as change in glass transition temperature) was investigated through 1NMR and DSC tests. Under multi-hysteresis stress-strain cycles, it was found that a stable filler networking can reduce hysteresis losses.
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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.
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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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Skulason, Helgi. "Optical properties of few and many layer graphene flakes." Thesis, McGill University, 2009. http://digitool.Library.McGill.CA:80/R/?func=dbin-jump-full&object_id=67024.
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This thesis reports, for the first time, measurements of optical properties of graphene as a function of layer number up to 700 layers. Optical reflection, optical transmission and atomic force microscopy was performed on graphene exfoliated on glass. Universal optical conductance of graphene arising from pi−pi^* interband transitions was used to identify and count up to 9 layer graphene samples with optical reflection microscopy alone. The optical properties of graphene are best described by refractive index of 1.88−1.59i at 550 nm up to 90 layers. For thicker graphene flakes, we present a model for calculating conductance due to sigma−sigma^* transitions. Incorporating both transitions, we find a refractive index of 2.70−1.11i at 550 nm, which shows good agreement to 250−700 layer graphene flakes.Cette thèse rapporte, pour la première fois, des mesures des propriétés optiques du graphene en fonction du nombre de couches et ce allant jusqu'à 700 couches. La réflexion et la transmission optique ainsi que la microscopie par force atomique ont été utilisés sur du graphene déposé sur de la vitre. La conductance optique universelle du graphene provenant des transitions entre les bandes pi-pi^* a été utilisée afin de compter jusqu'à 9 couches de graphene avec seulement la microscopie à réflexion optique. Les propriétés optiques du graphene sont bien décrites par un index de réfraction de 1.88-1.59i à 550 nm et ce jusqu'à 90 couches. Pour des échantillons plus épais, nous présentons un modèle servant à calculer la conductance causée par les transitions entre les bandes sigma-sigma^*. En incorporant les deux transitions, nous trouvons un index de 2.70-1.11i à 550 nm, ce qui démontre un bon accord avec les échantillons de graphene de 250-700 couches.
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Jeon, Intak. "Synthesis of functionalized few layer graphene via electrochemical expansion." Thesis, Massachusetts Institute of Technology, 2015. http://hdl.handle.net/1721.1/101797.
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Thesis: S.M., Massachusetts Institute of Technology, Department of Materials Science and Engineering, 2015.This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.
Cataloged from student-submitted PDF version of thesis.
Includes bibliographical references (pages 59-62).
Single layer graphene is a nearly transparent two-dimensional honeycomb sp2 hybridized carbon lattice, and has received immense attention for its potential application in next-generation electronic devices, composite materials, and energy storage devices. This attention is a result of its desirable and intriguing electrical, mechanical, and chemical properties. However, mass production of high-quality, solution-processable graphene via a simple low-cost method remains a major challenge. Recently, electrochemical exfoliation of graphite has attracted attention as an easy, fast, and environmentally friendly approach to the production of high-quality graphene. This route solution phase approach complements the original micromechanical cleavage production of high quality graphite samples and also involved a chemically activated intermediate state that facilitates functionalization. In this thesis we demonstrate a highly efficient electrochemical exfoliation of graphite in organic solvent containing tetraalkylammonium salts, avoiding oxidation of graphene and the associated defect generation encountered with the broadly used Hummer's method. The expansion and charging of the graphite by intercalation of cations facilitates the functionalization of the graphene basal surfaces. Electrochemically enhanced diazonium functionalization of the expanded graphite was performed. The exfoliated graphene platelets were analyzed by Raman spectroscopy, to quantify defect states and the degree of exfoliation. Additional microscopy techniques provided additional insight into the chemical state and structure of the graphene sheets.
by Intak Jeon.
S.M.
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Sole, C. G. "Application of few layer graphene and exfoliated graphite materials in lithium ion batteries." Thesis, University of Liverpool, 2017. http://livrepository.liverpool.ac.uk/3019844/.
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Wang, Xiong, Russell S. Witte, and Hao Xin. "Thermoacoustic and photoacoustic characterizations of few-layer graphene by pulsed excitations." AMER INST PHYSICS, 2016. http://hdl.handle.net/10150/615111.
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We characterized the thermoacoustic and photoacoustic properties of large-area, few-layer graphene by pulsed microwave and optical excitations. Due to its high electric conductivity and low heat capacity per unit area, graphene lends itself to excellent microwave and optical energy absorption and acoustic signal emanation due to the thermoacoustic effect. When exposed to pulsed microwave or optical radiation, distinct thermoacoustic and photoacoustic signals generated by the few-layer graphene are obtained due to microwave and laser absorption of the graphene, respectively. Clear thermoacoustic and photoacoustic images of large-area graphene sample are achieved. A numerical model is developed and the simulated results are in good accordance with the measured ones. This characterization work may find applications in ultrasound generator and detectors for microwave and optical radiation. It may also become an alternative characterization approach for graphene and other types of two-dimensional materials. (C) 2016 AIP Publishing LLC.
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Limmer, Thomas. "Influence of carrier density on the ultrafast optical response of graphene and few-layer graphene." Diss., lmu, 2012. http://nbn-resolving.de/urn:nbn:de:bvb:19-149006.
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Reina, Ceeco Alfonso. "Single- and few-layer graphene by ambient pressure chemical vapor deposition on nickel." Thesis, Massachusetts Institute of Technology, 2010. http://hdl.handle.net/1721.1/59230.
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Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2010.Includes bibliographical references (p. 169-177).
An ambient pressure chemical vapor deposition (APCVD) process is used to fabricate graphene based films consisting of one to several graphene layers across their area. Polycrystalline Ni thin films are used and the graphene can be transferred from the Ni surface to dielectric substrates in order to integrate them to graphene device prototypes. Uniform single layer graphene can be grown with the same process by using single crystalline Ni with a (111) surface orientation. Raman spectroscopy and electron diffraction characterization is undertaken in order to determine the nature of the layer stacking for the case of multilayer graphene.
by Alfonso Reina Ceeco.
Ph.D.
13
Graf, Davy. "Electrons in reduced dimensions : from finite lateral superlattices in AlGaAs heterostructures to few-layer graphene /." Zürich : ETH, 2007. http://e-collection.ethbib.ethz.ch/show?type=diss&nr=17241.
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Limmer, Thomas Verfasser], and Jochen [Akademischer Betreuer] [Feldmann. "Influence of carrier density on the ultrafast optical response of graphene and few-layer graphene / Thomas Limmer. Betreuer: Jochen Feldmann." München : Universitätsbibliothek der Ludwig-Maximilians-Universität, 2012. http://d-nb.info/1027066275/34.
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Ni, Yuxiang. "Thermal contact resistance between molecular systems : an equilibrium molecular dynamics approach applied to carbon nanotubes, graphene and few layer graphene." Phd thesis, Ecole Centrale Paris, 2013. http://tel.archives-ouvertes.fr/tel-00969185.
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This thesis is devoted to the calculation of thermal contact resistance in various molecular systems based on carbon nanotubes (CNTs) and few layer graphene (FLG). This work has been performed through equilibrium molecular dynamics (EMD) simulations. We adopted the temperature difference fluctuations method in our EMD calculations. This method only needs the input of the temperatures of the subsystems whereas the heat flux, which is involved in all the other approaches, remains more difficult to compute in terms of simulation time and algorithm. Firstly, three cases were studied to validate this method, namely: (i) Si/Ge superlattices; (ii) diameter modulated SiC nanowires; and (iii) few-layer graphenes. The validity of the temperature difference fluctuations method is proved by equilibrium and non-equilibrium MD simulations. Then, by using this method, we show that an azide-functionalized polymer (HLK5) has a lower contact resistance with CNT than the one between CNT and PEMA, because HLK5 could form covalent bonds (C-N bonds) with CNT through its tail group azide, while only weak Van der Waals interactions exist in the case of CNT-PEMA contact. The data from our EMD simulations match with the results from experiments in a reasonable range. We then report the thermal contact resistance between FLG and a SiO2 substrate, which could be tuned with the layer number. Taking advantage of the resistive interface, we show that a SiO2 /FLG superlattices have a thermal conductivity as low as 0.30 W/mK, exhibiting a promising prospect in nano-scale thermal insulation. In the last part, we investigated the layer number dependence of the cross-plane thermal resistances of suspended and supported FLGs. We show that the existence of a silicon dioxide substrate can significantly decrease the cross-plane resistances of FLGs with low layer numbers, and the effective thermal conductivities were increased accordingly. The Frenkel-Kontorova model was introduced to explain the substrate-induced band gaps in FLG dispersion relations and the corresponding thermal energy transfer. The enhanced thermal conduction in the cross-plane direction is ascribed to the phonon radiation that occurs at the FLG-substrate interface, which re-distributes the FLG in-plane propagating energy to the cross-plane direction and to the substrate.
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Herziger, Felix [Verfasser], Janina [Akademischer Betreuer] Maultzsch, and Stéphane [Gutachter] Berciaud. "Double-resonant Raman scattering in graphene, few-layer graphene, and carbon nanotubes / Felix Herziger ; Gutachter: Stéphane Berciaud ; Betreuer: Janina Maultzsch." Berlin : Technische Universität Berlin, 2015. http://d-nb.info/1156012309/34.
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Chen, Zhesheng. "Novel two dimensional material devices : from fabrication to photo-detection." Thesis, Paris 6, 2015. http://www.theses.fr/2015PA066595/document.
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Au delà du graphène, de nouveaux semiconducteurs 2D tels que MoS2, GaS, GaSe et InSe deviennent pertinents pour les applications et dispositifs émergents. Dans cette thèse, nous fabriquons des échantillons de quelques feuillets atomiques de ces matériaux pour des dispositifs de photo-détecteurs et les caractérisons par microscopie optique, AFM et TEM. L'interaction de la lumière avec le substrat et le dispositif ultra-mince étant critique pour son fonctionnement, nous calculons et mesurons le contraste et l'intensité de la lumière diffusée par le dispositif. Nous caractérisons également la réponse Raman et la photoluminescence en fonction du nombre de couches pour étudier les propriétés vibrationnelles et électroniques. Plusieurs dispositifs ont été fabriqués et analysés. Nous examinons d'abord les dispositifs homogènes basés sur MoS2, GaSe ou InSe, et trouvons une excellente photosensibilité pour notre photo-détecteur MoS2. Nous examinons ensuite plusieurs hétéro-structures pour combiner les propriétés de chaque matériau et atteindre de meilleures performances. Le premier exemple est un photo-détecteur graphène/InSe dont la photosensibilité augmente de quatre ordres de grandeur par rapport à un dispositif basés sur InSe seul. Nous montrons également que la couche supérieure de graphène prévient la dégradation de couches atomiques ultra-minces dans l'air. Des hétéro-structures plus complexes graphène/InSe/graphène et graphène/InSe/Au montrent un effet photovoltaïque. Enfin, nous combinons InSe avec MoS2 et obtenons un dispositifs avec photo-réponse rapide, un comportement de type photo-diode, une distribution de photo-courant uniforme et un fort effet photovoltaïqueNovel two dimensional (2D) semiconductors beyond graphene such as MoS2, GaS, GaSe and InSe are increasingly relevant for emergent applications and devices. In this thesis, we fabricate these 2D samples for photo-detector applications and characterize them with optical microscopy, atomic force microscopy, Raman and photoluminescence (PL) spectroscopy and transmission electron microscopy. Since the interaction of light with the substrate and the ultra-thin photodetector device is critical for its functioning we calculate and measure optical contrast and intensity of light scattered from the device. We also characterize the Raman and PL response as a function of number of layers to study both vibrational properties and the band gap transition. For the device application, we first examine homogenous devices based on few-layer MoS2, GaSe and InSe respectively and find an excellent photoresponsivity in our few-layer MoS2 photo-detector. We then examine several geometries for heterostructure devices, which have the advantage of combining favorable properties of each material to reach better performances. The first example is a graphene/InSe photo-detector where the photoresponsivity increases by four orders of magnitude with respect to a few-layer InSe device while the top graphene layer is also shown to prevent degradation of ultra-thin atomic layers in air. Still more complex graphene/InSe/graphene and graphene/InSe/Au heterostructures show a photovoltaic effect. Finally for the first time, we combine InSe with MoS2 and obtain a high performance device with fast photo-response, photodiode like behavior, uniform photocurrent distribution and high photovoltaic effect
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David, Lamuel Abraham. "Synthesis of large-area few layer graphene films by rapid heating and cooling in a modified apcvd furnace." Thesis, Kansas State University, 2011. http://hdl.handle.net/2097/13115.
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Master of ScienceDepartment of Mechanical and Nuclear Engineering
Gurpreet Singh
Graphene because of its unique electrical (electron mobility = 2 x 10[superscript]5 cm[superscript]2 V[superscript]-1 s[superscript]-1), mechanical (E = 1 TPa), optical, thermal and chemical properties has generated a lot of interest among the research community in recent years. One of the most notable methods of synthesizing large area pristine graphene sheets, which are several 100 micrometers wide, is through thermal chemical vapor deposition (CVD). But very little has been known about the effects of heating and cooling rate of the substrate on the quality of graphene produced. Hence we varied various growth parameters to understand the process of graphene growth on Cu and Ni substrates when subjected to fast heating and quenching. This allowed optimization of the CVD process to achieve large-area graphene films consistently and repeatedly. This work provides new insights on synthesis of graphene at atmospheric pressures and the effect of (a) fast heating and fast cooling of substrates, (b) catalyst type and (c) gas flow rates on quality of the graphene produced. A carbon nanotube CVD furnace was restored and modified to accommodate graphene synthesis. We started with synthesis of graphene on Cu substrate following procedures already available in the literature (heating rate ~ 15 °C/min and cooling rate ~ 5 °C/min; total processing time 7 hours). This provided a good reference point for the particular furnace and the test setup. The best results were obtained for 15 minutes of growth at a CH4:H2 ratio of 1:30 at 950 °C. SEM images showed full coverage of the substrate by few layer graphene (FLG), which was indicated by the relatively high I[subscript]2D/I[subscript]G ratio of 0.44. The furnace was further modified to facilitate fast cooling (~4 °C/sec) of substrate while still being in inert atmosphere (Argon). The effect of growth time and concentration of CH[subscript]4 was studied for this modified procedure (at H[subscript]2 flow rate of 300 SCCM). SEM images showed full coverage for a CH[subscript]4 flow rate of 10 SCCM in as little as 6 minutes of growth time. This coupled with the fast cooling cycle effectively reduced the overall time of graphene synthesis by 7 times. The I[subscript]2D/I[subscript]G ratio in Raman spectrum was 0.4 indicating that the quality of graphene synthesized was similar to that obtained in conventional CVD. This modification also facilitated introduction of catalyst substrate after the furnace has reached growth temperature (fast heating ~8 °C/sec). Hence, the overall time required for graphene synthesis was reduced to ~6 % (30 minutes) when compared to the traditional procedure. SEM images showed formation of high concentration few layer graphene islands. This was attributed to the impurities on the catalyst surface, which in the traditional procedure would have been etched away during the long heating period. The optimum process parameters were 30 minutes of growth with 20 SCCM of CH[subscript]4 and 300 SCCM of H[subscript]2 at 950 °C. The Raman spectrum for this condition showed a relatively high I[subscript]2D/I[subscript]G ratio of 0.66. We also studied the effect of Ni as a catalyst. Similar to Cu, for Ni also, traditional procedure found in the literature was used to optimize the graphene growth for this particular furnace. Best results were obtained for 10 minutes of growth time with 120 SCCM of CH[subscript]4 in 300 SCCM of H[subscript]2 at 950 °C. SEM images showed large grain growth (~50 μm) with full coverage. The Raman spectrum showed formation of bi-layer graphene with a I[subscript]2D/I[subscript]G ratio of 1.03. Later the effect of growth time and concentration of the hydrocarbon precursor for Ni substrate subjected to fast heating (~ 8 °C/sec) was studied. It was found that because the process of graphene synthesis on Ni is by segregation, growth period or gas flow rate had little effect on the quality and size of the graphene sheets because of the presence of impurities on the substrate. This procedure yielded multilayer graphite instead of graphene under all conditions. Future work will involve study of changing several other parameters like type of hydrocarbon precursor and pressure in the chamber for graphene synthesis. Also various other substrates like Cu or Ni based alloys will be studied to identify the behavior of graphene growth using this novel procedure.
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Hamze, Samah. "Graphene based nanofluids : development, characterization and application for heat and energy systems." Thesis, Rennes 1, 2020. http://www.theses.fr/2020REN1S010.
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Dans notre vie quotidienne, le transfert de la chaleur et de l’énergie constitue la base de nombreux processus industriels. L’épuisement progressif des énergies fossiles conduit à améliorer et optimiser les rendements de ces échanges par de nouveaux procédés. Pour cela, une idée d’améliorer la performance thermique des fluides dans les échangeurs de chaleur a été proposée pour réduire l’énergie consommée pour l’échange de chaleur. Cette idée est basée sur l’introduction des nanoparticules solides qui présentent des propriétés thermiques beaucoup plus importantes que les liquides caloporteurs dans ces derniers, en obtenant un nanofluide. Cette introduction a pour effet d’augmenter la conductivité thermique du fluide mais d’autre part provoque une augmentation défavorable de sa viscosité qui résulte en une augmentation de la puissance de pompage. Alors il faut faire un compromis entre la stabilité, la conductivité thermique et la viscosité des nanofluides. Dans cette étude, des nanofluides à base de graphène à quelques couches et un fluide commercial, Tyfocor® LS, ont été préparés dans la gamme de concentration massique 0,05-0,5% en utilisant trois surfactants différents. Une étude complète sur ces nanofluides est présentée, y compris la synthèse des feuillets de graphène, la préparation des nanofluides et l’étude de leur stabilité, ainsi que l’évaluation expérimentale de leurs propriétés thermophysiques en fonction de la concentration en graphène, du type de surfactant utilisé et de la température dans la gamme 283,15-323,15 K. Finalement, sur la base de ces résultats et par une approche qualitative, le potentiel applicatif des nanofluides dans des systèmes énergétiques est déterminé pour sélectionner le meilleur candidat. Les résultats ont montré une bonne amélioration de la performance thermique par rapport aux fluides de base dans la gamme de température testée et surtout le nanofluide de la série du surfactant Pluronic® P-123 de concentration massique 0,25%In our daily lives, the heat and energy transfer forms the basis of many industrial processes. The gradual depletion of fossil fuels leads to improving and optimizing the efficiency of these exchanges through new processes. To this end, the idea of improving the thermal performance of fluids in heat exchangers has been proposed forward to reduce the energy consumed for heat exchange. This idea is based on the introduction of solid nanoparticles, which have much greater thermal properties than heat-transfer fluids in the latter, obtaining a nanofluid. This introduction has the effect of increasing the thermal conductivity of the fluid but on the other hand causes an unfavorable increase in its viscosity, which results in an increase in pumping power. So a compromise has to be made between the stability, thermal conductivity and viscosity of nanofluids. In this study, few layer graphene based nanofluids and a commercial fluid, Tyfocor® LS, were prepared in the weight concentration range 0.05-0.5% using three different surfactants. A complete study on these nanofluids is presented, including the synthesis of the graphene sheets, the preparation of the nanofluids and the study of their stability, as well as the experimental evaluation of their thermo-physical properties as a function of the graphene concentration, the type of surfactant used and the temperature in the range 283.15-323.15 K. Finally, on the basis of these results and through a qualitative approach, the potential application of nanofluids in energy systems is determined in order to select the best candidate. The results showed a good improvement of the thermal performance compared to the base fluids in the tested temperature range and especially the nanofluid of the Pluronic® P-123 surfactant series with a mass concentration of 0.25%
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Pirzado, Azhar Ali Ayaz. "Integration of few kayer graphene nanomaterials in organic solar cells as (transparent) conductor electrodes." Thesis, Strasbourg, 2015. http://www.theses.fr/2015STRAD016/document.
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Dans cette thèse, des films à base de graphène ont été étudiés comme alternatives viables dans la fabrication d'électrodes transparentes (TCE). Elle met l'accent sur des couches fines de graphène (FLG), sur l'oxyde de graphène réduit (RGO) et leurs hybrides avec des nanotubes de carbone (NTCs) pour être utilisé comme TCE dans les cellule solaires organiques (OSC). Les FLGs et RGO ont été préparés par des méthodes d'exfoliation mécanique ou en phase liquide assistée par micro-ondes. Ces nanomatériaux dilués dans un solvant liquide ont été déposé en couche mince par aérographe. Des caractérisations de transport de charge ont été obtenues grâce à la méthode des 4 pointes. Ces échantillons ont été caractérisés: leur transparence(UV-Visible), leur morphologie et leur topographique (MEB, MET, AFM) ainsi que le travail de sortie (UPS). Pour obtenir des informations sur la qualité structurelle des échantillons, nous avons utilisés les méthodes de spectroscopie XPS, Raman et la photoluminescenceGraphene mate rials have been researched as viable alternatives of transparent conductors electrodes (TCEs) in this thesis. Current study focuses on few layer graphene (FLG), reduced graphene oxide (rGO) and their hybrids with carbon nanotubes (CNTs) for TCE applications inorganic solar cells (OSCs). FLGs and rGOs have been prepared by mechanical and microwave-assisted exfoliation methods. This films of these materials have been produced by hot-spray method. Results of charge transport characterizations by four-point probes, transparency (UV-Vis), measurements, along with morphological (SEM, TEM) and topgraphic (AFM) studies of films have been presented. UPS studies were performed to determine for a work-function. XPS,Raman and Photoluminescence studies have been employed to obtain the information about the structural quality of the samples
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Zhang, Liping. "Immobilisation de catalyseurs moléculaires de polymérisation d’oléfines sur nanomatériaux." Thesis, Toulouse, INPT, 2014. http://www.theses.fr/2014INPT0013/document.
Full textAbstract:
Le présent travail de thèse décrit le développement de systèmes actifs de polymérisation d’oléfines basés sur des métaux de fin de transition (nickel et fer) supportés sur des nanomatériaux. Le chapitre I décrit l’état de l’art des systèmes catalytiques supportés ou non pour la polymérisation d’oléfines. Dans le chapitre II, nous décrivons la polymérisation de l’éthylène en utilisant des catalyseurs de nickel contenant un groupement –NH2 pour leur immobilisation covalente sur nanotubes de carbone ; montrant l’influence positive de l’immobilisation : les catalyseurs ainsi supportés sont en effet à la fois plus actifs et conduisant à des polymères de plus haut poids moléculaire. Dans le chapitre III, des complexes de fer contenant un groupement pyrène sont décrits et immobilisés sur nanotubes de carbone par interaction non covalente π-π. Dans ce cas, à la fois les systèmes homogènes et leurs analogues supportés catalysent la réaction de polymérisation de l’éthylène avec des activités particulièrement élevées. Il a également pu être mis en évidence l’importante influence du support carboné sur les performances du système catalytique ainsi que sur la structure des polymères obtenus. Différents types de complexes de nickel contenant un ligand imino-pyridine et différents groupes polyaromatiques ont été synthétisés et leur utilisation en polymérisation de l’éthylène est décrite dans le chapitre IV. L’influence de l’addition de faibles quantités de matériaux nanocarbonés (nanotubes de carbone ou graphène) au milieu réactionnel a ainsi été étudiée. Le graphène s’est dans ce cas révélé particulièrement bénéfique sur les performances du catalyseur. Enfin, le chapitre V décrit la polymérisation de l’isoprène à l’aide de catalyseurs de fer contenant des groupements polyaromatiques permettant leur immobilisation à la surface de nanoparticules de fer. Ces systèmes ont ensuite pu être confinés dans des nanotubes de carbone. Les systèmes catalytiques décrits sont particulièrement actifs produisant des polyisoprènes à température de transition vitreuse élevée et avec une haute sélectivité trans-1,4-polyisoprèneThis present thesis deals with the development of active olefin polymerization catalysts based on late transition metal (nickel and iron) imino-pyridine complexes supported on nanomaterial. Chapter I gives a comprehensive literature review of unsupported and supported ethylene polymerization catalyst. In Chapter II we report the ethylene polymerization studies using nickel complexes containing an –NH2 group for covalent immobilization on multi-walled carbon nanotubes (MWCNTs) of the corresponding precatalysts. Comparison of the homogeneous catalysts with their supported counterparts evidenced higher catalytic activity and higher molecular weights for the polymers produced. In Chapter III, iron complexes containing a pyrene group have been synthesized and immobilized on MWCNTs through non-covalent π-π interactions between pyrene group and surface of MWCNTs. Activated by MMAO, both the iron complexes and immobilized catalysts show high activities for ethylene polymerization. It was possible to evidence that MWCNTs have a great influence on the catalytic activity and on the structure of the resulting polyethylenes. Imino-pyridine nickel complexes containing various kinds of aromatic groups have been synthesized in Chapter IV and polymerization conditions in the presence and in the absence of nanocarbon materials, such as MWCNTs or few layer graphene (FLG), are discussed. For those nickel catalysts bearing 1-aryliminoethylpyridine ligands, the presence of MWCNTs in the catalytic mixture allows the formation of waxes of lower molecular weight and polydispersity, whereas the presence of FLG proved to be beneficial for the catalytic activity. In Chapter V, isoprene polymerization catalyzed by iron complexes containing polyaromatic groups and non-covalently supported on nanoparticles and confined into the inner cavity of MWCNTs (Cat@NPs and Cat@NPs@MWCNTs) are investigated. Iron complexes show excellent activity for the isoprene polymerization and produced high glass temperature polyisoprene with a high trans-1,4-polyisoprene selectivity. Polymer nanocomposites are produced by supported catalysts and, transmission electron microscopy (TEM) evidenced efficient coating of the resulting polyisoprene around the oxygen sensitive iron nanoparticles
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Zhang, Fan 1983. "Electronic correlations in few layer graphene." Thesis, 2011. http://hdl.handle.net/2152/ETD-UT-2011-12-4452.
Full textAbstract:
In this thesis we investigate the electronic band structures and the correlations
in chirally (ABC) stacked N-layer graphene with N ≥ 2. We use ab initio
density-functional theory and k · p theory to fit the parameters of a p-band tightbinding
model. External potential differences between top and bottom layers are
strongly screened by charge transfer but still open an energy gap at overall neutrality.
Perpendicular magnetic field drives the system into the quantum Hall region
with 4N-fold zero energy Landau levels. We predict that Coulomb interactions
spontaneously break the SU(4N) symmetry and drive quantum Hall effects at all
integer fillings n from −2N to 2N with exotic spin and pseudospin polarizations.
Based on mean-field theory and perturbative renormalization group analysis,
we predict that the ground state of bilayer graphene spontaneously breaks inversion
symmetry for arbitrarily weak electron-electron interactions and conclude that this
instability is not suppressed by quantum fluctuations but that, because of trigonal
warping, it may occur only in high quality suspended bilayers. Remarkably flat
conduction and valence bands that touch at charge neutrality point and Bloch states
with large pseudospin chirality combine to make the bilayer graphene gapless band
state strongly susceptible to a family of broken symmetry states in which each spinvalley
flavor spontaneously transfers charge between layers. We explain how these
states are distinguished by their charge, spin, and valley Hall conductivities, by
their orbital magnetizations, and by their edge state properties. We further analyze
how these competing states are influenced by Zeeman fields that couple to spin
and by interlayer electric fields that couple to layer pseudospin, and comment on
the possibility of using response and edge state signatures to identify the character
of the bilayer ground state experimentally. We demonstrate that similar insulating
broken symmetry states and spontaneous topological orders also occur in bilayer’s
thicker cousins, chirally stacked multilayer graphene systems.text
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Kuan-YuChen and 陳冠宇. "Coulomb excitations of few-layer graphene." Thesis, 2017. http://ndltd.ncl.edu.tw/handle/5x48nw.
Full textAbstract:
碩士國立成功大學
物理學系
105
In recent years, the two-dimensional structure materials have become many scientists’ objectives of the study due to its unique physical properties. Graphene, the first two-dimensional condensed matter to be researched theoretically and verified experimentally, is the foundation for studying two-dimensional materials. In this thesis, I will use the tight-binding model to calculate band structures, and use the Random Phase Approximation method (RPA) to simulate the Coulomb excitations of monolayer and multilayer doped graphene systems. The Coulomb excitations of electrons which obey the momentum and energy conservations and Fermi-Dirac distribution, can be classified as electron-hole single particle excitations and plasmon excitations. These phenomena would be shown in the excitation spectra. The properties of excitation spectra are influenced by the variables, such as transferred momentum, excitation frequency and Fermi energy. Besides, for multilayer systems, the number of layers and the stacking sequence have an impact on the interlayer atomic and electronic Coulomb interactions, and result in the changes in the excitation spectra. These predictions can be verified by the inelastic light scattering spectroscopy and the electron energy loss spectroscopy (EELS), and are useful for understanding many-body physics.
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Alzahrani, Areej A. "Production of High-quality Few-layer Graphene Flakes by Intercalation and Exfoliation." Thesis, 2017. http://hdl.handle.net/10754/626356.
Full textAbstract:
Graphene, a two-dimensional nanomaterial, has been given much attention since it was first isolated in 2004. Driving this intensive research effort are the unique properties of this one atom thick sheet of carbon, in particular its electrical, thermal and mechanical properties. While the technological applications proposed for graphene abound, its low-cost production in large scales is still a matter of interrogation. Simple methods to obtain few-layered graphene flakes of high structural quality are being investigated with the exfoliation of graphite taking a prominent place in this arena. From the many suggested approaches, the most promising involve the use of liquid media assisted by intercalants and shear forces acting on the basal layers of graphite.
In this thesis, it is discussed how a novel method was developed to produce flakes with consistent lateral dimensions that are also few-layered and retain the expected structural and chemical characteristics of graphene. Here, the source material was a commercially available graphiteintercalated compound, also known as expandable graphite. Several exfoliation-inducing tools were investigated including the use of blenders, homogenizers, and ultrasonic processors. To aid in this process, various solvents and intercalants were explored under different reactive conditions. The more efficient approach in yielding defect-free thin flakes was the use of thermally expanded graphite in boiling dimethylformamide followed by ultrasonic processing and centrifugation. In parallel, a method to fraction the flakes as a function of their lateral size was developed. Ultimately, it was possible to obtain samples of graphene flakes with a lateral dimension of a few micrometers (<5 μm) and thickness of 1-3 nm (i.e. <10 layers).
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Huang, Hung-Cheng, and 黃弘政. "The Study of Fabrication for Few-Layer Graphene Thin Films." Thesis, 2014. http://ndltd.ncl.edu.tw/handle/6gw689.
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Lo, Shih-Han, and 羅詩涵. "Development of an environmental process for exfoliating few-layer graphene." Thesis, 2016. http://ndltd.ncl.edu.tw/handle/80876839558755537052.
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LIN, SHAO-YU, and 林劭宇. "Fabrication and Analysis of Few-layer Graphene Films with Stacked Nanoparticles." Thesis, 2018. http://ndltd.ncl.edu.tw/handle/s8syy6.
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Min-ChengYang and 楊閔丞. "Phase transition of AA-stacking few-layer graphene under the uniaxial pressure." Thesis, 2012. http://ndltd.ncl.edu.tw/handle/76567436301041619801.
Full textAbstract:
碩士國立成功大學
物理學系碩博士班
100
In this thesis, the first-principles method based on the Density Functional Theory (DFT) and the Local Density Approximation (LDA) was used. The Projector Augmented Wave Method (PAW) is employed for describing the electron-ion interaction. The structural properties and electronic structure of the AA stacked few layered graphene were studied under uniaxial pressure. The lateral equilibrium lattice constant under a uniaxial pressure was also investigated to estimate the possible experimental process. It is found the bonding type of the carbon atoms in the AA stacked graphene was changed from sp2 to sp3 under high uniaxial pressure, whereas the sp3 bonding came back to sp2 when overcome an energy barrier. A new covalent bond was formed under these mechanical deformation and the thermal process. The electronic structure changed dramatically under uniaxial pressure and a thermal relaxation. A semimetal-insulator transition was observed for even layer AA stacked graphene.
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CHENG-YANG, HU, and 胡承暘. "Growth of Few-layer Graphene Film On Copper Foils by Magnetron Sputtering." Thesis, 2019. http://ndltd.ncl.edu.tw/handle/vgy894.
Full textAbstract:
碩士國立高雄科技大學
電機工程系
107
Graphene has been discovered since 2004. Numerous scientists are continuing doing research and trying to apply it in different fields because many of its features are better than those materials that have been found so far. However, there are a lot of difficulties on graphene preparation. That is why graphene still cannot be widely used in different fields today. It is quite difficult to produce graphene. For example, the process requires an ultra-high temperature environment which forced higher cost. Moreover, it is hard to control the area of graphene and numbers of film layers when it produces. Besides, due to low capacity d that, graphene can’t be mass-produced and transference still cause some problems to solve. Today, the common preparation method for graphene is Chemical Vapor Deposition(CVD). This method is able to produce high quality and large size of graphene, but the problems are related to an ultra-high temperature environment which causes higher cost and needs more process time. Therefore, we are looking forward to having the preparation method with low cost, efficient process and non-transfer. Rechargeable lithium-ion batteries as the main power source, the portable electronic device, either or both of concern in the scientific industry. In order to make the lithium ion battery having the advantages of high capacity, high life and low cost, we use the reactive magnetron sputtering system to prepare the anode material of the excellent solid lithium-ion battery, and proceed to the commercial research direction. In this study, few–layer graphene (below ten layers) were prepared at low temperature, we could efficiently fabricate the graphene lower then ten layers under the following conditions: Silicon as catalytic material at bottom deposited for 3 min, CH4 flow with 3 sccm, CuC process time of 120 seconds, annealing temperature at 900℃ and annealing time of 60 minutes which direct growth on copper foils can be used as anode material for lithium-ion batteries.
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Wu, Chih-Yu, and 吳至彧. "The Study of Synthesizing Few Layer Graphene by Thermal-CVD and Graphene Based Transparent Conductive Thin Film." Thesis, 2009. http://ndltd.ncl.edu.tw/handle/11852708717314367137.
Full textAbstract:
碩士國立清華大學
工程與系統科學系
97
Graphene, a 2D-planner material which is composed of carbons with only one-atom-thick has a stable chemical stability, excellent electron mobility, and high mechanical strength. Therefore, graphene has paid more and more attention for the application on flexible transparent conductive thin film. However, due to the lower free electron concentration (or free electron quantity) of Single-layer graphene, it’s difficult to apply on the flexible transparent conductive thin film. According to our calculation, the free electron quantity can be enhanced by adding the layers of graphene. The optimum layers of graphene for transparent conductive thin film is about 10~20 layers by our calcutation. Therefore, we have synthesized few-layer graphene (FLG) on nickel foil by chemical vapor deposition method. By changing the amounts of diluation gases and the times of carbon sources, we can synthesize large area (centimeter scale) FLG with high coverage on nickel foil under optimum condition. The numbers of layers is controlled for 1~30 layers. To further investigate the growth mechanism of graphene in chemical vapor deposition method, we analyze the FLG/Ni with Raman mapping, SEM, AFM, EDX, and the EBSD on the same location. Therefore, we can provide the direct experimental analysis results. The preliminary experimental results can’t be explained by segregation growth mechanism and deposition growth mechanism would be an appropriate for our cases. In transfer sections, FLG can be transferred by etching nickel foil with dilute HCl solution to SiO2/Si. FLG/SiO2/Si sample is also analyzed with Raman spectra, SEM, AFM, and EDX on the same location. The thickness of our FLG is about several nm by AFM analysis results. To synthesize FLG flexible transparent conductive thin film, FLG is transferred by roll-to-roll (R2R) process after CVD process. The transmittance and sheet resistance for our best FLG flexible transparent conductive thin films are 50~55 % and several thounds Ω/□.
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Huang, Jian-Wei, and 黃建維. "High quality few-layer graphene grown by Chemical Vapor Deposition and electric properties." Thesis, 2010. http://ndltd.ncl.edu.tw/handle/96683676240128329954.
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Lui, Chun Hung. "Investigations of the electronic, vibrational and structural properties of single and few-layer graphene." Thesis, 2011. https://doi.org/10.7916/D8BC44BK.
Full textAbstract:
Single and few-layer graphene (SLG and FLG) have stimulated great scientific interest because of their distinctive properties and potential for novel applications. In this dissertation, we investigate the mechanical, electronic and vibrational properties of these remarkable materials by various techniques, including atomic-force microscopy (AFM) and Raman, infrared (IR), and ultrafast optical spectroscopy. With respect to its mechanical properties, SLG is known to be capable of undergoing significant mechanical deformation. We have applied AFM to investigate how the morphology of SLG is influenced by the substrate on which it is deposited. We have found that SLG is strongly affected by the morphology of the underlying supporting surface. In particular, SLG deposited on atomically flat surfaces of mica substrates exhibits an ultraflat morphology, with height variation essentially indistinguishable from that observed for the surface of cleaved graphite. One of the most distinctive aspects of SLG is its spectrum of electronic excitations, with its characteristic linear energy-momentum dispersion relation. We have examined the dynamics of the corresponding Dirac fermions by optical emission spectroscopy. By analyzing the spectra of light emission induced in the spectral visible range by 30-femtosecond laser pulses, we find that the charge carriers in graphene cool by the emission of strongly coupled optical phonons in a few 10's of femtoseconds and thermalize among themselves even more rapidly. The charge carriers and the strongly coupled optical phonons are thus essentially in thermal equilibrium with one another on the picosecond time scale, but can be driven strongly out of equilibrium with the other phonons in the system. Temperatures exceeding 3000 K are achieved for the subsystem of the charge carriers and optical phonons under femtosecond laser excitation. While SLG exhibits remarkable physical properties, its few-layer counterparts are also of great interest. In particular, FLG can exist in various crystallographic stacking sequences, which strongly influence the material's electronic properties. We have developed an accurate and convenient method of characterizing stacking order in FLG using the lineshape of the Raman 2D-mode. Raman imaging allows us to visualize directly the spatial distribution of Bernal (ABA) and rhombohedral (ABC) stacking in trilayer and tetralayer graphene. We find that 15% of exfoliated graphene trilayers and tetralayers are comprised of micrometer-sized domains of rhombohedral stacking, rather than of usual Bernal stacking. The accurate identification of stacking domains in FLG allows us to investigate the influence of stacking order on the material's electronic properties. In particular, we have studied by means of IR spectroscopy the possibility of opening a band gap by the application of a strong perpendicular electric field in trilayer graphene. We observe an electrically tunable band gap exceeding 100 meV in ABC trilayers, while no band gap is found for ABA trilayers. We have also studied the influence of layer thickness and stacking order on the Raman response of the out-of-plane vibrations in FLG. We observe a Raman combination mode that involves the layer-breathing vibrations in FLG. This Raman mode is absent in SLG and exhibits a lineshape that depends sensitively on both the material's layer thickness and stacking sequence.
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Chen, Yi-Liang, and 陳奕良. "Preparation and Capacitive Characteristics of Ruthenium Oxide/Few Layer Graphene Hybrid Electrodes for Supercapacitor Application." Thesis, 2015. http://ndltd.ncl.edu.tw/handle/ppgjk8.
Full textAbstract:
碩士國立高雄應用科技大學
化學工程與材料工程系博碩士班
103
In this study, tantalum pentoxide (Ta2O5)-ruthenium oxide (RuO2)/few layer graphene (FLG)/titanium foil (Ti)/sulfuric acid (H2SO4)/Ta2O5-RuO2/FLG/Ti symmetry supercapacitor (Ta2O5-RuO2 /FLG-SC) were fabricated by using sandwich assembling technique, where Ta2O5- RuO2/FLG/Ti, H2SO4 behaved as the bipolar hybrid electrode and liquid electrolyte, respectively. Ta2O5-RuO2/FLG/Ti hybrid electrodes were further prepared by dipping Ti foil in the TaCl3-RuCl3-FLG bath with different concentrations. Result showed that the synthesized FLG exhibits a transparent with some folded morphologies, the thickness of synthesized FLG is ca. 1~ 3 nm, which is prepared FLG possesses 3~10 layers caused by the Van der Waals force. As for Raman spectroscopy analysis, G-band intensity is decreased after the reduction of graphene oxide, which is attributed to the increase in graphene edges fraction. Without TaCl5, the deposited amount of RuO2 on Ti with increasing the drawing rate, but the deposited layer of RuO2 reveals an extreme uniform characteristic. In the presence of TaCl5, the uniform characteristic of RuO2 deposited layer can be significantly improved. When TaCl3/RuCl3 molar ratio is 0.6, drawing rate is 14.3 mm/s, Ta2O5-RuO2 layer is successfully deposited, which exhibits the best uniform characteristic. In the presence of FLG, Ta2O5-RuO2/FLG -SCs show lower resistance and leakage current, the cycle stability is also significantly improved. At 30 wt.% of FLG, dipping for three times, Ta2O5-RuO2/FLG-SCs show the best capacitance performance, the energy density and power density are 12.72 Wh/kg and 262kW/kg, respectively.
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Mansour, Ahmed. "Versatile and Tunable Transparent Conducting Electrodes Based on Doped Graphene." Diss., 2016. http://hdl.handle.net/10754/621935.
Full textAbstract:
The continued growth of the optoelectronics industry and the emergence of wearable and flexible electronics will continue to place an ever increasing pressure on replacing ITO, the most widely used transparent conducting electrode (TCE). Among the various candidates, graphene shows the highest optical transmittance in addition to promising electrical transport properties. The currently available large-scale synthesis routes of graphene result in polycrystalline samples rife with grain boundaries and other defects which limit its transport properties. Chemical doping of graphene is a viable route towards increasing its conductivity and tuning its work function. However, dopants are typically present at the surface of the graphene sheet, making them highly susceptible to degradation in environmental conditions. Few-layers graphene (FLG) is a more resilient form of graphene exhibiting higher conductivity and performance stability under stretching and bending as contrasted to single-layer graphene. In addition FLG presents the advantage of being amenable bulk doping by intercalation.
Herein, we explore non-covalent doping routes of CVD FLG, such as surface doping, intercalation and combination thereof, through in-depth and systematic characterization of the electrical transport properties and energy levels shifts. The intercalation of FLG with Br2 and FeCl3 is demonstrated, showing the highest improvements of the figure of merit of TCEs of any doping scheme, which results from up to a five-fold increase in conductivity while maintaining the transmittance within 3% of that for the pristine value. Importantly the intercalation yields TCEs that are air-stable, due to encapsulation of the intercalant in the bulk of FLG. Surface doping with novel solution-processed metal-organic molecular species (n- and p-type) is demonstrated with an unprecedented range of work function modulation, resulting from electron transfer and the formation of molecular surface dipoles. However, the conductivity increases compared modestly to intercalation as the electron transfer is limited to the uppermost graphene layers. Finally, a novel and universal multi-modal doping strategy is developed, thanks to the unique platform offered by FLG, where surface and intercalation doping are combined to mutually achieve high conductivity with an extended tunability of the work function. This work presents doped-FLG as a prospective and versatile candidate among emerging TCEs, given the need for efficient and stable doping routes capable of controllably tuning its properties to meet the criteria of a broad range of applications.
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WANG, PIN-HAN, and 王彬翰. "The effect of electric field on the electronic properties of few layer graphene nanoribbons:A first-principle study." Thesis, 2015. http://ndltd.ncl.edu.tw/handle/21600007157235912586.
Full textAbstract:
碩士國立中正大學
物理學系暨研究所
103
The purpose of this thesis was to use Density Functional Theory (DFT) to determine both geometry and electric properties of few-layer graphene nanoribbons and applied an electric field to study the influence of electric properties. More than two layer Zigzag graphene nanoribbon will make the structure bend.If fit the structure in Z-Y plane, we will get a quadratic linear regression model and it’s equation (z=a+by+cy^2 ).Then,we can judge the curvature by magnitude of c. Results indicate that wider width of Zigzag graphene nanoribbon got lower magnitude of c. Different stack of few-layer Zigzag graphene nanoribbon have different electric properties.Two-layer Zigzag graphene nanoribbon magnetic moment will vanish, and its bandgap decrease into zero. AAA-stack Zigzag graphene nanoribbon band structure cross through fermi level and change into a metal-semiconductor. Same result appearin ABC’A and AAAA-stack of Zigzag graphene nanoribbon. Armchair graphene nanoribbon more than two layer did not bend. Different layer-stack Armchair graphene nanoribbon got different electric properties. When layer increase, Armchair graphene nanoribbon band gap will decrease for N=3p and 3p+1. In order to get the relation between width and electric field with different stack,we applied an electric field on Zigzag graphene nanoribbon. The calculation show that some value of electric field will made moment vanish or total moment unequal zero. Causing the band structure irregular vary. The effective electric field alter the band cross fermi level,and causes the metal-semiconductor transition. Electric field cause no apparent change to Armchair graphene nanoribbon,N=3pwill tiny decrease band gap when electric field increase.
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Peng, Kaung-Jay, and 彭冠傑. "Hydrogen-free PECVD growth of few-layer graphene at threshold temperature for optical modulation and mode-locking applications." Thesis, 2011. http://ndltd.ncl.edu.tw/handle/18827894701872429224.
Full textAbstract:
碩士國立臺灣大學
光電工程學研究所
100
The synthesis of few-layer graphene sheet on ultra-thin nickel film coated SiO2/Si substrate by using hydrogen-free plasma-enhanced chemical vapor deposition with in-situ low-temperature carbon dissolution is preliminarily demonstrated. The deposited carbon atoms are initially dissolved into the nickel matrix and subsequently participated out on nickel film surface. The threshold carbon dissolution temperature for synthesizing few-layer graphene is observed as low as 475oC, and the critical thickness of host nickel film is at least 30 nm. Due to the ultra-low solubility of carbon atoms into nickel film at threshold temperature of 475oC, the layer number of few-layer graphene can be precisely controlled. Raman scattering analysis indicates almost identical D and 2D peak intensities for nickel films with different thickness, whereas the G peak enhances with increasing layer number of graphene precipitated from thicker nickel films. The saturation of G peak at 50-nm thick nickel film due to the finite carbon dissolution within a limited deposition time is observed to preserve a stabilized quality of precipitated few-layer graphene. The linear transmittance of few-layer graphene at 550 nm is increased from 83 to 93% when shortenening the deposition time from 600 to 100s, corresponding to a decrease of graphene layer number from 8 to 3 layers. The Raman scattering peak ratio of ID/IG decreases from 1.8 to 0.2 and the G-band linewidth shrinks from 67 to 37.2 cm-1 accordingly, providing strong evidence for the improved quality of few-layer graphene synthesized with the hydrogen-free and threshold temperature on ultra-thin nickel host. There are three type graphene based optical switches demonstrated by us. Two of the modulators controlled the signals by the voltage and the other controlled the signals by the continuous wave laser power. The first one is transmission type graphene modulator. For 20 % of modulation depth, the drive voltage can be decreased from 30 V to 5 V when the insulator thickness decreases from 200nm to 10 nm. The reflection type modulator doesn’t performed well than that of transmission type modulator. The modulation depth is only 5% at 8V with the insulator thickness of 50 nm. That is caused by the thicker insulator and the device short easier than that of transmission type modulator. The last one is graphene on side polished fiber optical switch. The input power enhanced from 500 mW to 525, 519 and 516 mW for 1520, 1550 and 1580 nm of laser under 85 mW of exposition and the modulation depths are 5, 3.8 and 3.2 % respectively. The modulation depth increases when the wavelength decreases. There are more states available at higher energy stage for graphene so 1520 nm of laser can excite more carriers than that of 1550 and 1580 nm. When the input power is 1000 mW and the power of continuous wave laser is 85 mW, the maximum modulation depth can be obtained. The modulation depth for 1520, 1550 and 1580 nm are 5.8, 5.1 and 4.6 % respectively. The modulation depth for 1550 nm with the intensity of 1000 mW increases from 1.2 to 5.1 % when the continuous wave laser power increases from 24 to 85 mW. More carriers can be excited when the power of continuous waved laser increases. The modulation depth of 1550 nm under 85 mW of exposition decreases from 5.1 to 3.3 % when the input power increase from 1000 mW to 2000 mW. The states are almost filled at 1000 mW, so 2000 mW can not further excited much more carriers. Graphene is served as mode locker for both transmission type and reflection type passively mode-locked fiber laser of the erbium-doped fiber lasers (EDFLs). In transmission type system, the pulsewidth increases from 441 to 483 fs and the spectral FWHM decreases from 6 to 4.2 nm when the pumping current decreases from 900 to 200 mA. The repetition rate is 28.57 MHz. In reflection type system the pulsewidth increases from 796 to 874 fs and the spectral FWHM decrease from 3.25 to 2.2 nm. The repetition rate is 16.66 MHz. The smaller repetition rate of reflection type passively mode-locked EDFL is cause by the extra cavity length contributing by circulator. The shorter pulsewidth could be obtained from transmission type passively mode-locked EDFL because the inevitable loss 1.8 dB caused by circulator in reflection type passively mode-locked EDFL. The time bandwidth product for both system is 0.31 under high pumping current (800~900 mA). It is really close to the transform limited. The pumping threshold for both system is 400 mA. The time bandwidth product decrease very fast when the pumping current is smaller than 400 mA.
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Deng, Jun-Hao, and 鄧君浩. "Growth of few layer graphene on SiC surface and characterization of their electronic structure by angle-resolved photoemission." Thesis, 2013. http://ndltd.ncl.edu.tw/handle/44980402927973960632.
Full textAbstract:
碩士國立清華大學
物理系
101
Graphene consists of a single layer carbon atoms packed in a 2D honeycomb structure. It is very highly promising as a low-dimensional material for the next generation devices owing to its high carrier mobility, high crystal quality and economic price. It will be playing an important role to improve the nanotechnology in the future. In this thesis, I successfully grow high quality bilayer graphene on 6H-SiC and use the Angle-resolved photoemission spectroscopy (ARPES) to probe its electronic structure. We obatin a good fitting result by a tight-binding (TB) model in SLG/SiC and BLG/SiC. Importantly, the on-site energy difference caused the gap opening in SLG/SiC and BLG/SiC can be realized by TB fitting. The reason of the gap opening in BLG/SiC is not only attributed to the difference in on-site energy between different carbon atom sites in the same graphene layer, but also originates from the energy difference between different graphene layers. The interlayer spacing of BLG/SiC can be determined from the oscillation period of photoemission intensity for different photon energies. Our result of interlayer spacing d = 3.31 Å for BLG/SiC is slightly smaller than the value in bulk graphite.
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Wu, Jhao-Yi, and 吳兆益. "Scalable and Facile Production of Few-Layer Graphene and its Application as Conductive Additives for Lithium Ion Battery." Thesis, 2018. http://ndltd.ncl.edu.tw/handle/fc2w8h.
Full textAbstract:
碩士中原大學
化學工程研究所
106
In this study, a green, facile, low-cost and scalable industrial method using jet cavitation (JC) is utilized to prepare graphene conductive additive and applied to a lithium-ion battery. The study is divided into three parts. In the first part, delamination of artificial graphite and natural graphite by jet cavitation to prepare few layer graphene (MoCPCB and Mo8) is studied. According to AFM analysis, more than 80% the layer of few layer graphene is less than 5 nm (10 layers). In the Raman analysis, the D/G ratio of MoCPCB is 0.24, and the D/G ratio of Mo8 is 0.19, which is lower than the graphene produced by Hummers method. Next, the as-prepared graphene is applied as conductive additive for lithium ion battery. Graphene is also mixed with commercial conductive carbon black for comparison. The battery with MoCPCB 7% SP 3% has an average capacity of 144.6 mAh/g at 0.1 C and 131.0 mAh/g at 1 C. The battery with Mo8 3% SP 7 % has an average capacity 154.0 mAh/g at 0.1 C and 142.1 mAh/g at 1 C. These two conductive additive ratios deliver the best C-rate performance compared to commercial KS6. In the second part, the effects of different methods to add conductive additives to improve the C-rate performance is investigated. The battery with liquid-phase conductive additives has an average capacity 149.8 mAh/g at 0.1 C, 142.9 mAh/g at 1 C and 91.8 mAh/g at 10 C with a retention of 61.2%. This method is found to be more effective than using solid-phase method to prepare the conductive additive. Lastly, the effect of different amount conductive additives on the C-rate performance through liquid-phase method is examined. It is found that the optimum electrochemical performance of the battery is achieved by adding 4% conductive additive. An average capacity of 137.3 mAh/g(electrode) and 125.3 mAh/g(electrode) are obtained at 0.1C and 1C, respectively. At 10 C, the average capacity is 56.2 mAh/g(electrode) with a capacity retention of 41.0 %. A continuous, facile and scalable approach to prepare graphene for conductive additive applications is successfully obtained through jet-cavitation technique. It is effectively applied as conductive additives for the cathode in lithium-ion batteries. The obtain results showed potential commercial applications in the future.
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Lin, Yu-Hsiang, and 林郁翔. "Raman investigation of pyramid-like hexagonal single-crystal domains of few layer graphene grown via low pressure chemical vapor deposition." Thesis, 2011. http://ndltd.ncl.edu.tw/handle/54834292787365286956.
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CHIANG, PIN-HSUAN, and 江品璇. "Preparation and Optoelectric Properties of Few-Layer Reduced Graphene Oxide Conjugated with Self Welding Silver Nanowire Junctions as Flexible Transparent Conducting Hybrid Films." Thesis, 2018. http://ndltd.ncl.edu.tw/handle/m676a4.
Full textAbstract:
碩士國立高雄應用科技大學
化學工程與材料工程系博碩士班
106
In this study, flexible transparent conducting hybrid films (TCFs) based on few layer reduced graphene oxide (FrGO) conjugated with hydrolyzed-polyethylene terephthalate (H-PET)-based self-welding (SW) commercial silver nanowires (AgNWs) were fabricated by water-bath assisted dipping coating method. H-PET-based SW-AgNW networks were controlled by the mirror silver reaction with different reaction rates and followed by dip-coated on the H-PET film. Few layer graphene oxide (FGO) were prepared by modified Hummers and low speed centrifuge method. FrGO/SW-AgNW TCFs were further prepared by reduced under sodium borohydride and followed by dip-coated on the H-PET-based SW- AgNWs. Effects of mirror silver reaction rate and FrGO layer on the conducting networks, surface morphology, sheet resistance and transmittance of FrGO/SW-AgNW TCFs are systematically studied. The interaction between AgNWs and FrGO was also further discussed. Results showed that SW-AgNW TCFs can be successfully prepared by water-bath assisted dip-coated and mirror silver reaction. As for optical and electrical characteristics analysis, the gain value of transmittance (GPS) can reach 1.88% (the transmittance is slightly increased from 76.06% to 77.49% ) which induced by the self-welding effect. However, GPS value is decreased with increasing the mirror silver reaction temperature and time. The Gain values of sheet resistance (GES) exhibit mostly negative in nature, the maximum value of GES can remark reduce to 61.06%, confirm the truth of mirror silver reaction with the excellent self-welding effect, as well GES value is also decreased with decreasing the mirror silver reaction temperature and time. Furthermore, the results revealed that FrGO/SW-AgNW TCFs can be successfully prepared by water-bath assisted dip-coated. As for optical and electrical analysis, the gain value of transmittance (GPF) can reach 1.92% (the transmittance is slightly increased from 70.80% to 72.16%). The gain value of sheet resistance (GEF) can remark reduce to 59.9% (the sheet resistance dramatically dropped from 123.6 Ω/sq to 49.5 Ω/sq). For Raman and XPS analysis, the charge transfer behavior between FrGO and SW-AgNWs is observed, which attributed to the bridging effect between FrGO and SW-AgNWs, leading to the increase the number of conductive paths in the networks.
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Yang, Chun-Yu, and 楊竣宇. "Fabrication of high-quality few-layer graphene nano-sheet by ultraslow electrochemical exfoliation for application in passive mode-locking of Erbium-doped fiber lasers." Thesis, 2013. http://ndltd.ncl.edu.tw/handle/28261668002567389915.
Full textAbstract:
碩士國立臺灣大學
光電工程學研究所
101
High-quality few-layer graphene nano-sheet without the need of post annealing procedure is obtained with ultra-slow electrochemical exfoliation of graphite. A decreasing D/G Raman scattering band intensity ratio with reducing exfoliation bias from +6 to +3 volts implies suppression on the structural defect with controlling the exfoliation bias to the threshold. In contrast, the I(2D)/I(G) ratio is concurrently enlarged by twice to support the decrease in layer number of exfoliated graphene nano-sheets with exfoliation bias lowered to the threshold. The X-ray photoelectron spectroscopy also confirms the suppression of the anode oxidation in the few-layer graphene nano-sheets with low exfoliation bias. After extracting by centrifugation, the average diameter of the exfoliated graphene nano-sheets extracted from the acetone solution is ranged from 7 um to 0.5 um as the exfoliation bias decreasing from 6 to 3 volts. The few-layer graphene nano-sheets electrochemically exfoliated at 3 volt is then demonstrated to passively mode-lock the erbium-doped fiber lasers. By spreading and drying the aqueous solution, the graphene nano-sheets are directly imprinted onto the connector end-face of a single-mode fiber patchcord inside the erbium-doped fiber laser (EDFL) ring cavity with the total GDD of -0.154 ps2. The linear transmittance and modulation depth of the inserted graphene nano-sheets are respectively 92.5% and 53%. The mode-locked EDFL can deliver a pulse-train with a pulsewidth of 454.5 fs and a bandwidth of 5.6 nm, at a repetition rate of 26.7 MHz and an average output power of 8.91 mW. The time-bandwidth product of 0.32 is close to the transform limit. The graphene nano-sheets exfoliated under different biases are next employed as saturable absorbers, and the modulation depth is decreased from 53% to 17% with the exfoliation bias increased from 3 to 6 volts. As being used to passively mode-lock the EDFLs at low gain condition, the different samples show clearly distinct mode-locking performances which is related to the modulation depth. However, such divergence is attenuated when the currents of the pumping laser diodes increased from 200 to 900 mA. The numerical simulation by using Haus’ mater equation is employed to describe the EDFL pulse formation, and the interaction between the anomalous GDD and the SPM effects is investigated to become the dominating mechanism with the increasing gain. The compression ratio with the cooperation of SPM and GDD effects is about 0.3 under the pumping currents of two laser diodes of 900 mA.
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Wu, Guanhung, and 吳官晃. "Visibility of few-layer graphenes." Thesis, 2012. http://ndltd.ncl.edu.tw/handle/75519916084564426235.
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碩士國立中正大學
光機電整合工程研究所
100
We investigated the colors of grapheme multilayers that were grew various layers. There are two sample had been analyzed. Firstly, we used silicon wafer grew with 300nm SiO2. When we observed the silicon wafer and glass substrates under optical microscope, we can found that the effects of the substrate were analyzed. The colors of graphene layers on a SiO2/Si and glass substrates were found to change as the material thickness increased. However, the color diffenence among three layers of grapheme on silicon/SiO2 is significant. The constructive interference at 560nm, the constructive also increase as the number of layers increase. On the other hand, the destructive interference at 680nm, the interference also increase as the number of layers increase. Secondly, the color diffenence among three layers of grapheme on glass are very close to each other. The constructive interference at 615nm, the constructive also increase as the number of layers increase Otherwise, we can’t get any useful information from the spectrum so that we make the information meaningful. We turn the spectrum into quantify by principle component analysis. We compare between first spindle and second spindle, we can identify the different layers among the community successfully. In addition, we can proceed between the different layers of the first spindle and the second spindle to classify by the community. In the future, when we get an un know layers of grapheme that we can make identify much more easier by multi-spectral technology. And thus can be between different layers of analog video graphics by determined the threshold. Eliminating validation process by the raman spectroscopy and transmission spectra. We believe that these results demonstrate the possibility of utilizing color as a simple tool for detecting and estimating the thicknesses of multi layers graphene. It’s a convenience tool for the detection and assessment of graphene layers.
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Ming-JiuHu and 胡明炬. "Landau levels in AA-stacked few-layer graphenes." Thesis, 2010. http://ndltd.ncl.edu.tw/handle/80483409099625099972.
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碩士國立成功大學
物理學系碩博士班
98
In this paper,we investigate the low energy landau levels (LLs) in few layer graphene.The LL distribution strongly depends on the magnetic field strength, the layer number,and the stacking structure. The interlayer atomic interations lead to more low LLs and the asymmetry LLs about the zero Fermi energy. For AA-stacked bilayer graphene, the low lying LLspectrum cannot be accounted by a simple relation as that was made in AB-stacked bilayer graphene.However, it can be categorized into two groups of LLs according to the two sets of effective quantum numbers and . The two groups of LLs show the similar dependence on the quantum number and field strength. For AA-stacked trilayer graphene, there are three groups of LLs. By examining the carrier density distribution, we know that they are attributed to the outmost layers and the middle layer respectively. These results are helpful to understand how the interlayer couplings affect the low-energy magnetoelectronic properties and could be used to analyze the optical experiments.
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Ho, Jon-Hsu, and 何烱煦. "Electronic Excitations and Deexcitations of Few-Layer Graphenes." Thesis, 2006. http://ndltd.ncl.edu.tw/handle/30066540490587739723.
Full textAbstract:
博士國立成功大學
物理學系碩博士班
94
In this thesis, coulomb excitations and deexcitations of a monolayer graphite are studied within random-phase approximation. A monolayer graphite exhibits rich low-frequency excitation spectra, mainly owing to the zero-gap characteristic. There exist interband e-h excitations, intraband e-h excitations, and plasmon. The latter two are purely caused by temperature. Interband e-h excitations are the only deexcitations mechanism for Fermi-momentum state. As to other states, three kinds of excitations are responsible for decay rates. The formulas of coulomb excitations for multilayered graphite are derived within random-phase approximation. Coulomb excitations of two typical bilayer graphites with different stacking sequences are studied. They are strongly affected by the stacking sequence, the interlayer coulomb interaction, and the momentum transfer. However, they hardly depend on the direction of momentum transfer and the temperature. The electronic structure of a monolayer graphite under the spatially modulated field is studied by tight-binding model. Electronic properties strongly depend on the strength, the direction, and the period of modulated fields. Such fields could lead to the drastic changes in state degeneracy, energy dispersions, band spacings, wave functions, and band-edge states. Hence, density of states exhibits several special structures.
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Lin, Hsiao-Po, and 林孝柏. "Electronic structures of few-layer graphenes under deformation." Thesis, 2007. http://ndltd.ncl.edu.tw/handle/19052816005191598381.
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碩士國立成功大學
物理學系碩博士班
95
In this thesis, the electronic structures of few-layer graphenes (~1-4 layers) are investigated by the tight-binding model. They are significantly affected by the number of layers, the stacking sequences, and the intensity of stress along armchair and zigzag directions. The effects of interlayer interactions would cause the drastic changes in the energy dispersions, the band widths, and the new band-edge states. The electronic properties could reflect in the density of states (DOS) per unit area. The linear subbands correspond to finite values in DOS. The band-edge states exhibit the peaks of logarithmic divergences or square-root divergences. The deformation effects of graphene lattices under external stress are applied to the elasticity theory. The strain along x-axis is relevant to different values along y-axis and z-axis. Both intralayer and interlayer hopping integrals would be adjusted by the deformed C-C bond lengths, according to the Harrison’s rule. The linear bands intersection of graphene shifts under deformation, but it still remains zero-gap semiconductors. The deformation influences energy bands and causes the semimetal-semiconductor transition in AB-stacked bilayer graphene. The stacking consequence would be more complicated in tri- and quadri- layers due to the increasing number of layers. In addition, the geometric structures of the tension (compression) along armchair direction are similar to those of the compression(tension) along zigzag direction for few-layer graphenes. The deformation effects would alter intralayer and interlayer hopping integrals between C-C atoms, the large shift of the Fermi momenta, the strong modification of low band structures, and the change of free carrier concentrations. It also leads to the prominent peaks in DOS. The predicted electronic properties could be verified by the scanning tunneling spectroscopy.
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Chen, Po-han, and 陳柏翰. "The electronic and structural properties of few-layer graphenes." Thesis, 2007. http://ndltd.ncl.edu.tw/handle/e9smy3.
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碩士國立中山大學
物理學系研究所
95
The first-principles calculation method has been used to obtain electronic and structural properties of few-layer-graphenes (FLG), the layer spacing for N = 2, 3, 4, 5, 6, 7 and 8 AB stacked FLG’s are calculated. It is found that the AB stacking is more favorable than the AA stacking and the layer spacing for the two-layer FLG is only 2.725Å, which is substantially reduced from that of the graphite. The average layer spacing for 3-, 4-, 5-, 6-, 7-, and 8-layer AB stacked FLG’s are 3.389Å, 3.331Å, 3.317Å, 3.192Å, 3.220Å, and 3.220Å, respectively, which show that the average layer spacing approaches the bulk value when the number of layers is increased. For all 2- to 8-layer AB stacked FLG’s energy bands overlap near EF and near K, which show that FLG’s are semi-metallic.
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Shih-HaoLee and 李偲豪. "Deformation effects on electronic structures and optical excitations of few-layer graphenes." Thesis, 2010. http://ndltd.ncl.edu.tw/handle/80729555131163242205.
Full textAbstract:
博士國立成功大學
物理學系碩博士班
98
The electronic properties of monolayer- , AA-stacked and AB-stacked multilayer graphenes under deformation are studied by the tight-binding model. The optical excitation spectra, directly reflecting the main characteristics of band-edge states, are evaluated within the gradient approximation. The mechanical effects of strain on graphene are based on the elasticity theory. The uniaxial stress drastically changes the energy dispersion, band-edge states, Fermi momenta, state degeneracies, which reflects on the density of states and absorption spectra A(w). The interlayer atomic interactions induce prominent peaks, shoulder structures, and transition gaps in optical excitations. The stacking sequences respectively lead to linear and parabolic low-energy bands for AA- and AB-stacked graphenes, resulting in extremely different optical structures. These optical features are also influenced by the layer number. In the absence of uniaxial stress, the first Brillouin zones (1st BZ) of both AA- and AB-stacked few-layer graphenes exhibit a hexagonal symmetry; nevertheless, their band structures show strong anisotropy. Such anisotropic properties are not revealed in absorption spectra, owing to the relations between the velocity matrix elements and the joint density of states, even though the integrals of square of velocity matrix elements possess anisotropy for all frequency. Under the uniaxial stress, deformation breaks the hexagonal symmetry of the 1st BZ, and changes the relations between the energy dispersions and the velocity matrix elements. Thus isotropic spectra are also destroyed by strain. The predicted results would be useful in identifying the experimental measurements on two-dimensional few-layer graphenes.
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Tseng, Po Tang, and 曾柏棠. "Field Emission Characteristic Study on Vertical Few-layer Graphite/Diamond Composite Film." Thesis, 2015. http://ndltd.ncl.edu.tw/handle/d5t7pw.
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碩士國立清華大學
動力機械工程學系
103
The purpose of this study is to produce the diamond tip arrays composite vertical few-layer graphite with high field emission effect. Because of its lower energy cost and high power advantages. It can be applied to thin the panel or make better cooling components. The diamond tip arrays with octagonal cone are formed by microwave plasma enhanced chemical vapor deposition (MPCVD) on silicon. Through the diamond and vertical few-layer graphite deposition, change the amount of nitrogen which leads to improve the diamond tip field emission. The diamond tip arrays and vertical few-layer graphite morphology are examined by Raman spectroscopy and scanning electron microscopy (SEM), respectively. The field emission effect with different diamond composite vertical few-layer graphite arrays are examined by field emission meter. The research first proposed the diamond tip composite vertical few-layer graphite structure with novel octagonal cone array to improve the field emission effect and lifetime because of the high pitch height ratio of vertical few-layer graphite and the high stability of diamond. The microcrystalline and ultrananocrystalline diamond are growth in this research. Besides, the staggered arrangement of vertical few-layer graphite and the diamond structure growth inside the vertical graphite flake is different from previous studies. In this study, the diamond tip arrays composite vertical few-layer graphite growth by N2/H2/CH4 = 40/80/20 shows good filed emission stability and high field emission characteristics with low turn-on field of 2.60 V/μm and field enhancement factor of 1921.