Дисертації з теми "Fluorinated graphite"

Sa conductivité électronique ou encore sa transparence optique sont autant de propriétés physico-chimiques singulières du graphène qui expliquent le nombre accru de méthodes d’exfoliation de précurseurs graphitiques développées pour l’obtention de ce matériau. Pour palier à l’utilisation d’un oxyde de graphite/graphène caractérisé par une chimie de surface mal maitrisée, des graphites fluorés, de cristallinité mais aussi de concentration en fluor variables, ont été préparés par fluoration de graphite sous fluor moléculaire pur après optimisation des paramètres. Les précurseurs, que ce soit par fluoration dynamique ou statique, ainsi obtenus ont été caractérisés finement : diffraction des rayons X, spectroscopies IR et Raman et leur texture sondée par Microscopie Electronique à Balayage et à Transmission. Suite à cela, trois méthodes d’exfoliation ont été mises en place, basées sur des mécanismes différents : i) une exfoliation par choc thermique, déjà connue mais dont les mécanismes de décomposition ont été affinés dans cette étude, ii) une exfoliation en voie liquide, avec l’utilisation pour la première fois d’un graphite fluoré pour la synthèse de graphène fluoré multi feuillets par voie électrochimique pulsée, et enfin iii) une méthode originale, peu conventionnelle, basée sur l’interaction laser femtoseconde/graphite hautement fluoré pour induire des mécanismes de réduction contrôlée, et surtout d’exfoliation de la matrice. Ces méthodes ont permis de mettre en évidence l’intérêt de la présence de fluor dans la course actuelle pour la synthèse de graphène, et ont montré l’obtention de matériaux graphéniques,possédant une fonction résiduelle fluorée intéressante pour certaines applications
Its electronic conductivity or its optical transparency are unequaled physicochemicalproperties of graphene which explain the increased number of exfoliation methods based ongraphitic precursors to obtain this material. To overcome the use of a graphite/graphene oxidecharacterized by a poorly controlled surface chemistry, graphite fluorides, with variablecrystallinity and also fluorine concentration, were prepared by fluorination of graphite under puremolecular fluorine atmosphere after optimization of the process parameters. The obtainedprecursors, whether by dynamic or static fluorination, were characterized : X-Ray diffraction, FTIRand Raman spectroscopies for the structure, and their texture probed by Scanning andTransmission Electron Microscopy. After that, three methods of exfoliation were developed, basedon different mechanisms: i) a thermal shock, already known but decomposition mechanisms wererefined in this study, ii) an exfoliation within liquid medium by pulsed electrochemical treatment,using for the first time a fluorinated graphite for the synthesis of few-layered fluorinated grapheneand finally iii) an unconventional method, based on the interaction between femtosecond laser andhighly fluorinated graphite to induce mechanisms like controlled reduction, and especially for thisstudy exfoliation of the matrix. These methods have permit to highlight the interest of fluorine inthe current race for the synthesis of graphene, and have shown the production of graphenematerials, having an interesting fluorinated residual functionalization for some applications

L'objectif de ces travaux de thèse était de synthétiser des nanomatériaux hautement diffusant à faible absorption neutronique, constitués uniquement de carbone et de fluor, ceci afin de combler le gap de réflectivité des neutrons lents dans la plage de vitesse des neutrons 90-600 m/s (se situant entre le meilleur supermiroir et le graphite). Les matériaux retenus étaient des nanodiamants de diamètre calibré (5 nm) et des graphites fluorés. Parmi les graphites fluorés, la phase (C2F)n, possède la distance interfeuillet la plus élevée (9 Å) permettant une réflectivité jusqu'à 220 m/s. Malgré la difficulté d'obtention de cette phase (C2F)n, de par son domaine de température de fluoration restreint (350-400 °C), la tendance à la surfluoration et le risque d'exfoliation, nous sommes parvenus à synthétiser des poudres de graphites fluorés à haute teneur en (C2F)n dont la teneur maximale obtenue est de 96 % de (C2F)n, avec une distance interplannaire d'environ 9 Å. Les mesures de réflectivité des neutrons sur ces échantillons ont révélé qu'un graphite fluoré riche en (C2F)n peut être utilisé comme un réflecteur efficace pour les neutrons lents. Des feuilles de graphites ont également été fluorées afin de palier l'impossibilité de densifier les poudres de graphite fluoré, critère essentiel pour la création de réflecteurs de neutrons. Des taux élevés en (C2F)n ont été obtenus, soit ~75 % et aucune exfoliation n'a été constatée. Toutes ces caractéristiques font que les feuilles de graphite fluoré se révèlent très prometteuses en tant que réflecteurs de neutrons lents. Les nanodiamants de détonation choisis pour ce travail, car disponibles en quantité industrielle, contiennent en surface une couche de carbone sp2 et des groupements hydrogénés et oxygénés absorbeurs de neutrons, ainsi que des impuretés métalliques activables sous flux neutronique. Il est montré dans ces travaux de thèse que l'utilisation de dichlore permet d'éliminer efficacement les impuretés métalliques des nanodiamants de détonation, et que la combinaison avec du fluor moléculaire convertie les groupements hydrogénés et oxygénés à la surface de ces nanoparticules en liaisons C-F apportant un caractère hydrophobe empêchant tout adsorption de molécules d'eau. Une méthode de concentration des impuretés métalliques a été développée au cours de ces travaux de thèse, ce qui a permis de lever le verrou que représente la limite des détections des appareils de caractérisation utilisés. Il s'est également avéré que le suivi de la température de combustion des nanodiamants de détonation constituait un bon indicateur de la pureté de ces derniers. Par ailleurs, la combinaison des traitements de chloration et de fluoration a permis d'augmenter de 200 °C la stabilité thermique de ces composés
The aim of this PhD work was to synthesize highly scattering nanomaterials with low neutron absorption, consisting solely of carbon and fluorine, in order to close the slow neutron reflectivity gap in the 90-600 m/s neutron velocity range (between the best supermirror and graphite). The materials selected were nanodiamonds with a calibrated diameter (5 nm) and fluorinated graphites. Among fluorinated graphites, the (C2F)n phase has the highest interplanar distance (9 Å), enabling reflectivity of up to 220 m/s. Despite the difficulty of obtaining this (C2F)n phase, due to its restricted fluorination temperature range (350-400°C), the tendency to over-fluorinate and the risk of exfoliation, we successfully synthesized fluorinated graphite powders with a high (C2F)n content, with a maximum content of 96 % (C2F)n and an interplanar distance of around 9 Å. Neutron reflectivity measurements of these samples revealed that (C2F)n-rich fluorinated graphite can be used as an effective reflector for slow neutrons. Graphite foils have also been fluorinated to overcome the impossibility of densifying fluorinated graphite powders, an essential criterion for the creation of neutron reflectors. High levels of (C2F)n were also obtained, i.e. ~75%, and no exfoliation was observed. All these characteristics make fluorinated graphite foils very promising as slow neutron reflectors. The detonation nanodiamonds are chosen for the purpose of developing new slow neutron reflectors because they are available in industrial quantities. They unfortunately contain a sp2 carbon shell on their surface and hydrogenated and oxygenated neutron-absorbing impurities, as well as metallic impurities that can be activated under neutron flux. It was shown in this study that the use of chlorine effectively eliminates metallic impurities from detonation nanodiamonds, and that the combination with molecular fluorine converts the hydrogenated and oxygenated groups present on the surface of these nanoparticles into C-F bonds, providing a hydrophobic character that prevents any subsequent adsorption of water molecules. A method for concentrating metallic impurities was developed during the course of this PhD work, which made it possible to overcome the detection limit of the characterization equipment used. It also proved that the combustion temperature of detonation nanodiamonds was a good indicator of their purity. In addition, the combination of chlorination and fluorination treatments increased the thermal stability of these compounds by 200 °C

The discovery of unique properties of graphene has led to the development of graphene for a variety of applications like integrated circuits, organic electronic devices, supercapacitors, sensors, and composite materials. Fluorination of graphene enables control of its physical, chemical, and electronic properties. Our initial studies demonstrated the viability of sulfur hexafluoride plasmas to fluorinate epitaxial graphene as a safer alternative to the commonly reported techniques of fluorination that include exposures to fluorine and xenon difluoride gas. Formation of carbon-fluorine bonds after SF6 plasma-treatment was confirmed by x-ray photoelectron spectroscopy. Raman spectroscopy and low-energy electron diffraction studies suggest that the framework of sp2-hybridized carbon atoms remains intact after the plasma-treatment. Increase in work function after the fluorination was determined by ultra-violet photoelectron spectroscopy. The findings of our subsequent investigation to controllably modify the work function of epitaxial graphene via plasma-fluorination indicate that the work function of fluorinated epitaxial graphene is controlled by the polarity of carbon-fluorine bonds. Further studies to investigate the effect of the surface topography of epitaxial graphene on the work function of plasma-fluorinated epitaxial graphene were performed using scanning Kelvin probe microscopy (SKPM). The results of SKPM characterization of plasma-fluorinated epitaxial graphene demonstrated that the increase in the work function of epitaxial graphene after plasma-treatment is independent of its surface topography, but non-uniform fluorination may result from non-uniformities in plasma density.

In this thesis investigations into chemically modified graphene structures are presented. Chemical functionalization of graphene is the chemical attachment of molecules or atoms to the graphene surface via covalent or Van der Waals bonds, this process offers a unique way to tailor the properties of graphene to make it useful for a wide range of device applications. One type of chemical functionalization presented in this thesis is fluorination of graphene which is the covalent attachment of fluorine to the carbon atoms of graphene and the resultant material is fluorographene which is a wide band-gap semiconductor. For low fluorine coverage the low temperature electron transport is through localized states due to the presence of disorder induced sub-gap states. For high fluorine coverage the electron transport can be explained by a lightly doped semiconductor model where transport is through thermal activation across an energy gap between an impurity and conduction bands. On the other hand, at low temperatures the disorder induced sub-gap density of states dominates the electrical properties, and the conduction takes place via hopping through these localized states. In this thesis it is also shown that electron beam irradiation can be used to tune the coverage of fluorine adatoms and therefore control energy gap between the impurity and conduction bands. Futhermore, electron beam irradiation also offers a valuable way to pattern conductive structures in fluorinated graphene \textit{via} the irradiation-induced dissociation of fluorine from the fluorinated graphene. This technique can be extended to the patterning of semiconducting nano-ribbons in fluorinated graphene where the spatial localization of electrons is just a few nm. The second type of chemical functionalization presented in this thesis is the intercalation of few layer graphene with ferric chloride which greatly enhances the electrical conductivity of few layer graphene materials making them the best known transparent conductors.

In this thesis a variety of topics concerning 2D materials that have been separated from bulk layered crystals are discussed. Throughout the thesis, single and few layers of graphene, fluorinated graphene, MoS2 and WS2 are used. Two new methods of freely suspending 2D materials are presented as well as a method of removing the background from optical images. This aids contrast measurements for the determination of the number of layers. Fluorinated graphene is found to be sensitive to beta radiation; the resistance of fluorinated graphene transistors is shown to decrease upon exposure to the radiation. This happens due to the carbon-fluorine bond breaking. The sp3 hybridised structure of the fluorinated graphene is reduced back into the sp2 hybridised structure of pristine graphene. The superconducting properties of molybdenum-rhenium are characterised. It is shown to have a transition temperature of 7.5 K. It is also discovered that the material has a resistance to hydrofluoric acid; the acid etches nearly all other superconducting materials. This makes MoRe a possible candidate to explore superconductivity in conjunction with high mobility suspended graphene. To see if the material is compatible with graphene, a supported Josephson junction is fabricated. A proximity induced super current is sustained through the junction up to biases of ∼ 200 nA. The temperature dependence of the conductivity is measured for both suspended MoS2 and WS2 on a hexagonal boron nitride substrate. The dominant hopping mechanism that contributes to the conductivity at low temperatures is found to be Mott variable range hopping, with the characteristic T−1/3 dependence. The hopping transport is due to impurities that are intrinsic to the crystals, this is confirmed by comparing the results with those of supported devices on SiO2.

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

碩士
長庚大學
電子工程學系
101
Graphene, it is a 2D material which the film is composed of carbon atoms in a hexagonal honeycomb lattice. graphene is also triggered a lively academic attention since it seperated from graphite. Until now, graphene is still the hotest research topic. Firt part of this article is the synthesis of graphene by chemical vapor deposition ,next analysis by Raman spectrum, we made the monolayer and defect free graphene. Next, we put the sample to the CF4 plasma system to fluorination, with fluorine atom bonded to carbon atom(from 27.4 to 5.6), to the effect of the insulation(sheet resistance from 1.6 K ~1 M ohm/sq). we control the fluorinated graphene's conduction(semi-metal -> semiconductor -> insulator). Finally we concluded the bandgap of grapheme can be tuned to 2.9eV. Second part of this article will combine the method of gold bufflayer , we can control the degree of fluorination by one-step fluorination. With the electrical measurement, we can know the change of current's on/off ratio is 10.1,it’s 3-fold higher than that of the device made from pristine graphene. The third part come up with the fluorination of multilayer grapheme,and use the multilayer graphene as insulator,finally, we hope to creat a graphene transistor.

碩士
國立交通大學
平面顯示技術碩士學位學程
103
In this study, we describe the preparation of fluorinated graphene nanosheets (FGS) through photoexfoliation from fluorinated graphite (FG) in liquid phase. We discovered that the use of UV radiation upon the FG dispersion in N-methyl-2-pyrolidone could facilitate the exfoliation of FGS. From the analysis of the images obtained from transmission electron microscopy and atomic force microscopy, the average thickness of the FGSs was ca. 3 nm, which was thinner than that of the nanosheets prepared using conventional sonication approach. Furthermore, the FGS can be uniformly deposited on the substrates by spin coating and behaved as an effective electron transport layer of polymer solar cells (PSCs). The as-prepared inverted PSC exhibited an open circuit voltage of 0.53 V, a short circuit current density of 10.22 mA cm-2 and a fill factor up to 53.7%., resulting in a power conversion efficiency (PCE) of 2.91%. The PCE of the PSC containing FGS prepared through sonication was 1.73%. The lower efficiency was probably due to the higher contact resistances, which may be originated from the higher thickness of the FGS.

碩士
國立東華大學
光電工程學系
106
In this study, PERC solar cell components are laser-fired contact to promote the combination of aluminum and silicon to form a BSF layer for improved efficiency. This laboratory has studied the derivatives of graphene. Graphene is a single-layer two-dimensional structure graphene extracted from three-dimensional structure graphite. Its characteristics will be introduced in Chapter 3 as solar cell auxiliary and future direction. The solar cell of this article is based on the industry PERC, and the unopened test piece is sputtered with aluminum, and then the laser fired contact. The aluminum can be penetrated through the passivation layer to almost completely contact with the silicon and diffuse to form the BSF layer. After the test, the efficiency is obviously improved. Such a low-temperature process can have such high efficiency, and it is worthwhile to continue the research in the future, and can also avoid the damage of the graphene oxide caused by the high-temperature diffusion process. Subsequently, the target is placed on the laser parameters, and the passivation of the SiON passivation layer by laser sintering is used to combine the aluminum into the silicon to find an optimum value. After the above process is feasible, the graphene oxide is added between the PERC substrate and the aluminum. The graphene oxide has a fixed negative charge which can repel a few carrier electrons, reduce the recombination rate, and further improve the efficiency. Keywords: PERC solar cell、laser fired contact、BSF layer、graphene oxide、fluorinated graphene