L’immersion de plaquettes de pyrographite dans des alliages liquides à base de lithium permet l’intercalation de métaux alcalino-terreux et d’europium dans cette structure hôte. Cette méthode de synthèse liquide-solide conduit à la formation de composés ternaires graphite-Li-Ca et graphite-Li-Eu ainsi qu’à celle des binaires CaC6, BaC6 et EuC6 à l’état massif. L’étude approfondie du système graphite-lithium-calcium a permis de déterminer les mécanismes de formation des phases CaC6 et Li3Ca2C6 qui comportent systématiquement deux étapes. La structure cristalline de CaC6 a été entièrement résolue. Cette phase adopte une structure rhomboédrique de groupe d’espace . Parallèlement, un complément d’information a pu être apporté à celle de Li3Ca3C6. L’étude des propriétés magnétiques de CaC6 et de Li3Ca2C6 indique que ces phases sont supraconductrices avec des températures critiques proches de 11 K. Une caractérisation électrodynamique, entreprise dans le but d’identifier le mécanisme responsable de la supraconductivité de CaC6 a montré que ce composé apparaît comme un supraconducteur conventionnel. A l’aide de la méthode de synthèse en milieu alliage fondu, le composé binaire EuC6 et une phase ternaire originale ont été isolés au cours de l’étude du système graphite-lithium-europium. Ce composé ternaire possède le long de l’axe un empilement atomique polycouche, contenant principalement deux plans d’europium. La quantité de lithium n’a en revanche pas été estimée, en raison de la grande difficulté du dosage de cet élément. La formule chimique s’écrit donc LixEuC4. Les mesures magnétiques réalisées sur ce composé ternaire révèlent un comportement particulièrement complexe en dessous de 225 K. Bien que la structure ne soit pas entièrement résolue, des considérations géométriques simples associées à une analogie avec le composé EuC6 permettent d’expliquer qualitativement le comportement magnétique de LixEuC4. / The immersion of pyrographite platelets in lithium based liquid alloys allows the intercalation of alkaline earth metals and europium in this host structure. This liquid-solid synthesis method led to the formation of graphite-Li-Ca and graphite-Li-Eu ternary compounds as well as bulk samples of the CaC6, BaC6 and EuC6 binaries. The study of the graphite-lithium-calcium system has made it possible to determine the formation mechanism of the phases CaC6 and Li3Ca2C6, which includes two stages systematically. The crystal structure of CaC6 has been fully resolved. This phase adopt a rhombohedral structure with the space group . Meanwhile, additional information has been provided to the crystal structure of Li3Ca3C6. The study of magnetic properties of CaC6 and Li3Ca2C6 indicates that these phases are superconductors with critical temperatures close to 11 K. An electrodynamics characterization, undertaken in order to identify the mechanism responsible of the superconductivity of CaC6, has shown that this compound appears to be a conventional superconductor. By using the synthesis method in molten alloy media, EuC6 binary compound and an original ternary phase were isolated during the study of the graphite-lithium-europium system. This ternary compound has along the axis an atomic stacking containing several sheets but with two main layers of europium. The lithium has however not been estimated, due to the experimental difficulty related to the determination of its amount. The chemical formula is then written LixEuC4. Magnetic measurements performed on this ternary compound reveal a complex behaviour below 225 K. Although the structure is not fully resolved, some simple geometric considerations associated with an analogy with the EuC6 compound can explain the magnetic behaviour of LixEuC4 qualitatively.

No description available.

In this research work, we have investigated the band engineering of epitaxial graphene using first principles calculations. Epitaxial graphene on SiC (0001) surface is modified by using different methods such as intercalation, doping, passivation and oxidation. The calculations are done using Density functional theory which is implemented in quantum espresso package. In the presence of H intercalation, epitaxial graphene is shown to have p type behavior with monolayer graphene. However this behavior is different for multilayer epitaxial graphene systems, and it depended on the concentration of the H atoms. When epitaxial graphene is intercalated with Ge atoms, the Ge atoms make clusters and these clusters are responsible for the electronic properties of the epitaxial graphene systems. As a result of oxidation of epitaxial SiC surface, the graphene layer is mostly stable on the surface for both silicates and oxynitrides structures. For silicate/SiC configurations, the epitaxial graphene is shown to be less n type. For oxynitrides/ SiC configurations, epitaxial graphene is shown to be neutral. In the presence of oxygen intercalation with silicate/SiC, epitaxial graphene is shown to have p type behavior. These systematic studies of epitaxial graphene will opens up great potential for electronic applications. Additionally the resultant models can be used to guide further studies.

Graduation date: 2005

Graduation date: 1999

Le graphène épitaxié sur des substrats métalliques est un modèle prometteur pour le développement de nouveaux systèmes hybrides, dans lesquelles les effets d'interface peuvent être exploités pour concevoir de nouvelles propriétés. L'insertion d'espèces entre le graphène et son substrat, une opération connues sous le nom d'-«intercalation», est une approche très puissante à cet égard. Avec l'aide des outils de la physique des surfaces, nous avons étudié trois systèmes graphène/métal, dont deux sont des systèmes hybrides intercalés, et l'autre est un candidat pour un tel système : (i) le graphène/Ir(111) intercalé avec un oxyde ultra-mince, (ii) graphène/Ir(111) intercalé avec des couches sub-atomiques du cobalt et (iii) de graphène sur Re(0001). Nous avons montré que certains défauts, en particulier les ridules (délamination du graphène de son substrat) et d'autres régions courbées du graphène, jouent un rôle crucial, non anticipé, dans le processus d'intercalation. Nous avons également observé que l'intercalation se déroule d'une manière nettement différente sous ultravide et à pression atmosphérique. Dans le premier système, des espèces contenant de l'oxygène entrent à l'extrémité ouverte des ridules et diffusent au long de ces ridules pour former des nano-rubans d'oxyde. Ces rubans modifient le dopage électronique du graphène, ce qui se traduit également par des changements substantiels dans la réponse optique inélastique (Raman) du graphène. Dans le second système, l'efficacité de l'intercalation est apparue dépendante de l'interaction graphène-métal, laquelle varie entre les domaines de graphène orientés différemment sur_(111). Dans ce système, les sites d'entrée pour les espèces intercalées, des régions courbées dans le graphène, ont pu être identifiés grâce à l'observation in_operando (en cours de croissance) du processus. Enfin, la croissance de graphène dans un troisième système (graphène/Re(0001)), a été étudiée afin de permettre le développement de futurs systèmes graphène/Re hybrides supraconducteurs. Dans ce système, nous avons proposé deux voies de croissance, l'une étant basé sur un processus de croissance en surface d'un monocristal massif de Re(0001), l'autre reposant sur la ségrégation en surface, activée thermiquement, du carbone dissout à haute température dans des films minces de Re sur saphir. / Epitaxial graphene grown on metal substrates is a promising platform for developing new hybrid systems, in which interface effects can be exploited to engineer novel properties. The insertion of foreign species between graphene and its substrate, referred to as “intercalation”, was shown very powerful in this respect. With the help of surface science tools, we have studied three graphene/metal systems, two of which are intercalated hybrid systems, and the other is a candidate for such a system: (i) graphene/Ir(111) intercalated with an ultrathin oxide, (ii) graphene/Ir(111) intercalated with cobalt (sub) atomic layers, and (iii) graphene on Re(0001). We found that some defects, especially wrinkles (linear delaminations of graphene from its substrate) and other curve graphene regions, play a crucial, yet unanticipated role in the intercalation process. We also found that the intercalation proceeds in a markedly different fashion under ultra-high vacuum and under atmospheric pressure. In the first system, oxygen-containing species were found to intercalate via the open end of wrinkles, to diffuse along then, and to form oxide nanoribbons along wrinkles accordingly. These ribbons modify the charge density of graphene, which also translates into substantial changes in the inelastic (Raman) optical response of graphene. In the second system, the efficiency of intercalation proved to be dependent on the graphene-metal interaction, which varies between differently oriented graphene domains on Ir(111). In this systems the entry sites for intercalated species could be identified, thanks to in operando observation of the process, as curved regions in graphene. Finally, graphene growth in a third system, graphene on Re(0001), was addressed in order to enable the future development of graphene/Re superconducting hybrids. In this system, we proposed two growth routes, one being a surface-confined process, on bulk single-crystal Re(0001), and the other being a temperature-induced segregation of carbon dissolved at high temperature in thin Re(0001) films on sapphire.

No description available.

The presented PhD thesis is focused on the synthesis, characterization, and modifications of zeolites and zeolitic materials. The main interests are two-dimensional (2D) zeolites and modification of their interlamellar space. Presented work was performed at the Department of Synthesis and Catalysis at J. Heyrovský Institute of Physical Chemistry in Prague, Czech Republic under the supervision of Prof. Jiří Čejka. Zeolites are inorganic crystalline solids with a microporous framework structure. They are widely used as catalysts, sorbents, and ion-exchangers. Conventional zeolites have been recognized as three-dimensional (3D) tetrahedrally-connected frameworks. However, some of them are also known to exist in various layered forms (2D zeolites). Recently, the transformation of 3D germanosilicate UTL into layers (IPC-1P) has started a new branch in 2D zeolites chemistry. This chemically selective degradation of UTL framework was performed via acid hydrolysis. In the structure of this germanosilicate, Ge atoms are preferentially located in specific building units, double-four-rings (D4R), which connect dense silica layers. Modifications of the layered precursor IPC-1P led to discovery of the two novel 3D zeolites: IPC-4 (PCR) and IPC-2 (OKO). This novel approach in the zeolite synthesis, called ADOR...

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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).