>Magister Scientiae - MSc / In the past few decades, the studies of f-electron materials have revealed unusual physical properties such as Fermi-liquid, non-Fermi-liquid behaviour at low temperatures, heavy- Fermion behaviour, valence fluctuation, Kondo effect, superconducting and magnetic ordering. These materials include binary and ternary compounds and alloys with Cerium (Ce) or Ytterbium (Yb) based rare earth elements or Uranium (U) based actinide element. In these systems the localized magnetic moments formed by Ce, Yb or U ions transform the electronic properties of these compounds leading to quasiparticles with masses in excess to 1000 times the bare electron mass. These materials are known as heavy-fermion materials. Two well known heavy – Fermion compounds with Ce based rare earth elements of interest in this thesis are CePt₂Si₂ and CeCu₅In. The effect of substituting Ce with moment bearing Tb or Dy in these two compounds, are reported through measurements of electrical resistivity, magnetic susceptibility and magnetization. The three alloy systems (Ce₁₋ₓREₓ)Pt₂Si₂ (RE = Tb, Dy) and (Ce₁₋ₓTbᵪ)Cu₅In under investigation in the present thesis, was synthesized and characterized by x-ray diffraction. The alloy systems (Ce₁₋ₓREₓ)Pt₂Si₂ (RE = Tb, Dy, 0≤ ᵡ ≤1) formed a single phase in the P4/nmm tetragonal CaBe₂Ge₂ – type structure across the whole series while the (Ce₁₋ₓTbᵪ)Cu₅In alloy system formed a single phase in the Pnma orthorhombic CeCu₆ – type crystal structure up to 40% Ce substitution. The physical properties of these systems is reported and discussed through the measurements of electrical resistivity, magnetic susceptibility and magnetization. The variables of this study are: the doping concentration of Tb or Dy, the applied magnetic field and the sample temperature. Electrical resistivity studies for all the systems revealed coherence effect at Ce – rich alloys (0≤ ᵡ ≤0.2) and single-ion Kondo scattering with further increased RE concentration ( ᵡ ≥ 0.3). The magnetic property studies indicate antiferromagnetic ordering only for the (Ce₁₋ₓREₓ)Pt₂Si₂ alloy system in the concentration range 0.7≤ ᵡ ≤ 1. The present thesis is comprised of six chapters, which are arranged as follows: The first chapter deals with the theoretical background of the physical properties of Ce based intermetallics compounds and alloys. Experimental techniques constitutes chapter II and explains the techniques used in this study. The theoretical overview of the two parent compounds of interest in this thesis (CePt₂Si₂ and CeCu₅In) is presented in chapter III. The fourth and the fifth chapters of this study deals with results and discussion. The thesis is completed with a conclusion in chapter six.

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The dissertation consists of the following three sections: 1. Hydroxyapatite (HA) coatings. In this work, we deposited HA precursor films from weak basic electrolytic solution (pH= 8-9) via an electrochemical approach; the deposits were changed into crystallite coatings of hydroxyapatite by sintering at specific temperatures (600-800 ºC). The formed coatings were mainly characterized by powder X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and scanning electron microscopy (SEM). XRD patterns show well-defined peaks of HA when sintered under vacuum conditions. FTIR measurements indicate the existence of hydroxyl groups, which were confirmed by the characteristic intensity of the stretching and bending bands at ~3575 and ~630 cm-1, respectively. The SEM shows an adhesive, crack free morphology for the double-layer coating surface of the samples sintered in a vacuum furnace. 2. Silver/polymer/clay nanocomposites. Silver nanoparticles were prepared in layered clay mineral (montmorillonite)/polymer (PVP: poly (vinyl pyrrolidone)) suspension by an electrochemical approach. The silver particles formed in the bulk suspension were stabilized by the PVP and partially exfoliated clay platelets, which acted as protective colloids to prevent coagulation of silver nanoparticles together. The synthesized silver nanoparticles/montmorillonite/PVP composite was characterized and identified by XRD, SEM, and TEM (transmission electron microscopy) measurements. 3. Ce-doped lead zirconate titanate (PZT) thin films. In this study, we fabricated cerium-doped PZT films (molar ratio of Zr/Ti:: 0.5:0.5) via cathodic electrodeposition on the indium tin oxide ( ITO) coated glass substrate. In the preparation process, the PZT films were modified by adding a small amount of cerium dopants, which led to the formation of Ce-doped PZT films after sintering at high temperatures. The fabricated PZT films on the ITO coated glass substrate may be used as electro-optic devices in the industrial application.

Hexaferrites have a broad scope for application based on their physical properties. To study the influence of replacing cerium and gadolinium ions in hexaferrites, it is very important to understand their structural, electrical, and magnetic properties of hexaferrites. In this regard, the W-type strontium hexaferrite doped with Ce3 + and Gd3+ ions was prepared using a proteic sol-gel process under a pressure of 3000 psi and sintered at 1200º C for 2 h at a heating rate of 1 and 2º C/min. A pure W-type strontium hexaferrite was identified using X-ray diffraction (XRD); the XRD patterns exhibited increasing lattice parameters and unit cell volume as a function of doping. Scanning electron microscopy analysis indicated growth on plates with hexagonal divisions. The inserted Gd3+ and Ce3+ ions caused imbalance in the lattice. Therefore, some of the Fe3+ ions present in the octahedral site were converted to Fe2+ ions. Thus, the Curie temperature was reduced to 474º C, increasing the coercivity and reducing the superexchange interactions between Fe-O. The high saturation magnetization and low coercivity in the samples indicate that the hexaferrites are potential candidates for applications involving microwave absorption. In addition, impedance spectroscopy measurements indicated the presence of a replacement threshold with rare earth ions, which results in an increased resistivity; this is predominantly mediated by oxygen vacancies in the Ce3+-doped hexaferrite. / As hexaferritas apresentam um vasto campo de aplicações a depender de suas propriedades físicas. Estudar a influência da substituição dos íons de cério e gadolínio é de suma importância para entender como funciona o mecanismo de obtenção das propriedades estruturais, elétricas e magnéticas nas hexaferritas. Dessa forma, a hexaferrita de estrôncio tipo W foi dopada com íons de Ce3+ e Gd3+ foi obtida via processo sol-gel proteico à 3000 psi e sinterizada a 1200º C/2h com taxa de aquecimento de 1 e 2 /C/min. A fase pura da hexaferrita de estrôncio tipo W foi identificada por difração de raios X com aumento dos parâmetros de rede e volume da cela unitária em função da dopagem. As análises de microscopia eletrônica de varredura indicam o crescimento em placas com clivagens hexagonais. . Os íons de Gd3+ e Ce3+ quando inseridos provocou um desbalanceamento de cargas, levando a transformação de certa quantidade de íons Fe3+, presente no sítio octaédrico, para Fe2+. Tal fato leva a redução da temperatura de Curie para 474º C, aumento da coercividade e redução das interações de super troca entre Fe-O. A alta magnetização de saturação e baixa coercividade apresentadas pelas amostras, indicam que as hexaferritas são considerados candidatos em potencial para fins de absorção de micro-ondas. Além disso, as medidas de espectroscopia de impedância indicam que existe um limiar de substituição com os íons terra rara para que haja aumento da resistividade e a condução ocorra predominantemente mediada por vacâncias de oxigênio na hexaferrita dopada com íon de Ce3+. Palavras

This diploma thesis deals with cerium-based conversion coatings for aluminium alloys. At first the mechanism of protection of metals by inhibitory substances is described in the theoretical section. The next chapter is devoted to application of lanthanide salts as cathodic corrosion inhibitors. Then the characterization of alloy AA 2024 follows. The main part is dedicated to the growth mechanism of the cerium-based mixed conversion coating on the alloy AA 2024. Finally, the factors affecting the character and the quality of obtained coatings are commented. In the experimental part an influence of the thermal activation together with the activation by a hydrogen peroxide was observed. The created film was characterized using SEM and EDS. The level of corrosion protection was evaluated by the polarization resistance measurement. erized using SEM and EDS. The level of corrosion protection was evaluated by the polarization resistance measurement.

Cette thèse porte sur les applications optiques de films d’oxynitrures de silicium dopés cérium et co-dopés cérium-ytterbium élaborés par pulvérisation cathodique magnétron radiofréquence. Les paramètres de dépôt ont été optimisés afin d’obtenir une intense émission visible à l’œil nu des ions Ce3+ dans la matrice hôte SiOxNy. Il est démontré que le flux d’azote est un paramètre crucial pour obtenir cette émission. Nous avons montré aussi que les ions Ce3+ peuvent être incorporés en grande quantité dans cette matrice, sans clusterisations jusqu’à de très hautes températures de recuits (1200°C). Ces excellentes propriétés optiques ont mené à une première application : la tentative d’élaboration de DEL bleue. Les premiers résultats obtenus montrent une électroluminescence peu intense, mais restent encourageants pour une étude plus approfondie. La deuxième application étudiée est le développement de couches à conversion de fréquence basse pour augmenter le rendement des cellules solaires à base de silicium. En effet les cellules solaires sont limitées par le recouvrement du spectre solaire et la plage d’absorption de la cellule. L’élaboration de films SiOxNy co-dopés Ce/Yb pour convertir un photon ultra-violet (300-400 nm) en deux photons infra-rouges (980 nm) permet de passer outre la limite théorique des cellules solaires. Les systèmes élaborés montrent une émission des ions Yb3+ en présence d’ions Ce3+ dans la matrice hôte SiOxNy. Les ions Ce3+ permettent d’excités les ions Yb3+ sur une large gamme spectrale et le mécanisme de transfert d’énergie entre ces deux terres rares est détaillé. Un rendement de conversion de 185% est obtenu pour la plus forte concentration en ions Yb3+. Pour améliorer ce système, l'ajout de miroirs de Bragg entre la couche à conversion et le substrat de silicium, représentant la cellule solaire, a été étudié théoriquement. Leur but est double : maximiser le flux de photons ultraviolets piégé dans la couche à conversion de fréquence et transmettre un maximum de photons infrarouges, qui sont facilement absorbables, vers la cellule solaire. / This thesis is dedicated to cerium doped and cerium-ytterbium co doped oxynitride silicon films optical applications grown by radio frequency magnetron sputtering. Growth parameters have been optimized in order to obtain a strong cerium emission visible to the naked eye in the matrix host SiOxNy, especially the nitrogen flux has a dominant role. We showed that cerium ions have a high solubility without clustering at very high annealing temperature (1200°C). Those excellent properties lead to a first application: the elaboration of blue LED. First results show a weak electroluminescence signal but are still encouraging for further study. The second application is the elaboration of frequency conversion layers to increase the efficiency of Si based solar cell. Indeed solar cell are limited by the mismatch between the solar spectrum and the cell absorption range. The elaboration of Ce/Yb co doped SiOxNy films to convert a UV photon (300-400 nm) into two infrared photons (980 nm) allows to overcome the solar cell theoretical limit. Layers grown show an Yb3+ ion emission in presence of Ce3+ ions in the host matrix SiOxNy. Ce3+ ions sensitize Yb3+ ions on a large spectral range and their cooperative energy transfer mechanism is detailed. An efficiency of 185% is obtained for the higher dopants atomic concentration. In order to improve this system, the effect of adding Bragg mirrors placed between the conversion layer and the silicon substrate, which represents the solar cell, is theoretically studied. Their aim is double: increase the maximum flux of UV photons trapped in the frequency conversion layer and transmit a maximum of infrared photons, which are easily absorbable, toward the solar cell.

In this work ways of preparation of thin epitaxial cerium oxide film on Cu(110) surface were studied. X-ray photoelectron spectroscopy (XPS), X-ray photoelectron difraction (XPD), low energy electron difraction (LEED), ion scattering spectroscopy (ISS) and scanning tunneling microscopy (STM) were used for the characterization of prepared systems. The island structure of CeO2 was prepared by the method of reactive evaporation in oxygen atmosphere. The influence of temperature on the electronic structure and morphology was studied. At the temperature above 550 ˚C partial reduction to Ce2O3 and reordering of the islands to the CeO2(331) structure was observed. The ceria promoted oxidation of copper surface was approved, since the clean c(6x2) reconstruction of the surface was observed at the oxygen exposure 1,5 order of magnitude lower then on Cu(110) alone. The other model system was prepared by cerium evaporation to the oxygen precovered Cu(110) surface. The mix of (2x1) and c(6x2) surface reconstruction was formed by oxygen exposition at 300 ˚C. Cerium was deposited on this surface, also at 300 ˚C. During the following heating to 500 ˚C the formation of epitaxial film Ce2O3(0001) was observed, accompanied by the formation of large hundreds nm long smooth band structures in the [11̄0] direction.

Title: Study of new catalytic materials for proton exchange membrane fuel cells Author: Petr Homola Department: Department of Surface and Plasma Science Supervisor: Prof. RNDr. Vladimír Matolín, DrSc. Abstract: Submitted thesis deals with study of thin layers based on platinum and cerium oxides in order to use them in fuel cells with polymer membrane (PEM- FC). A set of samples with different amount of platinum was prepared by means of magnetron sputtering. Samples were investigated by X - ray Photoelectron Spectroscopy (XPS) and results were confronted with sputtering parameters. It was found out that chemical state of platinum is related to its amount in thin layer. The less platinum was contained in thin layer, the less amount of Pt0 state was observed and amounts of Pt2+ and Pt4+ states increased. Furthermore the temperature stability of prepared layers in the interval from room temperature to 250 ◦ C was studied by means of XPS. The adsorption of carbon monoxide was measured by infrared absorption spectroscopy (IRAS). Increasing degree of adsorption on sample probably related to platinum reduction with increased tem- perature was observed. Measurements of other samples were devaluated by strong contamination with nickel carbonyls. Keywords: PEMFC, cerium oxide, magnetron sputtering, XPS, CO adsorption

Title: Study of CeOX / Rh inverse catalyst Author: Bc. Miroslav Kettner Department: Department of Surface and Plasma Science Supervisor: doc. RNDr. Václav Nehasil, Dr., Department of Surface and Plasma Science Abstract: Inverse catalysts of cerium oxide deposited on polycrystalline and monocrystalline (111) rhodium substrates were studied by means of surface science experimental methods. Growth characteristics of cerium oxide were investigated in dependence on deposition conditions and different thickness determination methods were compared. Acording to oxidizing or reducing exposition conditions changes in degree of oxidation of CeOX were observed. Further spectra analysis showed additional Ce-Rh alloy formation. Adsorption positions CO on-top and CO hollow on Rh (111) surface were differentiated by spectra fitting procedures. Oxygen absorption and reverse desorption in CeOX was confirmed. Performed experiments indicate that this process occurs through oxide-metal interface. CO oxidation reaction mechanisms at room temperature were proposed. Cerium oxide presence is necessary for reaction occurrence. Significant influence of deposited CeOX on proposed CO oxidation reaction mechanisms was proved by this way. Keywords: Rhodium, cerium oxide, alloy, inverse catalyst, CO oxidation.