Melanins are a class of bio-macromolecules that are found throughout the biosphere. They fulfill various functions in human beings, which makes them a long studied substance in medicine and biology. Furthermore they possess a set of rare and special physico-chemical properties which include featureless broad band absorption in the UV-Vis spectrum and condensed phase electrical conduction. Many scientists have interpreted their findings in terms of an amorphous semiconductor model, but this was done under the a priori assumption that charge transport in melanin is electronic. However, a very strong dependence of melanin’s electrical properties on its level of hydration has recently led to speculations that the dominant charge carrier for high hydration is of protonic rather than electronic nature. This thesis will present direct evidence for electronic charge transport, found by investigating the influence of different environmental parameters on the conductivity of melanin. It will furthermore be shown that the hydration dependent conductivity of melanin can be understood in terms of a dielectric response model for an amorphous semiconductor. This establishment of the major charge carrier is an important step in the on-going effort to fully map the structure-property relationship of melanin and will help to understand its function in vivo. With the ultimate goal to make use of melanin’s fascinating properties, thin films, a new class of device has been characterized and investigated. These thin films, while exhibiting melanin’s characteristics, show improved mechanical stability, a very uniform morphology and a much faster response than standard pellet samples. They are furthermore applicable to standard polymer processing techniques which brings technological applications within reach.

Ion irradiation of tetrahedral amorphous carbon (ta-C) thin films induces a carbon phase transformation from the electrically insulating sp3 hybridization into the conducting sp2 hybridization. In this work, a detailed study on the electrical resistivity and the microstructure of areas, irradiated with several ion species at 30 keV energy is presented. Continuous ion bombardment yields a drastic drop of the resistivity as well as significant structural modifications of the evolving sp2 carbon phase. It is shown that the resistivity lowering can be attributed to the degree of graphitization in the film. Furthermore, the structural ordering processes are correlated with the ion deposited energy density. It is therefore revealed that the ion-induced phase transformation in ta-C films is a combination of sp3-to-sp2 conversion of carbon atoms and ion-induced ordering of the microstructure into a more graphite-like arrangement. All experiments were done with focused ion beam (FIB) systems by applying FIB lithography of electrical van-der-Pauw test structures. FIB lithography on ta-C layers is presented as a fast and easy technique for the preparation of electrically active micro- and nanostructures in an insulating carbon matrix.

Cette étude est dédiée au développement d’une anode pour une pile à combustible SOFC monochambre. Ce dispositif ne présente pas de séparation physique entre les compartiments anodique et cathodique, contrairement à une pile conventionnelle. Un mélange contenant de l’oxygène et un hydrocarbure est directement injecté sur l’ensemble du dispositif comprenant électrolyte, anode et cathode. La cathode doit être sélective à la réduction de l’oxygène et l’anode à l’oxydation de l’hydrocarbure. Ce dispositif permet donc de s’affranchir des problèmes d’étanchéité des dispositifs conventionnels mais les matériaux d’électrode doivent répondre à des critères catalytiques restrictifs. L’étude a été réalisée avec une anode de type cermet composée de nickel et d’oxyde de cérium gadolinié (CGO). L’hydrocarbure choisi est le propane. L’objectif du travail est de comprendre les phénomènes physico-chimiques se produisant à l’anode afin d’optimiser les conditions de fonctionnement de la pile monochambre, sous mélange O2 / C3H8. Pour cela, les propriétés catalytiques des poudres de nickel et de CGO ont été déterminées en fonction de la température et du rapport O2 / C3H8. Après avoir optimisé les paramètres de préparation des anodes par sérigraphie sur électrolyte support (CGO), des mesures de conductivité électrique par méthode Van der Pauw ont été réalisées également sous différents mélanges O2 / C3H8 en fonction de la température. La confrontation des résultats (propriétés catalytiques et électriques) a permis de mettre en évidence l’existence d’une température limite de fonctionnement, qui dépend du rapport O2 / C3H8. En-dessous de cette température limite, le nickel s’oxyde et l’anode catalyse principalement l’oxydation totale du propane. Au-dessus de cette température limite, le nickel reste sous la forme métallique et catalyse préférentiellement l’oxydation partielle du propane. Ces résultats sont également corroborés à des calculs thermodynamiques, qui mettent en évidence que la réaction préférentielle à basse température est l’oxydation du nickel. Des phénomènes d’oscillations de la conductivité électrique et de la température ont permis de proposer un mécanisme réactionnel basé sur le dépôt et l’oxydation de carbone. Enfin, des dispositifs complets monochambre ont été élaborés en utilisant une cathode BSCF (Ba0,5Sr0.5Co0,8Fe0,2O3) et testés. / This work is devoted to the development of a single chamber SOFC. Unlike a conventional SOFC, this type of fuel cell has no physical separation between the anodic and cathodic compartments. An oxygen and hydrocarbon mixture is injected directly on the overall fuel cell, including the electrolyte, the anode and the cathode. The cathode must have a high selectivity to the oxygen reduction, and the anode to the hydrocarbon oxidation. This device allows to avoid the conventional devices sealing problems, but the electrodes materials must fit with restrictive catalytic criterions. The study has been conducted with a cermet anode type, composed of nickel and gadolinia doped ceria (CGO). The selected hydrocarbon was the propane. The goal of this work is to understand physico-chemical processes taking place at the anode in order to optimize the operating conditions of the fuel cell, in an O2 / C3H8 mixture. The catalytic properties of nickel and CGO powders were determined as a function of temperature and O2 / C3H8 ratio. After optimization of the anode screen-printing conditions preparation on a CGO support, electrical measurements were done using the Van der Pauw method under different O2 / C3H8 mixtures as a function of temperature. The results of catalytic and electrical measurements show a limiting operating temperature, which depends on the O2 / C3H8 ratio. Below this limiting temperature, the nickel is oxidized and principally catalyses the total oxidation of the propane. Above this limiting temperature, nickel remains into its metallic state and preferentially catalyses the partial oxidation of the propane. These results are also corroborated to thermodynamics calculations, which show that at low temperature, the oxidation of nickel is the most favourable reaction. Some oscillations of the conductivity have been observed. Oscillations of electrical conductivity and temperature allowed proposing a reaction mechanism based on carbon deposition and oxidation. Finally, complete single chamber devices have been elaborated using a BSCF (Ba0,5Sr0.5Co0,8Fe0,2O3) cathode and tested.

Ion irradiation of tetrahedral amorphous carbon (ta-C) thin films induces a carbon phase transformation from the electrically insulating sp3 hybridization into the conducting sp2 hybridization. In this work, a detailed study on the electrical resistivity and the microstructure of areas, irradiated with several ion species at 30 keV energy is presented. Continuous ion bombardment yields a drastic drop of the resistivity as well as significant structural modifications of the evolving sp2 carbon phase. It is shown that the resistivity lowering can be attributed to the degree of graphitization in the film. Furthermore, the structural ordering processes are correlated with the ion deposited energy density. It is therefore revealed that the ion-induced phase transformation in ta-C films is a combination of sp3-to-sp2 conversion of carbon atoms and ion-induced ordering of the microstructure into a more graphite-like arrangement. All experiments were done with focused ion beam (FIB) systems by applying FIB lithography of electrical van-der-Pauw test structures. FIB lithography on ta-C layers is presented as a fast and easy technique for the preparation of electrically active micro- and nanostructures in an insulating carbon matrix.:Contents List of Figures iii List of Tables v List of Abbreviations vii 1. Introduction 1 2. Fundamentals 5 2.1. Ion-solid interactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 2.1.1. Scattering and stopping . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 2.1.2. Ion range distribution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 2.1.3. Target modifications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 2.1.4. Thermal driven segregation . . . . . . . . . . . . . . . . . . . . . . . . . . 18 2.2. Focused ion beams (FIBs) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 2.2.1. Commercial gallium FIB (Ga + -FIB) . . . . . . . . . . . . . . . . . . . . . 24 2.2.2. Mass-separated FIB . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 2.3. Tetrahedral amorphous carbon (ta-C) . . . . . . . . . . . . . . . . . . . . . . . . 26 2.3.1. Composition, microstructure and film properties . . . . . . . . . . . . . . 26 2.3.2. Growth mechanisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30 2.3.3. Electronic properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34 3. Experimental 39 3.1. Samples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39 3.2. Apparatus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40 4. Ion induced surface swelling 43 4.1. Fluence and energy dependence . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44 4.2. Calculations of the swelling height . . . . . . . . . . . . . . . . . . . . . . . . . . 48 5. Electrical properties of irradiated ta-C 55 5.1. Electrical resistivity of as-implanted ta-C . . . . . . . . . . . . . . . . . . . . . . 55 5.1.1. Resistance of Ga + implanted micropatterns . . . . . . . . . . . . . . . . . 55 5.1.2. Sheet resistance of Ga + irradiated ta-C . . . . . . . . . . . . . . . . . . . 59 5.1.3. Determination of the sp 3 content . . . . . . . . . . . . . . . . . . . . . . . 62 5.1.4. The effect of different ion species . . . . . . . . . . . . . . . . . . . . . . . 65 5.1.5. Low temperature resistivity – The peculiarity of gallium . . . . . . . . . . 71 5.2. The effect of annealing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74 5.3. Irradiation at elevated substrate temperatures . . . . . . . . . . . . . . . . . . . . 79 6. The microstructure of irradiated ta-C 87 6.1. Raman investigations of ion irradiated ta-C . . . . . . . . . . . . . . . . . . . . . 88 6.1.1. Fundamentals of Raman spectroscopy on amorphous carbon . . . . . . . . 88 6.1.2. Raman spectra of as-implanted ta-C . . . . . . . . . . . . . . . . . . . . . 93 6.1.3. Thermally driven graphitization of the microstructure . . . . . . . . . . . 98ii Contents 6.1.4. The correlation between microstructure and resistivity . . . . . . . . . . . 101 6.2. TEM investigations of ion irradiated ta-C . . . . . . . . . . . . . . . . . . . . . . 104 7. FIB lithography on ta-C layers 107 7.1. Graphitic nanowires . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107 7.1.1. Nanowire dimensions – The resolution of FIB lithography . . . . . . . . . 108 7.1.2. Nanowire resistance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110 7.2. Electrical insulation between conducting structures . . . . . . . . . . . . . . . . . 113 8. Conclusions and Outlook 117 A. Gallium nanoparticles on ta-C layers 121 Bibliography 123

Cette étude est dédiée au développement d'une anode pour une pile à combustible SOFC monochambre. Ce dispositif ne présente pas de séparation physique entre les compartiments anodique et cathodique, contrairement à une pile conventionnelle. Un mélange contenant de l'oxygène et un hydrocarbure est directement injecté sur l'ensemble du dispositif comprenant électrolyte, anode et cathode. La cathode doit être sélective à la réduction de l'oxygène et l'anode à l'oxydation de l'hydrocarbure. Ce dispositif permet donc de s'affranchir des problèmes d'étanchéité des dispositifs conventionnels mais les matériaux d'électrode doivent répondre à des critères catalytiques restrictifs. L'étude a été réalisée avec une anode de type cermet composée de nickel et d'oxyde de cérium gadolinié (CGO). L'hydrocarbure choisi est le propane. L'objectif du travail est de comprendre les phénomènes physico-chimiques se produisant à l'anode afin d'optimiser les conditions de fonctionnement de la pile monochambre, sous mélange O2 / C3H8. Pour cela, les propriétés catalytiques des poudres de nickel et de CGO ont été déterminées en fonction de la température et du rapport O2 / C3H8. Après avoir optimisé les paramètres de préparation des anodes par sérigraphie sur électrolyte support (CGO), des mesures de conductivité électrique par méthode Van der Pauw ont été réalisées également sous différents mélanges O2 / C3H8 en fonction de la température. La confrontation des résultats (propriétés catalytiques et électriques) a permis de mettre en évidence l'existence d'une température limite de fonctionnement, qui dépend du rapport O2 / C3H8. En-dessous de cette température limite, le nickel s'oxyde et l'anode catalyse principalement l'oxydation totale du propane. Au-dessus de cette température limite, le nickel reste sous la forme métallique et catalyse préférentiellement l'oxydation partielle du propane. Ces résultats sont également corroborés à des calculs thermodynamiques, qui mettent en évidence que la réaction préférentielle à basse température est l'oxydation du nickel. Des phénomènes d'oscillations de la conductivité électrique et de la température ont permis de proposer un mécanisme réactionnel basé sur le dépôt et l'oxydation de carbone. Enfin, des dispositifs complets monochambre ont été élaborés en utilisant une cathode BSCF (Ba0,5Sr0.5Co0,8Fe0,2O3) et testés.

L'auto-assemblage de polymères a été étudié afin d'élaborer des films multicouches possédant des propriétés conductrices. Dans un premier temps, l'influence de différents paramètres physico-chimiques (pH , force ionique, nature et charge des ions, concentrations en polymères...) sur la croissance de couche polymères isolants (PSS ou PDDA) / polymères conducteurs ( PEDOT : PSS ou P3HT-R) a été étudiée par des mesures in-situ de réflectométrie laser à angle fixe. La compréhension de ses paramètres a permis de contrôler l'assemblage des matériaux afin d'obtenir des films multicouches aux propriétés souhaitées ( épaisseur, morphologie, conductivité électrique...)les épaisseurs des films obtenus ont été déterminées par ellipsométrie ou profilométrie, la morphologie de la morphologie de la surface de ces films a été observé par MEB et leur conductivité a été mesurée par la méthode de van Pauw. Dans un second temps, des films multicouches polymères / particules composites ont été construits après avoir optimisé les conditions d'élaboration des particules composites Al2-PEDOT : PSSph ou SiO2-P3HT-R).Les films multicouches obtenus présentent des conductivités équivalentes à celles des films à base de polymères mais sont construits avec moins d'étapes d'adsorption. Enfin des couches multicouches à bases de deux polymères conducteurs (P3HT-R et PEDOT:PSS) ont été construits sur substrats souples. Ils présentent la plus haute conductivité mesurée (1,5 S.m-1) et conservent leurs propriétés électriques lorsqu'ils sont soumis à des contraintes mécaniques de flexion , ce qui est prometteur quant à leur utilisation dans le domaine de l'électronique déformable. / Self-assembly of polymers has been studied in order to produce multilayer films having conductive properties. First, the influence of various physicochemical parameters (pH, ionic strength, nature and charge of ions, polymer concentrations, etc.) on the growth of insulating polymer (PSS or PDDA) / conductive polymers (PEDOT : PSS or P3HT-R) was studied by in-situ measurements of fixed-angle laser reflectometry. The understanding of its parameters allowed to control the assembly of the materials in order to obtain multilayer films with the desired properties (thickness, morphology, electrical conductivity...) The thicknesses of the films obtained were determined by ellipsometry or profilometry, morphology Of the surface morphology of these films was observed by SEM and their conductivity was measured by the method of van Pauw. Second, polymer multilayer films / composite particles were built after optimizing the conditions for the production of the composite particles Al2-PEDOT: PSSph or SiO2-P3HT-R). The multilayer films obtained have conductivities equivalent to those of Films are based on polymers but are constructed with fewer adsorption steps. Finally, multilayer layers based on two conductive polymers (P3HT-R and PEDOT: PSS) were built on flexible substrates. They have the highest conductivity measured (1.5 Sm-1) and retain their electrical properties when subjected to mechanical bending stresses, which is promising for their use in the field of deformable electronics.

Anti-ferromagnetic interlayer exchange coupling between iron layers in Fe/MgO superlatices have recently been of interest among researchers for its potential application in 3D memory storage in computers (hard drives) and in the research field of spintronics. Thinfilms and bi-layers between the 10-100 ångström range in thicknesses were analyzed for its resistive properties during different conditions. Both mono-layers and superlatices were experimented with.The resistivity of the films was measured with both the four-probe method and the Van der Pauw method. Because of mechanical limitations with respect to the direction of the external magnetic field, the Van der Pauw method had to be used for the superlattice measurements. The collected data from all the measurments of the superlattice suggest that the electric current is not only passing the capping layer of the lattice but did not however show any sign of resisitive properties changing with changing magnetic profile. The resistivity of thesuperlattice was similar to that of the resistivity calculated from data when the external magnetic field was off. The results from this study show for the first time that it is possible to measure the electronic transport within an Fe/MgO superlattice which provides a basis for futher investigations.

Selbstorganisierte InAs Nanostrukturen in InP, wie Quantendrähte, Quantenpunkte, und Quantengräben als Referenz, werden bezüglich ihres Wachstums, ihrer Struktur, optischen Eigenschaften und Transporteigenschaften untersucht. Das Stranski-Krastanov Wachstum der Nanostrukturen auf exakt orientiertem und vizinalem InP(001) wird mittels Gasquellen-Molekularstrahlepitaxie untersucht. Ich zeige, dass die Missorientierung des vizinalen InP, weitestgehend unabhängig von den Wachstumsparametern, den Nanostrukturtyp definiert. Optische Polarisation der Interbandübergänge (im 1.55 Mikrometer Bereich) aufgrund des Nanostrukturtyps wird mittels Photolumineszenz- und Transmissionsspektroskopie wird nachgewiesen. Die experimentell unaufwändige 4-Kontakt van der Pauw Hall Messung wird erweitert, um anisotrope Transporteigenschaften zu bestimmen. Der Ladungstägertransport in einer Schicht dicht gepackter, lateral gekoppelter InAs Nanostrukturen ist stark anisotrop mit der Hochbeweglichkeitsrichtung [-110], was parallel zur Richtung der Quantendrähte ist. Die maximalen Anisotropien übersteigen 30 für Elektronen und 100 für Löcher. Die extreme Anisotropie im Falle der Löcher basiert auf diffusem Transport in der [-110], und Hoppingtransport in der [110] direction. Die Elektronenbeweglichkeit bei niedrigen Temperaturen wird duch Grenzflächenrauhigkeitsstreuung in der [110] direction, und Streuung an entfernten Störstellen in der [-110] dominiert. Im Kontext gekoppelter Nanostrukturen, zeige ich, dass die Transportanisotropie auf anisotroper Tunnelkopplung zwischen benachbarten Nanostrukturen beruht, und weniger durch die Form der Nanostruktur bestimmt wird. Transport im Quanten-Hall Regime, und die Schwache Lokalisierung werden untersucht. Ein neuartiges Baulelement basierend auf Gate-konrollierter Transportanisotropie wird vorgeschlagen. Es wird gezeigt, dass modulationsdotierte InAs Quantendrähte für eine Implementierung des Bauelements geeignet sind. / Self-assembled InAs nanostructures in InP, comprising quantum wires, quantum dots, and quantum wells as reference, are studied in terms of their formation, structural properties, optical properties, and anisotropic transport properties. The Stranski-Krastanov growth of InAs nanostructures is studied by gas-source molecular beam epitaxy on both nominally oriented and vicinal InP(001). I demonstrate that the off-cut direction of vicinal substrates - largely independent of growth conditions - determines the nanostructure type. Optical polarization of the interband transitions (in the 1.55 micron wavelength range) arising from the nanostructure type is demonstrated by photoluminescence and transmission spectroscopy. The experimentally convenient four-contact van der Pauw Hall measurement is extended to yield the anisotropic transport properties. The in-plane transport in large ensembles of closely spaced, laterally coupled InAs nanostructures is highly anisotropic with the high-mobility direction [-110], which is parallel to the direction of the quantum wires. The maximum anisotropies exceed 30 for electrons, and 100 for holes. The extreme anisotropy for holes is due to diffusive transport in the [-110], and hopping transport in the [110] direction. The principal electron mobilities at low temperature are dominated by interface roughness scattering in the [110] direction, and by remote impurity scattering in the [-110] direction. In the context of coupled nanostructure, I demonstrate that the transport anisotropy results from directionally anisotropic tunnel coupling between adjacent nanostructures rather than from the nanostructure shape anisotropy. The Quantum-Hall regime, and the weak-localization contribution to conductivity is studied. A novel 5-terminal electronic switching device based on gate-controlled transport anisotropy is proposed. Modulation-doped InAs/InP quantum wires are demonstrated to be a candidate for implementation of the device.

Increasing interest in an effciency enhancement of existing energy sources led to an extended research in the field of thermoelectrics. Especially skutterudites with their high power factor (electric conductivity times Seebeck coefficient squared) are suitable thermoelectric materials. However, a further improvement of their thermoelectric properties is necessary. The relatively high thermal conductivity can be decreased by introducing loosely bound guest ions, whereas atom substitution or nanostructuring (as thin films) could yield an increased power factor. The present work proves the feasibility to deposit single phase skutterudite thin films by MBE technique. In this regard CoSby and FeSby film series were deposited with three different methods: i) codeposition at elevated temperatures, ii) codeposition at room temperature followed by post-annealing, and iii) modulated elemental reactant method. The structural and thermoelectric properties of these films were investigated by taking the thermal stability of the film and the substrate properties into account. Compared to the stoichiometric Sb content of skutterudites of 75 at.%, a small excess of Sb is necessary for achieving single phase skutterudite films. It was found, that the deposited single phase CoSb3 films reveal bipolar conduction (and therefore a low Seebeck coefficient), whereas FeSb3 films show p-type conduction and very promising power factors at room temperature. The need of substrates with a low thermal conductivity and a suitable thermal expansion coefficient is also demonstrated. A high thermal conductivity influences the measurements of the Seebeck coefficient and the obtained values will be underestimated by thermal shortening of the film by the substrate. If the thermal expansion coefficient of film and substrate differ strongly from each other, crack formation at the film surface was observed. Furthermore, the realization of controlled doping by substitution as well as the incorporation of guest ions was successfully shown. Hence, this work is a good starting point for designing skutterudite based thin film structures. Two successful examples for such structures are given: i) a thickness series, where a strong decrease of the resistivity was observed for films with a thickness lower than 10nm, and ii) a FexCo1-xSb3 gradient film, for which the gradient was maintained even at an annealing temperature of 400°C.

Terahertz time-domain spectroscopy systems were developed and used for the anaylsis and characterization of various materials. By using ultra-fast Ti:Sapphire and Er-doped fiber lasers, terahertz time-domain spectrometers of different configurations were constructed and tested. To increase the accuracy and sensitivity of the measurements, the systems were optimized for spectroscopic analysis. MBE grown nominally undoped epitaxial GaAs samples were used for the spectroscopic measurements. These samples were first charactrized by electrical measurements in order to check the accuracy of the terahertz time-domain experiments. We have shown that the terahertz time-domin spectroscopic techniques provides a quick way of the determining the real ( ) and complex () components of the refractive index of material. In addition, we have investigated the photoexcitation dynamics of these GaAs samples. We have demonstrated that direct and photoexcited terahertz time-domain measurements give an estimate of the carrier densities and both the hole and electron mobility values with good precision. rnin An algorithm is developed to prevent the unwanted Fabry-Perot reflections which is commonly encountered in Terahertz Spectroscopy systems. We have performed terahertz time-domain transmission measurements on ZnTe &lt / 110&gt / crystals of various thicknesses to test the applicability of this algorithm. We have shown that the algorithm developed provides a quick way of eliminating the &ldquo / etalon&rdquo / reflections from the data. In addition, it is also shown that these &ldquo / etalon&rdquo / effects can be used for the frequency calibration of terahertz time-domain spectrometers.