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Publication Type: search.filters.UBSTyp.DissertationSubject: search.filters.subject.540Institute: search.filters.UBSInstitut.Institut für MaterialwissenschaftStart date: 2020

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Now showing 1 - 8 of 8
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    Bioinspired structure tailoring of tin oxide based materials for high capacity electrodes
    (2021) Jahnke, Timotheus; Bill, Joachim (Prof. Dr.)
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    Atom probe study on CuNi thin films : miscibility gap and grain boundary segregation
    (2023) Duran, Rüya; Schmitz, Guido (Prof. Dr. Dr. h. c.)
    In dieser Arbeit wurde die Lage der Mischungslücke, und die Korngrenzsegregation im Legierungssystem, Kupfer-Nickel, per Atomsondentomographie (APT) analysiert. Zur Untersuchung der Mischungslücke eines binären Systems mit langsamer Diffusion wurde ein neues Verfahren verwendet. Multilagen aus Cu- und Ni- Dünnschichten wurden mittels Ionenstrahlbeschichtung (IBS) auf Wolframpfosten beschichtet und durch fokussierte Ionenstrahlung (FIB) geformt. Bei drei unterschiedlichen Temperaturen, zwischen 573 und 673 K, wurden isotherme Auslagerungssequenzen an einem Ultrahochvakuumofen (UHV) durchgeführt und der Mischungsprozess analysiert. Ein Modell des Diffusionsprozesses wurde mittels mathematischer Überlegungen erstellt. Durch das Fitten der experimentellen Kompositionsprofile mittels dieses Modells konnten die Gleichgewichtskonzentrationen der Schichten auch mit relativ kurzen Auslagerungszeiten ermittelt werden. Darüber hinaus konnten aus den diffusionskontrollierten Zeit- und Temperaturdaten physikalische Eigenschaften wie der effektive Diffusionskoeffizient (Gitterdiffusion einschließlich Defektdiffusion) bestimmt werden. Dieser betrug Deff = 1.86 ∙ 10-10 m2/s ∙ exp(-164 kJ mol-1/RT). Während dem Vermischen wurde die Änderung der multilagigen Mikrostruktur bis zur vollständigen Mischung bei 623 und 673 K beobachtet, wobei Korngrenzen als schneller Diffusionsweg eine wichtige Rolle spielen. Bei 573 K wurde Nichtmischbarkeit experimentell deutlich nachgewiesen, wobei die Phasengrenzen bei cNi=26 at.% und cNi=66 at.% liegen. Mit diesen Phasengrenzen wurde die Mischungslücke über eine Redlich-Kister-Parametrisierung der Gibbs‘schen freien Energie über den gesamten Konzentrationsbereich rekonstruiert. Hierin wurde für die kritische Temperatur, TC, 608 K bei einer Konzentration von 45 at% Ni gefunden. Im zweiten Teil wurde die Korngrenzsegregation durch die FIB/tEBSD- (Transmissions-Elektronen-Rückstreubeugung) Technik, in Korrelation zu APT-Messung charakterisiert. Vier Legierungen mit einem Ni-Anteil zwischen 25 und 85 at.% wurden auf Wolframpfosten per IBS beschichtet, und bei 700 K für 24 h wärmebehandelt. Die Segregation von Cu in die Korngrenzen wurde beobachtet. Durch die Verwendung eines theoretischen Models wurde die Exzess-Kurve über den gesamten Konzentrationsbereich, und die Korngrenz-Formationsenergie auf Basis der experimentellen Daten berechnet. Die tEBSD-Analyse während der FIB-Präparation erlaubt die Identifikation der Körner und deren Orientierung. Ein neues Verfahren wurde entwickelt, um mithilfe der Orientierung benachbarter Körner, Berechnungen zur Ermittlung der Korngrenzorientierung durchzuführen und somit die Orientierung natürlicher Korngrenzen zu bestimmen. Mit diesem Verfahren konnte der zeitliche Aufwand dieser anspruchsvollen Auswertung (verglichen zur herkömmlichen Methode mittels TEM-Untersuchung) stark reduziert werden, so dass eine quantitative Analyse vieler Korngrenzen möglich wurde. Aus den einzelnen Korngrenzorientierungen wurde die Korngrenzrotation, und die jeweiligen Anteile an Kippung und Drehung berechnet. Eine Abhängigkeit der Feststoffsegregation vom Kipp- und Drehanteil der Korngrenze wurde beobachtet, die am kleinsten für die reine Kipp- und Drehrotation war. Die ermittelten Segregationsweiten sind signifikant größer als die strukturellen Korngrenzweiten und bewegen sich zwischen 12 und 85 Å. Dieses Verhalten wurde durch eine künstliche Verbreiterung der Korngrenze erklärt, die durch eine Flugbahnabweichung der Korngrenzatome während der Verdampfung verursacht wurde. Eine Korngrenzweite von w0 = (10.1 ± 1.5) Å wurde für eine unverfälschte Korngrenze gefunden.
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    Ultrathin organic thin-film transistors : investigating hybrid gate dielectrics and stable semiconductor monolayers
    (2021) Acharya, Rachana; Schmitz, Guido (Prof. Dr. Dr. h.c.)
    Organic electronics is an emerging field of research which includes the investigation of novel materials such as organic semiconductors and the development of devices such as organic thin-film transistors (TFTs). The unique properties of organic semiconductors, such as the ability to process them at relatively low temperatures, enable the development of potential applications of organic TFTs in flexible and wearable electronics such as rollable and foldable displays, conformable sensors and electronic skin. In order to facilitate the portable and lightweight nature of flexible electronics by powering them with small batteries or solar cells, a low operating voltage and an overall low power consumption are some of the main requirements of organic TFTs. High-capacitance gate dielectrics, such as hybrid gate dielectrics with an ultrathin metal oxide and an organic self-assembled monolayer (SAM), are an essential choice towards fulfilling these operation requirements. Organic TFTs are fabricated by depositing different materials as thin films by a variety of processes, and the individual film properties of the different components influence the overall electrical characteristics of organic TFTs. The main contribution of this thesis is to establish a correlation between the material properties of the individual components and the electrical properties of the organic TFTs, and moreover, suitably modify the fabrication process to achieve better electrical characteristics in organic TFTs. In this thesis, hybrid gate dielectrics consisting of an ultrathin aluminum oxide (AlOx) film and a phosphonic acid SAM are investigated. The AlOx films are fabricated by exposing the surface of the underlying aluminum gate electrode to an oxygen plasma, and the SAMs are processed from solution. Phosphonic acid molecules with an alkyl or a fluoroalkyl chain with different chain lengths have been chosen to form the SAMs. Two small-molecule organic semiconductors are selected as the active material in the organic TFTs: dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene (DNTT) and 2,7-diphenyl[1]benzothieno[3,2-b][1]benzothiophene (DPh-BTBT). The significance of both components of the hybrid gate dielectric in simultaneously achieving a leakage-current density of 10-7A/cm2 as well as an operating voltage of 3 V has been established. The thickness of the AlOx films was measured by Transmission Electron Microscopy and the films were characterized by Electron Energy Loss Spectroscopy and Atom Probe Tomography to determine the thin-film composition. Depending on the parameters of the oxygen-plasma process, AlOx films with a thickness ranging between 4 nm and 7 nm were fabricated, and consequently, organic TFTs with a gate-dielectric capacitance between 1 μF/cm2 and 1.6 μF/cm2. In particular, charge carrier mobilities ranging from 1.8 to 2.3 cm2/Vs were obtained for a number of favorable combinations of the plasma power and plasma duration that produce AlOx films with a small surface roughness and thus promote the formation of high-quality SAMs and well-ordered DNTT films on these gate dielectrics. The influence of the thickness of the SAM by employing different chain-length phosphonic acid molecules on different TFT characteristics such as the threshold voltage, gate-leakage current, charge-carrier mobility and the subthreshold swing has been examined in DNTT and DPh-BTBT TFTs. By employing the medium-chain-length phosphonic acid molecules, an optimum charge carrier mobility of 2 cm2/Vs for the DNTT TFTs, and a turn-on voltage of 0 V for the DPh-BTBT TFTs was achieved. The growth and morphology of the organic semiconductor DNTT on different gate-dielectric surfaces was observed by Scanning Electron Microscopy and Atomic Force Microscopy and was correlated with surface properties of the SAMs and the electrical characteristics of TFTs based on those films. The stability of ultrathin films with a thickness of one-two molecular monolayers of the organic semiconductor DNTT was investigated, and spontaneous morphological changes occurring in the films were observed and correlated with the stability of organic TFTs based on these films. The structural reconfiguration of the ultrathin DNTT films and the degradation of the charge-carrier mobility of organic TFTs based on these films were prevented to a certain extent by cryogenic cooling and in-situ encapsulation. The hybrid gate dielectric with its two components, the organic semiconductor and the gate dielectric-semiconductor interface are the focal points in this thesis. The material and surface properties of the individual components of the gate dielectric have been correlated with the film properties of the organic semiconductor and further with the electrical characteristics of organic TFTs.
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    Growth of oxide materials with Ruddlesden-Popper- and garnet-type structures via the optical float zone method: investigations of scintillation, optical, and magnetic properties, and post-growth modifications of single crystals via topochemical routes
    (2025) Yilmaz, Hasan; Clemens, Oliver (Prof. Dr.)
    With the continuous progress in materials science, the global market demand for high-performance functional materials, especially those with customized optical and magnetic properties, has increased significantly due to their critical role in next-generation technologies such as photonic devices, data storage systems, quantum computing, and biomedical applications. With this motivation, this dissertation is focused on the growth of high-quality oxide single crystals with Ruddlesden-Popper (RP) and garnet-type structures using the optical floating zone (OFZ) method and the detailed investigation of their scintillation, optical and magnetic properties. Furthermore, the use of chemical doping and post-growth topochemical modifications as strategies to modify material properties and stabilise metastable structures is also covered. With these approaches, we can classify this study into two parts: optical and magnetic properties. In the first part of this work, RP-type n = 1 LaSrGaO4 (LSGO) and Garnet-type Gd3In2Ga3O12 (GIGG) single crystals doped with different Rare Earths (RE) including Eu, Sm, Ho, Nd have been grown via OFZ method to investigate their luminescence, decay time kinetics and scintillation properties. To compare the scintillation behaviour, a well-known commercial scintillator, Gd2.98Ce0.02Al2Ga3O12 (0.02Ce:GAGG) single crystal, was synthesised and characterised using the same methods. This limits deviations in scintillation properties caused by different synthesis methods. Powder X-ray diffraction (XRD) and Single Crystal X-ray diffraction (SC-XRD) measurements together with Rietveld analysis confirmed phase purity and crystallinity, while Scanning Electron Microscopy (SEM-EDX) and backscatter electron imaging (BSE) confirmed the homogeneous distribution of the dopants. In addition to these analytical methods, Photoluminescence Spectroscopy (PL) and luminescence decay measurements have demonstrated that these RE-doped single crystals have strong emission, which is related to the low phonon energy of their host lattices. However, scintillation measurements under 137Cs gamma-ray point source excitation resulted in a limited gamma ray response. This response was significantly weaker than observed for the well-known commercial scintillator 0.02Ce:GAGG. Inefficient absorption of high-energy excitation or inadequate energy transfer to the activator ions in these materials are responsible for this relatively low scintillation performance. The second part of the thesis is focused on bilayer Ruddlesden-Popper nickelates. Single crystals of Sr3Ni2-xAlxO7-δ (SNAO) and its Y-substituted modification YᵧSr3-yNi2-xAlxO7-δ (YSNAO) have been grown by using strategically chemical substitutions and high oxygen pressure conditions to stabilize the RP phase. XRD and SC-XRD analysis confirmed the existence of the n = 2 RP-type structure by doping with Al, while thermogravimetric analysis (TGA) and X-ray photoelectron spectroscopy (XPS) indicated successful oxygen incorporation and corresponding changes in the Ni oxidation state. Magnetic susceptibility measurements (SQUID magnetometry) have demonstrated the antiferromagnetic ordering transitions in these doped nickelate single crystals, and transport measurements have shown enhanced electrical conductivity compared to the undoped parent compound. In addition, a topochemical fluorination process has been investigated as a post-growth modification for n = 1 RP-type La2NiO4+δ single crystals. Fluorine has been introduced into the crystal lattice at moderate temperatures using polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), and CuF2 as fluorination reagent. PTFE has been found to be the most effective fluorinating agent, resulting in partially fluorinated La2NiO4+δ with a diffusion-limited penetration pattern. EDX and EDX-mapping analyses have demonstrated an inhomogeneous F distribution, indicating that fluorine incorporation is limited by diffusion kinetics. This inhomogeneity should be carefully considered in future structural and functional analyses as it may affect the accurate characterisation of intrinsic properties in fluorinated crystals. In summary, the results of this thesis demonstrate the flexibility of the optical floating zone technique for the growth of high-quality functional oxides and show how structural stability, and functional properties can be modified using specific chemical doping approaches. To optimise scintillation performance in novel garnet-type hosts and to improve the magnetic and electronic behaviour of RP-type nickelates, the combined approach of crystal growth and post-growth modification could provide an additional parameter for material modification.