Effects of ionizing radiation on nanomaterials and III-V semiconductor devices.

Devices based on III-V semiconductors and nanomaterials are expected to be critical components of future microsystems as the demand for greater functionality, range of application, and robustness continue to increase. There currently is a need for small-scale power supplies which can be used to power microsystems thereby enabling autonomous functionalities. The use of III-V semiconductor-based solid state devices and nanomaterials to convert the radiant energy of a radioisotope source into electricity has been investigated as a viable option to fulfill this demand. The energy imparted to a material by incident alpha-particles, resulting in electron-hole pair formation and ionization, may be converted into usable electrical power by a radioisotope microbattery (RIMB). A model describing the spatially varying rate of ionizing energy deposited in an absorber material held in close proximity to an isotropic alpha-emitting radioisotope source has been developed. The alpha-particle energy deposition model (ADEP) allows the total energy exciting the RIMB devices to be calculated and thereby provides a means to determine the efficiency of the experimentally measured devices. Two RIMB designs are investigated including a direct conversion microbattery based on a nipi-diode structure and an indirect conversion microbattery employing radioluminescent nanophosphors. The multi-functional nature of microsystems may best be exploited by deploying them in extreme environments, such as space, where a low power consumption, small volume, and superior functionality are required. Expanding microsystems into such environments requires a full understanding of the effects that ionizing radiation will have on the optoelectronic properties of the devices and the materials which they are composed of. Irradiating devices with an isotropic alpha-particle flux is a good method for simulating the radiation damage encountered by III-V devices or nanomaterials employed in space. The large mass of alpha particles, in comparison to beta particles, leads to higher momentum transfers in nuclear interactions corresponding to a larger displacement damage dose near the surface of a material for comparably lower fluences. The effects of such irradiation on the optoelectronic properties of III-V semiconductor devices and epitaxially grown InAs quantum dots arrays are investigated. A crystal binding model based on the Tersoff interatomic potentials is developed and used to explain the increased radiation tolerance observed in the InAs quantum dot material system

Structural and Electronic Investigation of Strongly Correlated Transition Metal Oxide Perovskite Thin Films and Interfaces using In-situ Transmission Electron Microscopy

Discussing the necessity as well as possible details of global strategies to reduce and eventually eliminate the anthropogenic climate change (ACC) is a delicate matter, which easily leads to statements based on ressentiments rather than on scientific facts. Indeed, public polls revealed the volatility of individual beliefs in the existence of ACC correlating with short-term weather phenomena [1] well after a scientific consensus about its impact was found [2–5]. Naturally, the models presented in the cited references do not cover all facets of the cybernetic global system at once and the assessment of resulting forecast uncertainties is part of the careful work of colleagues [6, 7]. However, the included and certainly possible scenario of turning the Earth in an increasingly hostile planet appears to be an unreasonably high stake when betting on the future. Consequently, in order to reduce the emission of greenhouse gases being indisputably linked to global warming [7, 8], progress in sustainable energy sources as well as their actual usage is indispensable. In essence, establishing a prevailing renewable energy supply is a threefold problem as primary conversions need to be followed by storage and transport in order to bridge temporal as well as spatial source heterogeneities [9]. One candidate to master the former challenge is solar energy conversion, which has recently gained a lot in global energy market share as module efficiencies resp. prices are constantly rising resp. falling [10, 11]. In detail, both the optimisation of established solar cells, i.e. most prominently silicon modules, as well as the inclusion of innovative concepts and materials is pursued. The latter approach is also referred to as third-generation photovoltaics and aims for solar cell efficiencies beyond the famous Shockley-Queisser limit [12], which describes the theoretical thermodynamic conversion limit under the assumption of transmission of sub-bandgap photons and complete thermalisation of hot charge carriers before power extraction. A particularly promising example, in which these assumptions are no longer valid, is given by the material class of organic halide perovskites showing an extraordinarily fast increase of related efficiencies over the past years [13–15]. In this work, the inorganic counterpart of transition metal oxide perovskites will be the main subject of study. Certainly, currently achieved solar energy conversion efficiencies in this material class are significantly lower compared with organic halides [16–18]. However, because of their rich phase diagrams emerging due to strong correlations [19–22] they serve as a well- suited model system to study underlying mechanisms (lifting the previously mentioned Shockley-Queisser limit) on a fundamental level. Importantly, this statement is not limited to the context of photovoltaics, but holds also for additional fields such as the study of catalysis [23–25] or resistive random access memory (RRAM) [26]. In more detail, this dissertation focuses on the structural and electronic investigation of transition metal oxide perovskite thin films, being typically the basis of technological devices [26]. It includes significant contributions to the phase diagram of Pr1−xCaxMnO3 epitaxial layers – grown on SrTi1−yNbyO3 substrates – in doping and temperature regimes where ordered phases occur due to correlative exchange interactions of lattice, orbital, and spin degrees of freedom [20]. Importantly, these ordered phases have been demonstrated to correlate with an enhanced photovoltaic acitvity [17, 18, 27, 28] emphasizing the importance of such studies in the context of solar energy conversion. In fact, as nicely described in the cited references, the underlying mechanism of the enhanced photovoltaic activity was found to be a prolonged lifetime of hot carriers due to phonon interactions and, thus, reaches beyond the assumptions of the Shockley-Queisser limit. Consequently, the materials in question are well-suited to explore fundamental processes in third generation photovoltaics. In order to study the mentioned phase transitions in thin films as well as electric properties relevant for solar energy conversion such as the excess charge carrier diffusion length, which happens to be located on the nanoscale [29], high-resolution techniques are needed. Therefore, the transmission electron microscope is employed enabling for versatile and highly- resolved real and reciprocal space signal extraction as well as a large variety of in-situ techniques, e.g. heating, cooling, biasing, and environmental control. In fact, facilitated by the outstanding advances in scientific instrumentation, such in-situ methods are ever-increasingly applied on the micro- and nanoscale and successfully correlated to macroscopic physical, chemical, and biological properties [30–33]. In this study, in-situ heating, cooling, biasing, and environmental control are combined with established techniques such as selective area electron diffraction [34] and electron energy loss spectroscopy [35] as well as with recently emerging methods like four-dimensional scanning transmission electron microscopy [36] and scanning transmission electron beam induced current [29]. Additionally, a substantial part of this thesis focuses on further developments of the latter techniques. Selected highlights are the successful extraction of ordering parameters as well as critical temperatures of phase transitions in Pr1−xCaxMnO3 during heating (in a gaseous environment) and cooling. The observed transitions, i.e. charge ordering for x = 0.34 and an orthorhombic to pseudo- tetragonal (or cubic) transition for x = 0.1, are discussed thoroughly in the context of correlation phenomena and photovoltaic activity and differences to the bulk such as decreased critical temperatures will be pointed out. Furthermore, a structural model is presented linking atomic configurations with the material’s lattice parameters. In addition, experimental and modelling advances in the field of scanning transmission electron beam induced current are demonstrated enabling the observation of diffusion and recombination properties of excess charge carriers in perovskites on the nanoscale as well mapping of a sub-0.1 ppm concentration line of boron in a textured silicon solar cell. Lastly, first interpretations of atomic modulations in electron beam induced current signals are presented.2021-09-1

Experimental and Theoretical Analyses of Adiabatic Two-phase Flows in Horizontal Feed Pipes

The majority of technical separation processes for fluid mixtures utilize the principle of rectification. If a two-phase mixture is fed into the column, possibly undesirable flow morphologies or severe droplet carry-over may occur, which detrimentally affect separation efficiency and equipment integrity. Currently, the two-phase flow behavior in feed pipes is hardly predicable and mostly based on empirical or heuristic methods, which do not properly account for a broad range of possible fluid properties and plant dimensions. As a consequence, costly safety margins are applied. Feed pipes to separation columns are often characterized by horizontal inlet nozzles, small length-to-diameter ratios and complex routing, involving elbows or bends. The pipe lengths are too short to enable the two-phase flow to fully develop, which thus, enters the column with unknown flow morphology. Since developing flows have rarely been studied, today’s engineering practice relies on existing predictive methods for fully developed two-phase flows. Graphical methods can hardly represent gradual transitions between flow regimes. Analytical models provide only simplified flow representations of the two-phase flow that have not yet been qualified for developing pipe flow. In this work, a comprehensive experimental database of horizontal water-air flows in two test sections with nominal pipe diameters of D = 50 mm and D = 200 mm and feed pipe lengths in the range 10 < L/D < 75 was established. This way, the data cover developing pipe flows with entrance lengths typical for two-phase feeds of separation columns and more developed flows that are comparable with the extensively studied reference system water-air. A particular focus was put on the effect of pipe bends on the flow morphology up- and downstream. The flow morphology was captured using imaging wire-mesh sensors. A 4D fuzzy algorithm was applied to objectively identify the flow two-phase morphologies. Based on their fuzzy representation, the flow morphologies were classified and a novel 2D visualization technique is proposed to discuss the flow development along the feed pipes. Undesired flow morphologies (intermittent flow and entrainment) during the operation of two-phase feeds are hardly predictable by conventional design tools. The inception of intermittent flows was analyzed using the experimental data. Consequently, the inception criteria based on the required liquid levels for fully developed intermittent flows were adapted for short entrance lengths. The characteristic dynamics of flow morphologies that are known to cause the onset of entrainment were analyzed. Based on wave frequencies, a predictive criterion for the susceptibility of wavy flows for the onset of entrainment is introduced and applied to straight feed pipes and horizontal 90° bends. Among the dozens available, 66 reduced-order models for the prediction of the void fraction were tested for straight feed pipes and horizontal 90° pipe bends. Thereof, the ones most suitable for variable operating conditions and pipe geometries were identified and adapted. Complementary 3D simulations were performed to verify the applicability of numerical codes (VoF, AIAD) for flows with free interfaces. The flow morphologies were successfully reproduced at macroscopic scale, however, the simulation results rank behind reduced-order models considering their quantitative predicting capabilities.:Abstract II Kurzfassung IV Acknowledgement VI Nomenclature VIII Table of Contents XIII 1 Introduction 1 1.1 Thermal separation in view of the 21st century 1 1.2 Engineering and design of rectification plants 2 1.3 Outline of the thesis 4 2 State of the art 5 2.1 Two-phase feeds in thermal separation 5 2.1.1 Feed condition as adjustable parameter 5 2.1.2 Thermohydraulic optimization 8 2.1.3 Hydrodynamic conditioning 9 2.2 Hydrodynamics of two-phase feeds 11 2.2.1 Flow morphologies in feed pipes 11 2.2.2 Droplet entrainment 14 2.2.3 Flow regime maps 17 2.2.4 Consequences for two-phase feeds 19 2.3 Modelling of two-phase feeds 23 2.3.1 Basic definitions 23 2.3.2 Fundamentals of the two-fluid model 25 2.3.3 The interfacial level gradient 29 2.3.4 Analytical models 32 2.3.5 CFD simulations for commercial feed pipes 34 2.4 Objectives of this thesis 36 3 Experimental method and algorithms for flow characterization 37 3.1 Experimental setups 37 3.2 Wire-mesh sensors 40 3.3 Experimental procedure 42 3.4 Data processing 44 3.4.1 Fuzzy flow morphology classification 45 3.4.2 Power spectral density 48 3.5 Measurement uncertainty 49 4 Flow morphologies in different feed pipe geometries 53 4.1 Developing two-phase flow in straight pipes 53 4.2 Effect of pipe curvatures on the flow morphology 55 4.3 Morphology recovery 57 4.4 Conclusions 60 5 Prediction of undesirable flow morphologies in feed pipes 61 5.1 Initiation of intermittent flows 61 5.2 Onset of droplet entrainment 62 5.2.1 Vulnerable flow morphologies 62 5.2.2 Derivation of a criterion for onset of entrainment 64 5.2.3 Adjustment of the criterion for the investigated pipe geometries 67 5.3 Conclusions 70 6 Reduced-order modelling of two-phase feeds 71 6.1 Prediction of void fraction 71 6.2 Liquid levels 75 6.3 Conclusions 78 7 CFD modelling of two-phase feeds 79 7.1 Simulation setup 79 7.2 Multiphase models 82 7.3 Comparison with experimental data 83 7.3.1 Straight pipes 83 7.3.2 Horizontal 90° bends 85 7.4 Conclusions 88 8 Summary and recommendations for future work 89 8.1 Summary 89 8.2 Recommendations for future work 91 References 94 List of figures 113 List of tables 118 Appendix i Scientific publications and contributions xxxiii Eidesstattliche Erklärung xxxviiDie meisten technischen Verfahren zur Trennung von Flüssigkeitsgemischen beruhen auf dem Prinzip der Rektifikation. Wird ein Zweiphasengemisch in die Trennkolonne eingespeist, können unerwünschte Strömungsmorphologien oder ausgeprägte Tröpfchenverschleppung auftreten, welche sich nachteilig auf die Trennleistung und die Integrität einzelner Anlagenkomponenten auswirken. Derzeit lässt sich das Verhalten solcher Zweiphasenströmungen in Einspeiseleitungen kaum vorhersagen und basiert meist auf empirischen oder heuristischen Methoden, die ein breites Spektrum möglicher Stoffeigenschaften und Anlagendimensionen nicht angemessen berücksichtigen. Infolgedessen müssen kostspielige Sicherheitszuschläge angewendet werden. Einspeiseleitungen von Trennkolonnen sind häufig durch horizontale Eintrittsstutzen, ein geringes Länge-zu-Durchmesser-Verhältnis und eine komplexe Leitungsführung mit Bögen und anderen Normteilen gekennzeichnet. Typische Rohrlängen sind zu kurz, um eine vollständig entwickelte Zweiphasenströmung auszubilden, welche daher mit unbekannter Strömungs-morphologie in die Trennkolonne eintritt. Da derartige Strömungen jedoch bisher nur selten untersucht wurden, verlässt man sich gegenwärtig in der technischen Praxis auf bestehende Vorhersagemethoden für voll entwickelte Zweiphasenströmungen. Grafische Methoden können jedoch die allmählichen Übergänge zwischen Strömungsformen kaum darstellen. Analytische Modelle liefern nur vereinfachte Näherungswerte der Zweiphasenströmung, die noch nicht für sich entwickelnde Rohrströmung qualifiziert wird. In dieser Arbeit wurde eine umfangreiche experimentelle Datenbasis horizontaler Wasser-Luft-Strömungen in zwei Versuchsstrecken mit Rohrinnendurchmessern von D = 50 mm und D = 200 mm und Einlauflängen im Bereich 10 < L/D < 75 erstellt. Auf diese Weise decken die Daten sowohl sich entwickelnde Rohrströmungen mit typischen Einlauflängen für Einspeiseleitungen ab, als auch weiter (in axialer Richtung) entwickelte Strömungen, die mit dem umfangreich untersuchten Referenzsystem Wasser-Luft vergleichbar sind. Die Auswirkung von Rohrbögen auf die Strömungsmorphologie stromauf- und stromabwärts wurde gezielt untersucht. Die Strömungsmorphologie wurde mit bildgebenden Gittersensoren erfasst. Ein 4D-Fuzzy-Algorithmus wurde zur objektiven Identifizierung der Strömungsmorphologien eingesetzt. Auf Grundlage dieser Fuzzy-Darstellung der Strömung wurden die Strömungsmorphologien klassifiziert, und es wurde eine neuartige 2D-Visualisierungstechnik entworfen, mit der die Strömungsentwicklung entlang der Einspeiseleitungen diskutiert wurde. Unerwünschte Strömungsmorphologien (intermittierende Strömung und Tropfenmitriss) während des Betriebs zweiphasiger Einspeisungen sind mit herkömmlichen Auslegungswerkzeugen kaum vorherzusagen. Das Einsetzen intermittierender Strömungen wurde auf Grundlage der experimentellen Daten analysiert. Daraufhin wurden existierende Kriterien, basierend auf den notwendigen Mindestfüllständen, für das Einsetzen intermittierender Strömungen in Abhängigkeit von den untersuchten Einlauflängen angepasst. Die charakteristische Dynamik von Strömungsmorphologien, die Tropfenmittriss hervorrufen, wurde analysiert. Voraussagemethoden zur Vorhersage der Anfälligkeit welliger Strömungen für das Auftreten von Tropfenmitriss wurden auf der Grundlage von Wellenfrequenzen entwickelt und für gerade Einspeiserohre und horizontale 90°-Bögen angewandt. Von den zahlreichen verfügbaren Modellen zur Vorhersage des Gasanteils wurden 66 Modelle reduzierter Ordnung für gerade Einspeiseleitungen und horizontale 90°-Rohrbögen getestet. Davon wurden die für variable Betriebsbedingungen und Rohrgeometrien am besten geeigneten Modelle ermittelt und angepasst. Komplementäre 3D-Simulationen wurden durchgeführt, um die Anwendbarkeit numerischer Codes (VoF, AIAD) für Strömungen mit freien Grenzflächen zu bestätigen. Die Strömungsmorphologien wurden im makroskopischen Maßstab erfolgreich reproduziert, die Simulationsergebnisse bleiben jedoch hinsichtlich ihrer quantitativen Vorhersagekraft hinter den Modellen reduzierter Ordnung zurück.:Abstract II Kurzfassung IV Acknowledgement VI Nomenclature VIII Table of Contents XIII 1 Introduction 1 1.1 Thermal separation in view of the 21st century 1 1.2 Engineering and design of rectification plants 2 1.3 Outline of the thesis 4 2 State of the art 5 2.1 Two-phase feeds in thermal separation 5 2.1.1 Feed condition as adjustable parameter 5 2.1.2 Thermohydraulic optimization 8 2.1.3 Hydrodynamic conditioning 9 2.2 Hydrodynamics of two-phase feeds 11 2.2.1 Flow morphologies in feed pipes 11 2.2.2 Droplet entrainment 14 2.2.3 Flow regime maps 17 2.2.4 Consequences for two-phase feeds 19 2.3 Modelling of two-phase feeds 23 2.3.1 Basic definitions 23 2.3.2 Fundamentals of the two-fluid model 25 2.3.3 The interfacial level gradient 29 2.3.4 Analytical models 32 2.3.5 CFD simulations for commercial feed pipes 34 2.4 Objectives of this thesis 36 3 Experimental method and algorithms for flow characterization 37 3.1 Experimental setups 37 3.2 Wire-mesh sensors 40 3.3 Experimental procedure 42 3.4 Data processing 44 3.4.1 Fuzzy flow morphology classification 45 3.4.2 Power spectral density 48 3.5 Measurement uncertainty 49 4 Flow morphologies in different feed pipe geometries 53 4.1 Developing two-phase flow in straight pipes 53 4.2 Effect of pipe curvatures on the flow morphology 55 4.3 Morphology recovery 57 4.4 Conclusions 60 5 Prediction of undesirable flow morphologies in feed pipes 61 5.1 Initiation of intermittent flows 61 5.2 Onset of droplet entrainment 62 5.2.1 Vulnerable flow morphologies 62 5.2.2 Derivation of a criterion for onset of entrainment 64 5.2.3 Adjustment of the criterion for the investigated pipe geometries 67 5.3 Conclusions 70 6 Reduced-order modelling of two-phase feeds 71 6.1 Prediction of void fraction 71 6.2 Liquid levels 75 6.3 Conclusions 78 7 CFD modelling of two-phase feeds 79 7.1 Simulation setup 79 7.2 Multiphase models 82 7.3 Comparison with experimental data 83 7.3.1 Straight pipes 83 7.3.2 Horizontal 90° bends 85 7.4 Conclusions 88 8 Summary and recommendations for future work 89 8.1 Summary 89 8.2 Recommendations for future work 91 References 94 List of figures 113 List of tables 118 Appendix i Scientific publications and contributions xxxiii Eidesstattliche Erklärung xxxvi

Computational model of an Astrocyte as spatial potassium buffer at the neurovascular unit

Gli astrociti sono cellule presenti nel cervello e nel midollo spinale con una forma caratteristica radiale, la quale richiama una stella. Queste cellule contribuiscono all'omeostasi e alla regolazione del sistema nervoso centrale, inoltre, dimostrano notevoli capacità di adattamento all'ambiente che le circonda. Quest'ultima caratteristica permette di attribuire alla loro presenza la capacità di mantenimento funzionale del sistema nervoso durante l'invecchiamento. Gli astrociti svolgono molte funzioni attive e di supporto, incluse il controllo biochimico delle cellule endoteliali, la fornitura di nutrienti al tessuto nervoso, il mantenimento dell'equilibrio ionico e la regolazione del flusso sanguigno cerebrale. Negli ultimi anni, si è assistito ad un crescente interesse per le comunicazioni neurone-glia, considerando la loro partecipazione alle funzioni cognitive e al loro coinvolgimento in molti disturbi cerebrali e malattie neurodegenerative. Gli astrociti agiscono sulla cosiddetta unità neurovascolare reagendo alla attività neurale locale, e mediando la concentrazione di potassio nello spazio perivascolare in prossimità delle cellule muscolari liscie. Alta attività neurale richiede un maggiore afflusso di nutrienti e ossigeno, il quale trova risposta in una propagazione di calcio attraverso gli astrociti vicini, che portano ad attivare scambi di potassio tra il cosiddetto endfoot e lo spazio perivascolare. Questo fenomeno si rappresenta in un controllo dell'attività di cellule muscolari liscie, le quali hanno il compito di regolare la dilatazione e costrizione dei vasi sanguigni vicini. \\ L'obiettivo è di sviluppare un modello computazione riproducibile di un astrocita in grado di cooperare con l'unità neurovasculare al livello della barriera emato-encefalica. Questo lavoro presenta un modello multi-compartimentale di stato in grado di descrivere l'astrocita come una cellula singola, in grado di interagire con ambienti perisinaptici e perivascolari. Il modello consiste in un insieme di equazioni differenziali, dipendenti tra loro, in grado di rappresentare la dinamica di ogni variabile di stato. Implementa svariati fenomeni biologici e integra caratteristiche di studi passati, concentrandosi sul fenomeno di attivazione di specifiche proteine di trasporto di potassio, una volta raggiunte da una propagazione di calcio. Tra le proteine studiate, un maggiore interesse è riposto sui canali $Kir_{4.1}$ e $BK$. In aggiunta, il modello permetterà di definire una organizzazione geometrica dell'astrocita, conferendo la possibilità di studiare pattern spaio temporali delle varie variabili di stato durante una stimolazione esterna di glutamato, la quale rappresenta un'alta attività nervosa locale. I risultati mostrano come, durante una stimulazione esterna di glutamato, sia presente un rilascio di potassio nell'ambiente perivascolare, controllato dalla dinamica del calcio proveniente da regioni distanti dell'astrocita. Tutte le analisi sono state implementate usando linguaggi di programmazione come \emph{Python} e \emph{MATLAB}.Astrocytes are sponge-like cells present in brains and spinal cord that provide homeostasis and regulation of the central nervous system. Astrocytes are highly heterogeneous in morphological appearance, demonstrating remarkable adaptive plasticity capabilities to their surroundings, the same ones that define the functional maintenance of the nervous system through aging. They perform many supporting and active functions, including biochemical control of endothelial cells, provision of nutrients to the nervous tissue, maintenance of ion balance and regulation of the cerebral blood flow. Recent years have witnessed an increasing interest in neuron–glia communication due to the realization of their participation in cognitive functions and information processing, as well as being involved in many brain disorders and neurodegenerative diseases. Astrocytes act in neurovascular coupling by reacting to neural activity and by mediating potassium concentration in the perivascular space around smooth muscle cells. High neural activity demands a larger supply of nutrients and oxygen, which is answered by a propagation of calcium through the astrocyte, that ultimately triggers potassium exchanges between endfeets and perivascular space. This results in a control of the local smooth muscle cell's activity, bringing to regulation of dilation and constriction of nearby blood vessels and therefore regulation of the cerebral blood flow. The aim of the thesis project is to develop a reproducible computational model of an astrocyte's endfoot cooperating with the neurovascular unit at the blood-brain barrier. This work presents a multi-compartmental state space model describing the astrocyte as a single cell that interacts with different domains, such as the perisynaptic and perivascular spaces. The model consists of a set of coupled ordinary differential equations that represent the dynamics of all states. It implements several biological phenomenon and merges together characteristics of past studies by focusing on how the calcium signaling would trigger the activation of specific potassium transporters, such as inward rectifying channel ($Kir_{4.1}$) and big conductance ($BK$) channels. Additionally, the model allows to define a geometrical organization of how many astrocyte's processes and how they are qualitatively distributed in space are present in a simulation. This, in order to investigate on spatial-temporal patterns during external glutamate stimulations, which simulate high neural activity. The results showed that there is actually, buffering of potassium during external glutamate stimuli, led by calcium dynamics that propagate the information from synaptic areas to the blood-brain barrier. All analysis are made implementing a computational model using \emph{Python} and \emph{MATLAB} as coding languages

Delay-Line 3D Position Sensitive Radiation Detection

High-resistivity silicon(Si) in large volumes and with good charge carrier transport properties has been produced and achieved success as a radiation detector material over the past few years due to its relatively low cost as well as the availability of well-established processing technologies. One application of that technology is in the fabrication of various position-sensing topologies from which the incident radiation’s direction can be determined. We have succeeded in developing the modeling tools for investigating different position-sensing schemes and used those tools to examine both amplitude-based and time-based methods, an assessment that indicates that fine position-sensing can be achieved with simpler readout designs than are conventionally deployed. This realization can make ubiquitous and inexpensive deployment of special nuclear materials (SNM) detecting technology becomes more feasible because if one can deploy position-sensitive semiconductor detectors with only one or two contacts per side. For this purpose, we have described the delay-line radiation detector and its optimized fabrication. The semiconductor physics were simulated, the results from which guided the fabrication of the guard ring structure and the detector electrode, both of which included metal-field-plates. The measured improvement in the leakage current was confirmed with the fabricated devices, and the structures successfully suppressed soft-breakdown. We also demonstrated that fabricating an asymmetric strip-line structure successfully minimizing the pulse shaping and increases the distance through which one can propagate the information of the deposited charge distribution. With fabricated delay-line detectors we can acquire alpha spectra (Am-241) and gamma spectra (Ba-133, Co-57 and Cd-109). The delay-line detectors can therefore be used to extract the charge information from both ion and gamma-ray interactions. Furthermore, standard charge-sensitive circuits yield high SNR pulses. The detectors and existing electronics can therefore be used to yield imaging instruments for neutron and gamma-rays, in the case of silicon. For CZT, we would prefer to utilize current sensing to be able to clearly isolate the effects of the various charge-transport non-idealities, the full realization of which awaits the fabrication of the custom-designed TIA chip.Ph.D.Nuclear Engineering & Radiological SciencesUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/91556/1/mhjeong_1.pd

Evidence of two-dimensional flat band at the surface of antiferromagnetic kagome metal FeSn

The kagome lattice has long been regarded as a theoretical framework that connects lattice geometry to unusual singularities in electronic structure. Transition metal kagome compounds have been recently identified as a promising material platform to investigate the long-sought electronic flat band. Here we report the signature of a two-dimensional flat band at the surface of antiferromagnetic kagome metal FeSn by means of planar tunneling spectroscopy. Employing a Schottky heterointerface of FeSn and an n-type semiconductor Nb-doped SrTiO3, we observe an anomalous enhancement in tunneling conductance within a finite energy range of FeSn. Our first-principles calculations show this is consistent with a spin-polarized flat band localized at the ferromagnetic kagome layer at the Schottky interface. The spectroscopic capability to characterize the electronic structure of a kagome compound at a thin film heterointerface will provide a unique opportunity to probe flat band induced phenomena in an energy-resolved fashion with simultaneous electrical tuning of its properties. Furthermore, the exotic surface state discussed herein is expected to manifest as peculiar spin-orbit torque signals in heterostructure-based spintronic devices

Study of Computational Image Matching Techniques: Improving Our View of Biomedical Image Data

Image matching techniques are proven to be necessary in various fields of science and engineering, with many new methods and applications introduced over the years. In this PhD thesis, several computational image matching methods are introduced and investigated for improving the analysis of various biomedical image data. These improvements include the use of matching techniques for enhancing visualization of cross-sectional imaging modalities such as Computed Tomography (CT) and Magnetic Resonance Imaging (MRI), denoising of retinal Optical Coherence Tomography (OCT), and high quality 3D reconstruction of surfaces from Scanning Electron Microscope (SEM) images. This work greatly improves the process of data interpretation of image data with far reaching consequences for basic sciences research. The thesis starts with a general notion of the problem of image matching followed by an overview of the topics covered in the thesis. This is followed by introduction and investigation of several applications of image matching/registration in biomdecial image processing: a) registration-based slice interpolation, b) fast mesh-based deformable image registration and c) use of simultaneous rigid registration and Robust Principal Component Analysis (RPCA) for speckle noise reduction of retinal OCT images. Moving towards a different notion of image matching/correspondence, the problem of view synthesis and 3D reconstruction, with a focus on 3D reconstruction of microscopic samples from 2D images captured by SEM, is considered next. Starting from sparse feature-based matching techniques, an extensive analysis is provided for using several well-known feature detector/descriptor techniques, namely ORB, BRIEF, SURF and SIFT, for the problem of multi-view 3D reconstruction. This chapter contains qualitative and quantitative comparisons in order to reveal the shortcomings of the sparse feature-based techniques. This is followed by introduction of a novel framework using sparse-dense matching/correspondence for high quality 3D reconstruction of SEM images. As will be shown, the proposed framework results in better reconstructions when compared with state-of-the-art sparse-feature based techniques. Even though the proposed framework produces satisfactory results, there is room for improvements. These improvements become more necessary when dealing with higher complexity microscopic samples imaged by SEM as well as in cases with large displacements between corresponding points in micrographs. Therefore, based on the proposed framework, a new approach is proposed for high quality 3D reconstruction of microscopic samples. While in case of having simpler microscopic samples the performance of the two proposed techniques are comparable, the new technique results in more truthful reconstruction of highly complex samples. The thesis is concluded with an overview of the thesis and also pointers regarding future directions of the research using both multi-view and photometric techniques for 3D reconstruction of SEM images

On the development of memristive devices for electroforming-free and analog memristive crossbar arrays

Memristive devices can reversibly change their resistance by applying an electrical voltage or current. These thin-film devices have the potential to serve as central components in novel neuromorphic circuits, similar to synapses in the human brain. Unlike traditional neuromorphic systems, they enable a state-based and non-volatile weight between two neurons. This comes very close to the natural model of the human brain, where information is stored and processed together. The aim of this thesis was the development of novel memristive devices and the integration into crossbar arrays. An essential requirement was an analogous resistance change, which allows continuous changes in resistance. It was found, that devices with a combination of tunnel and Schottky barriers are best suited for this purpose. These double barrier devices show an analogous and homogeneous resistance change. As a reference system, filament-based memristive devices have been developed that alter their resistance due the migration of silver. Since the formation of filaments is almost random, they have a significantly higher device variability and very few states between the off- and on-state. Only the high quality of the double barrier component allowed the circuit integration without the need to individually adjust circuit parameters for each memristive device. Due to the non-linear switching characteristics and the advantageous I-V characteristics, the devices were integrated into a space-saving crossbar architecture, which increased the packing density tenfold. Due to the simultaneously simplified electrical connection, it was possible to realize a circuit for pattern classification with 180 memristive devices. The construction of an automated measuring system enabled the characterization of a large number of devices. The development of database-supported measurement and evaluation programs facilitated the analysis of the device and switching properties