2,437 research outputs found

    Grain boundaries in polycrystalline materials for energy applications : First principles modeling and electron microscopy

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    Polycrystalline materials are ubiquitous in technology, and grain boundaries have long been known to affect materials properties and performance. First principles materials modeling and electron microscopy methods are powerful and highly complementary for investigating the atomic scale structure and properties of grain boundaries. In this review, we provide an introduction to key concepts and approaches for investigating grain boundaries using these methods. We also provide a number of case studies providing examples of their application to understand the impact of grain boundaries for a range of energy materials. Most of the materials presented are of interest for photovoltaic and photoelectrochemical applications and so we include a more in depth discussion of how modeling and electron microscopy can be employed to understand the impact of grain boundaries on the behavior of photoexcited electrons and holes (including carrier transport and recombination). However, we also include discussion of materials relevant to rechargeable batteries as another important class of materials for energy applications. We conclude the review with a discussion of outstanding challenges in the field and the exciting prospects for progress in the coming years

    Exploring van der Waals materials with high anisotropy: geometrical and optical approaches

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    The emergence of van der Waals (vdW) materials resulted in the discovery of their giant optical, mechanical, and electronic anisotropic properties, immediately enabling countless novel phenomena and applications. Such success inspired an intensive search for the highest possible anisotropic properties among vdW materials. Furthermore, the identification of the most promising among the huge family of vdW materials is a challenging quest requiring innovative approaches. Here, we suggest an easy-to-use method for such a survey based on the crystallographic geometrical perspective of vdW materials followed by their optical characterization. Using our approach, we found As2S3 as a highly anisotropic vdW material. It demonstrates rare giant in-plane optical anisotropy, high refractive index and transparency in the visible range, overcoming the century-long record set by rutile. Given these benefits, As2S3 opens a pathway towards next-generation nanophotonics as demonstrated by an ultrathin true zero-order quarter-waveplate that combines classical and the Fabry-Perot optical phase accumulations. Hence, our approach provides an effective and easy-to-use method to find vdW materials with the utmost anisotropic properties.Comment: 11 pages, 5 figure

    Ohmic Behavior in Metal Contacts to n/p-Type Transition-Metal Dichalcogenides: Schottky versus Tunneling Barrier Trade-off

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    High contact resistance (RC) between 3D metallic conductors and single-layer 2D semiconductors poses major challenges toward their integration in nanoscale electronic devices. While in experiments the large RC values can be partly due to defects, ab initio simulations suggest that, even in defect-free structures, the interaction between metal and semiconductor orbitals can induce gap states that pin the Fermi level in the semiconductor band gap, increase the Schottky barrier height (SBH), and thus degrade the contact resistance. In this paper, we investigate, by using an in-house-developed ab initio transport methodology that combines density functional theory and nonequilibrium Green’s function (NEGF) transport calculations, the physical properties and electrical resistance of several options for n-type top metal contacts to monolayer MoS2, even in the presence of buffer layers, and for p-type contacts to monolayer WSe2. The delicate interplay between the SBH and tunneling barrier thickness is quantitatively analyzed, confirming the excellent properties of the Bi-MoS2 system as an n-type ohmic contact. Moreover, simulation results supported by literature experiments suggest that the Au-WSe2 system is a promising candidate for p-type ohmic contacts. Finally, our analysis also reveals that a small modulation of a few angstroms of the distance between the (semi)metal and the transition-metal dichalcogenide (TMD) leads to large variations of RC. This could help to explain the scattering of RC values experimentally reported in the literature because different metal deposition techniques can result in small changes of the metal-to-TMD distance besides affecting the density of possible defects

    Beam scanning by liquid-crystal biasing in a modified SIW structure

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    A fixed-frequency beam-scanning 1D antenna based on Liquid Crystals (LCs) is designed for application in 2D scanning with lateral alignment. The 2D array environment imposes full decoupling of adjacent 1D antennas, which often conflicts with the LC requirement of DC biasing: the proposed design accommodates both. The LC medium is placed inside a Substrate Integrated Waveguide (SIW) modified to work as a Groove Gap Waveguide, with radiating slots etched on the upper broad wall, that radiates as a Leaky-Wave Antenna (LWA). This allows effective application of the DC bias voltage needed for tuning the LCs. At the same time, the RF field remains laterally confined, enabling the possibility to lay several antennas in parallel and achieve 2D beam scanning. The design is validated by simulation employing the actual properties of a commercial LC medium

    Evaluation of Multi-frequency Synthetic Aperture Radar for Subsurface Archaeological Prospection in Arid Environments

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    The discovery of the subsurface paleochannels in the Saharan Desert with the 1981 Shuttle Imaging Radar (SIR-A) sensor was hugely significant in the field of synthetic aperture radar (SAR) remote sensing. Although previous studies had indicated the ability of microwaves to penetrate the earth’s surface in arid environments, this was the first applicable instance of subsurface imaging using a spaceborne sensor. And the discovery of the ‘radar rivers’ with associated archaeological evidence in this inhospitable environment proved the existence of an earlier less arid paleoclimate that supported past populations. Since the 1980’s SAR subsurface prospection in arid environments has progressed, albeit primarily in the fields of hydrology and geology, with archaeology being investigated to a lesser extent. Currently there is a lack of standardised methods for data acquisition and processing regarding subsurface imaging, difficulties in image interpretation and insufficient supporting quantitative verification. These barriers keep SAR technology from becoming as integral as other remote sensing techniques in archaeological practice The main objective of this thesis is to undertake a multi-frequency SAR analysis across different site types in arid landscapes to evaluate and enhance techniques for analysing SAR within the context of archaeological subsurface prospection. The analysis and associated fieldwork aim to address the gap in the literature regarding field verification of SAR image interpretation and contribute to the understanding of SAR microwave penetration in arid environments. The results presented in this thesis demonstrate successful subsurface imaging of subtle feature(s) at the site of ‘Uqdat al-Bakrah, Oman with X-band data. Because shorter wavelengths are often ignored due to their limited penetration depths as compared to the C-band or L-band data, the effectiveness of X-band sensors in archaeological prospection at this site is significant. In addition, the associated ground penetrating radar and excavation fieldwork undertaken at ‘Uqdat al-Bakrah confirm the image interpretation and support the quantitative information regarding microwave penetration

    Radio Measurements of Cosmic Rays at the South Pole

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    Die ultrahochenergetische kosmische Strahlung, die in der Erdatmosphäre massive Teilchenkaskaden (ausgedehnt Luftschauer) auslöst, kann am Erdboden mit Hilfe von Detektorfeldern gemessen werden. Unter den verschiedenen Detektoren, die zum Einsatz kommen, haben Radioantennen im letzten Jahrzehnt an Bedeutung gewonnen, da sie eine einzigartige Möglichkeit bieten diese Luftschauer zu untersuchen. Die Radioemission, die während der Entwicklung des Luftschauers hauptsächlich durch die Ablenkung der Elektronen und Positronen in der Teilchenkaskade durch das Erdmagnetfeld entsteht, enthält Informationen über die Art der Teilchen, die den Schauer ausgelöst haben. Insbesondere können Radioantennen zusammen mit Fluoreszenzteleskopen die Position des Maximums der Entwicklung des Luftschauers XmaxX_\mathrm{max} rekonstruieren. Dieser rekonstruierte Parameter ist abhängig von der Art des primären Atomkerns der kosmischen Strahlung, die den Luftschauer ausgelöst hat. Die Kenntnis des Typs der kosmischen Strahlung wiederum trägt zu einem besseren Verständnis der Beschleunigungsprozesse astrophysikalischer Quellen in unserem Universum bei. Das IceCube Neutrino Observatorium am geografischen Südpol ist ein Mehrzweckdetektor, der sowohl astrophysikalische Neutrinos, als auch Luftschauer nachweisen kann, insbesondere mit seinem Oberflächendetektor, IceTop. Um IceTop als Detektor für kosmische Strahlung zu verbessern und die Auswirkungen der Schneeansammlung abzuschwächen, soll in den kommenden Jahren ein hybrider Dektector aus anhebbaren Szintillationsplatten und Radioantennen installiert werden. Dieser Sub-Detektor wird aus 32 Stationen bestehen, die jeweils 8 Szintillationspaneele und 3 Antennen umfassen und eine Fläche von 1 km2^2 abdecken. Die Radioantennen nutzen mit 70 bis 350 MHz statt 30 bis 80 MHz ein höheres Frequenzband als bisher üblich. Der erste vollständige Prototyp einer Hybridstation wurde im Januar 2020 in Betrieb genommen. Diese Arbeit behandelt die Hardware der Prototyp-Station und der zukünftigen geplanten Stationen, die Inbetriebnahme der Daten der Prototyp-Station sowie eine Methode zur Energie- und XmaxX_\text{max}-Rekonstruktion, die auf der Grundlage gemessener Ereignisse und Monte-Carlo-Simulationen entwickelt wurde. Insbesondere wurde eine Struktur zum Anheben der Antennen über dem Schnee entworfen, gebaut, im Feld getestet und produziert, zusammen mit einer Radio-Frontend-Platine für die analoge Vorverarbeitung des von den Antennen empfangenen Signals. Die Kalibrierung der anderen Radiosignalkomponenten bei verschiedenen Temperaturen erreicht eine Amplitudenunsicherheit von nur 3,9%, was deutlich unter der geforderten Unsicherheit von 10% für die Radio-Signalkette liegt. Die Funktionsweise der Detektoren wurde durch die Analyse des Radio-Untergrunds unter Verwendung der entwickelten Radio-Datenanalysekette bestätigt. Es wurden insgesamt 121 Luftschauer nachgewiesen, von denen 5 auch durch die anderen Detektoren nachgewiesen wurden. Sechszehn Luftschauer wurden verwendet, um die erste Energie- und XmaxX_\text{max}-Rekonstruktionsmethode für die Radiokomponente der Detektorerweiterung zu entwickeln. Diese Rekonstruktionsmethode basiert auf dem neuesten Stand der Technik für Radio-Detektoren. Es wurde eine Analyse des Einflusses des Radio-Untergrundes auf das Signal durchgeführt. Anschließend wird die üblicherweise verwendete Methode der χ2\chi^2-Minimierung durch eine Log-Likelihood-Minimierung mit einer Parametrisierung des Rauschens ersetzt, und es wird gezeigt, dass diese Technik mit den gemessenen Daten funktioniert. Darüber hinaus zeigt sich, dass bei denselben rekonstruierten Ereignissen das Hochfrequenzband mit den nur drei Antennen der Prototypstation eine deutlich bessere Genauigkeit als das traditionelle Niedrigfrequenzband aufweist. Sobald der gesamte Detektor fertiggestellt ist, wird die erwartete Rekonstruktionsgenauigkeit auf 15 g/cm2^2 für XmaxX_\text{max} und besser als 10% für die Energie geschätzt

    Mechanical tailoring of dislocations in ceramics at room temperature: A perspective

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    The potential of dislocations (line defects) in ceramics may have been greatly underrated until most recently. Promising proofs-of-concept have been demonstrated for dislocation-tuned functional and mechanical properties, revealing a new research front for dislocations in ceramics for a wide range of potential applications. However, it is commonly known that ceramics are hard (difficult to deform) and brittle (easy to fracture), particularly at room temperature. It remains a great challenge to mechanically tailor dislocations in ceramics. To address this pressing bottleneck, this article discusses the mechanics-based dislocation engineering in ceramics by examining the three fundamental factors of dislocation nucleation, multiplication, and motion. Successful experimental approaches to tune dislocation density and plastic zone size on single-crystal strontium titanate are demonstrated. The dislocation-based competition between plastic deformation and crack formation is discussed. The aspects of coupling external fields to manipulate dislocations are highlighted, which may hold the key to modulating the charged dislocation cores in ceramics and opening new routes for mechanical tailoring of dislocations at room temperature. Some open questions and challenges for engineering dislocations in ceramics are discussed

    Estimating Solar Energy Production in Urban Areas for Electric Vehicles

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    Cities have a high potential for solar energy from PVs installed on buildings\u27 rooftops. There is an increased demand for solar energy in cities to reduce the negative effect of climate change. The thesis investigates solar energy potential in urban areas. It tries to determine how to detect and identify available rooftop areas, how to calculate suitable ones after excluding the effects of the shade, and the estimated energy generated from PVs. Geographic Information Sciences (GIS) and Remote Sensing (RS) are used in solar city planning. The goal of this research is to assess available and suitable rooftops areas using different GIS and RS techniques for installing PVs and estimating solar energy production for a sample of six compounds in New Cairo, and explore how to map urban areas on the city scale. In this research, the study area is the new Cairo city which has a high potential for harvesting solar energy, buildings in each compound have the same height, which does not cast shade on other buildings affecting PV efficiency. When applying GIS and RS techniques in New Cairo city, it is found that environmental factors - such as bare soil - affect the accuracy of the result, which reached 67% on the city scale. Researching more minor scales, such as compounds, required Very High Resolution (VHR) satellite images with a spatial resolution of up to 0.5 meter. The RS techniques applied in this research included supervised classification, and feature extraction, on Pleiades-1b VHR. On the compound scale, the accuracy assessment for the samples ranged between 74.6% and 96.875%. Estimating the PV energy production requires solar data; which was collected using a weather station and a pyrometer at the American University in Cairo, which is typical of the neighboring compounds in the new Cairo region. It took three years to collect the solar incidence data. The Hay- Devis, Klucher, and Reindl (HDKR) model is then employed to extrapolate the solar radiation measured on horizontal surfaces β =0°, to that on tilted surfaces with inclination angles β =10°, 20°, 30° and 45°. The calculated (with help of GIS and Solar radiation models) net rooftop area available for capturing solar radiation was determined for sample New Cairo compounds . The available rooftop areas were subject to the restriction that all the PVs would be coplanar, none of the PVs would protrude outside the rooftop boundaries, and no shading of PVs would occur at any time of the year; moreover typical other rooftop occupied areas, and actual dimensions of typical roof top PVs were taken into consideration. From those calculations, both the realistic total annual Electrical energy produced by the PVs and their daily monthly energy produced are deduced. The former is relevant if the PVs are tied to a grid, whereas the other is more relevant if it is not; optimization is different for both. Results were extended to estimate the total number of cars that may be driven off PV converted solar radiation per home, for different scenarios

    Band structure renormalization at finite temperatures from first principles

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    In dieser Doktorarbeit untersuchen wir den Einfluss von Elektron-Phonon-Wechselwirkungen (EPW) auf die Bandlueckenrenormierung in kristallinen Festkoerpern bei endlichen Temperaturen. Das Hauptziel besteht darin, den Einfluss der Kernbewegung und der thermischen Ausdehnung des Gitters auf die Bandstruktur in einer Vielzahl von Materialien zu quantifizieren. Zu diesem Zweck wird der Temperatureinfluss auf das EPW in harmonischen Naeherungen unter Verwendung der stochastischen Abtastmethode und vollstaendig anharmonisch durch Durchführung von ab initio Molekulardynamiksimulationen (aiMD). Die Bandluecke bei endlichen Temperaturen wird aus der thermodynamisch gemittelten Spektralfunktion extrahiert, die unter Verwendung der Bandentfaltungstechnik berechnet wird. Waehrend die Verwendung von aiMD bereits fuer Berechnungen von EPW verwendet wurde, wurde die Kombination von aiMD und Bandentfaltung zur Behandlung der Bandluecken renormalisierung erst kuerzlich verwendet. In dieser Doktorarbeit haben wir eine verbesserte Bandentfaltungstechnik verwendet, um die Berechnung effektiv zu verwalten. Diese verbesserte Methode enthaelt mehrere methodische Neuerungen, die dazu dienen, den Rechenaufwand zu verringern und das statistische Rauschen in den Endergebnissen zu minimieren. Die aktualisierte Methode wurde gruendlich bewertet, dokumentiert und mit einer benutzerfreundlichen Oberflaeche gestaltet. Wir praesentieren eine umfassende Untersuchung der numerischen Aspekte der thermodynamischen Mittelung, der Schaetzung von Fehlerbalken und der Bewertung der Konvergenz in Bezug auf die Groesse der Simulationssuperzelle. Unser etabliertes Protokoll ermoeglicht die Berechnung der Bandlückenrenormierung bei endlichen Temperaturen, was in guter Uebereinstimmung mit frueheren theoretischen Studien und experimentellen Daten steht.In this thesis, we investigate the influence of electron-phonon interactions (EPI) on the band gap renormalization in crystalline solids at finite temperatures. The main goal is to identify the impact of the nuclear motion and the lattice thermal expansion on the band structure in a wide range of materials. For this purpose, the temperature influence on the EPI is calculated in the harmonic approximations by utilizing the stochastic sampling methodology and fully anharmonically, by performing ab initio molecular dynamics simulations (aiMD). The band gap at finite temperatures is extracted from the thermodynamically averaged spectral function, which is calculated using band-unfolding technique. While utilization of aiMD was already used for calculations of EPI the combination of aiMD and band-unfolding to treat the band gap renormalization was used only recently. In this thesis, we employed an improved band unfolding technique in order to effectively manage the calculations. This improved method incorporates several methodological innovations that serve to mitigate computational cost and minimize statistical noise in the final results. The updated method was thoroughly benchmarked, documented, and designed with a user-friendly interface. We present a comprehensive examination of the numerical aspects of thermodynamic averaging, the estimation of error bars, and the evaluation of convergence with respect to the size of the simulation supercell. Our established protocol enables the calculation of band gap renormalization at finite temperatures, which is in good agreement with prior theoretical studies and experimental data

    The study of renal function and toxicity using zebrafish (Danio rerio) larvae as a vertebrate model

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    Zebrafish (Danio rerio) is a powerful model in biomedical and pharmaceutical sciences. The zebrafish model was introduced to toxicological sciences in 1960, followed by its use in biomedical sciences to investigate vertebrate gene functions. As a consequence of many research projects in this field, the study of human genetic diseases became instantly feasible. Consequently, zebrafish have been intensively used in developmental biology and associated disciplines. Due to the simple administration of medicines and the high number of offspring, zebrafish larvae became widely more popular in pharmacological studies in the following years. In the past decade, zebrafish larvae were further established as a vertebrate model in the field of pharmacokinetics and nanomedicines. In this PhD thesis, zebrafish larvae were investigated as an earlystage in vivo vertebrate model to study renal function, toxicity, and were applied in drug-targeting projects using nanomedicines. The first part focused on the characterization of the renal function of three-to four-dayold zebrafish larvae. Non-renal elimination processes were additionally described. Moreover, injection techniques, imaging parameters, and post-image processing scripts were established to serve as a toolbox for follow-up projects. The second part analyzed the impact of gentamicin (a nephrotoxin) on the morphology of the pronephros of zebrafish larvae. Imaging methodologies such as fluorescent-based laser scanning microscopy and X-ray-based microtomography were applied. A profound comparison study of specimens acquired with different laboratory X-ray-based microtomography devices and a radiation facility was done to promote the use of X-ray-based microtomography for broader biomedical applications. In the third part, the toxicity of nephrotoxins on mitochondria in renal epithelial cells of proximal tubules was assessed using the zebrafish larva model. Findings were compared with other teleost models such as isolated renal tubules of killifish (Fundulus heteroclitus). In view of the usefulness and high predictability of the zebrafish model, it was applied to study the pharmacokinetics of novel nanoparticles in the fourth part. Various in vivo pharmacokinetic parameters such as drug release, transfection of mRNA/pDNA plasmids, macrophage clearance, and the characterization of novel drug carriers that were manipulated with ultrasound were assessed in multiple collaborative projects. Altogether, the presented zebrafish model showed to be a reliable in vivo vertebrate model to assess renal function, toxicity, and pharmacokinetics of nanoparticles. The application of the presented model will hopefully encourage others to reduce animal experiments in preliminary studies by fostering the use of zebrafish larvae
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