Gezielte Manipulation einzelner Defekte in Schichtkristallen und 2D-Materialien

The structure-property relationship has always been one of the focal topics of materials science. Of the different structural aspects of a material that need to be considered, defects take a central role. Defects can fundamentally alter the intrinsic behavior of a material, which is why there has always been a major stride towards the control of defects. While many established routes exist to control the ensemble-characteristics of defects, the direct control of individual defects is still in its infancy. Different technological advances such as new microscopy techniques and nanomanipulation are vital in this development. Using direct control, the fundamental characteristics of defects can be understood and, in the future, used as design space for novel materials. In this work new routes are explored towards the direct manipulation of individual defects in crystalline solids using advanced in situ electron microscopy. Layered and 2D-materials are used as model systems, due to their well-defined nature. Individual sp³-type point defects in graphene, induced by covalently attached moieties, are made visible using high resolution transmission electron microscopy (HRTEM). A mechanical cleaning method, based on piezo-driven micromanipulation, is conceived which allows for the selective removal of adsorbed contaminants while leaving covalent bonds intact. The method reaches site-specific atomic cleanliness, which allows for an extended discussion of re-contamination effects and the question of whether clean is actually clean. Using the gathered knowledge about cleaning, an electron beam induced synthesis of a turbostratic graphene layer is observed in situ. The sp³-type point defects are confirmed by the comparison against complementary molecular dynamics simulations and dynamic HRTEM image simulations. Using electron beam illumination, a dynamic movement of the defects in the lattice can be stimulated. Going up in dimensionality, line defects, known as dislocations are analyzed in bilayer graphene, which is the thinnest material that can host those defects. Using an advanced in situ setup in scanning electron microscopy (SEM), that involves three independent micro-manipulators, the direct manipulation of individual dislocations becomes feasible. To understand the contrasts obtained from SEM in transmission, diffraction patterns are obtained. To facilitate this process, a novel diffraction setup for SEM is devised. Low energy nano diffraction (LEND) can be used to acquire spot-like diffraction patterns in SEM in transmission when the sample is thin enough. The method is non-invasive, cost-effective and retro-fittable to virtually any SEM. The possibilities of the system are shortly explored and discussed. The dislocation manipulation is used to confirm the fundamental properties of those defects, such as line tension, interaction with free edges and mutual interactions. While all expected interactions could be demonstrated, also a new switching mechanism was found that allows for the reversible division and connection of areas of different stacking orders. This finding is used to propose a functional device purely based on dislocations. An attempt is made at measuring the mechanical forces exerted by individual dislocations during manipulation. A nanowire-based approach of force measurement is used, giving promising, but as of yet inconclusive, results. In the third main part, individual twist grain boundaries are created and manipulated. As the material, layered vanadium diselenide (VSe2) is used. To generate the interfaces, first inclined micro-pillars are cut using focused ion beam (FIB) techniques. A subsequent compression step performed in situ in SEM leads to slip along a single interface. In certain cases, before sliding, a rotation is introduced that facilitates the slip process. After the creation of individual grain boundaries, they are manipulated to measure the intrinsic shear strength of the material as well as grain boundary sliding friction. A state of very low friction called superlubricity was found for certain rotation angles and the intrinsic interlayer adhesion energy of VSe2 was determined. Different routes towards a more precise control of the grain rotation as well as the possibility to perform the experiments in situ in TEM are explored. The fundamental movements of rotation and translation of a single twist grain boundary are demonstrated in situ in plan-view TEM. All in all, this work demonstrates several viable routes towards the control of individual defects using in situ microscopy. The next step towards true defect engineering lies in the measurement of the functional properties of single defects.Der Zusammenhang zwischen Struktur und Eigenschaften von Materialien stellt ein zentrales Thema in den Materialwissenschaften dar. Von den vielen strukturellen Aspekten, die einen Einfluss auf die Eigenschaften eines Materials haben, nehmen Defekte eine herausragende Rolle ein. Durch die Untersuchung und das kontrollierte Einbringen von Defekten können Materialien für bestimmte Anwendungen gezielt angepasst werden. In der Vergangenheit wurden viele unterschiedliche Verfahren entwickelt, Defekte in ihrer Gesamtheit zu beeinflussen. Eine neuere Entwicklung stellt hingegen der Versuch dar, direkt einzelne Defekte zu manipulieren. Die Manipulation von einzelnen Defekten ermöglicht es, deren individuelle Eigenschaften zu verstehen und ist somit ein bedeutender Beitrag für die zukünftige Forschung an neuen Materialsystemen. In dieser Dissertation werden verschiedene Wege aufgezeigt, einzelne Defekte direkt zu beeinflussen. Als Hilfsmittel dient dabei die in situ Elektronenmikroskopie, welche die nötige Auflösung mitbringt, Defekte überhaupt erst sichtbar zu machen. Die Studie wurde an Schichtkristallen und 2D-Materialien durchgeführt, da diese eine einzigartige, hochanisotrope Struktur besitzen. Zunächst werden einzelne, an Graphen kovalent angebundene Moleküle mittels hochauflösender Transmissions-elektronenmikroskopie dargestellt. Die an Graphen angebundenen Moleküle induzieren einen lokalen Defekt in der Hybridisierung der Molekülorbitale im Graphen. Um die Abbildung zu ermöglichen, wird eine neue mechanische Reinigungsmethode eingeführt, die selektiv Adsorbate auf einer Oberfläche entfernen kann, während kovalente Bindungen intakt bleiben. Die Methode erreicht atomare Sauberkeit, was weiterführende Experimente bezüglich Rekontamination zulässt. Zusätzlich kann die Frage, was atomar „sauber“ bedeutet, geklärt werden. Das Wissen darüber wird verwendet, um eine zusätzliche Graphenschicht in situ wachsen zu lassen. Die Kontraste der kovalent angebundenen Moleküle werden mit simulierten Bildern verglichen, um die Struktur der funktionellen Gruppen zu bestätigen. Durch den Elektronenstrahl kann kann eine Migration der Moleküle in Form von Platzwechseln im Graphengitter. Weiterführend werden Liniendefekte, sogenannte Versetzungen, in zweilagigem Graphen untersucht. Um diese direkt beeinflussen zu können, wurde ein komplexer Versuchsaufbau in einem Rasterelektronenmikroskop (REM) entwickelt. In diesem Aufbau werden drei unabhängige Mikromanipulatoren eingesetzt, die einerseits für die beidseitige Reinigung des Graphen und andererseits für die Manipulation der Versetzungen zuständig sind. Um die Versetzungskontraste im REM in Transmission zu verstehen, wurde ein neuartiges Beugungssystem entwickelt. Dieses System kann in beliebige Rasterelektronenmikroskope integriert werden und ermöglicht die Aufnahme von Punktbeugungsbildern. Die Möglichkeiten, die sich mit dieser Technik ergeben, werden anhand von Beispielen dargestellt und diskutiert. Die direkte Manipulation von Versetzungen zeigt einige der fundamentalen Eigenschaften dieser Defekte auf. Dabei werden die Effekte von Linienspannung, die Interaktion mit freien Oberflächen und die gegenseitige Beeinflussung von Versetzungen untersucht. Auf der Basis gezielter Versetzungsreaktionen wird ein neuer Schaltmechanismus postuliert, der Flächen mit gleicher Stapelfolge reversibel zerteilen oder vereinen kann, was als Grundlage für ein elektronisches Bauelement dienen könnte. Darüber hinaus zeigte ein erster Versuch, die rücktreibende Kraft einzelner Versetzungen zu messen, vielversprechende Ergebnisse. Der dritte Hauptteil befasst sich mit der gezielten Erzeugung von Drehkorngrenzen. Hierbei wird das Material Vanadium Diselenid verwendet, in welches Mikrosäulen mittels fokussiertem Ionenstrahl geschnitten werden. Werden diese Mikrosäulen komprimiert, ergibt sich ein Abgleiten entlang einer einzelnen Kristallebene. Durch eine zusätzliche Rotation beim Abgleiten entsteht eine Drehkorngrenze. Weiterführende Manipulationen dieser Drehkorngrenzen demonstrieren den Effekt des Korngrenzgleitens. Je nach Rotationswinkel ist dieser Prozess mit unterschiedlich hohen Reibungskräften verbunden, die analysiert werden. Zuletzt wird eine Testgeometrie vorgeschlagen, mittels derer eine Korngrenzmanipulation in situ im Transmissionselektronenmikroskop durchgeführt werden kann. Die vorliegende Arbeit zeigt verschiedene Wege auf, einzelne Defekte unterschiedlicher Dimensionalität auf unterschiedlichen Längenskalen gezielt zu manipulieren. Diese Manipulation ermöglicht neuartige Einsichten in die fundamentalen Eigenschaften von Defekten und Materialien

Laboratory Experiments on the Effects of Blade Strike from Hydrokinetic Energy Technologies on Larval and Juvenile Freshwater Fishes

There is considerable interest in the development of marine and hydrokinetic energy projects in rivers, estuaries, and coastal ocean waters of the United States. Hydrokinetic (HK) technologies convert the energy of moving water in river or tidal currents into electricity, without the impacts of dams and impoundments associated with conventional hydropower or the extraction and combustion of fossil fuels. The Federal Energy Regulatory Commission (FERC) maintains a database that displays the geographical distribution of proposed HK projects in inland and tidal waters (FERC 2012). As of March 2012, 77 preliminary permits had been issued to private developers to study HK projects in inland waters, the development of which would total over 8,000 MW. Most of these projects are proposed for the lower Mississippi River. In addition, the issuance of another 27 preliminary permits for HK projects in inland waters, and 3 preliminary permits for HK tidal projects (totaling over 3,100 MW) were under consideration by FERC. Although numerous HK designs are under development (see DOE 2009 for a description of the technologies and their potential environmental effects), the most commonly proposed current-based projects entail arrays of rotating devices, much like submerged wind turbines, that are positioned in the high-velocity (high energy) river channels. The many diverse HK designs imply a diversity of environmental impacts, but a potential impact common to most is the risk for blade strike to aquatic organisms. In conventional hydropower generation, research on fish passage through reaction turbines at low-head dams suggested that strike and mortality for small fish could be low. As a consequence of the large surface area to mass ratio of small fish, the drag forces in the boundary layer flow at the surface of a rotor blade may pull small fish around the leading edge of a rotor blade without making physical contact (Turnpenny 1998, Turnpenny et al. 2000). Although there is concern that small, fragile fish early life stages may be unable to avoid being struck by the blades of hydrokinetic turbines, we found no empirical data in the published literature that document survival of earliest life-stage fish in passage by rotor blades. In addition to blade strike, research on passage of fish through conventional hydropower turbines suggested that fish mortalities from passage through the rotor swept area could also occur due to shear stresses and pressure chances in the water column (Cada et al. 1997, Turnpenny 1998). However, for most of the proposed HK turbine designs the rotors are projected to operate a lower RPM (revolutions per minute) than observed from conventional reaction turbines; the associated shear stress and pressure changes are expected to be lower and pose a smaller threat to fish survival (DOE 2009). Only a limited number of studies have been conducted to examine the risk of blade strike from hydrokinetic technologies to fish (Turnpenny et al. 1992, Normandeau et al. 2009, Seitz et al. 2011, EPRI 2011); the survival of drifting or weakly swimming fish (especially early life stages) that encounter rotor blades from hydrokinetic (HK) devices is currently unknown. Our study addressed this knowledge gap by testing how fish larvae and juveniles encountered different blade profiles of hydrokinetic devices and how such encounters influenced survivorship. We carried out a laboratory study designed to improve our understanding of how fish larvae and juvenile fish may be affected by encounters with rotor blades from HK turbines in the water column of river and ocean currents. (For convenience, these early life stages will be referred to as young of the year, YOY). The experiments developed information needed to quantify the risk (both probability and consequences) of rotor-blade strike to YOY fish. In particular, this study attempted to determine whether YOY drifting in a high-velocity flow directly in the path of the blade leading edge will make contact with the rotor blade or will bypass the blade while entrained in the boundary layer of water flowing over the blade surface. The study quantified both immediate and delayed mortalities (observed immediately, 3 hours, and 24 hours after encountering the blade) among freshwater YOY fish resulting from contact with the blade or turbulent flows in the wake of the blade

In situ ultrafine force measurement with nanowire based cantilevers in SEM

In nanomechanics the measurement of ultrafine forces becomes increasingly important for unravelling subtle details of elastic and plastic deformation processes. In particular, achieving high force resolution in combination with in situ imaging is a major challenge which is becoming exceedingly difficult with conventional methods. In this work, we introduce a novel systematic method to measure ultrafine forces using well-defined nanowires as cantilever beams in situ in the Scanning Electron Microscope (SEM). Forces can be measured variably in the range from micro-newtons (mN) down to femto-newtons (fN), depending on the chosen reference nanowire. The reference wires are picked with a manipulator tip without the use of FIB (see Figure 1 a). Please click Additional Files below to see the full abstract

The Application of Traits-Based Assessment Approaches to Estimate the Effects of Hydroelectric Turbine Passage on Fish Populations

One of the most important environmental issues facing the hydropower industry is the adverse impact of hydroelectric projects on downstream fish passage. Fish that migrate long distances as part of their life cycle include not only important diadromous species (such as salmon, shads, and eels) but also strictly freshwater species. The hydropower reservoirs that downstream-moving fish encounter differ greatly from free-flowing rivers. Many of the environmental changes that occur in a reservoir (altered water temperature and transparency, decreased flow velocities, increased predation) can reduce survival. Upon reaching the dam, downstream-migrating fish may suffer increased mortality as they pass through the turbines, spillways and other bypasses, or turbulent tailraces. Downstream from the dam, insufficient environmental flow releases may slow downstream fish passage rates or decrease survival. There is a need to refine our understanding of the relative importance of causative factors that contribute to turbine passage mortality (e.g., strike, pressure changes, turbulence) so that turbine design efforts can focus on mitigating the most damaging components. Further, present knowledge of the effectiveness of turbine improvements is based on studies of only a few species (mainly salmon and American shad). These data may not be representative of turbine passage effects for the hundreds of other fish species that are susceptible to downstream passage at hydroelectric projects. For example, there are over 900 species of fish in the United States. In Brazil there are an estimated 3,000 freshwater fish species, of which 30% are believed to be migratory (Viana et al. 2011). Worldwide, there are some 14,000 freshwater fish species (Magurran 2009), of which significant numbers are susceptible to hydropower impacts. By comparison, in a compilation of fish entrainment and turbine survival studies from over 100 hydroelectric projects in the United States, Winchell et al. (2000) found useful turbine passage survival data for only 30 species. Tests of advanced hydropower turbines have been limited to seven species - Chinook and coho salmon, rainbow trout, alewife, eel, smallmouth bass, and white sturgeon. We are investigating possible approaches for extending experimental results from the few tested fish species to predict turbine passage survival of other, untested species (Cada and Richmond 2011). In this report, we define the causes of injury and mortality to fish tested in laboratory and field studies, based on fish body shape and size, internal and external morphology, and physiology. We have begun to group the large numbers of unstudied species into a small number of categories, e.g., based on phylogenetic relationships or ecological similarities (guilds), so that subsequent studies of a few representative species (potentially including species-specific Biological Index Testing) would yield useful information about the overall fish community. This initial effort focused on modifying approaches that are used in the environmental toxicology field to estimate the toxicity of substances to untested species. Such techniques as the development of species sensitivity distributions (SSDs) and Interspecies Correlation Estimation (ICE) models rely on a considerable amount of data to establish the species-toxicity relationships that can be extended to other organisms. There are far fewer studies of turbine passage stresses from which to derive the turbine passage equivalent of LC{sub 50} values. Whereas the SSD and ICE approaches are useful analogues to predicting turbine passage injury and mortality, too few data are available to support their application without some form of modification or simplification. In this report we explore the potential application of a newer, related technique, the Traits-Based Assessment (TBA), to the prediction of downstream passage mortality at hydropower projects

Microwave plasma-assisted reactive HiPIMS of InN films: Plasma environment and material characterisation

This work focuses on the low temperature fabrication process of InN thin films via microwave plasma-assisted reactive high power impulse magnetron sputtering (MAR-HiPIMS). The influence of microwave plasma on the HiPIMS discharge process at various nitrogen flows and microwave powers was monitored and characterised through in situ diagnostics, including following HiPIMS I(V,t) curves, optical emission spectroscopy (OES), as well as performing time-resolved Langmuir probe and time-of-flight mass spectroscopy (ToF-MS) measurements. This was followed by the deposition of InN films via standard reactive HiPIMS (reference sample) and MAR-HiPIMS and their characterisation via X-ray diffraction (XRD), reflectometry (XRR), as well as scanning and transmission electron microscopy (SEM, TEM). It was found that the microwave plasma facilitates the dissociation/activation of nitrogen species and supplies seed electrons to the magnetron discharge plasma. Furthermore, the energy of the incoming ions was determined via ToF-MS, and it was possible to identify their plasma origin and temporal behaviour. The produced R-HiPIMS sample was highly metallic, with no nitride phase detected. The MAR-HiPMS film, however, was stoichiometric and exhibited (0002) direction texturing, with an optical bandgap of approx. 1.5 eV, electron concentration of 2.72 × 1020 cm−3 and electron mobility of 7.16 cm2V−1 s−1 (in the range for polycrystalline InN)

1028-63 Cardiac Troponin T as a Marker for Perioperative Myocardial Ischemia in Noncardiac Surgical Patients

Episodes of perioperative myocardial ischemia (PMI) occur in 18 to 74% of noncardiac surgical patients with or at risk for coronary artery disease (CAD). PMls correlate with adverse postoperative cardiac outcome. To determine the diagnostic value of cardiac troponin T(TnT) in PMI, we studied 28 patients (63.9±8.9 years) undergoing peripheral vascular surgery (n=16) or carotid endarterectomy (n=12). Patients included had either documented CAD (n=16) or two (n=7) or more (n=5) risk factors (age >65 years, smoking. diabetes mellitus. hypertension, or hypercholesterolemia >240 mg/dL). Patients with uninterpretable ECG for PMI were excluded. 12-lead ECG recordings and blood sampling for measurement of CK-MB activity and TnT levels (ELISA troponin 1. Boehringer Mannheim, Germany) were carried out preoperatively, and immediately, 20 h, 48 h, 72 h, and 84 h postoperatively. ECG recordings were analysed by an independent blinded cardiologist for signs of PMI (new ST segment depression >0.1 mV 60 ms after the J point, new T inversion). We found an overall incidence of ECG documented PMI of 54% (n=15), 93% occuring immediately postoperatively. Patients undergoing peripheral vascular surgery developed significantly less PMI than carotid surgical patients (38% vs. 75%; p<0.05, Fisher's Exact test). TnT levels >0.1 μg/L were found in 80% (n=12) of patients with PMI (ECG). Only one patient without ECG-documented PMI had TnT levels >0.1 μg/L. Thus, comparing a TnT cut off level of 0.1 μg/L with intermittent 12-lead ECG recording, we found a sensitivity of 80% and a specifity of 92%. We were unable to detect elevated levels of CK-MB in any patient (tab.).overall (n=28)per. vase. surg. (n=16)carotid surg. (n=12)CK-MB >6 U/L000ECG(PMI)15(54%)6 (38%)9(75%)TnT >0.1 μg/L13(46%)5(31%)8(67%)ConclusionIn contrast to CK-MB, TnT is a specific and sensitive marker of PMI in patients with or at risk for CAD undergoing noncardiac surgery