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Surface phonon polaritons in silicon carbide nanostructures revealed by near-field imaging and spectroscopy
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Entwicklung von emissionsarmen kondensationsvernetzenden Silicondichtstoffen
Get PDFEs wurden verschiedene Konzepte zur Reduktion der bei der Kondensationsvernetzung von Silicondichtstoffen entstehenden Emissionen erarbeitet und getestet. Die Ansätze die bei der Vernetzung freiwerdenden Moleküle durch modifizierte Kieselsäure oder durch den Zusatz von Substanzen, die diese Gruppen chemisch umwandeln sollten, erwiesen sich als nicht praktikabel. Auch der Versuch Härter einzusetzen, deren Abgangsgruppen anschließend polymerisieren und so im Dichtstoff-Bulk gehalten werden, schlug fehl. Der Einsatz von neu entwickelten Härtern auf Basis von Silatranen und strukturell verwandten Verbindungen sowie eines mehrfach cyclisierten Silans brachten den gewünschten Erfolg. Dabei werden die Abgangsgruppen entweder durch deren chemisch-physikalischen Eigenschaften nicht emittiert oder sie sind kovalent an die Härtermoleküle gebunden. So konnten die Emissionen auf 5-15 Prozent eines etablierten Dichtstoffs gesenkt werden.Several different concepts for the reduction of the emissions caused by the condensation crosslinking of silicone sealants were developed and tested. Approaches where the molecules which are released during the crosslinking should be catched by modified pyrogenic silica or chemically converted by the addition of some substances showed no practical use. Also the trial to use crosslinkers which have leaving groups that can polymerize and remain in the sealant bulk failed. The use of new developed crosslinkers based on silatranes and structural related compounds as well as a multi cycled silane showed the wanted effect. With this concept, leaving groups are not emitted because of their chemical-physical properties or they are covalent bonded to the crosslinker molecule. It was possible to reduce the emissions to 5-15 percent of a established sealant
Transferable organic/inorganic nanosheets for van der Waals thin-film transistors on trap-passivated dielectrics
Get PDFHeutige Silizium-MOSFET nutzen die besonderen Eigenschaften von Silizium-Nanosheets für leistungsfähige Mikroprozessoren bei geringem Stromverbrauch. Parallel dazu wird an natürlich vorkommenden Übergangsmetall-Dichalcogeniden als ebenfalls vielversprechende Kandidaten für die nächste Generation von Nanosheet-Transistoren geforscht. Anders als beide Vorgenannten ist die Rolle von organischen Nanosheets in diesem Zusammenhang trotz ihrer überragenden Eigenschaften als Photodioden schwer zu fassen. Im Rahmen dieser Arbeit beschäftigen wir uns mit einer neuartigen Transfer-Technologie für organische Halbleiter, die wir selbst entwickelt haben und untersuchen den Ladungsträgertransport in transferierbaren, organischen und inorganischen van der Waals Nanosheets. Wir verwenden ein wasserlösliches Polymer als Unterlage für den Wachstumsprozess, das in einer geeigneten Anordnung benetzt und aufgelöst wird. Diese Methode gestattet die Ablösung dieser Nanosheets von Drei-Zoll-Wafer. Mit der Hilfe von Rasterkraftmikroskopie und Röntgenstreuexperimenten können wir die hohe Kristallinität der transferierten Nanosheets bestätigen. Wir untersuchen die Leistungsfähigkeit transferierter organischer Transistoren mit Kontakt auf der Unterseite, welche aufgrund der im Übertragungsprozess erhaltenen hohen Kristallinität bessere Ergebnisse liefern als solche aus bloß abgeschiedenen Schichten. Anschließend stellen wir Transistoren mit van der Waals Hetero-Übergängen auf Siliziumdioxid mit transferierten organischen Nanosheets und atomardünnen Halbleitern her. Die erstgenannten dienen als p-Leiter, die zweitgenannten als n-Leiter. Sowohl Elektronen als auch Löcher tragen zum Ladungstransport bei und zeigen eine ambipolare Charakteristik mit vergleichbaren Werten für die Ladungsträgermobilität.
Die quantitative Analyse des ambipolaren Transistors und eines unipolaren Transistors aus MoS2 auf einer unbehandelten 100 nm-dicken Siliziumdioxid-Schicht ergibt eine Unterschwellensteilheit von 2,400 mVdec-1 bis 1,000 mVdec-1. Bei Raumtemperatur kann die Unterschwellensteilheit einen Abfall von bis zu 60 mV pro Dekade des Drain-Stroms erreichen. Jede signifikante Abweichung vom idealen Verhalten in Richtung höherer Werte deutet auf lokale Grenzflächen-Zustände hin. Das Passivieren der Grenzflächenzustände des Aluminiumoxids bei unipolaren, organischen Nanosheet-Transistoren mit kovalent gebundenen, selbstorganisierten Monoschichten ermöglichen steilere Werte der Unterschwellensteilheit. Jedoch verhindern Benetzungsprobleme der SAM mit Chemikalien, die üblicherweise bei der Photolithographie zum Einsatz kommen, die zur Herstellung ambipolarer und unipolarer MoS2- Transistoren verwendete Oxid-Passivierung. Zur Lösung dieses Problems entwickeln wir eine neue, lithographie-kompatible Methode zur Grenzschichtpassivierung mit Hilfe einer ultra-dünnen Polymerschicht. Unsere Untersuchungen ergeben, dass der Ladungsträgertransport in Transistoren durch sog. Springen zwischen durch nichtabgesättigte Bindungen hervorgerufenen tiefen Störstellen dominiert wird. Diese treten bei unbehandelten Oxiden auf und führen zu schlechtem Unterschwellen-Verhalten. Hinzu kommt, dass das “Hopping” von Ladungsträgern in tiefen Störstellen die Bestimmung einer korrekten Transistor-Charakteristik verhindert. Hingegen zeigen Transistoren mit einer passivierten 100 nm-dicken Siliziumdioxidschicht Hopping-Transport über Randzustände mit wenigen kBT unterhalb des Leitungsbands und eine substantiell stärkere Unterschwellensteilheit von 189 mVdec-1. Wenngleich durch die Passivierung die benötigte Drain-Spannung auf 5 V reduziert wird, um den Sättigungsbereich zu erreichen, so zeigt der MoS2-Transistor doch eine nicht-ideale Sättigungscharakteristik.
Dieses nicht-ideale Sättigungsverhaltenen der Transistoren auf unpassivierten, 100 nm- dicken Siliziumdioxidschichten weist darauf hin, dass eine weitere Verbesserung der Unter- schwellensteilheit erforderlich ist, um die Dominanz der Grenzflächenzustände gegenüber den intrinsischen Eigenschaften des Halbleiters zu beseitigen. Wir verwenden eine lokale Gatter-Struktur mit Aluminium als Gatter-Metall und verstärken das native Oxid des Aluminiums mit einem Sauerstoff-Plasma-Prozess zur Erzeugung eines hochwertigen, ultradünnen Aluminiumoxids. Aluminiumoxid ermöglicht die Verwendung von konvalent bindenden SAM zur Grenzflächenpassivierung. Durch Verkleinerung der Passivierungsfläche innerhalb der Architektur des lokalen Gatters können wir die Bentzungsprobleme lösen, die zuvor bei solchen SAM-Oberflächen beobachtet worden sind. Die Kapazität des Plasma-Oxids vergrößert sich, verglichen mit dem passivierten 100 nm-dicken Siliziumdioxid, um das Zwanzigfache. Dadurch reduziert sich der Bereich für die Gate-Spannung auf ±0.75 V, was dazu führt, dass die Sättigungsbedingungen bereits bei einer Drain-Spannung von 1 V erreicht werden können. Mit solch ultradünnen Gate-Oxiden weisen MoS2-Transistoren eine lehrbuchhafte Charakteristik auf: geringe Betriebsspannung, starke Unterschwellensteilheit von 61.6 mVdec-1 und Ladungsträgerbeweglichkeiten von ungefähr 5 cm2V-1s-1.
Diese Ergebnisse betonen, dass die Herstellung effizienter Nanosheet-Transistoren mit atomardünnen Halbleitern auf ultradünnen Oxiden möglich ist, sofern die Oberflächen-Eigenschaften gründlich angepasst werden. Im Rahmen dieser Arbeit ist die elektronische Charakteristik von Einzel-Transistor-Bauelemente vorangetrieben worden, was zur Herstellung komplexerer organisch/anorganischer, ambipolarer Transistoren und integrierter Schaltkreise ermutigen soll.The latest silicon metal-oxide-semiconductor field-effect transistor design utilizes silicon nanosheets for high-performance, low-power microprocessors. In parallel, natural nanosheets like transition metal dichalcogenides are studied as promising candidates for the next-generation materials in nanosheet transistors. So far, organic nanosheets have remained elusive, despite superior performance of organic molecules in light-emitting diodes. In this thesis, we develop a novel transfer technique for organic semiconductors and investigate the charge transport in transferable organic/inorganic van der Waals nanosheets. We used a water-soluble polymer as a growth template, which was dissolved in a proper wetting geometry following the deposition of the organic semiconductor, allowing the release of wafer-scale (3-inch) nanosheets from their growth substrates. Using an atomic force microscope and X-ray diffraction, we confirmed that the nanosheets preserved their high crystallinity after transfer. We investigated the performance of bottom-contact transferred organic transistors, which outperformed bottom-contact as-evaporated organic transistors due to the transfer-enabled high crystallinity. Afterward, we fabricated van der Waals heterojunction transistors on silicon oxide with transferable organic nanosheets and atomically thin semiconductors, the former serving as a p-conductor and the latter as an n-conductor. We discovered that electrons and holes simultaneously contribute to charge transport, exhibiting ambipolar characteristics with similar mobility values.
Quantitative analysis of the ambipolar transistors and unipolar MoS2 transistors on un- treated 100 nm-thick silicon oxide revealed subthreshold slopes of 2,400 mVdec-1 to 1,000 mVdec-1 . At room temperature, the subthreshold slope can be as steep as 60 mV over a decade of drain current. Any significant deviation from this ideal behavior towards higher values indicates local interface states. Passivating the interface states of the aluminum oxide in unipolar organic nanosheet transistors with chemically bonded self-assembly monolayers (SAM) allowed us to achieve steeper subthreshold slopes. However, the wetting problems of SAM with lithography chemicals prevented the use of this oxide passivation in the fabrication of ambipolar transistors and unipolar MoS2 transistors. To resolve this issue, we introduced a new lithography-compatible interface state passivation by an ultrathin polymer. We observed that the charge transport was dominated by hopping in dangling bond-induced deep trap states for transistors on untreated oxides, resulting in poor subthreshold performance. In addition, the deep trap state hopping of charges made it impossible to determine the proper transistor characteristics. Contrarily, the transistors on passivated 100 nm-thick oxide demonstrated hopping transport in tail states a few kBT below the conduction band and substantially steeper subthreshold slopes of 189 mVdec-1. Although the passivation reduced the required drain voltages to 5 V to establish the saturation region conditions, the MoS2 transistors exhibited non-ideal saturation characteristics.
Non-ideal saturation characteristics of the transistors on passivated 100 nm-thick silicon oxide indicated the need for further improvement of the subthreshold slope to eliminate the domination of interface states’ over the intrinsic properties of the semiconductor. We adopted a local gate architecture, used patterned aluminum as gate metal, and enhanced its oxide by oxygen plasma to produce high-quality ultrathin aluminum oxide. Aluminum oxide enabled using chemically bonded SAMs as an interface state passivation. By decreasing the passivation area through the local gate architecture, we could eliminate the wetting problems that had previously occurred for such SAM surfaces. The capacitance of the plasma-enhanced oxide increased by 20 times compared to the passivated 100 nm-thick silicon oxide. As a result, the gate voltage range was lowered to ±0.75 V, enabling saturation conditions to occur at drain voltages of 1 V. MoS2 transistors on such ultrathin gate oxides exhibited textbook thin-film transistor characteristics; low operation voltages, steep subthreshold slopes of 61.6 mVdec-1, and mobilities around 5 cm2V-1s-1.
The results emphasize that efficient nanosheet transistor fabrication is possible with atomically thin semiconductors on ultrathin oxides if the surface properties are carefully adjusted. This work pushed the electronic properties of single transistor device characteristics, encouraging building of more complex organic/inorganic ambipolar transistors and integrated circuits
Same same but different: plasticity of a 'conserved' reflex
Get PDFTransformation of sensory percepts into motor output form a core element of how any animal interacts with their environment. While some such sensorimotor transformations can be very elaborate and depend on the lifestyle of a species, others serve basic functions and are ubiquitous across vertebrates. Among the latter ones are gaze-stabilizing reflexes, which serve to maintain stable vision during head motion through compensatory eye movements. Despite this conservation throughout evolution, these reflexive behaviors must remain plastic
depending on context or past experience to maintain functionality after e.g. impairments of motor or sensory systems through compensation, or to changes in the environment through adaptation. In this thesis, I employ tadpoles of the frog Xenopus laevis to investigate how neuronal circuits contribute to either adaptive or compensatory plasticity on otherwise conserved gaze-stabilizing reflexes.
My first study centers on the role of bilateral visual pathways in the development of the optokinetic reflex (OKR). In early embryos, I unilaterally remove the precursor of the eye, the optic vesicle. Tadpoles that develop under such monocular conditions display pathfinding errors of retinal ganglion cells at the optic chiasm. Tadpoles with near normal contralateral projections functionally compensate for the loss of one eye and show consistent responses to both leftward and rightward moving stimuli. In animals with an induced aberrant ipsilateral projection, compensation is increasingly impaired with more pathfinding errors. Combined,
this study shows that binocular eyes are required for appropriate visual circuit formation, and that resulting anatomical aberrations impose limitations on compensatory plasticity.
In my second study I focus on the role of the cerebellum in plasticity. Combinations of prolonged, repetitive stimulation with lesions of the cerebellum revealed adaptive plasticity of the OKR, where initially very variable OKR responses converge towards a homeostatic motor output by selective increase and decrease of response magnitude. The cerebellum is specifically associated only with response increases, and only starts to exert this influence well after initial OKR onset. This study therefore shows that multiple brain areas differentially contribute to plasticity of eye movements, leading to heterogenous appearance of different
modes of plasticity throughout development.
Combined, these studies contribute to the understanding of development and purpose of plasticity in Xenopus OKR. Multiple brain areas are involved with plasticity, and their formation depends on canonical, bilateral visual input. Once functional, plasticity mechanisms serve to maintain homeostasis of the OKR response in response to both adaptation and compensation
Cosmic-ray all-electron spectrum with MAGIC
Get PDFStudying high-energy cosmic-ray electrons and positrons is crucial in understanding nearby cosmic-ray sources. These particles experience significant energy loss during their propaga- tion through the Interstellar Medium (ISM) via synchrotron radiation and inverse-Compton scattering, leading to a short and energy dependent path-length in our Galaxy. Electrons and positrons with energies in the TeV ranges are expected to originate from sources within a distance of approximately one kiloparsec. In addition to the astrophysical origin, the possibility of a Dark Matter scenario makes the study of these particles even more intriguing.
Numerous experiments, including balloon-borne and satellite missions, as well as ground- based Imaging Atmospheric Cherenkov Telescopes (IACTs), have extensively studied the energy spectrum of cosmic-ray electrons and positrons. Although IACTs are designed to detect γ-rays, electrons and positrons can also be detected due to the similar air shower development process. Due to their large collection areas, IACTs can provide large statistics electrons and positrons at TeV energies. However, extracting the electron and positron events against the dominating hadronic background remains a significant challenge for IACTs. Additionally, the ability to reconstruct the spectrum of cosmic-ray electrons and positrons demonstrates the capabilities of IACTs to study diffuse sources.
In this thesis, two methods are introduced for estimating the background cosmic-ray hadron events in order to further extract the electron and positron events based on Random Forest (RF) algorithm: the RF-Fit method, which is a template fit method, and the Two- Step RF method, which is a hard cut method based on two steps of RF training. The RF-Fit method is a commonly used technique for analyzing the cosmic-ray electron and positron spectrum with IACTs, which has been validated by instruments like H.E.S.S. and VERITAS. To adapt this method for use with MAGIC, I designed a Monte-Carlo (MC) tracking simulation method to ensure that the simulated background template precisely matches the Field of View (FoV) of the observation data. The Two-Step RF method is a novel approach that uses RF to accurately train between signal events and signal-like background events. By applying a tight cut of a few percent for the electron survival rate, the background events can be reduced to approximately 20%. After thorough evaluations of the systematics, both methods yield consistent reconstructed cosmic-ray electron and positron spectra in the energy range between 300 GeV and 6 TeV. The spectra can be described by a broken power-law and confirm the presence of an energy break around 900 GeV from MAGIC data for the first time, which is consistent with previous measurements.
Afterwards, the contribution to the cosmic-ray electron and positron spectrum from Supernova Remnant (SNR) and pulsar models are compared with MAGIC data. The model preferred by MAGIC suggests that a Monogem pulsar is a source of the broken power law spec- trum, where the pulsar appears above the background but experiences suppression at high energies
Biomolecules and material-tissue interactions in regenerative dentistry
Get PDFPeriodontal disease is a prevalent condition affecting a substantial proportion of the global population. It has a significant impact on the quality of life and its incidence is projected to increase as the population ages.
This habilitation work focuses on various aspects of periodontal regeneration, personalized periodontics, and the influence of materials and interventions on periodontal and peri-implant health. The first part of the research explores the role of biomolecules in periodontal regeneration and repair. While common periodontal treatments result in tissue repair, the ultimate objective is achieving complete regeneration. Regenerative procedures that aim to restore lost or injured tissues in periodontal disease are being extensively studied. Two specific biomolecules, amelogenin (component of EMD) and hyaluronic acid (HA), were examined for promoting the regeneration of periodontal tissues. The studies evaluated the effects of these biomolecules on cell proliferation, migration, and differentiation, highlighting their potential in improving periodontal tissue regeneration. One study specifically focused on a recombinant version of the main protein found in EMD, amelogenin, investigating the effects of the full-length protein on periodontal wound healing and its interaction with oral keratinocytes. The results show that amelogenin inhibits the motility and proliferation of keratinocytes, suggesting its potential in preventing the occupation of periodontal ligament space by these cells. Another study explored the influence of different molecular weights of hyaluronic acid on periodontal ligament cells. Hyaluronic acid fragments induce osteogenic differentiation in these cells, with medium molecular weight hyaluronic acid showing the most significant effects. The study highlights the importance of considering the molecular weight of hyaluronic acid in its clinical application for periodontal therapy.
The second part of the research focuses on the use of biomolecules in the diagnosis, monitoring, and treatment of periodontal conditions. It discusses the potential of cytokines, such as interleukin-8 (IL-8), as diagnostic markers for periodontitis. The study showed a strong correlation between IL-8 levels in gingival crevicular fluid and the clinical severity of periodontitis. The research also investigated the correlation between IL-8 levels and smoking habits, revealing that for this group IL-8 cannot serve as a biomarker of periodontitis.
Additionally, the research explored the effects of prostaglandins E2 (PGE2) and D2 (PGD2) on cell proliferation and osteogenic capacity of human mesenchymal stem cells. It demonstrated that both PGE2 and PGD2 negatively affect osteogenic differentiation and metabolism, suggesting their involvement in periodontitis-induced tissue damage.
The third part of the research examines the influence of materials and iatrogenic interventions on periodontal and peri-implant health. One study investigated the ultrastructural changes of titanium implant surfaces caused by metal and plastic periodontal probes. Although slight changes in surface roughness were observed, they did not reach statistical significance. Further studies need to investigate how routine probing might affect the reattachment of osteoblasts after peri-implant defect treatment.
Two other studies focused on the cytotoxicity of 3D printed resin materials used for temporary dental restorations. The research evaluated the effects of these materials on human periodontal ligament cells and gingival keratinocytes. The results indicate a higher cytotoxicity of 3D printed resin materials compared to conventional and subtractive manufacturing materials.
Overall, this research provides valuable insights into the biological principles of regenerative materials, the potential of biomolecules in periodontal therapy, the use of molecules as diagnostic markers, and the influence of materials and interventions on periodontal and peri-implant health. The findings contribute to the advancement of periodontal treatment and personalized dentistry, aiming to improve patient care and outcomes in the field of periodontology
Antioxidantien als neuartige dentale Kompositkomponente und intranukleäre Zellaufnahme und Toxizität von Titandioxid- und Zirkonoxid-Partikeln sowie bakterielle Adhäsion auf dentalen Titan- und Zirkonoxid-Implantaten
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Rheology dependent on the distance to a propagating fault tip
Get PDFThe microfabric of fault rocks from the base of the seismogenic zone, i.e., for the continental crust at depths of greenschist facies conditions, is crucial for the understanding of the seismic cycle. It provides information about the rheology during episodic deformation, controlling the strength of crustal rocks, and is thus relevant for large-scale geological processes in tectonic active regions.
In this thesis, fault rocks of the Silvretta basal thrust in the central Alps and the Defereggen-Antholz-Vals (DAV) strike-slip shear zone in the Eastern Alps were analyzed to unravel the deformation, as well as stress and strain-rate histories within fundamentally different geological settings. Various fault rocks, containing pseudotachylytes and (ultra-)mylonites, were selected for the model case of the Silvretta basal thrust and compared to mylonitic pegmatites and cataclasites of a strike-slip tectonic setting (DAV), to evaluate the question of whether there is more than one transient high-stress deformation event recorded in one rock and the relation to the long-term shear zone activity. Fault rocks from both localities were deformed at greenschist facies conditions, i.e., they originate from the base of the seismogenic zone. The focus is on the microstructural record displaying a specific sequence of deformation mechanisms, to infer different stress and strain-rate conditions at hypocentral depth, not directly accessible for in situ measurements. The findings are compared to gneisses of the Vredefort impact structure in South Africa, which were shocked to relatively low shock conditions at depths comparable to those at the base of the seismogenic zone. These deformation conditions are compared and contrasted to those at hypocentral depth.
The rock record of the different geological settings gives new insights into the deformation processes of rupturing events at hypocentral depth. The microstructures preserved within pseudotachylyte-related (fault) rocks indicate high stresses (>400 MPa), that prevail only transiently at the base of the seismogenic zone, and are diagnostic for coseismic deformation. These coseismically high stresses are associated with stress redistribution, controlled by the distance to the tip of the propagating fault instead of depending on local heterogeneities, as evident by systematic variations of deformation mechanisms at the same greenschist facies conditions. The specific stress and strain-rate conditions are decisive regarding the rheological rock behavior during seismic faulting, as opposed to a change in pressure and temperature. Deformation conditions during major earthquakes, revealed by these commonly modified microstructures, are comparable to those realized during impact cratering at relatively deep parts of the impact structures characterized by relatively low shock conditions and allow evaluation of the material behavior at non-steady state conditions. Pseudotachylytes, representing the last imprint after the passage of the rupture, are therefore not the only microstructural marker of coseismic deformation and ancient earthquakes. The findings from this thesis highlight the importance of the different strengths of crustal rocks and the time-dependent rheology at specific stress/strain-rate conditions during the seismic cycle at the base of the seismogenic zone