Transmissionselektronenmikroskopie an memristiven Bauteilen

The aim of the research unit Memristive Devices for Neuronal Systems is to develop, manufacture and understand microelectronic devices that can change their electrical resistance repeatedly and reversibly dependent on their history of operation, commonly called memristors. In the framework of this research unit this work was conducted and encompasses the complete procedure of micro- and nanostructural analysis starting with the sample preparation for transmission electron microscopy (TEM), the methods of analysis and concludes with the results and their interpretation. Besides conceptualization, fabrication and functional probing, the analysis described in this study is essential for understanding the mode of action and validation of expected results. Out of the various concepts for memristors, the most promising ones pursued within this research unit are a combined aluminum oxide tunnel barrier and a niobium oxide Schottky barrier – in short called double barrier device – and a bimetallic nanoparticle based electrochemical metallization cell (EMC) approach. Besides all-electronic memristors other approaches utilize non-linear sensor devices – e.g. gas or pressure sensors – to combine memristors and sensors into memsensors. The devices allow a sensor to "acclimatize" to baseline stimuli but still react to changes from this baseline input to report deviations. First concepts for these devices based on highly porous CdTe or InP microstructures have been investigated for the crystallinity and chemical impurities under different growth conditions. The investigations led to a deeper understanding of the fabrication process and the mechanisms of memristive switching ultimately enabling improvement of device design and fabrication.Das Ziel der Forschergruppe „Memristive Bauelemente für neuronale Systeme“ ist es, mikroelektronische Bauelemente zu entwickeln und produzieren, welche ihren elektrischen Widerstand wiederholt und reversibel ändern können; im Sprachgebrauch werden diese Bauelemente Memristoren genannt. Im Rahmen dieser Forschergruppe entstand diese Arbeit, welche das komplette Vorgehen der mikro- und nanostrutkurellen Analyse umreißt, von der Probenpräparation für das Transmissionelektronenmikroskop (TEM) über die analytischen Methoden bis hin zu den Ergebnisse und ihrer Interpretation. Neben der konzeptionellen Entwicklung, der Herstellung und der funktionellen Untersuchung der Memristoren sind die hier beschriebenen Forschungsergebnisse essentiell, um deren Wirkweise zu verstehen und die nach der Herstellung erwarteten Ergebnisse zu validieren. Von den vielen verschiedenen Konzepten, welche es für Memristoren gibt, stechen zwei als besonders vielversprechend hervor, welche in der Forschergruppe fokussiert untersucht wurden. Zum einen handelt es sich dabei um ein sogenanntes „Doppelbarrieren“-Bauelement, welches eine Aluminiumoxid-Tunnelbarriere mit einer Nioboxid-Schottkybarriere kombiniert, zum anderen ein auf bimetallischen Nanopartikeln basierte elektrochemische Metallisierungszelle. Neben den rein elektrischen Memristoren gibt es auch Ansätze, welche Sensoren und Memristoren miteinander zu verheiraten versuchen. Hierzu zählen zum Beispiel Gas- und Drucksensoren, welche sich an ein Grundniveau „gewöhnen“ können, und trotzdem auf Veränderungen von diesem Niveau anspringen. Erste Konzepte für diese Bauelemente bestehen aus hochporösen CdTe- oder InP-Mikrostrukturen und wurden auf ihre Kristallinität und Zusammensetzung unter verschiedenen Herstellungsbedingungen untersucht. Die Ergebnisse führten insgesamt zu einem tieferen Verständnis der Herstellungsprozesse und der zugrundeliegenden memristiven Schaltmechanismen. Ultimativ ebnet dies den Weg, sowohl Design als auch Herstellung der Bauelemente zu verbessern

Late Holocene climate changes in the Altai Region based on a first high‐resolution biomarker isotope record from Lake Khar Nuur

The Late Holocene marks a substantial cultural and economic transition in the eastern Eurasian Steppe and Altai Region with the dispersal of nomadic pastoralism. So far, paleoclimate conditions during this time remain unclear and controversial. Here, we present a high-resolution 4.2 ka paleoclimate record from Lake Khar Nuur in the Mongolian Altai that is based on lake sediment proxies and biomarker compound-specific δ2H analyses. Our results document increased aridity before ∼3.7 cal. ka BP, followed by two pronounced phases of warm and wet conditions from ∼3.5–2.8 to ∼2.3–1.5 cal. ka BP, and a strong increase in aridity since ∼1.5 cal. ka BP. Phases of warmer and wetter conditions coincide with a negative North Atlantic Oscillation, which has been responsible for advecting moisture into the region by more southerly-displaced Westerlies and possibly favored the expansion of mobile nomadic pastoralism in the region

Holocene Temperature Variations in Semi-Arid Central Mongolia—A Chronological and Sedimentological Perspective From a 7400-year Lake Sediment Record From the Khangai Mountains

Semi-arid Mongolia is a highly sensitive region to climate changes, but the region’s Holocene paleoclimatic evolution and its underlying forcing mechanisms have been the subject of much recent debate. Here we present a continuous 7.4 ka sediment record from the high-altitude Shireet Naiman Nuur (Nuur = lake) in the central Mongolian Khangai Mountains. We extensively dated the sediments and analyzed elemental composition and bulk isotopes for lake sediment characterization. Our results show that 14C-dating of bulk organic carbon and terrestrial macrofossils provide a robust and precise chronology for the past 7.4 ± 0.3 cal ka BP at Shireet Naiman Nuur and 14C-ages are mostly in stratigraphic order. The 14C-based chronology is confirmed by paleomagnetic secular variations, which resemble the predictions of spherical harmonic geomagnetic field models. The very good chronological control makes paleomagnetic secular variation stratigraphy a powerful tool for evaluating and refining regional 14C-chronologies when compared to the record presented here. The lake sediment proxies TOC, N, log (Ca/Ti) and log (Si/Ti) reveal increased lake primary productivity and high growing season temperatures from 7.4 ± 0.3 to 4.3 ± 0.2 cal ka BP, which is likely the result of stronger summer insolation and pronounced warming. Reduced summer insolation thereafter results in decreased productivity and low growing season temperatures at Shireet Naiman Nuur from 4.3 ± 0.3 cal ka BP until present day. The globally acknowledged 4.2 ka event also appears as a pronounced cooling event at Shireet Naiman Nuur, and additional abrupt cooling events occurred during minima in total solar irradiance at ~3.4, 2.8 and 2.4 ka BP. Low lake primary productivity and growing season temperatures are likely the result of longer ice cover periods at the high-altitude (2,429m a.s.l.) Shireet Naiman Nuur. This leads to shorter mixing periods of the lake water which is supported by more positive δ13CTOC because of increased incorporation of dissolved HCO3− by aquatic producers during periods of longer ice cover

Summer paleohydrology during the Late Glacial and Early Holocene based on δ2H and δ18O from Bichlersee, Bavaria.

Isotope-based records provide valuable information on past climate changes. However, it is not always trivial to disentangle past changes in the isotopic composition of precipitation from possible changes in evaporative enrichment, and seasonality may need to be considered. Here, we analyzed δ2H on n-alkanes and δ18O on hemicellulose sugars in sediments from Bichlersee, Bavaria, covering the Late Glacial and Early Holocene. Our δ2Hn-C31 record documents past changes in the isotopic composition of summer precipitation and roughly shows the isotope pattern known from Greenland. Both records show lower values during the Younger Dryas, but at Bichlersee the signal is less pronounced, corroborating earlier suggestions that the Younger Dryas was mainly a winter phenomenon and less extreme during summer. δ18Ofucose records the isotopic composition of the lake water during summer and is sensitive to evaporative enrichment. Coupling δ2Hn-C31 and δ18Ofucose allows calculating lake water deuterium-excess and thus disentangling changes in the isotopic composition of precipitation and evaporative enrichment. Our deuterium-excess record reveals that the warm Bølling-Allerød and Early Holocene were characterized by more evaporative enrichment compared to the colder Younger Dryas. Site-specific hydrological conditions, seasonality, and coupling δ2H and δ18O are thus important when interpreting isotope records

Wet-Chemical Assembly of 2D Nanomaterials into Lightweight, Microtube-Shaped, and Macroscopic 3D Networks

Despite tremendous efforts toward fabrication of three-dimensional macrostructures of two-dimensional (2D) materials, the existing approaches still lack sufficient control over microscopic (morphology, porosity, pore size) and macroscopic (shape, size) properties of the resulting structures. In this work, a facile fabrication method for the wet-chemical assembly of carbon 2D nanomaterials into macroscopic networks of interconnected, hollow microtubes is introduced. As demonstrated for electrochemically exfoliated graphene, graphene oxide, and reduced graphene oxide, the approach allows for the preparation of highly porous (> 99.9%) and lightweight (<2 mg cm-3) aeromaterials with tailored porosity and pore size as well as tailorable shape and size. The unique tubelike morphology with high aspect ratio enables ultralow-percolation-threshold graphene composites (0.03 S m-1, 0.05 vol%) which even outperform most of the carbon nanotube-based composites, as well as highly conductive aeronetworks (8 S m-1, 4 mg cm-3). On top of that, long-term compression cycling of the aeronetworks demonstrates remarkable mechanical stability over 10 000 cycles, even though no chemical cross-linking is employed. The developed strategy could pave the way for fabrication of various macrostructures of 2D nanomaterials with defined shape, size, as well as micro- and nanostructure, crucial for numerous applications such as batteries, supercapacitors, and filters

Central Mongolian lake sediments reveal new insights on climate change and equestrian empires in the Eastern Steppes

The repeated expansion of East Asian steppe cultures was a key driver of Eurasian history, forging new social, economic, and biological links across the continent. Climate has been suggested as important driver of these poorly understood cultural expansions, but paleoclimate records from the Mongolian Plateau often suffer from poor age control or ambiguous proxy interpretation. Here, we use a combination of geochemical analyses and comprehensive radiocarbon dating to establish the first robust and detailed record of paleohydrological conditions for Lake Telmen, Mongolia, covering the past ~ 4000 years. Our record shows that humid conditions coincided with solar minima, and hydrological modeling confirms the high sensitivity of the lake to paleoclimate changes. Careful comparisons with archaeological and historical records suggest that in the vast semi-arid grasslands of eastern Eurasia, solar minima led to reduced temperatures, less evaporation, and high biomass production, expanding the power base for pastoral economies and horse cavalry. Our findings suggest a crucial link between temperature dynamics in the Eastern Steppe and key social developments, such as the emergence of pastoral empires, and fuel concerns that global warming enhances water scarcity in the semi-arid regions of interior Eurasia

Proimmunogenic impact of MEK inhibition synergizes with agonist anti-CD40 immunostimulatory antibodies in tumor therapy

Cancer types with lower mutational load and a non-permissive tumor microenvironment are intrinsically resistant to immune checkpoint blockade. While the combination of cytostatic drugs and immunostimulatory antibodies constitutes an attractive concept for overcoming this refractoriness, suppression of immune cell function by cytostatic drugs may limit therapeutic efficacy. Here we show that targeted inhibition of mitogen-activated protein kinase (MAPK) kinase (MEK) does not impair dendritic cell-mediated T cell priming and activation. Accordingly, combining MEK inhibitors (MEKi) with agonist antibodies (Abs) targeting the immunostimulatory CD40 receptor results in potent synergistic antitumor efficacy. Detailed analysis of the mechanism of action of MEKi shows that this drug exerts multiple pro-immunogenic effects, including the suppression of M2-type macrophages, myeloid derived suppressor cells and T-regulatory cells. The combination of MEK inhibition with agonist anti-CD40 Ab is therefore a promising therapeutic concept, especially for the treatment of mutant Kras-driven tumors such as pancreatic ductal adenocarcinoma. Immune checkpoint inhibitors have limited efficacy in tumors with lower mutational burden and non-permissive microenvironment. Here, the authors show that combining MEK inhibition with an agonist anti-CD40 immunostimulatory antibody improves antitumor treatment by inducing immunogenic changes in the tumor microenvironment

Spracherhalt oder Sprachverlagerung? Erstsprachgebrauch und Zweitsprachkompetenzen bei Jugendlichen mit Migrationshintergrund

Geringere Zweitsprachkompetenzen bei Jugendlichen mit Migrationshintergrund werden häufig mit einer herkunftssprachlichen Kommunikation innerhalb der Familie in Verbindung gebracht. Der vorliegende Beitrag erweitert den bisherigen Fokus. Er beschreibt den Sprachgebrauch in weiteren alltagsrelevanten Kontexten über drei Zuwanderergenerationen hinweg und untersucht, wie dieser mit den Zweitsprachkompetenzen zusammenhängt. Die Verwendung der Zweitsprache wird über den damit einhergehenden Zugang und eine erhöhte Lernmotivation mit den Deutschkompetenzen verknüpft. Anhand der Daten des Nationalen Bildungspanels lässt sich zeigen, dass die Herkunftssprache mit den Eltern häufiger verwendet wird als mit Geschwistern. Mit Peers wird überwiegend Deutsch gesprochen, während sich beim Medienkonsum eine komplementäre Nutzung beider Sprachen abzeichnet. Türkischstämmige Jugendliche verwenden in allen Kontexten häufiger die Herkunftssprache als diejenigen aus Polen oder der ehemaligen Sowjetunion. Wird mit den Eltern ausschließlich Deutsch gesprochen, dann auch meist in allen anderen Kontexten. Jugendliche mit einer nicht-deutschen Elternsprache variieren hingegen stark hinsichtlich des Sprachgebrauchs in weiteren Kontexten. Die mit der Familiensprache assoziierten Sprachdefizite können durch den Deutschgebrauch mit Peers reduziert werden