Paleo-ice sheet dynamics on the northwest Greenland continental shelf

Increasing temperatures in the ocean and atmosphere result in the decline of the modern ice sheets in both hemispheres, causing global sea-level rise. Direct records on these ice-sheet dynamics are sparse as most observations were first initiated in the last century. Although ice-core analyses and satellite data from the polar regions increase our knowledge of modern ice flow and ice-sheet dynamics, the reconstruction and investigation of these dynamics during past glacial cycles is required to predict future scenarios for ice-sheet dynamics. Erosional and depositional glacial landforms that developed under the paleo-ice sheet and at its margin are used to reconstruct past ice-sheet extends and dynamics. These glacial landforms indicate the flow direction of past ice-streams that drained the former ice-sheet. They indicate the maximum ice-sheet extent as well as ice-sheet stabilizations or re-advances during ice-sheet retreat. This thesis focuses on the analysis of the Greenland ice-sheet dynamics since the last glacial maximum (LGM, 26.5-19 ka BP). The analysis is based on the identification and investigation of glacial landforms on the seabed of Melville Bay, on the northwest Greenland continental shelf. This region in northeast Baffin Bay hosts three large crossshelf troughs. They were formed by ice-streams advancing to the shelf edge during past glacial cycles. They are among the widest and deepest cross-shelf troughs of the continental shelf of Greenland. High-resolution bathymetry, which is necessary for identification of glacial landforms at the seabed in and adjacent to these troughs is limited in this remote region due to harsh sea-ice conditions. Thus, detailed reconstructions of LGM ice-sheet dynamics and their timing in Melville Bay are incomplete. In this thesis, I present new high-resolution bathymetry data from the northeast Baffin Bay that I investigated for submarine glacial landforms. The data have been recorded in 2010 with RV Polarstern and in 2015 with RV Maria S. Merian. This study aims to reconstruct the Greenland Ice Sheet dynamics since the LGM by inferring glacial processes and their relative chronology from the distribution and configuration of glacial landforms. The presented results show that the ice streams extended to the shelf edge and subsequent retreat of the ice streams varied between the glacial cross-shelf troughs in Melville Bay. Local ice domes on the shallow banks between the cross-shelf troughs are inferred from the distribution of glacial landforms. A newly discovered trough network on the inner continental shelf extends from the large cross-shelf troughs towards the fjords under the modern ice sheet. It was likely formed by a former ice sheet. Steep ridges in the north of Melville Bay are likely related to Proterozoic volcanic dyke swarms that to some extent confined former meltwater and ice-stream pathways. The results of this thesis improve our understanding of the northwest Greenland ice-sheet dynamics during past glacials and indicate that at least some glacial processes are related to underlying bedrock morphology

Closing the yellow gap with Eu- and Tb-doped GaN: one luminescent host resulting in three colours

Gallium nitride (GaN) is a key material when it comes to light-emitting diodes (LEDs) and has pushed the LED revolution in lighting and displays. The concept of down-conversion of a GaN-based blue LED offers the possibility to provide efficient generation of monochromatic, high-color purity light resulting in a highly efficient warm-white all-nitride phosphor-converted light emitting diode (pc-LED). Although the down conversion of blue light from InGaN LEDs has become a dominant technique for producing white light, there are still some technical challenges, e.g. the immiscibility of GaN and InN and the lattice mismatch between the substrate and InGaN, that have to be overcome. Here we demonstrate the doping of bulk GaN with europium, terbium and the combination of both resulting in intriguing luminescence properties, pushing the role of GaN:Eu,Tb as a chief component in future light emitting diodes. This colour tuning proves that one luminescence host can provide three colours (red, green and orange) and that even the so called “yellow gap” could be closed with a III-nitride. By using one material for all colours, it will be possible to overcome the technical challenges in building up LED devices, which will open up new capabilities for modern highly efficient phosphors

¹⁴N, ¹³C, and ¹¹⁹Sn solid-state NMR characterization of tin(II) carbodiimide Sn(NCN)

We report the first magic-angle spinning (MAS) nuclear magnetic resonance (NMR) study on Sn(NCN). In this compound the spatially elongated (NCN)$^{2-}$ ion is assumed to develop two distinct forms: either cyanamide (N≡C–N$^{2-}$) or carbodiimide ($^{-}$N=C=N$^{-}$). Our $^{14}$N MAS NMR results reveal that in Sn(NCN) the (NCN) $^{2-}$ groups exist exclusively in the form of symmetric carbodiimide ions with two equivalent nitrogen sites, which is in agreement with the X-ray diffraction data. The $^{14}$N quadrupolar coupling constant ∣∣C$_{Q}$∣∣ ≈ 1.1 MHz for the $^{-}$N=C=N$^{-}$ ion in Sn(NCN) is low when compared to those observed in molecular compounds that comprise cyano-type N≡C– moieties ( ∣∣C$_{Q}$∣∣ > 3.5 MHz). This together with the information from $^{14}$N and $^{13}$C chemical shifts indicates that solid-state NMR is a powerful tool for providing atomic-level insights into anion species present in these compounds. The experimental NMR results are corroborated by high-level calculations with quantum chemistry methods

Eu- and Tb-adsorbed Si3_{3}N4_{4} and Ge3_{3}N4_{4}: tuning the colours with one luminescent host

Phosphor-converted white light emitting diodes (pc-LEDs) are efficient light sources for applications in lighting and electronic devices. Nitrides, with their wide-ranging applicability due to their intriguing structural diversity, and their auspicious chemical and physical properties, represent an essential component in industrial and materials applications. Here, we present the successful adsorption of Eu and Tb at the grain boundaries of bulk β-Si$_{3}$N$_{4}$ and β-Ge$_{3}$N$_{4}$ by a successful combustion synthesis. The adsorption of europium and terbium, and the synergic combination of both, resulted in intriguing luminescence properties of all compounds (red, green, orange and yellow). In particular, the fact that one host can deliver different colours renders Eu,Tb-β-M$_{3}$N$_{4}$ (M = Si, Ge) a prospective chief component for future light emitting diodes (LEDs). For the elucidation of the electronic properties and structure of β-Si$_{3}$N$_{4}$ and β-Ge$_{3}$N$_{4}$, Mott–Schottky (MS) measurements and density functional theory (DFT) computations were conducted for the bare and RE adsorbed samples