Fak. 2 Mathematik und Naturwissenschaften

2485 Items

Recent Submissions
Vibronic optical spectroscopy of cryogenic flavin ions: the O2+ and N1 tautomers of protonated lumiflavin

Müller, David ; Dopfer, Otto (2020-08-04)

Flavins are key compounds in many photochemical and photophysical processes used by nature, because their optical properties strongly depend on the (de-)protonation site and solvation. Herein, we present the vibronic optical spectrum of protonated lumiflavin (H+LF), the parent molecule of the flavin family, obtained by visible photodissociation (VISPD) spectroscopy in a cryogenic ion trap. By c...

Light-induced phase transition of gold-microgel hybrids

Lehmann, Maren (2020)

The aim of this thesis was the development and characterisation of a hybrid system based on poly(N-isopropylacrylamide) (PNIPAM) microgels and spherical gold nanoparticles (AuNPs), which undergoes a reversible photo-thermal volume phase transition (VPT). In order to understand the relationship between the microgel properties and the stimuli-responsive behaviour of AuNP-microgel hybrid particle...

On Weak (Measure-Valued)–Strong Uniqueness for Compressible Navier–Stokes System with Non-monotone Pressure Law

Chaudhuri, Nilasis (2020-02-24)

In this paper our goal is to define a renormalized dissipative measure-valued (rDMV) solution of compressible Navier–Stokes system for fluids with non-monotone pressure–density relation. We prove existence of rDMV solutions and establish a suitable relative energy inequality. Moreover we obtain the weak (measure-valued)–strong uniqueness property of this rDMV solution with the help of relative ...

Cobalt‐Exchanged Poly(Heptazine Imides) as Transition Metal–Nx Electrocatalysts for the Oxygen Evolution Reaction

Ye, Meng‐Yang ; Li, Shuang ; Zhao, Xiaojia ; Tarakina, Nadezda V. ; Teutloff, Christian ; Chow, Wing Ying ; Bittl, Robert ; Thomas, Arne (2020-01-27)

Poly(heptazine imides) hosting cobalt ions as countercations are presented as promising electrocatalysts for the oxygen evolution reaction (OER). A facile mixed‐salt melt‐assisted condensation is developed to prepare such cobalt poly(heptazine imides) (PHI‐Co). The Co ions can be introduced in well‐controlled amounts using this method, and are shown to be atomically dispersed within the imide‐l...

Structures in three-dimensional Euclidean space from hyperbolic tilings

Kolbe, Benedikt Maximilian (2020)

In this thesis we introduce a theory of isotopy classes of tilings with given symmetry group on hyperbolic surfaces, possibly punctured, nonorientable, and with boundary. We first generalise combinatorial tiling theory to incorporate tilings with tiles that are not closed disks. To establish a combinatorial theory for isotopy classes of tilings, based on Delaney-Dress tiling theory, we subseque...

Physicochemical Salt Solution Parameters Limit the Survival of Planococcus halocryophilus in Martian Cryobrines

Waajen, Annemiek C. ; Heinz, Jacob ; Airo, Alessandro ; Schulze-Makuch, Dirk (2020-07-07)

Microorganisms living in sub-zero environments can benefit from the presence of dissolved salts, as they significantly increase the temperature range of liquid water by lowering the freezing point. However, high concentrations of salts can reduce microbial growth and survival, and can evoke a physiological stress response. It remains poorly understood how the physicochemical parameters of brine...

Ultrafast spin dynamics of a ferrimagnet revealed by femtosecond soft X-ray and XUV radiation

Hennecke, Martin Heinrich (2020)

In this thesis, we will present time-resolved experimental investigations of ultrafast spin phenomena in ferrimagnetic metallic alloys of GdFeCo. Recent studies of such ferrimagnetic systems have revealed that a femtosecond optical excitation is not only able to demagnetize such a system on a subpicosecond timescale, but also to permanently reverse its magnetization without any other external s...

Techno-economic assessment – methodology development and the case of CO2-containing polyurethane rubbers

Buchner, Georg Alexander (2020)

Well-founded project decisions on the continuation of research, development, and deployment (RD&D) activities are crucial to the success of an economic entity. Decision-makers rely heavily on sound techno-economic assessment (TEA), a systematic approach for judging the economic viability of a technology. A multitude of individual technology characteristics and practitioners’ backgrounds result ...

Group III-nitride based UVC LEDs and lasers with transparent AlGaN:Mg layers and tunnel junctions grown by MOVPE

Kuhn, Christian (2020)

In this work, AlGaN-based light-emitting diodes (LEDs) and lasers with emission wavelengths in the deep ultraviolet (UVC) spectral range are produced, analyzed, and optimized. Here, the focus is on the UV transparency of the structures, enabling high light extraction efficiency for UVC LEDs and being a necessary condition for UVC laser diodes, however at the same time challenging due to low ele...

Heterostructure design and epitaxial growth of AlGaN-based light emitting diodes emitting in the UVC wavelength range

Susilo, Norman (2020)

AlGaN-based ultraviolet-C (UVC) light emitting diodes (LEDs) can be applied to a wide range of applications. However, the efficiency of UVC-LEDs is still relatively modest, which provides the focus of this thesis: The development of efficient UVC-LEDs with emission wavelengths near 265 nm. This thesis presents a systematic study of the influence of the heterostructure design and the layer prope...

Twofold gain enhancement by elongation of QDs in polarization preserving QD-SOAs

Greif, Ludwig Albrecht Thorsten ; Mittelstädt, Alexander ; Jagsch, Stefan Thomas ; Schliwa, Andrei (2019-06-06)

The impact of quantum dot (QD) elongation on key parameters of QD-based semiconductor optical amplifiers (SOAs) is investigated using a combination of 8-band k·p-theory including strain and piezoelectricity up to second order and a rate equation model describing the population of QD ground, excited and wetting layer states. By considering columnar QDs of selected aspect ratios, we show that chi...

Tuning the Emission Directionality of Stacked Quantum Dots

Greif, Ludwig Albrecht Thorsten ; Jagsch, Stefan Thomas ; Wagner, Markus R. ; Schliwa, Andrei (2018-11-02)

The emission directionality of stacks of coupled quantum dots (QDs) is investigated within the framework of 8-band k·p-theory including strain and strain-induced piezoelectricity up to second order. Using an artificial cuboidal QD, we show that the degree of radiation anisotropy can be tuned from −33% to nearly +60% via the structure’s vertical aspect ratio. We then demonstrate that these findi...

A comparative perspective of electrochemical and photochemical approaches for catalytic H2O2 production

Sun, Yanyan ; Han, Lei ; Strasser, Peter (2020-08-06)

Hydrogen peroxide (H2O2) has a wide range of important applications in various fields including chemical industry, environmental remediation, and sustainable energy conversion/storage. Nevertheless, the stark disconnect between today's huge market demand and the historical unsustainability of the currently-used industrial anthraquinone-based production process is promoting extensive research on...

Recent progress in the performance of HAPG based laboratory EXAFS and XANES spectrometers

Schlesiger, Christopher ; Praetz, Sebastian ; Gnewkow, Richard ; Malzer, Wolfgang ; Kanngießer, Birgit (2020-07-15)

New developments in the description and modeling of Highly Annealed Pyrolytic Graphite (HAPG) mosaic crystals have led to the possibility of designing optimized optical solutions for X-ray absorption fine structure (XAFS) spectroscopy. XAFS is a very versatile method that is usually divided into two sub methods: extended X-ray absorption fine structure (EXAFS) and X-ray absorption near edge str...

Spectroscopic identification of fragment ions of DNA/RNA building blocks: the case of pyrimidine

Chatterjee, Kuntal ; Dopfer, Otto (2020-07-07)

Pyrimidine (Pym, 1,3-diazine, 1,3-diazabenzene) is an important N-heterocyclic building block of nucleobases. Understanding the structures of its fragment and precursor ions provides insight into its prebiotic and abiotic synthetic route. The long-standing controversial debate about the structures of the primary fragment ions of the Pym+ cation (C4H4N2+, m/z 80) resulting from loss of HCN, C3H3...

H2 as a fuel for flavin- and H2O2-dependent biocatalytic reactions

Al-Shameri, Ammar ; Willot, Sébastien J.-P. ; Paul, Caroline E. ; Hollmann, Frank ; Lauterbach, Lars (2020-07-14)

The soluble hydrogenase from Ralstonia eutropha provides an atom efficient regeneration system for reduced flavin cofactors using H2 as an electron source. We demonstrated this system for highly selective ene-reductase-catalyzed C[double bond, length as m-dash]C-double bond reductions and monooxygenase-catalyzed epoxidation. Reactions were expanded to aerobic conditions to supply H2O2 for perox...

Establishing reactivity descriptors for platinum group metal (PGM)-free Fe–N–C catalysts for PEM fuel cells

Primbs, Mathias ; Sun, Yanyan ; Roy, Aaron ; Malko, Daniel ; Mehmood, Asad ; Sougrati, Moulay-Tahar ; Blanchard, Pierre-Yves ; Granozzi, Gaetano ; Kosmala, Tomasz ; Daniel, Giorgia ; Atanassov, Plamen ; Sharman, Jonathan ; Durante, Christian ; Kucernak, Anthony ; Jones, Deborah ; Jaouen, Frédéric ; Strasser, Peter (2020-06-24)

We report a comprehensive analysis of the catalytic oxygen reduction reaction (ORR) reactivity of four of today's most active benchmark platinum group metal-free (PGM-free) iron/nitrogen doped carbon electrocatalysts (Fe–N–Cs). Our analysis reaches far beyond previous such attempts in linking kinetic performance metrics, such as electrocatalytic mass-based and surface area-based catalytic activ...

Squirmer rods as elongated microswimmers: flow fields and confinement

Zantop, Arne W. ; Stark, Holger (2020-06-25)

Microswimmers or active elements, such as bacteria and active filaments, have an elongated shape, which determines their individual and collective dynamics. There is still a need to identify what role long-range hydrodynamic interactions play in their fascinating dynamic structure formation. We construct rods of different aspect ratios using several spherical squirmer model swimmers. With the h...

Isothermic constrained Willmore tori

Tervooren, Jonas (2020)

This dissertation treats the classification of isothermic constrained Willmore tori. Jörg Richter proved in his dissertation that for every immersion f : M → R³ of an isothermic constrained Willmore torus, there exists a conformal change of the euclidean metric of R³ such that the surface has constant mean curvature (CMC) in a space form. We extended his proof such a way that it remains true if...

Towards a deterministic cavity-integrated III-nitride single-photon source with two-photon resonant control

Jagsch, Stefan Thomas (2020)

Wide band gap III-nitride semiconductors already form the basis of today's industrial-scale diode lighting technology. In this two-part thesis, we explore their potential for cavity-enhanced single-photon applications and high-beta nanolasers at elevated temperatures. The overarching topic is the development of a fully deterministic single-photon source with ground-state resonant two-photon con...