Master Theses

Noble metal chemistry for noble physics. The high transition temperatures of Cuprate high-T_C superconductors under ambient pressure remain an unsolved problem in physics. Cu²⁺ with its 3d⁹ electronic configuration in combination with hole doping into the oxygen 2p states is the main feature shared by all known Cuprate superconductors. Chemically, the 4d⁹ analog of Cu²⁺ is Ag²⁺, which leads to the fundamental question of this project: Can Ag replace Cu in high-T_C superconductors? However, these materials are currently chemically inaccessible due to the high electronegativity of Ag and the low stability of Ag₂O. This study aims to oxidize silver and stabilize a novel type of complex oxide, the Argentate. From a physical point of view, Argentates are interesting materials to study the effect of spin-orbit coupling on superconductivity. (supervisor: Nicolas Bonmassar)

Superstructure ordering in doped quantum materials. Superconductivity and ferromagnetism are typical examples of quantum phenomena in complex oxides. These phenomena often occur upon doping of non-superconducting and non-ferromagnetic materials. The phase transition to the quantum state of interest is thus a function of doping. A prime example is the formation of the well-studied superconducting dome, where superconductivity is found only at low and intermediate doping levels, but not at high doping levels. So far, it is not understood in which way the dopants are ordered inside the crystal structure. This study aims to search for a possible ordering of these dopants using atomically resolved scanning transmission electron microscopy (STEM) with novel characterization methods such as 4D STEM (4-dimensional STEM) and differential phase contrast imaging. (supervisor: Nicolas Bonmassar)

Construction of an interatomic potential for the Li₄Ti₅O₁₂ battery material system. Transition metal oxides are known to exhibit variable oxidation states. Therefore, they are preferred as storage materials in lithium-ion batteries. We aim to construct a state-of-the-art interatomic potential for the Li₄Ti₅O₁₂ system which shall be used in atomistic Monte Carlo/molecular dynamics simulations. The atomistic framework incorporates Coulomb interactions, metallic bonding character via an embedded-atom formalism, as well as charge transfer due to the different oxidation states of the species. Varying oxidation states may induce different crystal structures which we could indeed observe in recent experiments. We want to further elucidate this from an atomistic point of view (Supervisor: Sebastian Eich)

Machine learning concepts in atom probe tomography. In impressive cases, concepts of machine learning (ML) have been effective tools for materials science. The project tries to develop a computational algorithm for the measurement of particle/precipitate sizes in atom probe tomography data. The preferred machine learning strategy will be the “support vector regression”, which should be trained with data sets that are generated by simulated field evaporation. A simulation tool (TAPSim) is already available for this step. So, the project has to develop suitable Python scripts that control the basic software. After training the KI with simulated data, the ML code should be applied to experimental data sets (supervisors: Guido Schmitz, Sebastian Eich) 

Imaging alkali atoms in energy materials. Li-doped oxides are among the most studied battery materials. Using state-of-the-art scanning transmission electron microscopy (STEM) techniques, we aim to image the atomic columns of Li to derive structure-property relationships between the observed crystal structure and the Li+-ion conductivity. We are looking for a highly motivated master student to use the new high-end transmission electron microscope in Stuttgart to shed light on the diffusion and migration of Li at the atomic scale. The microscope is equipped with 4D STEM (4-dimensional STEM) and differential phase contrast detectors, allowing for the highest possible spatial resolution. (supervisor: Nicolas Bonmassar)

Thin film deposition of LiNiFePO₄ and its optical behavior. LiFePO₄ is a battery cathode material used in the mass market of low-cost passenger cars, but is also foreseen for future electrical lorries. Partial replacement of Fe by Ni should increase the energy density significantly. For materials science it is important to determine the random statistics of Fe and Ni distribution. Also, the phase transitions and transport kinetics in this material would be highly interesting. The thesis will try to follow these questions by optical spectroscopy. We have ample experience with this spectroscopy from other thin film cathodes and anodes. LiMn₂O₄ [1], Li₄Ti₅O₁₂ [2] and LiCoO₂ [3]. Thin films of varying Ni:Fe composition will be determined, electrochemically characterized and the optical response exploited for a measurement of phases and Li transport kinetics. (supervisor Monica Mead and Guido Schmitz)

[1] https://doi.org/10.1002/adom.201701362
[2] https://doi.org/10.1021/acsami.9b19683
[3] https://doi.org/10.1149/1945-7111/ac63f6

Measurement of hydrogen permeation by high-resolution gravimetry. Hydrogen in materials can be both an effective concept of storage but also deleterious for the mechanical properties of construction steels. Thus, measurement of hydrogen transport velocity in materials is of utmost importance, but difficult. We have developed a highly sensitive method to measure the absorption hydrogen via oscillating quartz microbalances. Now the method should be applied to permeation methods through materials proposed for protective against hydrogen uptake. Thin coatings of Cr-Oxide or Cr-Nitride are deposited on Ti-layers. Comparing the absorption rates of hydrogen into Ti with and without the protective coatings will deliver the permeation coefficient and thus the effectivity of the protection films. (supervisor: Guido Schmitz)

Carbon structures in anodes for Li, Na and K ion batteries. Na and K ions are considered as economic alternatives in the design of batteries, since they are way more abundant than Li. Interestingly in all cases, carbon is suggested/used as the battery anode, but this in different preferred structures: as graphite with Li, as hard carbon with Na and as a mixture of soft and hard carbon with K. The project shall explore the reasons for this interesting difference. Various carbon films will be deposited by sputter deposition, CVD or pyrolysis. The films will be checked in battery function, i.e. charging kinetics and cycle stability, and the respective microstructures investigated by TEM. What are the mysteries between structure and performance? (supervisor: Guido Schmitz)

Reactive wetting on quantitatively evaluated rough surfaces. In a previous PhD work, we have developed a new thermodynamic model to understand the spreading of liquid solder metals on rough substrates when downscaling the size of the solder joints. We shall critically check this theory. Nano-structured rough surfaces are produced by electron beam lithography in our FIB-microscope, for which we exactly know the amplitude (RMS) and periodicity of the roughness. On these, the spreading of solder droplets with a size downward to about 10 micrometers is studied by in-situ SEM investigation. (Supervision: Guido Schmitz)

Evaluation of alloy Gibbs energy via compositional fluctuations in Pd-Pt.  Often, the measurement of equilibrium thermodynamics is prevented by too slow diffusion. Remarkably, the phase diagram of the the most important system of catalysis, Pd-Pt, is unknown at relevant temperatures below about 300°C. We developed in our team a new concept of measuring Gibbs energies via short-range composition fluctuations that establish equilibrium even at low diffusivity.  Confirmation experiments will be done with atom probe tomography. PdPt samples will be prepared by focused ion beam microscopy. In case of success, the impact on basics alloy physics would be ground breaking and at the same time important knowledge on the alloy system would be generated.  (Supervisors: Sebastian Eich, Guido Schmitz)

Pulse-probe experiments in atom probe tomography. In Atom Probe Tomography (APT), very fine samples are evaporated as tiny molecule fractions by superposition of a high electric field and short laser pulses. Especially with organic matter, the size and the charge state of the evaporated fractions have not been understood, yet. Likewise, oxygen and nitrogen can be lost due to insufficient ionization. The aim of the project is to investigate whether a second, subsequent laser pulse, that hits the species right after leaving the sample surface, can positively influence the level of ionization and fragmentation through multi-photon ionization. This requires laser powers > 10²⁰ W/cm², which we plan to obtain with ultra-short (40 fs), highly focused laser pulses. (Supervisors: Parisha Diwan, Guido Schmitz)

Development of cryo-APT Preparation techniques. Samples for Atom Probe Tomography must be needle-lshaped with a radius of curvature of 50 nm at the apex. Preparation of conventional materials is available by using Focused Ion Beam Techniques. Currently, liquids are of great interest, with the goal of studying processes at interfaces like cell membranes, viruses or bacteria in atomic resolution. Aqueous solutions are cryo-frozen and transferred into the FIB for tip shaping and finally to the atom probe. The sample must be permanently cooled to preserve them in the solid state. To avoid artefacts, crystallization has to be prevented. By innovative ink-jet printing, we try to avoid the crystallization of ice crystals (Supervisor: Guido Schmitz, Sakshi Sinha)