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Abstract: MAX phases are an extremely versatile family of layered compounds that usually consist of an early to-mid transition metal (M-element), a main group element (mainly groups 13–15) or late transition metal (A-element) and carbon and/or nitrogen (X-element). It is therefore not too surprising that in addition to the roughly 70 compounds with 211 stoichiometry, there exist many solid solutions with mixed elements on the M- and A-site, respectively. Much less common are solid solution phases with mixed elements on both M- and A-site simultaneously (double-site solid solutions), as well as solid solutions on the X-site (carbonitride MAX phases). Challenging these restrictions in the chemical composition space, we present here for the first time (V0.2Cr0.8)2(Ga0.5Ge0.5)(C0.6N0.4) as a new carbonitride member of the MAX phase family, containing solid solutions on all three lattice sites simultaneously. This triple-site solid solution MAX phase is synthesized by high-temperature solid-state methods, and we demonstrate that it is possible to use two different nitrogen-containing precursors (VN and Cr2N), respectively. Structure, morphology and chemical composition are characterized by X-ray powder diffraction (XRD), electron microscopy (SEM/TEM), secondary ion mass spectrometry (SIMS), and X-ray photoelectron spectroscopy (HAXPES).

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Abstract: Metallic, two-dimensional molybdenum disulfide (MoS2) nanosheets show promise for energy storage and catalysis applications. However, current chemical exfoliation methods require more than 48 h to produce milligrams of material, and result in an impure mixture of metallic (1T/1T′, approximately 50%–70%) and semiconducting (2H) phases. Here we demonstrate large-scale and rapid (>600 g h−1) production of nearly pure-phase metallic two-dimensional MoS2 nanosheets using microwave irradiation. Atomic-resolution imaging and X-ray photoelectron spectroscopy show nearly 100% metallic phase in the basal plane. This high purity leads to a large exchange current density (0.175 ± 0.030 mA cm−2) and low Tafel slopes (39–47 mV dec−1) for hydrogen evolution reaction. In supercapacitors and lithium–sulfur pouch-cell batteries, the resulting nanosheets enable a high volumetric capacitance of 753.0 ± 3.6 F cm−3 and a specific capacity of 1,245 ± 16 mAh g−1 (electrolyte-to-sulfur ratio, 2 µl mg−1), respectively. Our method provides a practical pathway for producing high-quality metallic two-dimensional materials for high-performance energy devices.

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Abstract: Kagome metals are an intriguing class of quantum materials as the presence of both flat bands and Dirac points provides access to functional properties present in strongly correlated and topological materials. To fully harness these electronic features, the ability to tune the Fermi level relative to the band positions is needed. Here, we explore the structural, electronic, and magnetic impacts of substitutional alloying within ferromagnetic kagome metal Fe3Sn2 in thin films grown by molecular beam epitaxy. Transition metals, Mn and Co, are chosen as substitutes for Fe to reduce or increase the d-band electron count, thereby moving the Fermi level accordingly. We find that Co is not incorporated into the Fe3Sn2 structure but instead results in a two-phase Fe–Co and (Fe,Co)Sn composite. In contrast, Fe3−xMnxSn2 films are realized with x of up to 1.0, retaining crystalline quality comparable with the parent phase. The incorporation of Mn repositions the flat bands relative to the Fermi level in a manner consistent with hole-doping, as revealed by hard x-ray photoemission and density functional theory. The Fe3−xMnxSn2 films retain room temperature ferromagnetism, with x-ray magnetic circular dichroism measurements confirming that the Fe and Mn moments are ferromagnetically aligned. The ability to hole-dope this magnetic kagome metal provides a platform for tuning properties, such as anomalous Hall and Nernst responses.

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Abstract: Superconducting microwave circuits are a promising physical implementation of quantum computing. A significant challenge with the development of superconducting qubits is the materials source of loss leading to qubit decoherence. Much of this loss is dielectric loss or quasiparticle dissipation at superconductor surfaces and interfaces. In order to understand the loss mechanisms microscopically, it is necessary to have an accurate picture of the atomic structure and microstructure. In this work, we explore the structure of three interfaces. First, we present non- destructive X-ray characterization of the NbH surface precipitation in Nb thin films. Unwanted hydride precipitation in niobium-based superconducting circuits is a side effect of hydrofluoric acid etching of the Nb surface oxide. The precipitate microstructure is challenging to probe because of the high mobility of hydrogen in niobium. Using X-rays diffraction, we show evidence supporting phase-field simulations that the nucleation of NbH occurs at free surfaces. Using darkfield X-ray nanoprobe microscopy, we identify a complex microstructure suggesting a martensitic nucleation that transitions to a dendritic growth. Next, we present X-ray standing wave excited X-ray photoelectron spectroscopy of the annealed, Nb(110) ordered oxide surface layer. We discover the existence of an oxygen interstitial rich subsurface layer and identify the origin of two distinct oxygen chemical states at the surface: one coming from this subsurface layer, and the other from the surface NbO termination. Last, we present the heteroepitaxy of single crystal Al2O3 with a TiN. We identify TiN as an ideal substrate for the epitaxial growth of Al2O3 in a capacitor geometry based on the high degree of crystallinity and sharp interfaces with minimal diffusion and 3 we measure the two-level-state loss of the Al2O3 junction dielectric layer. We present this interface as an alternative to the commonly used amorphous alumina in Josephson Junctions.

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Abstract: Hydrogen shows promise as the energy vector of the future, but problems with storage and transport are significant. Storage of hydrogen as ammonia has the potential to solve these problems, but current catalysts for its cracking are not efficient enough to enable the large-scale application of ammonia. Carbon materials, such as carbon nanotubes (CNTs), have shown potential as supports for ammonia decomposition catalysts. This thesis investigates the use of graphitised nanofibers (GNFs), which offer high purity and graphitisation, as a support material for Ru catalysts. Ru/GNF was synthesised using magnetron sputtering and tested for catalytic activity in ammonia decomposition and the catalyst exhibited self-improvement over the course of the reaction. The evolution of the Ru nanoclusters on GNF was studied by Identical Location Scanning Transmission Electron Microscopy (IL-STEM). The analysis revealed that the Ru nanoclusters undergo significant morphological changes during the reaction - transforming from flat and amorphous structures to more three-dimensional crystalline nanoclusters. The step-edges on the GNF surface help to stabilise the Ru nanoclusters, preventing excessive growth and maintaining a high density of active sites. Spectroscopic analysis using in-operando EXAFS and ex-situ XPS provide further insights into the mechanism behind the self-improvement. EXAFS data suggest that the Ru nanoparticles undergo bulk nitridation during the reaction. This is supported by XPS analysis, which confirms the formation of a metal nitride species. It is proposed that the formation of bulk nitrided Ru nanoclusters leads to a change in the reaction mechanism, increasing the number of active sites and enhancing the catalyst’s activity. This thesis highlights the importance of studying the dynamic behaviour of catalysts and provides an understanding of the self-improvement mechanism in Ru/GNF. This knowledge can contribute to the design of more efficient and stable catalysts for low-temperature ammonia cracking, advancing sustainable hydrogen production technologies.