Abstract: During the past decades, metallic nanoparticles (NPs) have attracted great attention in materials science due to their specific optical properties based on surface plasmon resonances. Because of these phenomena, plasmonic NPs (or nanoplasmonics) are very promising for application in biosensing, photocatalysts, medicine, data storage, solar energy conversion, etc. Currently, colloidal synthesis techniques enable scientists to routinely produce mono and bimetallic NPs of various shapes, sizes, composition, and elemental distribution, with superior properties for plasmonic applications. Two primary directions for further advancing nanoplasmonic-based technologies include synthesizing novel morphologies, such as highly asymmetric chiral NPs, and gaining deeper insights into the factors affecting the stability of produced nanoplasmonics. With the increasing complexity of nanoplasmonics morphologies and higher stability requirements, there is a pressing need for thorough investigations into their 3D structures and their evolution under different conditions, with high resolution. Electron tomography (ET) emerges as an ideal tool to retrieve shape and element-sensitive information about individual nanoparticles in 3D, achieving resolutions down to the atomic level. Moreover, ET techniques can be combined with in situ holders, enabling detailed studies of processes mimicking real applications of nanoplasmonic-based devices. The first part of this thesis will focus on detailed studies of chiral Au NPs, promising for spectroscopy techniques based on the differential absorption of left- and right-handed circularly polarized light. Specifically, I will discuss the primary strategies for wet-colloidal growth of the various types of intrinsically chiral Au NPs. Advanced ET methods will be demonstrated as powerful tools for characterizing the final helical morphologies of the produced Au NPs and for studying the chiral growth mechanisms by examining intermediate structures obtained during chiral growth. The second part will focus on the heat-induced stability of various Au@Ag core-shell NPs. Operating in real conditions, such as elevated temperatures, may cause particle reshaping and redistribution of metals between the core and shell, gradually altering nanoplasmonics properties. Hence, a thorough understanding of the influence of size, shape, and defects on these processes is crucial for further developments. Recently developed techniques, combining fast ET with in-situ heating holders, have allowed me to evaluate the influence of various parameters (size, shape, defect structure) on heat-induced elemental redistribution in Au@Ag core-shell nanoparticles qualitatively and quantitatively. Additionally, I will discuss the prospects of high-resolution ET for visualizing the diffusion of individual atoms within complex nanostructures.

Abstract: Production of struvite (MgNH4PO4·6H2O) from waste streams is increasingly implemented to recover phosphorus (P), which is listed as a critical raw material in the European Union (EU). To facilitate EU-wide trade of P-containing secondary raw materials such as struvite, the EU issued a revised fertilizer regulation in 2019. A comprehensive overview of the supply of struvite and its quality is presently missing. This study aimed: i) to determine the current EU struvite production volumes, ii) to evaluate all legislated physicochemical characteristics and pathogen content of European struvite against newly set regulatory limits, and iii) to compare not-regulated struvite characteristics. It is estimated that in 2020, between 990 and 1250 ton P are recovered as struvite in the EU. Struvite from 24 European production plants, accounting for 30% of the 80 struvite installations worldwide was sampled. Three samples failed the physicochemical legal limits; one had a P content of <7% and three exceeded the organic carbon content of 3% dry weight (DW). Mineralogical analysis revealed that six samples had a struvite content of 80–90% DW, and 13 samples a content of >90% DW. All samples showed a heavy metal content below the legal limits. Microbiological analyses indicated that struvite may exceed certain legal limits. Differences in morphology and particle size distribution were observed for struvite sourced from digestate (rod shaped; transparent; 82 mass% < 1 mm), dewatering liquor (spherical; opaque; 65 mass% 1–2 mm) and effluent from upflow anaerobic sludge blanket reactor processing potato wastewater (spherical; opaque; 51 mass% < 1 mm and 34 mass% > 2 mm). A uniform soil-plant P-availability pattern of 3.5–6.5 mg P/L soil/d over a 28 days sampling period was observed. No differences for plant biomass yield were observed. In conclusion, the results highlight the suitability of most struvite to enter the EU fertilizer market.

Abstract: To provide for the globally increasing demand for proteinaceous food, microbial protein (MP) has the potential to become an alternative food or feed source. Phosphorus (P), on the other hand, is a critical raw material whose global reserves are declining. Growing MP on recovered phosphorus, for instance, struvite obtained from wastewater treatment, is a promising MP production route that could supply protein-rich products while handling P scarcity. The aim of this study was to explore struvite dissolution kinetics in different MP media and characterize MP production with struvite as sole P-source. Different operational parameters, including pH, temperature, contact surface area, and ion concentrations were tested, and struvite dissolution rates were observed between 0.32 and 4.7 g P/L/d and a solubility between 0.23 and 2.22 g P-based struvite/L. Growth rates and protein production of the microalgae Chlorella vulgaris and Limnospira sp. (previously known as Arthrospira sp.), and the purple non‑sulfur bacterium Rhodopseudomonas palustris on struvite were equal to or higher than growth on conventional potassium phosphate. For aerobic heterotrophic bacteria, two slow-growing communities showed decreased growth on struvite, while the growth was increased for a third fast-growing one. Furthermore, MP protein content on struvite was always comparable to the one obtained when grown on standard media. Together with the low content in metals and micropollutants, these results demonstrate that struvite can be directly applied as an effective nutrient source to produce fast-growing MP, without any previous dissolution step. Combining a high purity recovered product with an efficient way of producing protein results in a strong environmental win-win.

Abstract: In iets meer dan 30 jaar moeten we tot 50% meer eiwit produceren, terwijl onze conventionele lineaire voedselproductieketen reeds de grenzen van duurzaamheid overschrijdt. Microbieel eiwit (MP), de eiwitrijke biomassa van micro-organismen, werd onderzocht als nieuw voeder en voedselingrediënt in een circulaire eiwitproductieketen. MP-productie kan gebaseerd zijn op primaire grondstoffen om zo de inefficiënte stappen in de huidige eiwitproductie over te slaan, of de productie kan gebaseerd zijn op secundaire grondstoffen afkomstig van de verliezen in de conventionele productieketen (bijvoorbeeld afvalwater). Zowel indirecte als directe strategiën voor nutriëntenherwinning kunnen geïdentificeerd worden. Indirecte nutriëntenherwinning werd onderzocht door struviet toe te passen als fosfor- en stikstofbron voor MP-productie. Struviet, een herwinningsproduct in de afvalwaterzuivering, bleek een hoge zuiverheidsgraad te hebben waardoor het een uitstekende selectieve barrière vormt tussen afvalstroom en MP productie. Oplossingssnelheden en MP groei-experimenten toonden aan dat struviet gemakkelijk gedoseerd kan worden in functie van de microbiële nutriëntenbehoeftes, waarnaast problemen vermeden worden gerelateerd aan de turbiditeit van nevenstromen, wat de productie van fototrofe MP mogelijk maakt. Daarnaast werd ook directe nutriëntenherwinning onderzocht. Hierbij werden de voedingswaarde en veiligheid geanalyseerd van biomassa bestaande uit aerobe heterotrofe bacteriën (AHB) afkomstig van effluentzuivering van 25 bedrijven in de voedings- en drankensector. Verder werd ook de temporele variabiliteit in kaart gebracht. Er werd een veelbelovend eiwitgehalte geobserveerd waarvan de variabiliteit aanzienlijke was. Verder werd aangetoond dat het eiwitgehalte positief gecorreleerd was met stikstofbelasting en negatief met slibverblijftijd, wat een indicatie is dat de implementatie van een geoptimaliseerd productieproces, de biomassakwaliteit gevoelig kan verhogen. Bovendien was de veiligheid als veevoeder gegarandeerd op vlak van de meeste contaminanten. In het daarop volgende hoofdstuk werd de variabiliteit in voedingswaarde en veiligheid bestudeerd van commerciële fototrofe MP (de microalgen Chlorella en Spirulina), gekweekt op primaire grondstoffen. De waargenomen variabiliteit in voedingswaarde vereist verdere optimalisatie van het productieproces. Er werd ook waargenomen dat een hoog eiwitgehalte geen hoge totale voedingswaarde impliceert, omdat de verteerbaarheid en de eiwitkwaliteit nog steeds ongunstig kunnen zijn. Op basis van gemeten contaminanten geeft een veilige consumptiedosis aan dat microalgen perfect kunnen worden geconsumeerd als volwaardige eiwitbron in plaats van als supplement, hun huidige hoofdtoepassing. In een laatste experimenteel hoofdstuk werd onderzocht hoe de variabiliteit in biomassakwaliteit van microalgen kan worden gereduceerd en hoe een stabiele, hoogwaardige biomassaproductie kan worden bekomen. De invloed van oogsttijd, operationele modus en fotoperiode werd bepaald op de productiviteit van biomassa, eiwit en essentiële aminozuren en er werden optimale productieparameters geïdentificeerd. Samengevat is MP-productie op basis van indirecte en directe nutriëntenherwinning veelbelovend. Verdere technologische ontwikkelingen en het verhogen van bewustwording en sociale acceptatie, moeten een verdere introductie van MP in de voeder- en voedingsmarkt faciliteren. Hier kan MP een belangrijke oplossing vertegenwoordigen om de exponentieel groeiende wereldbevolking op een duurzame manier te voeden.

Abstract: Lead halide perovskites have been widely explored
in the field of photovoltaics, light-emitting diodes, and lasers due to
their outstanding linear and nonlinear optical (NLO) properties.
But, the presence of lead toxicity and low chemical stability remain
serious concerns. Lead-free double perovskite with excellent
optical properties and chemical stability could be an alternative.
However, proper examination of the NLO properties of such a
material is crucial to identify their utility for future nonlinear device
applications. Herein, we have made use of femtosecond (fs) Z-scan
technique to explore the NLO properties of Cs2AgIn0.9Bi0.1Cl6
nanocrystals (NCs). Our measurements suggest that under
nonresonant fs excitation, perovskite NCs exhibit strong twophoton
absorption (TPA). The observed saturation of TPA at high
light intensities has been explained by a customized model. Furthermore, we have demonstrated a change in the nonlinear refractive
index of the NCs under varying input intensities. The strong TPA absorption of lead-free double perovskite NCs could be used for
Kerr nonlinearity-based nonlinear applications such as optical shutters for picosecond lasers.

Abstract: Using a numerical tight-binding approach based on the Chebyshev–Bogoliubov–de Gennes method we describe Josephson junctions made of multilayer graphene contacted by top superconducting gates. Both Bernal (ABA) and rhombohedral (ABC) stacking are considered and we find that the type of stacking has a strong effect on the proximity effect and the supercurrent flow. For both cases the pair amplitude shows a polarization between dimer and nondimer atoms, being more pronounced for rhombohedral stacking. Even though the proximity effect in nondimer sites is enhanced when compared to single-layer graphene, we find that the supercurrent is suppressed. The spatial distribution of the supercurrent shows that for Bernal stacking the current flows only in the topmost layers while for rhombohedral stacking the current flows throughout the whole structure.

Abstract: Using highly efficient GPU-based simulations of the tight-binding Bogoliubov-de Gennes equations we solve self-consistently for the pair correlation in rhombohedral (ABC) and Bernal (ABA) multilayer graphene by considering a finite intrinsic s-wave pairing potential. We find that the two different stacking configurations have opposite bulk/surface behavior for the order parameter. Surface superconductivity is robust for ABC stacked multilayer graphene even at very low pairing potentials for which the bulk order parameter vanishes, in agreement with a recent analytical approach. In contrast, for Bernal stacked multilayer graphene, we find that the order parameter is always suppressed at the surface and that there exists a critical value for the pairing potential below which no superconducting order is achieved. We considered different doping scenarios and find that homogeneous doping strongly suppresses surface superconductivity while nonhomogeneous field-induced doping has a much weaker effect on the superconducting order parameter. For multilayer structures with hybrid stacking (ABC and ABA) we find that when the thickness of each region is small (few layers), high-temperature surface superconductivity survives throughout the bulk due to the proximity effect between ABC/ABA interfaces where the order parameter is enhanced. DOI: 10.1103/PhysRevB.87.134509

Abstract: We report the mapping of polarization-induced internal electric fields in AlN/GaN nanowire heterostructures at unit cell resolution as a key for the correlation of optical and structural phenomena in semiconductor optoelectronics. Momentum-resolved aberration-corrected scanning transmission electron microscopy is employed as a new imaging mode that simultaneously provides four-dimensional data in real and reciprocal space. We demonstrate how internal mesoscale and atomic electric fields can be separated in an experiment, which is verified by comprehensive dynamical simulations of multiple electron scattering. A mean difference of 5.3 +- 1.5 MV/cm is found for the polarization-induced electric fields in AlN and GaN, being in accordance with dedicated simulations and photoluminescence measurements in previous publications.

Abstract: The Zn-carboxylate-based porous coordination polymer MOF-5 [Zn4O(bdc)3] and the metal oxide loaded materials ZnO@MOF-5 and TiO2@MOF-5 were loaded in a second step with the precursor [ClAuCO] to yield intermediate materials denoted as [ClAuCO]@MOF-5, [ClAuCO]/ZnO@MOF-5 and [ClAuCO]/TiO2@MOF-5. These composites were decomposed to Au@MOF-5, Au/ZnO@MOF-5 and Au/TiO2@MOF-5 under hydrogen at 100 °C. The nanoparticle-loaded hybrid materials were characterised by powder X-ray diffraction (PXRD), IR spectroscopy, X-ray photoelectron spectroscopy (XPS) and N2 sorption measurements, which reveal an intact MOF-5 structure that maintains a high specific surface area. For Au@MOF-5, crystalline Au nanoparticles were distributed over the MOF matrix in a homogeneous fashion with a size of ca. 13 nm, evidenced by high resolution transmission electron microscopy. In the case of Au/ZnO@MOF-5, the Au and metal oxide particles of a few nm in size were coexistent in a given volume of the MOF-5 matrix and were not separated in different crystalline MOF particles. For the TiO2 loaded materials the oxide is preferentially located near the outer surface of the MOF particles, leading to an increase of larger exterior Au particles in comparison to very small interior Au particles as observed for the other materials. Au@MOF-5, Au/ZnO@MOF-5 and Au/TiO2@MOF-5 were tested in liquid-phase oxidation of alcohols. Preliminary results show a high activity for the Au loaded materials in this reaction. This observation is attributed to the microstructure of the composites with very small Au particles distributed homogeneously over the MOF matrix.

Abstract: We introduce a new method to measure structure factors from parallel beam electron diffraction (PBED) patterns. Bloch wave refinement routines were developed which can minimise the difference between simulated and experimental Bragg intensities via variation of structure factors, Debye parameters, specimen thickness and -orientation. Due to plane wave illumination, the PBED refinement is highly efficient not only in computational respect, but also concerning the experimental effort since energy filtering is shown to have no significant effect on the refinement results. The PBED method was applied to simulated GaAs diffraction patterns to derive systematic errors and rules for the identification of plausible refinement results. The evaluation of experimental GaAs PBED patterns yields a 200 X-ray structure factor of -6.33±0.14. Additionally, we obtained -6.35±0.13 from two-dimensional convergent beam electron diffraction refinements. Both results confirm density functional theory calculations published by Rosenauer et al. and indicate the inaccuracy of isolated atom scattering data, which is crucial e.g. for the composition evaluation by lattice fringe analysis.

Abstract: We describe investigations of the largely unexplored field of mesoscopic type-I superconductors. Micromagnetometry and 3D Ginzburg-Landau simulations of our single crystal β-tin samples in this regime reveal size- and temperature-dependent supercritical fields whose behavior is radically different from the bulk critical field HcB. We find that complete suppression of the intermediate state in medium-size samples can result in a surprising reduction of the critical field significantly below HcB. We also reveal an evolution of the superconducting-to-normal phase transition from the expected irreversible first order at low temperatures through the previously unobserved reversible first-order to a second-order transition close to Tc, where the critical field can be many times larger than HcB. Finally, we have identified striking correlations between the mesoscopic Hc3 for nucleation of surface superconductivity and the thermodynamic Hc near Tc. All these observations are entirely unexpected in the conventional type-I picture.