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Item type:Publication, Access status: with files , Enhancing 3D reconstruction accuracy of FIB tomography data using multi-voltage images and multimodal machine learningFIB-SEM tomography is a powerful technique that integrates a focused ion beam (FIB) and a scanning electron microscope (SEM) to capture high-resolution imaging data of nanostructures. This approach involves collecting in-plane SEM images and using FIB to remove material layers for imaging subsequent planes, thereby producing image stacks. However, these image stacks in FIB-SEM tomography are subject to the shine-through effect, which makes structures visible from the posterior regions of the current plane. This artifact introduces an ambiguity between image intensity and structures in the current plane, making conventional segmentation methods such as thresholding or the k-means algorithm insufficient. In this study, we propose a multimodal machine learning approach that combines intensity information obtained at different electron beam accelerating voltages to improve the three-dimensional (3D) reconstruction of nanostructures. By treating the increased shine-through effect at higher accelerating voltages as a form of additional information, the proposed method significantly improves segmentation accuracy and leads to more precise 3D reconstructions for real FIB tomography data.Publicationtype: Journal Article76 128 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Chemical Interface Damping by Electrochemical Gold Oxidation(American Chemical Society (ACS), 2024-09-19); ; ; ; Chemical interface damping is a change in the effective collision frequency of conduction band electrons in metal originating from a chemical change in the metal interface. In this work, we present in situ ellipsometric measurements that reveal the chemical interface damping effect from the electrochemical oxidation of single-crystal and polycrystalline gold films. We observed an increase in collision frequency of up to 21 meV for single-crystalline gold. To compare with literature results obtained with thiols and metal oxides on gold nanoparticles, we normalize the collision frequency by the electron mean free path to the surface of the structure. We show that electrochemical gold oxidation provides a stronger effect on the collision frequency than these coatings. Similar ellipsometric experiments have previously been conducted to investigate the optical properties of gold oxide but without taking chemical interface damping into account. The change in reflection from the oxidation of gold was solely attributed to the oxide coating. We also show that the chemical interface damping effect saturates at a larger effective oxide thickness, which is attributed to the stabilization of the gold-oxide interface. Our results are important for the design of optoelectronic and photochemical experiments involving nanostructured gold. Electrochemical gold oxidation can be used to significantly change the optical response of such structures and to increase the efficiency of the light-matter interaction.Publicationtype: Journal Article33 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Demystifying the semiconductor-to-metal transition in amorphous Vanadium pentoxide: the role of substrate/thin film interfaces(Wiley-VCH, 2024-07-24); ; ; The precise mechanism governing the reversible semiconductor-to-metal transition (SMT) in V2O5 remains elusive, yet its investigation is of paramount importance due to the remarkable potential of V2O5 as a versatile “smart” material in advancing optoelectronics, plasmonics, and photonics. In this study, distinctive experimental insights into the SMT occurring in amorphous V2O5 through the application of highly sensitive, temperature-dependent, in situ analyses on a V2O5 thin film deposited on soda-lime glass are presented. The ellipsometry measurements reveal that the complete SMT occurs at ≈340 °C. Remarkably, the refractive index and extinction coefficients exhibit reversible characteristics across visible and near-infrared wavelengths, underscoring the switch-like behavior inherent to V2O5. The findings obtained from ellipsometry are substantiated by calorimetry and in situ secondary ion mass spectrometry analyses. In situ electron microscopy observations unveil a separation of oxidation states within V2O5 at 320 °C, despite the thin film retaining its amorphous state. The comprehensive experimental investigations effectively demonstrate that alterations in electronic state can trigger the SMT in amorphous V2O5. It is revealed for the first time that the SMT in V2O5 is solely contingent upon electronic state changes, independent of structural transitions, and importantly, it is a reversible transformation within the amorphous state itself.Publicationtype: Journal Article24 54 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Water, not salt, causes most of the Seebeck effect of nonisothermal aqueous electrolytes(American Physical Society (APS), 2024-05-03); ; ; A temperature difference between two electrolyte-immersed electrodes often yields a voltage Δψ between them. This electrolyte Seebeck effect is usually explained by cations and anions flowing differently in thermal gradients. However, using molecular simulations, we found almost the same Δψ for cells filled with pure water as with aqueous alkali halides. Water layering and orientation near polarizable electrodes cause a large temperature-dependent potential drop χ there. The difference in χ of hot and cold electrodes captures most of the thermovoltage, Δψ≈χhot-χcold.Publicationtype: Journal Article11 83 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Iridium-based selective emitters for thermophotovoltaic applications(Elsevier, 2023-10-12); ; ; ; Raudsepp, RagleThe long-term operation of refractory-metal-based metamaterials is crucial for applications such as thermophotovoltaics. The metamaterials based on refractory metals like W, Mo, Ta, Nb, and Re fail primarily by oxidation. Here, the use of the noble metal Ir is proposed, which is stable to oxidation and has optical properties comparable to gold. The thermal endurance of Ir in a 3-layer-system, consisting of HfO2/Ir/HfO2, by performing annealing experiments up to 1240 °C in a pressure range from 2 × 10−6 mbar to 1 bar, is demonstrated. The Ir layer shows no oxidation in a vacuum and inert gas atmosphere. At temperatures above 1100 °C, the Ir layer starts to agglomerate due to the degradation of the confining HfO2 layers. An in situ X-ray diffraction experimental comparison between 1D multilayered Ir/HfO2 and W/HfO2 selective emitters annealed at 1000 °C, 2 × 10−6 mbar, over 100 h, confirms oxidation stability of Ir while W multilayers gradually disappear. The results of this work show that W-based metamaterials are not long-term stable even at 1000 °C. However, the oxidation resistance of Ir can be leveraged for refractory plasmonic metamaterials, such as selective emitters in thermophotovoltaic systems with strong suppression of long wavelength radiation.Publicationtype: Journal ArticleScopus© Citations 6 29 90 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Thermal stability of magnetron sputtered W/HfO₂ and Ir/HfO₂ thin films used as selective emitter in thermophotovoltaic applicationsHochtemperaturstabilität und Langlebigkeit eines selektiven Emitters sind die beiden entscheidenden Parameter, um hohe Strahlungsleistung und einen höheren Wirkungsgrad beim Einsatz in einem thermophotovoltaischen System (TPV) zu erzielen. Bei hohen Temperaturen sind thermisch-aktivierte Prozesse wie Diffusion, Kornwachstum, Phasentransformation, Erholung, chemische Reaktionen insbesondere Oxidation sowie Sublimation von Metalloxiden wirksam und somit stellt die thermische Stabilität eines Dünnschichtsystems eine besondere Herausforderung dar. In dieser Arbeit wurden die thermische Stabiliät und die Versagensmechanismen einer 1D-Metamaterial-Vielfachschicht als selektiver Emitter untersucht und analysiert. Zwei Dünnschichtsysteme: W/HfO2 und Ir/HfO2 standen aufgrund ihrer besonderen Materialeigenschaften (hoher Schmelzpunkt und niedriger Vakuumdampfdruck) im Fokus. Sie wurden mittels Magnetron sputtering hergestellt und durch Röntgenbeugung (XRD) untersucht. Die in-situ Hochtemperaturuntersuchungen (bis zu 2000 ⁰C) unter Verwendung von Heizbändern und die Abschätzung der Probentemperatur waren herausfordernd, dadurch wurde ein spezielles Wissen für die Hochtemperaturvalidierung unterschiedlicher Materialien aufgebaut. Der erste Teil der Arbeit zeigt die Heizexperimente an selektiven W/HfO2-Emitterschichten in verschiedenen Temperatur- (1000 ⁰C bis 1520 ⁰C) und Vakuumdruckbereichen (2 × 10-2 mbar bis 2 × 10-6 mbar). Eine selektive W/HfO2-Emitterschicht ist bis 6 h bei 1400 ⁰C und 2 × 10-5 mbar stabil, welches bisher einer der weltweit höchsten erreichten Werte ist. Darüber hinaus erfolgte eine detaillierte in-situ Röntgenanalyse am Schichtsystem HfO2/W/HfO2 zwischen 300 ⁰C und 1520 ⁰C im Hinblick auf die Mechanismen, die zu strukturellen und mikrostrukturellen Veränderungen wie Relaxation, Phasenumwandlung, Kornwachstum und schließlich zum Versagen (Degradation) führen. Der Hauptgrund des Versagens in einer selektiven W/HfO2-Emitterschicht ist auf die Oxidation des Wolframs gefolgt durch eine Sublimation des Suchtigen Wolframoxids zurückzuführen. Für diesen Prozess wurde eine Aktivierungsenergie von 1.2 eV abgeschätzt. Im Falle des HfO2 wurde eine polymorphe Phasentransformation beobachtet. Die strukturelle Umwandlung der Phasen verursacht einen Anstieg des Volumens der Einheitszelle, der wiederum zur Bildung von Leerstellen, Poren sowie der Entstehung von Transportkan alen entlang der Korngrenzen in der keramischen Schicht führt. Im zweiten Teil wird ein systematischer Ansatz verfolgt, um die thermische Stabilität des Metalls Iridium gegen Oxidation und die Lebensdauer der selektiven Ir/HfO2-Emitterschicht zu testen. Zunächst wurden in-situ XRD-Untersuchungen am Schichtsystem: HfO2/Ir/HfO2 zwischen 800 ⁰C - 1100 ⁰C bei 2 × 10-6 mbar durchgeführt. Iridium zeigte eine sehr gute thermische Stabilität bis zu 1000 ⁰C unterstützt durch HfO2 als hervorragende Deckschicht. Bei Temperaturen über 1000 ⁰C begann die Iridium-schicht zu agglomerieren, was bedeutet, dass die treibende Kraft eine Minimierung der Oberflächenenergie ist. Die thermische Stabilität wurde auch durch exsitu XRD-Untersuchungen bei unterschiedlichen Drücken von 1 bar bis zu 2 × 10-5 mbar im Schichtsystem HfO2/Ir/HfO2 verifiziert, welche bis zu 100 h bei 1000 ⁰C und 2 × 10-2 mbar stabil ist. Satellitenreflexe wurden nahe am primären (111) Ir-Peak beobachtet, die als wichtiger Hinweis für die thermische Stabilität dienen können. Ein finaler Vergleich erfolgte zwischen verschiedenen Metamaterialschichten bestehend aus W/HfO2 und Ir/HfO2 bei 1000 ⁰C und 2 × 10-2 mbar. Über 100 h weist Iridium eine exzellente thermische Stabilität im Vergleich zu Wolfram auf. Die in dieser Arbeit durchgeführten Hochtemperaturexperimente und - analysen bieten eine neue Perspektive auf die thermische Stabilität, die in Dünnschichtsystemen praktisch erreicht werden kann und ermöglichen die Entwicklung selektiver Emitter für TPV-Systeme der nächsten Generation.Publicationtype: Doctoral Thesis115 184 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Machine learning models for photonic crystals band diagram prediction and gap optimisation(2022-12); ; ; ; Data-driven methods of machine learning (ML) have attracted a lot of interest in various fields of physics. Inverse design and optimisation of structured optical metamaterials such as photonic crystals, metasurfaces, and other nanostructured components seem to benefit a lot from this approach in the nearest future. Here we develop several approaches to use ML methods to predict and optimise properties of photonic crystals (e.g. size of bandgaps) effectively. We use a dataset of 2D photonic crystals produced recently in [T.Christinsen et al., Nanophotonics 9, 4183 (2020)]. For improving performance of predictive models, we apply symmetry-aware augmentations and hybrid ML-solver approaches. As a result, considerable improvement in prediction accuracy could be achieved as compared to baseline models. For generative models, we apply variational autoencoders (VAEs) combined with predictor architecture, inspired by related works in chemical design realm. By using latent space optimisation, we achieve good results in the task of increasing bandgaps of photonic structures. The approach seems to be very promising and can be extended to 3D geometries.Publicationtype: Journal Article84 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Which factor determines the optical losses in refractory tungsten thin films at high temperatures?(Elsevier, 2022-03-04); ; ; ; Refractory tungsten (W) plays an important role in high temperature photonic/plasmonic applications. Previously room temperature bulk single/poly-crystalline optical constants were extensively used to calculate the optical properties of W nanostructures at high temperatures. This might lead to a significant deviation between the predicted and measured optical properties due to the exclusion of electron–phonon interaction, as well as grain-boundary and surface-scattering. Herein, we show experimentally, how film thickness and temperature affects the optical losses in W. A drastic increase in the effective electron collision frequency is observed with decreasing the film thickness down to 5 nm, due to the grain-boundary and surface-scattering mechanisms. At sufficiently high temperatures (greater than 200 °C for W), the electron–phonon interaction eventually becomes the dominant mechanism, linearly increasing collision frequency with temperature, and it is independent of the geometry of the thin film structure. The impact of thickness and temperature-dependent optical properties of W is showcased with a hyperbolic 1D metamaterial structure acting as a thermophotovoltaic emitter. This work opens new directions in accurate prediction of the optical properties of nanostructures and design of efficient devices in various applications, such as aerospace, energy-efficient lighting, radiative cooling and energy harvesting, by incorporating thickness and temperature-dependent optical constants.Publicationtype: Journal ArticleScopus© Citations 6 87 445 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Spectrally selective emitters based on 3D Mo nanopillars for thermophotovoltaic energy harvesting(Elsevier, 2021-08-10); ; ; ; High-temperature stable emitters with spectral selective functionality are an absolute condition for efficient conversion of thermal radiation into electricity using thermophotovoltaic (TPV) systems. Usually, spectral selective emitters are made up of multilayered materials or geometrical structures resulting from complex fabrication processes. Here, we report a spectrally selective emitter based on a single metal layer coating of molybdenum (Mo) over a 3D dielectric pillar geometry. 3D Mo nanopillars are fabricated using large-area and cost-effective hole-mask colloidal lithography. These nanostructures show an absorptivity/emissivity of 95% below the cut-off wavelength of an InGaAsSb PV cell at 2.25 μm, and a sharp decline in absorptivity/emissivity in the near-infrared regions, approaching a low emissivity of 10%. The 3D Mo nanopillars show outstanding thermal/structural stability up to 1473 K for 24 h duration under Ar atmosphere and polarization and angle invariance up to 60° incidence angles. With a low-cost and scalable fabrication method, 3D Mo nanostructures provide tremendous opportunities in TPV and high temperature photonic/plasmonic applications.Publicationtype: Journal ArticleScopus© Citations 25 116 282 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Direct Plasmonic Excitation of the Hybridized Surface States in Metal Nanoparticles(2021-07-21); ; ; Plasmon-driven chemical reactions are a subject that is currently capturing the attention of the research community and generates a fair amount of arguments about their origin. Taking into account that the lifetime of excited hot carriers in metals is very short, some mechanism is required to store carriers long enough and in sites that allow chemical reactions with the environment. One established mechanism is the injection of charges into either the valence or conduction band of a semiconductor, followed by a chemical reaction at the semiconductor surface. Here, we consider a somewhat less-explored pathway by which plasmon decay can cause a chemical reaction: the direct excitation of hybridized surface states by plasmons. Using a simple model, we evaluate theoretically the rate of direct excitation and find that it can be comparable and often exceed the rate of indirect excitation of surface states. Our findings correspond to prior experimental results. We also identify the conditions under which one can enhance the direct excitation efficiency and, thus, bring plasmon-driven photochemistry closer to practical applications.Publicationtype: Journal Article90