FG Angewandte Physik und Halbleiterspektroskopie
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Cerium oxide-tin oxide (CeOx/SnOx) thin films with varying Sn content were deposited using RF magnetron sputtering and investigated for hydrogen sensing applications. Structural, compositional, and morphological properties were characterized using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), transmission electron microscopy (TEM), and energy-dispersive X-ray spectroscopy (EDX). Gas sensing measurements showed effective hydrogen detection at room temperature, with the sensitivity strongly influenced by Sn content and oxygen vacancy concentration. Higher Sn concentration enhanced the sensing response, which was correlated with microstructural features obtained from AFM and EDX, as well as with the presence of Ce3+ and Ce4+ oxidation states identified by XPS. This study highlights the potential of CeOx/SnOx thin films for possible back-end-of-line integration and provides proof-of-principle for room-temperature hydrogen sensing.
Here we report an investigation of ultrathin tin oxide films on Pt3Sn(111) using low-energy electron microscopy (LEEM), microspot low-energy electron diffraction (𝜇-LEED), scanning tunneling microscopy (STM), surface X-ray diffraction (SXRD), and high-resolution X-ray photoelectron spectroscopy (XPS). Oxidation at ∼390–410 ◦C produces triangular, two-dimensional oxide islands that nucleate rapidly and exhibit self-limited lateral growth, attributed to limited Sn diffusion from the subsurface of the crystal. 𝜇-LEED shows that the initially formed (4 × 4) Sn oxide is subsequently converted to a more oxygen-rich (2 × 2𝑛) ‘‘stripe’’ phase. At 630 ◦C, enhanced Sn mobility enables a closed (4 × 4) film. The (2 × 2𝑛) phase is shown to consist of a (2 × 2) Sn lattice modulated by 1D stripe defects with spacings of 𝑛 = 4–6 atomic rows; LEED and SXRD measurements show diffraction features corresponding to this striped superstructure. The two oxides can be distinguished in XPS by their O 1s lineshapes: the (4 × 4) phase shows a clear doublet attributable to distinct O species, whereas the (2 × 2𝑛) phase exhibits a broader envelope consistent with a distribution of O coordination environments. The Sn 3d5∕2 spectra are similar for both phases, reflecting closely related Sn bonding motifs. The spectra are consistent with those of previous near-ambient-pressure XPS measurements, suggesting that the surface oxides forming under CO oxidation conditions are similar to those studied here.
Development of detailed surface reaction mechanism for methanation process based on experiments
(2026)
The pressure to reduce greenhouse gas emissions is growing, which demands new and innovative technologies to produce mobile as well as stationary energy. The methanation offers a pathway to reduce greenhouse gas emissions by directly converting to . This also plays a crucial role in “power‐to‐gas” (P2G) technologies by providing an approach to store excess renewable energy in the form of methane in an existing natural gas infrastructure. However, methanation is a complex process due to its exothermic nature, interaction of the gas species with the catalyst, and possible catalyst degradation. Therefore, a deeper understanding is required for the methanation reaction, its different reaction pathways, and side reactions. In this work, we aim to understand the direct production of synthetic natural gas from and in a Sabatier process with the help of experiments over a Ni/ catalyst. A detailed surface reaction mechanism is developed to extend the study numerically by validating the simulation results with the experimental data. A one‐dimensional model, LOGEcat, based on a single‐channel catalyst model, is used for kinetic modeling. Experiments as well as simulations have been performed at various conditions, such as temperature variation and dilution to the inlet composition. We have successfully captured the experimental trends using the kinetic model developed for the conditions considered for the analysis.
The surface of β-Ga₂O₃ single crystals cleaved along their (100) plane is investigated using surface x-ray diffraction and atomic force microscopy. The results show the surface to consist of a single, so-called B-termination, which means that the crystal cleaves at planes formed by edge-sharing oxygen octahedra, thereby breaking the longest and weakest Ga–O bonds. Refinement of the atomic positions results in small displacements from the bulk structure, at most approximately 0.01 Å. Atomic force microscopy suggests that relatively large terraces form together with steps of half the a-axis length of approximately 0.6 nm, which means that terraces have the same atomic termination, related by the crystal symmetry. These results are important as a fundamental property of β-Ga₂O₃ when processed or used in various semiconductor applications.
Cerium oxide thin films were prepared by electron beam evaporation (EBE) under three deposition conditions: standard process, substrate heating at 200 °C, and ion beam assisted deposition (IBAD). The high optical transparency of the PVD-prepared coatings enables integration with transparent microelectronic devices, an aspect seldom explored in ceria-based gas sensors. Raman and optical analyses revealed various levels of oxygen vacancy-related defects in all films. Gas sensing measurements of the Pd/CeOx structures were correlated with their structural and electronic characteristics, including changes in Ce4+/Ce3+ ratios and band alignment during exposure to reducing gases, providing insight into the redox-driven sensing mechanism. All structures exhibit high ethanol sensitivity, while the highest response achieved for the films deposited with substrate heating and IBAD is consistent with their larger defect density and modified morphology. These findings demonstrate that tailoring EBE conditions provides an effective route to optimize CeOx thin films for improved gas sensing performance.
Nitrogen doping is a widely-used strategy for enhancing the electronic and electrocatalytic properties of graphene. On single-sheet graphene electrodes, substrate-induced charge density effects obscure their intrinsic behavior and limit the efficiency of surface-sensitive electron transfer. Here, we report a Janus bilayer electrode architecture that decouples a nitrogen-doped graphene top layer from the substrate via a pristine graphene bottom layer. This design allows the presentation of catalytically active nitrogen sites while simultaneously suppressing substrate effects, enabling a direct evaluation of the intrinsic effect of nitrogen-doping on the electron transfer (ET) properties of graphene. Using ferricyanide as a redox-probe, we show that the ET kinetics of pristine bilayer graphene are pH-dependent, while nitrogen-doped bilayer graphene displays stable, pH-independent redox behavior. As an application avenue of biological relevance, we demonstrate that nitrogen-doped bilayer graphene exhibits a reduced overpotential for NADH oxidation. These results demonstrate that the synergy between substrate decoupling and controlled nitrogen incorporation yields a robust electrode architecture with enhanced stability and improved electrocatalytic activity.
This study explores the role of atomic layer deposition (ALD) as an enabling technique for the defect engineering of catalytically active ultrathin deposits. In particular, we demonstrate the feasibility of tuning the O/Ce ratio in thermal ALD-based cerium oxide layers grown on silicon-based or alumina substrates by using the organometallic precursor tris(N, N’-diisopropyl-2-dimethylamido-guanidinato)cerium(III) ([Ce(dpdmg)3]) with H2O, O2, or O3 as coreactants. As revealed by in situ X-ray photoelectron spectroscopy (XPS), the Ce3+ concentration, i.e., the concentration of oxygen vacancies, depends strongly on three factors: the type of oxygen source, the chosen substrate, and the film thickness. The fixation of Ce3+ states during the early stages of growth is primarily determined by interface formation and the appearance of silicate and aluminate species, along with changes in morphology and surface-to-volume ratio. For thicker deposits (>5 nm), the intrinsic oxygen vacancies are coreactant-dependent. Furthermore, the chosen oxygen source also influences the morphology of ultrathin deposits, enabling potential surface functionalization with ceria nanoislands of varying composition and size. We point to a likely connection between this chemical and morphological tuning and changes in the ALD reaction pathway. The evolution of different nitrogen and carbon species depends on the oxygen source and the number of ALD cycles, indicating a shift in the ALD reaction mechanism from ligand exchange using H2O to ligand combustion for O3. The comprehensive investigation of these growth parameters is crucial for tailoring film properties via precise defect engineering.
The internal chemical potential Γ of mixed covalent-ionic systems represents the potential differences between the covalent and the ionic intrinsic defect states located within the ionic gap. It is the key parameter to control the carrier densities, the stability regimes, and the photosensitive properties of materials. In this work, we describe first the quantitative analysis of the carrier densities in dependence on the internal potential Nπ(Γ) based on the common features of the electronic structure of mixed covalent-ionic materials. Subsequently, this method is applied on two mixed covalent-ionic materials, i.e., formamidinium lead triiodide and gallium oxide, as representatives of the respective families of perovskites (halides) and transparent conducting oxide thin films. Based on this analysis, the carrier densities as well as the photosensitivity mechanisms and the related specific properties of these materials in dependence on their internal chemical potential are discussed.
Ruthenium is emerging as a promising candidate to replace copper in highly integrated electronics by enabling barrierless metallization in ultrathin interconnects. From this perspective, the study of graphene growth on such surface templates is of paramount importance as a platform for graphene integration in electronic devices. In particular, graphene growth on the Ru (10-10) surface allows selective growth of different graphene orientations, one-dimensional structures, and reduced substrate interaction compared to the well-established hexagonal Ru(0001) substrate. Real-time growth observations using low-energy electron microscopy and micro-diffraction highlight the influence of substrate symmetry on graphene growth, leading to the formation of rectangular islands with distinct zigzag- or armchair-terminated edges. Bilayer formation on Ru(10-10) occurs by nucleation of graphene nanoribbons under the monolayer. Micro-spot angle-resolved photoemission spectroscopy shows significantly less charge-transfer doping in these freestanding, zigzag-terminated bilayer graphene nanoribbons, indicating reduced graphene-substrate interaction and hence more effective decoupling as compared to graphene/Ru(0001). Our results show that the growth of graphene on non-hexagonal substrates opens new pathways for tailoring the graphene-substrate interaction at the interface, and thus the properties of graphene beyond the limits imposed by hexagonal substrates.
This work presents a comprehensive reaction and kinetic model of the pyrite thin films formation by sulfuration of Fe monosulfides when a molecular sulfur (S2) atmosphere is used. This investigation completes the results already published on the explanation and interpretation of the sulfuration process that transforms metallic iron into pyrite. It was previously shown that the monosulfide species (i.e., orthorhombic and hexagonal pyrrhotite phases) are intermediate phases in the sulfuration reaction. Based on experimental data we now show that the sulfuration of pyrrhotite to pyrite takes place in two distinct stages: (i) conversion of orthorhombic pyrrhotite to pyrite (Fe1–xSO → FeS2) while the hexagonal pyrrhotite (Fe1–xSH) phase remains unaltered, and (ii) final transformation of hexagonal pyrrhotite to pyrite (Fe1–xSH → FeS2). Both processes occur via interstitial sulfur diffusion through the previously formed pyrrhotite layer. Consequently, the monosulfide is sulfurated at the internal Fe1–xS/FeS2 interface. The reaction mechanism at each stage has been validated using the corresponding kinetic model to fit the experimental data on time evolution of Fe1–xS and FeS2 layers thicknesses and some of the film transport properties. The concluding global reaction mechanism proposed in some of our former papers and completed here (Fe → Fe1–xS → FeS2) can explain the resulting microstructure of the pyrite films (i.e., Kirkendall effect and formation of a porous layer in the film). Simultaneously, it also justifies the presence of intrinsic defects, such as iron and sulfur vacancies, and the accumulation of interstitial sulfur at the film grain boundaries. The conductivity of pyrite films is tentatively explained using a two-band model where the changes in the Seebeck coefficient and the S/Fe ratio during the pyrite recrystallization stage can be successfully explained.