Jan Ingo Flege

Van der Waals (vdW) heterostructures are attracting enormous attention due to their remarkable physical properties governed by moiré superlattices (MSLs) directly controlled by interlayer twist angles or a lattice mismatch. We investigated surface properties of moiré heterostructures made up of commercially available chemical vapor deposited (CVD) graphene and exfoliated MoS 2 layers on silicon oxide, by means of spectroscopy, microscopy, and diffraction. High resolution Auger elemental maps and spectra revealed composition inhomogeneity due to thickness variations in CVD graphene and signatures of oxidation of bare MoS 2 surface exposed to the ambient atmosphere. Low-energy electron microscopy and diffraction disclosed local stacking and crystallographic information in the graphene- MoS 2 moiré heterostructure, including evidence for small-angle twisted bilayer MSL regions in CVD monolayer graphene. This systematic study helps us to understand the overall configuration of the vdW heterostructures and correlate their structure and performance in transport experiments.

The redox chemistry of mixed ceria-based oxides is commonly interpreted in terms of direct charge transfer between neighboring cations. In the Ce1–xUxO2 system, this corresponds to the oxidation of U4+ into U5+ accompanied by the reduction of Ce4+ into Ce3+. In this work, epitaxial Ce1–xUxO2(111) thin films with different uranium contents (with x ranging from 0.05 to 1) were grown under ultra-high vacuum conditions and investigated using angle-resolved X-ray photoelectron spectroscopy. The measurements reveal a substantial excess of Ce3+ relative to U5+, particularly at low uranium fractions and in the topmost surface region, indicating that the reduction of Ce4+ cations cannot be explained solely by direct U4+ to Ce4+ charge transfer. Quantitative analysis further shows deviations in oxygen stoichiometry when lattice oxygen (Ol) is assumed to remain entirely in the O2– state. The effect is stronger in epitaxial films and largely absent in structurally disordered counterparts. These observations support a picture in which oxygen ions participate in the redox process through lattice distortion and polaron-mediated charge redistribution. The results suggest that (i) oxygen-mediated electronic coupling, (ii) strain, and (iii) local structural order collectively govern the reduction behavior in Ce1–xUxO2(111), with important implications for ceria-based redox materials used in catalysis and thermochemical fuel production.

Ceria-based inverse catalysts derive their functionality from Ce3+–oxygen vacancy chemistry, yet strategies to deliberately control vacancy formation and distribution at metal–oxide interfaces remain limited. Here, we demonstrate that aliovalent samarium doping provides a direct and tunable route to vacancy engineering in the prototypical CeO2/Ru(0001) inverse catalyst. Using real-time, nanometer-resolved low-energy electron microscopy in combination with low-energy electron microdiffraction, X-ray absorption spectroscopy in photoemission electron microscopy, and density functional theory, we directly correlate lattice structure, oxidation state, and vacancy formation under reducing conditions. Sm incorporation generates extrinsic oxygen vacancies, which are accompanied by Ce3+ formation, which induces a pronounced lattice expansion that in turn fosters the formation of additional intrinsic oxygen vacancies, substantially enhancing reducibility and stabilizing reduced ceria phases relative to undoped CeO2. The doped system exhibits accelerated reduction kinetics, ordered intrinsic vacancy formation, and modified phase-transition behavior. By establishing how aliovalent dopants govern the vacancy landscape and redox response in inverse ceria catalysts, this work provides atomic-level mechanistic insight and concrete design principles for engineering reducible oxides at metal–oxide interfaces with tailored redox properties.

This study focused on evaluating how different substrates for depositing an Au electrode influence the stable formation of a bioreceptor layer. Substrates tested included ceramic, glass, and epoxy laminate, onto which thin gold coatings were applied using the EBE method. EBE allows producing extremely smooth layers, and the surface properties of the produced sensor structures depend on the type of substrate. To build the bioreceptor layer, the Au-electrode was modified with gold nanoparticles, and laccase for further testing. The resulting biosensing systems show potential in aquaculture for detecting pathogens using Electrochemical Impedance Spectroscopy (EIS). The layers characteristics were examined using optical profilometry, scanning electron microscopy (SEM), and atomic force microscopy (AFM), including Kelvin Probe measurements, to analyze surface topography before and after bioreceptor layer formation. The essential stage of the study involved analyzing surface oxidation levels via photoelectron spectroscopy, specifically through the Au4f, O1s, and C1s states. This helped determine oxidation levels post-deposition and estimated the coverage area of the bioreceptor layer. The results have great commercial impact, as they relate to performance of sensors that have been commercially available for a short time, and understanding the mechanism that determines their reliability and reproducibility of the production process.

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.

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 work presents the results of investigating the influence of experimental parameters on the growth and properties of pyrite thin films produced by sulfurating metallic Fe layers. We use in-situ monitoring of the Seebeck coefficient and electrical conductivity of the films to study the influence of three variables that directly control the partial pressure of molecular sulfur (S2) and, consequently, the growth of pyrite films. These variables are the sample temperature during their sulfuration (Ts), the final temperature of the basket holding the elemental sulfur (Tsb), and its heating ramp (TRsb). Compared with selected reference conditions in which the global sulfuration reaction is slowed down, our results show that higher Ts and S2 partial pressures increase the kinetics of the sulfuration process. This acceleration helps to put in evidence the two main stages of Fe sulfuration to pyrite: (i) first diffusion of Fe vacancies (Fe → Fe1-x S) and (ii) diffusion of interstitial sulfur (Fe1-xS → FeS2), with some overlapping between both steps. For Ts values below 540 K, the resulting sulfurated layers present traces of pyrrhotite. We also link differences in the stages of pyrite film growth to the formation of various types of defects, primarily iron and sulfur vacancies and interstitial sulfur atoms. These defects are correlated with the macroscopic transport properties of the films through a two-band conduction model. Our results highlight that precise control of specific experimental conditions enables tuning of the defects and the microstructure of the obtained pyrite films and, therefore, their macroscopic properties.

The intrinsic effect of nitrogen doping on the electrochemical performance of a graphene sheet is investigated by substrate decoupling using a Janus bilayer electrode design.