Christoph Baeumer

Understanding HER activity on MXenes requires probing their evolving solid–liquid interface. Operando spectroscopy can link dynamic surface chemistry, electronic structure, and interfacial water to catalytic performance.

The development of In‐free transparent conductive oxides (TCOs) is essential for sustainable photovoltaics. For undoped tin oxide (SnO _x ), achieving high conductivity requires precise control over intrinsic defect chemistry. This work explores the impact of deposition pressure and oxygen stoichiometry on the electrical and optical properties of SnO _x films grown by pulsed laser deposition (PLD). We identify a narrow conductivity window achieved at oxygen pressures between 0.015 and 0.020 mbar, beyond which a substantial drop in conductivity is observed. Within this optimal window, the SnO _x films exhibit a 3‐fold mobility enhancement after 300°C air annealing, despite preserving an amorphous phase. This improvement is primarily attributed to the reduction of ionized impurity scattering, evidenced by a simultaneous decrease in free carrier density. To demonstrate functionality, the inherent soft‐landing nature of PLD was exploited to preserve the passivation quality of textured tunnel oxide passivating contacts (TOPCon) silicon substrates, as implied open‐circuit voltage (iV _oc ) mapping confirmed the absence of deposition‐induced damage. Finally, we demonstrate the compatibility of this In‐free TCO by fabricating proof‐of‐concept perovskite solar cells on textured SnO _x /n‐TOPCon substrates, achieving open‐circuit voltage of 1.21 V. These results highlight the potential of SnO _x for integration as an interconnection layer in fully textured perovskite/Si tandem solar cells.

The ability to rationally design materials with specific properties is a major motivator in materials science and condensed matter physics. Driven by this motivation and by the potential to open new technological avenues, researchers are continually pushing the boundaries of established materials while pioneering new ones. This push has led to newly developed techniques for fabricating, handling, and transferring freestanding oxide membranes (FOMs), providing a new avenue to overcome traditional oxide epitaxial growth limitations, which relies on specific substrates and synthesis conditions. These advances enable the design and creation of novel materials with previously inaccessible interface symmetries, stacking configurations, and atomic reconstructions, opening exciting possibilities for exploring new physics and developing innovative device architectures using atomically engineered complex oxides. At the same time, these advancements present new challenges in the synthesis, membrane transfer, characterization, and final functionalization of FOMs. The field is still in its early stages and necessitates further scientific and technological progress. This roadmap offers an overview of the current state of the field and perspectives on its future, highlighting opportunities, challenges, and guiding principles for the research community.

Understanding the intricate interplay between electronic and lattice dynamics is indispensable for controlling photoinduced functionality in correlated oxides. Here, we investigate the ultrafast relaxation pathways in textured LaFeO3 using femtosecond extreme-ultraviolet (XUV) absorption spectroscopy at the iron 3p edge. These nanosheets facilitate oriented perovskite growth on the amorphous Si3N4 membrane, giving rise to local epitaxy while preserving XUV transparency. By combining table-top time-resolved measurements with crystal-field multiplet calculations, we track the evolution from rapid ligand-to-metal charge transfer (LMCT) excitation to a lattice-stabilized polaronic state. The early-time response is dominated by the population of a mixed charge-transfer state with pronounced t2g3eg3 (5E) character, which indicates the initial electronic redistribution that follows photoexcitation. On subpicosecond time scales, the emergence of lattice-assisted carrier localization leads to partial charge localization and a lowering of the symmetry of Fe3+ sites. These changes suggest the formation of a polaronic state. At later delays, the transient spectra are governed by distorted Fe3+ configurations with residual charge-transfer character, without evidence for long-lived Fe2+ formation. Comparison with α-Fe2O3 reveals distinct material dependence in both charge-transfer relaxation and polaron-formation dynamics, demonstrating the roles of lattice topology and iron–oxygen covalency.

Tantalum dioxide (TaO2) is a metastable tantalum compound. Here, we report the epitaxial stabilization of TaO2 on Al2O3 (11̄02) (r-plane sapphire) substrates using suboxide molecular-beam epitaxy and thermal laser epitaxy, demonstrating single-oriented, monodomain growth of anisotropically strained thin films. Microstructural investigation is performed using synchrotron x-ray diffraction and scanning transmission electron microscopy. The tetravalent oxidation state of tantalum is confirmed using x-ray absorption and photoemission spectroscopy as well as electron energy-loss spectroscopy. Optical properties are investigated via spectroscopic ellipsometry and reveal a 0.3 eV Mott gap of the tantalum 5d electrons. Density-functional theory and group theoretical arguments are used to evaluate the limited stability of the rutile phase and reveal the potential to unlock a hidden metal–insulator transition concomitant with a structural phase transition to a distorted rutile phase, akin to NbO2. Our study expands the understanding of tantalum oxides and paves the way for their integration into next-generation electronic and photonic devices.

High-entropy oxide (HEO) catalysts are promising candidates for overcoming intrinsic activity limitations in the oxygen evolution reaction (OER), a key bottleneck in electrochemical conversion. Thin-film model systems provide a controlled platform for uncovering the underlying catalytic mechanisms. Here, we develop and compare four deposition templates for LaNi 0.2 Co 0.2 Fe 0.2 Mn 0.2 Cr 0.2 O 3 model systems: (i) epitaxial films on Nb:SrTiO 3 , (ii) sacrificial-layer-released films transferred to platinized Si wafers, (iii) locally epitaxial films grown on oxide nanosheets, and (iv) films directly grown on platinized Si wafers. These geometries support a wide range of operando techniques while enabling direct comparison of materials grown under identical pulsed laser deposition conditions. Using electrochemical measurements and ex-situ characterization, we evaluate crystallinity, stoichiometry, surface morphology, and OER activity across the different templates. The observed variations highlight the important role of template selection in interpreting catalytic performance. We find that the transferred epitaxial thin film and the film on Nb:SrTiO 3 are comparable in crystallinity, stoichiometry and electrocatalytic activity, while the films on nanosheets and directly grown on Pt have a reduced crystallinity, different stoichiometry and a lower catalytic activity. Overall, this set of deposition templates facilitates the integration of well-controlled model systems with operando spectroscopy, advancing mechanistic insight and guiding the rational design of next-generation high-entropy OER catalysts.

A strong societal and political drive is motivating the development and optimization of novel energy conversion and storage systems for decarbonization. The successful implementation of solid state devices such as fuel cells and secondary batteries depends, however, on achieving ambitious targets in terms of performance, reliability and cost competitiveness. Research and technology are addressing these needs through a holistic approach including exploration of new materials and nanoarchitectures, as well as system engineering. These significant efforts require the support of appropriate characterization tools capable of assessing nanometer-scale phenomena such as concentration profiles of ionic and electronic charges, local chemical compositions and their evolution over time across interfaces. This roadmap provides an overview of selected advanced characterization techniques for energy materials and devices. Specific focus is put on in situ/operando methods for probing electrochemical phenomena in real-time under realistic working conditions. Experts in the field provide an extensive review of the current state of the art in 2025 and the current and future challenges for the characterization of local chemistry and kinetics in the bulk of the material, in nanoarchitectures (e.g. thin films) and at the interfaces (e.g. grain boundaries, phase contacts, solid/liquid and solid/gas interfaces) . The aim is to provide a detailed guide to the techniques, describing opportunities and bottlenecks for their practical deployment and examples of successful applications. This roadmap provides an overview of selected advanced characterization techniques for energy materials and devices. Specific focus is put on in situ/operando methods for probing electrochemical phenomena in real time under realistic working conditions. Experts in the field provide an extensive review of the current state of the art in 2024 and the current and future challenges for the characterization of local chemistry and kinetics in the bulk of the material, in nanoarchitectures (e.g. thin films) and at the interfaces (e.g. grain boundaries, phase contacts, solid/liquid and solid/gas interfaces) . The aim is to provide a detailed guide to the techniques, describing opportunities and bottlenecks for their practical deployment and examples of successful applications.

Perovskite oxides are a versatile class of materials with tunable electronic structures, making them attractive for catalytic applications, including the oxygen evolution reaction (OER). The surface reactivity of these oxides is closely tied to the electronic structure of transition metal cations, particularly their 3d orbital occupation, which can be modulated by interfacial engineering. In this work, we investigate how subsurface engineering influences the interaction of ultrathin LaCoO₃ films with water vapor. Using (near) ambient pressure core-level spectroscopy, we observe distinct differences in hydroxyl affinity and Co valence response depending on the electronic structure imposed by the underlying layer. Ultrathin LaCoO₃ films with a higher initial Co oxidation state show stronger hydroxyl affinity, while those with a lower Co valence show more significant electronic changes upon water exposure. Our findings demonstrate a form of "remote control" in surface chemistry, where subsurface electronic engineering dictates hydroxyl affinity and electronic response at the surface. This concept offers a new degree of freedom to optimize oxide-adsorbate interactions for (electro)catalysis.