Iris C. G. van Den Bosch

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.

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.

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.

We investigated the valence dynamics of thin films during the electrocatalytic oxygen evolution reaction using operando high-energy-resolution fluorescence-detected (HERFD) X-ray absorption spectroscopy (XAS) in a 0.1 M KOH solution under an applied potential. We show that it is possible to measure the 10 unit cell (∼4 nm) bimetallic perovskite oxide La₂CoMnO₆ thin film grown on Pt-covered SiN_x membranes. Operando Co and Mn K-edge HERFD XAS spectra on the active surface species were recorded from the backside of the SiN_x membrane. The HERFD XAS Co K-edge spectra reveal a (partial) increase in the oxidation state from Co²⁺ in ex situ conditions to mainly Co³⁺ in the solid-liquid open-circuit voltage (OCV). Voltage-dependent Co K-edge HERFD XAS white line intensities show that Co is oxidized toward the higher oxidation state, while Mn remains Mn⁴⁺. Interestingly, the Co and Mn main edge shifts with the average valence change of 0.20; the valence of Mn is constant, implying that the valence of Co increases by 0.40. The OCV spectra were reversible when switching off the potentials. These observations clearly identify element-specific valence dynamics in the intrinsic oxygen evolution reaction.

Innovative approaches to study buried interfaces and heterogeneous interactions under reaction conditions are crucial for advancing energy and catalytic materials. Our near-ambient pressure x-ray photoelectron spectroscopy (NAP-XPS) setup is equipped with a tricolor x-ray source, with Al Kα, Ag Lα, and Cr Kα excitation energies, enabling information depth–selective operando and in situ analysis of solid-liquid, solid-gas, and solid-solid interfaces. We present three case studies to demonstrate the systems’ capabilities. First, we compare experimental depth profiling of a LaMnO ₃ /LaFeO ₃ /Nb:SrTiO ₃ multilayer with SESSA (simulation of electron spectra for surface analysis) simulations. Second, we examine the oxidation and reduction of Fe _x O _y as a function of environment and temperature. Last, the Pt/liquid electrolyte interface is examined, revealing surface oxidation in the absence of bulk oxidation. As our results confirm, the unique combination of a NAP-XPS with the tricolor x-ray source empowers laboratory-based in situ and operando XPS characterization of advanced materials under reaction conditions in a wide range of applications.

The development of efficient electrocatalysts in water electrolysis is essential to decrease the high overpotentials especially at the anode where the oxygen evolution reaction (OER) takes place. However, establishing catalyst design rules to find optimal electrocatalysts is a substantial challenge. Complex oxides, which are often considered as suitable OER catalysts, can exhibit vastly different conductivity values, making it challenging to separate intrinsic catalytic activities from internal transport limitations. Here, we systematically decouple the limitations arising from electrical bulk resistivity, contact resistances to the catalyst support, and intrinsic OER catalytic properties using a systematic epitaxial thin film model catalyst approach. We investigate the influence of the resistivity of the three perovskite oxides LaNiO3-δ (3.7x10-4 Ωcm), La0.67Sr0.33MnO3-δ (2.7x10-3 Ωcm) and La0.6Ca0.4FeO3-δ, (0.57 Ωcm) on the observed catalytic activity. We tune the electron pathway through the catalyst bulk by comparing insulating and conductive substrates. The conducting substrate reduces the electron pathway through the catalyst bulk from millimeter to nanometer length scale. As we show, for the large electron pathways, the observed catalytic activity scales with the resistivity because of a highly inhomogeneous lateral current density distribution. At the same time, even on the conducting substrate (Nb-doped SrTiO3), large contact resistances occur that limit the determination of intrinsic catalytic properties. By inserting interfacial dipole layers (in this case LaAlO3) we lifted these interface resistances, allowing us to reveal the intrinsic catalytic properties of all examined catalysts. We find that La0.6Ca0.4FeO3-δ and LaNiO3-δ exhibit a similar intrinsic overpotential of 0.36 V at 0.1 mA/cm2, while their resistivities differ by three orders of magnitude. This finding shows that optimizing the electron pathway of OER catalyst can lead the way to find new structure-activity relationships and to identify high-activity catalysts even if the electronic resistance is high.