arXiv subjects

Iris C. G. van den Bosch

Publications and source records attributed to Iris C. G. van den Bosch.

4 recordsLinked to original sources

arXiv2026

Synthesis of epitaxial TaO$_2$ thin films on Al$_2$O$_3$ by suboxide molecular-beam epitaxy and thermal laser epitaxy

Tantalum dioxide (TaO2) is a metastable tantalum compound. Here, we report the epitaxial stabilization of TaO2 on Al2O3 (1-102) (r-plane sapphire) substrates using suboxide molecular-beam epitaxy (MBE) and thermal laser epitaxy (TLE), 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 work expands the understanding of tantalum oxides and paves the way for their integration into next-generation electronic and photonic devices.

cond-mat.mtrl-sci↗

arXiv2025

Interfacial Control of Orbital Occupancy and Spin State in LaCoO$_3$

Transition metal oxides exhibit a wide range of tunable electronic properties arising from the complex interplay of charge, spin, and lattice degrees of freedom, governed by their $d$ orbital configurations, making them particularly interesting for oxide electronics and (electro)catalysis. Perovskite oxide heterointerfaces offer a promising route to engineer these orbital states. In this work, we tune the Co $3d$ orbital occupancy in LaCoO$_3$ from a partial $d^7$ to a partial $d^5$ state through interfacial engineering with LaTiO$_3$, LaMnO$_3$, LaAlO$_3$ and LaNiO$_3$. Using X-ray absorption spectroscopy combined with charge transfer multiplet calculations, we identify differences in the Co valence and spin state for the series of oxide heterostructures. LaTiO$_3$ and LaMnO$_3$ interfaces result in interfacial charge transfer towards LaCoO$_3$, resulting in a partial $d^7$ orbital occupancy, while a LaNiO$_3$ interface results in a partial Co $d^5$ occupancy. Strikingly, a LaAlO$_3$ spacer layer between LaNiO$_3$ and LaCoO$_3$ results in a Co $d^6$ low spin state. These results indicate that the Co spin state, like the valence state, is governed by the interfacial environment. High-resolution scanning transmission electron microscopy imaging reveals a clear connection between strain and spin configuration, emphasizing the importance of structural control at oxide interfaces. Overall, this work demonstrates that interfacial engineering simultaneously governs orbital occupancy and spin state in correlated oxides, advancing spin-engineering strategies in correlated oxides and offering new insights for the rational design of functional oxide heterostructures.

cond-mat.mtrl-sci↗

arXiv2025

Ultrathin oxide freestanding membranes with large-scale continuity and structural perfection

Freestanding oxide membranes offer integration with advanced semiconductor platforms, unlocking opportunities for flexible electronics, silicon-based spintronics, neuromorphic computing, and high-performance energy technologies. Scalable fabrication of such membranes is essential for bridging fundamental discoveries in complex oxides with practical device deployment. However, the lateral dimensions of crack- and wrinkle-free membranes remain limited to millimeter scales, forming a critical bottleneck for large-area architectures. Overcoming this challenge demands strategies that preserve crystalline quality while suppressing defect transfer during release. Here, we demonstrate an approach based on a water-soluble sacrificial layer of super-tetragonal Sr4Al2O7, enabling the fabrication of ultrathin, crack-free, and wrinkle-free free-standing oxide membranes spanning centimeter-scale areas. This method is broadly applicable to a wide range of oxides and establishes a new pathway toward large-scale silicon integration and flexible oxide technologies. Nevertheless, dissolution of the sacrificial layer introduces oxygen vacancies into the SrRuO3 membranes, with diffusion depths reaching six unit cells, leading to anomalous "up-and-down" transport behavior. Although post-annealing can eliminate these vacancies, the required temperatures are incompatible with CMOS processes. Therefore, ultrathin freestanding membranes fabricated by water-assisted lift-off still face critical challenges for integration into miniaturized silicon-based oxide devices.

cond-mat.mtrl-sci↗

arXiv2022

Operando X-ray characterization of interfacial charge transfer and structural rearrangements

Key technologies in energy conversion and storage, sensing and chemical synthesis rely on a detailed knowledge about charge transfer processes at electrified solid-liquid interfaces. However, these interfaces continuously evolve as a function of applied potentials, ionic concentrations and time. We therefore need to characterize chemical composition, atomic arrangement and electronic structure of both the liquid and the solid side of the interface under operating conditions. In this chapter, we discuss the state-of-the-art X-ray based spectroscopy and diffraction approaches for such 'operando' characterization. We highlight recent examples from literature and demonstrate how X-ray absorption spectroscopy, X-ray photoelectron spectroscopy and surface X-ray diffraction can reveal the required interface-sensitive information.

cond-mat.mtrl-sci↗