Xiaoyan Li

Colors in biominerals often arise from the occlusion of low-molecular-weight organic pigments within calcium carbonate (CaCO ₃ ) mineral phases during biomineralization processes; however, the physicochemical origin of these colors has been overlooked. Here, inspired by the growth of colored sea urchin spines that occurs through amorphous CaCO ₃ (ACC) precursors, we show an in-vitro halochromic modulation of ACC crystallization. In this mechanism, the deprotonation of a natural red pigment naphthazarin (NZ) is driven by pH variations during mineral formation and subsequently stabilized within CaCO ₃ - hybrid pigments. The latter show lavender-blue to violet-blue hues upon crystallization. Deprotonated NZ has a minimal impact on ACC formation and crystallization, which occurs through local dissolution and reprecipitation mechanisms; however, it limits the formation of vaterite, and calcite distorted nanodomains. Deprotonated NZ nano-inclusions that interact with calcite through water are thus formed. Overall, this study brings insights into the processes behind the coloration of biomineralized organisms.

Magnetoelectric multiferroics are foreseen as paramount materials to control magnetism with an electric-field, targeting energy-efficient spintronics. The archetypal room-temperature antiferromagnetic ferroelectric, BiFeO3, harbors an incommensurate antiferromagnetic cycloid whose propagation direction is locked to ferroelectric domains. Epitaxial strain was shown to affect this antiferromagnetic ordering, stabilizing different cycloidal propagation directions, or a collinear antiferromagnetic state. Here we demonstrate the reversible, nonvolatile, electric-field triggered magnetic phase transition between two distinct antiferromagnetic states at room temperature. Using SrTiO3 vicinal substrates, we stabilize a single ferroelectric domain associated with a single antiferromagnetic cycloidal state in BiFeO3 epitaxial thin films. Electrically reversing the ferroelectric polarization deterministically within the same ferroelastic domain induces a reversible transition from a cycloidal to a collinear antiferromagnetic state, as directly visualized by scanning nitrogen vacancy (NV) magnetometry. These results bring insights into magnetoelectric devices for ultrafast and low-power spintronics.

Surface plasmon resonances in nanoparticles are intensely studied in fundamental and applied research as they permit investigating the interplay between atomic and electronic structures and generate strongly enhanced local electric fields. For the commonly used noble metals it is not possible to extend this resonance below ∼400 nm, into the deep ultraviolet, even though such an extension is highly desirable for numerous applications. We report the successful fabrication and characterization of surfactant-free In2Au nanoparticles in the size range of 5–10 nm diameter which, if properly protected against oxidation, are entirely metallic, highly crystallized in the blue gold structure and exhibit plasmon resonances down to 280 nm. We characterized the crystalline and the chemical structure of these particles as well as their plasmonic response on both the ensemble-averaged and the single particle level. Machine-learning assisted data treatment of the single-particle experiments allowed identifying and separating chemical phases as well as the unequivocal separation of the surface plasmon resonance from the background signal. The possibility of unambiguously measuring plasmonic signals in individual particles in the size range <10 nm using STEM-EELS constitutes a major advance in nanoalloy nano-optics. These results introduce the In2Au nanoalloy as a material for the tailored fabrication of alloy-based UV-plasmonic nanostructures and their implementation in applications such as photocatalysis, biosensing and high-energy hot carrier generation.

Layered perovskite‐based compounds offer a range of unconventional properties enabled by their naturally anisotropic structure. Among these, the Carpy‐Galy phases (A_nB_nO_3n+2), characterized by (110)‐oriented perovskite planes interleaved with additional oxygen layers, stand out for robust in‐plane polarization. However, the challenges associated with the synthesis of ultrathin Carpy‐Galy films and understanding the impact of strain on their properties limit their integration into devices. Here, La₂Ti₂O₇ (n = 4) films grown on substrates imposing tensile, compressive, or negligible epitaxial strains are investigated. Surprisingly, a 3% tensile strain from DyScO₃ (100) substrates facilitates layer‐by‐layer growth mode, whereas compressive (LaAlO₃‐Sr₂TaAlO₆ (110)) or negligible (SrTiO₃ (110)) epitaxial strains require post‐deposition annealing to reach comparable crystallinity. Using density‐functional theory calculations, scanning probe microscopy, X‐ray diffraction, scanning transmission electron microscopy, and polarization switching experiments, it is confirmed that these films possess exceptional ferroelectric properties, including a polarization of 18 µCcm⁻² – more than three times higher than previously reported – as well as persistence of ferroelectricity down to a single‐unit‐cell thickness. This study not only advances the understanding of Carpy‐Galy phases as epitaxial thin films but also lays a foundation for their integration into advanced ferroelectric device architectures.

Thermal transport in nanostructures plays a critical role in modern technologies. As devices shrink, techniques that can measure thermal properties at nanometer and nanosecond scales are increasingly needed to capture transient, out-of-equilibrium phenomena. We present a novel pump-probe photon-electron method within a scanning transmission electron microscope (STEM) to map temperature dynamics with unprecedented spatial and temporal resolutions. By combining focused laser-induced heating and synchronized time-resolved monochromated electron energy-loss spectroscopy (EELS), we track phonon, exciton, and plasmon signals in various materials, including silicon nitride, aluminum thin film, and transition metal dichalcogenides. Our results demonstrate the technique's ability to follow temperature changes at the nanometer and nanosecond scales. The experimental data closely matched theoretical heat diffusion models, confirming the method's validity. This approach opens new opportunities to investigate transient thermal phenomena in nanoscale materials, offering valuable insights for applications in thermoelectric devices and nanoelectronics.

We develop here a comprehensive experimental approach to independently determine charge carrier parameters, namely, carrier density and mass, in plasmonic indium tin oxide nanocrystals. Typically, in plasmonic nanocrystals, only the ratio between these two parameters is accessible through optical absorption experiments. The multitechnique methodology proposed here combines single particle and ensemble optical and magneto-optical spectroscopies, also using ¹¹⁹Sn solid-state nuclear magnetic resonance spectroscopy to probe the surface depletion layer. Our methodology overcomes the limitations of standard fitting approaches based on absorption spectroscopy and ultimately gives access to carrier effective mass directly on the NCs, discarding the use of literature value based on bulk or thin film materials. We found that mass values depart appreciably from those measured on thin films; consequently, we found carrier density values that are different from reported literature values for similar systems. The effective mass was found to deviate from the parabolic approximation at a high carrier density. Finally, the dopant activation and defect diagram for ITO NCs for tin doping between 2.5 and 15% are determined. This approach can be generalized to other plasmonic heavily doped semiconductor nanostructures and represents, to the best of our knowledge, the only method to date to characterize the full Drude parameter space of 0-D nanosystems.

Superconducting qubits have arisen as a leading technology platform for quantum computing, which is on the verge of revolutionizing the world's calculation capacities. Nonetheless, the fabrication of computationally reliable qubit circuits requires increasing the quantum coherence lifetimes, which are predominantly limited by the dissipations of two-level system defects present in the thin superconducting film and the adjacent dielectric regions. In this paper, we demonstrate the reduction of two-level system losses in three-dimensional superconducting radio frequency niobium resonators by atomic layer deposition of a 10 nm aluminum oxide Al2O3 thin films, followed by a high vacuum heat treatment at 650 °C for few hours. By probing the effect of several heat treatments on Al2O3-coated niobium samples by x-ray photoelectron spectroscopy plus scanning and conventional high resolution transmission electron microscopy coupled with electron energy loss spectroscopy and energy dispersive spectroscopy, we witness a dissolution of niobium native oxides and the modification of the Al2O3-Nb interface, which correlates with the enhancement of the quality factor at low fields of two 1.3 GHz niobium cavities coated with 10 nm of Al2O3.

Atomic Fe in N-doped C (Fe-N-C) catalysts provide the most promising non-precious metal O2 reduction activity at the cathodes of proton exchange membrane fuel cells. However, one of the biggest remaining challenges to address towards their implementation in fuel cells is their limited durability. Fe demetallation has been suggested as the primary initial degradation mechanism. However, the fate of Fe under different operating conditions varies. Here, we monitor operando Fe dissolution of a highly porous and >50% FeNx electrochemical utilization Fe-N-C catalyst in 0.1 M HClO4, under O2 and Ar at different temperatures, in both flow cell and gas diffusion electrode (GDE) half-cell coupled to inductively coupled plasma mass spectrometry (ICP-MS). By combining these results with pre- and post-mortem analyses, we demonstrate that in the absence of oxygen, Fe cations diffuse away within the liquid phase. Conversely, at -15 mA cm-2geo and more negative O2 reduction currents, the Fe cations reprecipitate as Fe-oxides. We support our conclusions with a microkinetic model, revealing that the local pH in the catalyst layer predominantly accounts for the observed trend. Even at a moderate current density of -15 mA cm-2geo and under O2 at 25 oC, a significant H+ consumption and therefore pH increase (pH = 8-9) within the bulk Fe-N-C layer facilitate precipitation of Fe cations. This work provides a unified view on the Fe degradation mechanism for a model Fe-N-C in both high-throughput flow cell and practical operating GDE conditions, underscoring the crucial role of local pH in regulating the stability of the active sites.

The metal-hydride-based "topochemical reduction" process has produced several thermodynamically unstable phases across various transition metal oxide series with unusual crystal structures and nontrivial ground states. Here, by such an oxygen (de-)-intercalation method we synthesis a samarium nickelate with ordered nickel valences associated with tri-component coordination configurations. This structure, with a formula of Sm9Ni9O22 as revealed by four-dimensional scanning transmission electron microscopy (4D-STEM), emerges from the intricate planes of {303}(pc) ordered apical oxygen vacancies. X-ray spectroscopy measurements and ab initio calculations show the coexistence of square planar, pyramidal, and octahedral Ni sites with mono-, bi-, and tri-valences. It leads to an intense orbital polarization, charge-ordering, and a ground state with a strong electron localization marked by the disappearance of ligand-hole configuration at low temperature. This nickelate compound provides another example of previously inaccessible materials enabled by topotactic transformations and presents an interesting platform where mixed Ni valence can give rise to exotic phenomena.