Na-ion batteries have emerged as viable candidates for large-scale energy storage applica- tions due to resource abundance and cost advantages. The constraints imposed on their performance and durability, for instance, by complex phase transformations in positive electrode materials during electrochemical cycling, can be addressed and are thus not detrimental to their development. However, diffusion-limited Na-ion transport can drive spatially heterogeneous phase nucleation and propagation, leading to multiphase coexis- tence and locally non-uniform electrochemical activity, generating complex reaction path- ways that challenge both mechanistic understanding and predictive material optimization. These challenges can be addressed by investigating single-crystalline regions of materials, i.e. down to the scale of individual particles, although such analyses are often constrained by energetically and/or spatially sparse hyperspectral datasets. Here, we developed an AI-driven method to process hyperspectral data under sparse sampling conditions and generate multiphase maps with nanometer-scale resolution over a micrometer-scale field of view. We applied this processing on scanning transmission X-ray microscopy (STXM) data to determine the distribution and coexistence of phases in individual particles of NaxV2(PO4)2F3 cathode materials, at different states of charge. The methodology relies on a workflow which combines a Gaussian mixture variational autoencoder (GMVAE) algorithm with the Pearson corre- lation coefficient to identify the sodium content and map their spatial distribution. Our approach reveals nanoscale phase heterogeneity and evolution within individual particles, and improves the reliability of phase detection by identifying ambiguity zones, false assign- ments, and transition phases localized at grain boundaries.

Two-dimensional (2D) transition metal carbides, nitrides, or carbonitrides known as MXenes form a class of inherently functionalized layers. The large variety of surface terminations plays a pivotal role in MXene properties and governs the interactions with their environment. In particular, numerical simulations suggest that these functional groups could be key players in gas sensing applications (toward, e.g., humidity, volatile organic compounds─VOCs─or NH3) for which MXenes have been identified as highly promising. Focusing on the benchmark Ti3C2Tx MXene (T being OH, O, F, or Cl), we here use electron energy-loss spectroscopy (EELS) in environmental transmission electron microscopy (ETEM) to characterize in situ, and on the nanometer scale, the selective interactions of different standard surface terminations with two model gases: ethanol (a typical VOC) and water vapor. The quantitative analysis of the core-edge fine structure, supported by density functional theory simulations, demonstrates the much higher affinity of chlorine terminations toward ethanol than water vapor and their superior response for ethanol as compared to oxygen terminations. In addition, the analysis of the carbon K-edge brings evidence of the different modifications of the Ti3C2 conducting core electronic structure upon ethanol or water vapor adsorption, bringing fundamental elements for the understanding of the different sensing mechanisms in Ti3C2Tx layers. Our results highlight the benefits of MXene surface engineering for their rational design as gas sensors, as well as the high relevance of EELS in gas-phase TEM to reveal the intrinsic mechanisms at play in gas adsorption on nanomaterials.

The electrochemical performance of the spinel LiNi0.5Mn1.5O4, a high-voltage positive electrode material for Li-ion batteries (LIBs), is influenced by the transition metal arrangement in the octahedral network, leading to disordered ( F d 3 ̲ m S.G.) and ordered (P4332 S.G.) structures. However, widely used techniques lack the spatial resolution necessary to elucidate the ordering phenomenon at the particle scale. Using the 4D-STEM technique, we present the first direct observation of ordering distribution in individual LiNi0.5Mn1.5O4 particles with nanometric spatial resolution. We propose a quantification method for the local degree of ordering based on the ratio of ordered to disordered spinel lattices along the particle thickness extracted from electron diffraction spot intensities. In an ordered spinel LiNi0.5Mn1.5O4, the transition metal ordering is consistently observed throughout the primary particle. However, the extent of ordering in the spinel phase depends on its distribution at the particle scale, a factor influenced by the annealing conditions. The 4D-STEM analysis elucidates the boundary between highly-ordered and low-ordered LiNi0.5Mn1.5O4 particles.

Scalable quantum-photonic technologies require spatially separated emitters to emit at the same wavelength, yet epitaxially grown quantum dots naturally exhibit emitter-to-emitter variations in their emission energies. Here, we demonstrate a contact-free post-growth tuning approach for InAs quantum dots embedded in InP nanowires using the photo-induced transformation of an amorphous Sb2S3 shell. This photoinduced effect leads to a progressive relaxation of the strain imposed on the InP core, enabling controlled spectral tuning of the quantum-dot emission. The effect is observed both at room temperature and under cryogenic conditions for quantum dots emitting in the O-band of the telecommunications spectrum. Strain-induced redshifts of approximately 7 meV at room temperature and 28 meV under cryogenic conditions are observed, respectively. The use of an amorphous chalcogenide shell therefore provides a route to achieve local wavelength tuning at the single quantum-dot-in-a-nanowire level after growth. This approach could enable spectrally matched emitters for scalable quantum-photonic architectures.

The quantum confinement in III-V semiconductors displays important applications such as lasers or single-photon sources. III-V nanowires (NWs) are ideal candidates for the integration of direct bandgap material on silicon. However, to reach quantum confinement, small diameters are needed. In this work, we perform wet chemical etching of NWs to reduce their diameter. In contrast to plasma irradiation and high-temperature techniques, this approach is more energy-efficient and causes less physical damage. With accurate control of the etching rate in an acidic solution, we obtained NW diameters as small as 20 nm. A 60 nm-thick AlGaAs shell was regrown on the NWs after the etching, and STEM images showed a fully crystalline interface of the core and the shell. Finally, photoluminescence measurements reveal a blueshift of similar to 20 meV due to reduced diameters. This study demonstrates the potential of a top-down approach via wet chemical etching to tune the III-V NW morphology.

Engineering the properties of semiconductors by changing their crystalline phase is a technologically and economically relevant alternative to doping using foreign elements, with strong potential for photonic and electronic applications. Although major advances have been reported recently for crystal-phase engineering of III-V and group IV semiconductor nanowires, interfacing two mismatched crystalline phases in a nanostructure induces several deformation mechanisms, which remain largely unexplored. Here, using state-of-the-art synchrotron X-ray nanobeam diffraction and transmission electron microscopy, subtle twisting and bending is unveiled within an individual GaAs nanowire containing cubic and hexagonal segments. Their role is discussed in accommodating the inter-reticular spacing fluctuations, and their variations are correlated to the nanoscale phase distribution and to the effect of the NW support. This study brings direct evidence of a complex combination of deformation mechanisms in biphasic nanowires, which opens a new path to tune the nanowire properties with appealing perspectives for device engineering in nanophotonics and nanomechanics.

CeO2 nanoparticles possess catalytic activity for hydrolysis of organophosphates such as paraoxon and nerve agents used in chemical warfare. Intoxication with these highly toxic compounds can occur directly through inhalation or dermal contact and lead to rapid and severe consequences including death. It is therefore necessary to have effective means of skin decontamination, and CeO2 nanoparticles are promising. In this study, a wide range of CeO2 nanoparticles were synthesized and their efficiency against paraoxon degradation was evaluated. Nanocubes (NC), nanorods (NR), nano-octahedra (NO), and nanopolyhedra (NPO) were studied along with nanoparticles obtained after calcination (NC*, NR*, NO*, and nanotruncated octahedra NTO*). Results show an influence of calcination, specific surface area, and crystal facets with higher activity for {111} facets compared to {100} facets. pH also impacted POX degradation rates, with higher pH accelerating the degradation. In vitro tests using the Franz cell method demonstrated the skin decontamination efficacy of the CeO2 nanoparticles. NC* exhibited lower efficiency, possibly due to smaller surface area and limited {100} facet degradation. The most efficient nanoparticles were NR* and NO* followed by NTO* consistently with their degradation efficiency and specific surface area. Notably, NR* and NO* performed comparably to FE (Fuller's earth), the standard powder skin decontaminant on battlefield. Unlike FE that can only adsorb paraoxon, CeO2 nanoparticles can neutralize it into safer byproducts. This study highlights the interest of CeO2 nanoparticles and the influence of their physicochemical properties on organophosphorus compound degradation and cutaneous decontamination.

VO2 undergoes a metal‐insulator transition (MIT) at ≈70 °C, which induces large variations in its electrical and wavelength‐dependent optical properties. These features make VO2 a highly sought‐after compound for optical, thermal, and neuromorphic applications. To foster the development of VO2‐based devices for the microelectronic industry, it is also imperative to integrate VO2 on silicon. However, high lattice mismatch and the formation of silicates at the interface between VO2 and Si degrade the quality and functionality of VO2 films. Moreover, VO2's polymorphic nature and stable VO phases pose integration issues. To address these challenges, the MIT of VO2 thin films integrated on Si with a complementary metal‐oxide semiconductor‐compatible HfxZr1−xO2 (HZO) buffer layer is investigated. Using in situ high‐resolution X‐ray diffraction and synchrotron far‐infrared spectroscopy, combined with multiscale atomic and electronic structure characterizations, it is demonstrated that VO2 on the HZO buffer layer exhibits an unusually low thermal hysteresis of ≈4 °C. In these results, the influence of strain on M2 phase nucleation, which controls the hysteresis, is unraveled. Notably, the rate of phase transition is symmetric and does not change for the heating and cooling cycles, implying no incorporation of defects during cycling, and highlighting the potential of an HZO buffer layer for reliable operation of VO2‐based devices.

Batteries are complex systems operating far from equilibrium, relying on intricate reactions at interfaces for performance. Understanding and optimizing these interfaces is crucial, but challenges arise due to the diverse factors influencing their development, making comprehensive characterization essential despite experimental difficulties. Recent advancements in characterization tools offer new opportunities to explore interfacial evolution, particularly in the solid electrolyte interphase (SEI).In this perspective article, leading experts in physical-chemical characterization techniques for electrochemical systems discuss the current state-of-the-art and emerging approaches to study interfaces and their evolution in batteries. The focus here is on the capabilities, technical challenges, limitations, and requirements that these techniques must meet to advance our understanding of battery interfacial evolution. The emphasis is placed on techniques that enable probing interfaces under realistic conditions, close to commercial battery systems, and on the integration of multiple approaches within a single measurement (multimodal) to minimise variable effects.This article focuses on the most promising techniques for characterizing all phases relevant to interfacial processes, as well as their integration with correlative analyses and computational modelling. We discuss solid phase characterization with X-ray spectroscopies and microscopies (XPS, XAS, STXM, X-PEEM & XCT), Raman spectroscopies (SERS, TERS & SHINERS), solid-state NMR and electron microscopies and spectroscopies (STEM, EDXS, EELS & 4D-STEM). The liquid phase characterization is discussed in terms of solution NMR spectroscopy, TEM and optical spectroscopies, while the gas phase can be characterized using OEMS, pressure monitoring and GCMS. Computational modelling and simulation (DFT, ReaxFF & MLIP) are also discussed

Narrow gaps between coupled plasmonic nano-particles show strong optical field enhancements and spectrally adjustable resonance positions, making them attractive for surface enhanced spectroscopies. Gold nanorod dimers formed from nanorod solutions with narrow size distributions are intensely investigated in this context. However, the binding angle of rods coupled at their end faces is usually not controllable. Surprisingly, it is observed that this has only little effect on field enhancement and resonance energies. In this work, gold nanorod dimers are investigated by mapping their plasmon resonances using electron energy-loss spectroscopy in a scanning transmission electron microscope. For a wide range of dimer orientations, a negligible influence of the angle between the two rods on the bonding and antibonding longitudinal dipole resonances is confirmed, in good agreement with numerical simulations. The results are interpreted via the predominant end-coupling of the individual nanorod's plasmonic modes, as illustrated by an analytical charge coupling model. In addition, the simulations emphasize that conclusions from experimental data on the gap morphology on the size range of one nanometer can be ambiguous. In any case, the full understanding of the angle-invariant resonances of nano-rod dimers can further promote their controlled application in surface enhanced spectroscopy or -sensing. Narrow gaps between gold nanorods show strong optical field enhancement at spectrally adjustable plasmon resonances. The hardly controllable binding angle has little effect on the resonances. Electron energy-loss spectroscopy in a transmission electron microscopy (TEM) and numerical simulations are applied here to explain this phenomenon. The results suggest the application of the dimers for reproducible surface enhanced spectroscopy. image

The concept of electronic orbitals has enabled the understanding of a wide range of physical and chemical properties of solids through the definition of, for example, chemical bonding between atoms. In the transmission electron microscope, which is one of the most used and powerful analytical tools for high-spatial-resolution analysis of solids, the accessible quantity is the local distribution of electronic states. However, the interpretation of electronic state maps at atomic resolution in terms of electronic orbitals is far from obvious, not always possible, and often remains a major hurdle preventing a better understanding of the properties of the system of interest. In this review, the current state of the art of the experimental aspects for electronic state mapping and its interpretation as electronic orbitals is presented, considering approaches that rely on elastic and inelastic scattering, in real and reciprocal spaces. This work goes beyond resolving spectral variations between adjacent atomic columns, as it aims at providing deeper information about, for example, the spatial or momentum distributions of the states involved. The advantages and disadvantages of existing experimental approaches are discussed, while the challenges to overcome and future perspectives are explored in an effort to establish the current state of knowledge in this field. The aims of this review are also to foster the interest of the scientific community and to trigger a global effort to further enhance the current analytical capabilities of transmission electron microscopy for chemical bonding and electronic structure analysis.

An inhomogeneous grain coarsening distribution to explain the metamagnetic transition in a thick film of assembled B2 FeRh nanoclusters.

Phase change materials (PCMs) have gained a tremendous interest as a means to actively tune nanophotonic devices through the large optical modulation produced by their amorphous to crystalline reversible transition. Recently, materials such as Sb2S3 emerged as particularly promising low loss PCMs, with both large refractive index modulations and transparency in the visible and near-infrared. Controlling the local and reversible phase transition in this material is of major importance for future applications, and an appealing method to do so is to exploit pulsed lasers. Yet, the physics and limits involved in the optical switching of Sb2S3 are not yet well understood. Here, the reversible laser-induced phase transition of Sb2S3 is investigated, focusing specifically on the mechanisms that drive the optically induced amorphization, with multi-physics considerations including the optical and thermal properties of the PCM and its environment. The laser energy threshold for reversibly changing the phase of the PCM is determined through both theoretical analysis and experimental investigation, not only between fully amorphous and crystalline states but also between partially recrystallized states. Then, the non-negligible impact of the material's polycrystallinity and anisotropy on the power thresholds for optical switching is revealed. Finally, the challenges related to laser amorphization of thick Sb2S3 layers are addressed, as well as strategies to overcome them. These results enable a qualitative and quantitative understanding of the physics behind the optically-induced reversible change of phase in Sb2S3 layers.