Magnetic anisotropy engineering in onion-structured metal oxide nanoparticles combining dual exchange coupling and proximity effects.

A series of exchange-coupled magnetic nanoparticles combining several magnetic phases in an onion-type structure were synthesized by performing a three-step seed-mediated growth process. Iron and cobalt precursors were alternatively decomposed in high-boiling-temperature solvents (288-310 °C) to successively grow CoO and Fe3-δO4 shells (the latter in three stages) on the surface of Fe3-δO4 seeds. The structure and chemical composition of these nanoparticles were investigated in depth by combining a wide panel of advanced techniques, such as scanning transmission electron microscopy (STEM), electron energy-loss spectroscopy-spectrum imaging (EELS-SI), 57Fe Mössbauer spectrometry, and X-ray circular magnetic dichroism (XMCD) techniques. The size of the nanoparticles increased progressively after each thermal decomposition step, but the crystal structure of core-shell nanoparticles was significantly modified during the growth of the second shell. Indeed, the antiferromagnetic CoO phase was progressively replaced by the CoFe2O4 ferrimagnet due to the concomitant processes of partial solubilization/crystallization and the interfacial cationic diffusion of iron. A much more complex chemical structure than that suggested by a simple size variation of the nanoparticles is thus proposed, namely Fe3-δO4@CoO-CoFe2O4@Fe3-δO4, where an intermediate Co-based layer was shown to progressively become a single, hybrid magnetic phase (attributed to proximity effects) with a reduction in the CoO amount. In turn, the dual exchange-coupling of this hybrid Co-based intermediate layer (with high anisotropy and ordering temperature) with the surrounding ferrite (core and outer shells) stabilized the particle moment well above room temperature. These effects allow for the production of Fe oxide-based magnetic nanoparticles with high effective anisotropy, thus revealing the potential of this strategy to design rare-earth-free permanent nanomagnets at room temperature.

Toward Reliable Synthesis of Superconducting Infinite Layer Nickelate Thin Films by Topochemical Reduction.

Infinite layer (IL) nickelates provide a new route beyond copper oxides to address outstanding questions in the field of unconventional superconductivity. However, their synthesis poses considerable challenges, largely hindering experimental research on this new class of oxide superconductors. That synthesis is achieved in a two-step process that yields the most thermodynamically stable perovskite phase first, then the IL phase by topotactic reduction, the quality of the starting phase playing a crucial role. Here, a reliable synthesis of superconducting IL  nickelate films is reported after successive topochemical reductions of a parent perovskite phase with nearly optimal stoichiometry. Careful analysis of the transport properties of the incompletely reduced films reveals an improvement in the strange metal behavior of their normal state resistivity over subsequent topochemical reductions, offering insight into the reduction process.

Direct Quantifying Charge Transfer by 4D-STEM: A Study on Perfect and Defective Hexagonal Boron Nitride.

Four-dimensional scanning transmission electron microscopy (4D-STEM) offers an attractive approach to simultaneously obtain precise structural determinations and capture details of local electric fields and charge densities. However, accurately extracting quantitative data at the atomic scale poses challenges, primarily due to probe propagation and size-related effects, which may even lead to misinterpretations of qualitative effects. In this study, we present a comprehensive analysis of electric fields and charge densities in both pristine and defective h-BN flakes. Through a combination of experiments and first-principle simulations, we demonstrate that while precise charge quantification at individual atomic sites is hindered by probe effects, 4D-STEM can directly measure charge transfer phenomena at the monolayer edge with sensitivity down to a few tenths of an electron and a spatial resolution on the order of a few angstroms.

Valence-Ordered Thin-Film Nickelate with Tri-component Nickel Coordination Prepared by Topochemical Reduction.

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.

Charge Distribution across Capped and Uncapped Infinite-Layer Neodymium Nickelate Thin Films.

Charge ordering (CO) phenomena have been widely debated in strongly-correlated electron systems mainly regarding their role in high-temperature superconductivity. Here, the structural and charge distribution in NdNiO2 thin films prepared with and without capping layers, and characterized by the absence and presence of CO are elucidated. The microstructural and spectroscopic analysis is done by scanning transmission electron microscopy-electron energy loss spectroscopy (STEM-EELS) and hard X-ray photoemission spectroscopy (HAXPES). Capped samples show Ni1+ , with an out-of-plane (o-o-p) lattice parameter of around 3.30 Å indicating good stabilization of the infinite-layer structure. Bulk-sensitive HAXPES on Ni-2p shows weak satellite features indicating large charge-transfer energy. The uncapped samples evidence an increase of the o-o-p parameter up to 3.65 Å on the thin film top with a valence toward Ni2+ in this region. Here, 4D-STEM demonstrates (303)-oriented stripes which emerge from partially occupied apical oxygen. Those stripes form quasi-2D coherent domains viewed as rods in the reciprocal space with Δqz ≈ 0.24 reciprocal lattice units (r.l.u.) extension located at Q = ( ± 1 3 , 0 , ± 1 3 $\pm \frac{1}{3},0,\pm \frac{1}{3}$ ) and ( ± 2 3 , 0 , ± 2 3 $\pm \frac{2}{3},0,\pm \frac{2}{3}$ ) r.l.u. The stripes associated with oxygen re-intercalation concomitant with hole doping suggest a possible link to the previously reported CO in infinite-layer nickelate thin films.

Strontium-driven physiological to pathological transition of bone-like architecture: A dose-dependent investigation.

Whilst strontium (Sr2+) is widely investigated for treating osteoporosis, it is also related to mineralization disorders such as rickets and osteomalacia. In order to clarify the physiological and pathological effects of Sr2+ on bone biomineralization , we performed a dose-dependent investigation in bone components using a 3D scaffold that displays the hallmark features of bone tissue in terms of composition (osteoblast, collagen, carbonated apatite) and architecture (mineralized collagen fibrils hierarchically assembled into a twisted plywood geometry). As the level of Sr2+ is increased from physiological-like to excess, both the mineral and the collagen fibrils assembly are destabilized, leading to a drop in the Young modulus, with strong implications on pre-osteoblastic cell proliferation. Furthermore, the microstructural and mechanical changes reported here correlate with that observed in bone-weakening disorders induced by Sr2+ accumulation, which may clarify the paradoxical effects of Sr2+ in bone mineralization. More generally, our results provide physicochemical insights into the possible effects of inorganic ions on the assembly of bone extracellular matrix and may contribute to the design of safer therapies for treating osteoporosis. STATEMENT OF SIGNIFICANCE: Physiological-like (10% Sr2+) and excess accumulation-like (50% Sr2+) doses of Sr2+ are investigated in 3D biomimetic assemblies possessing the high degree of organization found in the extracellular of bone. Above the physiological dose, the organic and inorganic components of the bone-like scaffold are destabilized, resulting in impaired cellular activity, which correlates with bone-weakening disorders induced by Sr2+.

From Low-Field Sondheimer Oscillations to High-Field Very Large and Linear Magnetoresistance in a SrTiO3-Based Two-Dimensional Electron Gas.

Quantum materials harbor a cornucopia of exotic transport phenomena challenging our understanding of condensed matter. Among these, a giant, nonsaturating linear magnetoresistance (MR) has been reported in various systems, from Weyl semimetals to topological insulators. Its origin is often ascribed to unusual band structure effects, but it may also be caused by extrinsic sample disorder. Here, we report a very large linear MR in a SrTiO3 two-dimensional electron gas and, by combining transport measurements with electron spectromicroscopy, show that it is caused by nanoscale inhomogeneities that are self-organized during sample growth. Our data also reveal semiclassical Sondheimer oscillations arising from interferences between helicoidal electron trajectories, from which we determine the 2DEG thickness. Our results bring insight into the origin of linear MR in quantum materials, expand the range of functionalities of oxide 2DEGs, and suggest exciting routes to explore the interaction of linear MR with features like Rashba spin-orbit coupling.

Can Copper Nanostructures Sustain High-Quality Plasmons?

Silver, king among plasmonic materials, features low inelastic absorption in the visible-infrared (vis-IR) spectral region compared to other metals. In contrast, copper is commonly regarded as too lossy for actual applications. Here, we demonstrate vis-IR plasmons with quality factors >60 in long copper nanowires (NWs), as determined by electron energy-loss spectroscopy. We explain this result by noticing that most of the electromagnetic energy in these plasmons lies outside the metal, thus becoming less sensitive to inelastic absorption. Measurements for silver and copper NWs of different diameters allow us to elucidate the relative importance of radiative and nonradiative losses in plasmons spanning a wide spectral range down to <20 meV. Thermal population of such low-energy modes becomes significant and generates electron energy gains associated with plasmon absorption, rendering an experimental determination of the NW temperature. Copper is therefore emerging as an attractive, cheap, abundant material platform for high-quality plasmonics in elongated nanostructures.

Template-Free and Surfactant-Free Synthesis of Selective Multi-Oxide-Coated Ag Nanowires Enabling Tunable Surface Plasmon Resonance.

Without using templates, seeds and surfactants, this study successfully prepared multi-oxide-layer coated Ag nanowires that enable tunable surface plasmon resonance without size or shape changes. A spontaneously grown ultra-thin titania layer onto the Ag nanowire surface causes a shift in surface plasmon resonance towards low energy (high wavelength) and also acts as a preferential site for the subsequent deposition of various oxides, e.g., TiO2 and CeO2. The difference in refractive indices results in further plasmonic resonance shifts. This verifies that the surface plasma resonance wavelength of one-dimensional nanostructures can be adjusted using refractive indices and shell oxide thickness design.

Strontium Calcium Phosphate Nanotubes as Bioinspired Building Blocks for Bone Regeneration.

Calcium phosphate (CaP)-based ceramics are the most investigated materials for bone repairing and regeneration. However, the clinical performance of commercial ceramics is still far from that of the native tissue, which remains as the gold standard. Thus, reproducing the structural architecture and composition of bone matrix should trigger biomimetic response in synthetic materials. Here, we propose an innovative strategy based on the use of track-etched membranes as physical confinement to produce collagen-free strontium-substituted CaP nanotubes that tend to mimic the building block of bone, i.e., the mineralized collagen fibrils. A combination of high-resolution microscopic and spectroscopic techniques revealed the underlying mechanisms driving the nanotube formation. Under confinement, poorly crystalline apatite platelets assembled into tubes that resembled the mineralized collagen fibrils in terms of diameter and structure of bioapatite. Furthermore, the synergetic effect of Sr2+ and confinement gave rise to the stabilization of amorphous strontium CaP nanotubes. The nanotubes were tested in long-term culture of osteoblasts, supporting their maturation and mineralization without eliciting any cytotoxicity. Sr2+ released from the particles reduced the differentiation and activity of osteoclasts in a Sr2+ concentration-dependent manner. Their bioactivity was evaluated in a serum-like solution, showing that the particles spatially guided the biomimetic remineralization. Further, these effects were achieved at strikingly low concentrations of Sr2+ that is crucial to avoid side effects. Overall, these results open simple and promising pathways to develop a new generation of CaP multifunctional ceramics that are active in tissue regeneration and able to simultaneously induce biomimetic remineralization and control the imbalanced osteoclast activity responsible for bone density loss.

Tailored Nanoscale Plasmon-Enhanced Vibrational Electron Spectroscopy.

Atomic vibrations and phonons are an excellent source of information on nanomaterials that we can access through a variety of methods including Raman scattering, infrared spectroscopy, and electron energy-loss spectroscopy (EELS). In the presence of a plasmon local field, vibrations are strongly modified and, in particular, their dipolar strengths are highly enhanced, thus rendering Raman scattering and infrared spectroscopy extremely sensitive techniques. Here, we experimentally demonstrate that the interaction between a relativistic electron and vibrational modes in nanostructures is fundamentally modified in the presence of plasmons. We finely tune the energy of surface plasmons in metallic nanowires in the vicinity of hexagonal boron nitride, making it possible to monitor and disentangle both strong phonon-plasmon coupling and plasmon-driven phonon enhancement at the nanometer scale. Because of the near-field character of the electron beam-phonon interaction, optically inactive phonon modes are also observed. Besides increasing our understanding of phonon physics, our results hold great potential for investigating sensing mechanisms and chemistry in complex nanomaterials down to the molecular level.

Tailored ultra-small Prussian blue-based nanoparticles for MRI imaging and combined photothermal/photoacoustic theranostics.

Ultrasmall sub-10 nm nanoparticles of Prussian blue analogues incorporating GdIII ions at their periphery revealed longitudinal relaxivities above 40 mM-1 s-1 per GdIII regardless of the nature of the core and the polymer coating. Large T1-weighted contrast enhancements were achieved in addition to a highly efficient photothermal effect and in vivo photoacoustic imaging in tumors.

Room Temperature Blocked Magnetic Nanoparticles Based on Ferrite Promoted by a Three-Step Thermal Decomposition Process.

Exchange coupled nanoparticles that combine hard and soft magnetic phases are very promising to enhance the effective magnetic anisotropy while preserving sizes below 20 nm. However, the core-shell structure is usually insufficient to produce rare earth-free ferro(i)magnetic blocked nanoparticles at room temperature. We report on onion-type magnetic nanoparticles prepared by a three-step seed mediated growth based on the thermal decomposition method. The core@shell@shell structure consists of a core and an external shell of Fe3-δO4 separated by an intermediate Co-doped ferrite shell. The double exchange coupling at both core@shell and shell@shell interfaces results in such an increased of the magnetic anisotropy energy, that onion-type nanoparticles of 16 nm mainly based on iron oxide are blocked at room temperature. We envision that these results are very appealing for potential applications based on permanent magnets.

Formation of stable strontium-rich amorphous calcium phosphate: Possible effects on bone mineral.

Bone, tooth enamel, and dentin accumulate Sr2+, a natural trace element in the human body. Sr2+ comes from dietary and environmental sources and is thought to play a key role in osteoporosis treatments. However, the underlying impacts of Sr2+on bone mineralization remain unclear and the use of synthetic apatites (which are structurally different from bone mineral) and non-physiological conditions have led to contradictory results. Here, we report on the formation of a new Sr2+-rich and stable amorphous calcium phosphate phase, Sr(ACP). Relying on a bioinspired pathway, a series of Sr2+ substituted hydroxyapatite (HA) that combines the major bone mineral features is depicted as model to investigate how this phase forms and Sr2+ affects bone. In addition, by means of a comprehensive investigation the biomineralization pathway of Sr2+ bearing HA is described showing that not more than 10 at% of Sr2+, i.e. a physiological limit incorporated in bone, can be incorporated into HA without phase segregation. A combination of 31P and 1H solid state NMR, energy electron loss spectromicroscopy, transmission electron microscopy, electron diffraction, and Raman spectroscopy shows that Sr2+ introduces disorder in the HA culminating with the unexpected Sr(ACP), which co-exists with the HA under physiological conditions. These results suggest that heterogeneous Sr2+ distribution in bone is associated with regions of low structural organization. Going further, such observations give clues from the physicochemical standpoint to understand the defects in bone formation induced by high Sr2+ doses. STATEMENT OF SIGNIFICANCE: Understanding the role played by Sr2+ has a relevant impact in physiological biomineralization and provides insights for its use as osteoporosis treatments. Previous studies inspired by the bone remodelling pathway led to the formation of biomimetic HA in terms of composition, structures and properties in water. Herein, by investigating different atomic percentage of Sr2+ related to Ca2+ in the synthesis, we demonstrate that 10% of Sr2+ is the critical loads into the biomimetic HA phase; similarly to bone. Unexpectedly, using higher amount leads to the formation of a stable Sr2+-rich amorphous calcium phosphate phase that may high-dose related pathologies. Our results provide further understanding of the different ways Sr2+ impacts bone.

Magnetism and plasmonic performance of mesoscopic hollow ceria spheres decorated with silver nanoparticles.

We investigate the role of interfaces and surfaces in the magnetic and surface enhanced Raman spectroscopy (SERS) properties of CeO2 hollow spheres decorated with Ag nanoparticles (H-CeO2@Ag). The composites, H-CeO2@Ag, were synthesized using a newly developed two-step process. The CeO2 hollow sphere diameter ranges from 100 nm to 2 µm and the grafted Ag nanoparticle (NP) size varies from 5 to 50 nm with a controllable coverage ratio. Spectroscopic and microscopic characterization confirms the formation of an interface between the Ag and ceria and shows different charge rearrangements occurring at both the interface and the surface. Room temperature ferro-magnetism was observed in all composites, and is associated mostly with ceria surface defects. A strong SERS effect was reported with a detection limit down to 10-14 M for the rhodamine 6G analyte. Scanning transmission electron microscopy and electron energy loss spectroscopy investigation reveals that hot-spots are associated with the silver NP surfaces and also with the Ag/CeO2 interface. This interfacial hot spot occurs for metallic particles above 30 nm and is strongly red shifted with respect to the Ag surface plasmon. The strong SERS activity is then attributed to the presence of several types of hot-spots and the geometrical features (buoyant hollow sphere and size dispersion) of the composite.

Direct Mapping of Phase Separation across the Metal-Insulator Transition of NdNiO3.

Perovskite rare-earth nickelates RNiO3 are prototype correlated oxides displaying a metal-insulator transition (MIT) at a temperature tunable by the ionic radius of the rare-earth R. Although its precise origin remains a debated topic, the MIT can be exploited in various types of applications, notably for resistive switching and neuromorphic computation. So far, the MIT has been mostly studied by macroscopic techniques, and insights into its nanoscale mechanisms were only provided recently by X-ray photoemission electron microscopy through absorption line shifts, used as an indirect proxy to the resistive state. Here, we directly image the local resistance of NdNiO3 thin films across their first-order MIT using conductive-atomic force microscopy. Our resistance maps reveal the nucleation of ∼100-300 nm metallic domains in the insulating state that grow and percolate as temperature increases. We discuss the resistance contrast mechanism, analyze the microscopy and transport data within a percolation model, and propose experiments to harness this mesoscopic electronic texture in devices.

Interplay Between Cr Dopants and Vacancy Clustering in the Structural and Optical Properties of WSe2.

Here, we analyze the effect of Cr doping on WSe2 crystals. The topology and the chemistry of the doped samples have been investigated by atom-resolved scanning transmission electron microscopy combined with electron energy loss spectroscopy. Cr (measured to have formal valence 3+) occupies W sites (formal valence 4+), indicating a possible hole doping. However, single or double Se vacancies cluster near Cr atoms, leading to an effective electron doping. These defects organization can be explained by the strong binding energy of the CrW-Vse complex obtained by density functional theory calculations. In highly Cr-doped samples, a local phase transition from the 2H to the to 1T phase is observed, which has been previously reported for other electron-doped transition-metal dichalcogenides. Cr-doped crystals suffer a compressive strain, resulting in an isotropic lattice contraction and an anisotropic optical bandgap energy shift (25 meV in-plane and 80 meV out-of-plane).

Interface interactions and enhanced room temperature ferromagnetism of Ag@CeO2 nanostructures.

Enhancement of room temperature ferromagnetism (RTFM) has been achieved with core-shell metal-oxide nanoparticles (Ag@CeO2). To enhance the magnetic properties, interfacial charge transfer is achieved via the formation of a core-shell interface. Furthermore, by varying the shell thicknesses, additional control of the RTFM can be obtained. The Ag@CeO2 core-shell nanoparticles are synthesized successfully via a two-step method. Ag nanoparticles (NPs) are first synthesized on a TiO2 substrate by a thermally assisted photoreduction method, and then CeO2 NPs are deposited on the surface of Ag NPs by chemical reduction. No surfactants or organic compounds are used during the synthesis. At the interface between the core and the shell, electron transfers from the Ag-p orbital to the Ag-d and Ce-f orbitals are evidenced by X-ray absorption spectroscopy and electron energy loss spectroscopy. Such interfacial charge transfer results in enhanced room temperature ferromagnetism in the Ag@CeO2 core-shell NPs compared to the magnetism arising for bare Ag or CeO2 NPs. This study suggests that tailoring the interface, the surface and their coupling in nanostructured metal-oxide core shell nanoparticles is an effective way to enhance their magnetic properties.

Structural properties and thermal stability of cobalt- and chromium-doped α-MnO2 nanorods.

α-MnO2 nanorods were synthesized via the hydrothermal decomposition of KMnO4 in an acidic environment in the presence of Co2+ and Cr3+ ions. Reactions were carried out at three different temperatures: 90, 130 and 170 °C. All prepared samples exhibit a tetragonal MnO2 crystalline phase. SEM-EDS analysis shows that cobalt cations are incorporated to a higher degree into the MnO2 framework than chromium ions, and that the content of the dopant ions decreases with increasing reaction temperature. The oxidation of Co2+ to Co3+ during the reaction was proved by an XANES study, while EXAFS results confirm that both dopant ions substitute Mn4+ in the center of an octahedron. The K/Mn ratio in the doped samples synthesized at 170 °C is significantly lower than in the undoped samples. Analysis of an individual cobalt-doped α-MnO2 nanorod with HAADF-STEM reveals that the distribution of cobalt through the cross-section of the nanorod is uniform. The course of thermal decomposition of the doped nanorods is similar to that of the undoped ones. Dopant ions do not preserve the MnO2 phase at higher temperatures nor do they destabilize the cryptomelane structure.

Conductivity and Local Structure of LaNiO3 Thin Films.

A marked conductivity enhancement is reported in 6-11 unit cell LaNiO3 thin films. A maximal conductivity is also observed in ab initio calculations for films of the same thickness. In agreement with results from state of the art scanning transmission electron microscopy, the calculations also reveal a differentiated film structure comprising characteristic surface, interior, and heterointerface structures. Based on this observation, a three-element parallel conductor model is considered and leads to the conclusion that the conductivity enhancement for films of 6-11 unit cells, stems from the onset of intercompetition between the three local structures in the film depth.