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In situ study of the synthesis of lamellar metal chalcogenides by alternating deposition of organic & inorganic molecules
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Molybdenum effects on the stability of passive films unraveled at the nanometer and atomic scales
No full textInternational audienceData recently obtained on model FeCrNi(Mo), 316 L stainless steel, and FeCrNiCo(Mo) passivated surfaces by advanced surface analysis and density functional theory modeling are comprehensively discussed to unravel the multiple effects that molybdenum might have at the nanometer and atomic scales to enhance the stability of passive films. The key role played on corrosion protection by the compositional and structural nanoscale defects of the passive film that originate from the pre-passivation mechanisms of the surface is considered. It is shown how Mo, enriched together with Cr in the nanometer-thick passive film, can combine several effects to enhance the resistance to Cl - -induced passivity breakdown. Enriched as Mo(VI) species in the outer exchange layer of the passive film, Mo impedes the deep penetration of Cl - ions and limits their access to the inner barrier layer. Dispersed as Mo(IV) at the interface with the inner layer, Mo protects against the entry of Cl - ions into the defect sites of the Cr(III) oxide barrier. Present as Mo(IV + δ) in the Fe-rich compositional nanoscale defects self-generated by the local failure of Cr supply upon initial formation of the barrier layer, Mo enhances the selective dissolution of iron and its replacement by chromium and molybdenum. By impeding the formation of O vacancies, Mo also increases the resistance against chloride entry in the oxide matrix, thereby curing these the Fe-rich weak sites against Cl - -induced passivity breakdown
Covalent shaping of polyoxometalate molecular films onto ITO electrodes for charge trapping induced resistive switching
No full textInternational audienceAs nano-sized molecular oxides, polyoxometalates (POMs) hold great promise in non-volatile memory materials based on redox-active molecules. Materials processed from solution, by drop-casting, by embedding POMs in polymers, or using Layer-by-Layer deposition techniques have thus been reported and successfully investigated. Almost all these examples are electrostatically assembled materials. We herein propose an original route to the elaboration of robust covalent POM networks, to seek the influence of the shaping process on the POM-to-POM communication and the final device performance. Capitalizing on our experience in the handling of organic-inorganic POM hybrids, we have prepared diazonium hybrids to harness the propensity of diazonium salts to form multi-layered materials upon electrochemical reduction. A few nanometers thick materials have thus been grown onto ITO electrodes and have shown to be potentially suitable for Write-Once-Read-Many (WORM) devices, with a low set voltage
TWIP-assisted Zr alloys for medical applications: Design strategy, mechanical properties and first biocompatibility assessment
No full textInternational audienceThis study proposes a novel strategy for the design of a new family of metastable Zr alloys. These alloys offer improved mechanical properties for implantss, particularly in the applications where conventional stainless steels and Co-Cr alloys are currently used but lack suitability. The design approach is based on the controlled twinninginduced plasticity (TWIP) effect, significantly enhancing the ductility and strainhardenability of the Zr alloys. In order to draw a "blueprint" for compositional design of biomedical TWIP (Bio-TWIP) Zr alloys-using only non-toxic elements-the study combines d-electron phase stability calculations (specifically bond order (Bo) and mean d-orbital energy (Md)) with systematic experimental screening of active deformation mechanisms within the Zr-Nb-Sn alloy system. This research aids in accurately identify of the TWIP line, which signifies the mechanism shift between TWIP and classic slip as the primary deformation mechanism. To demonstrate the efficacy of the TWIP mechanism in enhancing mechanical properties, three alloy compositions-Zr-12Nb-2Sn, Zr-13Nb-1Sn, and Zr-14Nb-3Sn-are selected.. Results indicate that the TWIP mechanism leads to excellent average strain-hardening rates (~3.2 GPa) and uniform elongation of ~20% in Zr-12Nb-2Sn, which displays both {332} mechanical twinning and dislocation slip as the primary deformation mechanisms. Conversely, Zr-14Nb-3Sn exhibits the typical mechanical properties found in stable BCC alloys, characterized by the sole occurrence of dislocation slip. Cell viability tests confirm the superior biocompatibility of Zr-Nb-based alloys with deformation twins on the surface, in line with existing literature.. Based on the whole set of results, a comprehensive design diagram is proposed
Humidity‐Induced Degradation Processes of Halide Perovskites Unveiled by Correlative Analytical Electron Microscopy
No full textInternational audienceAbstract Improving the stability of lead halide perovskite solar cells (PSCs) for industrialization is currently a major challenge. It is shown that moisture induces changes in global PSC performance, altering the nature of the absorber through phase transition or segregation. Understanding how the material evolves in a wet environment is crucial for optimizing device performance and stability. Here, the chemical and structural evolution of state‐of‐the‐art hybrid perovskite thin‐film Cs 0.05 (MA 0.15 FA 0.85 ) 0.95 Pb(I 0.84 Br 0.16 ) 3 (CsMAFA) is investigated after aging under controlled humidity with analytical characterization techniques. The analysis is performed at different scales through Photoluminescence, X‐ray Diffraction Spectroscopy, Cathodoluminescence, Selected Area Electron Diffraction, and Energy Dispersive X‐ray Spectroscopy. From the analysis of the degradation products from the perovskite layer and by the correlation of their optical and chemical properties at a microscopic level, different phases such as lead–iodide (PbI 2 ), inorganic mixed halide CsPb(I 0.9 Br 0.1 ) 3 and lead‐rich CsPb 2 (I 0.74 Br 0.26 ) 5 perovskite are evidenced. These phases demonstrate a high degree of crystallinity that induces unique geometrical shapes and drastically affects the optoelectronic properties of the thin film. By identifying the precise nature of these specific species, the multi‐scale approach provides insights into the degradation mechanisms of hybrid perovskite materials, which can be used to improve PSC stability
Transient currents produced by mobile ions in thick Metal-Semiconductor-Metal devices
No full textInternational audienceIt is now well established that halide perovskite materials, such as Methyl Ammonium Lead Iodide (MAPI), contain low-mobility ions that affect the device operation and performance. Ionic motion is believed to be primarily responsible for the long transients observed in dark J-V measurements (hysteresis) of perovskite devices.In this work, we use a drift-diffusion numerical simulation to evaluate the main characteristics of the current transients produced by a mobile ion after biasing simple Metal-Semiconductor-Metal (MSM) structures. We compare the theoretical results with experimental measurements performed in monocrystals and thin-films devices of halide perovskites. We observe that most of the transient characteristics can be explained with our model and we present a discussion of the possible causes for some observed discrepancies.We relate the semiconductor parameters, such as the concentration of dopants and ionic species, with the current transient shape and time position. We deduce two analytical formulas for extracting the ionic mobility and the ratio between ionic and dopant concentrations from the current transient. Finally, we perform measurements at different temperatures for extracting the activation energy of the ionic mobility
Electrochemical and Chemical Reactivities of Titanium Oxide-Based Materials with a Chloroaluminate Ionic Liquid Electrolyte for Aluminum Batteries
No full textInternational audienceBy selecting three different types of electrode materials, we intended to better understand the Al3+ intercalation chemistry of titanium oxide-based frameworks with an acidic chloroaluminate electrolyte. In agreement with previous reports, we confirmed that the native interstitial sites of anatase TiO2 are less prone to accommodate Al3+ than Li+ or Na+ ions, while introducing cationic vacancies largely increases the electrochemical storage capacity. Upon the first cycle, the highest reversible capacity, up to 277 mAh/g, was obtained for a hydrated layered structure featuring cationic vacancies. Total scattering data showed that the insertion of Al3+ ions induced a strong distortion of the framework. In addition, combined 27Al MAS NMR and DFT calculations revealed that in oxy-hydroxylated vacant sites, the coordination mode of Al3+ ions depends on the arrangement of anions around vacancies inducing the occurrence of 4-, 5-, and 6-fold coordination modes. Further cycling experiments revealed a progressive capacity fading for all electrode materials. Using cyclic voltammetry on the used electrolyte, we evidenced that a partial dissolution has occurred, which is more pronounced for the layered hydrate compound, and that solubilized species are electrochemically active, giving rise to specific signatures in both CVs and galvanostatic experiments. Raman spectroscopy enabled us to characterize these species, which are derived from the Ti–Cl system. The solubilized species, however, eventually precipitated, as shown by a purple deposit observed on the separator and tentatively assigned to TiCl3, known to be insoluble in this medium. By providing further information on the Al3+ intercalation chemistry and a better understanding of the electrochemical and chemical reactivities of electrode materials, this work will enable progress to be made in the development of aluminum-ion batteries
Multifunctional Supramolecular Gels with Strong Mechanical Properties Formed by Self-Assembly of Polyoxometalate-Based Coordination Polymers
No full textInternational audienceMetallogels built in a bottom-up approach by metal coordination and supramolecular interactions have important potential for the elaboration of smart materials. In this context, we present here the formation of supramolecular coordination polymers driven by the complexation of cobalt(II) or zinc(II) ions with polyoxometalate-based hybrids displaying two terpyridine ligands in a linear arrangement. Thanks to the electrostatic interactions between the polyoxometalate cores and metal nodes, the polymer chains selfassemble into fibers that physically cross-link to form gels above a critical concentration. Using spectroscopy, microscopy, X-ray scattering, and rheometry, complemented by molecular dynamics simulations, we investigated the supramolecular organization of the chains in the fibers and the resulting processes leading to gelation. Compared to previously reported systems, these gels have improved rheological features and appealing properties, such as birefringence, luminescence, and spin crossover, paving the way for their use as building blocks for multifunctional smart materials.</div
First Electrodeposition of Silicon on Crumbled MXene (c‐Ti 3 C 2 T x ) for High‐Performance Lithium‐Ion Battery Negative Electrode
No full textInternational audienceThe demand for high energy density Li‐ion batteries requires electrode materials with high capacity and long cycling stability. Silicon is among the most promising negative electrode materials due to its high theoretical capacity, abundant resources, and low working potential. However, its poor conductivity and significant volume expansion during cycling limit its practical application. To overcome these issues, this study develops a two‐step synthesis method for a nanostructured composite based on silicon as the active material. First, a crumbled Ti 3 C 2 T x (c‐Ti 3 C 2 T x ) structure formed through electrostatic interaction between a Ti 3 C 2 T x suspension and 1 M KOH. Then, an amorphous silicon layer is electrodeposited onto the c‐Ti 3 C 2 T x flakes in a room‐temperature ionic liquid, creating the Si/c‐Ti 3 C 2 T x composite for the negative electrode of Li‐ion batteries. The c‐Ti 3 C 2 T x structure enhances conductivity, provides mechanical stability to accommodate silicon's expansion, and offers nanostructured porosity for lithium‐ion diffusion. The composite material demonstrates exceptional cycling stability, achieving a capacity of 1300 mAh g −1 at C/5 with 91 % capacity retention after 100 cycles
Rapid and Noise‐Resilient Mapping of Photogenerated Carrier Lifetime in Halide Perovskite Thin Films
No full textInternational audienceHalide perovskite materials offer significant promise for solar energy and optoelectronics yet understanding and enhancing their efficiency and stability require addressing lateral inhomogeneity challenges. While photoluminescence imaging techniques are employed for the measurement of their opto-electronic and transport properties, going further in terms of precision requires longer acquisition times. Prolonged exposure of perovskites to light, given their high reactivity, can substantially alter these layers, rendering the acquired data less meaningful for analysis. In this paper, a method to extract high-quality lifetime images from rapidly acquired, noisy time-resolved photoluminescence images is proposed. This method leverages concepts of the field of constrained reconstruction and includes the Huber loss function and a specific form of total variation regularization. Through both simulations and experiments, it is demonstrated that the approach outperforms conventional pointwise methods. Optimal acceleration and optimization parameters tailored for decay time imaging of perovskite materials, offering new perspectives for accelerated experiments crucial in degradation process characterization are identified. Importantly, this methodology holds the potential for broader applications: it can be extended to explore additional beam-sensitive materials, and other imaging characterization techniques and employed with more complex physical models to treat time-resolved decays.</div