Viscoélasticité et plasticité de polymères confinés dans des contacts

PARIS-BIUSJ-Thèses (751052125) / SudocPARIS-BIUSJ-Physique recherche (751052113) / SudocSudocFranceF

Achieving OER selectivity in the electrolysis of seawater through shielded carbon-supported iridium nanoclusters

International audienc

Oxide nanoparticles in water : amorphous to crystal conversion of luminescent yttrium vanadate

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Elucidating non-classical nucleation of nanocrystals from an amorphous intermediate state

International audienceWhile nanoparticles are attractive because of their peculiar properties, their production remains a challenge. Their attributes mostly depend on size and surface state, but also on their microstructure. However, this structure (size, shape, porosity, crystalline quality) is currently only controlled via trial-error experimentations and poorly described by the only tool available, the classical nucleation theory (CNT), especially for oxide nanoparticles synthesised in water. Luminescent, europium-doped yttrium vanadate (YVO$_4$: Eu) illustrates nicely how important tuning the properties at the nanometric scale is. For light emission applications, a good crystalline quality and low surface to volume ratio is required, whereas porosity and high surface area will be key for chemical sensor applications. This branching makes it crucial to understand the mechanisms of formation of these objects in order to have precise control on their structure and thus on their properties. In this work, we focused on elucidating how crystalline YVO4 is formed in water. A first striking result is that tuning the initial pH leads to two critical microstructures : (i) the "expected" [4], porous, one, with nanoparticles ~20nm wide composed of subunits of 2nm, (ii) a new, monocrystalline-like one, with particles 30nm large and no primary unit detected. To understand this difference, we conducted luminescence, pH, ICP-MS, and SAXS/WAXS/XRD studies during the reaction from reaction times as short as 5ms. In particular, we could show the existence of an amorphous intermediate state in both cases and its impact on the particles' size and microstructure, and measure nucleation rates within the disordered network to improve nucleation theories

NaYF4 Microstructure, beyond Their Well-Shaped Morphology

International audienceLanthanide doped nanoparticles are widely investigated for their optical properties. However, the sensitivity of the lanthanide ions to the local symmetry, useful when investigating structural environments, becomes a drawback for optimized properties in the case of poorly controlled crystallinity. In this paper, we focus on β-NaYF4 nanorods in order to provide a detailed description of their chemical composition and microstructure. The combination of detailed XRD analysis and TEM observations show that strong variation may be observed from particles from a same batch of synthesis, but also when considering small variations of synthesis conditions. Moreover, also the nanorods observed by SEM exhibit a very nice faceted shape, they are far from being monocrystalline and present significant local deviation of crystalline symmetry and orientation. All these structural considerations, sensitively probed by polarized emission analysis, are crucial to be analyzed for the development of optimal systems toward the targeted applications

Colloidal Rare Earth Vanadate Single Crystalline Particles as Ratiometric Luminescent Thermometers

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Crystallization within Intermediate Amorphous Phases Determines the Polycrystallinity of Nanoparticles from Coprecipitation

International audienceIntense research on nanocrystals synthesized in solution is motivated by their original physical properties, which are determined by their sizes and shapes on various scales. However, morphology control on the nanoscale is limited by our understanding of crystallization, which is challenged by the now well-established prevalence of noncrystalline intermediates. In particular, the impact of such intermediates on the final sizes and crystal quality remains unclear because the characterization of their evolution on the nanometer and millisecond scales with nonperturbative analyses has remained a challenge. Here we use in situ X-ray scattering to show that the nucleation and growth of YVO4:Eu nanocrystals is spatially restrained within amorphous, nanometer-scaled intermediates. The reactivity and size of these amorphous intermediates determine (i) the mono versus polycrystalline character of final crystals and (ii) the size of final crystals. This implies that designing amorphous intermediates themselves that form in <6 ms is one of the keys to controlled bottom-up syntheses of optimized nanoparticles

Investigation of LSPR Coupling Effects toward the Rational Design of Cs x WO 3– δ Based Solar NIR Filtering Coatings

International audienceAbstract The optical range of localized surface plasmon resonance (LSPR) is extended into the infrared region, thanks to the development of highly doped semiconductor nanocrystals. Particularly, the near‐infrared (NIR) range holds a significant interest in managing solar radiation. However, practical applications necessitate the arrangement of particles, which is known to possibly impact their optical properties through LSPR coupling effects. How such coupling modifies the LSPR response in semiconductor hosts remains largely unexplored. In this study, a protocol for producing composite coatings composed of cesium‐doped tungsten bronze nanocrystals embedded in a silica matrix is presented. Achieving individual dispersion of nanocrystals is made possible through careful selection of a surface polyglycerol ligand exchange. This allows to tune the interparticle distance by adjusting the nanocrystal volume fraction in the composite. The findings demonstrate that LSPR coupling effects significantly influence the LSPR intensity of nanocrystals in the composite when the nanocrystal‐to‐nanocrystal distance matches their size. Beyond elucidating the LSPR coupling effect, this study provides insights into the potential use of Cs‐HTB nanocrystals for solar control applications. Through the optimization of morphology and film structure, remarkable selectivity is obtained in terms of maintaining good transparency in the visible range while achieving high absorption in the NIR

Strain engineering of photo-induced phase transformations in Prussian blue analogue heterostructures

International audienceHeterostructures based on Prussian blue analogues (PBA) combining photo- and magneto-striction have shown a large potential for the development of light-induced magnetization switching. However, studies of the microscopic parameters that control the transfer of the mechanical stresses across the interface and their propagation in the magnetic material are still too scarce to efficiently improve the elastic coupling. Here, this coupling strength is tentatively controlled by strain engineering in heteroepitaxial PBA core–shell heterostructures involving the same Rb0.5Co[Fe(CN)6]0.8·zH2O photostrictive core and isostructural shells of similar thickness and variable mismatch with the core lattice. The shell deformation and the optical electron transfer at the origin of photostriction are monitored by combined in situ and real time synchrotron X-ray powder diffraction and X-ray absorption spectroscopy under visible light irradiation. These experiments show that rather large strains, up to +0.9%, are developed within the shell in response to the tensile stresses associated with the expansion of the core lattice upon illumination. The shell behavior is, however, complex, with contributions in dilatation, in compression or unchanged. We show that a tailored photo-response in terms of strain amplitude and kinetics with potential applications for a magnetic manipulation using light requires a trade-off between the quality of the interface (which needs a small lattice mismatch i.e. a small a-cubic parameter for the shell) and the shell rigidity (decreased for a large a-parameter). A shell with a high compressibility that is further increased by the presence of misfit dislocations will show a decrease in its mechanical retroaction on the photo-switching properties of the core particles

Measuring 3D orientation of nanocrystals via polarized luminescence of rare-earth dopants

International audienceAbstract Orientation of nanoscale objects can be measured by examining the polarized emission of optical probes. To retrieve a three-dimensional (3D) orientation, it has been essential to observe the probe (a dipole) along multiple viewing angles and scan with a rotating analyzer. However, this method requires a sophisticated optical setup and is subject to various external sources of error. Here, we present a fundamentally different approach employing coupled multiple emission dipoles that are inherent in lanthanide-doped phosphors. Simultaneous observation of different dipoles and comparison of their relative intensities allow to determine the 3D orientation from a single viewing angle. Moreover, the distinct natures of electric and magnetic dipoles originating in lanthanide luminescence enable an instant orientation analysis with a single-shot emission spectrum. We demonstrate a straightforward orientation analysis of Eu 3+ -doped NaYF 4 nanocrystals using a conventional fluorescence microscope. Direct imaging of the rod-shaped nanocrystals proved the high accuracy of the measurement. This methodology would provide insights into the mechanical behaviors of various nano- and biomolecular systems