Sintesi e caratterizzazione rilassometrica di agenti di contrasto paramagnetici-superparamagnetici ibridi per Risonanza Magnetica

E' stato realizzato un mezzo di contrasto ibrido paramagnetico-superparamagnetico per risonanza magnetica di imaging comprendente nanoparticelle di magnetite con parziale ricoprimento di Gd-DOTA. Si è quindi analizzato il sistema dal punto di vista magnetico e rilassometrico alla luce della teoria fisica del rilassamento sviluppata per i normali mezzi di contrasto. A supporto si è anche proceduto all'analisi dei sistemi mediante tecniche di spettroscopia TEM, Dynamic light scattering e ATR-FT-IR

Defects in Hybrid Perovskites: The Secret of Efficient Charge Transport

The interaction of free carriers with defects and some critical defect properties are still unclear in methylammonium lead halide perovskites (MHPs). Here, a multi-method approach is used to quantify and characterize defects in single crystal MAPbI(3), giving a cross-checked overview of their properties. Time of flight current waveform spectroscopy reveals the interaction of carriers with five shallow and deep defects. Photo-Hall and thermoelectric effect spectroscopy assess the defect density, cross-section, and relative (to the valence band) energy. The detailed reconstruction of free carrier relaxation through Monte Carlo simulation allows for quantifying the lifetime, mobility, and diffusion length of holes and electrons separately. Here, it is demonstrated that the dominant part of defects releases free carriers after trapping; this happens without non-radiative recombination with consequent positive effects on the photoconversion and charge transport properties. On the other hand, shallow traps decrease drift mobility sensibly. The results are the key for the optimization of the charge transport properties and defects in MHP and contribute to the research aiming to improve perovskite stability. This study paves the way for doping and defect control, enhancing the scalability of perovskite devices with large diffusion lengths and lifetimes

Couches minces nanoporeuses comme plateforme pour applications nanofluidiques

This thesis had a dual purpose: i) the development of nanofluidic devices through not lithographic, cheap and scalable bottom-up approach ii) the understanding of nanofluidic phenomena both through experiments and simulations. Mesoporous thin films, in particular Pillared Planar Nanochannels (PPNs), were prepared and utilized to study the capillary infiltration of liquids in nanostructures and have been tested for future nanofluidic applications like separations and nanoconfined reactions. Non organized mesoporous films have also been studied to determine the relationship between nanostructure characteristics and infiltration speed. It has been also demonstrated that in the case of porosities with reduced bottle-necks capillary penetration is performed through a vapor mediated mechanism The samples were prepared by dip-coating. A novel method of preparation based on the substitution of a large part of the deposing solution in dip-coating with an inert fluid has been developed in order to strongly reduce the fabrication costs and allow the preparation of larger samples. Moreover advancement in control of the dip-coating technique in “acceleration-mode” to produce thickness gradients has been developed and some potential application linked to fluidics shown. Finally a part of the effort of this thesis has been placed in the modeling of the electro-osmotic phenomenon in nanostructures through a rather novel simulation method, Stochastic Rotational Dynamics, which takes into account the hydrodynamics and the other interactions inside a nanofluidic system. Validations of the method and further investigations in particular nanofluidic conditions have been performed.Ces travaux de thèse ont eu deux objectifs: i) le développent de systèmes nanofluidique en utilisant une méthode non-lithographique, peu chère et facilement transposable à l'échelle industrielle ii) la compréhension des phénomènes nanofluidiques au travers des études expérimentales et de modélisation. Des couches minces mesoporeuses, en particulier des structures planaires avec des nanopiliers, ont été utilisé pour des études sur l'infiltration capillaire des liquides dans espaces confiné au niveau nanométrique. En plus des premiers tests pour des applications plus complexes comme des séparations et réactions nanoconfiné. Des structures mesoporeuses non-organisés ont aussi été étudiées pour déterminer la relation entre la nanostructure et la vitesse de remplissage capillaire. A été aussi démontré que pour des porosités avec des forts rétrécissements le remplissage capillaire se produit par l'intermédiaire d'une phase vapeur. Les échantillons ont été préparés par dip-coating. Une méthode de préparation basé sur une substitution de la plus grande parte de la solution à déposer par un fluide inerte a été développé. La méthode permet de réduire fortement le cout de procédé et, par conséquence, de faire des dépôts sur plus grande surface. Un effort dans la modélisation des phénomènes nanofluidiques a aussi été fait pendant cette thèse. Une méthode de simulation qui permet de décrire adéquatement les interactions hydrodynamiques dans un système nano a été utilisée pour simuler un flux électro-osmotique. La méthode, Stochastic Rotational Dynamics, a été valide par confrontation avec des résultats connus et l'influence des certains paramètres de simulation évaluée dans le détail

Nanomechanical signatures of degradation-free influence of water on halide perovskite mechanics

International audienceAbstract Humidity is often reported to compromise the stability of lead halide perovskites or of devices based on them. Here we measure the humidity dependence of the elastic modulus and hardness for two series of lead halide perovskite single crystals, varying either by cation or by anion type. The results reveal a dependence on bond length between, hydrogen bonding with, and polarizability/polarization of these ions. The results show an intriguing inverse relation between modulus and hardness, in contrast to their positive correlation for most other materials. This anomaly persists and is strengthened by the effect of humidity. This, and our overall findings are ascribed to the materials’ unique atomic-scale structure and properties, viz nano-polar domains and strong dynamic disorder, yet high-quality average order. Our conclusions are based on comparing results obtained from several different nano-indentation techniques, which separate surface from bulk elastic modulus, and probe different manifestations of the hardness

Defects in Hybrid Perovskites: The Secret of Efficient Charge Transport (Adv. Funct. Mater. 48/2021)

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The Saga of Water and Halide Perovskites: Evidence of Water in Methylammonium Lead Tri‐Iodide

International audienceThe environment humidity effects on performance of halide perovskites (HaPs), especially MAPbI 3 , are known. Nevertheless, it is hard to find direct experimental evidence of H 2 O in the bulk materials at the levels lower than that of Monohydrate (MAPbI 3 .H 2 O). Here, for the first time, direct experimental evidence of water being released from bulk (µm-s deep) of MAPbI 3 single crystal is reported. The thermogravimetric analysis coupled with mass spectrometry (TGA-MS) of evolved gases is used to detect the MS signal of H 2 O from the penetrable depth and correlate it with the TGA mass loss due to H 2 O leaving the material. These measurements yield an estimate of the average H 2 O content of 1 H 2 O molecule per three MAPbI 3 formula units (MAPbI 3 .0.33H 2 O). Under the relatively low temperature conditions no other evolved gases that can correspond to MAPbI 3 decomposition products, are observed in the MS. In addition to being direct evidence that there is H 2 O inside MAPbI 3 , the data show that H 2 O diffuses into it. With this article, a solid basis is proved for further studies on the mechanisms through which water modifies the properties of MAPbI 3 and all the other halide perovskites

Degradation and Self-Healing of FAPbBr3 Perovskite under Soft-X-Ray Irradiation

The extensive use of perovskites as light absorbers calls for a deeper understanding of the interaction of these materials with light. Here, the evolution of the chemical and optoelectronic properties of formamidinium lead tri-bromide (FAPbBr(3)) films is tracked under the soft X-ray beam of a high-brilliance synchrotron source by photoemission spectroscopy and micro-photoluminescence. Two contrasting processes are at play during the irradiation. The degradation of the material manifests with the formation of Pb-0 metallic clusters, loss of gaseous Br-2, decrease and shift of the photoluminescence emission. The recovery of the photoluminescence signal for prolonged beam exposure times is ascribed to self-healing of FAPbBr(3), thanks to the re-oxidation of Pb-0 and migration of FA(+) and Br- ions. This scenario is validated on FAPbBr(3) films treated by Ar+ ion sputtering. The degradation/self-healing effect, which is previously reported for irradiation up to the ultraviolet regime, has the potential of extending the lifetime of X-ray detectors based on perovskites