Christine Revenant

Synchrotron small-angle and wide-angle X-ray scattering (SAXS/WAXS) puts in evidence a hierarchical organization of ferroelectric poly(vinylidene fluoride–trifluoroethylene) [P(VDF–TrFE)] that rationalizes its anisotropic piezoelectric response. The material is organized into stacks with a periodicity of ∼60 nm along the c-axis. These stacks consist of grains separated by isotropic amorphous (IA) regions. After poling, the grains separated by 1.5 nm IA regions along the a-axis are composed of crystalline or ordered amorphous (OA) domains, with characteristic sizes of ∼16 nm along the a-axis and ∼22 nm along the c-axis. Complementary Raman scattering puts in evidence that the OA phase has an all-trans conformation with a lower density than the crystalline one. Ferroelectric domains connected along the b-axis give rise to a large d33 piezoelectric coefficient. An analytical elastic model allows linking successfully the local piezoelectric coefficients to the macroscopic ones.

Polyvinylidene fluoride‐trifluoroethylene (P(VDF‐TrFE)) exhibits outstanding electromechanical properties and is a material of choice for non‐volatile memories, energy‐harvesting systems, multifunctional actuators, and sensors. However, this polymer remains one of the least understood materials among ferroelectric materials due to its semi‐crystalline structure with chains folding in elongated grains through the lamellae and amorphous regions. Here, operando high‐resolution X‐ray diffraction is exploited to unravel the P(VDF‐TrFE) structural evolution upon the first electric field application, called poling. This X‐ray technique allows revealing drastic changes in the lamellae with the ordered amorphous (OA) emergence between crystalline nanodomains. The discovery of this OA emergence is critical because it significantly affects the structure, as well as the mechanical, ferroelectric, and piezoelectric properties of the polymer. After poling, even small nanostructural changes lead to measurable effects yielding the piezoelectric coefficients. The longitudinal and transverse piezoelectric effects can be explained by the complex P(VDF‐TrFE) structure. The elongated grains consist of lamellae separated by isotropic amorphous (IA) regions. This study not only sheds light on the fundamental mechanisms occurring in P(VDF‐TrFE), but also offers guidance for new ferroelectric organic materials for flexible, biocompatible sensor and energy applications.

To treat cardiovascular diseases (i.e., a major cause of mortality after cancers), endovascular-technique-based guidewire has been employed for intra-arterial navigation. To date, most commercially available guidewires (e.g., Terumo, Abbott, Cordis, etc.) are non-steerable, which is poorly suited to the human arterial system with numerous bifurcations and angulations. To reach a target artery, surgeons frequently opt for several tools (guidewires with different size integrated into angulated catheters) that might provoke arterial complications such as perforation or dissection. Steerable guidewires would, therefore, be of high interest to reduce surgical morbidity and mortality for patients as well as to simplify procedure for surgeons, thereby saving time and health costs. Regarding these reasons, our research involves the development of a smart steerable guidewire using electroactive polymer (EAP) capable of bending when subjected to an input voltage. The actuation performance of the developed device is assessed through the curvature behavior (i.e., the displacement and the angle of the bending) of a cantilever beam structure, consisting of single- or multi-stack EAP printed on a substrate. Compared to the single-stack architecture, the multi-stack gives rise to a significant increase in curvature, even when subjected to a moderate control voltage. As suggested by the design framework, the intrinsic physical properties (dielectric, electrical, and mechanical) of the EAP layer, together with the nature and thickness of all materials (EAP and substrate), do have strong effect on the bending response of the device. The analyses propose a comprehensive guideline to optimize the actuator performance based on an adequate selection of the relevant materials and geometric parameters. An analytical model together with a finite element model (FEM) are investigated to validate the experimental tests. Finally, the design guideline leads to an innovative structure (composed of a 10-stack active layer screen-printed on a thin substrate) capable of generating a large range of bending angle (up to 190°) under an acceptable input level of 550 V, which perfectly matches the standard of medical tools used for cardiovascular surgery.

Among lead-free ferroelectric materials, Barium Strontium Titanate (BST) has interesting ferroelectric, pyroelectric, piezoelectric, and energy-harvesting properties. This material can be advantageously solution-deposited. There is a need for homogeneous and dense films with optimized electrical properties. BST thin films are elaborated with a highly diluted precursor solution of less than 0.05 M. The electric properties are correlated with the morphology and structure of the films as a function of precursor solution dilution. The film growth is columnar with a tetragonal perovskite structure. As the dilution increases, the roughness of the top surface and of the columns decreases, facilitating grain coalescence. In addition, the permittivity of the grains and of the grain boundaries decreases. The highest dielectric strength (400 kV/cm) is obtained for the sample elaborated with the most diluted precursor solution of 0.008 M. For a given polarization, the necessary poling electric field decreases by a factor 2.3 as dilution increases by a factor 4. Finally, hysteresis loops are obtained with increasing saturation and remnant polarizations as the dilution increases. The saturation polarization is approximately doubled as the solution dilution is multiplied by four. To conclude, BST films elaborated with highly diluted solutions have enhanced ferroelectric properties.

The link between the morphology and the electrical properties of the sol–gel processed Ba_0.7Sr_0.3TiO₃ thin films is investigated. Previous studies have not fully explained the differences in growth morphology as a function of the elaboration conditions. The thin films were investigated by Grazing Incidence Small-Angle X-ray Scattering (GISAXS), x-ray diffraction, and scanning electron microscopy. More precisely, prototype films were studied as a function of the annealing temperature: at low temperatures (140 °C–200 °C) by in situ GISAXS and at high temperatures (600 °C–800 °C) by ex situ GISAXS. At ∼150 °C, self-organized domains with a preferential distance of approximately 14 nm are formed. At high annealing temperatures, the growing domains become either nanoparticles or pores with a preferential distance of approximately 85 nm at 600 °C. This growth evolution is successfully explained by a general model based on convection and evaporation. With thermal annealing, the characteristic lengths parallel to the surface increase due to convection and the characteristic lengths perpendicular to the surface decrease due to evaporation. In addition, two types of annealing were investigated at 700 °C. For annealing after each other layer, a growth with vertically shifted particles occurs with no ferroelectric behavior. On the contrary, for annealing after each deposited layer, a columnar growth occurs and a ferroelectric hysteresis loop is obtained. The ferroelectricity of the sol–gel barium strontium titanate thin films is definitely linked to the complete removal of organic constituents leading to columnar growth.

Grazing-incidence small-angle X-ray scattering (GISAXS) performed at several X-ray energies in the vicinity of the absorption edge of a considered element is called anomalous GISAXS (AGISAXS). This emerging technique takes advantage of the variation of the scattering factor near an absorption edge, allowing the morphology of multi-component nanomaterials to be unraveled. The selected model system for AGISAXS simulations is mainly an In₂O₃ layer containing Ga₂O₃-based nanoparticles. The transmission coefficients at the different X-ray energies near the Ga K edge are identical at one specific incident angle (near the critical angle). Hence, it could be relevant to perform AGISAXS at this incident angle in order to cancel the transmission modification as the X-ray energy changes. For buried nanoparticles, grazing-incidence effects are negligible with respect to the anomalous element-specific contribution provided that the experiments are performed at energies a few electronvolts below the absorption edge. Interestingly, AGISAXS has a clearly different intensity behavior versus X-ray energy for an embedded monodisperse spherical particle, a hole and a core–shell particle. Hence, AGISAXS can be used to unambiguously distinguish such embedded particles. Moreover, even for a dense layer of core–shell nanoparticles on a substrate, anomalous effects are much larger than grazing effects as the X-ray energy changes. Finally, it is shown that experimental anomalous scattering can be significant and can be satisfactorily simulated.

We study by X‐ray absorption spectroscopy the local structure around Zn and Ga in solution‐processed In–Ga–Zn–O thin films as a function of thermal annealing. Zn and Ga environments are amorphous up to 450 °C. At 200 °C and 450 °C, the Ga atoms are in a β‐Ga₂O₃ like structure, mostly tetrahedral gallium oxide phase. Above 300 °C, the Zn atoms are in a tetrahedral ZnO phase for atoms inside the nanoclusters. The observed formation of the inorganic structure above 300 °C may be correlated to the rise of the mobility for IGZO TFTs. The Zn atoms localized at the nanocluster boundary are undercoordinated with O. Such ZnO cluster boundary could be responsible for electronic defect levels. Such defect levels were put in evidence in the upper half of the band gap. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)

Self-organized nanoclusters were obtained in annealed solution-processed In–Ga–Zn–O thin films. STEM and anomalous grazing incidence small-angle X-ray scattering (GISAXS) reveal the formation of nanometer-sized nanoclusters together with the spontaneous creation of a mesopore pattern.