International audienceno abstrac

Amyloid fibres attract considerable interests due to their biological roles in neurodegenerative diseases and their potentials as functional biomaterials. We describe here a completely new finding about an intrinsic signal of amyloid fibres in the near infrared (NIR) range. When combined with their recently reported blue luminescence, it paves the way toward new blueprints for label-free detections of amyloid deposits within in vitro up to in vivo contexts. The blue luminescence allows for staining-free characterization of amyloid deposits within human samples. The NIR signal offers promising prospects for innovative diagnostic strategies of neurodegenerative diseases; a need to improve medical care and to develop new therapies. As a proof of concept, we demonstrate direct detection of amyloid deposits within brains of living aged "Alzheimer's" mice using non-invasive and contrast agent-free imaging. UV-Vis-NIR optical properties of amyloids opens new research avenues across amyloidoses as well as for next generation biophotonic devices.

Crystal defects in unintentionally doped ZnO nanowires grown by chemical bath deposition (CBD) play a capital role on their optical and electrical properties, governing the performances of many nanoscale engineering devices. However, the nature of these crystal defects is still highly debated. In particular, the hydrogen-related defects have not been explored in detail yet although the growth medium operates in aqueous solution. By using four-point probe resistivity measurements, we show that ZnO nanowires grown by CBD using zinc nitrate and hexamethylenetetramine exhibit a high electrical conductivity with electron densities ranging from 2.7 x 10 18 to 3.1 x 10 19 cm -3 . Most of them have a metallic electrical conduction. By combining density-functional theory calculations with cathodoluminescence and Raman spectroscopy, we reveal that the high electrical conductivity mostly originates from the formation of interstitial hydrogen in bond-centered sites (HBC) and of zinc vacancy -hydrogen (VZn-nH) complexes. In particular, the HBC and (VZn-3H) complex are found to act as two shallow donors with a very low formation energy, for which the most stable configurations are reported. Additionally, this combined theoretical and experimental approach allows us to revisit the highly debated origin of the visible and ultra-violet emission bands in the luminescence spectra. They are found to be mostly related to VZn and (VZn-nH) complexes located in the bulk and on the surfaces of ZnO nanowires. These findings represent an important step forward in the identification of the predominant native and extrinsic defects driving the electronic structure properties of ZnO nanowires grown by CBD. They further reveal the significance of hydrogen engineering to tune the source of crystal defects for optimizing the physical properties of ZnO nanowires.

The elucidation of the fundamental processes in aqueous solution during the chemical bath deposition of ZnO nanowires (NWs) using zinc nitrate and hexamethylenetetramine is of great significance: however, their extrinsic doping by foreign elements for monitoring their optical and electrical properties is still challenging. By combining thermodynamic simulations yielding theoretical solubility plots and speciation diagrams with in situ pH measurements and structural, chemical, and optical analyses, we report an in-depth understanding of the pH effects on the formation and aluminum doping mechanisms of ZnO NWs. By the addition of aluminum nitrate with a given relative concentration for the doping and of ammonia over a broad range of concentrations, the pH is shown to strongly influence the shape, diameter, length, and doping magnitude of ZnO NWs. Tuning the dimensions of ZnO NWs by inhibition of their radial growth only proceeds over a specific pH range, where negatively charged Al(OH) complexes are predominantly formed and act as capping agents by electrostatically interacting with the positively charged m-plane sidewalls. These complexes further favor the aluminum incorporation and doping of ZnO NWs, which only operate over the same pH range following thermal annealing above 200 °C. These findings reporting a full chemical synthesis diagram reveal the significance of carefully selecting and following the pH to control the morphology of ZnO NWs as well as to achieve their thermally activated extrinsic doping, as required for many nanoscale engineering devices.

No abstract

ZnO nanowires grown by chemical bath deposition (CBD) are of high interest, but their doping with extrinsic elements including gallium in aqueous solution is still challenging despite its primary importance for transparent electrodes and electronics, as well as mid-infrared plasmonics. We elucidate the formation mechanisms of ZnO nanowires by CBD using zinc nitrate and hexamethylenetetramine as standard chemical precursors, as well as gallium nitrate and ammonia as chemical additives. A complete growth diagram, revealing the effects of both the relative concentration of gallium nitrate and pH, is gained by combining a thorough experimental approach with thermodynamic computations yielding theoretical solubility plots as well as Zn(II) and Ga(III) species. The role of Ga(OH)4complexes is specifically shown as capping agents on the m-plane sidewalls of ZnO nanowires, enhancing their development and hence decreasing their aspect ratio. Additionally, the gallium incorporation into ZnO nanowires is investigated in details by chemical analyses and Raman scattering. They show the predominant formation of gallium substituting for zinc atoms (GaZn) in as-grown ZnO nanowires and their partial conversion into GaZn-VZn complexes after post-deposition annealing under oxygen atmosphere. The conversion is further related to a significant relaxation of the strain level in 2 ZnO nanowires. These findings reporting the physico-chemical processes at work during the formation of ZnO nanowires and the related gallium incorporation mechanisms offer a general strategy for their extrinsic doping and open the way for carefully controlling their physical properties as required for nanoscale engineering devices.

Mastering the properties of ZnO nanowires grown by the low temperature chemical bath deposition (CBD) is of crucial importance but is still challenging. We show that the shape, dimensions, and doping of ZnO nanowires can simultaneously be tuned by the addition of aluminum nitrate in the standard chemical system using zinc nitrate, hexamethylene­tetramine, and ammonia in aqueous solution. The formation and doping mechanisms of ZnO nanowires are thoroughly investigated by combining chemical, structural, and optical analyses with in situ pH measurements correlated with thermodynamic simulations. We reveal that the electrostatic interactions of Al­(OH)4 – complexes with the positive m-plane sidewalls of ZnO nanowires at a given pH favor their adsorption as capping agents, reducing the radial growth and promoting the elongation, while favoring the aluminum uniform incorporation. Importantly, the aluminum doping is found to be thermally activated above the low temperature of 200 °C under oxygen atmosphere, as indicated by the occurrence of six related additional modes in the range of 200–900 cm–1 in temperature-dependent Raman spectroscopy. These findings show that CBD using aluminum nitrate is of high potential for tuning both the morphology of ZnO nanowires and their physical properties via the aluminum doping, which paves the way for their more efficient use into sensing, electronic, and optoelectronic devices on both flexible and rigid substrates.

BiFeO3 (BFO) multiferroic oxide has a complex phase diagram that can be mapped by using appropriately substrate-induced strain in epitaxial films. By using Raman spectroscopy, we conclusively show that films of the so-called supertetragonal T-BFO phase, stabilized under compressive strain, display a reversible temperature-induced phase transition at about 100 °C, and thus close to room temperature.