Enhanced structural and magnetic properties of fcc colloidal crystals of cobalt nanoparticles.

We report the elaboration of supercrystals made up of dodecanoic acid-coated 8.1 nm-Co nanocrystals with controlled supercrystallinity, morphology and magnetic properties. Supercrystal growth is controlled using a solvent-mediated ligand-ligand interaction strategy. Either face-centered cubic supercrystalline films or single colloidal crystals composed of cobalt nanocrystals are obtained. The change in supercrystal morphology is explained by Flory-type solvation theory using Hansen solubility colloidal parameters. The use of the same batch of Co nanocrystals for the fabrication of supercrystalline films and colloidal crystals enables accurate comparative structural and magnetic studies using (high-resolution) transmission electron microscopy, field emission gun scanning electron microscopy, grazing incidence small-angle X-ray scattering and vibrating sample magnetometry. The nearest neighbor distance between nanoparticles is interpreted using theoretical models proposed in the literature. We evidence the increase in both geometric anisotropy and magnetic dipolar interactions for colloidal crystals compared to supercrystalline films.

Comparison between x-ray diffraction and quantitative surface calorimetry based on infrared thermography to evaluate strain-induced crystallinity in natural rubber.

The crystallinity of stretched crystallizable rubbers is classically evaluated using x-ray diffraction (XRD). As crystallization is a strongly exothermal phenomenon, quantitative surface calorimetry from infrared thermography offers an interesting alternative to XRD for determining the crystallinity. In this paper, the two measurement techniques have been used for evaluating the strain-induced crystallinity of the same unfilled natural rubber. This study provides the first comparison between the two techniques. The results obtained highlight the very satisfactory agreement between the two measurements, which opens a simple way for evaluating the strain-induced crystallinity from temperature measurements.

Freezing-induced self-assembly of amphiphilic molecules.

The self-assembly of amphiphilic molecules usually takes place in a liquid phase, near room temperature. Here, using small angle X-ray scattering (SAXS) experiments performed in real time, we show that freezing of aqueous solutions of copolymer amphiphilic molecules can induce self-assembly below 0 °C.

Memristive and neuromorphic behavior in a Li(x)CoO2 nanobattery.

The phenomenon of resistive switching (RS), which was initially linked to non-volatile resistive memory applications, has recently also been associated with the concept of memristors, whose adjustable multilevel resistance characteristics open up unforeseen perspectives in cognitive computing. Herein, we demonstrate that the resistance states of Li(x)CoO2 thin film-based metal-insulator-metal (MIM) solid-state cells can be tuned by sequential programming voltage pulses, and that these resistance states are dramatically dependent on the pulses input rate, hence emulating biological synapse plasticity. In addition, we identify the underlying electrochemical processes of RS in our MIM cells, which also reveal a nanobattery-like behavior, leading to the generation of electrical signals that bring an unprecedented new dimension to the connection between memristors and neuromorphic systems. Therefore, these LixCoO2-based MIM devices allow for a combination of possibilities, offering new perspectives of usage in nanoelectronics and bio-inspired neuromorphic circuits.

Hierarchy in Au nanocrystal ordering in supracrystals: a potential approach to detect new physical properties.

Here we describe the morphologies of Au nanocrystals self-assembled in fcc 3D superlattices called supracrystals. The average size of the nanocrystals is either 5 or 7 nm with a very small size distribution (<7%). The coating agents used to stabilize the nanocrystals are dodecanethiol (C12H25-SH), tetradecanethiol (C14H29-SH), and hexadecanethiol (C16H33-SH). The influences of the evaporation time, the volume of the chamber used to evaporate the toluene solvent, and the substrate temperature are studied. For nanocrystals characterized by the same size and coating agent, the supracrystal morphologies markedly change on increasing the evaporation time from 8 to 9 to 25 h whereas a slight change takes place on increasing the chamber volume. The nanocrystals' ability to self-order in supracrystals decreases upon increasing the chain length of the coating agent from dodecanethiol (C12) to tetradecanethiol (C14) to hexadecanethiol (C16). Decreasing the evaporation rate (25 h) and/or increasing the substrate temperature (50 °C) improves the nanocrystal ordering in fcc supracrystals. A hierarchy in nanocrystal ordering has the following sequence disordered assemblies, supracrystal film sitting on a disordered nanocrystal film, supracrystal films grown layer-by-layer, and finally supracrystals grown in solution with various well-defined shapes.

Shape-controlled platinum nanocubes and their assembly into two-dimensional and three-dimensional superlattices.

Liquid-liquid phase transfer has been used to synthesize platinum nanocrystals with a cubic morphology. By finely tuning the parameters controlling the nucleation and growth processes, nanometric truncated cubes or perfect cubes may be obtained. To our knowledge, this is the first time such shapes are obtained with this procedure. The importance of both the length of the capping agent to control the growth process and the bromide anions as poison for the (111) facet is shown. The low degree of size polydispersity allows these nanocrystals to self-assemble with a long-range ordering in two-dimensional and three-dimensional supracrystals. According to the nanocrystal shape, simple cubic or face-centered cubic supracrystals are observed. It is remarkable to notice that well-faceted supracrystals with sizes on the order of 10 microm may be obtained.

Tuning of solid phase in supracrystals made of silver nanocrystals.

Decanethiol-passivated silver nanocrystals are shown, by small-angle X-ray diffraction, to organize into hexagonal close packed or face centered cubic (fcc) structures depending on the substrate temperature. When the nanocrystals are passivated by dodecanethiols, fcc and body centered cubic lattices as well as disordered arrangements are observed. The different crystalline phases correspond to thermodynamic equilibrium states. The passivant chain length is shown to control the interactions between the nanocrystals and consequently the superlattice structure.

[Synchrotron radiation techniques for structural characterisation of biological entities: an example with renal stone analysis]. / Les techniques de rayonnement synchrotron au service de la caractérisation d'objets biologiques: un exemple d'application, les calculs rénaux.

This paper describes the opportunities given by the synchrotron radiation techniques regarding the structural characterisation of biological entities. After a short recall on the characteristics of the synchrotron radiation, are described the experimental devices based on fluorescence X, wide angle X-ray scattering and X-ray absorption spectroscopy, which may applied for biological samples, especially in the field of stone analysis. Recent progresses in medical research using synchrotron radiation will be also discussed.

Chain orientation in natural rubber, Part II: 2H-NMR study.

Stress-induced crystallisation (SIC) and stress-induced melting (SIM) in natural rubbers (NR), unfilled and filled with carbon black (CB) have been studied by (2)H-NMR measurements. Various materials have been swollen with small amount (< 2%) of deuterated alkane chains. The orientation of the amorphous chains, then the local deformation of the amorphous chains during deformation cycles and during stress relaxation, permits to clarify the SIC and SIM processes during hardening and recovery. By mechanical, WAXS and NMR measurements one determines the same critical draw ratio for appearance lambda(A) and disappearance lambda(E) of the crystallites. It is demonstrated that the hysteresis observed by the different techniques (stress sigma, crystallinity chi, NMR splitting Deltanu) are due to the supercooling effect ( lambda(A) > lambda(E), at constant temperature). During hardening at constant strain rate it is found that the local draw ratio remains constant and equal to lambda(A), whereas the crystallinity increases linearly with the macroscopic draw ratio lambda. The hardening sigma approximately (lambda - lambda(A))(2) is then interpreted as a reinforcement effect due to the crystallites, which act as new crosslinks. This confirms the prediction of Flory. In filled rubber the same effects are observed, and the stress amplification factor is determined as a function of the CB content. It is found that the fillers act as nucleation centres for the NR crystallites. The reinforcement of such materials is due principally to this nucleation effect and to the presence of a super network formed by both the NR crystallites and the CB fillers.

Chain orientation in natural rubber, Part I: the inverse yielding effect.

Inhomogeneous deformations are observed in stretched natural rubber of different crosslink density; the conditions of observation, nucleation and propagation are given in the first part of the paper. In samples of low crosslink density these inhomogeneities recall necking observed in others materials and in glassy polymers when the materials are drawn above a critical draw ratio. The difference is that in natural rubbers, NR, they nucleate and propagate at constant stress during unloading. This phenomenon, called inverse yielding appears during recovery only if the samples have been drawn previously in the hardening domain. During necking propagation the stress is constant. The mechanical and crystallinity properties of samples with and without inverse yielding are studied as a function of draw ratio, crosslink density and temperature. In the second part of the paper this transition zone (neck) of thickness 2 mm is studied by WAXS at the synchrotron source. From the orientation of NR crystallites and from the orientation of the stearic acid (2%, present in this type of rubber) we conclude that the deformation in the neck follows the flow lines. From the local crystallinity of the NR crystallites one deduces the local draw ratio across this transition zone. We suggest that in all these rubbers, which present a plateau of the recovery stress strain curve, micronecking exists. This effect is discussed in the framework of the Flory theory.

Vibrational coherence of self-organized silver nanocrystals in f.c.c. supra-crystals.

Fabrication of devices from inorganic nanocrystals normally requires that they are self-organized into ordered structures. It has now been demonstrated that nanocrystals are able to self-organize in a 'supra'-crystal with a face-centred cubic (f.c.c.) structure. The physical properties of nanocrystals self-organized into compact arrays are quite different from those of both isolated nanocrystals and the bulk phase. The collective optical and magnetic properties of these nanocrystal assemblies are governed mainly by dipolar interactions. Here, we show that nanocrystals vibrate coherently when they are self-organized in f.c.c. supra-crystals. Hence, a phase relation exists between the vibrations of all of the nanocrystals in a supra-crystal. This vibrational coherence can be observed by a substantial change of the quadrupolar low-frequency Raman scattering peak. Although a change in electronic transport properties has previously been observed on self-organization of silver nanocrystals, vibrational coherence represents the first intrinsic property of f.c.c. supra-crystals.

Intrinsic vibrational coherence in face-centered cubic supra-crystals of silver nanocrystals: Raman scattering measurements.

The ordering of silver nanocrystals is tuned from amorphous aggregates to highly well-ordered, face-centered cubic supra-crystals, using various substrates and controlling their temperature to obtain this. Low-frequency Raman scattering, for the first time, demonstrates vibrational coherence in fcc supra-crystals of nanocrystals. This is shown by a narrowing of the peak corresponding to the quadrupolar modes of the nanocrystals. However, this is obtained when the supra-crystals are smaller than the excitation wavelength. When the supra-crystals are larger, the narrowing cannot be observed. Furthermore, for any size of the supra-crystals, a shift to low frequency of the Raman peak due to the Lorentz field effect is seen.

Design and post-functionalisation of ordered mesoporous zirconia thin films.

Bidimensional hexagonal or centred-rectangular mesoporous zirconia thin films have been reproducibly prepared by evaporation-induced self-assembly (EISA), which are stable up to 300 degrees C, with pore size around 35 A; the films can be post-functionalised with organic ligands presenting different functions, opening a land of opportunities for the design of new hybrid mesostructured materials, based on the synergy of a transition metal oxide network and organic groups.