Protective Effect of Polyoxometalates in {Mo132}/Maghemite Binary Superlattices Under Annealing.

The binary assembly DDA-{Mo132}/OA-γ-Fe2O3 (DDA = didodecyldimethylammonium, {Mo132} = [Mo132O372(CH3COO)30(H2O)72]42-, OA = oleic acid) constitutes one of the two examples in the literature of binary superlattices made of a mixing of nanocrystals and oxo-clusters. In a precedent work, we reported in details the preparation of such magnetic binary systems and studied the effect of the nature of the polyoxometalates (POMs) on the magnetic properties. In the present paper, we study the stability of this model binary assembly under heating at various temperatures. Indeed, especially if magnetic and/or transport properties are targeted, an annealing can be essential to change the phase of the nanocrystals in a more magnetic one and/or to desorb the organic capping of the nano-objects that can constitute an obstacle to the electronic communication between the nano-objects. We gave evidence that the maghemite organization in the binary assembly is maintained until 370°C under vacuum thanks to the presence of the POMs. This latter evolve in the phase MoO3, but still permits to avoid the aggregation of the nanocrystals as well as preserve their periodical arrangement. On the contrary, an assembly made of pure γ-Fe2O3 nanocrystals displays a clear aggregation of the nano-objects from 370°C, as attested by transmission and scanning electronic microscopies and confirmed by magnetic measurements. The stability of the magnetic nanocrystals in such POMs/nanocrystals assemblies opens the way to (i) the elaboration of new binary assemblies from POMs and numerous kinds of nanocrystals with a good control on the magnetic properties and to (ii) the investigation of new physical properties as exchange coupling, or magneto-transport in such systems.

Detection of Biological Bricks in Space. The Case of Adenine in Silica Aerogel.

Space missions using probes to return dust samples are becoming more frequent. Dust collectors made of silica aerogel blocks are used to trap and bring back extraterrestrial particles for analysis. In this work, we show that it is possible to detect traces of adenine using surface-enhanced Raman spectroscopy (SERS). The method was first optimized using adenine deposition on glass slides and in glass wells. After this preliminary step, adenine solution was injected into the silica aerogel. Finally, gaseous adenine was successfully trapped in the aerogel. The presence of traces of adenine was monitored by SERS through its characteristic bands at 732, 1323, and 1458 cm-1 after the addition of the silver Creighton colloid. Such a method can be extended in the frame of Tanpopo missions for studying the interplanetary transfer of prebiotic organic compounds of biological interest.

Directed Organization of Platinum Nanocrystals through Organic Supramolecular Nanoporous Templates.

We propose a novel approach to trap 2 nm Pt nanocrystals using nanoporous two-dimensional supramolecular networks for cavity-confined host-guest recognition process. This will be achieved by taking advantage of two features of supramolecular self-assembly at surfaces: First, its capability to allow the formation of complex 2D architectures, more particularly, nanoporous networks, through noncovalent interactions between organic molecular building-blocks; second, the ability of the nanopores to selectively host and immobilize a large variety of guest species. In this paper, for the first time, we will use isotropic honeycomb networks and anisotropic linear porous supramolecular networks to host 2 nm Pt nanocrystals.

Binary Superlattices from {Mo132} Polyoxometalates and Maghemite Nanocrystals: Long-Range Ordering and Fine-Tuning of Dipole Interactions.

In the present article, the successful coassembly of spherical 6.2 nm maghemite (γ-Fe2O3) nanocrystals and giant polyoxometalates (POMs) such as 2.9 nm {Mo132} is demonstrated. To do so, colloidal solutions of oleic acid-capped γ-Fe2O3 and long-chain alkylammonium-encapsulated {Mo132 } dispersed in chloroform are mixed together and supported self-organized binary superlattices are obtained upon the solvent evaporation on immersed substrates. Both electronic microscopy and small angles X-ray scattering data reveal an AB-type structure and an enhanced structuration of the magnetic nanocrystals (MNCs) assembly with POMs in octahedral interstices. Therefore, {Mo132} acts as an efficient binder constituent for improving the nanocrystals ordering in 3D films. Interestingly, in the case of didodecyldimethylammonium (C12)-encapsulated POMs, the long-range ordered binary assemblies are obtained while preserving the nanocrystals magnetic properties due to weak POMs-MNCs interactions. On the other hand, POMs of larger effective diameter can be employed as spacer blocks for MNCs as shown by using {Mo132} capped with dioctadecyldimethylammonium (C18) displaying longer chains. In that case, it is shown that POMs can also be used for fine-tuning the dipolar interactions in γ-Fe2O3 nanocrystal assemblies.

Platinum and platinum based nanoalloys synthesized by wet chemistry.

Platinum nanocrystals and their derivatives with palladium and cobalt are of fundamental interest due to their wide field of application in chemistry and physics. Their properties are strongly dependent on their shape and composition. However the chemical route is far from allowing control of both shape and composition. In this paper, we show both experimentally and theoretically the important role of the interaction of small adsorbed molecules on the shape but also on the composition. This has been studied by comparing the case of pure palladium and platinum nanocrystals and the case of PtPd and PtCo nanoalloys synthesized by the liquid-liquid phase transfer method.

Role of the nanocrystallinity on the chemical ordering of Co(x)Pt(100-x) nanocrystals synthesized by wet chemistry.

Co(x)Pt(100-x) nanoalloys have been synthesized by two different chemical processes either at high or at low temperature. Their physical properties and the order/disorder phase transition induced by annealing have been investigated depending on the route of synthesis. It is demonstrated that the chemical synthesis at high temperature allows stabilization of the fcc structure of the native nanoalloys while the soft chemical approach yields mainly poly or non crystalline structure. As a result the approach of the order/disorder phase transition is strongly modified as observed by high-resolution transmission electron microscopy (HR-TEM) studies performed during in situ annealing of the different nanoalloys. The control of the nanocrystallinity leads to significant decrease in the chemical ordering temperature as the ordered structure is observed at temperatures as low as 420 °C. This in turn preserves the individual nanocrystals and prevents their coalescence usually observed during the annealing necessary for the transition to an ordered phase.

Synthesis of tin nanocrystals in room temperature ionic liquids.

The aim of this work was to investigate the synthesis of tin nanoparticles (NPs) or tin/carbon composites, in room temperature ionic liquids (RTILs), that could be used as structured anode materials for Li-ion batteries. An innovative route for the synthesis of Sn nanoparticles in such media is successfully developed. Compositions, structures, sizes and morphologies of NPs were characterized by high-energy X-ray diffraction (HEXRD), X-ray photoelectron spectroscopy (XPS) and high-resolution transmission electron microscopy (HRTEM). Our findings indicated that (i) metallic tetragonal ß-Sn was obtained and (ii) the particle size could be tailored by tuning the nature of the RTILs, leading to nano-sized spherical particles with a diameter ranging from 3 to 10 nm depending on synthesis conditions. In order to investigate carbon composite materials for Li-ion batteries, Sn nanoparticles were successfully deposited on the surface of multi-wall carbon nanotubes (MWCNT). Moreover, electrochemical properties have been studied in relation to a structural study of the nanocomposites. The poor electrochemical performances as a negative electrode in Li-ion batteries is due to a significant amount of RTIL trapped within the pores of the nanotubes as revealed by XPS investigations. This dramatically affected the gravimetric capacity of the composites and limited the diffusion of lithium. The findings of this work however offer valuable insights into the exciting possibilities for synthesis of novel nano-sized particles and/or alloys (e.g. Sn-Cu, Sn-Co, Sn-Ni, etc.) and the importance of carbon morphology in metal pulverization during the alloying/dealloying process as well as prevention of ionic liquid trapping.

Crack patterns in superlattices made of maghemite nanocrystals.

Here, 11 nm γ-Fe2O3 nanocrystals characterized by a low size distribution of 5% and dispersed in chloroform are deposited onto a substrate as well-defined films. The film thickness is controlled by the initial concentration of the colloidal solution. After drying, the film shows a network of well-defined cracks. It is demonstrated using scanning electron microscopy and small-angle X-ray diffraction that the nanocrystals within the film are self-assembled in ordered lattices. It is shown that the nanocrystals self-assemble in superlattices before the cracks appear. In contrast to what was previously observed with a disordered film of maghemite nanocrystals with large size distribution, the top surface of the films is covered with patterns formed before crack formation. The cracks preferentially follow the direction of the hexagonal array observed on the top surface of the film by using a field emission gun scanning electron microscope. The average crack distance as a function of the film height follows the same linear scaling law as in amorphous nanocrystal assemblies. The size of superlattices of nanocrystals evolves with film height, but is usually significantly smaller than the average crack distance except on the border of the films.

Do directional primary and secondary crack patterns in thin films of maghemite nanocrystals follow a universal scaling law?

Cracks due to a shrinking film restricted by adhesion to a surface are observed in nature at various length scales ranging from tiny crack segments in nanoparticle films to enormous domains observed in the earth's crust. Here, we study the formation of cracks in magnetic films made of maghemite (gamma-Fe2O3) nanocrystals. The cracks are oriented by an external magnetic field applied during the drying process which presents a new method to produce directional crack patterns. It is shown that directional and isotropic crack patterns follow the same universal scaling law with the film height varying from micrometer to centimeter scales. Former experimental studies of scaling laws were limited to small variations in height (1 order of magnitude). The large variation in height in our experiments becomes possible due to the combined use of nanocrystals and electron microscopy. A simple two-dimensional computer model for elastic fracture leads to structural and scaling behaviors, which match those observed in the experiments.

Cracks in magnetic nanocrystal films: do directional and isotropic crack patterns follow the same scaling law?

In this letter, we show that the use of nanocrystals enables new insights into the scaling law of crack patterns. Directional and isotropic crack patterns made of gamma-Fe2O3 nanocrystals follow the same scaling law, with the film height varying by 3 orders of magnitude. A simple two-dimensional computer model for elastic fracture also leads to the same scaling behavior for directional and isotropic cracks, in good agreement with the experiments.

Mesoscopic structures of maghemite nanocrystals: fabrication, magnetic properties, and uses.

Mesoscopic structures made of maghemite (gamma-Fe2O3) nanocrystals with different coatings and shapes are described. They are controlled by a combination of van der Waals and magnetic dipolar interactions and their shapes are responsible for the collective magnetic response of the nanocrystals. It is shown that spherical gamma-Fe2O3 nanocrystals can be used as a mask to reproduce the mesoscopic structure on a silicon wafer.