Odile Stéphan

Aligned Carbon Nanotube Arrays Formed by Cutting a Polymer Resin—Nanotube Composite

A simple technique is described here that produces aligned arrays of carbon nanotubes. The alignment method is based on cutting thin slices (50 to 200 nanometers) of a nanotube-polymer composite. With this parallel and well-separated configuration of nanotubes it should be possible to measure individual tube properties and to demonstrate applications. The results demonstrate the nature of rheology, on nanometer scales, in composite media and flow-induced anisotropy produced by the cutting process. The fact that nanotubes do not break and are straightened after the cutting process also suggests that they have excellent mechanical properties.

Doping Graphitic and Carbon Nanotube Structures with Boron and Nitrogen

Composite sheets and nanotubes of different morphologies containing carbon, boron, and nitrogen were grown in the electric arc discharge between graphite cathodes and amorphous boron-filled graphite anodes in a nitrogen atmosphere. Concentration profiles derived from electron energy-loss line spectra show that boron and nitrogen are correlated in a one-to-one ratio; core energy-loss fine structures reveal small differences compared to pure hexagonal boron nitride. Boron and carbon are anticorrelated, suggesting the substitution of boron and nitrogen into the carbon network. Results indicate that singlephaase CyBxNx as well as separated domains (nanosize) of boron nitride in carbon networks may exist.

Zeptomol Detection Through Controlled Ultrasensitive Surface-Enhanced Raman Scattering

SERS permits identifying the nature of molecules in extremely low concentrations, but it is hindered by poor enhancement or low reproducibility. We demonstrate controllable approximately 10(10) signal amplification reaching the zeptomol detection limit for a nonresonant molecule by sandwiching the analyte between the tips of star-shaped gold nanoparticles and a planar gold surface using a simple synthetic procedure. This unprecedented control over light-intensity amplification opens a new avenue toward high-yield, fully reproducible, SERS-based, zeptomol detection and holds promise for nonlinear optics applications at the single-particle level.

Spin-crossover coordination nanoparticles.

Spin-crossover coordination nanoparticles of the cyanide-bridged three-dimensional network Fe(pyrazine){Pt(CN) 4} were prepared at three different sizes using a microemulsion. The 14 nm particles present a transition centered around 265 K with a hysteresis of 6 K.

Evolution of Fe species during the synthesis of over-exchanged Fe/ZSM5 obtained by chemical vapor deposition of FeCl3

The evolution of iron in over-exchanged Fe/ZSM5 prepared via chemical vapor deposition of FeCl3 was studied at each stage of the synthesis. Different characterization techniques (EXAFS, HR-XANES, 57 Fe Mossbauer spectroscopy, 27 Al NMR, EELS, HR-TEM, XRD, N2 physisorption, and FTIR spectroscopy) were applied in order to correlate the changes occurring in the local environment of the Fe atoms with migration and aggregation phenomena of iron at micro- and macroscopic scale. Mononuclear isolated Fe-species are formed upon FeCl3 sublimation, which are transformed into binuclear Fe-complexes during washing. During calcination, iron detached from the Bronsted sites migrates to the external surface of the zeolite, finally leading to significant agglomeration. Nevertheless, agglomeration of Fe can be strongly suppressed by adequately tuning the conditions of the calcination.  2002 Elsevier Science (USA). All rights reserved.

Ultraviolet Photodetector Based on GaN/AlN Quantum Disks in a Single Nanowire

We report the demonstration of single-nanowire photodetectors relying on carrier generation in GaN/AlN QDiscs. Two nanowire samples containing QDiscs of different thicknesses are analyzed and compared to a reference binary n-i-n GaN nanowire sample. The responsivity of a single wire QDisc detector is as high as 2 x 10(3) A/W at lambda = 300 nm at room temperature. We show that the insertion of an axial heterostructure drastically reduces the dark current with respect to the binary nanowires and enhances the photosensitivity factor (i.e., the ratio between the photocurrent and the dark current) up to 5 x 10(2) for an incoming light intensity of 5 mW/cm(2). Photocurrent spectroscopy allows identification of the spectral contribution related to carriers generated within large QDiscs, which lies below the GaN band gap due to the quantum confined Stark effect.

Core–Multishell Magnetic Coordination Nanoparticles: Toward Multifunctionality on the Nanoscale

Three-dimensional Prussian Blue analogues (PBAs) and related cyano-bridged coordination networks have been at the forefront of the field of molecular magnetism for more than a decade because of the extraordinary variety of their physical properties (electrochromism, ferromagnetism, photomagnetism, piezomagnetism, spin crossover), which opens up prospects for original functional materials. The large metal–metal distance ( 5 ) across the cyano bridge leads to relatively large porosity, which may play a role in hydrogen storage, ion selection, catalysis, and sensors. 14] One important issue is the effect of size reduction on the physical and chemical behavior of cyano-bridged coordination networks and their possible application as molecule-based components in devices. 16] A unique way to take advantage of the physical behavior of PBAs stemming from their rich electronic properties and porosity is to synthesize multishell nanoparticles such that a single particle consists of a core of a given network surrounded by shells of networks that may contain other functionalities. We report here the design of core–multishell nanocrystals thanks to the stabilization of surfactant-free particles in water. Epitaxial growth of different shells on various charged cores is demonstrated, and the thickness of the shells can be fine-tuned. The synergy between the different components is illustrated with one selected magnetic core–shell system. During the last few years, several groups have attempted to establish chemical routes that allow the stabilization of coordination (or metal–organic) nanoparticles of various face-centered-cubic PBAs of the general formula AxM [M’(CN)6](2+x)/3, where A is an alkali-metal cation and M II and M’ are transition-metal ions (see the Supporting Information). Generally, a chemical agent (organic or inorganic) is used during the synthetic process to control the growth of the particles, preclude their aggregation, and ensure their dispersion in different solvents. However, the presence of such protective agents weakens, in most cases, the surface reactivity of the particles and their electronic coupling with other objects, consequently decreasing their multifunctional potential. This can be avoided by the stabilization in solution of surfactant-free nanoparticles. We have recently shown that such electrostatic stabilization can be achieved in the case of the Cs[NiCr(CN)6] network leading to quasi-monodisperse particles with a size of 6.5 nm in diameter. The stabilization of surfactant-free nanoparticles makes it possible to perform coordination chemistry on the particles surface and opens the possibility of the epitaxial growth of one or several shells on the preexisting cores in solution. Thus, the key requirement for the preparation of pure core–shell nanoparticles is 1) stabilization in solution of well-defined crystalline surfactant-free charged nanoparticles and 2) prevention of the side nucleation of the shell by controlling the addition rate and the concentration of the components. Inorganic multishell particles have been prepared on oxides, sulfides, and metallic cores; some interesting examples of shape control have been reported by epitaxial growth seed-mediated procedures involving surfactants. 34] However, this is the first example of coremultishell particles based on coordination networks. The general procedure for the simple growth process on the charged cores present in solution is straightforward and thus feasible on a large scale: a dilute solution containing the divalent metal salt (M(H2O)6Cl2) and CsCl, and another containing the hexacyanometalate(III) salt are added dropwise (1 mL s ) to a stirred solution containing the core particles. The thickness of the growing shell is finely controlled by adjusting the amount of material added in solution (see the Supporting Information). As the growth process occurs, the solution is diluted in order to avoid aggregation that may occur because of the increase of the ionic force. To show the versatility and the efficiency of this approach, we report the preparation and the characterization of surfactant-free Cs[FeCr(CN)6] and Cs [CoCr(CN)6] nanoparticles as well as the design of core–(multi)shell particles of three different systems: 1) bicomponent particles made of a shell of CoII[CrIII(CN)6]2=3 on top of the Cs[FeCr(CN)6] core (denoted CsFeCr@CoCr), 2) tricomponent particles made of two different shells of Cs[FeCr(CN)6] and then Cs [NiCr(CN)6] grown on [*] Dr. L. Catala, Dr. D. Brinzei, Y. Prado, Prof. T. Mallah Institut de Chimie Mol culaire et des Mat riaux d’Orsay Universit Paris-Sud 11, 91405 Orsay (France) Fax: (+ 33)1-6915-4754 E-mail: laurecatala@icmo.u-psud.fr mallah@icmo.u-psud.fr

Comparative Structural and Chemical Studies of Ferritin Cores with Gradual Removal of their Iron Contents

Transmission Electron Microscopy (TEM), X-ray Absorption Near Edge Spectroscopy (XANES), Electron Energy-Loss Spectroscopy (EELS), Small-Angle X-ray Scattering (SAXS), and SQUID magnetic studies were performed in a batch of horse spleen ferritins from which iron had been gradually removed, yielding samples containing 2200, 1200, 500, and 200 iron atoms. Taken together, findings obtained demonstrate that the ferritin iron core consists of a polyphasic structure (ferrihydrite, magnetite, hematite) and that the proportion of phases is modified by iron removal. Thus, the relative amount of magnetite in ferritin containing 2200 to 200 iron atoms rose steadily from approximately 20% to approximately 70% whereas the percentage of ferrihydrite fell from approximately 60% to approximately 20%. These results indicate a ferrihydrite-magnetite core-shell structure. It was also found that the magnetite in the ferritin iron core is not a source of free toxic ferrous iron, as previously believed. Therefore, the presence of magnetite in the ferritin cores of patients with Alzheimers disease is not a cause of their increased brain iron(II) concentration.

Two-Dimensional Quasistatic Stationary Short Range Surface Plasmons in Flat Nanoprisms

We report on the nanometer scale spectral imaging of surface plasmons within individual silver triangular nanoprisms by electron energy loss spectroscopy and on related discrete dipole approximation simulations. A dependence of the energy and intensity of the three detected modes as function of the edge length is clearly identified both experimentally and with simulations. We show that for experimentally available prisms (edge lengths ca. 70 to 300 nm) the energies and intensities of the different modes show a monotonic dependence as function of the aspect ratio of the prisms. For shorter or longer prisms, deviations to this behavior are identified thanks to simulations. These modes have symmetric charge distribution and result from the strong coupling of the upper and lower triangular surfaces. They also form a standing wave in the in-plane direction and are identified as quasistatic short range surface plasmons of different orders as emphasized within a continuum dielectric model. This model explains in simple terms the measured and simulated energy and intensity changes as function of geometric parameters. By providing a unified vision of surface plasmons in platelets, such a model should be useful for engineering of the optical properties of metallic nanoplatelets.

A Top‐Down Synthesis Route to Ultrasmall Multifunctional Gd‐Based Silica Nanoparticles for Theranostic Applications

New, ultrasmall nanoparticles with sizes below 5 nm have been obtained. These small rigid platforms (SRP) are composed of a polysiloxane matrix with DOTAGA (1,4,7,10-tetraazacyclododecane-1-glutaric anhydride-4,7,10-triacetic acid)-Gd(3+) chelates on their surface. They have been synthesised by an original top-down process: 1) formation of a gadolinium oxide Gd2O3 core, 2) encapsulation in a polysiloxane shell grafted with DOTAGA ligands, 3) dissolution of the gadolinium oxide core due to chelation of Gd(3+) by DOTAGA ligands and 4) polysiloxane fragmentation. These nanoparticles have been fully characterised using photon correlation spectroscopy (PCS), transmission electron microscopy (TEM), a superconducting quantum interference device (SQUID) and electron paramagnetic resonance (EPR) to demonstrate the dissolution of the oxide core and by inductively coupled plasma mass spectrometry (ICP-MS), mass spectrometry, fluorescence spectroscopy, (29)Si solid-state NMR, (1)H NMR and diffusion ordered spectroscopy (DOSY) to determine the nanoparticle composition. Relaxivity measurements gave a longitudinal relaxivity r1 of 11.9 s(-1)  mM(-1) per Gd at 60 MHz. Finally, potentiometric titrations showed that Gd(3+) is strongly chelated to DOTAGA (complexation constant logβ110 =24.78) and cellular tests confirmed the that nanoconstructs had a very low toxicity. Moreover, SRPs are excreted from the body by renal clearance. Their efficiency as contrast agents for MRI has been proved and they are promising candidates as sensitising agents for image-guided radiotherapy.