Alexandre Gloter

Ferroelectric Control of Spin Polarization

Spin into Control Spintronics—the use of the spin direction of subatomic particles to control on and off states, instead of electric charge—has the potential to create low-power electronics, because less energy is needed to flip spin states than to flip switches to create voltage barriers. Theoretical work hints that spin-polarized electrons from a ferromagnetic electrode can be controlled by a change in polarization created in a ferroelectric thin film. Garcia et al. (p. 1106, published online 14 January) fabricated an iron-barium titanate junction on a lanthanum strontium manganate substrate that acts as a spin detector. Local control of spin polarization was observed in the ferroelectric layer, which retained its polarization without any applied power. Ferroelectric tunnel junctions control the spin polarization of electrons emitted from iron electrodes. A current drawback of spintronics is the large power that is usually required for magnetic writing, in contrast with nanoelectronics, which relies on “zero-current,” gate-controlled operations. Efforts have been made to control the spin-relaxation rate, the Curie temperature, or the magnetic anisotropy with a gate voltage, but these effects are usually small and volatile. We used ferroelectric tunnel junctions with ferromagnetic electrodes to demonstrate local, large, and nonvolatile control of carrier spin polarization by electrically switching ferroelectric polarization. Our results represent a giant type of interfacial magnetoelectric coupling and suggest a low-power approach for spin-based information control.

Interface-induced room-temperature multiferroicity in BaTiO3

Multiferroic materials possess two or more ferroic orders but have not been exploited in devices owing to the scarcity of room-temperature examples. Those that are ferromagnetic and ferroelectric have potential applications in multi-state data storage if the ferroic orders switch independently, or in electric-field controlled spintronics if the magnetoelectric coupling is strong. Future applications could also exploit toroidal moments and optical effects that arise from the simultaneous breaking of time-reversal and space-inversion symmetries. Here, we use soft X-ray resonant magnetic scattering and piezoresponse force microscopy to reveal that, at the interface with Fe or Co, ultrathin films of the archetypal ferroelectric BaTiO₃ simultaneously possess a magnetization and a polarization that are both spontaneous and hysteretic at room temperature. Ab initio calculations of realistic interface structures provide insight into the origin of the induced moments and bring support to this new approach for creating room-temperature multiferroics.

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.

Improving energy resolution of EELS spectra: an alternative to the monochromator solution

In this paper, we propose a numerical method which can routinely improve the energy resolution down to 0.2-0.3eV of electron energy-loss spectra acquired in a transmission electron microscope. The method involves measurement of the point-spread function (PSF) corresponding to the spectrometer aberration and to the incident energy spread, and then an inversion of this PSF so as to restore the spectrum. The chosen algorithm is based on an iterative calculation of the maximum likelihood solution known to be very robust against small errors in the PSF used. Restorations have been performed on diamond and graphite C-K edges acquired with an initial energy resolution of around 1eV. After reconstruction, the sharp core exciton lines become clearly visible for both compounds and the final energy resolution is estimated to be about 200-300meV. In the case of graphite, restorations involving both energy resolution and angular resolution have been successfully conducted. Finally, restorations of Fe L(2,3) and O-K edges measured for various iron oxides will be shown.

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

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.

New host for carbon in the deep Earth

The global geochemical carbon cycle involves exchanges between the Earth’s interior and the surface. Carbon is recycled into the mantle via subduction mainly as carbonates and is released to the atmosphere via volcanism mostly as CO2. The stability of carbonates versus decarbonation and melting is therefore of great interest for understanding the global carbon cycle. For all these reasons, the thermodynamic properties and phase diagrams of these minerals are needed up to core mantle boundary conditions. However, the nature of C-bearing minerals at these conditions remains unclear. Here we show the existence of a new Mg-Fe carbon-bearing compound at depths greater than 1,800 km. Its structure, based on three-membered rings of corner-sharing (CO4)4- tetrahedra, is in close agreement with predictions by first principles quantum calculations [Oganov AR, et al. (2008) Novel high-pressure structures of MgCO3, CaCO3 and CO2 and their role in Earth’s lower mantle. Earth Planet Sci Lett 273:38–47]. This high-pressure polymorph of carbonates concentrates a large amount of Fe(III) as a result of intracrystalline reaction between Fe(II) and (CO3)2- groups schematically written as 4FeO + CO2 → 2Fe2O3 + C. This results in an assemblage of the new high-pressure phase, magnetite and nanodiamonds.

Ab initio study of bilateral doping within the MoS2-NbS2 system

We present a systematic study on the stability and the structural and electronic properties of mixed molybdenum-niobium disulphides. Using density-functional theory we investigate bilateral doping with up to 25% of MoS2 NbS2 by Nb Mo atoms focusing on the precise arrangement of dopants within the host lattices. We find that over the whole range of considered concentrations, Nb doping of MoS2 occurs through a substitutional mechanism. For Mo in NbS2 both interstitial and substitutional dopings can coexist depending upon the particular synthesis conditions. The analysis of the structural and electronic modifications of the perfect bulk systems due to the doping is presented. We show that substitutional Nb atoms introduce electron holes to the MoS2 leading to a semiconductor-metal transition. On the other hand, the Mo doping of NbS2 does not alter the metallic behavior of the initial system. The results of the present study are compared with available experimental data on mixed MoS2-NbS2 bulk and nanoparticles.

Shaping single walled nanotubes with an electron beam

We show that electron irradiation in a dedicated scanning transmission microscope can be used as a nano-electron-lithography technique allowing the controlled reshaping of single walled carbon and boron nitride nanotubes. The required irradiation conditions have been optimized on the basis of total knock-on cross sections calculated within density functional based methods. It is then possible to induce morphological modifications, such as a local change of the tube chirality, by sequentially removing several tens of atoms with a nanometrical spatial resolution. We show that electron beam heating effects are limited. Thus, electron beam induced vacancy migration and nucleation might be excluded. These irradiation techniques could open new opportunities for nanoengineering a large variety of nanostructured materials.

Cr(VI) detoxification by Desulfovibrio vulgaris strain Hildenborough: microbe–metal interactions studies

Toxic heavy metals constitute a worldwide environmental pollution problem. Bioremediation technologies represent efficient alternatives to the classic cleaning-up of contaminated soil and ground water. Most toxic heavy metals such as chromium are less soluble and toxic when reduced than when oxidized. Sulfate-reducing bacteria (SRB) are able to reduce heavy metals by a chemical reduction via the production of H2S and by a direct enzymatic process involving hydrogenases and c3 cytochromes. We have previously reported the effects of chromate [Cr(VI)] on SRB bioenergetic metabolism and the molecular mechanism of the metal reduction by polyhemic cytochromes. In the current work, we pinpoint the bacteria–metal interactions using Desulfovibrio vulgaris strain Hildenborough as a model. The bacteria were grown in the presence of high Cr(VI) concentration, where they accumulated precipitates of a reduced form of chromium, trivalent chromium [Cr(III)], on their cell surfaces. Moreover, the inner and outer membranes exhibited precipitates that shared the spectroscopic signature of trivalent chromium. This subcellular localization is consistent with enzymatic metal reduction by cytochromes and hydrogenases. Regarding environmental significance, our findings point out the Cr(VI) immobilization mechanisms of SRB; suggesting that SRB are highly important in metal biogeochemistry.