David Zitoun

Hybrid Organic–Inorganic Perovskites (HOIPs): Opportunities and Challenges

The conclusions reached by a diverse group of scientists who attended an intense 2-day workshop on hybrid organic-inorganic perovskites are presented, including their thoughts on the most burning fundamental and practical questions regarding this unique class of materials, and their suggestions on various approaches to resolve these issues.

Nickel nanocrystals: fast synthesis of cubes, pyramids and tetrapods

We report the use of organometallic precursors in microwave and microfluidic reactors to synthesize Ni nanoparticles of various shapes: spheres, cubes, trigonal pyramids and tetrapods. Each route has the ability to yield a preferential shape based on kinetic control of crystal growth. Tetrapods are obtained only in a microfluidic reactor. Trigonal pyramids crystallize in the metastable hexagonally close-packed (HCP) phase while spheres, cubes and tetrapods crystallize in the thermodynamically stable face-centred cubic (FCC) phase. The magnetic properties of nanocubes (in particular the magnetic coercive field) are found to be greatly enhanced compared to bulk and previous examples of nanostructured Ni materials.

One-pot fabrication and magnetic studies of Mn-doped TiO2 nanocrystals with an encapsulating carbon layer

Mn-doped TiO(2) nanocrystals encapsulated in a carbon layer (Ti(1-x)Mn(x)O(2)@C) were synthesized by the one-pot RAPET (reaction under autogenic pressure at elevated temperature) technique. Manganese was doped into the body-centered tetragonal TiO(2) anatase phase to give a Mn:Ti atomic ratio of 1%, 5% and 10%. The surface modification by carbon was achieved in order to make the cubic/tetragonal nanocrystals non-toxic and biocompatible. Electron paramagnetic resonance (EPR) studies revealed a broad resonance (centered at g = 1.9977 due to the interacting spins in the oxide matrix) with increased dopant concentration and the resonance due to carbon. Manganese is mainly present as +II or +III oxidation states. The magnetic behavior was found to be very dependent on the manganese concentration with a ferromagnetic behavior of the 1% doped sample due to the coupling between carriers and manganese spins. A predominant paramagnetic behavior was observed for the higher Mn-doped samples. This study opens up a new dimension for the carbon encapsulation of room-temperature ferromagnetic diluted magnetic semiconductor (DMS) nanomaterials.

Magnetic nanoparticles through organometallic synthesis: evolution of the magnetic properties from isolated nanoparticles to organised nanostructures.

Co and NiFe nanoparticles (2.7 to 3.3 nm mean diameter) of narrow size distribution have been obtained through the decomposition of organometallic precursors in organic solutions of long alkyl chain ligands, namely oleic acid and hexadecylamine. Materials of various volume fractions were produced. The particles have been structurally characterised by WAXS. Both adopt the bulk structure: HCP in the case of cobalt; a mixture of FCC and BCC for NiFe. Their aptitude to self-assemble either on flat supports or in bulk solid state has been investigated by means of TEM and SAXS. This study suggests the crystallisation of the nanoparticles upon solvent evaporation, especially a local FCC arrangement was observed for the NiFe material. Magnetic measurements (SQUID) confirm this tendency. The blocking temperature depends on the metal volume fraction, i.e. on the anisotropy generated by the dipolar couplings (Ki). We show that, for dense samples, the particles of high intrinsic anisotropy, Ku, (Co) still display an individual behaviour while the soft ones (NiFe) display a collective behaviour.

Tailoring the magnetic anisotropy in CoRh nanoalloys

CoRh alloy nanoparticles (NPs) show nontrivial correlations between chemical and magnetic order that lead to a remarkable nonmonotonous dependence of the magnetic anisotropy energy as a function of composition. Combining experiment and theory we demonstrate how the induced 4d moments and the 3d−4d interfaces control the magnetoanisotropic behavior. New possibilities of tailoring the magnetic characteristics of NPs are thus opened.

Operando electron magnetic measurements of Li-ion batteries

One of the challenges in the development of batteries consists of investigating new electrode materials and comprehending the mechanism of lithium uptake. Herein, we report on the first operando measurements of electron magnetism in a battery during cycling. We have succeeded in designing a non-magnetic cell and have investigated the lithiation mechanism of FeSb2, a high energy density anode material. The stepwise increase of the magnetic moment reveals an increase of amorphous Fe nanoparticle size, while Sb undergoes reversible alloying with Li.

Silver nanowires and nanoparticles from a millifluidic reactor: application to metal assisted silicon etching

Silver nanowires and nanoparticles are synthesized by a polyol method in a millifluidic reactor. We have been able to optimize the flow chemistry reaction conditions to get a high yield of nanowires in a continuous flow. By changing reaction parameters we have demonstrated the synthesis of single crystalline silver nanoparticles in a rapid reaction time of only 3 minutes. All results are compared with standard batch and microwave reactions. An example of application is provided through the silver nanowire assisted etching of silicon wafers. This colloidal approach of metal assisted silicon etching allows transferring of the nanowire shape to silicon.

Magnetic properties of CoNRhM nanoparticles: experiment and theory

The magnetism of Co–Rh nanoparticles is investigated experimentally and theoretically. The particles (≈2 nm) have been synthesized by decomposition of organometallic precursors in mild conditions of pressure and temperature, under hydrogen atmosphere and in the presence of a polymer matrix. The magnetic properties are determined by SQUID, Mossbauer spectroscopy, and X-ray magnetic circular dichroism (XMCD). The structural and chemical properties are characterized by wide angle X-ray scattering, transmission electronic microscopy and X-ray absorption near edge spectroscopy. All the studied Co–Rh clusters are magnetic with an average spin moment per atom µ that is larger than the one of macroscopic crystals or alloys with similar concentrations. The experimental results and comparison with theory suggest that the most likely chemical arrangement is a Rh core, with a Co-rich outer shell showing significant Co–Rh mixing at the interface. Measured and calculated magnetic anisotropy energies (MAEs) are found to be higher than in pure Co clusters. Moreover, one observes that the MAEs can be tuned to some extent by varying the Rh concentration. These trends are well accounted for by theory, which in addition reveals important spin and orbital moments induced at the Rh atoms as well as significant orbital moments at the Co atoms. These play a central role in the interpretation of experimental data as a function of Co–Rh content. A more detailed analysis from a local perspective shows that the orbital and spin moments at the Co–Rh interface are largely responsible for the enhancement of the magnetic moments and magnetic anisotropy.

Structural and magnetic study of bimetallic Co1−xRhx particles

Abstract We studied the structural and magnetic properties of cobalt-rhodium nanoparticles dispersed in a polymer matrix. It is shown that the structure depends on the composition, high cobalt content leading to a non-periodic structure. Magnetic measurements confirm the dispersion, particle size, and bimetallic character of the particles. Evidence is given of enhanced magnetisation for the composition x =0.5.

New topotactic synthetic route to mesoporous silicon carbide

Mesoporous silicon carbide (SiC) was synthesized by a one-pot thermal reduction of SiO2/C composites by metallic Mg at the remarkably low temperature of 800 °C. Two distinct mesostructured silica were used as hard templates for composite preparation: a hexagonal 3D close-packed assembly of Stober silica spheres and an ordered mesoporous SBA15 silica. In the latter case, SiC has crystallized in its 2H–SiC hexagonal phase, which is rather unique at such a low temperature. Composites were obtained by impregnation/polymerisation/carbonisation of a molecular carbon precursor within the porous structure of the silica template. After thermal treatment at a moderate temperature in the presence of Mg and subsequent by-products rinsing off, both prepared SiC showed distinct mesoporous structures related to the initial SiO2 architectures. By comparison of the mesoporous characteristics, resulting SiC was found to retain the carbon structures of the pristine composites. The description of the synthetic mechanism of this topotactic reaction contrasts with the usual assumption stating the templating role of silica.