Femtosecond Double-Pulse Laser Ablation and Deposition of Co-Doped ZnS Thin Films.

Nanostructured thin films of Co-doped zinc sulfide were synthesized through femtosecond pulsed laser deposition. The scheme involved ablation of physically mixed Co and ZnS with pairs of ultrashort pulses separated in time in the 0-300 ps range. In situ monitorization of the deposition process was carried out through a simultaneous reflectivity measurement. The crystallinity of generated nanoparticles and the inclusion of Co in the ZnS lattice is demonstrated by transmission electron microscopy and energy dispersive X-ray microanalysis (TEM-EDX) characterization. Surface morphology, Raman response, and photoluminescence of the films have also been assessed. The role of interpulse temporal separation is most visible in the thickness of the films obtained at the same total fluence, with much thicker films deposited with short delays than with individual uncoupled pulses. The proportion of Co in the synthesized doped ZnS nanoparticles is found to be substantially lower than the original proportion, and practically independent on interpulse delay.

Synthesis and Electron Microscopy Study of the Quaternary Misfit Layer Chalcogenides {(Bi,Nd)S}1+δCrS2 and {(Pb,Nd)Se}1+δ(NbSe2)2.

In the present work, two compounds, in the Bi-Nd-Cr-S and Pb-Nd-Nb-Se systems, not reported to date were synthesized. The chemical composition and the structural determination of these complex compounds, at atomic resolution, was performed through conventional and aberration-corrected electron microscopy including selected area electron diffraction, high resolution (HR) transmission electron microscopy (TEM), HR scanning TEM, and the analytical associated techniques X-ray energy-dispersive spectroscopy and electron energy-loss spectroscopy. The average compositions are [(Bi0.4,Nd0.6)S]1.25CrS2 and [(Pb0.5,Nd0.5)Se]1.15(Nb1.0Se2)2, respectively. By using these electron microscopy techniques, we confirmed that both compounds can be described in term of two interpenetrated sublattices that fit along a but do not fit along b, giving rise to an incommensurate modulation. A closer inspection along the stacking direction of the subcell has provided an ideal structural model for [(Bi0.4,Nd0.6)S]1.25CrS2 based on the intergrowth of one layer of CrS2, three atoms thick, (111) B1 type, and one layer of (Bi, Nd)Se, two atoms thick, (100) B1 type. In [(Pb0.5,Nd0.5)Se]1.15(Nb1.0Se2)2 we found that two layers of NbSe2, which adopt the 2H-NbSe2 polytype, alternate with one layer of (Pb, Nd)Se B1 type. In addition, crystals showing extended defects, associated with the weak interaction between the layers, were frequently found.

Organometallic-Derived Carbon (ODC)-Metal Nano-Oxide Composites as Improved Electrode Materials for Supercapacitors.

In the search for the new generation of electrochemical energy storage materials, a novel and straightforward synthetic route for porous carbons and metal oxide nanoparticle composites based on the chlorination of the organometallic compounds Ni(C5H5)2 and Mn(C5H7O2)2 at moderate temperatures, followed by hydrothermal treatment, has been developed. Electrochemical measurements in a three-electrode configuration show that, in both composites NiO@ODC and Mn3O4@ODC, a synergistic effect between the capacitive and pseudocapacitive energy storage mechanisms is observed, thereby improving their electrochemical performance vs pure carbon materials. Electrochemical evaluation of symmetric cells gave gravimetric capacitances of 124 and 130 F g-1 for NiO@ODC and Mn3O4@ODC, respectively. However, the porous structure of the carbon matrix and the higher conductivity of Mn3O4, together, were found to be responsible for the superior electrochemical performance of Mn3O4@ODC.

Crystal Structures at Atomic Resolution of the Perovskite-Related GdBaMnFeO5 and Its Oxidized GdBaMnFeO6.

Perovskite-related GdBaMnFeO5 and the corresponding oxidized phase GdBaMnFeO6, with long-range layered-type ordering of the Ba and Gd atoms have been synthesized. Oxidation retains the cation ordering but drives a modulation of the crystal structure associated with the incorporation of the oxygen atoms between the Gd layers. Oxidation of GdBaMnFeO5 increases the oxidation state of Mn from 2+ to 4+, while the oxidation state of Fe remains 3+. Determination of the crystal structure of both GdBaMnFeO5 and GdBaMnFeO6 is carried out at atomic resolution by means of a combination of advanced transmission electron microscopy techniques. Crystal structure refinements from synchrotron X-ray diffraction data support the structural models proposed from the TEM data. The oxidation states of the Mn and Fe atoms are evaluated by means of EELS and Mössbauer spectroscopy, which also reveals the different magnetic behavior of these oxides.

Microwave-Assisted Synthesis, Microstructure, and Magnetic Properties of Rare-Earth Cobaltites.

The series of perovskite rare-earth (RE) doped cobaltites (RE)CoO3 (RE = La-Dy) was prepared by microwave-assisted synthesis. The crystal structure undergoes a change of symmetry depending on the size of the RE cation. LaCoO3 is rhombohedral, S.G. R3̅c (No. 167), while, for the rest of the RE series (Pr-Dy), the symmetry is orthorhombic, S.G. Pnma (No. 62). The crystal structure obtained by X-ray diffraction was confirmed by high-resolution transmission electron microscopy, which yielded a good match between experimental and simulated images. It is further shown that the well-known magnetism in LaCoO3, which involves a thermally induced Co3+ (d6) low spin to intermediate or high spin state transition, is strongly modified by the RE cation, and a rich variety of magnetic order has been detected across the series.

Mechanosensitive Gold Colloidal Membranes Mediated by Supramolecular Interfacial Self-Assembly.

The ability to respond toward mechanical stimuli is a fundamental property of biological organisms at both the macroscopic and cellular levels, yet it has been considerably less observed in artificial supramolecular and colloidal homologues. An archetypal example in this regard is cellular mechanosensation, a process by which mechanical forces applied on the cell membrane are converted into biochemical or electrical signals through nanometer-scale changes in molecular conformations. In this article, we report an artificial gold nanoparticle (Au NP)-discrete π-conjugated molecule hybrid system that mimics the mechanical behavior of biological membranes and is able to self-assemble into colloidal gold nanoclusters or membranes in a controlled and reversible fashion by changing the concentration or the mechanical force (pressure) applied. This has been achieved by rational design of a small π-conjugated thiolated molecule that controls, to a great extent, the hierarchy levels involved in Au NP clustering by enabling reversible, cooperative non-covalent (π-π, solvophobic, and hydrogen bonding) interactions. In addition, the Au NP membranes have the ability to entrap and release aromatic guest molecules reversibly (Kb = 5.0 × 105 M-1) for several cycles when subjected to compression-expansion experiments, in close analogy to the behavior of cellular mechanosensitive channels. Not only does our hybrid system represent the first example of a reversible colloidal membrane, but it also can be controlled by a dynamic mechanical stimulus using a new supramolecular surface-pressure-controlled strategy. This approach holds great potential for the development of multiple colloidal assemblies within different research fields.

Double Double Cation Order in the High-Pressure Perovskites MnRMnSbO6.

Cation ordering in ABO3 perovskites adds to their chemical variety and can lead to properties such as ferrimagnetism and magnetoresistance in Sr2 FeMoO6 . Through high-pressure and high-temperature synthesis, a new type of "double double perovskite" structure has been discovered in the family MnRMnSbO6 (R=La, Pr, Nd, Sm). This tetragonal structure has a 1:1 order of cations on both A and B sites, with A-site Mn(2+) and R(3+) cations ordered in columns and Mn(2+) and Sb(5+) having rock salt order on the B sites. The MnRMnSbO6 double double perovskites are ferrimagnetic at low temperatures with additional spin-reorientation transitions. The ordering direction of ferrimagnetic Mn spins in MnNdMnSbO6 changes from parallel to [001] below TC =76 K to perpendicular below the reorientation transition at 42 K at which Nd moments also order. Smaller rare earths lead to conventional monoclinic double perovskites (MnR)MnSbO6 for Eu and Gd.

Ordering effects in the crystal structure and electrochemical properties of the Gd0.5Ba0.5Mn0.5Fe0.5O3-δ perovskite.

Layered-type ordering and oxygen vacancies ordering are revealed in GdBaMnFeO(6-δ) perovskite. Selected area electron diffraction and high-resolution transmission electron microscopy results indicate a modulation of the crystal structure. Ba and Gd ordering in (001)(p) layers is confirmed by high angle annular dark field scanning transmission electron microscopy and electron energy-loss spectroscopy. These techniques also revealed formation of layer-stacking defects in the crystals. Direct imaging of the oxygen sublattice is obtained by phase image reconstruction. Location of the oxygen vacancies in the (GdO)(x) layers is achieved by analysis of the intensity of the averaged phase image. Physical properties of the GdBaMnFeO(6-δ) perovskite, are likely to be strongly affected by its ordering effects and crystal microstructure. In this sense, layered-type GdBaMnFeO(6-δ) perovskite show better electrochemical properties as cathodes in SOFCs than ion disordered Gd(0.5)Ba(0.5)Mn(0.5)Fe(0.5)O(3-δ) perovskite.

Synthesis, structures and magnetic properties of the dimorphic Mn2CrSbO6 oxide.

The perovskite polymorph of Mn(2)CrSbO(6) compound has been synthesized at 8 GPa and 1473 K. It crystallizes in the monoclinic P21/n space group with cell parameters a = 5.2180 (2) Å, b = 5.3710(2) Å, c = 7.5874(1) Å and ß = 90.36(1)°. Magnetic susceptibility and magnetization measurements show the simultaneous antiferromagnetic ordering of Mn(2+) and Cr(3+) sublattices below TN = 55 K with a small canting. Low temperature powder neutron diffraction reveals a commensurate magnetic structure with spins confined to the ac-plane and a propagation vector κ = [1/2 0 1/2]. The thermal treatment of this compound induces an irreversible phase transition to the ilmenite polymorph, which has been isolated at 973 K and crystallizes in R3[combining macron] space group with cell parameters a = 5.2084 (4) Å and c = 14.4000 (11) Å. Magnetic susceptibility, magnetization and powder neutron diffraction data confirm the antiferromagnetic helical ordering of spins in an incommensurate magnetic structure with κ = [00 0.46] below 60 K, and the temperature dependence of the propagation vector up to κ = [00 0.54] at about 10 K.

Order, disorder and structural modulations in Bi-Fe-W-O-Br Sillén-Aurivillius intergrowths.

Transmission electron microscopy observations on a new complex oxybromide with nominal composition Bi(4)Fe(1/3)W(2/3)O(8)Br, heated at high temperature, reveal the transformation of its basic structure yielding two types of crystals. The first crystal type shows ordered and disordered extended defects leading to a new family of intergrowths between one Sillén block and n Aurivillius blocks and occasionally between one Aurivillius block and n Sillén blocks. The second type presents a compositionally modulated structure, determined by electron diffraction, with an average composition Bi(4)Fe(1/2)W(1/2)O(8 - delta)Br and unit-cell parameters a = (1/gamma) 3.8, b = 3.8, c = 14.5 A (gamma = 0.10-0.15) in the superspace group Immm[(1 - gamma)00] no. 71.1.

Electron microscopy characterization of nanostructured carbon obtained from chlorination of metallocenes and metal carbides.

In this work we report some new well-defined carbon nanostructures produced by direct chlorination of metallocenes (ferrocene and cobaltocene) and NbC, at temperatures from 100 to 900 degrees C. Thus, amorphous carbon nanotubes with variable dimensions depending on reaction temperature were produced from ferrocene. When cobaltocene is the carbon precursor the main product are solid amorphous nanospheres. The high refractory metal carbide NbC as carbon source favours the growth of nanospherical cabbage-like particles with a higher degree of graphene sheets order. Besides, NbC crystallites encapsulated in an amorphous carbon shell were also found at lower temperatures (T< or =700 degrees C).

Statistical estimation of atomic positions from exit wave reconstruction with a precision in the picometer range.

The local structure of Bi4W2/3Mn1/3O8Cl is determined using quantitative transmission electron microscopy. The electron exit wave, which is closely related to the projected crystal potential, is reconstructed and used as a starting point for statistical parameter estimation. This method allows us to refine all atomic positions on a local scale, including those of the light atoms, with a precision in the picometer range. Using this method one is no longer restricted to the information limit of the electron microscope. Our results are in good agreement with x-ray powder diffraction data demonstrating the reliability of the method. Moreover, it will be shown that local effects can be interpreted using this approach.