Influence of the Ion Coordination Number on Cation Exchange Reactions with Copper Telluride Nanocrystals

Cu2-xTe nanocubes were used as starting seeds to access metal telluride nanocrystals by cation exchanges at room temperature. The coordination number of the entering cations was found to play an important role in dictating the reaction pathways. The exchanges with tetrahedrally coordinated cations (i.e. with coordination number 4), such as Cd2+ or Hg2+, yielded monocrystalline CdTe or HgTe nanocrystals with Cu2-xTe/CdTe or Cu2-xTe/HgTe Janus-like heterostructures as intermediates. The formation of Janus-like architectures was attributed to the high diffusion rate of the relatively small tetrahedrally coordinated cations, which could rapidly diffuse in the Cu2-xTe NCs and nucleate the CdTe (or HgTe) phase in a preferred region of the host structure. Also, with both Cd2+ and Hg2+ ions the exchange led to wurtzite CdTe and HgTe phases rather than the more stable zinc-blende ones, indicating that the anion framework of the starting Cu2- xTe particles could be more easily deformed to match the anion framework of the metastable wurtzite structures. As hexagonal HgTe had never been reported to date, this represents another case of metastable new phases that can only be accessed by cation exchange. On the other hand, the exchanges involving octahedrally coordinated ions (i.e. with coordination number 6), such as Pb2+ or Sn2+, yielded rock-salt polycrystalline PbTe or SnTe nanocrystals with Cu2-xTe@PbTe or Cu2-xTe@SnTe core@shell architectures at the early stages of the exchange process. In this case, the octahedrally coordinated ions are probably too large to diffuse easily through the Cu2-xTe structure: their limited diffusion rate restricts their initial reaction to the surface of the nanocrystals, where cation exchange is initiated unselectively, leading to core@shell architectures.Comment: 11 pages, 7 figures in J. Am. Chem. Soc, 13 May 201

Development of a semiempirical potential for simulations of thiol-gold interfaces. Application to thiol-protected gold nanoparticles

A new semiempirical potential, based on density functional calculations and a bond-order Morse-like potential, is developed to simulate the adsorption behavior of thiolate molecules on non-planar gold surfaces, including relaxing effects, in a more realistic way. The potential functions include as variables the metal-molecule separation, vibrational frequencies, bending and torsion angles between several pairs of atom types and the coordination number of both the metal (Au) and thiolate groups. The potential was parameterized based on a set of density functional calculations of molecular adsorption in several surface sites (i.e. hollow, bridge, top, on-top Au adatom and the novel staple motif) for different crystalline facets, i.e. Au(111) and (100). Langevin dynamics simulations have been performed to study the capping effects of alkanethiolates molecules on Au nanoparticles in the range 1-4 nm. The simulation results reveal an enhancement of the coverage degree whilst the nanoparticles diameter decreases. A high surface disorder due to the strong S-Au bond was found, in very good agreement with very recent experimental findings [M. M. Mariscal, J. A. Olmos-Asar, C. Gutierrez-Wing, A. Mayoral and M. J. Yacaman, Phys. Chem. Chem. Phys., 2010, 12, 11785]. © 2011 the Owner Societies.Fil: Olmos Asar, Jimena Anahí. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Instituto de Investigaciones en Físico-química de Córdoba. Universidad Nacional de Córdoba. Facultad de Ciencias Químicas. Instituto de Investigaciones en Físico-química de Córdoba; ArgentinaFil: Rapallo, Arnaldo. Istituto per lo Studio delle Macromolecole del CNR; ItaliaFil: Mariscal, Marcelo. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Instituto de Investigaciones en Físico-química de Córdoba. Universidad Nacional de Córdoba. Facultad de Ciencias Químicas. Instituto de Investigaciones en Físico-química de Córdoba; Argentin

A Crystalline 2,3-<i>exo</i>-Disyndiotactic Dicyclopentadiene Tetramer

Cycloolefin oligomers and polymers have attracted attention for functional materials, but they are rarely seen in the crystalline state. A new crystalline cycloolefin dicyclopentadiene tetramer, having a 2,3-<i>exo</i>-disyndiotactic structure, is presented. By combining global optimization techniques based on configurational sampling in generalized statistical ensembles, with very high quality synchrotron X-ray diffraction data, the molecular mass of the crystallizing oligomer was first singled out, and its structure was then determined with a high degree of accuracy. The original procedures and methods developed for this particular case can be considered a suitable reference to tackle the difficult characterization problems posed by the crystalline powders of organic materials in general

Crystallographic structure and morphology of bithiophene-fluorene polymer nanocrystals

Nanocrystals of the polymer poly(9,9-dioctylfluorenyl-co-bithiophene) (F8T2) with a molecular weight of 3.2 kg/mol are grown in a para-xylene solution. The typical morphology of the crystals is needle like with typical widths of 50 nm and lengths of about 200 nm. The crystal structure and morphology are stable up to a temperature of 353 K. The structure solution is obtained by x-ray powder diffraction (XRD) pattern with data modelling by a stochastic global optimization procedure which allows simultaneous indexing and molecular packing determination. Final Rietveld refinement was applied on the most promising crystal structure with a = 1.376 nm, b = 3.105 nm, c = 2.690 nm and ß = 109.5° within the space group C2/c choosing the polymer backbone parallel to the b-axis. The structural motifs of the molecular packing could be identified: aromatic units within a single polymer chain are slightly bent relative to the chain axis, octyl side chains are aligned along the polymer backbone and aromatic units of neighbouring molecules display a strong tendency to stack parallel to each other. XRD results of F8T2 with a molecular weight of 19 kg/mol reveal the same peak positions compared to the 3.2 kg/mol material, showing that both materials crystallise similarly and can be described by the same crystallographic unit cell. The smaller peak intensities together with the broader peak widths, however, show that the ability of crystal formation for the 19 kg/mol material is reduced

Transseptal Approach for Mitral Valve Replacement

Background Although the access to the mitral valve through transseptal approach is technically more demanding, it allows a better exposure of its leaflets and subvalvular apparatus, specially, in small atria, in reoperations or when it is combined with tricuspid valve treatment.ObjectiveTo evaluate technical difficulties and complications associated with the transseptal approach for mitral valve replacement.Material and MethodsBetween 2006 and 2009, 62 consecutive patients who underwent mitral valve replacement alone or associated with myocardial revascularization through a transseptal approach extended to the left atrial roof were included. Technical difficulties and morbimortality of the procedure were evaluated.ResultsIn all patients, the transseptal approach could be done with no technical difficulties with an appropriate exposure of the mitral valve. The rate of conduction disorders in the postoperative period was 9.7% and the need of a permanent pacemaker implantation was 4.8%. From patients who had previous atrial fibrillation (n = 18), 83.3% recovered high sinus or junctional rhythm during the postoperative period.ConclusionsThe transseptal approach extended to the left atrial roof constituted an access alternative to the mitral valve, with an exposure better than the traditional approach, at the expense of a more demanding technique. Surgical times, mortality and complications were similar to those which belong to the conventional technique, except for a probable greater incidence of junctional rhythm and AV block. Possibly, there is certain benefit in the recovery of sinus rhythm in patients with previous chronic atrial fibrillation.REV ARGENT CARDIOL 2010;78:400-404.    Introducción A pesar de que es técnicamente más demandante, el acceso a la válvula mitral por vía transeptal permite una exposición mayor de sus valvas y del aparato subvalvular, en especial en aurículas pequeñas, en reoperaciones o cuando se combina con el tratamiento de la válvula tricuspídea. Objetivos Evaluar las dificultades técnicas y las complicaciones asociadas con el abordaje transeptal para el reemplazo valvular mitral.Material y métodos Entre 2006 y 2009 se incluyeron 62 pacientes consecutivos a los que se les realizó reemplazo valvular mitral solo o asociado con revascularización miocárdica a través de un abordaje transeptal extendido al techo de la aurícula izquierda. Se evaluaron las dificultades técnicas y la morbimortalidad del procedimiento