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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 163 records · Page 9
All sourcesNASA NTRSDOE OSTI
DOE OSTI2012

Tuning the Surface Structure and Optical Properties of CdSe Clusters Using Coordination Chemistry

A series of nonstoichiometric CdSe clusters with lowest energy electronic absorptions between 409 and 420 nm has been prepared from cadmium 1-naphthoate, 2-naphthoate, 4-thiomethyl-1-naphthaote, and 1-naphthalene thiolate complexes and diphenylphosphine selenide (DPPSe). Pair distribution function analysis of X-ray diffraction data, ligand exchange experiments, and NMR molecular weight analyses suggest the nanocrystal core changes minimally among these clusters despite significant changes to their absorption and luminescence spectra. Photoluminescence excitation spectra obtained at 77 K reveal an energy transfer process between the surface-trapped excited state and the naphthalene-containing ligands that leads to ligand phosphorescence. A Dexter energy transfer mechanism is proposed to explain the observation of ligand phosphorescence on excitation of the cluster. These compounds demonstrate that cluster absorption and trap luminescence can be controlled with surface coordination chemistry.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2011

Nano-engineered defect structures in Ce- and Ho-doped metal-organic chemical vapor deposited YBa{sub 2}Cu{sub 3}O{sub 3+{delta} }films : correlation of structure and chemistry with flux pinning performance.

This study reports on the fabrication of metal-organic chemical vapor deposited (MOCVD) YBa{sub 2}Cu{sub 3}O{sub 6+{delta}} (YBCO) films doped with varying amounts of Ce and Ho and the characterization of their electrical, microstructural, and chemical properties. The films are prepared by vapor phase deposition of a Y-Ba-Cu precursor mix containing controlled amounts of Ce and Ho onto buffered metal strip templates. The comprehensive characterization of these films by critical current measurement, transmission electron microscopy, x-ray diffraction, Raman microspectroscopy, and x-ray absorption spectroscopy provides detailed information about the structure/chemistry/performance relationships and how they vary with varying amounts of Ce and Ho in the YBCO films. The microstructure exhibited by both the Ce-doped and the Ho-doped films contains a high density of crystal basal-plane aligned, fluoritelike precipitates within the YBCO matrix. For optimally doped samples, the influence of these nanocrystalline phases on the flux pinning properties manifests itself as a significant improvement in the critical current density (J{sub c}) for magnetic field orientations that approach being parallel to the ab planes of the YBCO, while no appreciable change is observed in either self-field J{sub c} or applied-field J{sub c} performance in the vicinity of field orientations parallel to the YBCO c-axis. The Ce is almost exclusively concentrated in the fluoritelike nanoprecipitates, while the Ho incorporates into both the planar arrays of nanoprecipitates and the superconducting matrix, where it substitutes for Y in the YBCO lattice. The present findings for Ce and Ho doping are in interesting contrast with our prior findings for Zr-doped MOCVD films due to the fact that the Zr-doped films exhibit columnar precipitate arrays that produce a substantial improvement in J{sub c} for magnetic field orientations parallel to the YBCO c-axis, while no appreciable change is observed in either self-field or applied-field J{sub c} performance for field orientations parallel to the ab planes. These results suggest that improvement of the in-field J{sub c} of YBCO films over an extended range of field angles could be achieved by doping strategies that simultaneously produce both ab-plane aligned and c-axis aligned precipitate arrays.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗