Adrian Lita

Ligand-passivated Eu:Y2O3 nanocrystals as a phosphor for white light emitting diodes.

Eu(III)-doped Y(2)O(3) nanocrystals are prepared by microwave synthetic methods as spherical 6.4 ± 1.5 nm nanocrystals with a cubic crystal structure. The surface of the nanocrystal is passivated by acetylacetonate (acac) and HDA on the Y exposed facet of the nanocrystal. The presence of acac on the nanocrystal surface gives rise to a strong S(0) → S(1) (π → π*, acac) and acac → Ln(3+) ligand to metal charge transfer (LMCT) transitions at 270 and 370 nm, respectively, in the Eu:Y(2)O(3) nanocrystal. Excitation into the S(0) → S(1) (π → π*) or acac → Ln(3+) LMCT transition leads to the production of white light emission arising from efficient intramolecular energy transfer to the Y(2)O(3) oxygen vacancies and the Eu(III) Judd-Ofelt f-f transitions. The acac passivant is thermally stable below 400 °C, and its presence is evidenced by UV-vis absorption, FT-IR, and NMR measurements. The presence of the low-lying acac levels allows UV LED pumping of the solid phosphor, leading to high quantum efficiency (∼19%) when pumped at 370 nm, high-quality white light color rendering (CIE coordinates 0.33 and 0.35), a high scotopic-to-photopic ratio (S/P = 2.21), and thermal stability. In a LED lighting package luminosities of 100 lm W(-1) were obtained, which are competitive with current commercial lighting technology. The use of the passivant to funnel energy to the lanthanide emitter via a molecular antenna effect represents a new paradigm for designing phosphors for LED-pumped white light.

Stable efficient solid-state white-light-emitting phosphor with a high scotopic/photopic ratio fabricated from fused CdSe-silica nanocomposites.

1 ] Two approaches to the development of solid-state lighting have been successfully implemented in commercially available white-light light-emitting diodes (LEDs). One method is the use of a phosphor conversion diode, in which a photodiode, emitting UV light, excites a phosphor (or group of phosphors), which then emits, singularly or collectively, white light.

Synthesis and Characterization of Phase-Pure Manganese(II) and Manganese(III) Silicalite-2

Manganese silicalite-2 was synthesized at high pH using the molecular cluster Mn 12O 12(O 2CCH 3) 16 as a Mn source. The silicalite-2 (ZSM-11) materials were synthesized using 3,5-dimethyl- N, N-diethylpiperdinium hydroxide as a structure-directing agent to produce phase-pure ZSM-11 materials. No precipitation of manganese hydroxide was observed, and synthesis resulted in the incorporation of up to 2.5 mol % Mn into the silicalite-2 with direct substitution into the framework verified by the linear relationship between the unit cell volume and loading. The Mn is reduced to Mn (II) during hydrothermal synthesis and incorporated into the silicalite-2 framework during calcination at 500 degrees C. Further calcination at 750 degrees C does not affect the crystallinity but oxidizes essentially all of the Mn (II) to Mn (III) in the framework. The large difference in oxidation temperatures between the II and III oxidation states provides a means of producing relatively pure manganese(II) and manganese(III) silicalite-2 materials for applications such as catalysis.

Synthesis, characterization, and spectroscopic characteristics of chromium(6+) and -(4+) silicalite-2 (ZSM-11) materials.

The systematic incorporation of Cr ions into a phase-pure silicalite-2 lattice was accomplished through hydrothermal synthesis using 3,5-dimethylpiperidinium as a templating agent. The Cr ions, after calcination to remove the template, were in the 6+ oxidation state, with their incorporation into the lattice verified by the systematic expansion of the unit cell as a function of Cr loading. The structures of these materials as revealed by electronic spectroscopy and X-ray absorption near-edge spectroscopy (XANES) were consistent with the dioxo structure typically exhibited by Cr(6+) in an amorphous silica matrix. These materials were highly luminescent, with the emission spectra showing an unusually well-resolved vibronic structure characteristic of an emissive site with little inhomogeneous broadening. The site was reduced under flowing CO to Cr(4+), as characterized by XANES. The reduction of Cr from 6+ to 4+ resulted in unit-cell volumes that are systematically smaller than those observed with Cr(6+), even though the ionic radius of Cr(4+) is larger. This is attributed to the fact that the Cr(6+) site is not a simple metal ion but a significantly larger [CrO(2)](2+) unit, requiring a larger lattice expansion to accommodate it. Through analysis of the XANES preedge and assignment of the ligand-field spectrum of the Cr(4+) ions, it is possible to establish isomorphic substitution into the silicalite lattice.

Ca11E3C8 (E = Sn, Pb): New Complex Carbide Zintl Phases Grown from Ca/Li Flux

New carbide Zintl phases Ca(11)E(3)C(8) (E = Sn, Pb) were grown from reactions of carbon and heavy tetrelides in Ca/Li flux. They form with a new structure type in space group P2(1)/c (a = 13.1877(9)Å, b = 10.6915(7)Å, c = 14.2148(9)Å, β = 105.649(1)°, and R(1) = 0.019 for the Ca(11)Sn(3)C(8) analog). The structure features isolated E(4-) anions as well as acetylide (C(2)(2-)) and allenylide (C(3)(4-)) anions; the vibrational modes of the carbide anions are observed in the Raman spectrum. The charge-balanced nature of these phases is confirmed by DOS calculations which indicate that the tin analog has a small band gap (E(g) < 0.1 eV) and the lead analog has a pseudogap at the Fermi level. Reactions of these compounds with water produce acetylene and allene.

Metal Site-Mediated, Thermally Induced Structural Changes in Cr6+-Silicalite-2 (MEL) Molecular Sieves

Cr(6+) ions were incorporated into the lattice sites of phase-pure silicalite-2 made using 3,5-dimethylpiperidinium as a structure-directing agent. The materials exhibited a remarkably well-resolved vibronic emission consisting of a high frequency progression of 987 cm(-1), which was assigned to the fundamental symmetric stretching mode of the (Si-O-)(2)Cr(═O)(2) group dominated by the terminal Cr═O stretch. A low frequency progression at 214 cm(-1), which was assigned to a symmetric O-Cr-O bending mode, was built on each band of the 987 cm(-1) progression. Studies of the vibronic structure of the emission spectrum as a function of temperature and Cr ion concentration reveal an abrupt change in the Franck-Condon factor of the emission at 20 K for samples with very low Cr concentrations (0.03 mol %). The change in the Franck-Condon factor is attributed to a temperature-induced structural change in the coordination sphere of the metal ion. This structural change was found to be accompanied by a concomitant structural change in the lattice structure of the silicalite-2. This structural change, as studied by temperature-dependent X-ray diffraction, did not involve a crystallographic phase change but an abrupt decrease in the unit cell volume, caused specifically by a decrease in the c-axis. This structural change was not observed in pure silicalite-2, indicating that it is not intrinsic to the silicalite lattice. Moreover, no similar structural change was observed at higher Cr loading (1 mol %). This suggests that the presence of the Cr ions and the changes in the coordination geometry they undergo at low temperature induced the observed contraction in the silicalite-2 lattice, in effect acting as a thermal switch that decreases the unit cell volume.

Synthesis, characterization, and electrospinning of novel polyaniline–peptide polymers

Aniline-peptide (FLDQV, FLDQVC, Dansyl-FLDQV, Dansyl-FLDQVC, and FLDQV-AMC) mixtures underwent oxidative chemical and electrochemical polymerization in excess of aniline. The products of the chemical polymerization were low molecular weight polymers containing more than 70% peptide. Electrochemically polymerized species polyaniline-FLDQV (PANI-FLDQV) consisted mainly of polyaniline units containing about 10% peptide. The solubility of the latter in 1,1,1,3,3,3-hexafluoro-2-propanol (HFP) was similar to the camphorsulfonic acid (CSA) doped emeraldine base (PANI-CSA) solubility, however the weight composition of the electrospun fibers produced from the two polymers was significantly different. 2D 1H-13C HSQC analyses were employed to analyze the binding between the aniline and peptide moieties. Binding of peptide to polyaniline is reflected by the appearance of extra cross-peaks which display line broadening between the free polyaniline and the free pentapeptide. Peptides may be chemically bonded to the polymer molecules, but they may also act as doping agents to the nitrogen atoms via hydrogen bonding.