187 research outputs found
Synthesis of Colloidal Mn2+:ZnO Quantum Dots and High-TC Ferromagnetic Nanocrystalline Thin Films
We report the synthesis of colloidal Mn2+-doped ZnO (Mn2+:ZnO) quantum dots
and the preparation of room-temperature ferromagnetic nanocrystalline thin
films. Mn2+:ZnO nanocrystals were prepared by a hydrolysis and condensation
reaction in DMSO under atmospheric conditions. Synthesis was monitored by
electronic absorption and electron paramagnetic resonance (EPR) spectroscopies.
Zn(OAc)2 was found to strongly inhibit oxidation of Mn2+ by O2, allowing the
synthesis of Mn2+:ZnO to be performed aerobically. Mn2+ ions were removed from
the surfaces of as-prepared nanocrystals using dodecylamine to yield
high-quality internally doped Mn2+:ZnO colloids of nearly spherical shape and
uniform diameter (6.1 +/- 0.7 nm). Simulations of the highly resolved X- and
Q-band nanocrystal EPR spectra, combined with quantitative analysis of magnetic
susceptibilities, confirmed that the manganese is substitutionally incorporated
into the ZnO nanocrystals as Mn2+ with very homogeneous speciation, differing
from bulk Mn2+:ZnO only in the magnitude of D-strain. Robust ferromagnetism was
observed in spin-coated thin films of the nanocrystals, with 300 K saturation
moments as large as 1.35 Bohr magneton/Mn2+ and TC > 350 K. A distinct
ferromagnetic resonance signal was observed in the EPR spectra of the
ferromagnetic films. The occurrence of ferromagnetism in Mn2+:ZnO and its
dependence on synthetic variables are discussed in the context of these and
previous theoretical and experimental results.Comment: To be published in the Journal of the American Chemical Society Web
on July 14, 2004 (http://dx.doi.org/10.1021/ja048427j
Structure and Stability of the Iodide Elpasolite, Cs2AgBiI6
Iodide elpasolites (or double perovskites, A2B'B"I6, B' = M+, B" = M3+) are
predicted to be promising alternatives to lead-based perovskite semiconductors
for photovoltaic and optoelectronic applications, but no iodide elpasolite has
ever been definitively prepared or structurally characterized. Iodide
elpasolites are widely predicted to be unstable due to favorable decomposition
to the competing A3B2I9 (B = M3+) phase. Here, we report the results of
synchrotron XRD and X-ray total scattering measurements on putative Cs2AgBiI6
nanocrystals made via anion exchange from parent Cs2AgBiBr6 nanocrystals.
Rietveld refinement of XRD and PDF data shows that these nanocrystals indeed
exhibit a tetragonal (I4-m) elpasolite structure, making them the first example
of a structurally characterized iodide elpasolite. A series of experiments
probing structural relaxation and the effects of surface ligation or grain size
all point to the critical role of surface free energy in stabilizing the iodide
elpasolite phase in these nanocrystals.Comment: 7 figures, 1 scheme, plus supporting information fil
Cluster values of analytic functions on a Banach space
We investigate uniform algebras of bounded analytic functions on the unit ball of a complex Banach space. We prove several cluster value theorems, relating cluster sets of a function to its range on the fibers of the spectrum of the algebra. These lead to weak versions of the corona theorem for the open unit balls of c0 and ℓ2, in which all but one of the functions comprising the corona data extend to be weak-star continuous on the closed unit ball.Fil: Aron, Richard Martin. Kent State University; Estados UnidosFil: Carando, Daniel Germán. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Investigaciones Matemáticas "Luis A. Santaló". Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Investigaciones Matemáticas "Luis A. Santaló"; ArgentinaFil: Gamelin, T. W.. University of California at Los Angeles; Estados UnidosFil: Lassalle, Silvia Beatriz. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Investigaciones Matemáticas "Luis A. Santaló". Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Investigaciones Matemáticas "Luis A. Santaló"; ArgentinaFil: Maestre, Manuel. Universidad de Valencia; Españ
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