Friday, August 11, 2006

Joel's Monthly Journal Update

Andy - Some CdSe papers
Fabrication of Stable Low-Density Silica Aerogels Containing Luminescent ZnS Capped CdSe Quantum Dots.
L. Sorensen, G.F. Strouse, and A.E. Stiegman.
Advan. Mat. (2006)18, 1965.
Luminescent CdSe quantum dots of 2.5 and 6.0 nm dimension have been incorporated into a low-density silica aerogels matrix. The aerogels are formed from the supercritical CO2 extraction of an alcogel containing quantum dots surface passivated with 3-aminopropyltriethoxysilane. The resulting aerogels (see figure and cover) are low scattering and show intense, stable luminescence.

Growth of CdSe Quantum Rods and Multipods Seeded by Noble-Metal Nanoparticles.
K.T. Yong, Y. Sahoo, M.T. Swihart and P.N. Prasad
Advan. Mat. (2006)18, 1978.
CdSe quantum rods are prepared at much milder conditions than previously reported, using noble-metal seed particles to initiate growth. The CdSe nanocrystals initially form as multipods that cleave to yield freestanding quantum rods (see figure) with high photoluminescence quantum yields. This study provides a new direction in developing facile syntheses of semiconductor NCs with nonspherical morphology, thereby making available new building blocks for nanotechnology.

Bryan - Au Nanoparticle Stuff
Turkevich Method for Gold Nanoparticle Synthesis Revisited
J. Kimling, M. Maier, B. Okenve, V. Kotaidis, H. Ballot, and A. Plech.
The growth of gold nanoparticles by reduction by citrate and ascorbic acid has been examined in detail to explore the parameter space of reaction conditions. It is found that gold particles can be produced in a wide range of sizes, from 9 to 120 nm, with defined size distribution, following the earlier work of Turkevich and Frens. The reaction is initiated thermally or in comparison by UV irradiation, which results in similar final products. The kinetics of the extinction spectra show the multiple steps of primary and secondary clustering leading to polycrystallites.

Structural, Magnetic, and Spectroscopic Studies of YAgSn, TmAgSn, and LuAgSn
C.P. Sebastian, H. Eckert, C. Fehse, J.P. Wright, J.P. Attfield, D. Johrendt, S. Rayaprol, R.D. Hoffmann, R. Pottgen.
JSolidStateChem (2006) 179, 2376.
The rare earth–silver–stannides YAgSn, TmAgSn, and LuAgSn were synthesized from the elements by arc-melting and subsequent annealing. The three stannides were investigated by X-ray powder and single-crystal diffraction: NdPtSb type, P63mc, Z=2, a=468.3(1), Click to view the MathML sourcepm, wR2=0.0343, 353 F2 values, 12 variables for YAgSn, and ZrNiAl type, P6¯2 m, a=726.4(2), Click to view the MathML source, wR2=0.0399, 659 F2 values, 15 variables for TmAgSn, and a=723.8(2), Click to view the MathML source, wR2=0.0674, 364 F2 values, 15 variables for LuAgSn. Besides conventional laboratory X-ray data with monochromatized Mo radiation, the structures were also refined on the basis of synchrotron data with Click to view the MathML source, in order to clarify the silver–tin ordering more precisely. YAgSn has puckered, two-dimensional [AgSn] networks with Ag–Sn distances of 278 pm, while the [AgSn] networks of TmAgSn and LuAgSn are three-dimensional with Ag–Sn distances of 279 and 284 pm for LuAgSn. Susceptibility measurements indicate Pauli paramagnetism for YAgSn and LuAgSn. TmAgSn is a Curie–Weiss paramagnet with an experimental magnetic moment of 7.2 ?B/Tm. No magnetic ordering is evident down to 2 K. The local environments of the tin sites in these compounds were characterized by 119Sn Mössbauer spectroscopy and solid-state NMR (in YAgSn and LuAgSn), confirming the tin site multiplicities proposed from the structure solutions and the absence of Sn/Ag site disordering. Mössbauer quadrupolar splittings were found in good agreement with calculated electric field gradients predicted quantum chemically by the WIEN2k code. Furthermore, an excellent correlation was found between experimental 119Sn nuclear magnetic shielding anisotropies (determined via MAS-NMR) and calculated electric field gradients. Electronic structure calculations predict metallic properties with strong Ag–Sn bonds and also significant Ag–Ag bonding in LuAgSn.

Na->Li (and Li->Na) CP/MAS
Site Discrimination in Mixed-Alkali Glasses Studied by Cross-Polarization NMR
S.P. Puls and H. Eckert.
Cation-cation interactions are thought to play a significant role in shaping the nonlinear compositional dependence of ionic conductivity, known as the mixed-alkali effect (MAE) in glassy solid electrolytes. For providing a structural rationale of this effect, the discrimination of various cation sites in mixed-alkali glasses is of interest. In the present study, cross-polarization (CP) experiments have been applied to glasses in the system [(Li2O)x(Na2O)1-x]0.3[B2O3]0.7 to discriminate between alkali ions by virtue of different heteronuclear 7Li-23Na dipole-dipole coupling strengths. Cross-polarization studies involving two types of quadrupolar nuclei (both 7Li and 23Na have a spin-quantum number I = 3/2) are complicated by spin state mixing under radio frequency irradiation and magic-angle spinning (MAS). Therefore careful validation and optimization protocols are reported for the model compound LiNaSO4 prior to conducting the measurements on the glassy samples. 23Na 7Li CP/MAS NMR spectra have been obtained on glasses containing the Na+ ions as the dilute species. They reveal that those lithium species interacting particularly strongly with sodium ions have the same average 7Li chemical shift as the entire lithium population; the symmetrical situation applies to the 23Na nuclei at the sodium rich end of the composition range. On the other hand, a clear site discrimination is afforded by temperature-dependent static 23Na 7Li CP experiments, indicating that the Li ions that are most strongly interacting with sodium ions are strongly immobilized. This finding provides the first direct experimental evidence for the proposed secondary mismatch concept invoked for explaining the strong MAE in the dilute foreign ion limit

Cory - 17O NMR and Repetitive DFS experiments
Sensitivity Enhancement and Heteronuclear Distance Measurements in Biological 17O Solid-State NMR
A. Binkmann and P.M. Kentgens
In this contribution we present a comprehensive approach to study hydrogen bonding in biological and biomimetic systems through 17O and 17O-1H solid-state NMR combined with density functional theory calculations of 17O and 1H NMR parameters. We explore the signal enhancement of 17O in L-tyrosine·HCl using repetitive double-frequency swept radio frequency pulses in solid-state NMR. The technique is compatible with high magnetic fields and fast magic-angle spinning of the sample. A maximum enhancement by a factor of 4.3 is obtained in the signal-to-noise ratio of the selectively excited 17O central transition in a powdered sample of 17O-L-tyrosine·HCl at an external field of 14.1 T and a spinning frequency of 25 kHz. As little as 128 transients lead to meaningful 17O spectra of the same sample at an external field of 18.8 T and a spinning frequency of 50 kHz. Furthermore we employed supercycled symmetry-based pulse sequences on the protons to achieve heteronuclear longitudinal two-spin-order (IzSz) recoupling to determine 17O-1H distances. These sequences recouple the heteronuclear dipolar 17O-1H couplings, where dipolar truncation is absent, while decoupling the homonuclear proton dipolar interactions. They can be applied at fast magic-angle-spinning frequencies up and beyond 50 kHz and are very robust with respect to 17O quadrupolar couplings and both 17O and 1H chemical shift anisotropies, which makes them suitable for the use at high external magnetic fields. The method is demonstrated by determining the 17O-1H distance in L-tyrosine·HCl at a spinning frequency of 50 kHz and an external field of 18.8 T.

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