Deposition of tellurium films by decomposition of electrochemically-generated H2Te: application to radiative cooling devices

The preparation of homogenous, large area thin layers of tellurium on thin polyethylene foils is described. The tellurium was formed by room temperature decomposition of electrochemically generated H₂Te. Pre-treatment of the polyethylene substrates with KMnO₄ to give a Mn-oxide layer was found to improve the Te adhesion and homogeneity. Optical characterization of the layers was performed using UV/VIS/NIR spectroscopy. Such coatings have favorable characteristics for use as solar radiation shields in radiative cooling devices. The simplicity of generation of the very unstable H₂Te was also exploited to demonstrate formation of size-quantized CdTe nanocrystals.     

Absolute calibration method for nanosecond-resolved, time-streaked, fiber optic light collection, spectroscopy systems

This paper describes a convenient and accurate method to calibrate fast (<1 ns resolution) streaked, fiber optic light collection, spectroscopy systems. Such systems are inherently difficult to calibrate due to the lack of sufficiently intense, calibrated light sources. Such a system is used to collect spectral data on plasmas generated in electron beam diodes fielded on the RITS-6 accelerator (8-12MV, 140-200kA) at Sandia National Laboratories. On RITS, plasma light is collected through a small diameter (200 μm) optical fiber and recorded on a fast streak camera at the output of a 1 meter Czerny-Turner monochromator. For this paper, a 300 W xenon short arc lamp (Oriel Model 6258) was used as the calibration source. Since the radiance of the xenon arc varies from cathode to anode, just the area around the tip of the cathode ("hotspot") was imaged onto the fiber, to produce the highest intensity output. To compensate for chromatic aberrations, the signal was optimized at each wavelength measured. Output power was measured using 10 nm bandpass interference filters and a calibrated photodetector. These measurements give power at discrete wavelengths across the spectrum, and when linearly interpolated, provide a calibration curve for the lamp. The shape of the spectrum is determined by the collective response of the optics, monochromator, and streak tube across the spectral region of interest. The ratio of the spectral curve to the measured bandpass filter curve at each wavelength produces a correction factor (Q) curve. This curve is then applied to the experimental data and the resultant spectra are given in absolute intensity units (photons∕sec∕cm(2)∕steradian∕nm). Error analysis shows this method to be accurate to within +∕- 20%, which represents a high level of accuracy for this type of measurement.     

Formation and optical properties of gold nanoparticles synthesized in the presence of double-hydrophilic block copolymers

In this article, we present a systematic, controlled synthesis of water-dispersed, polymer-stabilized Au nanoparticles with different sizes by either self-reduction or ultraviolet (UV) irradiation-enhanced reduction in the presence of double hydrophilic block copolymers with different functional patterns. The morphology of the particles was found to be spherical, triangular, truncated triangular, and hexagonal under various conditions. In addition, both the particle size and shape could be controlled by variation of the functional group pattern of the block copolymers. With a hexacyclen functional block, we achieved a perfect match to the Au (111) face, resulting in extremely thin free-standing triangular and hexagonal nanoplatelets that show interference patterns in the electron beam due to deformations in the sub-Angstrom range. UV-visible spectra show that the plasmon absorption band can be fine tuned, depending on the type of the polymer and the irradiation power. The influence of the functional polymer groups and the irradiation on the formation of gold nanoparticles was investigated in more detail.     

Thiocyanate Ion Promoted E ⇌ Z Isomerization of a-Bromo-β,β-Diactivated Electrophilic Olefins: An Apparent High Nucleofugality of NCS−

(E)-β-Chloro-α-phenylcinnamaldehyde and (E)- and (Z)-methyl α-tert-butoxycarbonyl-p-nitro-β-bromocinnamates in the presence of NCS⁻ ion in acetonitrile undergo an (E) ⇌ (Z) isomerization which is faster than the vinylic substitution. (E)- and (Z)-methyl α-cyano-β-mesyloxy- (and β-chloro-) p-nitrocinnamates undergo substitution under similar conditions without preceding isomerization. MO calculations gave the energy differences between the eclipsed and the perpendicular conformers of carbanions ⁻CH₂CH₂X and (NC)₂ČH₂X (X = Cl, NCS). These rotational barriers denote the hyperconjugation stabilization energy resulting from interaction of the C-X bond and the 2p(C⁻) orbital. Both HCA values are high, those for X = Cl being 15–20% higher than for X = NCS. The limited literature data on the nucleofugality of NCS⁻ suggests that it can have a nucleofugality which is not much lower than that of Cl⁻ or Br⁻. Consequently, NCS⁻-catalyzed isomerizations of electrophilic vinyl chlorides and bromides, which involve a 180° internal rotation followed by a faster expulsion of NCS⁻ than of Cl⁻ from the initially formed intermediate carbanion Y'Y⁻C-C(R)(Cl)SCN, are not unreasonable, especially if solvation, which was not included in the calculations, plays an important role in the nucleofuge expulsion process.     

The Lise Meitner-Minerva Center for Computational Quantum Chemistry: 18 Years of Israeli-German Collaboration

We tell the story of the Lise Meitner-Minerva Center, its establishment and activities, its members and their scientific activity, and its instrumental role in weaving intense relationships with the theoretical community in Germany, and in amalgamating the Israeli community of computational quantum chemistry into a national center that enjoys a high international reputation.     

Supervisors' role conflict and role ambiguity differential relations with performance ratings of subordinates and the moderating effect of screening ability.

Survey data obtained from two independent samples of supervisors (Ns?=?68 and 109) supported the prediction that supervisors' experience of role conflict would correlate positively with their reported tendency to deliberately inflate subordinates' performance ratings. Contrary to expectation, in neither sample did supervisors' screening ability emerge as a moderator of the relation between role conflict and rating inflation. In a third independent sample of supervisors (N?=?25), in which rating inflation was operationalized on the basis of actual performance ratings of subordinates, strong support was obtained for both the predicted positive association between role conflict and rating inflation as well as the predicted moderating effect of screening ability on this relation. By contrast, the authors did not anticipate that supervisors' experience of role ambiguity would be directly associated with, or interact with screening ability to predict, rating inflation. The data from all three samples were, in general, in line with these expectations. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Thin semiconductor films for radiative cooling applications

Passive cooling systems use simple low-cost techniques to provide summer comfort in warm climates and can also be used to keep food, liquids and other materials at temperatures below ambient. Radiative cooling devices require a convective shield that should reject solar radiation but be transparent to mid-IR thermal radiation. In this paper, chemical solution deposition of thin semiconductor films (PbS and PbSe) onto polyethylene foils for radiative cooling applications is described. Optical and structural characterizations of the films were performed using UV–Vis, FTIR spectroscopies, X-ray diffraction and electron microscopy. Additionally, pigmented shields, which exhibit good radiation scattering properties, were prepared by incorporation of ZnS or ZnO into polyethylene. We also studied a combination of pigmented polyethylene foils coated with thin films of PbS that show improved optical properties for cooling purpose.     

Modeling the conductance and DNA blockade of solid-state nanopores

We present measurements and theoretical modeling of the ionic conductance G of solid-state nanopores with 5-100 nm diameters, with and without DNA inserted into the pore. First, we show that it is essential to include access resistance to describe the conductance, in particular for larger pore diameters. We then present an exact solution for G of an hourglass-shaped pore, which agrees very well with our measurements without any adjustable parameters, and which is an improvement over the cylindrical approximation. Subsequently we discuss the conductance blockade ΔG due to the insertion of a DNA molecule into the pore, which we study experimentally as a function of pore diameter. We find that ΔG decreases with pore diameter, contrary to the predictions of earlier models that forecasted a constant ΔG. We compare three models for ΔG, all of which provide good agreement with our experimental data.