Effect of plasma hydrogenation on the defect properties of CuInSe2 single crystals

The defect properties of p-type CuInSe₂ single crystals hydrogenated by diffusion from a plasma source are investigated using the Rutherford backscattering/channeling technique and photoacoustic spectroscopy. Plasma hydrogenation is found to produce severe lattice damage, to affect the intrinsic defect equilibrium in the near surface region of the crystals and to create an additional donor. A defect formation model including both defect passivation by hydrogen and damage induced defect creation is proposed to explain the experimental data. The thermal stability of the property modifications is limited to temperatures below about 100 °C.     

Theories in polymer science - helpful or inhibiting?

Theories in polymer science play an important role in how experiments are designed and results interpreted. Three areas of polymer chemistry are focussed on: step growth polymerization, particularly predictions of equal reactivity of functional groups; free radical chain polymerization with emphasis on structural irregularities and the consequences of these structures for previous theories; free radical copolymerization and the many models at present under discussion aimed at providing a theoretical basis for predicting the outcome of such copolymerization. The question of whether or not we as polymer chemists have been helped or hindered by theory is addressed.     

Improvements to air mass calculations for ground-based infrared measurements

High-resolution ground-based infrared solar spectra are routinely recorded at the Network for the Detection of Stratospheric Change (NDSC) stations. These data sets play a key role in providing a long-term record of atmospheric composition and their links to climate change. The analysis of observed infrared spectra involves comparison to a computer-modeled atmosphere where knowledge of the air mass distribution is an essential component. This note summarises improvements made to an existing and widely used computer code (FSCATM) to perform refractive ray-tracing and calculation of the air mass distribution. Changes were made towards higher vertical resolution in the troposphere and increased numerical precision. The revised FSCATM improves the analysis of infrared spectra mostly through the more accurate representation of the temperature profile. Air mass differences with respect to earlier versions are documented and are typically <0.7%, exceptions being extreme cases of inversion layers. The current version provides ray tracing and air mass calculations for any terrestrial observation site. The output files are reported in a format compatible with the SFIT and SFIT2 retrieval algorithms, which are widely used for NDSC infrared atmospheric studies. The improved computer code, documentation, reference profiles, and test cases are available electronically.     

Triangular to square flux lattice phase transition in YBa2Cu3O7

We have used the technique of small-angle neutron scattering to observe magnetic flux lines directly in a YBa2Cu3O7 single crystal at fields higher than previously reported. For field directions close to perpendicular to the CuO2 planes, we find that the flux lattice structure changes smoothly from a distorted triangular coordination to nearly perfectly square as the magnetic induction approaches 11 T. The orientation of the square flux lattice is as expected from recent d-wave theories but is 45 degrees from that recently observed in La(1.83)Sr(0.17)CuO(4+delta).     

Dielectron widths of the Gamma(1S,2S,3S) resonances

We determine the dielectron widths of the Gamma(1S), Gamma(2S), and Gamma(3S) resonances with better than 2% precision by integrating the cross section of e+e- -->Gamma over the e+e- center-of-mass energy. Using e+e- energy scans of the Gamma resonances at the Cornell Electron Storage Ring and measuring Gamma production with the CLEO detector, we find dielectron widths of 1.252+/-0.004(sigma(stat))+/-0.019(sigma(syst)) keV, 0.581+/-0.004+/-0.009 keV, and 0.413+/-0.004+/-0.006 keV for the Gamma(1S), Gamma(2S), and Gamma(3S), respectively.     

Base‐dependent selectivity of an SNAr reaction

A detailed kinetic analysis and computational study of an S_NAr reaction between 2,5,6‐trifluoronicotinonitrile, 2 , and the ambident 3‐isopropoxy‐1H‐pyrazol‐5‐amine, 3 , is presented. The selectivity with respect to the reaction at the primary amino group of 2 , to give the desired product, 2,5‐difluoro‐6‐[(3‐isopropoxy‐1H‐pyrazol‐5‐yl)amino]nicotinonitrile, 1 , is strongly dependent upon reaction conditions. Reaction is found to proceed via both uncatalysed and base catalysed routes, and selectivity towards 1 is strongly enhanced in the presence of the base diazabicyclo[2,2,2]octane (DABCO). Computational studies in tetrahydrofuran solution at the B3LYP/6‐31G* level of theory have provided valuable insight into alternative kinetically indistinguishable reaction pathways. The results suggest that for reaction at the primary amino group, proton removal by DABCO accompanying amine addition allows avoidance of a high‐energy, zwitterionic Meisenheimer intermediate. Reactions at the alternative pyrazole nitrogen atoms are less sensitive to the presence of base because of stabilisation of the Meisenheimer zwitterions by intramolecular hydrogen bonding. Copyright © 2010 John Wiley & Sons, Ltd.     

Energy-dependent tunneling from few-electron dynamic quantum dots

We measure the electron escape rate from surface-acoustic-wave dynamic quantum dots (QDs) through a tunnel barrier. Rate equations are used to extract the tunneling rates, which change by an order of magnitude with tunnel-barrier-gate voltage. We find that the tunneling rates depend on the number of electrons in each dynamic QD because of Coulomb energy. By comparing this dependence to a saddle-point-potential model, the addition energies of the second and third electron in each dynamic QD are estimated. The scale ( approximately a few meV) is comparable to those in static QDs as expected.     

Evidence for the decay D0-->K(-)pi(+)pi(-)e(+)nu(e)

Using a 281 pb{-1} data sample collected at the psi(3770) with the CLEO-c detector, we present the first absolute branching fraction measurement of the decay D0-->K(-)pi(+)pi(-)e(+)nu(e) at a statistical significance of about 4.0 standard deviations. We find 10 candidates consistent with the decay D0-->K(-)pi(+)pi(-)e(+)nu(e). The probability that a background fluctuation accounts for this signal is less than 4.1 x 10{-5}. We find B(D0-->K(-)pi(+)pi(-)e(+)nu(e)) = [2.8{-1.1}{+1.4}(stat)+/-0.3(syst)]x10{-4}. By restricting the invariant mass of the hadronic system to be consistent with K1(1270), we obtain the product of branching fractions B(D{0}-->K{1}{-}(1270)e{+}nu{e})xB(K1-(1270)-->K{-}pi{+}pi{-})=[2.5{-1.0}{+1.3}(stat)+/-0.2(syst)]x10{-4}. Using B(K1-(1270)-->K{-}pi{+}pi{-})=(33+/-3)%, we obtain B(D{0}-->K{1}{-}(1270)e{+}nu{e})=[7.6{-3.0}{+4.1}(stat)+/-0.6(syst)+/-0.7]x10{-4}. The last error accounts for the uncertainties in the measured K1-(1270)-->K{-}pi{+}pi{-} branching fractions.     

TEM characterization of chemically synthesized copper–gold nanoparticles

Dodecanethiol-capped Cu–Au nanoparticles, synthesized via a successive two-phase (water/toluene) and galvanic-exchange procedure, were characterized using transmission electron microscopy (TEM). The size range of the particles is around 1–7 nm. Electron-induced morphological evolution was observed under high resolution (HR) TEM. Cuboctahedral morphology was found to be thermodynamically stable. Electron-induced aggregation of two particles was also observed. Chemical ordering of cuboctahedral particles was studied by atomic-resolution high angle annular dark field (HAADF) imaging in scanning TEM (STEM) mode and energy dispersive X-ray (EDX) element mapping using a silicon drift detector (SDD). The particles were found to be Cu–Au mixed, and to be stable in air. Surface plasmon resonance (SPR), which is dependent on local structure and morphology, was investigated by electron energy loss spectroscopy (EELS).     

Entry distribution, fission barrier, and formation mechanism of 254102No

The entry distribution in angular momentum and excitation energy for the formation of 254No has been measured after the 208Pb(48Ca,2n) reaction at 215 and 219 MeV. This nucleus is populated up to spin 22Planck's over 2pi and excitation energy greater, similar6 MeV above the yrast line, with the half-maximum points of the energy distributions at approximately 5 MeV for spins between 12Planck's over 2pi and 22Planck's over 2pi. This suggests that the fission barrier is greater, similar5 MeV and that the shell-correction energy persists to high spin.