Electronic and optical properties of Si and Ge nanocrystals: An ab initio study

First-principles calculations within density functional theory and many-body perturbation theory have been carried out in order to investigate the structural, electronic and optical properties of undoped and doped silicon nanostructures. We consider Si nanoclusters co-doped with B and P. We find that the electronic band gap is reduced with respect to that of the undoped crystals, suggesting the possibility of impurity based engineering of electronic and optical properties of Si nanocrystals. Finally, motivated by recent suggestions concerning the chance of exploiting Ge dots for photovoltaic nanodevices, we present calculations of the electronic and optical properties of a Ge₃₅H₃₆ nanocrystal, and compare the results with those for the corresponding Si₃₅H₃₆ nanocrystals and the co-doped Si₃₃BPH₃₆.     

Method optimization by solid-phase microextraction in combination with gas chromatography with mass spectrometry for analysis of beer volatile fraction

A simple and sensitive method for the analysis of beer volatile compounds was optimised using headspace solid-phase microextraction (SPME) and gas chromatography with mass detection. Headspace SPME using a 75 microm Carboxen-polydimethylsiloxane (CAR-PDMS) fiber provided effective sample enrichment and enabled extraction of a wide variety of compounds. The reproducibility depended on the compounds, with a mean value of 1.4% for alcohols, 3.3% for ethers, 6.7% for aldehydes, 3.4% for acids, 1.7% for aromatic compounds, 2.4% for esters, 7.4% for hydrocarbons, 1.8% for alicyclic compounds, and 3.4% for heterocyclic compounds. The optimised methodology can be used to compare volatile profile from different types of beers and eventually to study the evolution of a particular beer during aging.     

Why local air pollution is more than daily peaks: modelling policies in a city in order to avoid premature deaths

We use a predator–prey representation of an urban system to analyse how policy interventions can prevent the adverse effects of air pollution on people’s health. The number of residents is treated as prey variable, and particulate matter that consists of particles with a diameter of up to 10 micrometres (PM10) as predator variable. This representation allows integration of population trends and the effects of environmental interventions on the average level of PM10 concentration (which establishes a baseline for the potential health burden for residents). For the case of Graz, Austria, we illustrate the insights generated regarding the interdependency of market-based and technological pollution controls, and propose an indicator that assesses the cost of delayed interventions by counting additional premature deaths caused by polluted environments.

     

Managing contribution and capital market risk in a funded public defined benefit plan: Impact of CVaR cost constraints

Using a Monte Carlo framework, we analyze the risks and rewards of moving from an unfunded defined benefit pension system to a funded plan for German civil servants, allowing for alternative strategic contribution and investment patterns. In the process we integrate a Conditional Value at Risk (CVaR) restriction on overall plan costs into the pension manager’s objective of controlling contribution rate volatility. After estimating the contribution rate that would fully fund future benefit promises for current and prospective employees, we identify the optimal contribution and investment strategy that minimizes contribution rate volatility while restricting worst-case plan costs. Finally, we analyze the time path of expected and worst-case contribution rates to assess the chances of reduced contribution rates for current and future generations. Our results show that moving toward a funded public pension system can be beneficial for both civil servants and taxpayers.     

Detection of nisin expression by Lactococcus lactis using two susceptible bacteria to associate the effects of nisin with EDTA

Nisin, a bacteriocin produced during the exponential growth phase of Lactococcus lactis ATCC 11454, inhibits the growth of a broad range of Grampositive bacteria. Gram-negative bacteria can also be inhibited by nisin with EDTA. In this study, nisin production was assayed by the agar diffusion method using Lactobacillus sake ATCC 15521 and a recombinant Escherichia coli DH5-α expressing the recombinant green fluorescent protein as the nisin-susceptible test organisms. The titers of nisin expressed and released in culture media were quantified and expressed in arbitrary units (AU/mL of medium) and converted to standard nisin concentration (Nisaplin^®, 25 mg of pure nisin with an activity of 1×10⁶ AU/mL). The expression and release of nisin by L. lactis in skimmed milk (9.09% total solids) with Man Rugosa Shepeer-Bacto Lactobacilli broth (1∶1) was monitored in a 5 L New Brunswick fermentor. Combining EDTA with nisin increased the bactericidal effect of nisin on the bacteria examined. The presence of EDTA was necessary to inhibit E. coli growth with nisin. L. sake was shown to be a good indicator for the evaluation of nisin release in the culture media, including with the addition of EDTA.     

Kinetics of phenol oxidation by peroxidase

Studies of the kinetic behavior of horseradish peroxidase (HRP) at pH 8 and at room temperature indicate that the reaction of phenol with H₂O₂ catalyzed by HRP exhibits normal Michaelis-Menten saturation kinetics. An irreversible reaction mechanism for the steady-state kinetics of HRP, which is consistent with the experimental data, is considered. The second-order rate constants for the reactions of HRP with H₂O₂ and compound II with phenol are 4.14 × 10⁵ M^-1s^-1 and 5.54 × 10⁴M^-1s^-1, respectively.     

Asymptotic properties of solutions to some nonlinear integral equations of convolution type

For a class of nonlinear integral equations of convolution type we give necessary and sufficient conditions for the boundedness of nonnegative solutions. Moreover, conditions for the solution to converge asymptotically to a determined limit are obtained.     

Reflectance anisotropy spectra of the diamond (100)-(2x1) surface: evidence of strongly bound surface state excitons

We compare the results of ab initio calculations with measured reflection anisotropy spectra and show that strongly bound surface-state excitons occur on the clean diamond (100) surface. These excitons are found to have a binding energy close to 1 eV, the strongest ever observed at a semiconductor surface. Important electron-hole interaction effects on the line shape of the optical transitions above the surface-state gap are also found.     

Effect of molecular binding to a semiconductor on metal/molecule/semiconductor junction behavior

Diodes made by (indirectly) evaporating Au on a monolayer of molecules that are adsorbed chemically onto GaAs, via either disulfide or dicarboxylate groups, show roughly linear but opposite dependence of their effective barrier height on the dipole moment of the molecules. We explain this by Au-molecule (electrical) interactions not only with the exposed end groups of the molecule but also with its binding groups. We arrive at this conclusion by characterizing the interface by in situ UPS-XPS, ex situ XPS, TOF-SIMS, and Kelvin probe measurements, by scanning microscopy of the surfaces, and by current-voltage measurements of the devices. While there is a very limited interaction of Au with the dicarboxylic binding groups, there is a much stronger interaction with the disulfide groups. We suggest that these very different interactions lead to different (growth) morphologies of the evaporated gold layer, resulting in opposite effects of the molecular dipole on the junction barrier height.     

Reaction of carbonyl compounds with dimethyl 3-ketoglutarate. Synthesis of bicyclic furan derivatives

Acid catalyzed reaction of cyelododecane-1,2-dione VI with dimethyl,3-ketoglutarate II led to the formation of methyl (E)-6,7,8,9,10,11,12,13-octahydro-3-(methoxycarbonyl)cyclododeca-[b]furan-2-aeetate IX, presumably formed through the dihydrodihydroxy furan VIII. This dihydrofuran intermediate VIII was shown to be characteristic of the reaction of 2-substituted carbonyl-compounds with 11 for both α-chlorocyclohexanone and α-chlorocyclopentanone furnished furans XVIa and XX, respectively, when treated with 11. The effect of dihydrodihydroxy-furan intermediates on the course of such reactions is discussed.