Influence of stoichiometry on the luminescent properties of InAs quantum dots grown on a heterostructure

We report photoluminescence (PL) spectra of InP/In_xGa_1-xAs/InAs/InP dot-in-a-well structures grown by MOVPE, with different compositions of the ternary layer. Measurements with atomic force microscopy showed that the largest quantum dot (QD) height is obtained when the InAs QDs are grown on the In_xGa_1-xAs layer with a mismatch of 1000 ppm, and the height decreases as the mismatch departs from this value. PL spectra of the QDs showed an asymmetric band, which involves transitions between dot energy levels and can be deconvoluted into two peaks. The highest energy PL peak of this band was observed for the sample with the QDs grown on top of the lattice-matched In_xGa_1-xAs layer and it shifted to lower energies for strained samples as the degree of mismatch increased. Theoretical calculations of the energy levels of the entire structure were used to interpret the obtained PL spectra and determine the possible detection tunability range.     

Revisitation of the molecular mobility of the amorphous solid 4,4′-methylenebis(N,N-diglycidylaniline) (MBDA): New contributions from the TSDC technique

In the present work we revisit the previously published study by Thermally Stimulated Depolarisation Currents (TSDC) of the slow molecular mobility in the amorphous solid state of 4,4′-methylenebis(N,N-diglycidylaniline), MBDA (H. P. Diogo, J. J. Moura Ramos, J. Mol. Liq. 129(2006)138–146) in order to add two important points dealing with recent uses of the TSDC technique. One of them concerns a new method to account for nonexponentiality in Thermally Stimulated Depolarisation Currents (TSDC) data treatment which affects the analysis of the alpha relaxation peak and the determination of the fragility index of the glass-forming system. The other point concerns the analysis of the secondary relaxations in MBDA. It is shown that two kinds of secondary relaxations are differently influenced by aging. The faster components of the secondary relaxation are negligibly dependent on aging and may be ascribed to intramolecular modes of motion, while the slower motional modes show a significant dependence on aging and correspond to the genuine Johari-Goldstein β-relaxation.     

Shape resonances in slow electron scattering by aromatic molecules. I. Anthraquinone derivatives

A series of anthraquinone (C(14)O(2)H(8)) derivatives has been studied by means of electron capture negative ion mass spectrometry (ECNI-MS), photoelectron spectroscopy (PES), and AM1 quantum chemical calculations. Mean lifetimes of molecular negative ions M(-.) (MNI) have been measured. The mechanism of long-lived MNI formation in the epithermal energy region of incident electrons has been investigated. A simple model of a molecule (a spherical potential well with the repulsive centrifugal term) has been applied for the analysis of the energy dependence of cross sections at the first stage of the electron capture process. It has been shown that a temporary resonance of MNI at the energy approximately 0.5 eV corresponds to a shape resonance with lifetime 1-2.10(-13) s in the f-partial wave (l = 3) of the incident electron. The next resonant state of MNI at the energy approximately 1.7 eV has been associated with the electron excited Feshbach resonance (whose parent state is a triplet npi* transition). In all cases the initial electron state of the MNI relaxes into the ground state by means of a radiationless transition, and the final state of the MNI is a nuclear excited resonance with a lifetime measurable on the mass spectrometry timescale. Copyright 1999 John Wiley & Sons, Ltd.     

Dynamics and Control Strategies for a Butanol Fermentation Process

In this work, mathematical modeling was employed to assess the dynamic behavior of the flash fermentation process for the production of butanol. This process consists of three interconnected units as follows: fermentor, cell retention system (tangential microfiltration), and vacuum flash vessel (responsible for the continuous recovery of butanol from the broth). Based on the study of the dynamics of the process, suitable feedback control strategies [single input/single output (SISO) and multiple input/multiple output (MIMO)] were elaborated to deal with disturbances related to the process. The regulatory control consisted of keeping sugar and/or butanol concentrations in the fermentor constant in the face of disturbances in the feed substrate concentration. Another objective was the maintenance of the proper operation of the flash tank (maintenance of the thermodynamic equilibrium of the liquid and vapor phases) considering that oscillations in the temperature in the tank are expected. The servo control consisted of changes in concentration set points. The performance of an advanced controller, the dynamic matrix control, and the classical proportional-integral controller was evaluated. Both controllers were able to regulate the operating conditions in order to accommodate the perturbations with the lowest possible alterations in the process outputs. However, the performance of the PI controller was superior because it showed quicker responses without oscillations.     

Thermal stability of aDesulfovibrio gigas periplasmic hydrogenase in reverse micelles

The thermal and operational stabilities ofDesulfovibrio gigas periplasmic hydrogenase were studied at 30 and 50°C in aqueous and micellar media. At the lower temperature the hydrogenase was more stable in reverse micelles of a cationic surfactant. No inactivation was detected over almost 16 d of incubation of the hydrogenase in the reverse micellar solution, during which the enzyme lost around 50% of the initial enzymatic activity in the aqueous solution. At 50°C the hydrogenase was more stable in aqueous medium, because of the changes that occur with the temperature in the organic phase—precipitation of surfactant and solvent evaporation. It was found that only micellar solutions of this enzyme can be repeatedly used, since the hydrogenase is inactivated after the first cycle of oxireduction in aqueous medium. The effect of glycerol and the electron carrier methylviologen as stabilizers of hydrogenase activity was also investigated. The results are interpreted on the grounds of hydrogenase and surfactant electrostatic properties.     

Interpolation of relative retentions as a tool for identification of perhalogenated compounds

Summary Separation of perhalogenated compounds resulting from thermolysis, photolysis or radiolysis of mixtures of halogens in carbon tetrachloride was carried out by programmed temperature gas chromatography. Plots of relative retentions (to CBr₄) vs the number of bromine atoms present for known compounds of the general formula C_xBr_yCl_z (x=1 and 2; y+z=4 or 6) gave three linear correlations (all with r>0.99). These correlations permitted projections of the identities of reaction products for which no standards were available. These projections were later confirmed by CG-MS. *** DIRECT SUPPORT *** A0732166 00010     

Thermodynamics of micellization of cationic surfactants in aqueous solutions: consequences of the presence of the 2-acylaminoethyl moiety in the surfactant head group

The enthalpies of micellization of the following surfactant series have been determined by calorimetry: benzyl (2-acylaminoethyl)dimethylammonium chlorides, RABzMe₂Cl, and alkyldimethylbenzylammonium chlorides, RBzMe₂Cl, where A, Bz and Me refer to amide, benzyl, and methyl groups, respectively and the acyl (for RABzMe₂Cl) and/or the alkyl (for RBzMe₂Cl) groups C₁₀, C₁₂, C₁₄, and C₁₆, respectively. For both series, the shapes of the calorimetric titration curves (enthalpograms) depend on the following micellar parameters: critical micelle concentration, aggregation number, and degree of counterion binding. The calorimetric-based critical micelle concentrations are in excellent agreement with those determined by conductivity. The Gibbs free energy, the enthalpy and the entropy of micellization were calculated, and divided into contributions from the CH₂ groups of the hydrophobic tail, and the terminal CH₃ plus head group of the surfactant. For both surfactant series, all thermodynamic parameters per CH₂ group were found to be similar, since their transfer (from bulk solution to the micelle) is independent of the surfactant head-group structure. The Gibbs free energy, the enthalpy, and the entropy of transfer of the head group of RABzMe₂Cl are more favorable than their counterparts for RBzMe₂Cl, because of direct and/or water mediated hygrogen bonding of the amide groups in the micelle.     

The heterogeneous formation of N2O in the presence of acidic solutions: Experiments and modeling

We investigated the heterogeneous processes that contribute towards the formation of N₂O in an environment that comes as closely as possible to exhaust conditions containing NO and SO₂ among other constituents. The simultaneous presence of NO, SO₂, O₂, and condensed phase water in the liquid state has been confirmed to be necessary for the production of significant levels of N₂O. The maximum rate of N₂O formation occurred at the beginning of the reaction and scales with the surface area of the condensed phase and is independent of its volume. The replacement of NO by either NO₂ or HONO significantly increases the rate constant for N₂O formation. The measured reaction orders in the rate law change depending upon the choice of the nitrogen reactant used and were fractional in some cases. The rate constants of N₂O formation for the three different nitrogen reactants reveal the following series of increasing reactivity: NO < NO₂ < HONO, indicating the probable sequential involvement of those species in the elementary reactions. Furthermore, we observed a complex dependence of the rate constant on the acidity of the liquid phase where both the initial rate as well as the yield of N₂O are largest at pH=0 of a H₂SO₄/H₂O solution. The results suggest that HONO is the major reacting N(III) species over a wide range of acidities studied. The N₂O formation in synthetic flue gas may be simulated using a relatively simple mechanism based on the model of Lyon and Cole. The first step of the complex overall reaction corresponds to NO oxidation by O₂ to NO₂ mainly in the gas phase, with the presence of both H₂O and active surfaces significantly accelerating NO₂ production. Subsequently, NO₂ reacts with excess NO to obtain HONO which reacts with S(IV) to result in N₂O and H₂SO₄ through a complex reaction sequence probably involving nitroxyl (HON) and its dimer, hyponitrous acid. © 1997 John Wiley & Sons, Inc. Int J Chem Kinet: 29 : 869–891, 1997.     

Catalytic oxidation of phenol in aqueous media over CuZSM-12 zeolite?

Summary The catalytic oxidation of phenol over CuZSM-12 (SAR = 200) has been studied using a semi-batch slurry reactor at temperatures of 110; 120 and 130^oC, and acidic neutral and basic pH. At 130^oC, the phenol undergoes total conversion to carbon dioxide and water in 1.5 h. The kinetic parameters were evaluated using a modified homogeneous-heterogeneous model for the experimental data. Using this modified model, the activation energy for the catalytic oxidation process was ca. 90 kJ mol^-1.     

A dynamic study of the quasi-one-dimensional hydrogen-bonded ferroelectric crystal CsH2PO4

The dynamics of the strong-anisotropic Ising model in a transverse field is used with the purpose to explain the dielectric critical slowing down observed experimentally in the quasi-one-dimensional hydrogen-bonded ferroelectric crystal CsH₂PO₄. A good agreement with the experimental data of the temperature dependence of the dielectric constant and the relaxation time is obtained.