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.     

A new approach to reduction of the Neumann problem in acoustic scattering to a non-hypersingular integral equation

The Neumann problem for the propagative Helmholtz equation inthe exterior of several bodies (obstacles) is studied in twoand three dimensions by a special modification of the boundaryintegral equation method. This modification can be called the'method of interior boundaries', because additional boundariesare introduced inside scattering bodies. The solution of theproblem is obtained in the form of a single layer potentialon the whole boundary. The density in the potential satisfiesthe uniquely solvable Fredholm equation of the second kind andcan be computed by standard codes. In fact our method holdsfor any positive wave numbers.     

Oxidation of gas-phase protactinium ions, Pa+ and Pa2+: formation and properties of PaO2(2+)(g), protactinyl

Oxidation reactions of bare and ligated, monopositive, and dipositive Pa ions in the gas phase were studied by Fourier transform ion cyclotron resonance mass spectrometry. Seven oxidants were employed, ranging from the thermodynamically robust N(2)O to the relatively weak CH(2)O-all oxidized Pa(+) to PaO(+) and PaO(+) to PaO(2)(+). On the basis of experimental observations, it was established that D[Pa(+)-O] and D[OPa(+)-O] > or = 751 kJ mol(-1). Estimates for D[Pa(+)-O], D[OPa(+)-O], IE[PaO], and IE[PaO(2)] were also obtained. The seven oxidants reacted with Pa(2+) to produce PaO(2+), indicating that D[Pa(2+)-O] > or = 751 kJ mol(-1). A particularly notable finding was the oxidation of PaO(2+) by N(2)O to PaO(2)(2+), a species, which formally comprises Pa(VI). Collision-induced dissociation of PaO(2)(2+) suggested the protactinyl connectivity, {O-Pa-O}(2+). The experimentally determined IE[PaO(2)(+)] approximately 16.6 eV is in agreement with self-consistent-field and configuration interaction calculations for PaO(2)(+) and PaO(2)(2+). These calculations provide insights into the electronic structures of these ions and indicate the participation of 5f orbitals in bonding and a partial "6p hole" in the case of protactinyl. It was found that PaO(2)(2+) catalyzes the oxidation of CO by N(2)O-such O atom transport via a dipositive metal oxide ion is distinctive. It was also observed that PaO(2)(2+) is capable of activating H(2) to form the stable PaO(2)H(2+) ion.     

Multiple square wave voltammetry for analytical determination of paraquat in natural water, food, and beverages using microelectrodes

This paper reports on the use of multiple square wave voltammetry (MSWV) for analytical determination of paraquat herbicide at gold microelectrode (Au-ME) in different samples of natural water, food, and beverages. In this work, the MSWV consisted in a sequence of four pairs of potential pulse in the same step and the interval potential evaluated was of the 0.0 V at −1.2 V versus Ag/AgCl 3.0 mol L⁻¹. The paraquat herbicide presented two reduction peaks, in −0.69 V and −0.99 V, with profile of the redox process totally reversible, and the use of multiple pulses allowed a detection of nanomolar levels after the optimization of experimental and voltammetric conditions. Analytical curves were constructed for pulse potential frequency of 250 s⁻¹, pulse amplitude of 50 mV, scan increment of 2 mV and pulse number of 8 pulses in a same step. The two reduction peaks showed that the peak currents were found to be directly proportional to the pesticide concentration in the range comprised between 5.0 × 10⁻⁷ mol L⁻¹ and 1.04 × 10⁻⁵ mol L⁻¹. With this, it was possible to determine detection limits (DL), which resulted in 0.044 μg L⁻¹ (0.044 ppb) and 0.146 μg L⁻¹ (0.146 ppb), respectively, for peak 1 and peak 2. DL results, obtained using MSWV, were 2–3 orders of magnitude lower (10⁻² to 10⁻³) less than those observed for traditional square wave voltammetry or published in literature, clearly pointing to the advantages arising from the possibility of using a MSWV for analytical purposes in contaminated matrices. In addition, the proposed methodology was applied in different samples of natural water, food and beverages without pre-treatment or pre-concentration step, where a recovery measurement indicated that the methodology could be employed to analyze paraquat in such matrices.     

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.     

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.