Alguns aspectos da transposição de uma sequencia didática sobre o comportamento de partículas e ondas

This work presents a partial analysis of the implementation of a didactic sequence aimed at undergraduate students of quantum chemistry. The sequence develops on some fundamental concepts to understand the behavior of the objects particles and waves in the doubleslit experiment. The analysis is based on the didactic transposition theory, proposed by Yves Chevallard, which allows reflection upon the scientific knowledge reorganized to be used in teaching contexts. Our interest in this study lays on the dynamics of transposition of knowledge-to-be-taught into knowledge-taught, particularly the importance of making concepts more comprehensible. Results showed that the communication of knowledge by the students is presented with interruptions, gaps and colloquial language. The analysis allowed the identification of needs and possibilities of learning, as well as a turn in the teacher practice, through a dynamic process of action and reflection.

Finite-difference calculation of donor energy levels in a spherical quantum dot subject to a magnetic field

Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)

Photoinduced hole-transfer in semiconducting polymer/low-bandgap cyanine dye blends: evidence for unit charge separation quantum yield

Power-conversion efficiencies of organic heterojunction solar cells can be increased by using semiconducting donor-acceptor materials with complementary absorption spectra extending to the near-infrared region. Here, we used continuous wave fluorescence and absorption, as well as nanosecond transient absorption spectroscopy to study the initial charge transfer step for blends of a donor poly(p-phenylenevinylene) derivative and low-band gap cyanine dyes serving as electron acceptors. Electron transfer is the dominant relaxation process after photoexcitation of the donor. Hole transfer after cyanine photoexcitation occurs with an efficiency close to unity up to dye concentrations of similar to 30 wt%. Cyanines present an efficient self-quenching mechanism of their fluorescence, and for higher dye loadings in the blend, or pure cyanine films, this process effectively reduces the hole transfer. Comparison between dye emission in an inert polystyrene matrix and the donor matrix allowed us to separate the influence of self-quenching and charge transfer mechanisms. Favorable photovoltaic bilayer performance, including high open-circuit voltages of similar to 1 V confirmed the results from optical experiments. The characteristics of solar cells using different dyes also highlighted the need for balanced adjustment of the energy levels and their offsets at the heterojunction when using low-bandgap materials, and accentuated important effects of interface interactions and solid-state packing on charge generation and transport.

Indirect optical transitions from carriers trapped on the delta doping and on the parabolic quantum well

In this work, doped AlGaAs/GaAs parabolic quantum wells (PQW) with different well widths (from 1000 angstrom up to 3000 angstrom) were investigated by means of photoluminescence (PL) measurements. In order to achieve the 2DEG inside the PQW Si delta doping is placed at both side of the well. We have observed that the thickness of this space layer plays a major rule on the characteristics of the 2DEG. It has to be thicker enough to prevent any diffusions of Si to the well and thin enough to allow electrons migration inside the well. From PL measurement, we have observed beside the intra well transitions, indirect transitions involving still trapped electron on the delta doping and holes inside the PQW. For the thinness sample, we have measured a well defined PL peak at low energy side of the GaAs bulk emission. With the increasing of the well thickness this peak intensity decreases and for the thickest sample it almost disappears. Our theoretical calculation indicated that carriers (electron and holes) are more placed at the center of the PQW. In this way, when the well thickness increases the distance between electrons on the delta doping and holes on the well also increases, it decreases the probability of occurrence of these indirect optical transitions. (C) 2012 Published by Elsevier B.V. Selection and/or peer-review under responsibility of Universidade Federal de Juiz de Fora, Brazil.

A quantum chemical study on the formation of phosphorus mononitride

Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)

Path integral quantization of generalized quantum electrodynamics

Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)

Quantum entanglement and fixed point Hopf bifurcation

We present the qualitative differences in the phase transitions of the mono-mode Dicke model in its integrable and chaotic versions. These qualitative differences are shown to be connected to the degree of entanglement of the ground state correlations as measured by the linear entropy. We show that a first order phase transition occurs in the integrable case whereas a second order in the chaotic one. This difference is also reflected in the classical limit: for the integrable case the stable fixed point in phase space undergoes a Hopf type whereas the second one a pitchfork type bifurcation. The calculation of the atomic Wigner functions of the ground state follows the same trends. Moreover, strong correlations are evidenced by its negative parts. (c) 2006 Elsevier B.V. All rights reserved.

Quantum Mechanics Insight into the Microwave Nucleation of SrTiO3 Nanospheres

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)

Energy-transfer mechanisms and emission quantum yields in Eu3+-based siloxane-poly(oxyethylene) nanohybrids

We report the energy-transfer mechanisms and emission quantum yield measurements of sol-gel-derived Eu3+-based nanohybrids. The matrix of these materials, classified as diureasils and termed U(2000) and U(600), includes urea cross-links between a siliceous backbone and polyether-based segments of two molecular weights, 2000 and 600, respectively. These materials are full-color emitters in which the Eu3+ (5)Do --> F-7(0-4) lines merge with the broad green-blue emission of the nanoscopic matrix's backbone. The excitation spectra show the presence of a large broad band (similar to 27000-29000 cm(-1)) undoubtedly assigned to a ligand-to-metal charge-transfer state. Emission quantum yields range from 2% to 13.0% depending on the polymer molecular weight and Eu3+ concentration. Energy transfer between the hybrid hosts and the cations arises from two different and independent processes: the charge-transfer band and energy transfer from the hybrid's emitting centers. The activation of the latter mechanisms induces a decrease in the emission quantum yields (relative to undoped nanohybrids) and permits a fine-tuning of the emission chromaticity across the Comission Internacionalle d'Eclairage diagram, e.g., (x, y) color coordinates from (0.21, 0.24) to (0.39, 0.36). Moreover, that activation depends noticeably on the ion local coordination. For the diureasils with longer polymer chains, energy transfer occurs as the Eu3+ coordination involves the carbonyl-type oxygen atoms of the urea bridges, which are located near the hybrid's host emitting centers. on the contrary, in the U(600)-based diureasils, the Eu3+ ions are coordinated to the polymer chains, and therefore, the distance between the hybrid's emitting centers and the metal ions is large enough to allow efficient energy-transfer mechanisms.

GENERATING FUNCTION FOR CLEBSCH-GORDAN-COEFFICIENTS, OF THE SUQ(2) QUANTUM ALGEBRA

Some methods have been developed to calculate the su(q)(2) Clebsch-Gordan coefficients (CGC). Here we develop a method based on the calculation of Clebsch-Gordan generating functions through the use of 'quantum algebraic' coherent states. Calculating the su(q)(2) CGC by means of this generating function is an easy and straightforward task.

Origin of photo luminescence in SrTiO3: a combined experimental and theoretical study

A joint experimental and theoretical study has been carried out to rationalize the photoluminescence properties of SrTiO3 perovskite thin films synthesized through a soft chemical processing. Only the amorphous samples present photoluminescence at room temperature. From the theoretical side, first principles quantum mechanical techniques, based on density functional theory at B3LYP level, have been employed to study the electronic structure of a crystalline (ST-c) and an asymmetric (ST-a) model. Electronic properties are analyzed in the light of the experimental results and their relevance in relation to the PL behavior of ST is discussed. (C) 2004 Elsevier B.V. All rights reserved.

Molecular orbital calculation for the model compounds of kainoid amino acids, agonists of excitatory amino acid receptors. Does the kainoid C4-substituent directly interact with the receptors?

Kainoid amino acids are agonists of the AMPA/kainate receptors and exhibit highly potent neuroexcitatory activity. From the results of extensive structure-activity relationship studies, we previously postulated that the C4-substituent of the kainoid amino acids interacts with an allosteric site of the glutamate receptor with electron-donating character. In order to investigate the mode of action in more detail, molecular orbital calculation for model compounds of the kainoid were performed. The results indicated that the HOMO energy level of the C4-substituent is involved in the potent neuroexcitatory activity, thus supporting our hypothesis. (C) 2002 Elsevier B.V. Ltd. All rights reserved.

Environmental coordination chemistry: Binary systems comprising some bivalent cations and monocarboxylates in aqueous solution. Ionic medium effects on equilibrium constants

The molar single activity coefficients associated with propionate ion (Pr) have been determined at 25 degrees C and ionic strengths comprised between 0.300 and 3.00 M, adjusted with NaClO4, as background electrolyte. The investigation was carried out potentiometrically by using a second class Hg/Hg2Pr2 electrode. It was found that the dependence of propionate activity coefficients as a function of ionic strength (I) can be assessed through the following empirical equation: log y(Pr) = -0.185 I-3/2 + 0.104 I-2. Next, simple equations relating stoichiometric protonation constants of several monocarboxylates and formation constants associated with 1:1 complexes involving some bivalent cations and selected monocarboxylates, in aqueous solution, at 25 degrees C, as a function of ionic strength were derived, allowing the interconversion of parameters from one ionic strength to another, up to I = 3.00 M. In addition, thermodynamic formation constants as well as parameters associated with activity coefficients of the complex species in the equilibria are estimated. The body of results shows that the proposed calculation procedure is very consistent with critically selected experimental data.

Drude's model calculation rule on electrical transport in Sb-doped SnO2 thin films, deposited via sol-gel

The evaluation of free carrier concentration based on Drude's theory can be performed by the use of optical transmittance in the range 800-2000 nm (near infrared) for Sb-doped SnO2 thin films. In this article, we estimate the free carrier concentration for these films, which are deposited via sol-gel dip-coating. At approximately 900 mn, there is a separation among transmittance curves of doped and undoped samples. The plasma resonance phenomena approach leads to free carrier concentration of about 5 x 1020 cm(-3). The increase in the Sb concentration increases the film conductivity; however, the magnitude of measured resistivity is still very high. The only way to combine such a high free carrier concentration with a rather low conductivity is to have a very low mobility. It becomes possible when the crystallite dimensions are taken into account. We obtain grains with 5 nm of average size by estimating the grain size from X-ray diffraction data, and by using line broadening in the diffraction pattern. The low conductivity is due to very intense scattering at the grain boundary, which is created by the presence of a large amount of nanoscopic crystallites. Such a result is in accordance with X-ray photoemission spectroscopy data that pointed to Sb incorporation proportional to the free electron concentration, evaluated according to Drude's model. (c) 2006 Elsevier Ltd. All rights reserved.

In Situ and Simultaneous UV-vis/SAXS and UV-vis/XAFS Time-Resolved Monitoring of ZnO Quantum Dots Formation and Growth

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)