Highly luminescent CdTe/CdSe colloidal heteronanocrystals with temperature-dependent emission color

In this work we present the preparation of highly luminescent anisotropic CdTe/CdSe colloidal heteronanocrystals. The reaction conditions used (low temperature, slow precursor addition, and surfactant composition) resulted in a tunable shape from prolate to branched CdTe/CdSe nanocrystals. Upon CdSe shell growth the heteronanocrystals show a gradual evolution from type-I to type-II optical behavior. These heteronanocrystals show a remarkably high photoluminescence quantum yield (up to 82%) and negligible thermally induced quenching up to temperatures as high as 373 K.     

Non-destructive method for determination of hydroxyl value of soybean polyol by LS-SVM using HATR/FT-IR

This paper presents the use of least-squares support vector machine (LS-SVM) for quantitative determination of hydroxyl value (OHV) of hydroxylated soybean oils by horizontal attenuated total reflection Fourier transform infrared (HATR/FT-IR) spectroscopy. A least-squares support vector machine (LS-SVM) calibration model for the prediction of hydroxyl value (OHV) was developed using the range 1805.1-649.9 cm(-1). Validation of the method was carried out by comparing the OHV of a series of hydroxylated soybean oil predicted by the LS-SVM model to the values obtained by the AOCS standard method. A correlation coefficient equal to 0.989 and RMSEP = 4.96 mg of KOH/g was obtained. This study demonstrates a better prediction ability of the LS-SVM technique to determine OHV in hydroxylated soybean oil samples by HATR/FT-IR spectra.     

Oppenauer oxidation of secondary alcohols with 1,1,1-trifluoroacetone as hydride acceptor

1,1,1-Trifluoroacetone (2a) reacts as a hydride-acceptor in the Oppenauer oxidation of secondary alcohols (1) in the presence of diethylethoxyaluminum. The oxidant allows for selective oxidation of secondary alcohols in the presence of primary alcohols.     

New luminescent polymers for leds and LECS

Statistical copolymers 5 containing poly(2-dimethyloctylsilyl-1,4-phenylenevinylene) (DMOS-PPV) and poly[2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) have been synthesized using the dehydrohalogenation condensation route. The copolymers show a shift of photoluminescence maxima to longer wavelengths as the proportion of the MEH-PV unit increases. This trend is accompanied by reduced efficiencies and lower turn-on voltages in single layer electroluminescent devices. Light-emitting electrochemical cells (LECs) have been prepared using a blend of DMOS-PPV 1 with poly(ethylene oxide)/lithium triflate and the homopolymer poly[2-methoxy-5-(triethoxymethoxy)-1,4-phenylene vinylene] (MTEM-PPV) 9 with lithium triflate. In comparison with single-layer devices which were fabricated using the homopolymers 1 and poly[2,5-bis(triethoxymethoxy)-1,4-phenylene vinylene] (BTEM-PPV) 10 , the LEC devices showed lower turn-on voltages.     

Description of the transport critical current density behavior of polycrystalline superconductors as a function of the applied magnetic field

A model to describe the critical current density behavior of high-T_c polycrystalline superconductors is proposed for all magnetic field values. The main features of the model are as follows: the transport critical current density is controlled by the weak-link network at grain boundaries. The size distribution of weak links is well represented by a Gamma-type distribution. Finally, the tunneling critical current between grains follows a Fraunhofer diffraction pattern or a modified pattern if the applied magnetic field is lower or higher than the first critical field H_c1.     

Femtosecond compensated pair-pulses generation with nonlinear SOI-MZI waveguides

In this paper, we propose and simulate the generation of femtosecond compensated pair-pulses (bright and dark pulses), obtained simultaneously at the same center wavelength. The proposed configuration is based on a silicon-on-insulator (SOI) Mach–Zehnder interferometer (MZI) waveguide. The ultrafast pair-pulses are pumped at the mid-wave infrared wavelength (MWIR) injected in one arm of MZI, and a continuous-wave (CW) at the near infrared wavelength (specific communication wavelength) is launched from the other arm of MZI and divided equally into the two MZI arms. Numerical results show that femtosecond compensated pair-pulses with little pedestal shoulders in optical communication window can be simultaneously generated. In addition, the output properties, including the extinction ratio (ER) and contrast of compensated pair-pulses, are studied. The relation between the launching optical powers and the length of the Nonlinear SOI-MZI Waveguides are also explored. Such a work is helpful to dual binary code optical communication systems, opto-electronic devices and spectroscopy, etc.     

Efficiency and sustainability assessment for a group of farmers in the Brazilian Amazon

The aim of this paper is to use DEA models to evaluate sustainability in agriculture. Several variables are taken into account and the resulting efficiency is measured by comparison. The performance of family farms is analysed here (variables: farmed area, work force, and production). As agricultural sustainability depends on the maintenance of systems of production for long periods of time, the models were run for the years of 1986 and 2002. Tiered DEA models were used to group farmers in sustainability categories. Non-parametric regression models were used to identify the factors affecting the efficiency measurements. All the results indicate that the majority of the farmers increased their efficiency along the time. These improvements may support the existence of sustainability.     

New central configurations for the planar 7-body problem

In this paper we study numerically the existence of families of planar central configurations for the 7-body problem with the following properties: five bodies are on the vertices of a regular pentagon and the other two bodies are symmetrically positioned in the interior of the pentagon.     

Electronic coupling and exciton energy transfer in CdTe quantum-dot molecules

Stable dispersions of molecularlike aggregates of CdTe quantum dots are prepared by chemical cross-linking. Cryo-TEM images confirm the presence of cross-linked quantum dots and show that the size of the small aggregates can be controlled by the amount of cross-linker added. Optical measurements reveal two types of interdot interactions within these quantum-dot molecules: exciton energy transfer and electronic coupling. Quantitative information on the energy transfer rates in quantum-dot molecules is obtained by photoluminescence lifetime measurements. The degree of electronic coupling is dependent on the size of the quantum dots, which is supported by quantum mechanical calculations.     

Side chain mediated electronic contact between a tetrahydro-4H-thiopyran-4-ylidene-appended polythiophene and CdTe quantum dots

The properties of a mixed CdTe quantum dot/tetrahydro-4H-thiopyran-4-ylidene-functionalized polythiophene system are reported. This system was prepared by exposing trioctylphosphine (TOP)-capped CdTe quantum dots to the polythiophene in solution. Strong fluorescence emission quenching and shortening of the fluorescence emission lifetimes of both the polythiophene and the quantum dots occur when they are mixed, indicating the occurrence of photoinduced charge separation. Photoinduced absorption spectroscopy reveals a considerable decrease in the population of the polythiophene triplet excited state in the mixed system. These results demonstrate that between the quantum dots and the polythiophene there is both physical and electronic contact, which is mediated by the tetrahydro-4H-thiopyran-4-ylidene side chains.