Radiative and non-radiative relaxation of excitons in strain-compensated quantum dots

We have investigated the population dynamics of excitons in strain-compensated InAs quantum dots (QDs) using a pump–probe technique under resonant excitation. Precise control of polarization directions of incident pulses enabled us to selectively estimate population lifetimes for two orthogonally polarized exciton ground states according to polarization selection rules. Measured decay times of the probe transmissions were highly dependent on the polarization directions of the exciton states. We found that the ratio of the decay times for the orthogonally polarized states is in quantitative agreement with the ratio of square of the transition dipole moments. This indicates that radiative recombination processes have a dominant effect on the population dynamics and that non-radiative and spin relaxations are negligible in our QDs. As a result, we can estimate the radiative lifetimes to be 1.0±0.1 and 1.7±0.2 ns for orthogonally polarized exciton ground states.     

All-optical serial-to-parallel conversion of T-bits/s signals using a four-wave-mixing process

We report a simple method of a serial-to-parallel conversion that does not use a Fourier-transform system. The method is based on directly picking up a narrow temporal range of the skewed input signal by using a nonlinear wave mixing with the reference pulse. We show theoretically that the pick-up method is less influenced by the relaxation times of the nonlinear material. We demonstrate our method experimentally using a self-organized quantum-well material, and confirm that T-bits/s pulses are clearly converted into spatial patterns with conversion rates of 140 GHz.     

Piezoelectric PVDF-TrFE nanocomposite mats filled with BaTiO3 nanofibers: The effect of poling conditions

BaTiO₃ nanofibers (BT NFs), prepared by electrospinning, were used as a filler for electrospun poly(vinylidene fluoride-co-trifluoroethylene) (PVDF-TrFE) nanocomposite mats. The phase structure and the effect of poling conditions on the piezoelectric properties of PVDF-TrFE/BT nanocomposites were investigated. The results showed an improved degree of crystallinity (78.6%) and a high β-crystal phase (up to 98.3%) in all electrospun samples, independent of the nanofiber content. The two-step poling method, applying electric fields of opposite polarity, led to significantly improved piezoelectric constants d₃₃ (−31.7 pC N⁻¹), strongly dependent on the added BaTiO₃ nanofibers. The inclusion of piezoelectric ceramic nanofibers into a polymer matrix, easily carried out by means of electrospinning, followed by an ad hoc optimized poling treatment, allowed to develop flexible materials with enhanced piezoelectric properties, potentially exploitable in innovative conversion systems used in wearable and sensing devices.     

Additivity and transferability of exchange energy

Exchange energy of molecules at equilibrium geometries is shown to be highly independent of the basis set and nearly equal to the sum of the exchange energies of isolated atoms. Furthermore, molecular exchange can be predicted along a chemical series with high accuracy in terms of empirical exchange contributions associated to the constituent atoms, whose values, obtained by fitting, are similar to those of the isolated atoms. It is concluded that molecular exchange is essentially a sum of atomic contributions whose differences from those of the isolated atoms are small and characteristic of the neighboring atoms. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2010     

Influence of ultrasonic processing on the macromolecular properties of poly (d,l-lactide-co-glycolide) alone and in its biocomposite with hydroxyapatite

In this work poly(d,l-lactide-co-glycolide) (PLGA) and a poly(d,l-lactide-co-glycolide)/hydroxyapatite (PLGA/HAp) composite processed in an ultrasonic field at higher (25 °C) and lower (8 °C) temperatures were studied with respect to the molecular properties of the obtained materials. The processing of the PLGA and the PLGA/HAp composite in an ultrasonic field resulted in a change of molar mass averages of the polymer/polymeric part of these materials, while an amorphous structure and a 50:50 lactide-to-glycolide co-monomer ratio were preserved without the formation of crystalline oligomers. However, mobility of polymeric chains obtained after ultrasonic processing was lower indicating ordering the structure of polymeric chains as a result of processing. Additionally, it was observed that the mobility of the PLGA macromolecules was lower within the composite in comparison with the mobility of the chains within the PLGA alone in the case when both were obtained after ultrasonic processing. This was a consequence of the structure formation through the interactions between the PLGA and the HAp. Based on these results different degradation rate of PLGA in composite can be expected, which is important in the application of this material for the controlled drug delivery of medicaments.     

Evaluation of bandgap energy and carrier density of InN nanocolumns

We have measured the optical properties of wurtzite InN nanocolumns and film by photoluminescence (PL) measurements at temperatures from 5 to 300 K and analyzed the PL spectra by fitting with the free-electron recombination bound (FERB) model. For the top-linked InN nanocolumns, we observed strong PL intensity compared to the InN film sample. The PL spectra were asymmetrical with low-energy tails and a red-shift of the PL peak energy position was observed with increasing temperature. However, for the separated InN nanocolumns, we observed weak PL intensity and symmetrical PL spectra. Analyzing the spectra shape of the top-linked InN nanocolumns at 5 K using the FERB model, we evaluated the intrinsic bandgap energy and carrier density of InN nanocolumns to be 0.69 eV and 2.5×10¹⁷ cm⁻³, respectively.     

Transient differential reflectivity of ferromagnetic and paramagnetic phases in the bilayered manganite La1.24Sr1.76Mn2O7

Photoinduced effects in a single crystal of bilayered manganites, La_2−2xSr_1+2xMn₂O₇ (x=0.38), were investigated in a wide range of temperatures by pump-probe measurement at a photon energy of 1.6 eV. In a ferromagnetic metallic state, significant enhancement of positive rise in differential reflectivity with a slow relaxing time of 100 ps was observed just below T_C=127 K, indicating that the reflectivity change with the slow relaxation time constant is induced by laser heating. We have also observed an unconventional fast relaxing component that has a time constant of the order of 10 ps. This fast relaxing component, whose absolute value has an asymmetric peak at T_C, is presumably due to short-range correlation of Jahn-Teller distortion.     

Formation and dichroism of poly(vinyl alcohol)–iodine complex in photocurable film

The formation of polyiodine complexes was investigated in a photocurable poly(vinyl alcohol) modified N‐methyl‐4(4′‐formylstyryl)pyridinium methosulfate acetal (PVA‐SbQ), which is a photofunctional group that causes photodimerization. PVA‐SbQ films with polyiodine complexes were prepared to be photocured, iodinated and soaked in a boric acid solution. The formation of PVA–polyiodine complexes was studied during iodinating and while in the boric acid treatment through UV–vis absorption spectrometry, resonance Raman spectrometry and IR absorption spectrometry. As a result, polyiodines were formed in the photocurable PVA‐SbQ films, and the formation of PVA–polyiodine complexes was enhanced by boric acid treatment. It was found that the SbQ‐ratio of PVA‐SbQ affects the formation of PVA–polyiodine complexes. The photocrosslinking by the dimerization of SbQ groups helps to form the PVA–I₅⁻ complex during the boric acid treatment. Based on this effect, we demonstrated a unique recording method by the PVA–polyiodine complex formation. The PVA‐SbQ film cured by the irradiation of the liner polarized light showed that dichroism of the PVA–polyiodine complexes formed after iodination. Copyright © 2015 John Wiley & Sons, Ltd.     

Discotic Liquid Crystals of Transition Metal Complexes VI. Monotropic Lamella Mesomorphism of Tetrakis(N-Alkyldithiolato)Dinickel(II) and Bis(N-Alkylxanthato)Nickel(II) Complexes

Abstract

It was found that each of the tetrakis(n-alkyldithiolato)dinickel(II), (C_n-DTA)₄-Ni₂, complexes where n-alkyl is n-pentyl through n-dodecyl, exhibits a broken-fan texture on cooling from an isotropic liquid, and that the phase gave a characteristic lamella structure X-ray diffraction powder pattern. Furthermore, the infrared spectrum of this phase is more similar to that of the isotropic liquid than that of the crystal. Therefore, the phase can be described as a monotropic lamella mesophase. Interestingly, each of the complexes of bis(n-alkylxanthato)nickel(II), (C_n-Xan)₂Ni, (n = 5, 7, 9, 11) exhibits double (triple) melting behavior via the isotropic liquid, whereas each of the complexes of (C_n-Xan)₂Ni (n = 4, 6, 8, 10, 12) shows ordinary single melting behavior. Such unique double melting accompanied by an even-odd effect appears to be the first example of this in the long chain substituted compounds. Each of the complexes of (C_n-Xan)₂ Ni (n = 9, 11, 12) has a monotropic lamella mesophase exhibiting a large broken fan texture.