Steady State and Time Resolved Spectroscopic Study of CdSe and CdSe/ZnS QDs:FRET Approach

This article highlights some physical studies on the relaxation dynamics and Förster resonance energy transfer (FRET) of semiconductor quantum dots (QDs) to proximal dye molecule and the way these phenomena change with core to core-shell QD is discussed. Efforts to understand the optical and carrier relaxation dynamics of CdSe and CdSe/ZnS QDs are made by using absorption, steady-state fluorescence and time-resolved fluorescence (TCSPC) techniques. Steady-state as well as time-resolved fluorescence measurements were employed to evaluate the QD PL quenching induced by the proximal Rhodamine 101 dye molecule and to examine the influence of deep trap states on energy transfer efficiency. The FRET parameters such as spectral overlap, Förster distance, intermolecular distance for each donor-acceptor pair are determined and variation of these parameters from core to core-shell QD is discussed.     

A photodimerization approach to crosslink and functionalize microgels

Microgels were prepared within reverse micelles via photocrosslinking. Gelation resulted from the [2 + 2] photodimerization reaction of nitrocinnamoyl (NC) groups on multi-arm polyethylene glycol (PEG) or gelatin. Because of the potential for biomedical and chemical applications, immobilization capacity within the microgels was investigated. Quantum dots (QDs), for example, share a similar size scale with proteins and can be physically trapped within the microgels. In addition, the optoelectronic properties of QDs could be utilized for analytical, imaging, and therapeutic purposes. Small molecules and recognition sequences (e.g. biotin) can also be covalently immobilized within the microgel networks through the photodimerization reaction. In the presence of biotin-PEG-NC, the resulting microgels added to streptavidin-coated plates. The microgel properties such as biodegradability and degree of swelling may be engineered for particular applications including targeted monitoring and controlled drug delivery systems.     

Lie-optics, geometrical phase and nonlinear dynamics of self-focusing and soliton evolution in a plasma

It is useful to state propagation laws for a self-focusing laser beam or a soliton in group-theoretical form to be called Lie-optical form for being able to predict self-focusing dynamics conveniently and amongst other things, the geometrical phase. It is shown that the propagation of the gaussian laser beam is governed by a rotation group in a non-absorbing medium and by the Lorentz group in an absorbing medium if the additional symmetry of paraxial propagation is imposed on the laser beam. This latter symmetry, however, needs care in its implementation because the electromagnetic wave of the laser sees a different refractive index profile than the laboratory observer in this approximation. It is explained how to estimate this non-Taylor paraxial power series approximation. The group theoretical laws so-stated are used to predict the geometrical or Berry phase of the laser beam by a technique developed by one of us elsewhere. The group-theoretical Lie-optic (or ABCD) laws are also useful in predicting the laser behavior in a more complex optical arrangement like in a laser cavity etc. The nonlinear dynamical consequences of these laws for long distance (or time) predictions are also dealt with. Ergodic dynamics of an ensemble of laser beams on the torus during absorptionless self-focusing is discussed in this context. From the point of view of new physics concepts, we introduce a stroboscopic invariant torus and a stroboscopic generating function in classical mechanics that is useful for long-distance predictions of absorptionless self-focusing.     

Generation of an arbitrary chirped microwave waveform with high time-bandwidth product for increasing range resolution of RADAR by using photonic technique

An approach for photonic generation of an arbitrary chirped microwave waveform with an increased time-bandwidth product (TBWP) is proposed and experimentally demonstrated. In the proposed model, light from the mode locked laser is splitted into two parts by using 1 × 2 power splitter: one is sent to linearly chirped fiber Bragg grating (LCFBG) through circulator and the other is time delayed by fiber delay line. The optical pulse in upper arm is time stretched by the LCFBG. Meanwhile, the optical pulse in lower arm experiences a time delay and then stretched by the dispersive single mode fiber. Temporal interference pattern is generated with an increasing or decreasing free spectral range by combination of two time-stretched optical pulses. Finally, the temporal interference pattern which is obtained at the output of optical coupler is transformed into an arbitrary chirped microwave waveform by using a photo-detector. The main advantage of this proposed model is high TBWP in the range of 750–1000 which ultimately results in an increased range resolution of radio detection and ranging.     

Modified phonon confinement model for size dependent Raman shift and linewidth of silicon nanocrystals

A modified phonon confinement model considering the size distribution, an improved phonon dispersion curve and a confinement function is developed for the calculation of size dependent Raman spectra of the silicon (Si) nanocrystals. The model is capable of simultaneous calculations of the Raman shift, intensity and linewidth. The calculated size dependent redshift and linewidth of Raman spectra are in good agreement with the available experimental data in literature and better than previously reported theoretical results. The rapid rise in the redshift and linewidth for relatively smaller Si nanocrystals are well reproduced. The asymmetric behavior of Raman spectra is also obtained from the present model.     

Water-soluble xanthobilirubinic acids?

Abstract  Xanthobilirubinic acid, a model dipyrrinone for one-half of the bilirubin molecule in photochemical and metabolism studies, is more polar than bilirubin and insoluble in water and in chloroform. Replacing the β-alkyl substituents on the lactam ring of xanthobilirubinic acid with methyl-capped ethylene glycol, diethylene glycol, and triethylene glycol (PEG) groups steadily increased the water solubility of the pigment so that the last is completely soluble in both water and chloroform. Synthesized by base-catalyzed condensation of the corresponding methyl-capped 3,4-diPEG-pyrrolin-2-one with 3,5-dimethyl-4(2-ethoxycarbonylethyl)-2-formyl-1H-pyrrole, these new PEGylated analogs of xanthobilirubinic acid are yellow-colored dipyrrinones that form intermolecular hydrogen-bonded dimers in chloroform solution but are monomeric in methanol and water, as revealed by ¹H NMR spectroscopy and vapor pressure osmometry. Methyl xanthobilirubinate has served as a synthetic precursor to bilirubinoids; its amphiphilic PEGylated analogs suggest a route to water-soluble bilirubinoids and biliverdinoids. Graphical abstract        

Effect of proton irradiation on electrical properties of a-As2S3

This paper reports the effect of proton irradiation on the electrical properties of a-As₂S₃ in the temperature range of 323–418 K and frequency range 0.1–100 kHz. The variation of transport property is studied with proton irradiation dose (1 × 10¹³ ions/cm² and 1 × 10¹⁵ ions/cm²). It has been observed that proton irradiation changes the dc conductivity (σ_dc), dc activation energy (ΔE_dc) and ac conductivity (σ_ac(ω)). The σ_dc and σ_ac(ω) increases with dose of proton irradiation. The value of frequency exponent (s) decreases with the temperature and irradiation dose. These results are explained in terms of change in density of defect states in these glasses.     

XAS and XMCD studies of amorphous FeCo-based ribbons

This paper reports x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD) studies at Fe L_3,2 and Co L_3,2-edges to investigate the electronic structure of (Fe_100 ? xCo_x)₇₈Si₉Nb₃B₉Cu₁ (0 < x < 95) alloys. The influence of controlled Co addition on electronic structural and magnetic properties of (Fe_100 ? xCo_x)₇₈Si₉Nb₃B₉Cu₁ (x = 0, 20, 40, and 60) alloys has been investigated and it has been observed that Co exists as Co²⁺/Co³⁺, while Fe exists as a mixture of Fe⁰ (metallic) and Fe²⁺. The XMCD studies confirm these results and reveal that Co-ions are responsible for the room temperature ferromagnetism (RTFM) in the system, while at Fe L_3,2-edge it shows a diamagnetic behavior.     

Effect of electrolytes on the cloud point of chlorpromazine hydrochloride solutions

Cloud point (CP) phenomenon occurring in amphiphilic drug chlorpromazine hydrochloride (CPZ) solutions with and without salts is reported herein. The CP of a 50mM CPZ solution (prepared in 10mM sodium phosphate, SP, buffer) was found to decrease with increasing pH, both in the absence as well as presence (50mM) of added salts (NaCl, NaBr, LiBr, KBr, tetra-n-butylammonium bromide). Whereas, at a fixed concentration of NaCl, the CP increased with increasing CPZ concentration, addition of increasing amounts of salts (NaF, NaCl, NaBr, LiCl, KCl) to 50mM CPZ solution (at pH 6.7) caused continuous increase in CP. On the basis of these studies the binding-effect orders of counterions and co-ions have been deduced, respectively, as: Br(-)>Cl(-)>F(-) and Li(+)>Na(+)>K(+). The similar trend of increasing CP with addition of increasing amounts of quaternary bromides (tetramethylammonium bromide, TMeAB; tetraethylammonium bromide, TEtAB; tetra-n-propylammonium bromide, TPrAB; tetra-n-butylammonium bromide, TBuAB; tetra-n-pentylammonium bromide, TPeAB) to 50mM CPZ solutions (at pH 6.7) was found to be dependent upon the alkyl chain length of the particular salt. The overall behaviour has been discussed in terms of electrostatic interactions, micellar growth, and mixed micelle formation.