Preparation and characterization of silver-poly(vinylidene fluoride) nanocomposites: formation of piezoelectric polymorph of poly(vinylidene fluoride)

In situ Ag nanoparticles are produced on reduction of Ag+ with N,N-dimethylformamide in the presence of poly(vinylidene fluoride) (PVF2). The plasmon band transition is monitored with time in the reaction mixture for three sets of experiments by UV-vis spectroscopy. The plasmon band absorbance increases sigmoidally with log(time). Analysis of the data with the Avrami equation yields an exponent n value between 1.5 and 2.0, indicating two-dimensional nucleation with linear or diffusion controlled growth. The TEM study of the polymer nanocomposites (PNC) indicates both spherical and rodlike morphology for PNC0.5 and PNC2.5 samples, whereas the PNC11 sample has spherical and agglomerated structures (the numerical number associated with PNC indicates percentage (w/w) of Ag in the nanocomposite). The WAXS and FTIR studies indicate the formation of piezoelectric beta-polymorphic PVF2 in the nanocomposites. The DSC study indicates some increase of the melting point and enthalpy of fusion of PVF2 in the nanocomposite, although with increase in Ag nanoparticle concentration the increase is smaller. The crystallization temperatures of PNCs also increased, indicating nucleating effect of Ag nanoparticles in the composite. In the TGA curves, the PNCs exhibit a three-step degradation process. The degradation temperatures of PNCs are lower than that of PVF2. The storage modulus data indicate a significant reinforcement of the mechanical property in the PNCs where also the reinforcement effect decreases with increasing nanoparticle concentration. Both the loss modulus and tan delta plots indicate two peaks; the lower temperature peak has been attributed for glass transition temperature, whereas the higher one has been attributed to a similar type relaxation process for the crystalline-amorphous interface. The increase in the glass transition is marginal for the PNCs, but the increase of later transition temperature is somewhat higher. The FTIR study shows that the dipolar interaction of the >CF2 dipole with the surface charges of Ag nanoparticle stabilizes the nanoparticle in the nanocomposite.     

Electrical bistability and memory switching phenomenon in Cu2FeSnS4 thin films: role of p-n junction

Journal of Solid State Electrochemistry - Electrical bistability of Cu2FeSnS4 (CFTS) thin films fabricated via successive ionic layer adsorption and reaction (SILAR) method was studied here. The...     

Human tryptophan dioxygenase: a comparison to indoleamine 2,3-dioxygenase

In contrast to the diverse superfamily of monooxygenases, there are only two classes of heme-containing dioxygenases in humans. One is tryptophan 2,3 dioxygenase (hTDO), and the other is indoleamine 2,3-dioxygenase (hIDO), both of which catalyze the oxidative degradation of Trp to N-formyl kynurenine. Although hTDO and hIDO catalyze the same reaction, they engage in distinct physiological functions. The molecular properties of hTDO, unlike hIDO, have never been explored in the past. Here, we report the first structural and functional characterization of hTDO with resonance Raman and optical absorption spectroscopies. We show that the proximal Fe-His stretching frequency of hTDO is 229 cm(-1), 7 cm(-1) lower than that of hIDO, indicating its weaker imidazolate character as compared to hIDO. In the CO derivative of the L-Trp-bound enzyme, the Fe-CO stretching and C-O stretching frequencies are 488 and 1972 cm(-1), respectively, suggesting that L-Trp binds to the distal pocket with its C2-C3 double bond facing the heme-bound ligand, in contrast to hIDO, in which the indole NH group forms an H-bond with the heme-bound ligand. Moreover, the Km values of hTDO for D-Trp and L-Trp are similar, but the kcat value for D-Trp is 10-fold lower than that for L-Trp. In contrast, in hIDO, the Km value for D-Trp is 700-fold higher than L-Trp, whereas the kcat values are comparable for the two stereoisomers. Taken together, the data indicate that the initial deprotonation reaction of the indole NH group in hTDO is carried out by the evolutionarily conserved distal His, whereas that in hIDO is performed by the heme-bound dioxygen; in addition, the stereospecificity of hTDO is determined by the efficiency of the dioxygen chemistry, whereas that in hIDO is controlled by the substrate affinity.     

Measurement of στ product of solid state laser materials by an alternative method: Application to Nd3+ doped YVO4 crystal for 4 F 3/2→4 I 11/2 transition

In this paper an alternative approach for measurement of στ product for ⁴ F _3/2→⁴ I _11/2 transition of Nd³⁺ doped YVO₄ crystal is reported. In this method a microchip laser is formed by keeping a small piece of the sample in plane-plane resonator and a diode laser (808 nm) is used for pumping. The pump power induced thermal lensing effect is used to make the cavity stable. The cavity mode area is estimated by measuring the thermal lens focal length at the threshold and the average pump area is measured by Gaussian fit to the intensity profiles of the pump beam. The value of στ product of Nd:YVO₄ crystal obtained by this method is within 10% of the reported values. The advantage of this method is that it is a simple method for direct measurement of στ product of laser crystals.     

Dielectric relaxation phenomena of rigid polar liquid molecules under giga hertz electric field

The dielectric relaxation phenomena of rigid polar liquid molecules chloral and ethyltrichloroacetate (j) in benzene, n-hexane and n-heptane (i) under 4.2, 9.8 and 24.6 GHz electric fields at 30°C are studied to show the possible existence of double relaxation times τ ₂ and τ ₁ for rotations of the whole and the flexible parts of molecules. The probability of showing double relaxation is more in aliphatic solvents indicating their nonrigidity. The symmetric and asymmetric distribution parameters γ and δ are obtained from X _ij ^′ /X _0ij and X _ij ^″ /X _0ij and w _j →0 where X _ij ^′ and X _ij ^″ are real and imaginary parts of the complex orientational susceptibility X _i ^* and X _0ij is the low frequency susceptibility which is real. X _ij ’s are involved with the measured dielectric relative permittivities ε _ij ^′ , ε _in ^″ , ε _0ij and ε _∞ij of solutions. The theoretical weighted contributions c ₁ and c ₂ towards dielectric dispersions by Fröhlich’s method are compared with the experimental ones obtained from the graphical variation of X _ij ^′ /X _0ij and X _ij ^″ /X _0ij with weight fractions w _j ’s at w _j → 0. The measured dipole moments μ ₂ and μ ₁ of the whole and the flexible part of a polar molecule in terms of the linear coefficients β’s of X _ij ^′ ’s with w _j ’s and the estimated τ ₂ and τ ₁ reveal their associations with aliphatic solvents. The theoretical dipole moments μ _theo’s from the available bond angles and bond moments of the substituent polar groups of the molecules with the estimated μ’s suggest the mesomeric, inductive and electromeric effects in them under GHz electric field.     

Morphological evolution of selenium micro‐crystals

A simple solution phase approach using dimethyl formamide (DMF) as solvent is utilized to obtain selenium (Se) microcrystals. Different morphological modifications of the products result from varied reaction environments. Solvothermal treatment of Se with DMF in an autoclave at 170°C yields rod‐like, and feather‐shaped microcrystalline structures while reduction of selenium dioxaide (SeO₂) by DMF generates microspheres at 30°C and 80°C, rods admixed with spherical particles at 120°C and predominantly micro rods at 150°C. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Study of hydrogenated amorphous silicon nitride films prepared by RF magnetron sputtering

Hydrogenated amorphous silicon nitride films have been deposited by the rf magnetron sputtering method with non-stoichiometric and stoichiometric compositions using a poly-Si target and a mixture of Ar, H₂ and N₂ as the sputtering gas. Data on optical and infrared absorption, electrical conductivity, breakdown voltage, capacitance measurements and thermal evolution of hydrogen have been presented as a function of nitrogen concentration in the films, especially in the stoichiometric region of composition. Attempts have been made to identify the roles of hydrogen and nitrogen in determining the electrical and optical properties and thermal stability exhibited by the films. Properties relevant for device application of the material have been shown to be comparable to those obtained by glow discharge or electron cyclotron resonance plasma chemical vapour deposition methods of deposition. RF magnetron sputtering has therefore been suggested as a viable alternative to the more widely adopted CVD methods for device applications of silicon nitride, where the use of hazardous process gases can be avoided.