S.K. De

Characterization and dielectric properties of polyaniline–TiO2 nanocomposites

Nanocomposites of polyaniline (PANI)–titanium dioxide (PANI–TiO2) are prepared from a colloidal sol of TiO2 nanoparticles. The dc and ac conductivities of samples with different concentrations of PANI have been investigated as a function of frequency and temperature. The dc conductivity follows three-dimensional variable range hopping. The ac conductivity has been interpreted as a power law of frequency. The temperature variation of the frequency exponent suggests a correlated barrier hopping conduction process in the nanocomposites. A very large dielectric constant of about 3700 at room temperature has been observed. An electric modulus presentation is used to interpret the dielectric spectra. The interface between polyaniline and TiO2 plays an important role in yielding a large dielectric constant in the nanocomposite.

Hopping transport in HCl doped conducting polyaniline

Abstract The present work includes a study on the transport property based on the measurement of the electrical conductivity of HCl doped conducting polyaniline in the presence as well as in the absence of a magnetic field in the temperature range 1.8 K≤ T ≤300 K. It has been observed that the conductivity of the samples increases with increasing temperature and the conductivity ratio [ r = σ (300 K)/ σ (1.8 K)] is large for the sample having higher dopant concentration. A crossover from Mott ( T −1/4 ) to Efros–Shklovskii (ES) ( T −1/2 ) in temperature dependent conductivity of the samples is observed at 10 K. In the presence of a magnetic field the electrical conductivity varies as T −3/4 . From the plot of conductivity versus temperature, different physical quantities like density of states, molecular vibrational frequency, hopping distance and localization length have been determined.

Bright white light emitting Eu and Tb co-doped monodisperse In2O3 nanocrystals

Bright light emitting monodisperse In2−(x+y)EuxTbyO3 (x = 0.05, 0.1, 0.15, 0.2 and y = 0.05, 0.1, 0.15) nanocrystals were successfully synthesized by a versatile hot-injection colloidal route. The X-ray diffraction and X-ray photoelectron spectroscopy studies clearly showed that Eu and Tb were incorporated into the In2O3 lattice. Surface adsorbed NO3− and Cl− anions of Eu3+ and Tb3+ precursors assist the formation of flower like morphology through the oriented attachment process. Optical absorption spectra have been interpreted in terms of 4f → 5d (Eu, Tb) and 2p(O) → 5d (Eu, Tb) interband transitions. The observation of characteristic emission peaks related to Eu3+ and Tb3+ by indirect excitation suggests that In2O3 is an efficient sensitizer. Proper tuning of the compositions, 15% of Eu and 10% of Tb, gives the most intense white light and excellent chromaticity co-ordinates (0.35, 0.35). The shortening of lifetime with Eu and Tb doping improves the energy transfer from the host In2O3 to dopant Eu3+ and Tb3+ ions.

Electronic structure and optical properties of neodymium monopnictides

Abstract The electronic structure and optical properties of neodymium monopnictides have been investigated. The linear muffin-tin orbital method in the atomic sphere approximation and the local density approximation to exchange and correlation in density functional theory has been used. The p–f mixing increases while f-band width decreases on going down the pnictogen column due to the increase in pnictogen size. The density of states at the Fermi energy increases on going from NdP to NdSb. The joint density of states and partial oscillator strength have been calculated. The optical conductivity at higher energy increases with the increase of pnictogen atomic number due to the increase in p→d and f→d oscillator strengths.

Low temperature transport properties of thin disordered vanadium oxide films

Low temperature magnetoresistance measurements of vanadium oxide films have been performed. Both the resistance and magnetoresistance are well explained by weak electron localization and electron-electron interaction effects. The magnetoresistance due to weak electron localization is used to investigate inelastic and spin-orbit scattering times. Spin-orbit scattering time is independent of temperature. The temperature dependence of inelastic scattering time is compared with the theoretical values. In the temperature range 1.5 K≤T≤5 K, the inelastic scattering occurs due to only electron-electron interaction.

Structural features of the La-Sr-Fe-Co-O system

Abstract:A structural study has been performed on the La0.8Sr0.2FexCo1-xO3 (x = 0.025 to 0.3) system displaying large magnetoresistance (MR) at room temperature. A detailed analysis of the crystal structure and microstructure was done by X-ray diffraction (XRD), transmission and scanning electron microscopy (TEM and SEM). The atomic resolution TEM images and the appearing superreflections in the corresponding SAED patterns revealed that a superstructure is formed due to the presence of iron. The correlation between the ordered microstructure and the observed large MR ratio is discussed. 57Fe Mössbauer spectroscopy was utilized to gain information on the valence state of iron in the sample with x = 0.3. The lattice parameters of Fe- doped La0.8Sr0.2FexCo1-xO3 compounds were found to increase monotonously with increasing Fe content. The valence state of iron was found to be Fe3+.

Magnetic scattering of Co on the surface of Al thin films

Abstract The magnetic scattering of Co on the surface of Al has been extensively investigated by weak localization method. The anomalous sharp decrease of magnetic scattering with lowering temperature is observed. This behaviour of magnetic scattering cannot be explained by the spin-glass, kondo and crystal field effects. The observed surface magnetic impurities behave quite differently from the bulk impurities in metals.

Structural stability and optical properties of nanocrystalline zirconia

Nanoparticles of the cubic phase of zirconia (ZrO2) of size range 4.5–8.7 nm have been synthesized by alkaline hydrolysis of a zirconium salt followed by solvothermal reaction. Subsequently, the room-temperature stability of cubic ZrO2 nanoparticles has been explored with the goal of understanding how crystal structure tends to transform into a structure of higher symmetry with decreasing crystallite size. The room-temperature-stable cubic phase in nanosized ZrO2 has been observed to transform into the monoclinic phase at 873 K. The crystalline phases of ZrO2 nanocrystals have been determined quantitatively by Rietveld refinement. Lattice constant and internal strain increase with decreasing particle size. Growth kinetics studies have established that cubic nanocrystals are more stable at smaller crystallite sizes. UV–visible absorption spectra show an absorption peak at 275 nm which indicates a lowering of the band gap energy. Photoluminescence spectra of zirconia nanoparticles show an emission peak at 305 nm at room temperature. The modification of the optical properties is explained on the basis of oxygen vacancies present within the samples.

High dielectric permittivity observed in Na and Al doped NiO

Dielectric properties of Na and Al doped NiO (NANO) ceramics were studied as a function of frequency and temperature. Microstructures of polycrystalline samples reveal the grain size of 100?200?nm. The grain conductivity is higher than the grain boundary region. The concentration of Al has an excellent effect on the dielectric properties of NANO ceramics. The sample with Al 2% has the highest dielectric constant of the four samples. The electric modulus spectrum indicates the absence of the electrode effect in the dielectric constant. The high dielectric constant of the NANO ceramic systems is due to the formation of an insulating barrier layer across the grain boundary.

Impedance and dielectric spectra in zirconia?polypyrrole hybrid nanocomposites

Zirconia (ZrO2) nanoparticles have been synthesized by a novel two-reverse emulsion technique and combined with polypyrrole (PPY) to form ZrO2?PPY nanocomposites. The complex impedance and dielectric permittivity of ZrO2?PPY nanocomposite have been investigated as a function of frequency and temperature for different compositions. The optical band gap of zirconia decreases with increase in polymer concentration. Two electrical responses in the impedance spectra corresponding to the grain and grain boundary effects are analysed by an equivalent circuit. Activation energies are very small (35?70?meV). The samples exhibit a large dielectric constant and at room temperature it becomes about 12?000 at 0.7?KHz for the highest content of zirconia. The high dielectric response is mainly originated by Maxwell?Wagner polarization.