Subhojyoti Sinha

Dielectric relaxation and ac conductivity behaviour of polyvinyl alcohol–HgSe quantum dot hybrid films

Here we report a comparative study on the dielectric relaxation and ac conductivity behaviour of pure polyvinyl alcohol (PVA) and PVA?mercury selenide (HgSe) quantum dot hybrid films in the temperature range 298?K???T???420?K and in the frequency range 100?Hz???f???1?MHz. The prepared nanocomposite exhibits a larger dielectric constant as compared to the pure PVA. The real and imaginary parts of the dielectric constants were found to fit appreciably with the modified Cole?Cole equation, from which temperature-dependent values of the relaxation times, free charge carrier conductivity and space charge carrier conductivity were calculated. The relaxation time decreases with the quantum dots inclusion in the PVA matrix and with an increase in temperature, whereas free charge carrier conductivity and space charge carrier conductivity increases with an increase in temperature. An increase in ac conductivity for the nanocomposites has also been observed, while the charge transport mechanism was found to follow the correlated barrier hopping model in both cases. An easy-path model with a suitable electrical equivalent circuit has been employed to analyse the temperature-dependent impedance spectra. The imaginary part of the complex electric modulus spectra exhibit an asymmetric nature and a non-Debye type of behaviour, which has been elucidated considering a generalized susceptibility function. The electric modulus spectra of the nanocomposite demonstrate a smaller amplitude and broader width, as compared to the pure PVA sample.

Semiconducting selenium nanoparticles: Structural, electrical characterization, and formation of a back-to-back Schottky diode device

Well crystalline selenium nanoparticles having an optical band gap of 2.95 eV have been synthesized using oxalic acid. Microstructural parameters such as crystallite size, lattice strain, cell parameters, and unit cell volume are estimated from X-ray diffraction line profile analysis by Rietveld refinement technique. dc and ac transport properties of the nanoparticles in the temperature range 300 K ≤ T ≤ 390 K and frequency range 20 Hz ≤ f ≤ 2 MHz have also been studied. The values of dc activation energies in the low and high temperature regions are found to be 0.083 eV and 0.382 eV, respectively. The charge transport mechanism of the sample follows correlated barrier hopping (CBH) model and the calculated value of barrier height and relaxation time is 0.786 eV and 2.023 × 10−11 s, respectively, while grain boundary contribution being greater than the grain contribution. Considering metal electrode-semiconductor contact as a back-to-back Schottky diode device, analysis of the current-voltage and capacita...

Structural characterization and observation of variable range hopping conduction mechanism at high temperature in CdSe quantum dot solids

We have used Rietveld refinement technique to extract the microstructural parameters of thioglycolic acid capped CdSe quantum dots. The quantum dot formation and its efficient capping are further confirmed by HR-TEM, UV-visible and FT-IR spectroscopy. Comparative study of the variation of dc conductivity with temperature (298 K ≤ T ≤ 460 K) is given considering Arrhenius formalism, small polaron hopping and Schnakenberg model. We observe that only Schnakenberg model provides good fit to the non-linear region of the variation of dc conductivity with temperature. Experimental variation of ac conductivity and dielectric parameters with temperature (298 K ≤ T ≤ 460 K) and frequency (80 Hz ≤ f ≤ 2 MHz) are discussed in the light of hopping theory and quantum confinement effect. We have elucidated the observed non-linearity in the I-V curves (measured within ±50 V), at dark and at ambient light, in view of tunneling mechanism. Tunnel exponents and non-linearity weight factors have also been evaluated in this re...

Linear mean free path and quadratic temperature dependence of electron-phonon scattering rate in V82Al18-xFex alloys at low temperature

We have reported a comprehensive study on temperature and disorder dependence of inelastic electron dephasing scattering rate in disordered V82Al18-xFex alloys. The dephasing scattering time has been measured by analysis of low field magnetoresistance using the weak localization theory. In absence of magnetic field the variation of low temperature resistivity rise follows the relation Δρ(T)∝−ρ05/2T, which is well described by three-dimensional electron-electron interactions. The temperature-independent dephasing rate strongly depends on disorder and follows the relation τ0−1∝le, where le is the electron elastic mean free path. The inelastic electron-phonon scattering rate obeying the anomalous relation τe−ph−1∝T2le. This anomalous behavior of τe−ph−1 cannot be explained in terms of current theories for electron-phonon scattering in impure dirty conductors.

AC electrical transport properties and current-voltage hysteresis behavior of PVA-CNT nanocomposite film

Polyvinyl alcohol (PVA) – Carbon nanotube (CNT) composite has been prepared and its electric modulus, ac conductivity, impedance spectroscopy and current-voltage characteristics have been studied, at and above room temperature, to understand the prevailing charge transport mechanism. Non-Debye type relaxation behavior was observed with activation energy of 1.27 eV whereas correlated barrier hopping was found to be the dominant charge transport mechanism with maximum barrier height of 48.7 meV above room temperature. The sample, under ±80 V applied voltage, exhibits hysteresis behavior in its current - voltage characteristics.

Synthesis and characterization of TbMnO 3 nanorods

We report the synthesis and characterization of multiferroic TbMnO3 nanorods by sol-gel route. From XRD analysis it was observed that annealing at 700°C for 2h is the most favorable condition for the fabrication of TbMnO3 nano rods. The TEM study revealed that the diameter of these rods varied from 12.6 – 38.1nm and the length of these nanorods varied from 88.6 – 292 nm. SAED pattern of the nano rods revealed single crystalline structure. Presence of TbMnO3 nano particles was also observed alongwith the nanorods. UV, PL, F.T.I.R study on the samples annealed at different calcination condition is also reported.

Electric modulus and impedance spectroscopy analysis on selenium nanoparticles at and above room temperature

Here we report the electric modulus and impedance spectroscopy study on selenium nanoparticles in the temperature (T) range of 298K ≤ T ≤ 364K and in the frequency (f) of range 20Hz ≤ f ≤ 2MHz. Variation of the imaginary part of the electric modulus (M”) with frequency yields well defined peaks (non–Debye type) which shift to higher frequency side with increasing temperature. From the response peaks of M” versus f graphs relaxation times (τ) are calculated which follows Arrhenius law with temperature with an activation energy of 0.27 eV. We observe that similar relaxation dynamics prevails at all temperatures with significant suppression of electrode effects. Role of grain and grain boundaries is also elucidated by the impedance spectroscopy analysis. We found that grain resistance varies from 0.07 MΩ to 0.48 MΩ whereas the grain boundary resistance varies in the range 0.09 MΩ to 0.82 MΩ. Both the grain and grain boundary resistances follow Arrhenius law with temperature having activation energies of 0.28...

Preparation and introduction of CdSe quantum dots in a 5CB twisted nematic liquid crystal cell: Observation of ordered array of nanostructures

Thioglycolic acid capped cadmium selenide (CdSe) quantum dots have been prepared by chemical synthesis method. HR-TEM and SAED characterization confirm the formation of CdSe quantum dots with approximate particle size of 3.8 nm which is also comparable to the crystallite size as determined from the XRD analysis. This II-VI semiconductor quantum dot is introduced in a positive dielectric anisotropic liquid crystal host and then injected into a twisted nematic liquid crystal cell where the dots agglomerates and sports ordered arrays. Effect of the application of dc voltage on the composite system is also observed with an optical polarizing microscope.

Introduction of BiFeO3 Nanoparticles in 5CB Nematic Liquid Crystal Host: Effect on Texture and the Non Responsive Behaviour of the Sample in a Twisted Nematic Configuration

Here we report the introduction of multiferroic BiFeO3 nanoparticles in a 5CB nematic liquid crystal host having positive dielectric anisotropy. The BiFeO3 nanoparticles had an average diameter of ~50 nm and were prepared by modified Pechini method. Its effect on the Schlieren Texture and the formation of the isolated stripe domains was observed under optical polarizing microscope. When the sample was poured into a 90° twisted nematic cell and observed under an optical polarizing microscope in cross polarizer configuration it yielded normally black state in contradiction to the expected normally white and remains unchanged even with the application of external voltage. The polarizing microscope photograph of the cell observed under parallel polarizer configuration suggests the accommodation of the nanoparticles along the cell wall grooves.

Effect of screening on the variation of transmittance with voltage and frequency for a twisted nematic liquid crystal cell

The transmittance (T) versus voltage (V) curve for a 90° twisted nematic liquid crystal cell with positive dielectric anisotropic liquid crystal sample is revisited to elucidate the ion charge effect or screening effect. It is shown explicitly why the application of ac voltage is more suitable to enhance the performance of the liquid crystal devices as compared to the dc voltage. The observed variation of transmittance with frequency shows an interesting “U” shaped behavior between 10Hz-300 kHz for a fixed ac voltage of 2.5 V (rms value).