Electro-optical response of the combination of two twisted nematic liquid crystal cells in series and the applicability of the extended Jones matrix

A typical electro-optical response is observed by connecting two twisted nematic liquid crystal cells in series and placed back to back such that the director orientation on the back glass substrate of the first cell coincides with the director orientation of the front glass substrate of the second cell and vice versa. The two cells connected in series show cholesteric rotation. It is observed that for a given frequency the output transmittance oscillates having distinct maxima and minima with varying voltage and no flickering of transmittance have been observed at low frequencies. Variation of transmittance with frequency for given voltages is also reported and it has been found that increase of voltage at high frequency does not alter the transmittance appreciably. A discussion on the extended Jones matrix formulation and the scope of its applicability for this case is also given

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 300K <= T <= 390K 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 x 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 capacitance-voltage characteristics is done to extract the Schottky barrier heights, ideality parameters, built in voltage, and charge density. With +/- 40V sweep the capacitance versus voltage characteristics of the sample shows hysteresis behavior which may be attributed to the presence of deep traps. (C) 2013 American Institute of Physics. [http://dx.doi.org/10.1063/1.4796106

Comparative study on the freezing temperature of synthesised CdSe nanoparticles before and after annealing

Thiyoglycolic acid-capped CdSe nanoparticles have been synthesised at 273-279 K. A comparative study on the structural and optical characteristics of the as-synthesised and annealed samples is done using state-of-the art instruments and software. Based on these studies here the authors report a colossal change in crystallinity, particle size and red shift in the absorption spectra upon annealing. Lattice strain is also found to increase by 18.77% upon annealing. Excitation wavelength-dependent photoluminescence with giant Stokes shift and narrow full-width at half-maxima of both the samples is also reported, which may be useful for device application

Electrical transport properties of polyvinyl alcohol-selenium nanocomposite films at and above room temperature

Here, we report the DC and AC electrical properties of polyvinyl alcohol (PVA)-selenium (Se) nanocomposite films in the temperature (T) range 298 K a parts per thousand currency sign T a parts per thousand currency sign 420 K and in the frequency (f) range 120 Hz a parts per thousand currency sign f a parts per thousand currency sign 1 MHz. The introduction of selenium nanoparticles into the PVA matrix slightly increases the values of DC conductivity whose temperature dependency obeys Vogel-Fulcher-Tammann law. The AC conductivity follows a power law with frequency in which the temperature dependence of the frequency exponent suggests that the correlated barrier hopping is the dominant charge transport mechanism for the nanocomposite films. Comparative discussions with Dyre's random free-energy barrier model have also been made in this regard. The increase in AC conductivity with increase in nanoparticles concentration was also observed and attributed to the corresponding increase in conducting channels in the PVA matrix. The real part of the dielectric constant increases either with increase in temperature or with increase in selenium nanoparticles loading into the polymer matrix, which may be attributed to the enhancement of interfacial polarization. The frequency dispersion of the dielectric spectra has been modeled according to the modified Cole-Cole equation. Well-defined peaks were appeared in the plotting of imaginary part of electric modulus with frequency above room temperature, which was fitted with suitable equations to account for the deviations from ideal Debye-type behavior. Though the current-voltage characteristics are linear at smaller voltages, it appreciably becomes nonlinear at higher voltages. This nonlinearity has been accounted in light of Werner's model and back to back Schottky diode model

Analysis of the Dielectric Relaxation and AC Conductivity Behavior of Polyvinyl Alcohol-Cadmium Selenide Nanocomposite Films

We adopt an analytical approach to study the dielectric relaxation and ac conductivity behavior of the polyvinyl alcohol (PVA) - Cadmium Selenide (CdSe) nanocomposite films in the frequency range 100 Hz <= f <= 1 MHz and in the temperature range 298 K <= T <= 420 K. The CdSe nanoparticles are synthesized via a simple wet chemical route and are then impregnated into the polyvinyl alcohol matrix by 0.5 wt%, 1 wt%, and 4 wt%. About 1.7 times increase in effective permittivity (at 100 KHz) for the 4 wt% nanoparticle impregnated sample is observed from the analysis of the dielectric reinforcement function, which is attributed to the occurrence of interfacial polarization. It is found that the Cole- Cole model with a dc conductivity correction term can well fit the permittivity data. We have found that the dielectric relaxation time increases with increase in nanoinclusion into the matrix but decreases with increase in temperature. However, the space charge carrier conductivity and free charge carrier conductivity increase either with increase in nano inclusions or with increase in temperature. AC conductivity behavior have been analyzed considering the Jonscher power law and Dyre's random free energy barrier model, which reveals lowering of the maximum barrier height and correlated barrier hopping as the prevalent charge transport mechanism for the nanocomposites. Electric modulus study indicates similar conductivity relaxation dynamics for the nanocomposites but the responses deviate appreciably from the simulated ideal Debye responses. (C) 2015 Society of Plastics Engineer

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 (298K <= 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 (298K <= T <= 460 K) and frequency (80 Hz <= f <= 2MHz) 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 regard. (C) 2013 American Institute of Physics. [http://dx.doi.org/10.1063/1.4794019

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 298K <= T <= 420K 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 dot's 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

Anomalous electrical transport properties of CdSe quantum dots at and below room temperature

Here we report the dielectric relaxation and anomalous ac conduction behavior of CdSe quantum dots in the temperature (T) range of 77 K <= T <= 300 K and in the frequency (f) range of 100 Hz <= f <= 1 MHz. Interfacial polarization exhibits strong temperature dependence only above 250 K and dielectric loss spectrum undergoes a shallow minimum. Superlinear power law behavior of the ac conductivity was observed above certain frequencies, along with the universal dielectric response at comparatively lower frequencies. Extraction of the temperature dependent dc contribution, hopping frequencies, frequency exponents and activation energies values were done by theoretical fitting and was observed that for T=77-200 K, classical hopping over barrier (HOB) is the prevailing charge transport mechanism whereas for T=250-300 K, the charge transport mechanism is correlated barrier hopping (CBH). Analysis of the Nyquist plotting and non-Debye electric modulus spectra have also been conferred in this context. (C) 2013 Elsevier B.V. All rights reserved

Electrical transport properties of consolidated ZnSe quantum dots at and above room temperature

Here we report a comprehensive study on the prevailing conduction mechanism and dielectric relaxation behavior of consolidated Zinc Selenide quantum dots in the frequency range of 1 kHz <= f <= 1.5 MHz and in the temperature range of 298K < T < 573 K. The ac conductivity increases either with increase in temperature or with increase in frequency, which is explained by the Jonscher Power law. At higher temperatures, correlated barrier hopping is found to be the prevalent charge transport mechanism with a maximum barrier height of 0.88 eV. The dielectric constant of the sample is found to exhibit weak temperature dependence. DC conductivity study reveals the semiconducting nature of the sample and it is discussed in the light of polaron hopping conduction. From the impedance spectroscopic study, role of the grains and grain boundaries in the overall electrical transport properties have been elucidated by considering an electrical equivalent circuit (composed of resistances and constant phase elements). Electric modulus study reveals non-Debye responses of the sample in the experimental range. (C) 2015 Elsevier B.V. All rights reserved

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 298K <= T <= 420K 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 dot's 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