Structural and AC conductivity study of CdTe nanomaterials

Cadmium telluride (CdTe) nanomaterials have been synthesized by soft chemical route using mercapto ethanol as a capping agent. Crystallization temperature of the sample is investigated using differential scanning calorimeter. X-ray diffraction and transmission electron microscope measurements show that the prepared sample belongs to cubic structure with the average particle size of 20 nm. Impedance spectroscopy is applied to investigate the dielectric relaxation of the sample in a temperature range from 313 to 593 K and in a frequency range from 42 Hz to 1.1 MHz. The complex impedance plane plot has been analyzed by an equivalent circuit consisting of two serially connected R-CPE units, each containing a resistance (R) and a constant phase element (CPE). Dielectric relaxation peaks are observed in the imaginary parts of the spectra. The frequency dependence of real and imaginary parts of dielectric permittivity is analyzed using modified Cole–Cole equation. The temperature dependence relaxation time is found to obey the Arrhenius law having activation energy ~0.704 eV. The frequency dependent conductivity spectra are found to follow the power law. The frequency dependence ac conductivity is analyzed by power law.     

Structure,dielectric and optical properties of p-type (PVA/CuI) nanocomposite polymer electrolyte for photovoltaic cells

A novel PVA/CuI nanocomposite polymer electrolyte layer synthesized via the reduction of CuCl₂ by NaI in an aqueous PVA solution. The as-prepared films were characterized by X-ray diffraction, scanning electron microscope, as well as impedance spectroscopy. The obtained results indicated the formation of hexagonal CuI nano particles of ≈55 nm sizes embedded in the PVA matrix. In addition, the study of dielectric parameters and conductivity of PVA/CuI nanocomposite in wide range of temperature and frequency are given and discussed. The frequency dependence of ac-conductivity suggests power law with an exponent 0.026 < s < 0.73 which predicts hopping of charge carriers. The bulk conductivity showed activation with temperature, significant values of activation energy are deduced and discussed. An average value of the energy gap width, 2.05 eV obtained using optical absorption in UV–visible spectra for PVA/CuI nanocomposite polymer electrolyte.     

Correlation between phosphorus concentration and opto-electrical properties of doped a-Si:H films

Phosphorus doped hydrogenated amorphous silicon (a-Si:H) films were prepared by decomposition of silane using RF plasma glow discharge. Both DC dark conductivity measurements, and spectrophotometric optical measurements through the range 200–3000 nm were recorded for the prepared films. The DC conductivity activation energy E_a decreased from 0.8 eV for the undoped sample to 0.34 eV for the highest used doping value. The optical energy gap E_g decreased ranging from 1.66 eV to 1.60 eV. The refractive index n, the density of charge carriers N/m* and the plasma frequency ω_p showed an opposite behavior, i.e. an increase in value with doping. Fitting the dispersion values to Sellmeier equation led to the determination of the material natural frequency of oscillating particles. A correlation between the changes in these parameters with the doping has been attempted.     

Effect of Al-doping on optical and electrical properties of spray pyrolytic nano-crystalline CdO thin films

CdO and Al-doped CdO nano-crystalline thin films have been prepared on glass at 300 °C substrate temperature by spray pyrolysis. The films are highly crystalline with grain size (18–32 nm) and found to be cubic structure with lattice constant averaged to 0.46877 nm. Al-doping increased the optical transmission of the film substantially. Direct band gap energy of CdO is 2.49 eV which decreased with increasing Al-doping. The refractive index and dielectric constant varies with photon energy and concentration of Al as well. The conductivity of un-doped CdO film shows metallic behavior at lower temperature region. This behavior dies out completely with doping of Al and exhibits semiconducting behavior for whole measured temperature range. Un-doped and Al-doped CdO is an n-type semiconductor having carrier concentration is of the order of ∼10²¹ cm⁻³, confirmed by Hall voltage and thermo-power measurements.     

Thermal,dielectric and conductivity studies on PVA/Ionic liquid [EMIM][EtSO4] based polymer electrolytes

Polymer electrolyte films of (PVA+15 wt% LiClO₄)+x wt% Ionic liquid (IL) 1-ethyl-3-methylimidazolium ethylsulfate [EMIM][EtSO₄] (x=0, 5, 10, 15) were prepared by solution cast technique. These films were characterized using TGA, DSC, XRD and ac impedance spectroscopic techniques. XRD result shows that amorphosity increases as the amount of the IL in PVA+salt (LiClO₄) is increased. DSC results confirm the same (except (PVA+15 wt% LiClO₄)+10 wt% IL). The dielectric and conductivity measurements were carried out on these films as a function of frequency and temperature. The addition of IL significantly improved the ionic conductivity of polymer electrolytes. Relaxation frequency vs. temperature plot for (PVA+15 wt% LiClO₄)+x wt% IL were found to follow an Arrhenius nature. The dielectric behavior was analyzed using real and imaginary parts of dielectric constant, dielectric loss tangent (tan δ) and electric modulus (M′ and M″).     

Ac and Dc conductivities of polyaniline/poly vinyl formal blend films

The synthesis and characterization of polyaniline (PANI)/poly vinyl formal (PVF) blend films were carried out in this work. Polyaniline base was doped using dodecylbenzene sulfonic acid (DBSA). These blend films were characterized by UV–Visible, FTIR spectra and scanning electron microscopy (SEM) to investigate their optical, structural and morphological properties. It was found that the percolation threshold of these blends is 4.4 wt% of PANI. The dc and ac conductivities of these blend films have been measured at a temperature range from 300 to 100 K in the frequency range of 10 kHz to 1 MHz. The electrical conductivity of the blend films enhanced with the increase of polyaniline amount up to a value of 2.5 × 10⁻⁴ S cm⁻¹ at 65 wt% of polyaniline. The dc conductivity of the PANI/PVF blend films follows the three-dimension variable range hopping. Temperature variation of frequency exponents in this blend suggests that ac conduction is attributed to be correlated barrier hopping.     

Dielectric relaxation of ZnO nanostructure synthesized by soft chemical method

Thioglycerol capped nanoparticles of ZnO have been prepared in methanol through chemical technique. Nanostructures of the prepared ZnO particles have been confirmed through X-ray diffraction measurement. The Debye–Scherrer formula is used to obtain the particle size. The average size of the prepared ZnO nanoparticles is found to be 50 nm. The frequency-dependent dielectric dispersion of the sample is investigated in the temperature range from 293 to 383 K and in a frequency range from 100 Hz to 1 MHz by impedance spectroscopy. An analysis of the complex permittivity (ε′ and ε′′) and loss tangent (tan δ) with frequency is performed assuming a distribution of relaxation times. The frequency-dependent maxima of the imaginary part of impedance are found to obey Arrhenius law with activation energy ∼1 eV. The scaling behavior of dielectric loss spectra suggests that the relaxation describes the same mechanism at various temperatures. The frequency-dependent electrical data are analyzed in the framework of conductivity and modulus formalisms. The frequency-dependent conductivity spectra obey the power law.     

Conductivity and dielectric relaxation phenomena in (NH4)2SO4 single crystal

AC impedance measurements have been carried out on (NH₄)₂SO₄ single crystals for the temperatures from 300 to 473 K and frequency range between 100 Hz and 4 MHz. The results reveal two distinct relaxation processes in the sample crystal. One is the dipolar relaxation with a peak at frequency slightly higher than 4 × 10⁶ Hz. The other is the charge carrier relaxation at lower frequencies. The frequency dependence of conductivity is described by the relation σ(ω) = Bωⁿ, and n = 1.32 is obtained at temperatures below 413 K. This value drops to 0.2 and then decreases slightly with increasing temperature. The dipolar response of the (NH₄)₂SO₄ single crystal under an ac field is attributed to the reorientation of dipoles. The contribution of charge carriers is increasing substantially with increasing temperature at temperatures above 413 K. The temperature variation of conductivity relaxation peaks follows the Arrhenius relation. The obtained activation energy for migration of the mobile ions for (NH₄)₂SO₄ single crystal was 1.24 eV in the temperature range between 433 and 468 K in this intrinsic region. It is proposed that the NH₄⁺ in the sample crystal has the contribution to the electrical conduction.     

Manganese doping influence on the plasmon energy of nickel films

Manganese doping in nickel films capped with copper have been prepared by evaporation in vacuum. The films are composed of grains with an average diameter of ~ 20 nm from scanning electron microscope scans. Optical absorption is measured over a wavelength range of 190–450 nm. Two plasmon peaks are observed at 3.30 eV and 4.45 eV for a range of concentrations of films. The 4.45 eV peak is a bulk plasmon peak that is enhanced by increasing the manganese in nickel. The 3.30 eV peak is a surface plasmon peak that increases in width or strength of plasmon resonance with increasing concentration of manganese. This may be a combination effect of charge carrier concentration and dielectric screening from the reformed electronic band structure caused by manganese doping. By adding manganese into nickel, the ferromagnetic order is further destroyed as a transition into a spin glass occurs. This spin glass behavior is seen in a coercivity measurement at 4 K where the coercivity drops precipitously as the doping concentration increases.     

Investigations on electrical properties of (PVA:NaF) polymer electrolytes for electrochemical cell applications

Solid polymer electrolytes based on poly (vinyl alcohol) (PVA) complexed with sodium fluoride (NaF) at different weight percent ratios were prepared using solution cast technique. The structural properties of these electrolyte films were examined by XRD studies. The XRD data revealed that the amorphous domains of PVA polymer matrix increased with increase of NaF salt concentration. The complexation of the salt with the polymer was confirmed by FT-IR studies. Electrical conductivity was measured in the temperature range of 303–373 K and the conductivity was found to increase with the increase of dopant concentration as well as temperature. The dielectric constant (ε′) increased with the increase in temperature and decreased with the increase in frequency. A loss peak was identified at 365 K in the dielectric loss spectra and is attributed to the orientation of polar groups. Measurement of transference number was carried out to investigate the nature of charge transport in these polymer electrolyte films using Wagner’s polarization technique and Watanabe technique. Transport number data showed that the charge transport in these polymer electrolyte systems was predominantly due to ions and in particular due to anions. Using these polymer electrolytes, solid state electrochemical cells were fabricated. Various cell parameters like open circuit voltage (OCV), short circuit current (SCC), power density and energy density were determined.     

Influence of Al doping on electrical properties of Ni–Cd nano ferrites

We have reported the structural and electrical properties of nano particles of Al doped Ni_0.2Cd_0.3Fe_2.5O₄ ferrite using X-ray diffraction, dielectric spectroscopy and impedance spectroscopy at room temperature. XRD analysis confirms that the system exhibits polycrystalline single phase cubic spinel structure. The average particle size estimated using Scherrer formula for Lorentzian peak (3 1 1), has been found 5(±) nm. The results obtained show that real (ε′), imaginary (ε″) part of the dielectric constant, loss tangent (tan δ), and ac conductivity (σ_ac) shows normal behaviour with frequency. The dielectric properties and ac conductivity in the samples have been explained on the basis of space charge polarization according to Maxwell–Wagner two-layer model and the Koop’s phenomenological theory. The impedance analysis shows that the value of grain boundary impedance increases with Al doping. The complex impedance spectra of nano particles of Al doped Ni–Cd ferrite have been analyzed and explained using the Cole–Cole expression.     

Dielectric and impedance studies of DBSA doped polyaniline/PVC composites

Polyaniline (PANI) was doped with dodecylbenzene sulfonic acid (DBSA) and then mixed with PVC by solution blending method to prepare DBSA doped PANI (PAND)/PVC composites. FTIR spectroscopy indicates the strong dipole–dipole interaction between the individual components of the composites. The ac electrical properties of the synthesized composites were investigated by complex impedance spectroscopy in the frequency range of 0.5–10⁶ Hz at room temperature. Both dielectric loss factor and permittivity increase with the decrease of frequency exhibiting strong interfacial polarization at low frequency. Addition of PAND in PVC reduces the charge trapping centers by increasing the number of conducting channels participating in the relaxation process; hence an increase in conductivity is observed.     

Dielectric relaxations in undoped,Ce-doped and Ce,Zr-codoped Lu3Al5O12 single crystals

Dielectric spectroscopy was performed on single crystals of pure, Ce-doped or Ce,Zr-codoped Lu₃Al₅O₁₂, before and after UV- or X-irradaiation, at various frequencies within the range 100 Hz–1 MHz as the temperature was scanned from 110 to 353 K. All samples previously subjected to ionising radiations gave spectra showing loss peaks with Arrhenius characteristics of permanent dipoles relaxation. We attribute the dipoles to defect-stabilised pairs of anion–anion vacancies (oxygen ions and oxygen vacancies) that have captured holes and photo-electrons separately, thus forming O^?- and F⁺-like centers. The dielectric relaxation peaks disappeared in undoped or doped samples annealed at 573 K, suggesting that charge carrier traps are relatively deep. UV–visible absorption spectra have also been measured, which tend to support our proposed interpretation. Further evidence for deep traps has come from thermally stimulated luminescence experiments.     

Studies of dielectric relaxation in natural fiber–polymer composites

Polymer composites of a polyester resin matrix filled with short palm tree lignocellulosic fibers were studies by means of dielectric spectroscopy in the frequency range 0, 1–100 kHz and temperature interval from 40 °C to 200 °C. Three relaxations processes were identified, namely the orientation polarization imputed to the presence of polar water molecules in Palm fiber, the relaxation process associated with conductivity occurring as a result of the carriers charges diffusion noted for high temperature above glass transition and low frequencies, and the interfacial relaxation that is attributable to the accumulation of charges at the Palm fibers/polyester interfaces.     

Structural and optical properties of chromotrope 2R thin films of different thicknesses

Chromotrope 2R (CHR) films of different thicknesses have been prepared using spin coater. The material has been characterized using FT-IR, DTA and X-ray diffraction. The XRD of the material in powder and thin film forms showed polycrystalline structure with triclinic phase. Preferred orientation at the (1 1 4) plane is observed for the deposited films. Initial indexing of the XRD pattern was performed using “Crystalfire” computer program. Miller indices, h k l, values for each diffraction line in X-ray diffraction (XRD) spectrum were calculated and indexed for the first time. The DTA thermograms of CHR powder have been recorded in the temperature range 25–350 °C with different heating rates. The spectra of the infra-red absorption allow characterization of vibration modes for the powder and thin film. The effect of film thickness on the optical properties has been studied in the UV-visible-NIR regions. The films show high transmittance exceeding 0.90 in the NIR region λ > 800 nm. The intensity of the absorption peaks for λ < 800 nm are enhanced as the film thickness increase. The absorption bands are attributed to the (π–π*) and (n–π*) molecular transitions. The optical properties have been analyzed according to the single-oscillator model and the dispersion energy parameters as well as the free charge carrier concentration have been determined. The optical energy gap as well as the oscillator strength and electric dipole strength have been calculated.     

Pulsed excimer laser ablation growth and characterization of Ba(Sn0.1Ti0.9)O3 thin films

Polycrystalline thin films of Ba(Sn_0.1Ti_0.9)O₃ were deposited on Pt coated silicon substrates by pulsed excimer laser ablation technique. The room temperature dielectric constant of the Ba(Sn_0.1Ti_0.9)O₃ films was 350 at a frequency of 100 kHz. The films showed a slightly diffused phase transition in the range of 275–340 K. The polarization hysteresis behavior confirmed the ferroelectric nature of the thin films. Remanent polarization (P_r) and saturation polarization (P_s) were 1.1 and 3.2 μC/cm², respectively. The asymmetric capacitance–voltage curve for Ba(Sn_0.1Ti_0.9)O₃ was attributed to the difference in the nature of the electrodes. Dispersion in both the real (ε′_r) and imaginary (ε″_r) parts of the dielectric constant at low frequencies with increase in temperature was attributed to space charge contribution in the complex dielectric constant.     

Impedance spectroscopy and conduction mechanism of multiferroic (Bi0.6K0.4)(Fe0.6Nb0.4)O3

Polycrystalline (Bi_0.6K_0.4) (Fe_0.6Nb_0.4)O₃ material has been prepared using a mixed-oxide route at 950 °C. It was shown by XRD that at room temperature structure of the compound is of single-phase with hexagonal symmetry. Some electrical characteristics (impedance, modulus, conductivity etc.) were studied over a wide frequency (1 kHz–1 MHz) and temperature (25–500 °C) ranges. The Nyquist plot (i.e., imaginary vs real component of complex impedance) of the material exhibit the existence and magnitude of grain interior and grain boundary contributions in the complex electrical parameters of the material depending on frequency, input energy and temperature. The nature of frequency dependence of ac conductivity follows Joncher׳s power law, and dc conductivity follows the Arrhenius behavior. The appearance of P–E hysteresis loop confirms the ferroelectric properties of the material with remnant polarization (2P_r) of 1.027 µC/cm² and coercive field (2E_c) of 16.633 kV/cm. The material shows very weak ferromagnetism at room temperature with remnant magnetization (2M_r) of 0.035 emu/gm and coercive field (2H_c) of 0.211 kOe.     

Structural,dielectric and impedance properties of NaCa2V5O15 ceramics

Polycrystalline sample of NaCa₂V₅O₁₅ (NCV) with tungsten bronze structure was prepared by a mixed oxide method at relatively low temperature (i.e. 630 °C). Preliminary structural analysis of the compound showed an orthorhombic crystal structure at room temperature. Microstructural study showed that the grains are uniformly and densely distributed over the surface of the sample. Detailed studies of dielectric properties showed that the compound has dielectric anomaly above the room temperature (i.e. 289 °C), and shows hysteresis in polarization study. The electrical parameters of the compound were studied using complex impedance spectroscopy technique in a wide temperature (23–500 °C) and frequency (10²–10⁶ Hz) ranges. The impedance plots showed only bulk (grain) contributions, and there is a non-Debye type of dielectric dispersion. Complex modulus spectrum confirms the grain contribution only in the compound as observed in the impedance spectrum. The activation energy, calculated from the ac conductivity of the compound, was found to be 0.20–0.30 eV. These values of activation energy suggest that the conduction process is of mixed type (i.e. ionic–polaronic).     

Structural and dielectric properties of Ba3V2O8 ceramics

Polycrystalline sample of Ba₃V₂O₈ was prepared by a high-temperature solid-state reaction technique. Preliminary X-ray diffraction (XRD) analysis confirms the formation of single-phase compound of hexagonal (rhombohedral) crystal structure at room temperature. Microstructural analysis by scanning electron microscope (SEM) shows that the compound has well defined grains, which are distributed uniformly throughout the surface of the sample. The dielectric properties of the compound studied in a wide frequency range (10²–10⁶ Hz) at different temperatures (25–400 °C), exhibits that they are temperature dependent. Detailed analysis of impedance spectra showed that the electric properties of the material are strongly dependent on frequency and temperature. The activation energy, calculated from the temperature dependence of ac conductivity (dielectric data), was found to be 0.23 eV at 50 kHz in the higher temperature region.     

Finite size effect and influence of temperature on electrical properties of nanocrystalline Ni–Cd ferrites

Electrical conductivity and dielectric measurements have been investigated for four different average grain sizes ranging from 3 to 7 nm of nanocrystalline Ni_0.2Cd_0.3Fe_2.5−xAl_xO₄ (0.0 ⩽ x ⩽ 0.5) ferrites. The impedance spectroscopy technique has been used to study the effect of grain and grain boundary on the electrical properties of the Al doped Ni–Cd ferrites. The analysis of data shows only one semi-circle corresponding to the grain boundary volume suggesting that the conduction mechanism takes place predominantly through grain boundary volume in the studied samples. The variation of impedance properties with temperature and composition has been studied in the frequency range of 120 Hz–5 MHz between the temperatures 300–473 K. The hopping of electrons between Fe³⁺ and Fe²⁺ as well as hole hopping between Ni³⁺ and Ni²⁺ ions at octahedral sites are found to be responsible for conduction mechanism. The dielectric constant and loss tangent (tan δ) are found to decrease with increasing frequency, whereas they increase with increasing temperature. The dielectric constant shows an anomalous behavior at selected frequencies, while the temperature increases, which is expected due to the generation of more electrons and holes as the temperature increases. The behavior has been explained in the light of Rezlescu model.