Hopping Mechanism Of Charge Carriers Research Articles

Laser-modified nanocrystalline NiMoO4 as an electrode material in hybrid supercapacitors

The nanocrystalline NiMoO4 obtained as a result of hydrothermal synthesis was exposed to laser radiation with a pulse energy of 70 mJ/cm2 for 5 minutes. The phase composition and size of crystallites of the triclinic structure of NiMoO4 were determined by X-ray analysis. The average crystallite size was 18 nm for laser-irradiated nickel molybdate. Impedance analysis was used to analyze the temperature dependence of the electrical conductivity of laser-modified NiMoO4. The frequency index of the power law, determined by the nonlinear approximation method, was 0.5-0.67, which corresponds to the hopping mechanism of charge carriers. The electrochemical behavior of NiMoO4 was studied using cyclic voltammetry and galvanostatic charge/discharge testing. The laser-irradiated NiMoO4 reaches a specific capacitance of 553 F/g at a scan rate of 1 mV/s. The hybrid electrochemical system based on electrodes of modified NiMoO4 and carbon material provides high Coulombic efficiency (95%) for a significant number of charge/discharge cycles.

Dielectric and electrical properties of Ca-modified BFN perovskite

The present work reports fabrication of Ca-modified Ba(Fe1/2Nb1/2)O3 perovskite by solid state route at optimized temperature of 1250 °C through repeated firing method. X-ray diffraction profile reveals formation of the perovskite in single phase having monoclinic crystal structure. Detailed dielectric behavior is examined within frequency 100 Hz−5 MHz and temperature 25–300 °C. The perovskite has high dielectric constant and low loss value at room temperature that makes it suitable for designing energy storage appliances. Non-destructive impedance spectroscopy technique suggests the occurrence of non-Debye type relaxation and negative temperature coefficient of resistance property in the compound. Conductivity study suggests that hopping mechanism of charge carriers is responsible for the conduction process.

Impedance spectroscopic study of charge transport and relaxation mechanism in MnCr2O4 ceramic chromite

In the present report, MnCr2O4 ceramic oxide has been prepared via the solution based self-combustion sol-gel route sintered at 900 °C. The Rietveld refinement of X-ray powder diffraction profile revealed the single-phase normal spinel structure of the ceramic crystallized in a face-centered cubic unit cell with Fd3m space group symmetry. The field emission scanning electron microscopy exposed well distribution of grains separated by grain boundaries as micro-constituents of the sample. Complex impedance spectroscopy deduced the distribution of relaxation times and thermal activation of charge carriers in the frequency and temperature ranges of 70−4×106 Hz and 543−643 K, respectively. The depressed semicircular arcs of the Nyquist plot modeled by an equivalent circuit with configuration RG׀׀CG+RGB׀׀CGB confirmed the bulk and interface contribution to the electrical response of the material. The correlated barrier hopping conduction addressed the temperature dependence of frequency exponent after simulation of the AC conductivity data by the equation of Jonscher. The complex modulus study identified the electro-active components of the material and determined the dominant role of grains in the conduction mechanism. The analysis of dielectric constant and loss factor stated thermally activated hopping mechanism of charge carriers between Cr2+ and Cr3+ ions at the octahedral sites of the crystal structure.

The comparative investigation on synthesis, characterizations of silver ion-imprinting and non-imprinting cryogels, their impedance spectroscopies and relaxation mechanisms

In the present study, a novel ion-imprinting and non-imprinting cryogel samples have been prepared using ion-imprinting technique and the dielectric properties have been investigated using an impedance spectroscopy method. In the preparation of ion-imprinted cryogel, at the first attempt, N-methacryloly-(l)-cysteine methyl ester was used as the metal complexing monomer. Ag+-imprinted poly(hydroxyethyl methacrylate-N-methacryloly-(l)-cysteine methyl ester) cryogel was produced by bulk polymerization. Poly(2-hydroxyethyl methacrylate) was selected as the basic matrix by considering properties, high chemical and mechanical stability. After removal of template (silver ions), the ion-imprinted cryogel was used for the removal of photo-film-containing materials. The dielectric properties of cryogel samples have also been investigated by impedance spectroscopy within the frequency range of 1 Hz–10 MHz. The real part of the permittivity increases at low frequencies as electrode effects become dominant. It shows a constant value at high frequencies due to dipole polarization. On the other hand, the imaginary part does not show a relaxation peak as the relaxation time of samples is very short. The frequency dependence of electrical modulus has also been investigated. The real part of electrical modulus (M′ (f)) is an indicative of negligible electrode polarization phenomenon in the test material. The behavior of the imaginary part of frequency dependent electrical modulus (M″ (f)) exhibit that the dielectric relaxation process is usually not frequency-activated state. Dielectric relaxation process occurs spontaneously due to the hopping mechanism of charge carriers.

Structurally modulated dielectric relaxation in rhombohedral Cr2O3 mediated by Mn addition

A simple, energy and time efficient, sol–gel based auto-combustion route was employed to synthesize phase pure nanoparticles of Cr2O3 and MnCr2O4 to study structural, dielectric, impedance and ferroelectric properties. Crystal structure, identified through X-ray diffraction, revealed that Cr2O3 had rhombohedral symmetry (R-3c), while MnCr2O4 possessed cubic spinel symmetry with space group, Fd-3m. Data obtained from X-ray diffraction was further utilized to refine the diffraction patterns using Rietveld’s refinement approach to confirm the structural parameters. Frequency dependent dielectric properties have been investigated systematically in a high frequency range from 20 Hz to 20 MHz. Dielectric constant, loss factor and tangent loss were found to decrease with increase in frequency due to hopping and mainly explained on the basis of Maxwell–Wagner’s model followed by Koop’s theory. Impedance studies helped to define the electro-active regions and Nyquist plot thereby was used to evaluate grain and grain boundary resistance of the synthesized samples. Conductivity behavior followed by the typical Jonscher’s power law was confirmed by calculating the exponent in the expression and thus the hopping mechanism of charge carriers inside these chromites was studied. Ferroelectric analysis showed the potential of the synthesized samples in multifunctional memory devices, used in the miniaturized industry.

Influence of nickel incorporation on structural, optical and electrical characteristics of SILAR synthesized CuO thin films

In this report, we have primarily studied the influence of nickel (Ni) incorporation on ac electrical conductivity, dielectric relaxation mechanism and impedance spectroscopy characteristics of copper oxide (CuO) thin films synthesized by successive ion layer adsorption and reaction (SILAR) technique. The materials has been characterized using X-ray diffraction and UV–VIS spectrophotometric measurements. Reduction in grain size in doped films up to a certain extent of doping (tentatively 6%) were confirmed from XRD analysis, beyond which there is a reverse tendency. Increase in band gap in doped films were observed up to 6% doping level which could be associated with enhanced carrier density in doped films. Impedance spectroscopy analysis confirmed enhancement of ac conductivity and dielectric constant for doped samples. The results are useful for capacitive application of the films. Beyond 6% doping level, AC conductivity and dielectric constant shows a reverse tendency indicating reduced density of charge carriers. Nyquist plot shows contribution of both grain and grain boundary towards total resistance and capacitance. Imaginary part of complex modulus and imaginary part of complex impedance was used to find the migration/activation energy to electrical conduction process. Nearly identical result was obtained from relaxation frequency/relaxation time approach suggesting hopping mechanism of charge carriers.

Structural, optical and ac electrical characterization of CBD synthesized NiO thin films: Influence of thickness

We have studied the electrical conductivity, dielectric relaxation mechanism and impedance spectroscopy characteristics of nickel oxide (NiO) thin films synthesized by chemical bath deposition (CBD) method. Thickness dependent structural, optical and ac electrical characterization has been carried out and deposition time was varied to control the thickness. The material has been characterized using X-ray diffraction and UV-VIS spectrophotometer. Impedance spectroscopy analysis confirmed enhancement of ac conductivity and dielectric constant for films deposited with higher deposition time. Decrease of grain size in thicker films were confirmed from XRD analysis and activation energy of the material for electrical charge hopping process was increased with thickness of the film. Decrease in band gap in thicker films were observed which could be associated with creation of additional energy levels in the band gap of the material. Cole-Cole plot shows contribution of both grain and grain boundary towards total resistance and capacitance. The overall resistance was found to decrease from ~ 14.6 × 105Ω for 30min deposited film (~ 120nm thick) to ~ 2.42 × 105Ω for 120min deposited film (~307nm thick). Activation energy value to electrical conduction process evaluated from conductivity data was found to decrease with thickness. Identical result was obtained from relaxation time approach suggesting hopping mechanism of charge carriers.

Selective contacts drive charge extraction in quantum dot solids via asymmetry in carrier transfer kinetics

Colloidal quantum dot solar cells achieve spectrally selective optical absorption in a thin layer of solution-processed, size-effect tuned, nanoparticles. The best devices built to date have relied heavily on drift-based transport due to the action of an electric field in a depletion region that extends throughout the thickness of the quantum dot layer. Here we study for the first time the behaviour of the best-performing class of colloidal quantum dot films in the absence of an electric field, by screening using an electrolyte. We find that the action of selective contacts on photovoltage sign and amplitude can be retained, implying that the contacts operate by kinetic preferences of charge transfer for either electrons or holes. We develop a theoretical model to explain these experimental findings. The work is the first to present a switch in the photovoltage in colloidal quantum dot solar cells by purposefully formed selective contacts, opening the way to new strategies in the engineering of colloidal quantum dot solar cells.

Metal-insulator transition and conduction mechanism in dysprosium doped Bi1.7Pb0.4Sr2Ca1.1Cu2.1O8+δ system

To explore the mechanisms behind the metal-insulator transition and charge transport in high temperature superconducting cuprates, a systematic study on the Bi1.7Pb0.4Sr2Ca1.1Cu2.1O8+δ compound was made by adding the rare-earth dysprosium at stoichiometric amounts (x) of 0.5≤x≤1. Phase analysis, determination of lattice parameters, microstructure analysis, and elemental analysis were carried out to evaluate the relative performance of the samples prepared by the solid state synthesis route. Charge transport in the insulating and superconducting samples were analyzed through resistivity measurements at (64–300 K). It is found that the x=0.5 sample is superconducting with a critical transition temperature of 94.8 K while for the samples with x>0.5 superconductivity is suppressed, along with an increase in their normal state resistivities. A metal-insulator transition is found to take place around 0.5<x≤6. A detailed analysis of the experimental data shows that the hole filling and disorder, induced by change in charge carrier concentration, lead to a metal-insulator transition in the present system. Also the conductivity of the semiconducting samples (x>0.5) at lower temperature is due to the two-dimensional variable range hopping mechanism of charge carriers between the spatially localized states.

Some electrical characteristics of Li 0.5Cr xFe 2.25− xO 4 spinel structures

Some of the electrical properties of chromium-substituted spinel Li 0.5Cr x Fe 2.25− x O 4 with 0.15≤ x≤1.85, synthesized at 1200 °C for 12 h, are examined through impedance measurements and analysis. The electrical resistivity, relaxation frequencies and dielectric constant of Li 0.5Cr x Fe 2.25− x O 4 have been investigated versus the composition x. The behavior of these can be satisfactorily described by a hopping mechanism of charge carriers between Fe 2+ and Fe 3+ ions on octahedral sites of the spinel lattice.

Charge transport in B-carotene

The absorption currents in the ‘cis-trans’ isomeric forms of β-carotene glass were studied over a wide range of temperature and field. Limited observations of the absorption currents were also made with different sample thicknesses and electrode materials. The results indicate that the low temperature absorption current (<273K) may be due to a dipolar mechanism with a distribution of relaxation times whereas the absorption current in the high temperature range (≃273K<T<333K) may be adequately explained by a hopping mechanism of charge carriers through the localized states distributed by the bulk.