Electronic transport mechanisms in tetraphenyleprophyrin thin films

Thermally-evaporated thin films of tetraphenylporphyrin, TPP, with thickness range from (175 to 735) nm had been prepared. Annealing temperatures ranging from (273–473) K do not influence the amorphous structure of these films. The influence of environmental conditions: film thickness, temperature and frequency on the electrical properties of TPP thin films had been reported. It was found that dc conductivity increases with increasing temperature and film thickness. The extrinsic conduction mechanism is operating in temperature range of (293–380) K with activation energy of 0.13 eV. The intrinsic one is in temperatures >380 K via phonon assisted hopping of small polaron with activation energy of 0.855 eV. The ac electrical conductivity and dielectric relaxation in the temperature range (293–473) K and in frequency range (0.1–100) kHz had been also studied. It had been shown that theoretical curves generated from correlated barrier hopping (CBH) model gives the best fitting with experimental results. Analysis of these results proved that conduction occurs at low temperatures (300–370) K by phonon assisted hopping between localized states and it is performed by single polaron hopping process at higher temperatures. The temperature and frequency dependence of both the real and imaginary parts of dielectric constant had been reported.     

Electrical properties and dielectric relaxation of thermally evaporated zinc phthalocyanine thin films

The electrical transport properties and dielectric relaxation of Au/zinc phthalocyanine, ZnPC/Au devices have been investigated. The DC thermal activation energy at temperature region 400-500 K is 0.78 eV. The dominant conduction mechanisms in the device are ohmic conduction below 1 V and space charge limited conduction dominated by exponential trap distribution in potentials >1 V. Some parameters, such as concentration of thermally generated holes in valence band, the trap concentration per unit energy range at the valence band edge, the total concentration of traps and the temperature parameter characterizing the exponential trap distribution and their relation with temperatures have been determined. The AC electrical conductivity, σ_ac, as a function of temperature and frequency has been investigated. It showed a frequency and temperature dependence of AC conductivity for films in the temperature range 300-400 K. The films conductivity in the temperature range 400-435 K increased with increasing temperature and it shows no response for frequency change. The dominant conduction mechanism is the correlated barrier hopping. The temperature and frequency dependence of real and imaginary dielectric constants and loss tangent were investigated.     

Electrical conductivity and dielectric properties of nanofibrillated cellulose thin films from bagasse

Nanocelluloses are potential candidates for applications in flexible electronic due to their unique physical and mechanical properties. However, electrical properties of these materials have not investigated thoroughly to study their electrical properties. In the current work, electrical properties of nanocellulose films prepared from bagasse pulp were studied and compared with those of bagasse pulp fibers. Two kinds of nanocelluloses were used in the current study: microfibrillated cellulose (MFC) and TEMPO‐oxidized nanofibrillated cellulose (NFC). The crystallinity, grain size, and morphology of the different nanocelluloses were studied using X‐ray diffraction and transmission electron microscopy techniques. The dc‐, ac‐ electrical conductivity, dielectric constant ?′, and dielectric loss ?″ of non‐plasticized and glycerol‐plasticized nanocellulose films were studied in the temperature range from 298 to 373 K and in the frequency range from 0.1 KHz to 5 MHz. The results showed that the dc‐ electrical conductivity verifies Arrhenius equation and the activation energies varied in the range of 0.9 to 0.42 eV. Ac‐electrical conductivity increased with frequency and fitted with power law equation, which ensures that the conduction goes through hopping mechanism. The dielectric constant decreased with increasing frequency and increased with increasing temperature, probably due to the free movement of dipole molecular chains within the cellulose fiber. Glycerol‐plasticized NFC (NFC‐G) film had the highest dielectric constant and ac‐electrical conductivity values of 79 800 and 2.80× 10^?3ohm^?1 cm^?1, respectively. The high values of dielectric constant and conductivity of the prepared films support their use in electronic components.     

Rietveld refinement and impedance spectroscopy of calcium titanate

Single phase perovskite CaTiO₃ has been synthesized by conventional solid state reaction technique. The ceramic was characterized by XRD at room temperature and its Rietveld refinement inferred orthorhombic crystal structure with the space group Pbnm. The field dependence of dielectric relaxation and conductivity was measured over a wide frequency range from room temperature to 673 K. Analysis of Nyquist plots of CaTiO₃ revealed the contribution of many electrically active regions corresponding to bulk mechanism, distribution of grain boundaries and electrode processes. The dc conductivity depicted a semiconductor to metal type transition. Frequency dependence of dielectric constant (ε′) and tangent loss (tan δ) show a dispersive behavior at low frequencies and is explained on basis of Maxwell-Wagner model and Koop's theory. Both conductivity and electric modulus formalisms have been employed to study the relaxation dynamics of charge carriers. The variation of ac conductivity with frequency at different temperatures obeys the universal Jonscher's power law (σ_ac α ω^s). The values of exponent ‘s’ lie in the range 0.13 ≤ s ≤ 0.33, which in light of CBH model suggest a large polaron hopping type of conduction mechanism.     

Some dielectric properties of novel nano‐s‐triazine derivatives

The evaluation of the dielectric properties of s‐triazine and its mono‐, di‐, tri‐(trityloxy)triazine derivates as a function of temperature from room temperature to 200°C, and frequency varying from 50 Hz to 5 MHz was performed. The dielectric constant increases with the increase of both temperature and frequency. Moreover, from the measured dielectric loss ε″ we found that there are different types of electric energy losses in the presence of an alternating electric field from which we calculate the entropy ΔS and the enthalpy change ΔH of the dielectric relaxation for each sample. The dielectric relaxation was attributed to the phase transition of the s‐triazine derivatives. Additionally, ac‐electrical conductivity as a function of frequency at different temperatures were studied. Analysis of ac conductivity data indicates that the correlated barrier hopping model is the most suitable mechanism for the ac‐conductance behavior. X‐ray diffraction and scanning electron microscopy were performed on the compounds under consideration to determine the grain size of each sample, which was found in the range of 3 to 100 nm.     

Dielectric relaxation and ac conductivity behavior of carboxyl functionalized multiwalled carbon nanotubes/poly (vinyl alcohol) composites

We study the dielectric relaxation and ac conductivity behavior of MWCNT-COOH/Polyvinyl alcohol nanocomposite films in the temperature (T) range 303–423 K and in the frequency (f) range 0.1 Hz–1 MHz. The dielectric constant increases with an increase in temperature and also with an increase in MWCNT-COOH loading into the polymer matrix, as a result of interfacial polarization. The permittivity data were found to fit well with the modified Cole-Cole equation. Temperature dependent values of the relaxation times, free charge carrier conductivity and space charge carrier conductivity were extracted from the equation. An observed increment in the ac conductivity for the nanocomposites was analysed by a Jonscher power law which suggests that the correlated barrier hopping is the dominant charge transport mechanism for the nanocomposite films. The electric modulus study revealed deviations from ideal Debye-type behavior which are explained by considering a generalized susceptibility function. XRD and DSC results show an increase in the degree of crystallinity.     

Effect of substitutions on dielectric behavior of La2NiMnO6

The substituted La₂NiMnO₆ (LNMO) double perovskite powder samples are prepared by hydroxide co-precipitation method. The electrical properties such as conductivity, dielectric constant and impedance analysis of the substituted sample, have been studied. The activation energy is determined by dc conductivity plot gives the fair idea of conduction mechanism. The dielectric and impedance analysis suggests the contribution of extrinsic and intrinsic effects in dielectric relaxation due to substitution at A and B site cations. The Nyquist plot reveals the role of grain and grain boundaries in charge conduction mechanism. The large grain and grain boundary resistance value and activation energy ～100 meV of substituted LNMO samples shows the presence of variable range hopping (VRH) conduction mechanism.     

Effect of substitution group on dielectric properties of 4H-pyrano [3,2-c] quinoline derivatives thin films

The AC electrical conductivity and dielectrical properties of 2-amino-6-ethyl-5-oxo-4-(3-phenoxyphenyl)-5,6-dihydro-4H-pyrano[3, 2-c]quinoline-3-carbonitrile(Ph-HPQ) and 2-amino-4-(2-chlorophenyl)-6-ethyl-5-oxo-5,6-dihydro-4H-pyrano [3, 2-c] quinoline-3-carbonitrile(Ch-HPQ) thin films were determined in the frequency range of 0.5 k Hz–5 MHz and the temperature range of 290–443 K. The AC electrical conduction of both compounds in thin film form is governed by the correlated barrier hopping(CBH) mechanism. Some parameters such as the barrier height, the maximum barrier height, the density of charges, and the hopping distance were determined as functions of temperature and frequency. The phenoxyphenyl group has a greater influence on those parameters than the chlorophenyl group. The AC activation energies were determined at different frequencies and temperatures. The dielectric behaviors of Ph-HPQ and Ch-HPQ were investigated using the impedance spectroscopy technique. The impedance data are presented in Nyquist diagrams for different temperatures. The Ch-HPQ films have higher impedance than the Ph-HPQ films. The real dielectric constant and dielectric loss show a remarkable dependence on the frequency and temperature. The Ph-HPQ has higher dielectric constants than the Ch-HPQ.     

Impact of MoO3 concentration,frequency and temperature on the dielectric properties of zinc phosphate glasses

Phosphate glasses with the chemical composition of 47P₂O₅–24ZnO-(29-x)Na₂O-xMoO₃, x = 0, 2, 4, 6, 8 and 10, have been prepared using the melt quenching technique. Dielectric properties of these phosphate glasses are carried out in the frequency range from 1 to 100 kHz at different temperatures. Dielectric parameters such as dielectric constant ε′, dielectric loss ε′′ and ac conductivity of the investigated glasses have been evaluated. The dependences of these dielectric parameters on frequency, composition and temperature have been discussed. It is found that dielectric constant decreases with increasing frequency due to the reduction of space-charge polarization and dipole polarization. The dependence of ac conductivity on the MoO₃ content indicates a competition between electronic and ionic conduction. The temperature dependence of the dielectric parameters reveals a rising trend of the dielectric parameters with temperature. This rising trend is indicated due to the increase of the amplitude of the thermal vibration of the charge carriers which facilitates the electron hopping and drifting of the mobile ions. The linear trend of the ln(σ_ac)-1000/T plot indicates that ac conductivity of the investigated glasses is thermally-activated transport process and follows the Arrhenius equation. The activation energy and its composition dependence have been reported.     

Impedance spectroscopy and dielectric analysis in KH2PO4 single crystal

Optical observation, differential scanning calorimetry, thermogravimetric analysis, and differential thermogravimetric measurements have been carried out on KH₂PO₄ single crystals. As compared with the optical observation of crystal under polarizing microscope, the dehydration process occurred gradually over the crystal surface at temperatures above 195 °C and then the interior of the sample. The ac impedance measurements were performed as a function of both frequency and temperature. The electrical conduction and dielectric relaxation have been studied. The activation energy of migration is 1.02 eV in the temperature range between 150 and 179 °C. The conduction mechanism in this temperature range is attributed to the hopping of proton among hydrogen vacancies. At temperatures above 186 °C, a higher conductivity activation energy with 2.94 eV is obtained. In addition to the proton conduction, the migration of the heavier ions (such as potassium ion) is also suggested.     

Dielectric property of CaCu3Ti4O12 thin film grown on Nb-doped SrTiO3(100) single crystal

Thin film of CaCu₃Ti₄O₁₂ (CCTO) has been deposited on Nb-doped SrTiO₃(100) single crystal using pulsed laser deposition. The dielectric constant and AC conductivity of CCTO film in the metal–insulator–metal capacitor configuration over a wide temperature (80 to 500 K) and frequency (100 Hz to 1 MHz) range have been measured. The small dielectric dispersion with frequency observed in the lower temperature region (<300 K) indicates the presence of small defects in the deposited CCTO thin film. The frequency-dependent AC conductivity at lower temperature indicates the hopping conduction. The dielectric dispersion data has been analyzed in the light of both conductivity relaxation and Debye type relaxation with a distribution of relaxation times. Origin of dielectric dispersion is attributed to the distribution of barrier heights such that some charge carriers are confined between long-range potential wells associated with defects and give rise to dipolar polarization, while those carriers which do not encounter long-range potential well give rise to DC conductivity.     

The dc and ac properties of potassium trioxalatoferrate (III) trihydrate

The complex potassium trioxalatoferrate (III) trihydrate {K₃(Fe(C₂O₄)₃ · 3H₂O)} was synthesised and characterised by energy dispersion X-ray fluorescence (XRF) and X-ray diffraction (XRD). The electrical and dielectric properties of the complex pellet were studied by ac- and dc-techniques in room temperature and in a temperature range of 293–373 K. The data of the ac conductivity as a function of frequency in a frequency range of 1–100 kHz follow the correlated barrier hopping CBH model and the parameters of the model were determined and connecting them with the optical properties. The temperature dependence of dc conductivity shows that the semiconducting behaviour of conduction phenomenon in the complex is realised by hopping mechanism between localised states and the minimum hopping distance was determined. High relative permittivity of about 30 at 100 kHz was obtained for the complex, which can find technological applications like alternative for the SiO₂ insulator in MOS devices.     

Electrical conductivity and dielectric properties of sulfamic acid doped polyaniline

The temperature and frequency dependence of dielectric constant (ε′) and dielectric loss (ε″) is studied for different samples of polyaniline (PANI), doped with different concentration of sulfamic acid in the frequency range (10–100 kHz) and temperature range (300–400 K). The dc conductivity has also been measured to see the effect of sulfamic acid and the conduction mechanism has been explained by the propagation of polaron through a conjugated polymer chain due to shifting of double bonds (alternation), which gives rise to electrical conduction.     

Temperature-dependent ac conductivity and dielectric response of vanadium doped CaCu3Ti4O12 ceramic

Successful incorporation of vanadium dopant within the giant dielectric material CaCu₃Ti₄O₁₂ (CCTO) through a  conventional solid-state sintering process is achieved and its influence on the dielectric as well as electrical properties as a function of temperature and frequency is reported here. Proper crystalline phase formation together with dopant induced lattice constant shrinkage was confirmed through X-ray diffraction. The temperature dependence of the dielectric constant at different constant frequencies was investigated. We infer that the correlated barrier hopping (CBH) model is dominant in the conduction mechanism of the ceramic as per the temperature-dependent ac conductivity measurements. The electronic parameters such as density of the states at the Fermi level, N(E _f) and hopping distance, R _ω of the ceramic were also calculated using this model.     

Effects of photoexcitation on the current transport mechanism in amorphous indium selenide thin films

The effect of photoexcitation on the current transport mechanism in amorphous indium selenide thin films was studied by means of dark and illuminated conductivity measurements as a function of temperature. Analysis of the dark electrical conductivity in the temperature range 110–320 K reveals behaviour characteristic of carriers excited to the conduction band and thermally assisted variable-range hopping (VRH) at the Fermi level above 280 K and below 220 K, respectively. In the temperature range 220–280 K, a mixed conduction mechanism was observed. A conductivity activation energy of ～300 meV (above 280 K), a density of localised states (evaluated assuming a localisation length of 5 Å) of 1.08 × 10²¹ cm^?3 eV^?1, an average hopping distance of 20.03 Å (at 120 K) and an average hopping energy of 27.64 meV have been determined from the dark electrical measurements. When the sample was exposed to illumination at a specific excitation flux and energy, the values of the conductivity activation energy, the average hopping energy and the average hopping range were significantly decreased. On the other hand, the density of localised states near the Fermi level increased when the light flux was increased. Such behaviour was attributed to a reversible Fermi level shift on photoexcitation.     

Electrical properties of a-antimony selenide

This paper reports conduction mechanism in a-Sb₂Se₃ over a wide range of temperature (238 to 338 K) and frequency (5 Hz to 100 kHz). The d.c. conductivity measured as a function of temperature shows semiconducting behaviour with activation energy ΔE=0.42 eV. Thermally induced changes in the electrical and dielectric properties of a-Sb₂Se₃ have been examined. The a.c. conductivity in the material has been explained using modified CBH model. The band conduction and single polaron hopping is dominant above room temperature. However, in the lower temperature range the bipolaron hopping dominates.     

Hopping and tunneling transport over a wide temperature range in chemically synthesized doped and undoped polypyrrole

Polypyrrole was synthesized by the chemical oxidation method in the presence of phosphoric acid by varying oxidant to monomer molar ratio for the optimization of electrical conductivity. The conductivity in doped polypyrrole reached up to a maximum value of 9.18 S/cm. Granular morphology was observed in chemically synthesized polypyrrole. Neutralization of doped polypyrrole was done with aqueous ammonium hydroxide and three orders of reduced conductivity were obtained in neutral polypyrrole. Doped and undoped samples of polypyrrole were then electrically characterized over a wide temperature range of 10–300 K. The measured electrical conductivity rises with the increase in temperature and shows the semiconducting nature of the material. Strong and weak temperature dependence of conductivity was revealed by undoped and doped polypyrrole samples respectively. An effort has been made to explore the electrical transport in doped and undoped polypyrrole by charge transport models. The experimental data obeys Kivelson’s hopping model in temperature range of 60–300 K and fluctuation assisted tunneling was the dominant conduction mechanism below 60 K.     

Electrical conduction properties of Si δ-doped GaAs grown by MBE

The temperature dependent Hall effect and resistivity measurements of Si δ-doped GaAs are performed in a temperature range of 25–300 K. The temperature dependence of carrier concentration shows a characteristic minimum at about 200 K, which indicates a transition from the conduction band conduction to the impurity band conduction. The temperature dependence of the conductivity results are in agreement with terms due to conduction band conduction and localized state hopping conduction in the impurity band. It is found that the transport properties of Si δ-doped GaAs are mainly governed by the dislocation scattering mechanism at high temperatures. On the other hand, the conductivity follows the Mott variable range hopping conduction (VRH) at low temperatures in the studied structures.     

Dielectric behavior of sodium borate glasses

The dielectric constant and the electrical conductivity of the transparent glasses in the composition 3Na₂O-7B₂O₃ (NBO) were investigated in the 100 Hz–10 MHz frequency range at various temperatures. The activation energy associated with the electrical relaxation determined from the electric modulus spectra was found to be 0.76 ± 0.02 eV, close to that (0.74 ± 0.02 eV) obtained from DC conductivity studies. The frequency-dependent electrical conductivity was analyzed using Jonscher’s power law. Temperature-dependent behavior of the frequency exponent (n) suggested that the correlated-barrier hopping model was the most appropriate to rationalize the electrical transport phenomenon in NBO glasses.