Bipolaron Hopping Research Articles

Evaluating the significance of rare earth and iron doping on structure-property of lead titanate

The samples (Pb0.8Ln0.2 Fe0.2 Ti0.8O3, (Ln= La, Nd, Sm, Gd)) were synthesized successfully by mixed oxide method. A significant reduction in tetragonality is observed with the modification of structure. The grain size from FESEM study was observed to be decreasing with increase in ionic radii of rare earth ions. Specifically, the dielectric constant is found to be maximum for the Sm-doped sample. Conductivity studies at varying temperature and frequencies reveal the NTCR behaviour of these samples. In Sm, Nd and Gd doped samples, tunnelling of small polarons is responsible for conduction process whereas for La doped sample, the conduction process is mediated through the combined effect of small polaron tunnelling and bipolaron hopping. The Sm-doped sample exhibits highest remanent polarization. All the results of this comprehensive study provide valuable insights into the effect of dopant ions on various properties of the studied materials, paving the way for device applications.

Capturing ion trapping and detrapping dynamics in electrochromic thin films

Ion trapping has been found to be responsible for the performance degradation in electrochromic oxide thin films, and a detrapping procedure was proved to be effective to rejuvenate the degraded films. Despite of the studies on ion trapping and detrapping, its dynamics remain largely unknown. Moreover, coloration mechanisms of electrochromic oxides are also far from clear, limiting the development of superior devices. Here, we visualize ion trapping and detrapping dynamics in a model electrochromic material, amorphous WO3. Specifically, formation of orthorhombic Li2WO4 during long-term cycling accounts for the origin of shallow traps. Deep traps are multiple-step-determined, composed of mixed W4+-Li2WO4, amorphous Li2WO4 and W4+-Li2O. The non-decomposable W4+-Li2WO4 couple is the origin of the irreversible traps. Furthermore, we demonstrate that, besides the typical small polaron hopping between W5+ ↔ W6+ sites, bipolaron hopping between W4+ ↔ W6+ sites gives rise to optical absorption in the short-wavelength region. Overall, we provide a general picture of electrochromism based on polaron hopping. Ion trapping and detrapping were demonstrated to also prevail in other cathodic electrochromic oxides. This work not only provides the ion trapping and detrapping dynamics of WO3, but also open avenues to study other cathodic electrochromic oxides and develop superior electrochromic devices with great durability.

Correlation between the density of defect states (DDS) and cross-linking of corner/edge sharing GeSe4 tetrahedral structural units

We have estimated the DDS in the STSG [Se78-xTe20Sn2Gex (x = 0, 2, 4, 6)] system by using the Correlated Barrier Hopping (CBH) model by performing A.C. conduction measurements in the frequency range (1 kHz–10 kHz) and temperature underneath the glass transition temperature (303–333) K. The detailed analysis reveals that bi-polaron hopping is a leading conduction mechanism over single-polaron hopping. Further, there is a noticeable reduction in DDS with increasing concentration of Ge beyond the composition x = 2. A close inspection indicates that cross-linking of Se with Ge has an important role in controlling the DDS in terms of the corner/edge sharing configurations in the structural unit of GeSe4 tetrahedral.

Comprehensive studies of dielectric behavior, resistive switching, and a.c. conduction in zinc-containing quaternary glasses of Se-Te-Sn

The dielectric behavior, thermally activated a.c. conduction and resistive switching form the basis for revealing new information about the conduction mechanics of glassy chalcogenide materials. Zinc has been chosen as a modifier to control different electrical properties in a parent ternary glass Se78Te20Sn2 of Se-Te-Sn system in the present work keeping in mind the exceptional physical properties of zinc chalcogenides. The detailed analysis indicates that dielectric constant (ε′), dielectric loss (ε″), and a.c. conductivity (σac) is changed appreciably with varying concentrations of zinc atoms in the parent glass. The a.c. conduction follows the correlated barrier hopping (CBH) with bi-polaron hopping as the leading conduction mechanism. We have also determined the density of localized states by using the CBH model. Further, the resistive switching is modified significantly after zinc incorporation. The results have been explained using the chemically ordered network (CON) model and heavy cross-linking of glass-matrix due to isomers of SeyZny nanoclusters.

Bi-polaron hopping process of some MoO3 doped glassy system: Efors-Shkolvshkii gap in the unoccupied states

New glassy nanocomposites, yMoO3 - (0.1 – y)V2O5 – 0.9 (0.05ZnO - 0.95CuO) is found significant not only for surveying their microstructures using X-ray diffraction, FT-IR and SEM techniques, but also for setting out to explore their electrical conduction mechanism in terms of bi-polaronhopping process. Different nanophases such as Cu4O3, ZnO, Cu3Mo2O9 etc. have been identified and the average size of estimated nanocrystallites is found to increase with MoO3content.As MoO3 content is incorporated gradually, orthorhombic phases are found to be the major phases containing TMIs. Formation of more orthorhombic phases with higher MoO3 incorporation may directly indicate the stable structure of the resultant glassy nanocomposites.Electrical conductivity of the present system has been analyzed using bi-polaron hopping process, which may take place among various localized states in the extended stable orthorhombic structures. Significant increase in density of states near Fermi level, N(EF) at higher temperature may directly indicate the expansion of the present glassy structure with more and more MoO3incorporation. All experimental data have been used to frame a schematic model to explore conduction mechanism inside the present glassy system.

Summerfield scaling model and electrical conductivity study for understanding transport mechanisms of a Cr3+ substituted ZnAl2O4 ceramic.

Solid-state and sol-gel procedures were used to prepare ZnAl1.95Cr0.05O4 nanocrystal spinels. From the results obtained by X-ray diffraction (XRD) and transmission electron microscopy (TEM), it can be concluded that the samples prepared by sol-gel synthesis are better crystallized than the ones resulting from the solid-state method. Studies by spectroscopy of impedance were done in function of frequency (40-107 Hz) and temperature (540-680 K) in the sample prepared by sol-gel synthesis. The electrical conductivity spectra obey Jonscher's law and two models were observed studying the variation of the exponent 's' as a function of temperature, Correlated Barrier Hopping (CBH) and Non-overlapping Small Polaron Tunnelling (NSPT). The predominant conduction mechanism is bipolaron hopping. The scaling behavior of conductivity spectra was checked by Summerfield scaling laws. The time-temperature superposition principle (TTSP) points to a common transport mechanism working for the low and middle frequency ranges. The scaling mechanism fails in the high-frequency ranges suggesting that conduction dynamics, and the usual hopping distance of mobile species, have changed. The values obtained for the activation energy from the hopping frequency, conductivity σ dc, bulk resistance R gb, and relaxation (f max), in the temperature range of 540-680 K, are very close. A higher and negative temperature coefficient of resistivity (TCR coefficient) equal to -2.7% K-1 is found at 560 K. This result shows that our compound is suitable for uncooled infrared bolometric applications and infrared detectors.

Electrical and thermal transport properties of Ni1-xCexO nanostructures

In this report, we discussed the structural, electric conduction (DC and AC) mechanisms and thermal properties such as specific heat (CP) and change in enthalpy (ΔH) of Ni1-xCexO (x = 0.00, 0.01, 0.03 and 0.05) nanostructures synthesized by the sol-gel method which have been explored through LCR and differential thermal analysis (DTA) measurements. The dielectric relaxation in this system has also been discussed as a function of both frequency (75 kHz–1 MHz) and temperature (30 °C to 300 °C). In the light of several theoretical models, the AC conductivity has been analyzed using a power law σac∝ωs (s < 1), where frequency exponent ‘s’ examined as a function of temperature. Our experimental investigations based on theoretical models suggest that the AC conduction mechanism in all samples successfully described by non-overlapping small polaron tunnelling model (NSPT) and correlated hopping model (bipolaron hopping over single polaron hopping) (CBH) which provide reasonable physical parameters such as polaron hopping energies (WH and WM), characteristic relaxation time (τo=10−13 s), tunnelling distance (Rw) and density of states at Fermi level N(EF) as well as the concentration of a pair of defects sites (N), respectively. Also, specific heat and change in enthalpy (ΔH) have been discussed and it is observed that value of change in enthalpy (ΔH) decreases with increasing Ce concentration in NiO lattice.

Dielectric relaxation and current conduction mechanism of Tb and Mn codoped bismuth ferrite grafted poly (vinyl alcohol) nanocomposite film

Abstract Investigation on current conduction mechanism through Tb and Mn codoped Bismuth Ferrite grafted polyvinyl alcohol (BTFMO-PVA) nanocomposite film above room temperature (300 K – 415 K) is reported here in detail. A detailed study on dielectric properties of the sample is done over a wide temperature range in a frequency range of 20 Hz - 2MHz. The conduction is attributed to correlated barrier hopping model. Bipolaron hopping dominates over single-polaron hopping in this system. Complex electric modulus spectra and complex modulus spectra are well explained by suitable models to understand the effective dielectric response. The sample responds to the externally applied magnetic field exhibiting negative magnetocapacitance at room temperature.

Structural, optical and dielectric investigations of electrodeposited p-type Cu2O

Electrodeposition technique is employed to prepare cuprous oxide (Cu2O) thin film on fluorine-doped tin oxide (FTO) conducting glass substrate through the reduction of copper lactate in alkaline solution at pH = 12.25. Structural, optical and dielectric properties of the prepared film is investigated by means of scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), UV–Visible absorbance, photoluminescence (PL) and broadband dielectric spectroscopy (BDS). The structural means (XRD, SEM and EDS) revealed the formation of self-assembled cubic microstructure of Cu2O with average grain size of around 1.5 μm. The UV–Vis absorbance spectrum gives optical band gap of 2.05 eV. The PL spectrums confirmed the presence of defect centers ascribed to various forms of oxygen $$(V_{O}^{1 + } ,\,V_{O}^{2 + } )$$ and copper ( $$V_{Cu}^{ 1 + }$$ ) vacancies which are responsible for the conduction in the Cu2O film. The conduction mechanism in the Cu2O film is successfully described by the correlated barrier hopping (CBH) model in which bipolaron hopping become prominent. The density of defect states N, the effective barrier height W and the hopping distance Rω are also calculated based on the CBH model. Two dielectric relaxation processes (β1 and β2) with Arrhenius temperature dependence and activation energies of 0.31 and 0.48 eV are observed. The fast β2-relaxation process with activation energy of 0.48 eV is attributed to the Maxwell–Wagner-Sillars (MWS) polarization while the slow β1-relaxation process with activation energy of 0.31 eV is due to the hopping of the oxygen and copper vacancies.

Study of dielectric relaxation and thermally activated a.c. conduction in glassy Se70Te30 and Se70Te28M2 (M = Ag, Zn and Cd) alloys

A detailed study of conduction mechanism of glassy Se70Te30 and Se70Te28M2 (M = Ag, Zn and Cd) alloys has been carried out in terms of dielectric properties and a.c. conductivity. Temperature and frequency dependent dielectric constant, dielectric loss and σac for the same system were measured in the frequency spectrum, 1 kHz to 1 MHz and temperature range, 303–338 K. The a.c. conductivity (σac) is found to be proportional to ωs (s < 1). The temperature dependence of both a.c. conductivity and frequency exponent (s) is reasonably well interpreted by the correlated barrier hopping (CBH) model. We showed that the bi-polaron hopping is dominant over the potential barrier and estimated the density of defect states (DOS) at 10 kHz. We found a good agreement between bi-polaron conductivity and experimental results.

The investigation of dielectric properties and ac conductivity of new ceramic diphosphate Ag0.6Na0.4FeP2O7 using impedance spectroscopy method

In this paper, Ag0.6Na0.4FeP2O7 has been synthesized by solid state reaction method. The ceramic compound was characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), vibrational spectroscopy and impedance measurements. In fact, the investigated sample has shown single phase type monoclinic structure with P21/C space group. The frequency-dependent electrical data are analyzed in the frame-work of conductivity and electric modulus formalisms. The real and imaginary parts of complex impedance are well fitted to equivalent circuit model based on the Z-View-software. Besides, the observed frequency dependence of conductivity is found to obey Jonscher's universal law. The temperature dependence of both ac conductivity and the parameter s is reasonably well interpreted by the correlated barrier hopping (CBH). The theoretical fitting between the proposed model and the experimental data showed good agreement. The contribution of single polaron and bipolaron hopping to a.c. conductivity in present compound is also studied. The ionic conductivity is discussed on the basis of the structural characteristics of the sample.

Random free energy barrier hopping model for ac conduction in chalcogenide glasses

The random free energy barrier hopping model is proposed to explain the ac conductivity (σac) of chalcogenide glasses. The Coulomb correlation is consistently accounted for in the polarizability and defect distribution functions and the relaxation time is augmented to include the overlapping of hopping particle wave functions. It is observed that ac and dc conduction in chalcogenides are due to same mechanism and Meyer-Neldel (MN) rule is the consequence of temperature dependence of hopping barriers. The exponential parameter s is calculated and it is found that s is subjected to sample preparation and measurement conditions and its value can be less than or greater than one. The calculated results for a − Se, As2S3, As2Se3 and As2Te3 are found in close agreement with the experimental data. The bipolaron and single polaron hopping contributions dominates at lower and higher temperatures respectively and in addition to high energy optical phonons, low energy optical and high energy acoustic phonons also contribute to the hopping process. The variations of hopping distance with temperature is also studied. The estimated defect number density and static barrier heights are compared with other existing calculations.

Correlated barrier hopping of CuO nanoparticles

The ac conduction mechanism in copper oxide nanoparticles with 8 nm size, synthesized by a precipitation method was studied by analyzing ac conductivity in the frequency range of 50 Hz–1 MHz and in the temperature range of 373–573 K. X-ray diffraction and transmission electron microscopy (TEM) were employed for the structural and morphological characterization of CuO nanoparticles. The experimental and theoretical investigations suggested that the ac conduction mechanism in CuO nanoparticles can be successfully explained by a correlated barrier hopping model, which provided reasonable values for the maximum barrier height and characteristic relaxation time. It was also found that bipolaron hopping become prominent up to a particular temperature and beyond that single polaron hopping predominates. Physical parameters such as hopping distance and density of defect states were also calculated. Photoluminescence studies confirm the presence of a surface defect in CuO nanoparticles.

A.C. conduction in glassy alloys of Se90Sb10-xAgx

Temperature and frequency dependence of a.c. conductivity is studied in glassy Se90Sb10-xAgx (x=2, 4, 6 and 8) in the temperature range (288–358K) and frequency range (1–500kHz). An agreement between experimental and theoretical results suggests that the behavior of glassy Se–Sb–Ag system can be successfully explained by correlated barrier hopping (CBH) model. The results show that bipolaron hopping dominates over single polaron hopping in present case, which is explained in terms of lower value of potential barrier. By fitting the experimental data to CBH model, the density of charged defect states is calculated for all the glasses studied. The composition dependence of the density of these states is studied and found to show a maxima at 6at% of Ag which is explained in terms of the mechanically stabilized structure at a particular average co-ordination number.

Electrical conduction mechanisms and dielectric constants of nanostructured methyl violet 2B thin films

The uniform thin films of methyl violet 2B, MV2B, with thicknesses ranged from 96 to 300 nm, have been successfully prepared by spin coating technique. X-ray diffraction showed that the powder and pristine thin film of MV2B have amorphous structure. The amorphous pristine films become polymorphous nanocrystallites after annealing at 433 K. The electrical properties of MV2B thin films have been studied. There are a number of operational environments where the performance of MV2B thin films is likely to be affected significantly on their electrical properties and dielectric constants such as the differences of film thicknesses, temperatures and frequencies. It was found that the DC conductivity of MV2B films increases with increasing temperature. The extrinsic conduction mechanism is operating in temperature range of 288–360 K with activation energy of 0.16 eV, and the conduction in extrinsic region is explained via applying Mott model for variable range hopping. The intrinsic conduction mechanism is operating in temperatures >360 K with activation energy of 0.91 eV. The conduction in intrinsic region is explained by applying band to band transitions theory. The AC electrical conductivity and dielectric relaxation of MV2B thin films in the temperature range 365–473 K and in frequency range 0.1–100 kHz has been also studied. It has 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 by phonon-assisted hopping between localized states and it is performed by bipolaron hopping mechanism. The temperature and frequency dependence of both the real and imaginary parts of dielectric constant have been investigated.

Growth and various characterizations of LiHSO4 single crystals

In the present work, we have grown the single crystals of lithium hydrogen sulfate (LiHSO4) by slow evaporation technique. These grown crystals were characterized by various techniques. For structural analysis powder X-ray diffraction has been used and these crystals were found to have monoclinic crystal structure. Fourier transform infrared spectroscopy was used to find various affiliated functional groups present in the system. Optical properties were studied by using UV–visible spectroscopy. Present system shows an indirect allowed transition with an optical band gap of around 5.36 eV. The dielectric constant and dielectric loss show a well dispersion with frequency. AC conductivity of the present crystals is controlled by the correlated barrier hopping model. Possible evidence for bipolaron hopping mechanism was also explored. From thermal study, a phase transition is shown at around 389 K. Various thermodynamic parameters ΔH, ΔS and ΔCp are also calculated. We observed a strong inter-relation between various parameters of the system.

Electrical conduction mechanism in ZnS nanoparticles

ZnS nanoparticles with hexagonal wurtzite crystal structure have been prepared by coprecipitation method at 70°C and subsequently annealed at 400°C for 4h. The average particle size has been found to be ∼20nm. ZnS nanopowder has been characterized by UV–Vis spectrophotometry. The band gap has been calculated in the range of 3.9eV. Impedance spectroscopic technique has been used to examine the electrical properties of ZnS nanoparticles pressed to pellet form in the temperature range of 300–400K. Correlated barrier hopping has been the prevailing conduction mechanism in ZnS. The activation energy calculated from the Arrhenius relation is consistent with bipolaron and single polaron hopping in correlated barrier hopping model.

Transparent wide band gap crystals follow indirect allowed transition and bipolaron hopping mechanism

Recently, we carried out structural, optical and dielectric studies on micro-crystals of Oxypeucedanin (C16H14O5), isolated from the roots of plant Prangos pabularia (Mir et al. (2014) [3,4]). The obtained trend in frequency exponent (s) with frequency (ω) indicates that the universal dynamic response is followed by this compound. From optical absorption spectroscopy, the optical band gap (Eg) was estimated around 3.76eV and system is showing indirect allowed transition. Using Eg in certain relation of s, a close value of s (as much close obtained by fitting ac conductivity) was obtained. This method was further used for other similar systems and again same trend was obtained. So a general conclusion was made that the high transmitting wide band insulators or semiconductors may follow bipolaron hopping transport mechanism.

Electrical properties and conduction mechanism in the sodium nickel diphosphate

The Na3.6Ni2.2(P2O7)2 compound was obtained by the conventional solid-state reaction. The sample was characterized by X-ray powder diffraction and vibrational and impedance spectroscopy. The AC electrical conductivity and the dielectric relaxation properties of this compound have been investigated by means of impedance spectroscopy measurements over a wide range of frequencies and temperatures, 209 kHz–1 MHz and 564–729 K, respectively. Dielectric data were analyzed using complex electrical modulus M* at various temperatures. The peak positions ωm of M″ spectra shift toward higher frequencies with increase in temperature. The AC conductivity data fulfill the power law. Application of the correlated barrier hopping model revealed that the ionic conduction takes place by single-polaron and bipolaron hopping processes.

Investigation of a.c. conductivity measurements in a-Se 80Te 20 and a-Se 80Te 10M 10 (M = Cd, in, Sb) alloys using correlated barrier hopping model

The a.c. conductivity of a-Se 80Te 20 and a-Se 80Te 10M 10 (M = Cd, In, Sb) alloys has been investigated as a function of temperature in the range from 280 to 330 K and frequency in the range from 10 2 to 10 4 Hz. The experimental results indicate that a.c. conductivity σ ac is proportional to ω s where s < 1 and decreases with increasing temperature. The results obtained are discussed in terms of the correlated barrier hopping (CBH) model. An agreement between experimental and theoretical results suggests that the a.c. conductivity behavior of a-Se 80Te 20 and a-Se 80Te 10M 10 (M = Cd, In, Sb) system can be successfully explained by CBH model. The contribution of single polaron and bipolaron hopping to a.c. conductivity in present alloys is also studied.