Showing papers on "Variable-range hopping published in 2021"

Tunable magnetoresistance of core-shell structured polyaniline nanocomposites with 0-, 1-, and 2-dimensional nanocarbons

Abstract: Core-shell structured polyaniline (PANI) nanocomposites with tunable magnetoresistance (MR) were obtained through the facial surface-initiated polymerization method with assistance of zero-, one-, and two-dimensional nanocarbons (carbon black, carbon fiber, carbon tube, and graphene). The improved dielectric properties and typical semiconducting behavior were observed in the PANI nanocomposites. And the quasi 3D electron conduction mechanism was observed in all the samples through Mott variable range hopping model, indicating that dimension of the nanocarbons does not affect the charge transport mechanism. Meanwhile, positive MR was observed in all the samples, and the MR value can be controlled by nanocarbons. When nanocarbon loading is 10.0 wt%, MR of graphene/PANI, carbon fiber/PANI, carbon black/PANI, and carbon tube/PANI were 15.6%, 14.7%, 9.5%, and 1.5%, respectively. The positive MR phenomenon was analyzed by the wave functional shrinkage model. The magnetic field and nanocarbons’ effects on the localization length, density of state at the Fermi level, average hopping length, and hopping energy were systematically studied. This work provides the guideline for the fabrication of tunable magnetic sensor or information storage device. Tunable magnetoresistance was reported in the polyaniline nanocomposites with zero-, one-, and two-dimensional nanocarbons as fillers.

One-dimensional variable range charge carrier hopping in polyaniline–tungsten oxide nanocomposite-based hydrazine chemiresistor

Abstract: Low-temperature direct current charge transport mechanism of charge carriers in polyaniline–tungsten oxide (PAN-WO3) nanocomposite has been investigated. Charge transport in pristine PAN was found to govern by Mott’s three-dimensional variable range hopping (3-D VRH) model. However, the inclusion of WO3 in nanocomposite shifts the dimension of hopping from 3-D to 1-D. The room-temperature conductivity of PAN-WO3 nanocomposite (5.55 × 10–3 S/cm) was also found to be enhanced compared to pristine PAN (1.27 × 10–5 S/cm). The reasons for crossover in hopping dimensionality and enhanced conductivity of PAN-WO3 nanocomposite have been explained in terms of Mott’s parameters, i.e., small hopping radius, lower hopping energy, high inter-chain distance, and prominent intra-chain transport. Furthermore, PAN-WO3 exhibited enhanced reversible sensing behaviour (27%) towards 10 parts per million of hydrazine at room temperature compared to that of pristine PAN (12%). Enhanced sensing characteristics of PAN-WO3 nanocomposite can be attributed to its higher conductivity and prominent intra-chain unidirectional charge transport. Present communication opens a new window for energy-saving, eco-friendly, cost-effective, recoverable and reproducible, easily processable and efficient PAN-WO3 nanocomposite-based hydrazine detecting device.

Polysubstituted High-Entropy [LaNd](Cr0.2Mn0.2Fe0.2Co0.2Ni0.2)O3 Perovskites: Correlation of the Electrical and Magnetic Properties.

Abstract: La-, Nd- and La/Nd-based polysubstituted high-entropy oxides (HEOs) were produced by solid-state reactions. Composition of the B-site was fixed for all samples (Cr0.2Mn0.2Fe0.2Co0.2Ni0.2) with varying of A-site cation (La, Nd and La0.5Nd0.5). Nominal chemical composition of the HEOs correlates well with initial calculated stoichiometry. All produced samples are single phase with perovskite-like structure. Average particle size is critically dependent on chemical composition. Minimal average particle size (~400 nm) was observed for the La-based sample and maximal average particle size (5.8 μm) was observed for the Nd-based sample. The values of the configurational entropy of mixing for each sample were calculated. Electrical properties were investigated in the wide range of temperatures (150–450 K) and frequencies (10−1–107 Hz). Results are discussed in terms of the variable range hopping and the small polaron hopping mechanisms. Magnetic properties were analyzed from the temperature and field dependences of the specific magnetization. The frustrated state of the spin subsystem was observed, and it can be a result of the increasing entropy state. From the Zero-Field-Cooling and Field-Cooling regimes (ZFC-FC) curves, we determine the average and Smax maximum size of a ferromagnetic nanocluster in a paramagnetic matrix. The average size of a ferromagnetic cluster is ~100 nm (La-CMFCNO) and ~60 nm (LN-CMFCNO). The Smax maximum size is ~210 nm (La-CMFCNO) and ~205 nm (LN-CMFCNO). For Nd-CMFCNO, spin glass state (ferromagnetic cluster lower than 30 nm) was observed due to f-d exchange at low temperatures.

Temperature dependent AC-Conduction and relaxation mechanism of spinel MgCo2O4 system by impedance spectroscopy

Abstract: MgCo2O4 a spinel ceramic was studied by complex impedance spectroscopy. The measurements were performed in the temperature and frequency ranges 83 to 323 K and 40 Hz to 1 MHz, respectively. The decreasing trend in Nyquist plots semicircles was observed as a function of temperature indicating the distribution of different relaxaion processes. Two equivalent circuit models (RgCg)(RgbQgb) below 173 K and above this temperature (Rg)(RgbQgb) were applied to fit the impedance data. The values obtained for Rgb and Rg from these model fitting displayed semiconducting like behavior in the material. The variable range hopping model was employed to explain electrical conduction mechanism of grains and grain-boundaries. The complex modulus analysis asserted the contribution of both bulk and grain boundaries in electrical conductivity at low temperatures. The frequency dependent ac-conductivity followed the universal Jonscher's power law. The frequency exponent n showed a decreasing trend with increasing temperature. The ac-conductivity data was further elaborated by Correlated Barrier Hopping Model (CBH).

Study of variable range hopping conduction mechanism in nanocrystalline carbon thin films deposited by modified anodic jet carbon arc technique: application to light-dependent resistors

Abstract: In this study, we report the deposition of nanocrystalline carbon thin films by modified anodic jet carbon arc technique assisted with inert-helium and reactive-nitrogen gaseous environments. The modified anodic jet technique facilitates to generate a very high localized pressure near the arc spot for the growth of nanocrystallites of carbon at high arc temperature and high pressure. The deposited films were analyzed for the growth of nanocrystallites in amorphous carbon structure under high-resolution transmission electron microscopy (HRTEM). The HRTEM studies reveal the distribution of nanocrystallites in the amorphous carbon matrix. The temperature-dependent conduction behavior of the deposited films has also been analyzed under Mott’s variable range hopping conduction mechanism. Both sets of the film are found to follow three-dimensional variable range hopping conduction mechanisms for the transport of charge carriers. The deposited films are also analyzed for their applications to energy-saving light-dependent resistors under an illumination intensity of ~ 100 mW/cm2 of white light. The reasonably high value of detectivity values ~ 1.26 × 106 and 3.12 × 106 Jones and relatively lower values of trap depth 0.6826 and 0.6834 eV for the nanocrystalline films deposited in helium and nitrogen environments suggest trapped state-assisted significantly high power conversion efficiency. This supports the suitability of the deposited films for light-dependent resistor applications.

Impedance Spectroscopy and ac Conductivity in Ba0.5Sr0.5TiO3-Ca10(PO4)6(OH)2 Ceramic Composites: An Electrical Approach to Unveil Biocomposites.

Abstract: We report bioceramic composites of varying concentrations of Ba0.5Sr0.5TiO3 (BST) and Ca10(PO4)6(OH)2 (HAP) for the analysis of electrical properties. The motivation is to predict the suitability of the composites for bio-electrets or the practical possibility in designing electro-active scaffolds. X-ray diffraction (XRD) and field-emission scanning electron microscopy (FESEM) are used to analyze the microstructural evolution of the composites. A systematic variation in the grain and crystallite sizes is noticed from the FESEM and XRD, along with the presence of Sr5(PO4)3(OH) (SAP). The temperature and frequency variations of the dielectric properties of the composites are studied. Modeling of the dielectric properties with the microstructural properties and at. % of the monolith BST is presented. Cole-Cole formalism is adopted to model the electrical behavior of the synthesized composites. Furthermore, the ac conductivity analysis reveals that Mott's variable range hopping (VRH) conduction is the most appropriate formalism that successfully describes the conduction process. The established Mott's VRH is also related to the polarization mechanisms active in the specimens. Our study projects a correlation between the electrical and biological properties by predicting the protein adsorption behavior from the perspective of impedance spectroscopy.

Density of states, DC conductivity and physical properties of Ag2S-Ge–Te–Se chalcogenide glassy system

Abstract: Here, development of some Ag2S mixed chalcogenide glassy systems has been discussed in the light of DC conductivity model and variable range hopping model. X-ray diffraction (XRD) pattern has been collected to explore microstructure of them. Various nanophases such as Ag2Se and GeSe2 have been pointed out from XRD studies. Defects have been identified in the form of dislocation, and their roles in the electrical transport properties have been established. FE-SEM micrographs exhibit their amorphous nature with distributed sharper crystal-like structure. Optical phonon frequency may cause to enhance the structural vibrations by means of increasing DC electrical conductivity. Significant enhancement of density of states near Fermi level at low and high temperatures have been explained. Present glassy structure is expected to expand to reduce the scattering cross-section for getting higher values of N(EF) at higher temperature up to a great extent.

Dielectric relaxation and variable range hopping conduction in sol-gel auto combustion derived La0.7Bi0.3Fe0.5Mn0.5O3 manganite

Abstract: In the present work, the polycrystalline La0.7Bi0.3Fe0.5Mn0.5O3 maganite was synthesized by sol-gel auto-combustion method. The room temperature X-ray powder diffraction pattern confirmed the orthorhombic phase of the sample with pbnm space group. Frequency and temperature dependent electric modulus and impedance spectroscopic analyses revealed two electro-active components of the material characterized by highly resistive grain boundaries and comparatively conducting grains, respectively. The normalized scaling behavior of modulus plots divulged the temperature independence of the dynamic process in electro-active regions. The fitting of impedance data by the equivalent circuit (RG||QG) + (RGB||QGB) was applied to explore the physical parameters associated with grains and grain boundaries. The dispersion in dielectric permittivity and loss factor at lower frequencies attributed to the space charge polarization were observed. The electrical conductivity obeyed Mott’s T1/4 law and the slope of frequency dependent AC conductivity spectra changed due to multiple activation barriers and active traps, consistent with the variable-range hopping conduction in the material.

Tunable positive magnetoresistance of magnetic polyaniline nanocomposites

Abstract: Tunable positive magnetoresistance (MR) of polyaniline (PANI) nanocomposites with different magnetic nanoparticles (Fe3O4 or CoFe2O4) have been prepared through the facile surface-initiated polymerization (SIP) method. The characterizations including Fourier transform infrared (FT-IR), X-ray diffraction (XRD), scanning electron microscope (SEM), and transmission electron microscope (TEM) indicate that the magnetic nanoparticles were successfully coated by PANI. A quasi 3D variable range hopping (VRH) mechanism was observed in the magnetic PANI nanocomposites through Mott VRH model. The effects of magnetic field and magnetic nanoparticle on the charge carrier’s behavior were studied by the wave function shrinkage model. The localization length and average hopping length decrease with increasing magnetic field, while the density of state at the Fermi level increases with increasing magnetic field. The enhanced dielectric property and magnetic property of the magnetic PANI nanocomposites were reported as well. Magnetic polyaniline nanocomposites with tunable positive magnetoresistance.

A crossover from Efros–Shklovskii hopping to activated transport in a GaAs two-dimensional hole system at low temperatures

Abstract: In this paper, we discuss low-temperature hopping-conductivity behavior in the insulating phase, in the absence of a magnetic field. We conduct a theoretical study of the crossover from hopping to activated transport in a GaAs two-dimensional hole system at low temperatures, finding that a crossover takes place from the Efros-Shklovskii variable-range hopping (VRH) regime to an activated regime in this system. This conductivity behavior in p-GaAs quantum wells is qualitatively consistent with the laws laid down in theories of localized electron interactions. Given sufficiently strong interactions, the holes in the localized states are able to hop collectively.

Investigation of physical properties of manganite on example of Sm0.35Pr0.2Sr0.45MnO3

Abstract: We have prepared a 20% Pr doped Sm0.55Sr0.45MnO3 sample by using the solid-state reaction and studied the magnetic and electrical properties. Arrott plots confirm the occurrence of a first-order phase transition. All the signatures of the FM first-order transition disappear above a critical magnetic field. Based on dc-electrical conductivity, a metal semi-conductor transition has been observed at TM-Sc = 180 K. Temperature-dependent conductivity measurement has been carried out to probe the electrical conduction mechanism, which revealed the change of conduction mechanism from the variable range hopping to the small polaron hopping around 180 K. The temperature dependence of e’ indicates a dielectric anomaly, implying a possible ME coupling in the vicinity of FM transition. The modulus spectra and complex impedance of the reported sample undoubtedly showed the non-Debye type relaxation process. We have obtained different activation energies values calculated from the conduction and relaxation mechanism. An electrical equivalent circuit model was suggested to explain the impedance contributions.

Electrical and magnetic properties of V2O5 microstructure formed by self-assembled nanorods

Abstract: In this study, the hydrothermal method is used for the synthesis of Vanadium pentoxide (V2O5) microstructure. Analysis stands for the growth of microstructure in only the orthorhombic phase. The dot pattern in SAED image shows the crystalline nature of microstructure. Raman analysis shows the layered structure of V2O5. The morphological study of V2O5 shows the flower-like microstructure formed by nanorods having diameter ~3 μm. The transport measurement study confirmed the semiconductor nature of the microstructure in the temperature range 200K ≤ T ≤ 380K, and by Arrhenius analysis, we find the activation energy as 185 meV in the temperature range 285K–380K. In the lower temperature range 200K–285K the system shows the variable range hopping conduction of the charge carriers. An anomalous magnetization response is also evidenced in these V2O5 specimen in a narrow temperature range of ~45–60K where the magnetic ground state is antiferromagnetic. Interestingly, this state is bounded by paramagnetic states both at high and low temperatures.

Effect of terbium doping on the optical, electrical and magnetic properties of nanocrystalline nickel ferrite

Abstract: The present study focuses on the preparation and the characterization of a series of rare-earth terbium (Tb3+) ion-doped nickel ferrite (NFO) nanoparticles [(NiTbxFe2−xO4, x = 0.10, 0.15, 0.20)]. These samples were synthesized using sol–gel method. Rare-earth Tb was doped to tune the structural, optical, dielectric and magnetic features of the spinel nickel ferrite nanoparticles. Therefore, the systematic impact of Tb3+ on the structural, optical, electrical and magnetic studies were carried out. X-ray diffraction (XRD) profile reveals the presence of a single spinel ferrite phase for small doping while in moderately doped NFO NPs, an additional orthorhombic phase along with the spinel phase exists. The strain is increased in Tb-doped NFO lattice. X-ray photoelectron spectroscopy (XPS) study also confirms the presence of Tb ions at the tetrahedral site of Tb-NFO nanoparticles. In absorption spectroscopy, the shift of absorption towards the visible region is significantly increased with the increase in doping amount of trivalent Tb ion. The empirical variable range hopping (VRH) model is introduced to evaluate the density of states and the hopping range, respectively. The electrical properties of Tb-doped NFO compounds exhibit high values of dielectric constant at room temperature with Tb doping. Electric modulus study confirms the presence of non-Debye type of relaxation behaviour in the samples. Two relaxation peaks are observed at higher doping concentration. Room temperature magnetization study confers hysteresis behaviour in Tb-doped nickel ferrite nanoparticle at high frequencies. Different magnetic parameters are evaluated from the magnetic hysteresis loops. There is a significant decrease in magnetization with Tb doping due to the lower magnetic ordering of rare-earth ions.

Anderson localization of electron states in a quasicrystal

Abstract: The influence of disorder on the critical electron states in a quasiperiodic lattice is a subject of intense research. In this work, we report the occurrence of Anderson localization in an icosahedral (i) polygrain quasicrystal Al-Pd-Re due to site disorder using hard x-ray photoemission spectroscopy, resistivity, and density functional theory (DFT). Photoemission spectroscopy shows that the density of states is enhanced at the Fermi level in polygrain $i$-Al-Pd-Re compared to single-grain $i$-Al-Pd-Re. In contrast, the conductivity of the former is an order of magnitude reduced compared to the latter, indicating that these electron states are localized. DFT shows that these states originate primarily from Re $5d$-Pd $4d$ hybridization and are enhanced in polygrain $i$-Al-Pd-Re due to compositional difference, but are broadened because of disorder that brings about Anderson localization. This is established by the Mott variable range hopping behavior of conductivity, and the estimated localization length is 23 \AA{}.

Evolution of transition metal charge states in correlation with the structural and magnetic properties in disordered double perovskites Ca2-xLaxFeRuO6 (0.5 ≤ x ≤ 2).

Abstract: A series of disordered Ca1.5La0.5FeRuO6, CaLaFeRuO6 and La2FeRuO6 double perovskites were prepared by the solid-state reaction method and investigated by neutron powder diffraction, X-ray absorption near-edge structure (XANES) analysis at the Ru–K edge, Mossbauer spectroscopy, DC magnetization and resistivity measurements. All compounds crystallize in the orthorhombic crystal structure with the space group Pbnm down to 3 K, showing a random distribution of Fe and Ru at the B site. Thermogravimetric analysis indicates oxygen deficiency in the Ca-rich and formal oxygen hyperstoichiometry in the La-rich members of the present series. While Mossbauer spectra verify the Fe3+ state for all compositions, the XANES study reveals a variable Run+ oxidation state which decreases with increasing La content. The end member actually is a Ru3+/Ru4+ compound with possibly some cation vacancies. From magnetic susceptibility and neutron diffraction measurements, the presence of a G-type antiferromagnetic ordering was observed with a drastic increase in transition temperature from 275 K (Ca1.5La0.5FeRuO6) to 570 K (La2FeRuO6). Mossbauer spectroscopy confirms the presence of long-range ordering but, due to local variations in the exchange interactions, the magnetic states are microscopically inhomogeneous. All the samples are variable range hopping semiconductors. A complex interplay between structural features, charge states, anion or cation defects, and atomic disorder determines the magnetic properties of the present disordered 3d/4d double perovskite series.

Analytical Surface Potential-Based Compact Model for Independent Dual Gate a-IGZO TFT

Abstract: A surface potential-based compact model for independent dual gate (IDG) amorphous In-Ga-Zn-O thin-film transistors (IDG a-IGZO TFTs) is proposed here. The transport theories of percolation conduction, trap-limited conduction (TLC), and variable range hopping (VRH) in extended and localized states are first considered simultaneously via Schroder method, obtaining a physical description of the transport mechanism under different conditions of temperature and gate voltage. Moreover, a single formulation of front and back surface potentials which is valid and extremely accurate in all operation regimes is developed. Based on the transport theories and surface potentials, the complete compact model is developed and verified using both numerical simulation and experiment with an excellent agreement, and the threshold compensation effect is also included. Finally, the compact model is coded in Verilog-A, and implemented in a vendor CAD environment, which suggested that the proposed model can be successfully applied to circuit design.

In-Situ Chemical Synthesis, Microstructural, Morphological and Charge Transport Studies of Polypyrrole-CuS Hybrid Nanocomposites

Abstract: Hybrid nanocomposites (HNCs) of polypyrrole (PPy) and CuS were synthesized by an in situ chemical polymerization method. The HNCs were prepared by varying the CuS nanoparticles weight percentage (10–40%) in PPy matrix. The XRD and FESEM characterization indicated the uniform distribution of CuS nanoparticles in PPy matrix. The XRD pattern revealed the presence of hexagonal CuS peaks overlapped with amorphous PPy pattern. Williamson Hall method was employed to estimate intrinsic strain in HNCs. FTIR spectrum revealed the shifting of 1025 cm−1 peak toward higher wavenumber, indicating insertion of CuS nanoparticles in PPy. The room temperature electrical conductivity of PPy is found to be increased from 1.15 × 10−1 to 3.70 S/cm as the content of CuS nanoparticles increases up to 40 wt% in HNCs. To explore the charge transport mechanism in HNCs,the conductivity was measured in the temperature range of 300–15 K. The measured conductivity data was analyzed with the help of Arrhenius model and 3d Mott’s variable range hopping (VRH) model in the temperature range of 35–300 K. The approximate values of Mott’s parameters at 300 K such as the density of states at Fermi level, average hopping distance and average hopping energy of HNCs were estimated as ~ 4×1024 cm−3eV−1, ~ 1.7 A and ~ 10 meV respectively.

Electrical Conduction Properties of Hydrogenated Amorphous Carbon Films with Different Structures

Abstract: Hydrogenated amorphous carbon (a-C:H) films have optical and electrical properties that vary widely depending on deposition conditions; however, the electrical conduction mechanism, which is dependent on the film structure, has not yet been fully revealed. To understand the relationship between the film structure and electrical conduction mechanism, three types of a-C:H films were prepared and their film structures and electrical properties were evaluated. The sp2/(sp2 + sp3) ratios were measured by a near-edge X-ray absorption fine structure technique. From the conductivity–temperature relationship, variable-range hopping (VRH) conduction was shown to be the dominant conduction mechanism at low temperatures, and the electrical conduction mechanism changed at a transition temperature from VRH conduction to thermally activated band conduction. On the basis of structural analyses, a model of the microstructure of a-C:H that consists of sp2 and sp3-bonded carbon clusters, hydrogen atoms and dangling bonds was built. Furthermore, it is explained how several electrical conduction parameters are affected by the carrier transportation path among the clusters.

The Undoped Polycrystalline Diamond Film—Electrical Transport Properties

Abstract: The polycrystalline diamonds were synthesized on n-type single crystalline Si wafer by Hot Filament CVD method. The structural properties of the obtained diamond films were checked by X-ray diffraction and Raman spectroscopy. The conductivity of n-Si/p-diamond, sandwiched between two electrodes, was measured in the temperature range of 90–300 K in a closed cycle cryostat under vacuum. In the temperature range of (200–300 K), the experimental data of the conductivity were used to obtain the activation energies Ea which comes out to be in the range of 60–228 meV. In the low temperature region i.e., below 200 K, the conductivity increases very slowly with temperature, which indicates that the conduction occurs via Mott variable range hopping in the localized states near Fermi level. The densities of localized states in diamond films were calculated using Mott’s model and were found to be in the range of 9×1013 to 5×1014eV−1cm−3 depending on the diamond’s surface hydrogenation level. The Mott’s model allowed estimating primal parameters like average hopping range and hopping energy. It has been shown that the surface hydrogenation may play a crucial role in tuning transport properties.

Plasma-assisted combustion synthesis of p-type transparent Cu incorporated NiO thin films—Correlation between deposition chemistry and charge transport characteristics

Abstract: Optically transparent and conducting Cu-incorporated NiO thin films are deposited by low-temperature plasma-assisted solution combustion synthesis. The hole conductivity, crystallographic nature, and the film morphology in this semiconductor highly depend upon the amount of Cu incorporation in the material, which is found to have a strong influence on the defect density in these films, ultimately leading to a larger variation in their optoelectronic properties. The change in conductivity with Cu incorporation follows the generalized effective medium theory model with a fitted percolation threshold near zero. Under optimal composition of the Cu to Ni ratio in the as-deposited thin film, the p-type electrical resistivity of 0.85 Ω cm is obtained with a hole concentration of 3.9 × 1018 cm−3 and hole mobility of 1.9 cm2 V−1 s−1, with an average visible transmittance of 76%. Hole transport over a wide range of temperatures and Cu incorporation can be successfully modeled by Mott variable range hopping.