Variable Range Hopping Model Research Articles

Observation of 2D Conduction in Ultrathin Germanium Arsenide Field-Effect Transistors.

We report the fabrication and electrical characterization of germanium arsenide (GeAs) field-effect transistors with ultrathin channels. The electrical transport is investigated in the 20–280 K temperature range, revealing that the p-type electrical conductivity and the field-effect mobility are growing functions of temperature. An unexpected peak is observed in the temperature dependence of the carrier density per area at ∼75 K. Such a feature is explained considering that the increased carrier concentration at higher temperatures and the vertical band bending combined with the gate field lead to the formation of a two-dimensional (2D) conducting channel, limited to few interfacial GeAs layers, which dominates the channel conductance. The conductivity follows the variable-range hopping model at low temperatures and becomes the band-type at higher temperatures when the 2D channel is formed. The formation of the 2D channel is validated through a numerical simulation that shows excellent agreement with the experimental data.

Structural, electrical and magnetic properties of Sr1-xBixTiO3-δ ceramics

ABSTRACTThe structural, electrical and magnetic properties of Sr1-xBixTiO3-δ, (x = 0.10, 0.20, and 0.30) ceramics have been investigated. The X-ray powder diffraction (XRD) patterns at room temperature indicate that the structure of the samples is cubic with Pm3 m space symmetry group. The conduction of all investigated samples follows the three-dimensional variable-range hopping (VRH) model within the temperature range 100 K ≤ T ≤ 300 K. It was noticed that the paramagnetic behaviour depends on the content of Bi ions. The effective paramagnetic magnetic moment increased as the Bi content increased.

Low-temperature electrical and magnetic properties of La0.6Sr0.4Co0.2Fe0.8O3-δ nanofibers prepared by electrospinning

The microstructural, magnetic, and electrical properties of La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCFO) nanofibers prepared by electrospinning and calcination were systematically investigated. It was found that the as-prepared nanofibers manifested ferromagnetism at low temperatures. Moreover, the resistance and magnetoresistance (MR) data conformed to the one-dimensional variable range hopping (1D VRH) model and revealed that LSCFO nanofibers had different electronic conduction behavior at different temperatures and magnetic fields. LSCFO nanofibers had small negative MR at low temperatures and large positive MR at high temperatures, and the transition temperature ranged between 240 and 260 K. The negative MR and the positive MR of LSCFO nanofibers manifested a fit to the 1D weak localization model and the 1D VRH model, respectively. Therefore, the proposed work can provide an important reference to the design of solid oxide fuel cells (SOFCs) based on the LSCFO cathode.

Experimentally Validated Hopping-Transport Model for Energetically Disordered Organic Semiconductors

Charge transport in disordered organic semiconductors occurs by hopping of charge carriers between localized sites that are randomly distributed in a strongly energy dependent density of states. Extracting disorder and hopping parameters from experimental data like temperature dependent current-voltage characteristics typically relies on parametrized mobility functionals that are integrated in a drift-diffusion solver. Surprisingly, the functional based on the extended Gaussian disorder model (eGDM) has been extremely successful at this, despite it being based on the assumption of nearest neighbor hopping (nnH) on a regular lattice. We here propose a variable range hopping (VRH) model that has been integrated in a freeware drift-diffusion solver. The mobility model has been calibrated using kinetic Monte Carlo calculations and shows good agreement with the Monte Carlo calculations over the experimentally relevant part of the parameter space. The model is applied to temperature-dependent space charge limited current (SCLC) measurements of different systems. In contrast to the eGDM, the VRH model provides a consistent description of both p-type and n-type devices. We find a critical ratio of aNN/$\alpha$ (mean inter-site distance / localization radius) of ~3 below which hopping to non-nearest neighbors becomes important around room temperature and the eGDM cannot be used for parameter extraction. Typical (Gaussian) disorder values in the range 45-120 meV are found, without any clear correlation with photovoltaic performance when the same active layer is used in an organic solar cell.

Enhancement of conductivity and transmittance of graphene oxide/PEDOT:PSS electrodes and the evaluation of charge transfer dynamics

Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate), PEDOT:PSS, is a polymeric composite that can substitute indium tin oxide (ITO), enabling ITO-free organic devices. However, PEDOT:PSS films have conductivities two orders of magnitude lower than ITO due to the presence of PSS, an insulator polymer added to provide water dispersion to PEDOT. To enhance the electrical performance of the films and overcome the insulator limitations, we prepared thin films of graphene oxide (GO) and PEDOT:PSS composites. The dried films were then treated with ethylene glycol (EG). An increment of two orders of magnitude in conductivity values was observed, as well as an increase in transmittance at the visible region. Also, GO:PEDOT:PSS thin films became more hydrophobic due to the partial removal of PSS. The mass ratio that is equal to 0.058 of GO/PEDOT provides higher electrical conductivity due to higher chain orientation of PEDOT as seen in the near edge X-ray absorption fine structure and resonant Auger spectroscopy measurements. The electrical transport follows the Mott Variable Range Hopping model for low temperature, showing that the conduction occurs in three dimensions. The solvent treatment increases the characteristic conductivity and decreases the activation hopping energy, with the characteristic temperature remaining almost unchanged. It indicates that the EG treatment promotes a decrease in charge transfer time and resistivity.

Correlation between magnetic and transport properties of rare earth doped perovskite manganites La0.6R0.1Ca0.3MnO3 (R = La, Nd, Sm, Gd, and Dy) synthesized by Pechini process

Here, we report a comparative study of rare earth doped La0.6R0.1Ca0.3MnO3 (R = La, Nd, Sm, Gd, and Dy) samples synthesized by sol-gel Pechini process. All the compounds are single-phased and crystallize in Pbnm space group with orthorhombic symmetry. Temperature dependent magnetization measurements revealed that all our compounds exhibit a paramagnetic (PM)–ferromagnetic (FM) transition with decrease in both ordering temperature TC and the freezing temperature TF with decrease in ionic radii of lanthanide ions. The compounds display metal-semiconductor transition and the temperatures (TMS) obtained from the electrical resistivity measurements are consistently higher than those of the Curie temperatures TC. The transport properties are dominated by the Variable-range hopping (VRH) model above TMS, showing an increase in characteristic VRH temperature with decreasing size of rare-earth ion. The estimated hopping distance (RH) in all the samples is close to nearest neighbor Mn–O–Mn distance (~5 Å) in the ab plane.

Variable-range hopping conduction with positive and negative magnetoresistance transformation in reduced graphene oxide mesostructures

Graphene is a two-dimensional (2D) material with excellent physical properties, and its special charge transport characteristics promote extensive exploration based on graphene electronic devices. In order to determine the effect of the mesostructure on the electromagnetic transport properties of graphene, the temperature dependencies of the resistance and the magnetoresistance (MR) of reduced graphene oxide paper (rGOP) and reduced graphene oxide aerogel fiber (rGAF) were systematically measured. The conduction mechanism was systematically studied, and the graphene paper and aerogel fiber were in accordance with Mott’s 2D and 3D variable-range hopping (VRH) models, respectively. With the decrease of temperature, the transition from small-negative MR to large-positive MR was observed, the influences of wave function shrinkage and quantum interference effects on positive and negative MR were discussed in the framework of hopping conduction.

Microstructure and electrical transport mechanisms of the Ca-doped LaMnO3 films grown on MgO substrate

La0.7Ca0.3MnO3 (LCMO) thin films have been grown on MgO substrate using the metal organic deposition process. The lattice mismatch between LCMO and MgO is relatively large around 8.14% imposing large in-plane-tensile strain and out-of-plane-compressive strain on the film. Hence the structural, microstructural and electrical properties have been found to be strongly correlated to the strain degree and relaxation. Cross-section transmission electron microscopy observations demonstrate the presence of microstructural defects due to the large lattice mismatch between the LCMO and MgO. In addition to structural and microstructural defects, X-ray diffraction and Raman spectroscopy measurements show the existence of a minor phase of MnO in the film. Magnetization versus temperature measurement show a relatively low Curie temperature around 75 K. The electrical behavior is found to be semiconducting over a large temperature interval. The electrical transport mechanisms have been investigated using the small polaron hopping and variable range hopping models and correlated to their microstructural properties.

Dominant role of trioctylphosphine on the particle size and various properties of CoO nanoparticles

The dominant role of trioctylphosphine (TOP) on the particle size and various properties of CoO nanoparticle (NP) samples having Scherrer size in the range 9.4–42.8 nm is reported here; samples have been characterized in detail systematically. Remarkably, while nanosized CoO or CoO/Co nanocomposites with monoclinic or face-centered cubic (fcc) phase can be prepared depending on the synthesis conditions and particle size systematically decreases with increase in concentration of TOP, there is an increase in optical band gap and electrical resistivity with decrease in particle size. The highly semiconducting electrical resistivity is explained with the variable range hopping and small polaron hopping models that indicate in turn enhanced localized density of states, but decrease in Debye temperature (θD) as the particle size decreases. Curiously, 25.9 nm sample shows anomalous behaviour. Moreover, the silica-coated 9.4 nm CoO NP and bare 10.8 nm CoO NP samples exhibit ferromagnetism at 300 K even though their bulk counterpart is antiferromagnetic, with attractive indications of possible potential biomedical applications of the former.

Tuning the Electrical and Thermoelectric Properties of N Ion Implanted SrTiO3 Thin Films and Their Conduction Mechanisms

The SrTiO3 thin films were fabricated by pulsed laser deposition. Subsequently ion implantation with 60 keV N ions at two different fluences 1 × 1016 and 5 × 1016 ions/cm2 and followed by annealing was carried out. Thin films were then characterized for electronic structure, morphology and transport properties. X-ray absorption spectroscopy reveals the local distortion of TiO6 octahedra and introduction of oxygen vacancies due to N implantation. The electrical and thermoelectric properties of these films were measured as a function of temperature to understand the conduction and scattering mechanisms. It is observed that the electrical conductivity and Seebeck coefficient (S) of these films are significantly enhanced for higher N ion fluence. The temperature dependent electrical resistivity has been analysed in the temperature range of 80–400 K, using various conduction mechanisms and fitted with band conduction, near neighbour hopping (NNH) and variable range hopping (VRH) models. It is revealed that the band conduction mechanism dominates at high temperature regime and in low temperature regime, there is a crossover between NNH and VRH. The S has been analysed using the relaxation time approximation model and dispersive transport mechanism in the temperature range of 300–400 K. Due to improvement in electrical conductivity and thermopower, the power factor is enhanced to 15 µWm−1 K−2 at 400 K at the higher ion fluence which is in the order of ten times higher as compared to the pristine films. This study suggests that ion beam can be used as an effective technique to selectively alter the electrical transport properties of oxide thermoelectric materials.

Optical properties of RAS (N–CH3-2-NH2-5Cl-Py)(TCNQ)(CH3CN) solvate

The optical, electro-physical and magnetic properties of the radical-anionic salt of the so-called simple composition – (N–CH3-2-NH2-5Cl-Py)(TCNQ) were investigated, (N–CH3-2-NH2-5Cl-Py – N-methyl-2-amine-5-chloro-pyridine). Raman’s scattering spectra and infrared reflection spectra are very similar to quasi-one-dimensional salts based on TCNQ. However, in our case, almost isolated π-dimers (TCNQ)22− are used instead of columns built with TCNQ. Supramolecular contacts involving the amine-cation group combine dimers into infinite chains and thereby reduce the local symmetry of the molecules TCNQ to C2h. These peculiarities of the structure may be the cause of this extraordinary behaviour and the manifestation of vibronic-interaction. The salt is a semiconductor, the temperature dependence of the electrical conductivity is subordinated to the Variable Range Hopping model. According to the ESR data, the salt exhibits weak antiferromagnetic properties.

Charge transport mechanism and thermoelectric behavior in Te:(PEDOT:PSS) polymer composites

Polymer thermoelectics are an emerging class of materials for realizing flexible thermoelectric devices due to its low cost, solution processability and bio-compatibility. We report a systematic investigation of the charge transport and thermoelectric properties of polymer composites comprising of tellurium (Te) nanorods and poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) spanning over a temperature range from 10 K up to 320 K. The temperature dependence of electrical resistivity for Te:(PEDOT:PSS) composites is dominated by the thermal assisted hopping following Mott’s and Efros-Shklovskii (ES) variable range hopping (VRH) mechanism. A cross-over from ES to Mott VRH model is observed near 105 K. The Seebeck coefficient and the electrical resistivity values for Te:(PEDOT:PSS) composites are found to decrease with the increase in PEDOT:PSS concentration at 300 K. Moreover, the thermal conductivity of the polymer composites is found to decrease due to enhanced phonon-phonon scattering ascribed to the acoustical mismatch between the Te-nanorods and PEDOT:PSS interface. The maximum thermoelectric figure of merit, ZT, of the polymer composites is estimated to be 0.013 ± 0.001 for Te:(PEDOT:PSS) (1:5) weight percentage. The bending test done on the polymer composite film and mechanical properties investigated using nanoindentation technique suggest the flexible nature of the composite sample which is promising for room temperature flexible thermoelectric application.

High-pressure synthesis of A-site ordered perovskite CaMn3(Fe3Mn)O12 and sequential long-range antiferromagnetic ordering and spin glass transition

An AA’3B4O12-type perovskite oxide CaMn3(Fe3Mn)O12 was synthesized at 8 GPa and 1473 K. X-ray diffraction shows a cubic crystal structure with space group Im-3. The charge states are verified by soft x-ray absorption spectroscopy to be CaMn3+3(Fe3+3Mn4+)O12, where the Ca2+ and Mn3+ are 1:3 ordered respectively at A and A′ sites, while the Mn4+ and Fe3+ are disorderly distributed at B site. The spin interaction of A′-site Mn3+ ions causes a long-range antiferromagnetic phase transition at about 39 K. Subsequently, a spin glass transition is found to occur around 14 K due to the randomly distributed Fe3+ and Mn4+ at B site. Moreover, the spin glass behavior follows a dynamic scaling power law. The temperature dependent resistivity can be well fitted by a 3D Mott variable-range hopping model, indicating the insulating nature of CaMn3(Fe3Mn)O12 due to the strong electron correlation effects.

Enhancement of Charge Transport in Polythiophene Semiconducting Polymer by Blending with Graphene Nanoparticles.

This paper describes a study on the charge transport in a composite of liquid-exfoliated graphene nanoparticles (GNPs) and a polythiophene semiconducting polymer. While the former component is highly conducting, although it consists of isolated nanostructures, the latter offers an efficient charge transport path between the individual GNPs within the film, overall yielding enhanced charge transport properties of the resulting bi-component system. The electrical characteristics of the composite layers were investigated by means of measurements of time-of-flight photoconductivity and transconductance in field-effect transistors. In order to analyze both phenomena separately, charge density and charge mobility contributions to the conductivity were singled out. With the increasing GNP concentration, the charge mobility was found to increase, thereby reducing the time spent by the carriers on the polymer chains. In addition, for GNP loading above 0.2 % (wt.), an increase of free charge density was observed that highlights an additional key role played by doping. Variable-range hopping model of a mixed two- and three-dimensional transport is explained using temperature dependence of mobility and free charge density. The temperature variation of free charge density was related to the electron transfer from polythiophene to GNP, with an energy barrier of 24 meV.

MnO2‐Magnetic Core‐Shell Structured Polyaniline Dependent Enhanced EMI Shielding Effectiveness: A Study of VRH Conduction

AbstractThe core‐shell structured based Polyaniline/MnO2 composites were synthesized by chemical oxidation method, wherein MnO2 particles acts as a magnetic core while polyaniline chains wrapped over these particles acts as a shell. Four different loading concentrations of MnO2 particles (10, 20, 30 and 40 wt %) were used to investigate the effect of MnO2 concentration on the EMI shielding effectiveness. X‐ray diffraction (XRD) studies performed on pristine MnO2 and composite samples reveal the polycrystalline nature of the MnO2 particles and semi‐crystalline nature of the composites with a sharp peak around 25° over a broad hump. Vibrating sample magnetometry (VSM) studies performed on the pristine MnO2 particles as well composite sample (S4) at higher loading (40 wt %) concentration of MnO2 suggest the ferromagnetic nature at room temperature. The charge transport properties of the prepared samples have been investigated under variable range hopping (VRH) conduction. Mott's three dimensional VRH model has been found to be applicable for conduction in the present samples. The dc conductivity of the prepared samples is found to increase with MnO2 loading concentrations, which suggest their possible validity for EMI shielding applications. Therefore, the prepared samples are analyzed for EMI shielding properties in the frequency range of 8.2 to 12.4 GHz (X‐band). The sample prepared with 30 wt% of MnO2 has as much as 50 dB shielding effectiveness, which is quite higher than that required for industrial application (∼30dB).

Structural and physical properties of MnO mixed lead zirconium silicate glass ceramics: Dielectric relaxation spectra and conduction phenomena

In this study we have investigated the influence of MnO content on electrical characteristics (viz., dielectric properties including impedance spectra over wide ranges of continuous frequencies and temperatures, a.c and d.c. conductivities) of lead zirconium silicate glass ceramic with a view to assess its insulating character. Such assessment is crucial for using the material in different electronic devices, for example, as insulating sheets in the display panels. The variations of ε, tan δ (measured at different temperatures and frequencies) observed with the concentration of MnO are analysed in terms of structural transformations taking place in the glass ceramics. Although, the dielectric constant and loss tangent with temperature and frequency have not exhibited any distinct dipolar relaxation effects, the electric moduli (evaluated using real and imaginary parts of dielectric constant) have exhibits such effects clearly. Moreover, the variation of relaxation time (evaluated from electric moduli spectra) indicated multiple relaxation times for dipoles. Relaxation effects were ascribed to the complexes of octahedral Mn2+ ions. The conductivity (both ac and dc) with concentration of MnO exhibited the minima at 0.6 mol% of MnO. This effect is attributed to predominant presence of Mn ions in tetrahedral positions that were assumed to alternate with SiO4 units in the glass network. In the studied samples, both ionic and polaronic components are predicted to contribute to the conductivity. The polaronic component (arises due to exchange of polarons between Mn2+ ↔ Mn3+ ions) seemed to be gradually dominating over the ionic component with progressive insertion of MnO content up to 0.6 mol% in the bulk glass ceramic sample. The near temperature invariant part of σac behaviour (in the low temperature range) is analysed using both quantum mechanical tunnelling (QMT) model and the concentration of defect energy states near the Fermi level (N(EF), evaluated in this range of temperatures, was found to be the minimal for the sample M6. Analysis of dc conductivity results suggested the small polaron hoping (SPH) model holds good in the high temperature region; however, for the low temperature part of dc conductivity variable range hopping (VRH) model seems to be more appropriate. Finally, with the increase of MnO concentration up to 0.6 mol% in the titled glass ceramics, the conductivity decreases and leads to increase in the insulating strength of the material. This unique functionality may also make the investigated glass ceramic material perspective for medical applications in addition to their usage as insulating sheets in the display panels.

Tuning magnetoresistance and electrical resistivity by enhancing localization length in polyaniline and carbon nanotube composites

We report low temperature electrical resistivity and magnetoresistance (MR) measurements of conducting polyaniline (PANI) and multiwalled-carbon nanotube (MWCNT) composites. We have used an in-situ oxidative polymerization method to synthesize hydrochloric acid-doped PANI composites with MWCNT weight percentages of 0, 5, 10 and 15. The temperature dependence of resistivity is studied from room temperature to 4.2 K and analysed by a Mott variable range hopping (VRH) model. The resistivity increases from \(1.1\times 10^{-3}\,\Omega \mathrm {m}\) at 300 K to \(65.75\,\Omega \mathrm {m}\) at 4.2 K, almost four orders of the magnitude change with temperature for pure PANI. Whereas the PANI composite with 15% MWCNTs shows less variation from \(4.6\times 10^{-4}\,\mathrm{}\) to \(3.5\times 10^{-2}\,\Omega \mathrm{m}\). The huge change in resistivity is due to the localization of charge carriers in the presence of disorder. At 4.2 K MR shows transition from positive to negative with higher MWCNT loading. Samples with 5 and 10% MWCNTs show positive MR, whereas the 15% MWCNT loaded sample shows negative MR. The positive and negative MR are discussed in terms of the wave function shrinkage effect and quantum interference effect on VRH conduction.

Growth of high quality Sr2IrO4 epitaxial thin films on conductive substrates**Project supported by the National Natural Science Foundation of China (Grant Nos. 51627901 and 11574287), the National Key Research and Development Program of China (Grant No. 2016YFA0401004), and Hefei Science Center-Chinese Academy of Sciences (Grant No. 2016HSC-IU004). X. Z. acknowledges the support of the Youth Innovation Promotion Association of Chinese

Ruddlesden–Popper iridium oxides have attracted considerable interest because of the many proposed novel quantum states that arise from the large spin–orbit coupling of the heavy iridium atoms in them. A prominent example is the single layer Sr2IrO4, in which superconductivity has been proposed under electron doping. However, the synthesis of Sr2IrO4 high quality thin films has been a huge challenge due to the easy formation of impurities associated with different numbers of SrO layers. Thus techniques to optimize the growth of pure phase Sr2IrO4 are urgently required. Here we report the deposition of high quality Sr2IrO4 thin films on both insulating SrTiO3 and conducting SrTiO3:Nb substrates using pulsed laser deposition assisted with reflective high-energy electron diffraction. The optimal deposition temperature of Sr2IrO4 epitaxial films on SrTiO3:Nb substrates is about 90 °C lower than that on SrTiO3 substrates. The electrical transports of high quality Sr2IrO4 films are measured, which follow the three-dimensional Mott variable-range hopping model. The film magnetizations are measured, which show weak ferromagnetism below ∼240 K with a saturation magnetization of /Ir at 5 K. This study provides applicable methods to prepare high quality 5d Sr2IrO4 epitaxial films, which could be extended to other Ruddlesden–Popper phases and potentially help the future study of exotic quantum phenomena in them.

Enhanced multiferroic, magnetodielectric and electrical properties of Sm doped Lanthanum ferrite nanoparticles

In this work, we observe an enhancement of multiferroic property correlated with magnetic, magnetodielectric and ferroelectric properties in lanthanum ferrite (LaFeO3) nanoparticles by virtue of Samarium (Sm) doping in La site. The pure and doped lanthanum ferrite nanoparticle was synthesized by a simple solgel auto combustion route. We observe the positive magnetodielectric coupling in the samples and maximum 0.9% enhancement at the 1 T field in 10% Sm doped LaFeO3 nanoparticles. Highest value of saturation magnetization (1.84 emu/gm) is achieved in Sm doped LaFeO3 system. Particle size is decreased from 58 nm to 44 nm because of doping which is confirmed by transmission electron microscopy (TEM). The purity and nanocrystallinity nature of the prepared samples are confirmed by X-ray diffraction spectroscopy (XRD) pattern. DC and AC conduction mechanism and charge transport mechanism are illustrated in association with Mott’s variable range hopping (VRH) model and correlated barrier hopping (CBH) model respectively. The resistivity (DC & AC) is increased with a markable amount by virtue of Sm substitution. An improvement of dielectric properties is observed where dielectric loss is reduced due to the reduction of oxygen vacancies. The enhancement of magnetic properties is attributed to smaller crystallite size, inhomogeneous and disorder magnetic spin. The Sm doped LaFeO3 has been presented here which has promising applications in spintronic devices.

Enhanced ferromagnetism and Mott variable-range hopping behavior in Cu doped pyrochlore iridate Y2Ir2O7

The electric and magnetic properties of doped iridates Y2Ir2-xCuxO7 have been studied in the present paper. It is found that the magnetic properties show an enhanced ferromagnetism with doping, which comes from Cu spins and doping-broken Ir networks. The M–T data of all the doped compounds show a magnetic order around 150 K, which should be rooted in antiferromagnetic (AFM) order of Ir spins. However, there is also another transition below 20 K for x ≥ 0.2 samples, which should be originated from AFM Cu-Ir interactions. The result of bulk magnetization can be further proved by Electron spin resonance (ESR) measurements which suggest ferromagnetic (FM) interactions of Cu ions and the occurrence of magnetic order below 20 K. From the data of M–H curves, we found that there is a metamagnetic-like transition at 20 kOe for all the doped samples, which is considered as reconstruction of the Ir electrons. While another metamagnetic-like transition at 60 kOe for x = 0.4 sample is observed and considered as a spin-reorientation Cu spins. Resistivity data indicate that all the doped samples can be fitted by Mott's variable-range hopping (VRH) model at low temperatures. This is the consequence of localization due to disorder on the Ir/Cu site.