Showing papers on "Variable-range hopping published in 2018"

Electronic and transport properties of Li-doped NiO epitaxial thin films

Abstract: NiO is a p-type wide bandgap semiconductor of use in various electronic devices ranging from solar cells to transparent transistors. Understanding and improving its optical and transport properties have been of considerable interest. In this work, we have investigated the effect of Li doping on the electronic, optical and transport properties of NiO epitaxial thin films grown by pulsed laser deposition. We show that Li doping significantly increases the p-type conductivity of NiO, but all the films have relatively low room-temperature mobilities (<0.05 cm2 V−1 s−1). The conduction mechanism is better described by small-polaron hoping model in the temperature range of 200 K < T < 330 K, and variable range hopping at T < 200 K. A combination of X-ray photoemission and O K-edge X-ray absorption spectroscopic investigations reveal that the Fermi level gradually shifts toward the valence band maximum (VBM) and a new hole state develops with Li doping. Both the VBM and hole states are composed of primarily Zhang-Rice bound states, which accounts for the small polaron character (low mobility) of hole conduction. Our work provides guidelines for the search for p-type oxide materials and device optimization.

Characterization of nickel manganite NTC thermistor films prepared by aerosol deposition at room temperature

Abstract: NiMn2O4+δ thermistor thick films have been successfully deposited by the so-called Aerosol Deposition Method (ADM) at room temperature on alumina substrates, Si-wafers, as well as on special planar four-wire interdigital electrode structures for high-precision electrical characterization. The NTCR films are homogeneous, completely dense and scratch resistant. Both as-deposited and tempered, the NTCR films exhibit a cubic spinel structure. Between 25 °C and 90 °C, the NTCR film resistance behaves as it is typical for variable range hopping (VRH) with parabolic density of states. As a result of moderate film tempering, the thermistor constant B and the specific resistance at 25 °C (ρ25) decrease from 4250 K to 4020 K and 65 Ω·m to 40 Ω·m respectively, and are close to bulk values. In combination with the excellent reproducibility of the ρ25 and B values, AD processing of films appears to be a promising alternative for classical ceramic bulk processes.

Variable range hopping and thermally activated transport in molybdenum-based MXenes

Abstract: The magnetotransport of freestanding, vacuum filtered, thin films of Mo2CTz, Mo133CTz, Mo2TiC2Tz, and Mo2Ti2C3Tz was measured in the 10-300-K temperature (T) range Some of the films were annealed

Influence of thermal and compositional variations on conduction mechanisms and localized state density of amorphous Cd 50 S 50-x Se x thin films

Abstract: This work reports the study of dc-electrical conductivity of the amorphous chalcogenide Cd50S50−xSex (30 ≤ × ≤ 50 at.%) thin films and its dependent upon the temperature and composition. Thin films were prepared by the thermal evaporation process onto normal glass substrates in a vacuum about 8.2 × 10−4 Pa. The deposition rate and the film thickness were maintained constant at about 8 nm/s and 200 nm, respectively. X-ray diffraction was used to check the amorphous nature of thin-film samples. The resistance of the film samples has been measured in the temperature range 293 K to 473 K by using the two-point probe technique. DC-electrical conductivity was determined from the resistance measurements. The sheet resistance, the conduction mechanisms, activation energies, Mott parameters, barrier potential energy, trapping state energy and the density of localized states near the Fermi level, were investigated and studied. Obtained electrical data of the ternary Cd-S-Se thin films were investigated and studied in terms of Mott's variable range hopping model. All studied electrical parameters were found to be strongly dependent on the Se-content.

Electrical Transport Signature of the Magnetic Fluctuation-Structure Relation in α-RuCl3 Nanoflakes.

Abstract: The small gap semiconductor α-RuCl3 has emerged as a promising candidate for quantum spin liquid materials. Thus far, Raman spectroscopy, neutron scattering, and magnetization measurements have provided valuable hints for collective spin behavior in α-RuCl3 bulk crystals. However, the goal of implementing α-RuCl3 into spintronic devices would strongly benefit from the possibility of electrically probing these phenomena. To address this, we first investigated nanoflakes of α-RuCl3 by Raman spectroscopy and observed similar behavior as in the case of the bulk material, including the signatures of possible fractionalized excitations. In complementary experiments, we investigated the electrical charge transport properties of individual α-RuCl3 nanoflakes in the temperature range between 120 and 290 K. The observed temperature-dependent electrical resistivity is consistent with variable range hopping behavior and exhibits a transition at about 180 K, close to the onset temperature observed in our Raman measure...

Dielectric properties of magnetorheological elastomers with different microstructure

Abstract: Composite materials containing magnetic particles organised within the polymer matrix by the means of an external magnetic field during the curing process were prepared, and their dielectric properties were compared with their isotropic analogues of the same filler concentration but homogeneous spatial distribution. A substantial dielectric response observed for anisotropic systems in a form of relaxation processes was explained as charge transport via the mechanism of variable range hopping. The changes in registered relaxations' critical frequency and shape of dielectric spectra with the filler concentration were discussed in terms of decreasing anisotropy of the system. The knowledge of the dielectric response of studied systems is essential for their practical applications such as piezoresistive sensors or radio-absorbing materials.

Temperature Dependent Electrical Transport Properties of High Carrier Mobility Reduced Graphene Oxide Thin Film Devices

Abstract: We report temperature dependent electrical transport properties of high mobility reduced graphene oxide (RGO) thin films fabricated by pulse laser deposition. The temperature dependent (5K–350K) four terminal electrical transport property measurements confirm variable range hopping and thermally activated transport mechanism of the charge carriers at low (5K–210K) and high temperature (210K–350K) regions, respectively. The calculated localization length, the density of states near the Fermi level ( $E_{F}$ ), hopping energy, and Arrhenius energy gap provide useful information to explain the excellent electrical properties of the RGO films. Hall mobility measurement confirms ${p}$ -type characteristics of the thin films. The charge carrier Hall mobility can be engineered by tuning the growth parameters, and the measured maximum mobility was 1596 cm2v−1s−1. The optimization of the improved electrical property is well supported by structural properties such as the defect density, average size of sp2 clusters and degree of reduction, which were investigated by Raman spectroscopy and X-ray diffraction analysis.

Large Magnetoresistance and Electrical Transport Properties in Reduced Graphene Oxide Thin Film

Abstract: We report a systematic study of room temperature large positive and negative magnetoresistance (MR) in the reduced graphene oxide (RGO) thin-film devices grown by pulsed-laser deposition (PLD) at high and low applied magnetic fields, respectively. Raman spectroscopy, X-ray photoelectron spectroscopy, and electrical measurements on the RGO films help to explain the observed MR properties in the device. The temperature-dependent (5–400 K) electrical characterization of the thin films shows two distinct transport regimes: at low temperature, it follows 2-D Efros-Shoklovoski variable range hopping (VRH) transport mechanism and above 200 K, the device shows Arrhenius-like transport behavior. The crossover from VRH transport to Arrhenius transport is due to shortening in the characteristic lengths in the disordered 2-D system. We interpret the source of negative MR by vacancy and disorder-induced magnetic moments and the diffuse scattering at crystallite boundaries. At the high applied magnetic field, the lifting in degeneracy due to the Lorentz force explains the large positive MR effect. The highest value of the measured MR (160%) is surprisingly high for a non-magnetic material at room temperature, which can be attributed to the greater inhomogeneity in the PLD grown wafer-scale RGO thin films.

Thermal and electrical properties of (B 2 O 3 -TeO 2 -Li 2 O-CoO) glasses

Abstract: Borotellurite glasses in the composition, (B2O3)0.2–(TeO2)0.3–(CoO)x–(Li2O)0.5-x; x = 0.05, 0.10, 0.15, 0.20, 0.25, 0.30, 0.35, 0.40, 0.45 and 0.50 have been synthesized and their non-crystalline nature confirmed by XRD. Density of the glasses increased and molar volume decreased up to x = 0.3 and reversed their trends for x > 0.3. The glass transition temperature is increased up to x = 0.30 and decreased for x > 0.3. Changes in density, molar volume and glass transition temperature around x = 0.3 may be ascribed to structural/topological modifications occurring in the glasses. Polaron parameters have been determined. Thermal stability is found to increase with increasing x. Activation energy for dc conduction at high temperature has been determined by applying Mott's small polaron hopping model. Conductivity and activation energy were found to pass through minimum and maximum respectively for x around 0.30, which may be due to a changeover of conduction mechanism from predominantly ionic regime to electronic regime. This transition can be attributed to the interaction between electrons and alkali ions. This result is of significance both scientifically and technologically. Mott's variable range hopping model has been employed to understand conductivity at low temperature for the present glasses.

Mn2(Fe0.8Mo0.2)MoO6: A Double Perovskite with Multiple Transition Metal Sublattice Magnetic Effects

Abstract: Transition-metal-only perovskite oxides can introduce additional magnetic functionality with robust magnetoelectric properties but are rare. In this work we prepared a new transition-metal-only perovskite Mn2(Fe0.8Mo0.2)MoO6 at high pressure and temperature. Uniquely, Mn2(Fe0.8Mo0.2)MoO6 was discovered as a line phase upon composition modulation that was motivated from the above-room-temperature multiferroic Mn2FeMoO6 corundum phase. It exhibits ferrimagnetic Fe–Mo sublattice (TC = 194 K) and Mn sublattice antiferromagnetic (Tm ∼ 45 K) transitions. Below Tm the two sublattice orderings are coupled and give rise to canted components in both. A first-order field-induced transition is also observed below 45 K. Mn2(Fe0.8Mo0.2)MoO6 is a Mott variable range hopping semiconductor. These findings for the first time show that either an exotic perovskite or a corundum phase can be achieved by composition modulation besides the pressure effect.

Grain size effect on the electrical and magneto-transport properties of nanosized Pr0.67Sr0.33MnO3

Abstract: In this study, nanosized of Pr0.67Sr0.33MnO3 prepared via sol-gel method followed by heat treatment at 600–1000 °C in intervals of 100 °C were synthesized. The structure, surface morphology, electrical, magneto-transport and magnetic properties of the samples were investigated. Rietveld refinements of X-ray diffraction patterns confirm that single phase orthorhombic crystal structure with the space group of Pnma (62) is formed at 600 °C. A strong dependence of surface morphology, electrical and magneto-transport properties on grain size have been observed in this manganites system. Both grain size and crystallite size are increases with the sintering temperature due to the congregation effect. Upon increasing grain size, the paramagnetic-ferromagnetic transition temperature increases from 278 K to 295 K. The resistivity drops and the metal-insulator transition temperature shifted from 184 K to 248 K with increases of grain size due to the grain growth and reduction of grain boundary. Below metal-insulator transition temperature, the samples fit well to the combination of resistivity due to grain or domain boundaries, electron-electron scattering process and electron-phonon interaction. The resistivity data above the metal-insulator transition temperature is well described using small polaron hopping and variable range hopping models. It is found that the negative magnetoresistance also increases with larger grain size where the highest %MR of – 26% can be observed for sample sintered at 1000 °C (245 nm).

Magnetothermopower and giant magnetoresistance in the spin-glass CuCrTiS4 thiospinel

Abstract: The transport properties of the thiospinel CuCrTiS4 have been investigated in detail, focusing on the possible interplay between charges and spins, and its consequence on transport, and more specifically on the thermoelectric properties. The chemical homogeneity of a CuCrTiS4 polycrystalline sample synthesized by a direct reaction of the constitutive elements has been carefully probed by X-ray diffraction and transmission electron microscopy techniques. This thiospinel exhibits below 75 K, in its paramagnetic regime, a variable range hopping (VRH) conduction, associated with a giant magnetoresistance (MR), which persists below the spin-glass transition temperature at 8 K, reaching -95% in 9 T at 5 K. All the MR data can be rescaled as a function of H/T and described using a Brillouin function with S = 3/2. This demonstrates that the paramagnetic fluctuations strongly impact the transport properties, and this is confirmed by the Seebeck coefficient which depends on the magnetic field at low temperature, with a large negative magnetothermopower (MTEP) effect of -26% at 20 K, i.e., the thermopower is reduced as the spin alignment reduces the spin entropy. This first report of MTEP in CuCrTiS4 shows that this thiospinel behaves rather similarly to strongly correlated oxides, with MR and MTEP enhanced in the case of VRH transport with paramagnetic spins, such as in cobalt misfits or colossal MR manganites.

Optical and electrical properties of nanostructured N,N ′-diphenyl- N,N′ -di- p -tolylbenzene-1,4-diamine organic thin films

Abstract: In this paper, we introduce an extensive study on nanostructured thin films of an organic small molecule N,N′-diphenyl-N,N′-di-p-tolylbenzene-1,4-diamine, (NTD). Further, the possibility of using NTD as a hole transport layer in optoelectronic devices is reported for the first time to the best of our knowledge. In this context, the thermal behaviour, crystal structure, optical absorption in ultraviolet–visible regions, and DC electrical conductivity of the as-deposited NTD thin films are investigated. The differential scanning calorimetry (DSC) investigations show that NTD has a phase transition at a high temperature of 190 °C which may not affect its morphological stability. Further, the XRD patterns reveal that NTD thin films have an as-amorphous nature with some crystals. Additionally, optical investigations indicate that the indirect electronic transition from valence to conduction band is the most probable transition which agrees well with the amorphous structure of NTD thin films. The value of the mobility gap decreases from 2.74 to 2.51 eV when the thickness of the films increases from 80 to 200 nm. Furthermore, the absorbance lies below 370 nm and the thinnest film of thickness 80 nm achieves the highest absorbance. Further, the DC electrical measurements show that the NTD film has an activation energy of 378 meV. The DC conductivity is interpreted in terms of the variable range hopping (VRH) model. Based on our studies, NTD thin films are proposed with thickness range 50–80 nm to serve as a hole transport layer in white organic light emitting diodes (OLEDs).

Effect of Cr3+ substitution at Mn-site on electrical and magnetic properties of charge ordered Bi0.3Pr0.3Ca0.4MnO3 manganites

Abstract: The effect of Cr3+ substitution at Mn-site on the structure, magnetic and electrical properties of Bi0.3Pr0.3Ca0.4Mn1-xCrxO3 manganites were studied by using X-ray diffraction method, AC susceptibility and electrical transport measurements. All the samples with starting composition of Bi0.3Pr0.3Ca0.4Mn1-xCrxO3 (x = 0, 0.04, 0.08, 0.1, 0.12 and 0.14) were prepared using solid state method. R versus T curve shows that x = 0 sample exhibits a strong insulating behavior while Cr3+substitution at x = 0.08 induced metal-insulator (MI) transition at MI transition temperature (TMI) of 56 K. The substitution for x = 0.08 successfully weakened the hybridization effect which may related to strong ferromagnetic (FM) interaction between Cr3+ O Mn3+ as well as Mn3+ O Mn4+. Further substitution of Cr3+ for x = 0.1 increased the TMI to 58 K but decreased to 36 K for x = 0.12. Rietveld Refinement of the X-ray diffraction data showed a decreased of unit cell volume which indicated partial substitution of Cr3+ (0.615 A) at Mn+3(0.64 A) site. Susceptibility, χ′ versus temperature, T measurements, showed an increase of ferromagnetic-paramagnetic, FM-PM transition temperature, TC from 66 K (x = 0.04) to 125 K (x = 0.14) which suggested that Cr3+ substitution enhanced the growth of FM phase. Fitting of the experimental data in the metallic region to scattering models suggested scattering involving a combination of electron-electron, electron-magnon, Kondo-like spin-dependent scattering, and electron-phonon interaction are responsible for the observed resistivity behavior. On the other hand, fitting in the insulating region suggests that resistivity behavior obeys the variable range hopping (VRH) model below the charge ordered temperature, TCO for (x = 0) and TMI T θ D / 2 for x = 0.04–0.14 while at high temperatures involved small polaron hopping, SPH mechanism. Therefore, the enhanced double exchange mechanism (DE) indicates the substitution of Cr3+ increased the delocalization of charge carrier and destabilization of the CO-AFM state. It is suggested that such an increased of DE mechanism was related to the reduction of blocking mechanism which generated by the hybridization effect due to Bi 6s2 lone pair.

Variable range hopping and relaxation mechanism in graphene oxide sheets containing sp 3 hybridization induced localization

Abstract: The temperature dependent electrical properties of graphene oxide prepared by the modified Hummers method are investigated using broadband dielectric spectroscopy. The morphology and structure are confirmed by X-ray diffraction pattern and scanning electron microscope images. In the present work, we have studied the electrical properties of graphene oxide employing a recently proposed novel approach of the combined conduction and dielectric Cole–Cole formalism. The extracted dc conductivity values varies from 1.9 × 10−8 to 3.5 × 10− 5 S cm−1 as a function of temperature (153–353 K), show power-law behaviour, which is explained through Mott’s variable range hopping conduction mechanism. The density of states was found to be 6.02 × 1018 cm−3 eV−1. The conduction relaxation timescales and dielectric relaxation timescales of GO are following the power law. The physical origin of the non-Arrhenius dc conductivity behaviour of charge carriers is explained through structural heterogeneity in graphene oxide introduced due to sp2 and sp3 hybridization of carbon atoms. The range of hop was calculated to be 4.7–3.8 nm with hopping energy changing from 0.37 to 0.69 eV as a function of temperature.

Physical properties of Ag/Ca doped Lantanium manganite

Abstract: La0.6Ca0.3Ag0.1MnO3 system has been synthesized using the cheap solid–solid process. Structural analysis indicates that the sample exhibits a single phase of intragranular. Electrical and dielectric data, over a wide temperature range and frequencies have been recorded with impedance spectroscopy. We visualize the conductivity, whose activation energy is low, as a successive hopping of electrons as polaron does. Further, the conductivity is governed by the small polaron hopping model at higher temperature region while it fits well with the variable range hopping model at a lower temperature range. Dielectric data has been represented in different alternative complex formalisms, from which complementary information has been extracted. The complex impedance diagram suggests the contribution of only two active microstructures, ruling out the electrode polarization effect. We adopt the spectrum of the imaginary component of the electric modulus M″ to distinguish long range hopping from short range hopping. The relative permittivity spectrum permits to identify the possibility of the existence of electric polarizations. Further, it reveals colossal static dielectric constant, which makes it suitable for multitude potential applications in electronic industrial fields.

Effect of structural disorder on transport properties of LaNiO3 thin films

Abstract: We have deposited LaNiO3 thin films on LaAlO3 (001), SrTiO3 (001), and Si (001) substrates using the pulsed laser deposition technique. Depositions were carried out at various substrate temperatures ranging from 0 to 800 °C. The effects of lattice mismatch and structural disorder on the transport properties of films deposited on various substrates and at different substrate temperatures are reported. X-ray diffraction confirms a highly c-axis oriented growth of LaNiO3 films on all the substrates at substrate temperatures of 600 and 800 °C, while at lower substrate temperatures deposited films are amorphous. Emergence of a new Raman mode indicates symmetry lowering in all the deposited crystalline films. Hardening of the Eg(3) (∼400 cm−1) mode is also observed with the rise of in-plane compressive strain. Resistivity curves for films on Si show a semiconducting behaviour and follow a variable range hopping mechanism. Crystalline films on LaAlO3 and SrTiO3 exhibit a metallic character along with a low-temperature resistivity upturn, which is attributed to the contribution of self-localization to resistivity at low temperatures as indicated by magnetotransport measurements.We have deposited LaNiO3 thin films on LaAlO3 (001), SrTiO3 (001), and Si (001) substrates using the pulsed laser deposition technique. Depositions were carried out at various substrate temperatures ranging from 0 to 800 °C. The effects of lattice mismatch and structural disorder on the transport properties of films deposited on various substrates and at different substrate temperatures are reported. X-ray diffraction confirms a highly c-axis oriented growth of LaNiO3 films on all the substrates at substrate temperatures of 600 and 800 °C, while at lower substrate temperatures deposited films are amorphous. Emergence of a new Raman mode indicates symmetry lowering in all the deposited crystalline films. Hardening of the Eg(3) (∼400 cm−1) mode is also observed with the rise of in-plane compressive strain. Resistivity curves for films on Si show a semiconducting behaviour and follow a variable range hopping mechanism. Crystalline films on LaAlO3 and SrTiO3 exhibit a metallic character along with a low-tempe...

Possible half-metallicity and variable range hopping transport in Sb-substituted Fe2TiSn Heusler alloys.

Abstract: The investigation of the magnetotransport properties on [Formula: see text] [Formula: see text]Sb x with 0 [Formula: see text] 0.6 are presented in this paper. The substitution of Sb in place of Sn decreases the anti-site disorder as evident from x-ray diffraction patterns as well as from transport properties measurement. The much-disputed upturn in low temperature electrical resistivity of [Formula: see text]TiSn has been demonstrated to be a result of weak localization induced by anti-site disorder. With increased Sb substitution (⩾25%) the metallic transport behavior of [Formula: see text]TiSn changes to semiconductor-like. At low temperature, carrier transport in such compositions occurs via the variable range hopping mechanism. Moreover, a systematic increase in the anomalous Hall voltage is observed with increasing Sb-content, attributable to a side jump or Berry phase curvature effect. Electrical resistivity in the entire temperature regime hints towards half metallicity of the system. Our ab initio electronic structure calculations using generalised gradient approximation formalism further supports the results of our magnetotransport study.

A tunable palladium nanoparticle film-based strain sensor in a Mott variable-range hopping regime

Abstract: Metal nanoparticle (NP) film, an important artificial material that exhibits electronic transport properties different from bulk metals, has often been employed to fabricate micro/nano electronic devices. In this paper, Pd NP film was formed on the surface of flexible polyethylene terephthalate (PET) sheet in order to fabricate a strain sensor by means of gas-phase cluster beam deposition. Three Pd NP film-based strain sensors with different NP coverages were fabricated, and their electronic transport properties and strain sensing behaviors were investigated. Despite the differences in inter-particle coupling strength induced by different NP coverages, the Mott variable-range hopping (VRH) transport was found to be dominant in all three sensors. Sensors made of Pd NP films with lower NP coverage were found to give strain gauges with greater sensitivity, and their strain sensing calibration curves deviated from the exponential law due to variations in the gauge factor during strain loading, which was originated from the topology change of the percolation network. Sensitivity could be optimized by regulating the NP coverage, which is readily tuned during device fabrication. The tunable sensitivity, up to 1000, as well as the variety in flexible substrate materials available make them extremely promising for developing micro- and nano-sized electromechanical devices. Besides, the investigations about the reliability and hysteresis performance of the sensors were carried out. Our strain sensors show an excellent performance to the long-term bending–unbending cyclic test.