Temperature Dependence Of Transport Properties Research Articles

Temperature-Dependent Transport in Ultrathin Black Phosphorus Field-Effect Transistors.

We studied the temperature-dependent transport properties of ultrathin black phosphorus (BP). We present measurements of BP Schottky barrier (SB) metal-oxide-semiconductor field-effect-transistors (MOSFETs) with various channel lengths, constructed from a single BP sample with nanoscale uniformity in thickness and width. The electrical characterization reveals a reversal in the temperature dependence of drain current as a function of gate voltage. This reversal indicates a transition in the charge conduction limiting mechanisms as the device is swept from the off-state into the on-state. In the off-state, charge transport is limited by thermionic emission over the energy barriers at the source/drain SB contacts, and drain current increases with temperature. In the on-state, carriers can easily tunnel across the SB at the contacts, and charge transport is limited by scattering in the channel. As a result, drain current decreases with temperature in the on-state, as scattering increases with temperature. Using Landauer transport theory, we derive a closed-form expression for thermionic emission current in SB-MOSFETs with two-dimensional channels. We use this expression to extract the SB height at metal contact interface with BP and demonstrate the impact of scattering on the extraction. We then use a comprehensive BP SB-MOSFET model to analyze on-state current as a function of temperature and demonstrate the effects of charged impurity and phonon scattering on the transport properties of BP through extractions of mobility at fixed carrier density.

Investigations on electrical and thermal transport properties of Cu2SnSe3 with unusual coexisting nanophases

The ternary diamond-like compound Cu2SnSe3 is a potential thermoelectric material. Its Cu-Se conducting network leaves Sn as a likely doping site to optimize the carrier concentration without much deterioration to the carrier mobility. Thus, the precise determination on the intricate phase structures of Cu2SnSe3 is critical. Here, we first use the atomically resolved scanning transmission electron microscopy and reveal an unusual phase coexistence (monoclinic and orthorhombic phases) in Ag-doped Cu2SnSe3. Owing to coexisting phases and the order–disorder transition in the orthorhombic phase, the Ag-doped Cu2SnSe3 shows an unusual three-stage behavior in its temperature-dependent electrical transport properties and achieves record high power factors in this system. The observed three-stage behavior due to the phase coexistence is supported by the first principle calculations, Hall measurements, and optical diffuse reflectance measurements. To fully understand the band structures and the coherent interface between these coexisting phases, a band contact model is proposed that could well explain the three-stage electrical transport behavior. Moreover, the phase coexistence is observed at the nanoscale regime, thus providing a high density of phase boundaries. Such coexisting nanophases play an important role in lowering the lattice thermal conductivity. As a result, the ZT value obtained in Cu2Sn0.93Ag0.07Se3 is double that of undoped Cu2SnSe3.

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

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.

Carrier transport behavior in as-deposited and iodine-doped plasma polymerized 2, 6-diethylaniline thin films

The temperature dependent electrical carrier transport properties of as-deposited and iodine-doped plasma polymerized 2, 6-diethylaniline (PPDEA) thin films of different thicknesses prepared using a capacitively coupled glow discharge reactor at room temperature were investigated. The FTIR analyses show that iodine doping process occurred at the quinoid units in the polyaniline backbone of PPDEA. The activation energy (ΔE) of both types of PPDEA thin films suggests hopping type conduction of carriers between the localized states at temperatures ranging from 298 K to 423 K. The ΔE in the higher temperature region suggests a thermally activated charge carrier transition mechanism in the energy band gap. The ΔE of as-deposited PPDEA thin films is higher for the applied voltages of 5 and 15 V as compared to those for the iodine-doped ones. The lower value of ∆E of iodine-doped PPDEA thin films indicates formation of charge-transfer-complex.

Temperature dependent spin transport investigations in single layer VTe2

We calculate the temperature dependent spin-polarized transport properties of ferromagnetic vanadium ditelluride (VTe2) single layer by using density functional theory (DFT) along with non-equilibrium Green's function (NEGF) formulation. The spin-dependent current and transmission spectrum are calculated to explain the spin filtration effect based on temperature and excellent spin Seebeck effect. A high value of Spin polarisation efficiency ∼100% is achieved at a lower value of temperature difference range. Due to the high-temperature difference between the left electrode (TL) and right electrode (TR) lower Spin polarisation efficiency is obtained. A thermal giant magneto resistance of ∼1.5 × 103% is obtained and it increases with increase in temperature difference between the left and right electrode (ΔT), and it also depends on TL. The excellent characteristics such as high MR and perfect spin polarisation makes VTe2 a prominent material for spin caloritronic applications.

Dielectric and ac Conductivity Studies on Nanocrystalline Mn Modified Cobalt Ferrite

The ac conductivity and dielectric properties on CoFe2−xMnxO4 for x = 0.00, 0.10, 0.15 and 0.20 have been studied in detail. All the samples were prepared in nanocrystalline size. These materials are found to be crystallized to Fd <mml:math display="block"> <mml:semantics> <mml:mover accent="true"> <mml:mi>3</mml:mi> <mml:mo>¯</mml:mo> </mml:mover> </mml:semantics> </mml:math> m space group in cubic spinel structure. The dielectric constant and ac conductivity has been discussed as a function of frequency, temperature and composition. The relation between dielectric constant and ac conductivity has been analyzed and the results validate each other. The frequency response of ac conductivity (σac) obeys Johnschers power law and the parameters obtained, explain the sources of ac and dc electrical conductivity in the material. The frequency response of σac follows Maxwell–Wagner two-layer model. The influence of frequency as pumping force on activation energy has been determined. The temperature dependent ac conductivity shows the Arrhenius behavior. The σac observed to be enhanced with increase in frequency as well as temperature. The semiconducting behavior (NTCR) was also evident from temperature dependent electrical transport properties study. The low value of ac conductivity suggests a possible use of this material in dielectric applications.

Inverse Temperature Dependence of Charge Carrier Hopping in Quantum Dot Solids.

In semiconductors, increasing mobility with decreasing temperature is a signature of charge carrier transport through delocalized bands. Here, we show that this behavior can also occur in nanocrystal solids due to temperature-dependent structural transformations. Using a combination of broadband infrared transient absorption spectroscopy and numerical modeling, we investigate the temperature-dependent charge transport properties of well-ordered PbS quantum dot (QD) solids. Contrary to expectations, we observe that the QD-to-QD charge tunneling rate increases with decreasing temperature, while simultaneously exhibiting thermally activated nearest-neighbor hopping behavior. Using synchrotron grazing-incidence small-angle X-ray scattering, we show that this trend is driven by a temperature-dependent reduction in nearest-neighbor separation that is quantitatively consistent with the measured tunneling rate.

Hardware-in-the-Loop Simulation on Fault Current Limiting Operation of RE-123 Coated Conductors Under the Influence of Spatial Inhomogeneity

We have investigated transient behavior of RE-123 coated conductors in fault current limiting operation based on experimental hardware-in-the-loop simulation (HILS). Fault current limiting function of superconducting electric power application is attractive to maximize the potential of power system; such a function can be realized by the transition to the resistive state of superconducting wire. However, the phenomena is relatively complicated: nonlinear current transport properties varying with time-dependent temperature rise. Furthermore, from practical point of view, the influence of spatially homogeneity of cooling and local critical current properties in RE-123 coated conductors must be clarified for reliable operation. To demonstrate this, we developed HILS system using real-time digital simulator. Fault current limiting operation was successfully demonstrated for dc electric railway system. This method enabled us to evaluate the real-time responses of RE-123 coated conductors such as nonlinear properties and thermal behaviors under a power grid network. In particular, we applied RE-123 coated conductors with spatially inhomogeneous cooling and local critical currents to the experiment to understand the influences of them on the fault current limiting and recovery functions. Furthermore, such transient phenomena were successfully described by an electrically and thermally coupled analysis taking into account of temperature dependent nonlinear transport properties of the RE-123 coated conductors based on the percolation transition model. The method and findings will become crucial for effective and reliable designs of superconducting fault current limiters.

Impact of ceria nanofillers on temperature dependent electrical and transport properties of PVA solid polymer electrolyte films

Thermally sensitive, ceria based proton conducting solid polymer electrolyte films were prepared by doping ammonium ceric sulfate complex salt in a PVA matrix using solution casting method. The prepared films were characterized using FTIR, FESEM, TGA, UTM and impedance analyzer in order to understand structural, morphological, thermal, mechanical and electrical properties CeO2 nanoparticle based PVA/ACS composites. The presence of spindle shaped CeO2 nanoparticle was confirmed from FESEM micrographs. The prepared films shown good thermal stability up to 200 °C. The 15 wt% ACS doped PVA electrolyte shown good conductivity, mechanical strength and thermal stability. The Rice and Roth model have been used to evaluate transport parameters and it reveals that enhancement in conductivity is due to increment in the number density of mobile ions as the increase in salt concentration.

Tunable Optical and Electrical Transport Properties of Size- and Temperature-Controlled Polymorph MoS2 Nanocrystals

The phase transition of chemically synthesized MoS2 nanocrystals (NCs) from the metallic 1T to the semiconducting 2H phase has been investigated in detail. The metallic 1T phase NCs were prepared by the Li+ intercalation–deintercalation exfoliation techniques followed by prolonged sonication. The effect of ex situ thermal annealing on MoS2 polymorphs and their transformation from the 1T to 2H phase has been extensively monitored by the X-ray photoelectron, Raman, and optical absorption spectroscopy techniques. Electrical conductivity measurements have also been carried out to probe the phase transition of the synthesized NCs. The temperature-dependent (10–350 K) electrical charge transport properties of variable-sized NCs have been investigated to probe the scaling of conductivity and activation energy with size, which are yet to be reported experimentally. The charge transport mechanisms through the NC assembly for different temperature regions have been modeled and it is observed that the electron trans...

Current Transport Properties of Monolayer Graphene/n-Si Schottky Diodes

The present work reports on the fabrication and the detailed macroscopic and nanoscale electrical characteristics of monolayer graphene/n-Si Schottky diodes. The temperature dependent electrical transport properties of monolayer graphene/n-Si Schottky diodes were investigated. Nanoscale electrical characterizations were carried out using Kelvin probe force microscopy and conducting atomic force microscopy. Most the values of ideality factor and barrier height are found to be in the range of 2.0–4.4 and 0.50–0.70 eV for monolayer graphene/n-Si nanoscale Schottky contacts. The tunneling of electrons is found to be responsible for the high value of ideality factor for nanoscale Schottky contacts.

Role of five-fold symmetry in undercooled Al-Cu binary alloys

We investigate the role of five-fold symmetry (FFS) in undercooled Al1-xCux liquids (x = 0.3 and 0.4) using ab initio molecular dynamics simulations. We show that the structure factors and pair-correlation functions display characteristic features which are compatible with the occurrence of FFS and the emergence of a medium range order (MRO) below a temperature TX located close to the liquidus temperature. Then, we demonstrate that the formation of MRO is associated with a strong increase in local FFS-motifs which become more and more connected with decreasing temperature. From the temperature dependence of dynamic properties, we find that TX corresponds also to the onset of dynamic phenomena, like the non-Arrhenius temperature dependence of transport properties and the emergence of dynamical heterogeneities (DHs). Finally, we clearly identify a relationship between the fivefold topology at the medium-range scale (IMRO) and the spatial distribution of DHs using isoconfigurational ensemble simulations. This questions the direct role of the connectivity of five-fold-based motifs found in IMRO in nucleation of the parent crystalline ground states, namely, Al2Cu and Al3Cu2, which also display local ordering with a significant degree of FFS.

Electron hopping integral renormalization due to anharmonic phonons

Interpretation of some experimental data of temperature-dependent transport properties of a solid material has led to a hypothesis that some renormalization has occurred to the electron hopping integral, making it decrease as temperature increases up to some limit. Such a renormalization may be attributed to the effect of coupling of electrons with anharmonic phonons. Namely, the presence of anharmonic vibration of two adjacent atoms yields an increase of their equilibrium distance as temperature increases, this in turn weakens the electron hopping integral connecting the two atomic sites. Despite that the above qualitative picture is pretty clear, little has been explored on the detail of how such an electron hopping integral evolves as a function of temperature. Motivated by this issue, here, we propose a tight-binding model incorporating the presence of anharmonic phonons and electron-phonon coupling, and solve it using Green function technique. We define the renormalized electron hopping integral from the band width of the interacting system, and perform self-consistent calculations to demonstrate the evolution of the electron hopping integral as a function of equilibrium distance between adjacent atoms and temperature.

Hydrothermal preparation and intrinsic transport properties of nanoscale Li2FeSiO4

Nanoscale Li2FeSiO4 cathode material is successfully prepared using a ball-milling assisted hydrothermal method with inorganic precursors. Structures, morphologies, and thermal stabilities of the samples are characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and thermogravimetric-differential scanning calorimetry (TG-DSC). The prepared Li2FeSiO4 samples are well crystallized and have an irregular cuboid morphology with particle sizes in the range of 50–300 nm. The crystal structure of pure Li2FeSiO4 is stable in air at room temperature, and no distinct new phase is observed for the sample annealed at 300 °C in argon. The temperature-dependent intrinsic transport properties of nanoscale Li2FeSiO4 are obtained using dynamic potential scanning (DPS) and electrochemical impedance spectroscopy (EIS); the ionic conductivity and electronic conductivity are 1.32 × 10−8 S/cm and 2.8 × 10−8 S/cm, respectively, at 20 °C. Nanoscale Li2FeSiO4 exhibits stable cycle performance at different rates from 0.1C to 2C, and at 0.1C its specific charge and discharge capacity remain 144.1 mAh/g and 138.8 mAh/g, respectively, after 50 cycles.

High-temperature formation phases and crystal structure of hot-pressed thermoelectric compounds with chalcopyrite-type structure

In this study, we introduced the temperature-dependent formation phases and crystallographic parameters of hot-pressed silver gallium telluride AgGaTe2 and copper gallium telluride CuGaTe2 with chalcopyrite structure from 300 to 800 K. These two compounds are potential thermoelectric materials in the intermediate temperature range; however, the temperature-dependent formation phases and crystallographic parameters of hot-pressed samples have not yet been analyzed in detail. The crystal structure analysis based on synchrotron X-ray diffraction (SXRD) measurements clarifies that impurity phases such as Te and Ag2Te in the AgGaTe2 matrix and Te and CuTe in the CuGaTe2 matrix appear at some temperature regions above 300 K. The existence of such impurity phases could be correlated with the increases of the electrical resistivity and Seebeck coefficient of the samples after multiple measurement cycles of the temperature-dependent transport properties from 300 to 800 K. The tetragonal lattice parameters a and c, tetragonal lattice volume, thermal expansion coefficients, tetragonal distortion, anion displacement parameter, and isotropic displacement parameter of the hot-pressed AgGaTe2 and CuGaTe2 were also analyzed. These crystallographic parameters are expected to substantially affect the thermoelectric properties of AgGaTe2 and CuGaTe2. Our results provide prospect of the long-term high-temperature stability and clues of the detailed analysis on the transport properties of hot-pressed AgGaTe2 and CuGaTe2, which should aid their development for thermoelectric applications.

Revealing the anisotropy in thermoelectric transport performances in CNT/PANI composites

Although the carbon nanotube (CNT) based organic composites thermoelectric materials arouses considerable attentions, up to now the anisotropy in their thermoelectric performances has been overlooked. Herein, strong anisotropy in the temperature-dependent thermal and electrical transport properties have been demonstrated for organic composites thermoelectric materials, made by cold pressing the CNT powders coated by in situ polymerized polyaniline (PANI). The Seebeck coefficient, electrical and thermal conductivity are larger along the direction perpendicular to the cold pressing axis compared to the direction parallel to pressing axis. The anisotropy observed in the thermal conductivity and electrical conductivity maximizes at a CNT content of 30–40% (expressed in mass%), while the one observed for the Seebeck coefficient is small. The highest thermoelectric performances of the thermoelectric composites are achieved at a CNT mass ratio of ∼40% for both multi-walled CNT (MCNT)/PANI and single-walled CNT (SCNT)/PANI at a direction perpendicular to the cold pressing axis. These results highlight the importance of optimizing the CNT contents during the in situ polymerization with PANI to achieve a homogeneous nanostructure and reducing the agglomerate of the uncoated CNTs, in order to obtain high thermoelectric properties.

Thermal conversion of Cu4O3 into CuO and Cu2O and the electrical properties of magnetron sputtered Cu4O3 thin films

Among the three oxides of copper (CuO, Cu2O, and Cu4O3), Cu4O3 phase (paramelaconite is a natural, and very scarce mineral) is very difficult to synthesize. It contains copper in both + 1 and + 2 valence states, with an average composition Cu21+Cu22+O3. We have successfully synthesized Cu4O3 phase at room temperature (300 K) by reactive DC magnetron sputtering by controlling the oxygen flow rate (Murali and Subrahmanyam in J Phys D Appl Phys 49:375102, 2016). In the present communication, Cu4O3 thin films are converted to CuO phases by annealing in the air at 680 K and to Cu2O phase when annealed in argon at 720 K; these phase changes are confirmed by temperature-dependent Raman spectroscopy studies. Probably, this is the first report of the conversion of Cu4O3–CuO and Cu2O by thermal annealing. The temperature-dependent (300–200 K) electrical transport properties of Cu4O3 thin films show that the charge transport above 190 K follows Arrhenius-type behavior with activation energy of 0.14 eV. From photo-electron spectroscopy and electrical transport measurements of Cu4O3 thin films, a downward band bending is observed at the surface of the thin film, which shows its p-type semiconducting nature. The successful preparation of phase pure p-type semiconducting Cu4O3 could provide opportunities to further explore its potential applications.

Crystal growth and magneto-transport behavior of PdS1−δ

PdS is theoretically proposed to novel topological material with eight-band fermions. Here, PdS1−δ crystals were successfully grown from KI as solvent by modified flux method. The single crystalline quality and compositional homogeneity of grown PdS1−δ are characterized by X-ray diffraction and energy dispersion spectroscopy. Temperature dependent electrical transport property of PdS1−δ demonstrates a semiconductor-like behavior. Analysis of temperature-dependent resistance indicates that there is variable-range-hopping behavior at low temperature. The clear negative MR of PdS1−δ single crystals is measured at the low temperature (<30 K), which may be ascribed to the interaction between conducting carriers and localized moments. however, the magneto-transport results have not shown the clues of topological feature of PdS.

Temperature dependent charge transport studies across thermodynamic glass transition in P3HT:PCBM bulk heterojunction: insight from J–V and impedance spectroscopy

Temperature dependent charge transport properties of P3HT:PCBM bulk heterojunction are analysed by dc and ac measurements under dark conditions across a wide temperature range of 110–473 K, which includes the thermodynamic glass transition temperature ( K) of the system. A change from Ohmic conduction to space charge limited current conduction at higher (⩾1.2 V) applied bias voltages above ⩾200 K is observed from J–V characteristics. From capacitance–voltage (C–V) measurement at room temperature, the occurrence of a peak near the built-in voltage is observed below the dielectric relaxation frequency, originating from the competition between drift and diffusion driven motions of charges. Carrier concentration (N) is calculated from C–V measurements taken at different temperatures. Room temperature mobility values at various applied bias voltages are in accordance with that obtained from transient charge extraction by linearly increasing voltage measurement. Sample impedance is measured over five decades of frequency across temperature range by using lock-in detection. This data is used to extract temperature dependence of carrier mobility (μ), and dc conductivity () which is low frequency extrapolation of ac conductivity. An activation energy of ∼126 meV for the carrier hopping process at the metal–semiconductor interface is estimated from temperature dependence of . Above Tg, μ levels off to a constant value, whereas starts to decrease after a transition knee at Tg that can be seen as a combined effect of changes in μ and N. All these observed changes across Tg can be correlated to enhanced polymer motion above the glass transition.

Analog Circuit Applications Based on Ambipolar Graphene/MoTe2 Vertical Transistors

AbstractThe current integrated circuit technology based on conventional metal–oxide–semiconductor field‐effect transistor (MOSFET) is approaching the limit of miniaturization with increasing demand on energy. Several analog circuit applications based on graphene FETs have been demonstrated with less components comparing to the conventional technology. However, low on/off current ratio caused by the semimetal nature of graphene has severely hindered its practical applications. Here, a graphene/molybdenum ditelluride (MoTe2) van der Waals (vdW) vertical transistor with V‐shaped ambipolar field‐effect transfer characteristics to overcome this challenge is reported. Investigations on temperature dependence of transport properties reveal that gate‐tunable asymmetric barriers of the devices are account for the ambipolar behaviors. Furthermore, to demonstrate the analog circuit applications of such vdW vertical transistors, output polarity controllable amplifier and frequency doubler are successfully realized. These results enable vdW heterojunction‐based electronic devices to open up new possibilities for wide perspective in telecommunication field.