Temperature-dependent Electrical Transport Research Articles

Synthesis, Transfer, and Properties of Layered FeTe2 Nanocrystals.

Different from layered two-dimensional (2D) transition metal dichalcogenides (TMDs), iron dichalcogenides crystallize in the most common three-dimensional pyrite or marcasite structures. Layered iron dichalcogenides are rarely reported and little is known about their structures and properties. Here, layered hexagonal phase iron ditelluride FeTe2 (h-FeTe2) nanocrystals are grown on mica by atmospheric pressure chemical vapor deposition (APCVD) method and are fully characterized by various methods. Like other 2D layered TMD materials, the FeTe2 nanoflakes exhibit regular hexagon, half hexagon, or triangle shapes with a controllable thickness of 6-95 nm and lateral length from a few to tens of micrometers. A simple and effective method is used to transfer the FeTe2 nanoflakes from the mica substrate onto any other substrates without quality deterioration by using polystyrene (PS) as a support polymer, which can also be operated in ethanol or ethylene glycol in a glovebox to avoid contact with water and air. Temperature-dependent electrical transport demonstrates that the FeTe2 nanoflake is a semiconductor with a variable range hopping (VRH) conduction, and its nonsaturated linear magnetoresistance (MR) reaches up to 10.4% under magnetic field of 9 T at 2 K, both probably due to its structure disorders. No signature of magnetic ordering is observed down to 2 K. The CVD growth of this layered FeTe2 represents an addition to the extensive library of 2D materials, particularly iron chalcogenides or alloys. Synthesis, properties, and even doping of phase pure h-FeTe2 call for further study in the future.

The pressure-dependence of the band gap of tellurium

Elemental tellurium is a semiconductor with a small band gap of around 330 meV. Under the application of hydrostatic pressure, the band gap narrows, and it has been suggested that below its crystal phase transition at 40 kbar, the gap might close, leading to a transition into a topologically non-trivial state. Here, we present a pressure dependent study of the gap size of tellurium up to 22kbar, studied by temperature dependent electrical transport. We identify the gap size to be still well above 100 meV at our maximum pressure. Extrapolation of our data, assuming a non-linear gap narrowing (in agreement with previous studies), leaves us to assume that the gap does not close within the ambient pressure crystal phase. However, when assuming a linear narrowing of the gap, we leave a small possibility that there exists a minor pressure window at which a topological phase transition could occur. Furthermore, as the second systematic study of the gap size via transport, we see that historical inconsistencies between the measured gap size via transport and the measured optical gap size seem to be systematic and probably reflect some deeper physics of the material.

Understanding the efficacy of Cu in creating oxygen vacancies and temperature dependent electrical transport in solution processed Cu:ZnO thin films

In this investigation, chemically synthesized 5% copper doped ZnO (Cu:ZnO) thin films were deposited onto ITO/Glass using sol-gel spin coating method. Firstly, the effects of inclusion of Cu doping on the electronic properties of ZnO were studied using Density Functional Theory (DFT) based first principle calculations. The comparative energy band (E-k) and orbital projections of density of states profiles were extensively analysed to understand the electronic activities of Cu induced defect states in Cu:ZnO. X-ray photoelectron spectroscopy (XPS) analysis was performed to confirm the oxidation states of Zn, Cu and O and also confirm the existence of oxygen vacancy oriented defects in the Cu:ZnO films. This analysis also indicates that Cu existed in +2 valence state. The experimentally obtained results were compared with the theoretical analysis and both were closely matched. Furthermore, extended x-ray absorption fine structure (EXAFS) and x-ray absorption near edge structure (XANES) characterizations were also employed to study the role of oxygen vacancies, ions and defect states in Cu:ZnO, which play a pivotal role in the operation of resistive switching devices. XANES and EXAFS have confirmed that Cu conveniently replaced the Zn atoms within the crystal lattice in the deposited films. To investigate the temperature dependent charge transport mechanism, the resistivity of the Cu:ZnO films was measured in the temperature range of 10–300 K. It was ascertained that above 115 K, thermally activated type of conduction governs the charge carrier transport, whereas Mott's variable range hopping type of conduction mechanism is prevalent below 40 K.

Temperature dependent impedance spectroscopy and electrical transport mechanism in sol-gel derived MgCr2O4 spinel oxide

Magnesium chromite (MgCr2O4) spinel oxide was synthesized by sol-gel auto-combustion technique. Room temperature X-ray powder diffraction pattern was refined by Fullprof which confirmed the formation of single phase cubic structure with Fd3m space group. The complex impedance spectroscopy was employed to investigate the charge transport mechanism within the temperature range of 423–573K and the impedance data were finely fitted for temperature dependent Nyquist plot to track down the equivalent circuit model [(RGCG)(RGbQGb)]. The temperature and frequency dependent AC conductivity was found to follow Jonscher double power law and the slope of conductivity indicated the correlated barrier hopping (CBH) as a dominant conduction model in the material. The grain and grain boundary polarization was analyzed by electric modulus which confirmed the two phase response of the material. The dielectric relaxation exhibited dispersion in the low frequency region which owed to the interfacial effects.

Tunable superconductivity of epitaxial TiN films through oxygen doping

Titanium nitride (TiN) film is a remarkable material for a variety of applications ranging from superhard coating to superconducting quantum devices, which can be easily oxidized when it works in the atmosphere. However, the study of its oxidation effect on the crystal and electronic structures of epitaxial TiN films is rare as yet. Here, we coherently synthesize TiN epitaxial films on MgO single crystal substrates via reactive magnetron sputtering and, then, dope oxygen into these films via a controllable oxidation process. The crystal and electronic structures are characterized by high-resolution x-ray diffraction, x-ray photoelectron spectra, and Raman spectra. It is revealed that the crystal structure remains to be of the rocksalt type in these films even with heavy oxygen doping. The data of temperature-dependent electrical transport measurements indicate that the superconducting critical temperature (kinetic inductance) decreases (increases) from 4.6 K (0.672 pH/□) in the pristine TiN film to 3.4 K (1.13 pH/□) in the film with a maximum oxygen doping level. Our work provides a controllable way to tune the superconductivity of TiN films, which enables the flexibility to engineer the resultant performance of TiN-based superconducting quantum devices.

Enhancement of thermoelectric performance by tuning selenium content in the Cu2SnSe3 compound

To investigate the thermoelectric properties, we have attempted to tune selenium content in Cu2SnSe3-δ (−0.04 ≤ δ ≤ 0.1) compounds synthesized via the conventional solid-state reaction. Electrical and thermal properties are presented in the low and near room temperature regime (10–350 K). The lowest electrical resistivity is observed for the compound with δ = 0.08. The electron-phonon coupling and the presence of small polarons are experimentally validated from temperature-dependent electrical and thermal transport data. A decrease in the Seebeck coefficient along with carrier concentration is observed, verifying the linear electron band dispersion. A downward shift of the Fermi level deep into the valence band is realized in the case of non-stoichiometric samples, thereby reducing the Seebeck coefficient. A maximum power factor of ~50 μWmK−2 is achieved for the sample with δ = 0.04, which is about twice that of the pristine compound. The highest ZT of ~0.01 is obtained for the compound with δ = 0.08. The observed results suggest that stoichiometry plays a vital role in tuning the thermoelectric properties in the Cu2SnSe3 compound.

Epitaxial stabilization of ultra thin films of high entropy perovskite

High entropy oxides (HEOs) are a class of materials, containing equimolar portions of five or more transition metal and/or rare-earth elements. We report here about the layer-by-layer growth of HEO [(La0.2Pr0.2Nd0.2Sm0.2Eu0.2)NiO3] thin films on NdGaO3 substrates by pulsed laser deposition. The combined characterizations with in situ reflection high energy electron diffraction, atomic force microscopy, and x-ray diffraction affirm the single crystalline nature of the film with smooth surface morphology. The desired +3 oxidation of Ni has been confirmed by an element sensitive x-ray absorption spectroscopy measurement. Temperature dependent electrical transport measurements revealed a first order metal-insulator transition with the transition temperature very similar to the undoped NdNiO3. Since both these systems have a comparable tolerance factor, this work demonstrates that the electronic behaviors of A-site disordered perovskite-HEOs are primarily controlled by the average tolerance factor.

Single-crystal synthesis and properties of the open-framework allotrope Si24

Si24 is a new, open-framework silicon allotrope that is metastable at ambient conditions. Unlike diamond cubic silicon, which is an indirect-gap semiconductor, Si24 has a quasidirect gap near 1.4 eV, presenting new opportunities for optoelectronic and solar energy conversion devices. Previous studies indicate that Na can diffuse from micron-sized grains of a high-pressure Na4Si24 precursor to create Si24 powders at ambient conditions. Remarkably, we demonstrate here that Na remains highly mobile within large (~100 µm) Na4Si24 single crystals. Na readily diffuses out of Na4Si24 crystals under vacuum with gentle heating (10−4 mbar at 125 °C) and can be further reacted with iodine to produce large Si24 crystals that are 99.9985 at% silicon, as measured by wavelength-dispersive x-ray spectroscopy. Si24 crystals display a sharp, direct optical absorption edge at 1.51(1) eV with an absorption coefficient near the band edge that is demonstrably greater than diamond cubic silicon. Temperature-dependent electrical transport measurements confirm the removal of Na from metallic Na4Si24 to render single-crystalline semiconducting samples of Si24. These optical and electrical measurements provide insights into key parameters such as the electron donor impurity level from residual Na, reduced electron mass, and electron relaxation time. Effective Na removal on bulk length scales and the high absorption coefficient of single-crystal Si24 indicate promise for use of this material in bulk and thin film forms with potential applications in optoelectronic technologies.

Impurity assisted hopping conduction and persistent photoconductivity in disordered carbon nanoparticle film

We report the temperature-dependent electrical transport and photoconductivity in carbon nanoparticle films. The electrical transport is dominated by thermally activated conduction at higher temperatures in the range of ∼350–285 K, whereas at lower temperatures <280 K, the conduction is mostly due to the hopping mechanism. A film of an n-type semiconductor with a carrier concentration of ∼1016 cm−3 is prepared by pulsed laser ablation in the scanning technique. The photoconductivity shows a persistent behavior that lasts for several hundreds of seconds on sub-bandgap laser excitations. A broad green luminescence spectrum suggests the presence of a large number of oxygenated-impurity states in the nanoparticles. An unusual behavior in the temperature-dependent photoluminescence is observed in which the photoluminescence intensity first increases up to ∼100 K with the increasing temperature followed by a continuous decrease at higher temperatures. The observed persistent nature of the photocurrent and anomalous temperature dependence in photoluminescence is attributed to the presence of a large number of trap states in the nanoparticles. Due to the ability to trap and retain charges within the disordered carbon nanoparticle films, it can be utilized in the memory applications.

Effect of Tb substitution in naturally layered LaMn2Si2: magnetic, magnetocaloric, magnetoresistance and neutron diffraction study

Evolution of physical and magnetic properties of La rich La1−xTbxMn2Si2 (x = 0, 0.1 and 0.225) compounds is studied using temperature and field dependent dc magnetization, electrical transport, and heat capacity measurements. LaMn2Si2 undergoes a paramagnetic to antiferromagnetic ordering at TN ~ 400 K and a long range ferromagnetic ordering below TC ~ 308 K. However, the substitution of Tb at La site results in the contraction of unit cell as well as decrease in the Mn–Mn spacing, which leads to an additional antiferromagnetic phase below TN ~ 87 K and 257 K for La0.9Tb0.1Mn2Si2 and La0.775Tb0.225Mn2Si2, respectively. Using magnetization isotherm results, we have constructed an H–T phase diagram for x = 0.225 and have found the coexistence of this additional antiferromagnetic phase with ferromagnetic phase. The phase coexistence at x ~ 0.225 is further analysed using magnetoresistance (MR) and magnetocaloric effect (MCE) studies and a reasonable correlation is established between MCE and MR. In order to analyse the nature of the magnetic transition at TC, a universal master curve is constructed by rescaling the magnetic entropy curves. Low temperature neutron diffraction measurements performed on the polycrystalline samples revealed a canted ferromagnetic structure for x ~ 0.1 and a canted antiferromagnetic structure for x ~ 0.225.

Anomalous Broadband Spectrum Photodetection in 2D Rhenium Disulfide Transistor

Abstract2D transition metal dichalcogenide (TMD)‐based phototransistors generally work under photoconductive, photovoltaic, or photogating mode, in which photocarriers are generated from band‐to‐band excitation. Nevertheless, due to the relatively large bandgap, most TMD phototransistors working under these modes are restricted in visible spectrum. Here, photodetection in 2D multilayer rhenium disulfide (ReS2) transistor via bolometric mode, which relies on light heating induced conductance change instead of band‐to‐band photoexcitation is reported, making it possible for sub‐bandgap photon detection. The bolometric effect induced photoresponse is first revealed by an anomalous sign switching of photocurrent from positive to negative while increasing gate voltage under visible light, which is further validated by the temperature dependent electrical transport measurements. The phototransistor exhibits remarkable photoresponse under infrared regime, beyond the optical bandgap absorption edge of the ReS2 flake. Additionally, it demonstrates a low noise equivalent power, less than 5 × 10−2 pW Hz−1/2, which is very promising for ultra‐weak light detection. Moreover, the response time is below 3 ms, nearly 3–4 orders of magnitude faster than previously reported ReS2 photodetectors. The findings promise bolometric effect as an effective photodetection mode to extend the response spectrum of large bandgap TMDs for novel and high‐performance broadband photodetectors.

Electrical transport properties of focused ion beam lithography Au contacts on PbTiO3 nanorods

PbTiO3 nanorods with tetragonal phase were synthesized by hydrothermal method and heat treatment, and temperature-dependent electrical transport properties of individual PbTiO3 nanorod were investigated. The results show that the conductivities of PbTiO3 nanorods are gradually enhanced with temperature increasing from 77.4 to 295[Formula: see text]K, and exhibit the typical nonlinear I–V characteristics. The barrier height between Au electrode and nanorod is reduced from 0.137 to 0.088[Formula: see text]eV with increasing bias from 0.2 to 1[Formula: see text]V. The corresponding values of thermal activation energies are 0.172 and 0.06[Formula: see text]eV below the conduction band for 180–295 and 77.4–180[Formula: see text]K, respectively. This semiconductor-like behavior may result from the larger number of surface defects or localized states in the amorphorized PbTiO3.

Design of Nano-Structured Micro-Thermoelectric Generator: Load Resistance and Inflections in the Efficiency.

In recent years the interest for the harvest of energy with micro thermoelectric generators (TEG) has increased, due to its advantages compared to technologies that use fossil fuels. There are three ways to improve the performance of the device, by modifying its structure, type of material and operation control. In this study, the role of the load resistance on the performance of a TEG with nanostructured materials is investigated. The interaction of the load resistance with the thermoelements exhibits interesting features, arising from the coupling of the temperature-dependent electrical and thermal transport properties at different temperature ranges and the architecture of nanostructured thermoelectric materials. This coupling results in inflections on the efficiency, i.e., maximum and minimum values of the efficiency at higher temperatures, 600–900 K. We show the explicit dependence of the performance of the TEG in terms of the load resistance and discuss the underlying physics. The unusual features of the efficiency of nanostructured thermoelectric materials are a result of the behavior of the power factor and the nonequilibrium properties of the system. We also analyze the effect of the geometric shape of the thermoelements on the device. We determine the performance of the TEG, evaluating the generation power and its efficiency. The results show that the efficiency of the device can decrease or increase depending on the value of , while the power decreases with an increase of the load resistance.

Impact of crystal stacking sequence on electrical transport and dielectric properties of the nanocrystalline BaCo0.9Mn0.1O3-δ

We investigate the electrical transport and dielectric properties of the nanocrystalline BaCo0.9Mn0.1O3-δ under 2H- and 12H-type hexagonal phases. Temperature dependent electrical transport properties under 2H- and 12H-type phases show semiconducting-like behavior above the magnetic transition. The numerical value of electrical resistivity of the 12H-type phase at low temperature is 10-fold higher than the 2H-type counterpart, following the trend of structural stacking sequence. The 12H-type phase exhibits rarely known characteristics of ferromagnetic-insulator. The conduction mechanism under both phases is studied using variable range and small polaron hopping conduction mechanism. The 12H-phase shows large magnetoresistance (∼60%) under an application of 5 T of magnetic field at 50 K. The relation between magnetization and electrical resistivity around the magnetic ordering temperature is studied. The relation has been used to correlate the magnetic entropy change derived by magnetization and electrical resistivity measurements. Temperature dependence and magnetic field induced dielectric permittivity of the 2H- and 12H-type hexagonal phases is studied around the magnetic ordering temperature. Both phases exhibit dielectric suppression by magnetic ordering. The field dependence dielectric behavior shows magnetocapacitance value of ∼4% for 2H-phase and ∼94% for 12H-phase at their magnetic ordering temperature under external magnetic field of 5 T. The existence of intrinsic magneto-dielectric coupling is studied under the framework of Ginzburg-Landau theory for ferroelectromagnet. The origin of extrinsic magneto-dielectric contribution is justified using combinational effect of large negative magnetoresistance and Maxwell-Wagner effect. The magnetic and electrical transport properties of the nanocrystalline BaCo0.9Mn0.1O3-δ compound are also studied using density of states. Nanocrystalline BaCo0.9Mn0.1O3-δ sample shows novel magnetic field-induced effect on the magnetic, electric and dielectric properties around the same temperature window which could be utilized for multi-functional applications.

Electronic and Magnetic Properties of Pt Based Intermetalic LaPtAs and LaPt2As Compounds

We report the synthesis of polycrystalline LaPtAs and LaPt2As via solid state reactions and their temperature dependent magnetic and electrical transport properties. Crystal structures of both samples are suggested to be orthorhombic with a Pmmm space group. Temperature and field dependent magnetization measurements show that the splitting of field cooling and zero field cooling curves (initiation of irreversibility) take place at temperatures just below the Curie temperatures for both samples. This behavior is a characteristic of magnetic nanoparticles. Therefore, we propose that the magnetization arises from nanosized platinum clusters due to partial filling of the available interstitial sites. Temperature (T) dependent resistivity measurements of both alloys show metallic behavior at high temperatures and resistivity curve bends away from classical linear T dependence toward the temperature axis in the intermediate temperature range. This behavior is attributed to phonon induced interband s–d scattering. The main contribution to the resistivity at low temperatures is suggested to arise from the scattering of conduction electrons by spin-waves, in the presence of strong spin–orbit coupling due to Pt atoms. We discuss the role of spin–orbit effect on the electronic structure and the magnetic state of both samples based on various nano-scale structures.

Carrier reduction studies of type-II superlattice materials for very long wavelength infrared sensing

There are continuing efforts to develop type-II superlattice (SL) materials for very long wavelength infrared (VLWIR) detector applications. However, the SLs have high residual electron background doping densities that depend on SL growth conditions, which lead to shorter minority carrier lifetime and lower performance parameters than theoretically predicted. In this study, the authors compare the technical advantages of using InAs/GaInSb over InAs/GaSb SL with respect to reducing the electron doping levels. Our temperature-dependent electrical transport measurements show that the InAs/GaInSb SL design has a lower electron density than the InAs/GaSb SL with the same bandgap and have electron densities (mobilities) on the order of the mid 1011 cm−2 (25 000 cm2/V s). Since small period InAs/GaInSb SLs also produce greater Auger recombination suppression for a given VLWIR gap than the large period InAs/GaSb SL, the InAs/GaInSb SL appears to be a better candidate for long lifetime IR materials for future very long wavelength infrared devices.

Temperature Dependent Electrical Transport Characteristics of Polyaniline Films Modified with Gold nanoparticles

Abstract The present work reports the modification of the electrical properties of conducting polymer polyaniline (PA) with the introduction of gold nanoparticles (Au NP) into the polymer matrix. The nanocomposites of polyaniline and gold nanoparticles have been successfully synthesized using conventional chemical polymerization technique, where gold nanoparticles have been added in different successive weight percents. The dielectric measurements as a function of temperature have been carried out on these composite samples in the frequency range 20 Hz-1 MHz. The dielectric relaxation characteristics of the prepared samples have been examined by analyzing dielectric spectroscopy, dielectric loss and electrical conductivity spectroscopy. Perusal of the dielectric data depicts that the dielectric permittivity of the samples increases with an increase in the temperature and nanoparticles content. The frequency dependent ac conductivity has been analyzed using Jonscher’s universal power law relation. The power law exponent (s) has been found to be composition and temperature dependent. The decrease in s with an increase in temperature suggested that the ac conduction mechanism of these samples follows the correlated barrier hopping (CBH) model. The improved dielectric constant of nanocomposite suggests its increased ability to store electric potential energy under the impact of alternating electric field

Barrier Height Inhomogeneity in Mixed-Dimensional Graphene/Single CdSe Nanobelt Schottky Junctions

Mixed-dimensional graphene/single CdSe nanobelt (NB) Schottky junctions are fabricated, and the influence of Schottky barrier inhomogeneity on the electrical transport mechanism is investigated. The ideality factor increases and the zero-bias Schottky barrier height (SBH) decreases monotonically, as temperature decreases from 300 to 80 K. The temperature-dependent electrical transport characteristics can be explained by the SBH inhomogeneity. We use a spatial potential fluctuation model to analyze the conduction mechanism, where the SBH with a Gaussian distribution is assumed. The standard deviations of SBH distribution are up to 13.06% and 14.09% of the mean value of zero-bias SBH in 80–140 and 140–300 K, respectively, implying strong SBH inhomogeneity in typical graphene/CdSe NB junctions.

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.

Intrinsic Carrier Transport of Phase-Pure Homologous 2D Organolead Halide Hybrid Perovskite Single Crystals.

This work reveals the intrinsic carrier transport behavior of 2D organolead halide perovskites based on phase-pure homologous (n = 1, 2, and 3) Ruddelsden-Popper perovskite (RPP) (BA)2 (MA)n -1 Pbn I3n+1 single crystals. The 2D perovskite field effect transistors with high-quality exfoliated 2D perovskite bulk crystals are fabricated, and characteristic output and transfer curves are measured from individual single-crystal flakes with various n values under different temperatures. Unipolar n-type transport dominated the electrical properties of all these 2D RPP single crystals. The transport behavior of the 2D organolead halide hybrid perovskites exhibits a strong dependence on the n value and the mobility substantially increases as the ratio of the number of inorganic perovskite slabs per organic spacer increases. By extracting the effect of contact resistances, the corrected mobility values for n = 1, 2, and 3 are 2 × 10-3 , 8.3 × 10-2 , and 1.25 cm2 V-1 s-1 at 77 K, respectively. Furthermore, by combining temperature-dependent electrical transport and optical measurements, it is found that the origin of the carrier mobility dependence on the phase transition for 2D organolead halide perovskites is very different from that of their 3D counterparts. Our findings offer insight into fundamental carrier transport behavior of 2D organic-inorganic hybrid perovskites based on phase-pure homologous single crystals.