Temperature-dependent Electrical Transport Research Articles

Influence of Ge-doping on the collinear and non-collinear antiferromagnetic phases of Mn[formula omitted]Si[formula omitted] alloy

Effect of Ge-doping at the non-magnetic site (Si-site) of the Mn-based binary alloy Mn5Si3 has been explored through temperature-dependent x-ray diffraction, dc-magnetic, and electrical transport measurements. All the doped alloys show D88 type hexagonal structure with space group P63/mcm at room temperature and undergoes two structural distortions, hexagonal (P63/mcm) → orthorhombic (Ccmm) → orthorhombic (Cc2m), on cooling. The magnetic characters of these orthorhombic (Cc2m), orthorhombic (Ccmm), and hexagonal (P63/mcm) phases are non-collinear antiferromagnetic (AFM1), collinear antiferromagnetic (AFM2), and paramagnetic (PM), respectively. Doping of Ge results in a significant increase in the AFM1 to AFM2 transition temperature (TN1). However, the AFM2 to PM transition point remains unchanged. In addition, a reasonable increase in the critical field values of the AFM1 to AFM2 transition via another non-collinear antiferromagnetic phase (AFM1′) with increasing Ge concentration has been observed, indicating the strengthening of AFM1 and AFM1′ phases over the AFM2 phase. Such behaviors make the observation of unusual magnetic properties of undoped Mn5Si3 alloys, like inverted hysteresis loop (IHL) and thermomagnetic irreversibility (TI), more evident for these Ge-doped alloys. Further, this study unfolds the presence of conventional and inverse magnetocaloric effect and they found to decrease with Ge doping. An interesting interplay for positive and negative magnetoresistance has also been observed in all the studied alloys.

Crystallographic dependence of the electrical transport mechanism in La-Mn-O thermosensitive thin films

Bimetallic oxide materials with amorphous state usually exhibit singular peculiarities due to their periodic disorder structure, implying great potential applications as high-performance electronic devices. In this report, La-Mn-O thin films with different crystallographic features (amorphous and polycrystalline) were deposited on Si (100) substrates using magnetron sputtering method. The as-deposited thin films were successfully convert from the amorphous state to the polycrystalline state by tuning the post-annealing temperatures. X-ray diffraction (XRD) confirmed that La-Mn-O was amorphous at annealing temperatures of 500 °C, while the films were polycrystalline at higher annealing temperatures (600 °C and 700 °C). Scanning electron microscopy (SEM) and high-magnification transmission electron microscopy (HRTEM) distinguished the microscopic features of LaMnO3 thin films in polycrystalline and amorphous states. The effects of annealing temperatures and structural disorder on the temperature-dependent electrical transport properties of La-Mn-O thin films are investigated. The resistance-temperature relationship curves of both polycrystalline and amorphous films show negative temperature coefficient behavior, and the amorphous films follow more than one conduction mechanism. Results show that the electrical transport of the crystalline La-Mn-O films is mainly controlled by thermal activation mechanism, while the electron transport behavior of the amorphous film is simultaneously influenced by thermal activation and Mott VRH variable range hopping conduction.

Temperature dependent magnetic and electrical transport properties of lanthanum and samarium substituted nanocrystalline nickel ferrite and their hyperthermia applications

Structural, optical, temperature dependent magnetic and electrical properties have been investigated for the nanocrystalline NiFe1.97RE0.03O4 (RE = La3+ and Sm3+) compound. Without any signs of a secondary REFeO3 phase, the Rietveld refinement reveals a single-phase spinel structure possessing cubic symmetry for current samples. The decrease in lattice constant has been observed as a result of RE ions substitution. The TEM micrographs reveal nanosized particles with an average size of 35 ± 3 nm and 32 ± 3 nm for La3+ and Sm3+ substituted samples. EDX spectra of both samples show good compositional homogeneity. HRTEM micrographs of both samples show well resolved lattice fringes and their inter-planar spacing matches with that obtained from the Rietveld analysis of the XRD patterns. The concentration of Fe2+ and Fe3+ ions has been estimated from the XPS spectra analysis and it is found to be ∼ 22% and 78% for NiFe1.97La0.03O4 and, 20% and 80% for NiFe1.97Sm0.03O4 compound. The optical energy band gap for rare earth substituted samples is found to be more than that of pure nickel ferrite. The value of saturation magnetization (Ms) and magneto-crystalline anisotropy constant (K1) estimated from the “Law of Approach to Saturation” equation for rare earth substituted samples are found to be less than that of pure nickel ferrite. However, an enhanced value of coercivity has been observed with RE substitution. ZFC (zero field cooling) and FC (field cooling) DC magnetization curves measured at 100 Oe in the temperature range of 60–400 K reveal a combination of weak and intermediate forms of magnetic interaction between the particles for both samples. Temperature dependent analysis of saturation magnetization and coercivity employing modified Bloch’s law and Kneller’s relation supports the nanomagnetic behavior of both samples. Under an AC magnetic field of 14.92 kA/m and frequency 337 kHz, the SAR and ILP values have been found to be ∼ 331 W/g and 4.22 nHm2/kg for NiFe1.97La0.03O4 and, ∼ 241 W/g and 3.21 nHm2/kg for NiFe1.97Sm0.03O4. The dielectric relaxation data have been analyzed at various temperatures ranging from 40 to 300 0C over the frequency range 100 Hz-1 MHz using electrical impedance spectroscopy. The dielectric constant for rare earth substituted samples is found to be more and their AC conductivity is observed to be less than that of pure nickel ferrite. The analysis of temperature dependent AC conductivity employing Jonscher’s power law suggests that the charge carrier’s conduction mechanism of both samples follows the small polaron hopping mechanism below 200 0C, thereafter; it follows the correlated barrier hopping mechanism. The activation energy estimated by imaginary impedance spectra is found to be 0.411 eV and 0.398 eV for NiFe1.97La0.03O4 and NiFe1.97Sm0.03O4 which is more than that of pure nickel ferrite. The Cole-Cole plots at different temperatures suggest the presence of non-Debye-type relaxation. The modeling of Cole-Cole plots with equivalent circuits for both samples confirms the contribution of both grain and grain boundary in electrical conduction. The estimated stretching exponential factor by fitting the modified KWW (Kohlrausch-Williams-Watts) equation to the imaginary modulus curve reveals relatively more dipole-dipole interaction in NiFe1.97Sm0.03O4 than that of the La3+ substituted sample.

Two-Step Flux Synthesis of Ultrapure Transition-Metal Dichalcogenides.

Two-dimensional transition-metal dichalcogenides (TMDs) have attracted tremendous interest due to the unusual electronic and optoelectronic properties of isolated monolayers and the ability to assemble diverse monolayers into complex heterostructures. To understand the intrinsic properties of TMDs and fully realize their potential in applications and fundamental studies, high-purity materials are required. Here, we describe the synthesis of TMD crystals using a two-step flux growth method that eliminates a major potential source of contamination. Detailed characterization of TMDs grown by this two-step method reveals charged and isovalent defects with densities an order of magnitude lower than those in TMDs grown by a single-step flux technique. For WSe2, we show that increasing the Se/W ratio during growth reduces point defect density, with crystals grown at 100:1 ratio achieving charged and isovalent defect densities below 1010 and 1011 cm-2, respectively. Initial temperature-dependent electrical transport measurements of monolayer WSe2 yield room-temperature hole mobility above 840 cm2/(V s) and low-temperature disorder-limited mobility above 44,000 cm2/(V s). Electrical transport measurements of graphene-WSe2 heterostructures fabricated from the two-step flux grown WSe2 also show superior performance: higher graphene mobility, lower charged impurity density, and well-resolved integer quantum Hall states. Finally, we demonstrate that the two-step flux technique can be used to synthesize other TMDs with similar defect densities, including semiconducting 2H-MoSe2 and 2H-MoTe2 and semimetallic Td-WTe2 and 1T'-MoTe2.

Temperature and frequency-dependent electrical transport studies of manganese-doped zinc ferrite nanoparticles

Manganese substituted zinc ferrite nanoparticles (Zn1-xMnxFe2O4, x ≤ 10%) have been successfully synthesized by the co-precipitation technique. The x-ray diffraction technique was employed to determine the structure of nano-ferrites at room temperature. SEM, EDX, and FT-IR spectroscopy were used to establish surface morphologies, elemental composition, and phase formation. Impedance spectral analysis was carried out at various temperature ranges varying from 600 K to 870 K in the frequency range of 100 Hz to 2 MHz. The dielectric parameter measurements were used to compute the types of conduction in the synthesized samples. The significance of the distribution of cations between A and B sites was analyzed using frequency and temperature-dependent ac electrical transport studies of the synthesized nanoparticles. The conduction mechanism, overlapping large polaron tunneling (OLPT) was found in Zn1-xMnxFe2O4 nano-ferrite system.

Temperature-dependent electrical transport, Hall effect, and Seebeck properties of bulk chemically reduced graphene oxide with bipolar charge carrier materials

The temperature-dependent electrical transport, Hall effect, and Seebeck properties of bulk-reduced graphene oxide (rGO) prepared by a chemical reduction process were investigated in a temperature range of 310–475 K. The bulk rGO contained bipolar charge carriers with p-type to n-type switching at a temperature of 420 K. The materials illustrated a p-type characteristic in the temperature range of 310–420 K and n-type characteristic in the temperature range of 420–475 K. The charge transport mechanism was that of the graphene-derived 2D material in the p-type regime and governed by polaronic charge carriers.

One-step post-treatment boosts thermoelectric properties of PEDOT:PSS flexible thin films

• High power factor of 64.4 µW m −1 K −2 is achieved in PEDOT:PSS films at 360 K. • 80 vol.% DMAC solution boosts the electrical conductivity from 5 to 964 S cm −1 . • 0.5 mol L −1 LAA improves the Seebeck coefficient from 18.7 to 25 µV K −1 . • Mott variable range hopping model explains the thermoelectric transport behavior. Developing high-performance poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) significantly widens the practical applications of flexible organic thermoelectric devices, while the water-based co-solvent dopants and/or post-treatments are still rarely studied so far. Here, we develop a one-step post-treatment to improve the power factor of PEDOT:PSS films by using a water-based solution, which is composed of co-solvent (polar solvent dimethylacetamide (DMAC) and deionized water) and organic reducing agent L-ascorbic acid (LAA). The 80 vol.% DMAC solution significantly boosts the room-temperature electrical conductivity of the films from 5 to 964 S cm −1 , while the Seebeck coefficient can be further enhanced from 18.7 to 25 µV K −1 by treating with 0.5 mol L −1 LAA, contributing to a significantly improved power factor of 55.3 µW m −1 K −2 . The boosted electrical conductivity is ascribed to the separated PEDOT and PSS phases triggered by the high dielectric constant and polarity of DMAC; while the improved Seebeck coefficient is attributed to the reduced oxidation degree of PEDOT from the reducing agent LAA, both confirmed by the comprehensive structural and morphological characterizations. Furthermore, a maximum power factor of 64.4 µW m −1 K −2 can be achieved at 360 K and the observed temperature-dependent electrical transport behavior can be well explained by the Mott variable range hopping model. Besides, a flexible thermoelectric device, assembled by the as-fabricated PEDOT:PSS films, exhibits a maximum output power of ∼23 nW at a temperature difference of 25 K, indicating the potential for applying to low-grade wearable electronics.

Inhomogeneous heterojunction performance of Zr/diamond Schottky diode with Gaussian distribution of barrier heights for high sensitivity temperature sensor

Herein, we demonstrated the fabrication and operation of vertical diamond Schottky barrier diode (SBD) as a high-performance temperature sensor. The current transport mechanism and temperature sensor properties of Zr/p-diamond SBD were comprehensively investigated with operating temperatures up to 448 K. At room temperature, Zr/p-diamond SBD exhibits outstanding performances with a high rectification ratio of 2.28 × 1010, a low specific on-resistance of 0.5 mΩ•cm2, a large Schottky barrier height (SBH) of 1.43 eV, a low ideality factor of 1.77, and an extremely low saturation current of 4.42 × 10−22 A. The enhanced-performance with a higher forward current and a lower reverse leakage current is observed to be across a wide temperature range from 323 to 448 K. Additionally, a significant improvement of rectifying current curve with a highest rectification ratio over 1012 is obtained at 423 K. The SBH increases whereas the ideality factor decreases with temperature increasing, indicating barrier inhomogeneities at Zr/p-diamond interface. The temperature-dependent electrical transport characteristics on spatially inhomogeneous Schottky contacts can be successfully described by potential fluctuations model. The extracted original value of Richardson constant (A*) is 0.0153 A/cm2•K2, which is much lesser than theoretical value of 96 A/cm2•K2. The discrepancy of A* value from theoretical value has been well explained by thermionic emission model with Gaussian distribution of barrier inhomogeneity. The values of mean SBH and A* extracted from modified Richardson plot depending on SBH inhomogeneity are 1.76 eV and 105.51 A/cm2•K2, respectively. With a Gaussian distribution take into account, the A* value is close to the theoretical value, and confirmed the existence of SBH inhomogeneity at Zr/p-diamond interface. Furthermore, for Zr/p-diamond SBD operating as a temperature sensor, the voltage drop across the diode decreases linearly with increasing temperature at a specific forward current level, contributing to a highest sensor sensitivity of 6.8 mV/K. The reported results in this work are among the best to date in diamond SBD temperature sensor. Although not optimized for temperature sensor, the high and thermally stable rectifying barriers make Zr/p-diamond SBD a strong candidates for high-performance diamond rectifying diode and temperature sensor.

X-ray photoelectron spectroscopy and temperature-dependent electrical transport properties of MnFe2O4-reduced graphene oxide nanocomposites

The reduced graphene oxide (rGO) sheets are assembled with MnFe2O4 (MFO) nanoparticles via an in-situ sol–gel autocombustion route to obtain MFO@rGO nanocomposites. The investigation of their structural and electrical properties is carried out through X-ray photoelectron spectroscopy (XPS) and impedance analyzer, respectively. XPS study discloses the presence of C, O, Fe, and Mn in the MFO@rGO composite, indicating the decoration of MnFe2O4 nanoparticles by rGO sheets. XPS study also reflects the mixed Fe3+/Fe2+ oxidation states of Fe and mixed Mn2+/Mn3+oxidation states of Mn in the composite. The highest value of ac conductivity (σac = 7.51 × 10−6 Ω−1 cm−1 at 1 MHz) at room temperature is obtained within the vicinity of the percolation threshold when rGO content is 20 wt%. The ac conductivity of composites increases with frequency up to 100 °C and then becomes independent of the frequency with a further increase in temperature. The impedance gradually decreases with temperature, revealing the occurrence of negative temperature coefficient of resistance effect in composites. The findings suggest that the MFO@rGO composites can be a potential nominee for the fabrication of electrodes for supercapacitors and flexible electronic devices of low resistance and high electrical conductivity for high-frequency applications.

Micropore Filling and Temperature Dependent Electrical Transport Aspects of Poly(3,4‐ethylenedioxythiophene) Polymerized Zr‐Based Metal–Organic Framework

This work reports the polymerization of poly(3,4‐ethylenedioxythiophene) (PEDOT) within a metal–organic framework (MOF), namely UiO‐66 containing pores of tetrahedral and octahedral geometry. Partial filling of micropores of UiO‐66 with PEDOT and occupancy up to 29.6% has brought up conformational changes, vibrational responses as well as altered carrier transport phenomena characterized through different techniques. X‐ray diffraction (XRD) and Brunner–Emmett–Teller (BET) surface area analysis have revealed the structural stability of the MOF after the inclusion of PEDOT while microporous nature was maintained with a primary filling of octahedral micropores. Interestingly, the PEDOT polymerization is believed to draw a conformational change from benzoid to quinoid structure as substantiated through the Raman studies. Moreover, a noticeable improvement in electrical conductivity has been realized (typically, ≈10−3 to 10−2 S cm−1) for the composite while its temperature dependency followed a semiconducting feature. Controlled insertion, limited space‐filling, and occupancy can strengthen the insights on soft matter models and reptation theory and would find scope in applications where porosity, stability, and conductivity are desired.

In-plane Cr2N−CrN metal-semiconductor heterostructure with improved thermoelectric properties

Epitaxial metal-semiconductor heterostructures with suitable Schottky barrier can lead to high thermoelectric figure-of-merit (zT) due to selective filtering of low-energy electrons as well as reduced thermal conductivity from phonon scattering at the interfaces. Lattice-matched vertical metal-semiconductor multilayer/superlattices as well as metallic nanoparticles embedded inside semiconducting hosts have been studied intensively to explore their thermoelectric properties. However, development of in-plane metal-semiconductor heterostructures and exploration of their physical properties have remained elusive primarily due to the growth and fabrication challenges. In-plane heterostructures are expected to be more suitable for planar integration and should exhibit unique properties. In this work, we demonstrate an in-plane ${\mathrm{Cr}}_{2}\mathrm{N}\text{\ensuremath{-}}\mathrm{CrN}$ metal-semiconductor heterostructure that exhibits an improved thermoelectric power factor. The in-plane heterostructure is deposited by controlling the Cr-flux during deposition that leads to an in-plane phase separation between the metallic-${\mathrm{Cr}}_{2}\mathrm{N}$ and semiconducting CrN grains. Temperature-dependent electrical transport exhibits an Arrhenius-type thermal activation behavior with an activation energy of 70 meV, and an in-plane electrical conductivity that is about two orders of magnitude higher than that of CrN. The Seebeck coefficient also remained moderately large at \ensuremath{-}150 \ensuremath{\mu}V/K at 700K leading to a very large power factor of $2.1\phantom{\rule{0.16em}{0ex}}\mathrm{mW}/\mathrm{m}{\mathrm{K}}^{2}$ at 700 K.

Temperature dependent electrical transport behavior of (100-x)Bi2O3-x(BaTiO3) glass system

Impedance measurements were performed as a function of temperature (RT to 700 K) in the frequency range (1 Hz-1MHz) on fabricated Bismuth Barium Titanate glass samples. Dielectric permittivity is found to be increased on increasing both the temperature and BaTiO3 (BT) content in highly polarizable glass matrix of Bi2O3. Experimental data obtained for imaginary part of electric modulus (M″)has been fitted theoretically using non-exponential Kohlrausch-Williams-Watts (KWW) function, however Jonscher's universal power law has been employed to analyze the frequency dependent ac conductivity. The calculated activation energy values for both the samples have been found to be in close agreement with each other and qualify for doubly ionized oxygen vacancies, proposing that the similar kind of charge carriers are involved in the relaxation and conduction mechanisms. Stretched exponent (β) of Modulus and power exponent (n) of ac conductivity, estimated from theoretical fitting of experimental data, have been found to be dependent on temperature and BT (BaTiO3) content. On increasing temperature, an increase in values of (n) suggests that non-overlapping small polaron tunneling (NSPT) model can be easily applied to explain ac conduction and hopping transport mechanisms of the fabricated glass samples.

Trap states induced hopping transport and persistent photoconductivity in WSe2/MoS2 nanocomposite thin films

In this work, temperature-dependent electrical transport and photoconductivity studies of WSe2/MoS2 nanocomposite thin films have been analyzed. An n-type semiconductor film of ≈150 nm with a carrier concentration of ≈1023cm−3 is prepared by a thermal evaporation technique. Temperature-dependent (150–350 K) conductivity results present more than one type of carrier transport mechanisms. Trap states from chalcogenide vacancies in WSe2/MoS2 play an important role in the formation of localized states. The electrical conductivity of the composite conforms to Mott’s variable range-hopping model due to these localized states in the temperature range of 150–250 K. Nearest-neighbor hopping and a thermally activated conduction mechanism are responsible for electrical transport in the higher temperature range. The role of defect/trap states and random local potential fluctuations in persistent photoconductivity has also been studied. Also, the effect of vacuum and air atmosphere shows that acceptor states resulting from incorporation of oxygen in sulfur vacancies results in more photocurrent in air than vacuum. The results deepen the understanding of the conduction mechanism in such multifunctional transition-metal dichalcogenide based nanocomposites.

Transport anomalies in the layered compound BaPt4Se6

We report a layered ternary selenide BaPt4Se6 featuring sesqui-selenide Pt2Se3 layers sandwiched by Ba atoms. The Pt2Se3 layers in this compound can be derived from the Dirac-semimetal PtSe2 phase with Se vacancies that form a honeycomb structure. This structure results in a Pt (VI) and Pt (II) mixed-valence compound with both PtSe6 octahedra and PtSe4 square net coordination configurations. Temperature-dependent electrical transport measurements suggest two distinct anomalies: a resistivity crossover, mimic to the metal-insulator (M-I) transition at ~150 K, and a resistivity plateau at temperatures below 10 K. The resistivity crossover is not associated with any structural, magnetic, or charge order modulated phase transitions. Magnetoresistivity, Hall, and heat capacity measurements concurrently suggest an existing hidden state below 5 K in this system. Angle-resolved photoemission spectroscopy measurements reveal a metallic state and no dramatic reconstruction of the electronic structure up to 200 K.

Electrical transport through pelletized tablet of silicene-like quasi 2D crystalline silicon

Silicene, the silicon-based 2D analogue of graphene, is predicted to have many exotic properties. However, most of the theoretically predicted phenomena have not yet been experimentally verified. In this work, we have prepared silicene-like, quasi 2D nanostructures via topochemical exfoliation of layered silicide and hot-pressed the dried powders to form bulk pellets. Temperature dependent electrical transport characteristics of pelletized silicene were investigated and found to exhibit evidence of some theoretically predicted features. The current–voltage (I-V) characteristics of the pelletized samples are non-linear, asymmetric about V = 0 and bears clear signature of negative differential resistance (NDR). The heuristic study provides interesting insights into the transport properties of bulk pellets of silicene and opens up the possibility of many applications.

One-step epitaxy of high-mobility La-doped BaSnO3 films by high-pressure magnetron sputtering

As unique perovskite transparent oxide semiconductors, high-mobility La-doped BaSnO3 films have been successfully synthesized by molecular beam epitaxy and pulsed laser deposition. However, it remains a big challenge for magnetron sputtering, a widely applied technique suitable for large-scale fabrication, to grow high-mobility La-doped BaSnO3 films. In this work, we developed a method to synthesize high-mobility epitaxial La-doped BaSnO3 films (with mobility up to 121 cm2 V−1 s−1 at the carrier density of ∼4.0 × 1020 cm−3 at room temperature) directly on SrTiO3 single crystal substrates using high-pressure magnetron sputtering. The structural and electrical properties of La-doped BaSnO3 films were characterized by combined high-resolution x-ray diffraction, x-ray photoemission spectroscopy, and temperature-dependent electrical transport measurements. The room-temperature electron mobility of La-doped BaSnO3 films achieved in this work is two to four times higher than the reported values of the films grown by magnetron sputtering. Moreover, in the high carrier density range (n > 3 × 1020 cm−3), the electron mobility value of 121 cm2 V−1 s−1 achieved in our work is among the highest values for all reported doped BaSnO3 films. It is revealed that high argon pressure during sputtering plays a vital role in stabilizing the fully relaxed films and inducing oxygen vacancies, which facilitates high mobility at room temperature. Our work provides an easy and economical way to massively synthesize high-mobility transparent conducting films for transparent electronics.

Multiple carrier transport in high-quality α-Sn films grown on CdTe (001) by molecular beam epitaxy

The epitaxial growth of tin in an alpha phase (α-Sn) is desired for its topological properties. In this study, we have successfully grown a series of α-Sn films on CdTe (001) substrates by molecular beam epitaxy with different thicknesses. A (2 × 1) surface reconstruction of CdTe is obtained due to efficient cleaning by atomic hydrogen, which favors the α-Sn growth. The high quality of the α-Sn films has been confirmed by x-ray diffraction, atomic force microscopy, etc. Thickness and temperature-dependent electrical transport properties have been studied. All the samples show a p-type transport at room temperature, but transitions in transport type are observed at lower temperatures. These transport behaviors can be well explained by a three-band model, and a phase diagram illustrating the transport behaviors in α-Sn is presented.

Understanding of intriguing metal to semiconductor transition in Ni0.5Zn0.5Fe2O4 nanoparticulates

Temperature-dependent electrical transport characteristics of un-milled and high-energy ball-milled samples (3 h (70 nm), 6 h (55 nm) and 9 h (45 mm)) of Ni0.5Zn0.5Fe2O4 spinel ferrite were explored. A well-defined metal to semiconductor transition exhibited by all the samples has been construed in view of direct and superexchange cationic interactions and delocalization to localization of charge carriers on increasing temperature. The peak temperature (Tmax) was found to shift towards a higher temperature side on milling principally governs by the lattice vibration scattering and intrinsic excitation. The crystallite size reduction, enhancement in strain and sudden decrease in the formation and octahedral site occupancy of Fe 2+ ions on milling found responsible for the prodigious rise (~ 250 times) in normalized resistivity values for the sample comminuted for 9 h. The spectrum of energies corresponds to charge trapping centers that cause small bump (3 h milled sample) and sharp cusp (9 h milled sample) for T>Tmax. These materials may be found suitable for thermal cutoff switching applications.

Electrical transport properties of thermally stable n-ZnO/AlN/p-Si diode grown using RF sputtering

The temperature dependent electrical transport properties of n-ZnO/AlN/p-Si heterojunction diode fabricated by RF sputtering system have been investigated over a wide temperature range of 303 K–413 K. The AlN buffer layer in-between ZnO and Si lowers the mismatch in thermal expansion coefficient/lattice constant for improved electrical and structural characteristics. XRD pattern and FESEM confirm the crystalline nature and good quality of ZnO thin film with uniform grain size and crack free structure respectively. As measured from the temperature dependent I–V curve, the apparent barrier height reduces as Ideality factor rises which shows good thermal stability within the temperature range studied here. The mean value of barrier height as well as the Richardson constant have been evaluated. Additionally, by taking into account the Gaussian distribution of inhomogeneous barrier heights, the modified Richardson constant has been estimated.

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.