Crystal Orientation and Transport Properties of a 633-nm-Diameter Bismuth Nanowire

The crystal orientation and resistivity of a bismuth nanowire (diameter 633 nm, length 1.91 mm) encased in quartz were measured. The nanowire surface was irradiated with a high-intensity, collimated x-ray beam through the quartz template, and several Laue spots were observed with no streak patterns. Therefore, we concluded that the nanowire was a single crystal. The crystal orientation could be determined by measuring the relationship between the Laue spot distribution and the location of the nanowire fixed by a goniometer. The direction along the wire length was strongly directed toward the bisectrix axis of bismuth. The temperature dependence of the nanowire resistivity was measured; the resistivity at 300 K was 1.40 μΩ m, which is somewhat greater than that of the bulk sample due to the lower mobility of the nanowire. The temperature coefficient of resistivity was positive in the temperature range from 300 K to 165 K, and it became negative below 165 K. The temperature dependence can be modeled by accounting for the limited electron mean free path in the bismuth nanowire based on the crystal orientation determined by the Laue measurements.     

Enhancement in Figure of Merit (ZT) by Annealing of BiTe Nanostructures Synthesized by Microwave-Assisted Flash Combustion

Uniform polycrystalline bismuth telluride (BiTe) nanowires of diameter 100 nm to 150 nm and hexagonal nanoplates with thickness of 50 nm to 100 nm have been successfully synthesized by the microwave-assisted flash combustion technique. The formation of BiTe nanostructures depends on the type of fuel and the oxidant-to-fuel ratio, which in turn affect the reaction time and reaction temperature. Spark plasma sintering has been employed for compaction and sintering of both as-synthesized as well as annealed BiTe powders. Increasing the sintering temperature while using faster sintering cycles reduced the porosity, resulting in high densification while preserving the nanostructures. The dimensionless figure of merit (ZT) was evaluated from the Seebeck coefficient, electrical resistivity, and thermal conductivity values over the range from 300 K to 600 K. The effect of annealing on the enhancement of ZT is discussed. These evaluations suggest that the rarely studied BiTe is a potential candidate for thermoelectric applications at low temperatures.     

Numerical Study of Effects of Scattering Processes on Transport Properties of Bi Nanowires

In this study, we investigated the effects of scattering on the transport properties of Bi nanowires. The electrical conductivities and Seebeck coefficients of Bi nanowires were calculated using the Boltzmann equation, with an energy-dependent relaxation time corresponding to the scattering process. Decreasing the wire diameter increased the Seebeck coefficient for all of the scattering processes examined, because a semimetal–semiconductor transition occurred. In 80-nm-diameter nanowires, the Seebeck coefficient for ionized impurity scattering was larger than that of the acoustic deformation potential. On the other hand, in 20-nm-diameter nanowires, the dependence of the Seebeck coefficient on the scattering process was negligible, compared with the influence of wire diameter.     

Four-Wire Resistance Measurements of a Bismuth Nanowire Encased in a Quartz Template Utilizing Focused Ion Beam Processing

Four-wire resistance measurements were performed using a bismuth nanowire, 750?nm in diameter, 1.96?mm in length, and encapsulated in a quartz template. One side of the quartz template was polished to allow focused ion beam (FIB) processing, and metal film layers were deposited on the polished side to form electrodes. Nanofabrication was employed to remove a selected portion of the quartz, and FIB processing was used to expose the surface of the bismuth nanowire. A local area of the bismuth wire was successfully exposed, and a carbon electrode was deposited on the bismuth wire in?situ by a chemical reaction between the ion beam and phenanthrene gas. Additional carbon deposition on the initial carbon electrode was used to connect to a metal film on the quartz template. In total, four nanofabrications were performed on the bismuth wire to create the desired electrical contacts. The resistivity of the nanowire was measured by a four-wire method to be 1.29??????m at 300?K, corresponding to that of bulk bismuth. The temperature dependence of the resistivity was also measured, and was qualitatively and quantitatively in good agreement with previous calculated and experimental results using other bismuth nanowires. The present results demonstrate the successful development of a technique to fabricate an electrode on a local area of a nanowire using FIB processing to form suitable electrical contacts.     

Thermoelectric Properties of a 593-nm Individual Bismuth Nanowire Prepared Using a Quartz Template

An individual bismuth nanowire sample, 593 nm in diameter and 1.64 mm in length, has been successfully grown using a quartz template. The resistivity and the Seebeck coefficient of the nanowire at 300 K were 1.35 μΩ m and −59 μV/K, respectively, similar to those of a bismuth bulk sample. The temperature dependence of the resistivity was found to decrease with temperature from 300 K to 175 K and then increase with further temperature reduction below 175 K. The absolute value of the Seebeck coefficient decreased with temperature from 300 K to 90 K, and the sign of the Seebeck coefficient changed from negative to positive near 90 K. This result indicated that there was a small amount of contamination in the bismuth. The carrier density was estimated from the resistivity and Seebeck coefficient on the basis of limitation of the mean free path and a two-carrier model, and the observed temperature dependences are discussed.     

Thermal Conductivity of an Individual Bismuth Nanowire Covered with a Quartz Template Using a 3-Omega Technique

Thermal conductivity is estimated using a 3-omega technique for an individual bismuth nanowire (diameter: 595 nm, length: 2.24 mm) covered with a quartz template. To evaluate the thermal conductivity of the nanowire, we propose a simple model of thermal and electrical transfer functions for the nanowire that assumes a linear combination of that of the line heater on the substrate and a suspended wire. Since the out-of-phase third-harmonic component of the electrical transfer function depends only on the thermal diffusivity of the nanowire, measurement of the frequency dependence is carried out. A distinct extreme value of the frequency has been observed, as expected, and estimation of the thermal conductivity of the nanowire covered with the quartz is attempted. Although the thermal conductivity at 300 K is 9.8 W/mK, somewhat smaller than that of bulk bismuth, the temperature dependence of the thermal conductivity is quite different from that of bulk bismuth, and decreased linearly with decreasing temperature. In particular, this shows that the thermal conductivity obtained is suppressed in the low-temperature region by phonon confinement in the nanowire.     

Thermoelectric Properties of Bismuth Micro/Nanowire Array Elements Pressured into a Quartz Template Mold

A quartz template having a length of several millimeters and with holes having diameters on the order of micro/nanometers was fabricated. Bismuth was injected into the template holes by high-pressure injection. A bismuth micro/nanowire array sample was prepared, and the temperature dependence of the Seebeck coefficient and the resistance were measured in the temperature range of 50 K to 300 K. Although the temperature dependence of the Seebeck coefficient is similar to that of polycrystalline bulk bismuth, the temperature coefficient of the resistance is much less than that of the bulk sample. The magnetic-field dependence of the Seebeck coefficient was also measured. The Umkehr effect was observed, demonstrating that the mixed micro/nanowires are a bundle of single-crystal wires. The magnitude of the absolute value of the Seebeck coefficient was found to be large in high magnetic field and at low temperature.     

Thickness Dependence Study of Electron Beam Evaporated LBMO Manganite Thin Films for Bolometer Applications

La_0.7Ba_0.3MnO₃ (LBMO) thin films with different thicknesses were deposited on Si substrates using an electron beam evaporation technique for bolometer applications. To evaluate the influence of the thickness on their structural, compositional, morphological, and electrical properties, the LBMO thin films were characterized by x-ray diffraction (XRD), energy-dispersive spectroscopy, atomic force microscopy, and a four-probe method. XRD measurements showed that the crystal quality of the LBMO films improved with increasing thickness. The surface morphology revealed that the grain size and surface roughness of the films increased with increasing thickness. The resistivity increased with increasing thickness of the film. The temperature coefficient of resistance of the LBMO films decreased from 5.15%/K to 4.12%/K with increase of the film thickness from 20 nm to 100 nm.     

Effect of Composition on Thermoelectric Properties of Polycrystalline CrSi2

Ingots with compositions CrSi_2?x (with 0 < x < 0.1) were synthesized by vacuum arc melting followed by uniaxial hot pressing for densification. This paper reports the temperature and composition dependence of the electrical resistivity, Seebeck coefficient, and thermal conductivity of CrSi_2?x samples in the temperature range of 300 K to 800 K. The silicon-deficient samples exhibited substantial reductions in resistivity and Seebeck coefficient over the measured temperature range due to the formation of metallic secondary CrSi phase embedded in the CrSi₂ matrix phase. The thermal conductivity was seen to exhibit a U-shaped curve with respect to x, exhibiting a minimum value at the composition of x = 0.04. However, the limit of the homogeneity range of CrSi₂ suppresses any further decrease of the lattice thermal conductivity. As a consequence, the maximum figure of merit of ZT = 0.1 is obtained at 650 K for CrSi_1.98.     

Electrical transport properties of single GaN and InN nanowires

The transport properties of single GaN and InN nanowires grown by thermal catalytic chemical vapor deposition were measured as a function of temperature, annealing condition (for GaN) and length/square of radius ratio (for InN). The as-grown GaN nanowires were insulating and exhibited n-type conductivity (n ≈ 2×10¹⁷ cm⁻³, mobility of 30 cm²/V s) after annealing at 700°C. A simple fabrication process for GaN nanowire field-effect transistors on Si substrates was employed to measure the temperature dependence of resistance. The transport was dominated by tunneling in these annealed nanowires. InN nanowires showed resistivity on the order of 4×10⁻⁴ Ω cm and the specific contact resistivity for unalloyed Pd/Ti/Pt/Au ohmic contacts was near 1.09×10⁻⁷ Ω cm². For In N nanowires with diameters <100 nm, the total resistance did not increase linearly with length/square of radius ratio but decreased exponentially, presumably due to more pronounced surface effect. The temperature dependence of resistance showed a positive temperature coefficient and a functional form characteristic of metallic conduction in the InN nanowires.     

The Effect of Heat Treatment on Structural and Multiferroic Properties of Phase-Pure BiFeO3

Phase-pure multiferroic BiFeO₃ (BFO) was prepared by the coprecipitation technique using diverse precursors bismuth oxide at temperature as low as 400°C. The dependence of structural, microstructural, thermal, electrical (AC and DC), and magnetic properties on sintering temperature was systematically investigated. Uniaxially pressed samples (Ø8 mm) were sintered in air at 500°C to 800°C for 4 h. X-ray diffraction analysis was used to determine the amorphous and perovskite nature of as-synthesized and calcined/sintered samples, respectively. The crystallite size of sintered powders increased from 47 nm to 67 nm. Scanning electron microscopy showed grain growth during sintering, which improved intergranular connectivity and decreased porosity in the samples. The ferroelectric to paraelectric Curie transition temperature (T _C) of pure BFO powder was detected by differential scanning calorimetry analysis and found to be 820°C ± 1°C. The samples exhibited high AC resistivity and dielectric constant, and low loss tangent values. The samples exhibited weak ferromagnetic behavior with an unsaturated magnetization versus magnetic field hysteresis loop at room temperature. Ferroelectric behavior and variation in remnant polarization and coercivity were observed from polarization versus electric field loops. Enhanced capacitance in the magnetic field revealed the magnetoelectric effect in the samples.     

Effect of Heat Treatment on the Thermoelectric Properties of Bismuth–Antimony–Telluride Prepared by Mechanical Deformation and Mechanical Alloying

In this work, p-type 20%Bi₂Te₃–80%Sb₂Te₃ bulk thermoelectric (TE) materials were prepared by mechanical deformation (MD) of pre-melted ingot and by mechanical alloying (MA) of elemental Bi, Sb, and Te granules followed by cold-pressing. The dependence on annealing time of changes of microstructure and TE properties of the prepared samples, including Seebeck coefficient, electrical resistivity, thermal conductivity, and figure-of-merit, was investigated. For both samples, saturation of the Seebeck coefficient and electrical resistivity were observed after annealing for 1 h at 380°C. It is suggested that energy stored in samples prepared by both MA and MD facilitated their recrystallization within short annealing times. The 20%Bi₂Te₃–80%Sb₂Te₃ sample prepared by MA followed by heat treatment had higher a Seebeck coefficient and electrical resistivity than specimens fabricated by MD. Maximum figures-of-merit of 3.00 × 10^?3/K and 2.85 × 10^?3/K were achieved for samples prepared by MA and MD, respectively.     

Thermoelectric Properties of Nanostructured Bismuth-Doped Lead Telluride Bi x (PbTe)1−x Prepared by Co-Ball-Milling

In this work we present a simple method to synthesize nanostructured, bismuth-doped lead telluride by co-ball-milling. The obtained nanopowders were compacted via either a cold pressing/annealing approach or by hot pressing. The two compacting methods were compared regarding sample density. Series with bismuth content up to 6 at.% were characterized by measuring the thermoelectric transport properties over a wide temperature range between 123 K and 773 K using two different techniques for the Seebeck coefficient and electrical conductivity. A decreasing thermal conductivity and increasing electrical conductivity were found with increasing doping level. The best results were obtained for samples with 5 at.% and 6 at.% bismuth, showing a maximum ZT value of 1.1 at 773 K. Transmission electron microscopy study was performed to analyze the microstructure of the nanopowders, suggesting that, in addition to n-type doping of the lead telluride matrix, segregation effects occur and the samples consist of multiple phases.     

Measurement of Thermoelectric Properties of Single Semiconductor Nanowires

We have measured the thermopower and the thermal conductivity of individual silicon and indium arsenide nanowires (NWs). In this study, we evaluate a self-heating method to determine the thermal conductivity λ. Experimental validation of this method was performed on highly n-doped Si NWs with diameters ranging from 20 nm to 80 nm. The Si NWs exhibited electrical resistivity of $\rho = (8\pm4)\, \hbox{m}\Upomega\,\hbox{cm}$ ρ = ( 8 ± 4 ) m Ω cm at room temperature and Seebeck coefficient of ?(250 ± 100) μV/K. The thermal conductivity of Si NWs measured using the proposed method is very similar to previously reported values; e.g., for Si NWs with 50 nm diameter, λ = 23 W/(m K) was obtained. Using the same method, we investigated InAs NWs with diameter of 100 nm and resistivities of $\rho = (25\pm5)\, \hbox{m}\Upomega\,\hbox{cm}$ ρ = ( 25 ± 5 ) m Ω cm at room temperature. Thermal conductivity of λ = 1.8 W/(m K) was obtained, which is about 20 to 30 times smaller than in bulk InAs. We analyzed the accuracy of the self-heating method by means of analytical and numerical solution of the one-dimensional (1-D) heat diffusion equation taking various loss channels into account. For our NWs suspended from the substrate with low-impedance contacts the relative error can be estimated to be ≤25%.     

Dependence of Site Occupancy and Structural and Electrical Properties on Successive Replacement of Co by Zn in CoFe2O4

The crystal structure and cation distribution at particular sites in the crystal lattice play the primary role in determining the properties of nanocrystalline transition-metal oxide materials. Nanocrystalline ferrite particles of Co_1?x Zn _x Fe₂O₄ with x varying from 0.0 to 1.0 were synthesized by a coprecipitation method. Samples synthesized at the reaction temperature of 70°C were sintered at 600°C for 3 h. The face-centered cubic (FCC) spinel structure of the synthesized particles was confirmed by x-ray diffraction patterns. The grain sizes calculated from the most intense peak (311) using the Scherrer equation were found to be in the range from 10 nm to 35 nm. Extended x-ray absorption fine-structure and x-ray absorption near-edge structure spectroscopy is a powerful tool for structural study of metal oxide materials. These techniques are element specific and sensitive to the local structure. These techniques were used at Fe, Co, and Zn K-edges to investigate the cation distribution in the crystal structure. The dependence of the electrical transport properties on the shift in the crystal structure due to successive replacement of Co by Zn in CoFe₂O₄ was examined. Direct-current (dc) electrical conduction measurements were carried out as a function of temperature from 313 K to 700 K. Activation energy values indicated the polaron hopping conduction mechanism. The alternating-current (ac) electrical transport properties were studied by measuring the dielectric constant as a function of frequency. A regular shift?in the electrical properties was observed depending upon the cation distribution.     

Thermoelectric Properties of InSb Nanowires Over a Wide Temperature Range

The thermoelectric power and electrical conductance of bundles of indium antimonide nanowires with a diameter of about 5 nm have been measured over the temperature range of 80 K to 400 K. In the range from 80 K to 300 K, the temperature dependence of the conductance of nanowires is close to power law, while the thermopower increases linearly with temperature. The thermoelectric properties of the nanowires are discussed in terms of the Luttinger liquid theory, taking into account enhancement of the electron–electron interaction in one-dimensional conductors.     

Effect of the deposition rate on the phase transition in silver telluride thin films

Silver telluride thin films of thickness 50 nm have been deposited at different deposition rates on glass substrates at room temperature and at a pressure of 2×10⁻⁵ mbar. The electrical resistivity was measured in the temperature range 300–430 K. The temperature dependence of the electrical resistance of Ag₂Te thin films shows structural phase transition and coexistence of low temperature monoclinic phase and high temperature cubic phase. The effect of deposition rate on the phase transition and the electrical resistivity of silver telluride thin films in relation to carrier concentration and mobility are discussed.     

The effects of incomplete annealing on the temperature dependence of sheet resistance and gage factor in aluminum and phosphorus implanted silicon on sapphire

The temperature coefficient of resistivity (TCR) of ion implanted silicon can be significantly reduced by partially annealing the crystal damage produced during implantation. The extent to which this method can be used to temperature compensate the resistivity and the gage factor has been determined for 300 ohm-cm silicon on sapphire implanted with either 100 keV Al²⁷ or P³¹ ions. The implantations were made at room temperature parallel to the 〈100〉 axis and in four fluences ranging from 1 × 10¹³cm^?2 to 1·25 × 10¹⁵ cm^?2. Sheet resistance, Hall coefficient, and effective mobility were measured from ?150°C to 150°C for various anneal temperatures. It was possible to obtain very low temperature dependences of sheet resistance at 300°K for all dopant fluences by appropriate partial annealing. On samples having the lowest temperature dependence of sheet resistance, the gage factor was measured from ?75°C to 75°C. The measurements were made along the 〈100〉 direction for phosphorus doped samples, and along the 〈110〉 direction for aluminum doped samples for all four fluences. The gage factor and its temperature dependence for these crystal orientations are not drastically affected by the crystal damage. These results are interpreted in terms of a model previously developed to explain the effect of electron damage on the temperature dependence of the resistivity and the piezoresistance of silicon.     

Microchips for the Investigation of Thermal and Electrical Properties of Individual Nanowires

This paper focuses on the determination of thermal and electrical properties of individual thermoelectric nanowires, primarily bismuth and bismuth compound nanowires, as functions of their crystallinity, diameter, and composition. For measurements of the Seebeck coefficient and the electrical and thermal conductivity, specially designed microchips have been developed and employed. Finite-element simulations demonstrate that the temperature profiles of the microchips provide suitable temperature gradients for Seebeck-effect measurements and heat-sink conditions for thermal conductivity investigations. First measurements of thermal conductivity of metallic nanowires and of Seebeck coefficients of granular nanowires prepared by focused electron-beam-induced deposition are presented. Some of these results are discussed in the framework of finite-size-effect theory.     

Dielectric and Impedance Spectroscopy of Barium Bismuth Vanadate Ferroelectrics

Structural, micro-structural and electrical properties of barium bismuth vanadate Ba(Bi_0.5V_0.5)O₃ ceramics were investigated. X-ray diffraction (XRD) analysis of the prepared material confirmed the formation of the compound with monoclinic crystal system. Scanning electron microscopy (SEM) of the compound exhibits well-defined grains that are uniformly distributed throughout the surface of the sample. Dielectric properties of the compound were studied as a function of temperature at different frequencies. An observation of dielectric anomaly at 295 °C is due to ferroelectric phase transition that was later confirmed by the appearance of hysteresis loop. Detailed studies of complex impedance spectroscopy have provided a better understanding of the relaxation process and correlations between the microstructure–electrical properties of the materials. The nature of frequency dependence of ac conductivity obeys the Debye power law. The dc conductivity, calculated from the ac conductivity spectrum, shows the negative temperature coefficient of resistance behavior similar to that of a semiconductor.