Very high thermopower of Bi nanowires with embedded quantum point contacts

Quantum confinement effects in bismuth (Bi) nanowires (NWs) are predicted to impart them with high thermopower values and hence make them efficient thermoelectric materials. Yet, boundary scattering of charge carriers in these NWs operating in the diffusion transport regime mask any quantum effects and impede their use for nanoscale thermoelectric applications. Here we demonstrate quantum confinement effects in Bi NWs by forming in their structure ballistic quantum point contacts (QPCs) leading to exceptionally high thermopower values (S > 2 mV/K). The power factor, S(2)G, of the QPCs is maximized at G ~ 0.25G(0) (where G(0) is the quantum of conductance) within agreement with a one-band model with step edge characteristics.     

电化学制备Bi2Te3纳米线用于微型温差发电器

借助于电化学沉积的方法,在氧化铝纳米孔内生长Bi2Te3材料,从而形成温差电纳米线阵列．利用SEM,XRD and TEM分析手段对制备的纳米线形貌和结构进行了分析,测量了纳米线的组成和温差电性能．p型和n型Bi2Te3纳米线材料的Seebeck系数经过测量分别为260μV／K和-188μV／K（307K）,比同类的块状温差电材料性能高．同时研究了沉积电位对氧化铝模板中纳米孔的填充率的影响,并对纳米线阵列的电阻进行了测量．尝试了利用n型和P型Bi2Te3纳米线阵列制备一种新型的微型温差发电器．     

Features of Lifshits Electron Topological Transitions Induced by Anisotropic Deformation in Thin Wires of Doped Bismuth

For the first time, glass-coated single-crystal Bi–0.05Sn wires with d=200 nm ÷ 3 μm with C ₃ oriented along the wire axis were obtained by the recrystallization method of the wires with standard (10 $\bar{1}$ 1) orientation, which made it possible to study the thermopower anisotropy. With the use of ShdH oscillations method, we have determined characteristics of changes in the Fermi surface topology at Lifshits electron topological transitions induced by tension in single-crystal Bi–0.05Sn wires with a standard (10 $\bar{1}$ 1) orientation and with the C ₃ orientation along the wire axis. It is found that the thermopower increase with deformation and thermopower anisotropy of the Bi–0.05Sn wires with trigonal orientation achieves a value of 130–150 μV/K which leads to a considerable increase of thermoelectric efficiency, which is important for practical applications, in particular, for their use as anisotropic thermoelectric energy converters.     

Tuning the crystallinity of thermoelectric Bi(2)Te(3) nanowire arrays grown by pulsed electrodeposition

Arrays of thermoelectric bismuth telluride (Bi(2)Te(3)) nanowires were grown into porous anodic alumina (PAA) membranes prepared by a two-step anodization. Bi(2)Te(3) nanowire arrays were deposited by galvanostatic, potentiostatic and pulsed electrodeposition from aqueous solution at room temperature. Depending on the electrodeposition method and as a consequence of different growth mechanisms, Bi(2)Te(3) nanowires exhibit different types of crystalline microstructure. Bi(2)Te(3) nanowire arrays, especially those grown by pulsed electrodeposition, have a highly oriented crystalline structure and were grown uniformly as compared to those grown by other electrodeposition techniques used. X-ray diffraction (XRD) analyses are indicative of the existence of a preferred growth orientation. High resolution transmission electron microscopy (HRTEM) and selected area electron diffraction (SAED) confirm the formation of a preferred orientation and highly crystalline structure of the grown nanowires. The nanowires were further analyzed by scanning electron microscopy (SEM). Energy dispersive x-ray spectrometry (EDX) indicates that the composition of Bi-Te nanowires can be controlled by the electrodeposition method and the relaxation time in the pulsed electrodeposition approach. The samples fabricated by pulsed electrodeposition were electrically characterized within the temperature range 240?K≤T≤470?K. Below T≈440?K, the nanowire arrays exhibited a semiconducting behavior. Depending on the relaxation time in the pulsed electrodeposition, the semiconductor energy gaps were estimated to be 210-290?meV. At higher temperatures, as a consequence of the enhanced carrier-phonon scattering, the measured electrical resistances increased slightly. The Seebeck coefficient was measured for every Bi(2)Te(3) sample at room temperature by a very simple method. All samples showed a positive value (12-33?μV?K(-1)), indicating a p-type semiconductor behavior.     

Thermoelectric properties of p-type PbSe nanowires

The thermoelectric properties of individual solution-phase synthesized p-type PbSe nanowires have been examined. The nanowires showed near degenerately doped charge carrier concentrations. Compared to the bulk, the PbSe nanowires exhibited a similar Seebeck coefficient and a significant reduction in thermal conductivity in the temperature range 20 K to 300 K. Thermal annealing of the PbSe nanowires allowed their thermoelectric properties to be controllably tuned by increasing their carrier concentration or hole mobility. After optimal annealing, single PbSe nanowires exhibited a thermoelectric figure of merit (ZT) of 0.12 at room temperature.      

Electrodeposited bismuth telluride nanowire arrays with uniform growth fronts

Bismuth telluride (Bi2Te3) nanowires were deposited into porous alumina templates with 35 nm diameter pores by a pulsed-potential electrodeposition method. For growth at temperatures between 1 and 4 degrees C, the nanowires filled 93% of the pores of the template, and the growth fronts were uniform with nanowire lengths of approximately 62-68 microm. There are over ten billion nanowires per square centimeter with aspect ratios approaching 2000:1. Samples were characterized by scanning and transmission electron microscopy, X-ray diffraction, and electron microprobe analysis. The crystalline nanowire arrays are highly oriented in the [110] direction, which is optimal for thermoelectric applications.     

The Modulation and Enhancement of Thermopower in a Luttinger liquid

We investigate the thermoelectric transport properties of the Luttinger liquid coupled to the quantum dot by tunnel junctions. A general thermopower expression is derived by using nonequilibrium Green function method. At low temperature the thermopower is of linear temperature dependence and conductance is of a temperature-dependent power-law behavior. There is a peak in the curve of the thermopower vs. gate voltage. Intralead electron interaction enhances thermopower at low temperatures and the large slope is interpreted as a character of the Luttinger liquid. In the limit of strong electron interaction thermopower S(T) at low temperature can be represented by the noninteracting electrons S ₀(T) as: S(T)=3S ₀(T)/(2g).     

Surface faceting dependence of thermal transport in silicon nanowires

Surface faceting on sidewalls is ubiquitously observed during crystal growth of semiconductor nanowires. However, predicting the thermal transport characteristics of faceted nanowires relevant to thermoelectric applications remains challenging. Here, direct molecular dynamics simulations show that thermal conductivity is considerably reduced in crystalline <111> Si nanowires with periodic sawtooth faceting compared to nanowires of same size with smooth sidewalls. It is discovered that surface phonon scattering is particularly high with {100} facets, but less pronounced with {113} facets and remarkably low with {111} facets, which suggests a new means to optimize phonon dynamics for nanoscale thermoelectric devices. This anomaly is reconciled by showing that the contribution of each facet to surface phonons is due to diffuse scattering rather than to backward scattering. It is further shown that this property is not changed by addition of an amorphous shell to the crystalline core, similar to the structure of experimental nanowires.     

n-Type nanostructured thermoelectric materials prepared from chemically synthesized ultrathin Bi2Te3 nanoplates

We herein report on the large-scale synthesis of ultrathin Bi(2)Te(3) nanoplates and subsequent spark plasma sintering to fabricate n-type nanostructured bulk thermoelectric materials. Bi(2)Te(3) nanoplates were synthesized by the reaction between bismuth thiolate and tri-n-octylphosphine telluride in oleylamine. The thickness of the nanoplates was ~1 nm, which corresponds to a single layer in Bi(2)Te(3) crystals. Bi(2)Te(3) nanostructured bulk materials were prepared by sintering of surfactant-removed Bi(2)Te(3) nanoplates using spark plasma sintering. We found that the grain size and density were strongly dependent on the sintering temperature, and we investigated the effect of the sintering temperature on the thermoelectric properties of the Bi(2)Te(3) nanostructured bulk materials. The electrical conductivities increased with an increase in the sintering temperature, owing to the decreased interface density arising from the grain growth and densification. The Seebeck coefficients roughly decreased with an increase in the sintering temperature. Interestingly, the electron concentrations and mobilities strongly depended on the sintering temperature, suggesting the potential barrier scattering at interfaces and the doping effect of defects and organic residues. The thermal conductivities also increased with an increase in the sintering temperature because of grain growth and densification. The maximum thermoelectric figure-of-merit, ZT, is 0.62 at 400 K, which is one of the highest among the reported values of n-type nanostructured materials based on chemically synthesized nanoparticles. This increase in ZT shows the possibility of the preparation of highly efficient thermoelectric materials by chemical synthesis.     

Nanostructures for Thermoelectric Applications: Synthesis,Growth Mechanism,and Property Studies

Both heterostructures and hollow nanostructures have been predicted as candidates with excellent thermoelectric performance. In this Research News areticle, recent advances with regard to synthetic strategies, growth mechanisms, and thermoelectric properties of one‐dimensional heterostructures (segmented and core/shell) and tubular nanostructures are reported. The thermoelectric property studies of Te/Bi core/shell heterostructured nanowires and Bi₂Te₃ nanotubes indicate that the Seebeck coefficient and thermal conductivity of these materials can be optimized to improve their thermoelectric performance. In addition, the current issues and future research directions for promising thermoelectric nanostructures will be discussed on the basis of these experimental results.     

Rational synthesis of ultrathin n-type Bi2Te3 nanowires with enhanced thermoelectric properties

A rational yet scalable solution phase method has been established, for the first time, to obtain n-type Bi(2)Te(3) ultrathin nanowires with an average diameter of 8 nm in high yield (up to 93%). Thermoelectric properties of bulk pellets fabricated by compressing the nanowire powder through spark plasma sintering have been investigated. Compared to the current commercial n-type Bi(2)Te(3)-based bulk materials, our nanowire devices exhibit an enhanced ZT of 0.96 peaked at 380 K due to a significant reduction of thermal conductivity derived from phonon scattering at the nanoscale interfaces in the bulk pellets, which corresponds to a 13% enhancement compared to that of the best n-type commercial Bi(2)Te(2.7)Se(0.3) single crystals (~0.85) and is comparable to the best reported result of n-type Bi(2)Te(2.7)Se(0.3) sample (ZT = 1.04) fabricated by the hot pressing of ball-milled powder. The uniformity and high yield of the nanowires provide a promising route to make significant contributions to the manufacture of nanotechnology-based thermoelectric power generation and solid-state cooling devices with superior performance in a reliable and a reproducible way.     

Thermoelectric Transport Properties of a Quantum Wire,Coupled to a Quantum Dot: Finite Band Effects

We study the thermoelectric transport properties through a quantum wire, modeled on a tight-binding linear chain, with an embedded gate-defined quantum dot. We obtain the thermopower, thermal conductance and electrical conductance with a lateral Fano resonance, linked to a many-body renormalized quantum dot resonant level at the edge of the conduction band strongly hybridized with the van Hove singularity of the one-dimensional density of states of the lead; this resonance appears above the Kondo temperature and is due to a quantum interference thermally activated. We discuss the possibility of practical application of the system to a mesoscopic cooling process and thermopower generators, based on the thermoelectric figure of merit and thermal conductance values. Our results for the thermal transport properties are consistent with those obtained previously for electronic transport.     

制备条件对Bi2Te3和Bi0.5Sb1.5Te3材料热电性能的影响

采用水热法制备Bi2Te3/Sb2Te3纳米粉体并热压制备块体热电材料.用X射线衍射分析产物的相结构和成分,扫描显微镜与透射电镜观察产物的形貌.测量试样从室温到700K的塞贝克系数与电导率.实验结果表明,使用水热法制备了Bi2Te3与Sb2Te3纳米粉体,经热压后部分氧化.热压温度对于块体试样热电性能影响显著.     

Observation of superconductivity in single crystalline Bi nanowires

Bi nanowires have been fabricated by electrochemical deposition into the pores of ion track etched polycarbonate membranes. Transmission electron microscopy and selected area electron diffraction measurements reveal that these Bi nanowires are single crystalline with the rhombohedral lattice structure of bulk Bi at ambient pressure. We have measured the temperature dependence of the resistance and I-V characteristics at various magnetic fields on?these Bi nanowires. These measurements show clear evidence for superconductivity below?0.64?K.     

Thermoelectric and mechanical properties of the Bi0.5Sb1.5Te3 solid solution prepared by melt spinning

This paper reports the preparation and characterization of pressed microcrystalline materials based on a p-type Bi_0.5Sb_1.5Te₃ solid solution produced from a melt-spun powder. We have examined the effect of melt spinning conditions (temperature, disk rotation rate, and purity of the inert gas in the heat treatment chamber) on the particle size and morphology of the powders and the microstructure and thermoelectric properties of hot-pressed samples and investigated the mechanical properties (compression and bend tests) of materials prepared by various methods. The thermoelectric properties of the materials (thermopower, electrical conductivity, and thermal conductivity) were studied at room temperature and in the range 100–700 K. The highest thermoelectric figure of merit ZT of the materials prepared by pressing the melt-spun powder was 1.3, whereas the ZT of the materials prepared by the other methods did not exceed 1.1. The higher ZT of the materials studied was due to their lower lattice thermal conductivity and slightly higher thermopower.     

Morphology controlled synthesis of spherical Bi2S3 flowers

The spherical Bi2S3 flowers have been fabricated by a facile environmentally friendly hydrothermal method. It was found that the flowers are composed of pure orthorhombic phase Bi2S3, the nanorods (nanowires) composed of the flowers grow radically from a center toward all directions to form a spherical structure, and the nanowires are single-crystalline and grow along the [001] direction. The reaction time, reaction temperature and thiourea play key roles for the formation of the flowers. The morphology of the Bi2S3 flowers (e.g., honeycombs, porous nanorods, nanorods, and nanowires) can be controlled simply by controlling the reaction time without varying experimental parameters or addition of other surfactant. The formation mechanism of Bi2S3 flowers is self-assembly and the intrinsic splitting character of the Bi2S3 structure. The spherical Bi2S3 flowers could be found potential applications in optical, catalysts and sensor devices.     

Solvothermal synthesis of Bi2Te3 nanotubes by the interdiffusion of Bi and Te metals

Bi₂Te₃ nanotubes with a one-dimensional structure have been synthesized by a solvothermal method using Te nanowires and Bi metals formed by the liquid phase reduction of Bi³⁺. The alloying between Bi and Te was formed by the diffusion process through an interface of two joined metals. The void formed in the diffusion process induced the formation of binary-nanotubes from the single-component nanowires. The microstructures of the Te nanowires, such as its particle morphology and crystal density, were a function of the temperature and had considerable influence on the alloying into Bi₂Te₃ nanotubes. When the reduction rate from Bi³⁺ to Bi is constant, the alloying between Bi and Te metals was affected by the rate of diffusion of the Te atoms. The aggregation of the Te nanowires and diffusion resistance caused by the cross packed crystal structure of the Te nanowires disrupted migration of Te atoms from the inside of the Te nanowire.     

Anisotropy of the thermoelectric power in superlattices

Anisotropy of the thermoelectric power in superlattices on the scattering of charge carriers by impurity ions is studied. The thermopower anisotropy is found to be significant at the small degree of band filling. It has been shown that the concentration dependence of the transverse component of the thermopower is nonmonotonic.     

Effect of interference between the electron-impurity and electron-phonon interactions upon the thermoelectric properties of alloys

We calculate the thermoelectric power of a metal by considering particularly the particle-hole transformation symmetry. If there is complete symmetry with respect to this transformation, the thermopower is absent. However, the elec- tron-phonon and the electron-impurity interactions are intrinsically such that they change sign under this transformation. Thus a finite thermopower arises due to an interference between different orders in a perturbation expansion in these interactions.     

Bi／Bi2O3晶格复合热电薄膜的制备及其性能研究

利用射频磁控溅射法在玻璃基片上制备Bi／Bi2O3晶格复合热电薄膜,考察了溅射功率对单层Bi薄膜表面粗糙度和热电性能的影响,结果表明,当溅射功率为22W时,薄膜具有最小的表面粗糙度16.3nm,电导率和功率因子分别为2．9×10^4S／m和5．74μV／k^2m,单层Bi薄膜最佳的工作温度为85～105℃。Bi／Bi2O3晶格复合热电薄膜最佳的溅射层数为5层,其电导率和功率因子分别为9．0×10^4S／m和21．0μN／k^2m,分别比单层Bi薄膜提高了2．1倍和2．65倍。