(Ag2Se)1-x(Bi2Se3)x的热电性能研究

具有本征低晶格热导率的I-V-VI₂族三元硫属化合物在热电领域引起了广泛关注。AgBiSe₂作为这类化合物中少有的n型半导体, 成为一种有潜力的热电材料。本工作系统研究了AgBiSe₂的热电性能。基于Ag₂Se-Bi₂Se二元相图, 单相的(Ag₂Se)_1-x(Bi₂Se₃)_x的成分在x=0.4~0.62范围可调, 使得该材料载流子浓度具有可调性。结果表明, 通过组分调控获得了较宽范围的载体浓度1.0×10¹⁹~5.7×10¹⁹ cm^-3, 并基于声学声子散射的单一抛物带模型对其电传输性能进行了综合评估。本研究获得的最高载流子浓度接近理论最优值, 在700 K实现了最高ZT值0.5。本研究有助于深入理解AgBiSe₂的传输特性和决定热电性能的基本物理参数。     

Enhanced thermoelectric properties of solution grown Bi2Te(3-x)Se(x) nanoplatelet composites

We report on the enhanced thermoelectric properties of selenium (Se) doped bismuth telluride (Bi(2)Te(3-x)Se(x)) nanoplatelet (NP) composites synthesized by the polyol method. Variation of the Se composition within NPs is demonstrated by X-ray diffraction and Raman spectroscopy. While the calculated lattice parameters closely follow the Vegard's law, a discontinuity in the shifting of the high frequency (E(g)(2) and A(1g)(2)) phonon modes illustrates a two mode behavior for Bi(2)Te(3-x)Se(x) NPs. The electrical resistivity (ρ) of spark plasma sintered pellet composites shows metallic conduction for pure Bi(2)Te(3) NP composites and semiconducting behavior for intermediate Se compositions. The thermal conductivity (κ) for all NP composites is much smaller than the bulk values and is dominated by microstructural grain boundary scattering. With temperature dependent electrical and thermal transport measurements, we show that both the thermoelectric power S (-259 μV/K) and the figure of merit ZT (0.54) are enhanced by nearly a factor of 4 for SPS pellets of Bi(2)Te(2.7)Se(0.3) in comparison to Bi(2)Te(3) NP composites. Tentatively, such an enhancement of the thermoelectric performance in nanoplatelet composites is attributed to the energy filtering of low energy electrons by abundant grain boundaries in aligned nanocomposites.     

Ambipolar field effect in the ternary topological insulator (Bi(x)Sb(1-x))2Te3 by composition tuning

Topological insulators exhibit a bulk energy gap and spin-polarized surface states that lead to unique electronic properties, with potential applications in spintronics and quantum information processing. However, transport measurements have typically been dominated by residual bulk charge carriers originating from crystal defects or environmental doping, and these mask the contribution of surface carriers to charge transport in these materials. Controlling bulk carriers in current topological insulator materials, such as the binary sesquichalcogenides Bi2Te3, Sb2Te3 and Bi2Se3, has been explored extensively by means of material doping and electrical gating, but limited progress has been made to achieve nanostructures with low bulk conductivity for electronic device applications. Here we demonstrate that the ternary sesquichalcogenide (Bi(x)Sb(1-x))2Te3 is a tunable topological insulator system. By tuning the ratio of bismuth to antimony, we are able to reduce the bulk carrier density by over two orders of magnitude, while maintaining the topological insulator properties. As a result, we observe a clear ambipolar gating effect in (Bi(x)Sb(1-x))2Te3 nanoplate field-effect transistor devices, similar to that observed in graphene field-effect transistor devices. The manipulation of carrier type and density in topological insulator nanostructures demonstrated here paves the way for the implementation of topological insulators in nanoelectronics and spintronics.     

Mechanical properties of Bi(x)Sb(2-x)Te3 nanostructured thermoelectric material

Research on thermoelectric (TE) materials has been focused on their transport properties in order to maximize their overall performance. Mechanical properties, which are crucial for system reliability, are often overlooked. The recent development of a new class of high-performance, low-dimension thermoelectric materials calls for a better understanding of their mechanical behavior to achieve the desired system reliability. In the present study we investigate the mechanical behavior of nanostructure bulk TE material p-type Bi(x)Sb(2-x)Te(3) by means of nanoindentation and 3D finite element analysis. The Young's modulus of the material was estimated by the Oliver-Pharr (OP) method and by means of numerically assisted nanoindentation analysis yielding comparable values about 40 GPa. Enhanced hardness and yield strength can be predicted for this nanostructured material. Microstructure is studied and correlation with mechanical properties is discussed.     

Crystal growth and thermoelectric properties of tetradymite-like layered chalcogenides and (Bi2Te3)1−x−y (Sb2Te3) x (Sb2Se3) y solid solutions

Thermoelectric materials for segmented n-and p-legs of thermoelectric generators have been prepared by Czochralski growth with melt supply through a floating crucible. Two-segment ingots have been obtained using melt compositions corresponding to ternary layered compounds in the PbTe-Bi₂Te₃ and PbTe-Sb₂Te₃ systems, with (Bi₂Te₃)_1?x?y (Sb₂Te₃) _x (Sb₂Se₃) _y solid solutions as seed materials. Seeded growth of the ternary compounds makes it possible to fabricate legs without joining segments by soldering. Using scanning hot point microprobe measurements, we have studied the thermoelectric power distribution across the seed-crystal interface. The results attest to a steep thermoelectric power gradient across the seed-crystal interface in a narrow region. Quantitative analysis of the distribution of the number of measurements with respect to thermoelectric power has revealed peaks corresponding to individual segments.     

Physical properties of Bi2 (Te,Se)3 and Bi2Se1.2Te1.8 prepared using solid-state microwave synthesis

Bi₂(Te, Se)₃ and Bi₂Se_1.2Te_1.8 bulk products were synthesised using standard solid-state microwave synthesis. The Bi₂(Te, Se)₃ and Bi₂Se_1.2Te_1.8 were then deposited thermally onto glass substrates at a pressure of 10^? 6 Torr. The structure of the samples was analysed using X-ray diffraction (XRD), and the powders and thin films were observed to be polycrystalline and rhombohedral in structure. The surface morphology of the samples was determined using scanning electron microscopy (SEM). From the measurements of optical properties, the energy gap values for the Bi₂Te₃, Bi₂Se₃, and Bi₂Se_1.2Te_1.8 thin films were 0.43, 0.73, and 0.65 eV, respectively.     

Structural and Transport Properties of Sn-Doped Bi2Te3 – x Se x Single Crystals

The transport properties of Bi_{2 – y} Sn _y Te_{3 – x} Se _x solid solutions are studied. The results demonstrate that doping with Sn has a strong effect on the temperature dependences of the thermoelectric power and electrical conductivity of the crystals. This suggests that the valence band of the crystals contains Sn-related resonance states. The point defects and dislocation system in Bi₂Te₃ and Bi_{2 – y} Sn _y Te_{3 – x} Se _x solid solutions are studied by transmission electron microscopy. It is shown that the predominant defects in the crystals studied, grown by the Czochralski technique, are dislocations lying in the (0001) plane. The estimated dislocation density is 10⁸ to 10⁹ cm^–2, and the primary slip plane is (0001). Electron-microscopic examination indicates the presence of stacking faults and very small dislocation loops in both Bi₂Te₃ and Bi_{2 – y} Sn _y Te_{3 – x} Se _x single crystals. Since all of the crystals are highly degenerate semiconductors, it is reasonable to assume that structural defects have an insignificant effect on their electrical properties.     

Cu-intercalated (Bi0.5Sb1.5Te3)0.96(Bi2Se3)0.04 single crystals

A procedure is described for converting the conductivity type (p → n) of single crystals of the Bi_0.5Sb_1.5Te₃-4 mol % Bi₂Se₃ solid solution via copper intercalation. Using this procedure, we have produced high-performance single-crystal n-legs with a room-temperature thermoelectric power α = ?200 μV/K. This procedure facilitates the fabrication of thermoelectric coolers because the same solid solution can be used to produce p-and n-legs, the only difference being the presence of copper in the n-legs. We have fabricated thermolements and determined their characteristics in the range 100–300 K.     

Te-seeded growth of few-quintuple layer Bi2Te3 nanoplates

We report on a Te-seeded epitaxial growth of ultrathin Bi2Te3 nanoplates （down to three quintuple layers （QL）） with large planar sizes （up to tens of micrometers） through vapor transport. Optical contrast has been systematically investigated for the as-grown Bi2Te3 nanoplates on the SiO2/Si substrates, experimentally and computationally. The high and distinct optical contrast provides a fast and convenient method for the thickness determination of few-QL Bi2Te3 nanoplates. By aberration-corrected scanning transmission electron microscopy, a hexagonal crystalline structure has been identified for the Te seeds, which form naturally during the growth process and initiate an epitaxial growth of the rhombohedral- structured Bi2Te3 nanoplates. The epitaxial relationship between Te and Bi2T% is identified to be perfect along both in-plane and out-of-plane directions of the layered nanoplate. Similar growth mechanism might be expected for other bismuth chalcogenide layered materials.     

Structural and electrical properties of rapidly quenched Bi2-x Sb x Te3 foils

Liquid-quenched Bi_2-xSb_xTe₃ foils are found to have a pronounced (0225) texture. At high antimony concentrations, (1120) texturing also becomes significant. The compositions at which the magnitude of thermoelectric power is maximal are established. The electrical transport in the foils is shown to be dominated by holes at largex and electrons at smallx.     

Gate-controlled surface conduction in Na-doped Bi2Te3 topological insulator nanoplates

Exploring exciting and exotic physics, scientists are pursuing practical device applications for topological insulators. The Dirac-like surface states in topological insulators are protected by the time-reversal symmetry, which naturally forbids backscattering events during the carrier transport process, and therefore offers promising applications in dissipationless spintronic devices. Although considerable efforts have been devoted to controlling their surface conduction, limited work has been focused on tuning surface states and bulk carriers in Bi(2)Te(3) nanostructures by external field. Here we report gate-tunable surface conduction in Na-doped Bi(2)Te(3) topological insulator nanoplates. Significantly, by applying external gate voltages, such topological insulators can be tuned from p-type to n-type. Our results render a promise in finding novel topological insulators with enhanced surface states.     

Study of the strain-resistance properties of massive samples of Bi2Te3-Sb2Te3 and Bi2Te3-Bi2Se3

The coefficients of strain sensitivity of polycrystalline samples of ternary alloys based on bismuth and antimony chalcogenides were measured and the strain sensitivity of Peltier thermocouples of low height under real working conditions were evaluated.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 56, No. 1, pp. 97–99, January, 1989.     

Composition and thermoelectric power factor variation of (Bi2Te3)0.96(Bi2Se3)0.04 crystal in growth direction

(Bi₂Te₃)_0.96(Bi₂Se₃)_0.04 crystal, which is an n-type thermoelectric semiconductor, has many applications in thermoelectric cooling systems. Single crystal of this composition was grown by Traveling Heater Method. A sensible gradient in thermoelectric power factor was observed in the first quarter length of the prepared crystalline ingot. Characterizing the crystallization procedure and ingot composition, the gradient was attributed to the variation of the Bi₂Se₃ concentration of Bi₂Te₃–Bi₂Se₃ quasi-binary solid solution system. The structural properties were characterized by means of XRD analyses. Results of composition variation (Bi₂Se₃ distribution function) were in good correlation with experimental thermoelectric power factor measured along the grown rod.     

Thermoelectric properties of fine grained (75% Sb2Te3-25% Bi2Te3)p-type and (90% Bi2Te3-5% Sb2Te3-5% Sb2Se3)n-type alloys

The thermoelectric properties of fine-grained alloys prepared by either cold pressing and sintering or hot pressing in the range 5–50 m are compared with single-crystal best-direction values. It is shown that for thep-type alloy, almost the entire thermoelectric properties are recovered, i.e. the figure of merit for the finest grain size is almost the same as the best single-crystal value. The same trend is observed in then-type alloy except that 90% of the single-crystal figure of merit is recovered. These results are discussed in terms of a model which suggests that degradation of favourable thermoelectric properties by powdering the alloys is compensated by (1) decrease of thermal conductivity due to scattering of phonons at grain boundaries for grain sizes that are comparable to the mean free path of phonons; and (2) retention of some of the anisotropic properties of the single crystal in the fine-grained compacts.     

Effects of magnetic doping on weak antilocalization in narrow bi(2)se(3) nanoribbons

We report low-temperature, magnetotransport measurements of ferrocene-doped Bi(2)Se(3) nanoribbons grown by vapor-liquid-solid method. The Kondo effect, a saturating resistance upturn at low temperatures, is observed in these ribbons to indicate presence of localized impurity spins. Magnetoconductances of the ferrocene-doped ribbons display both weak localization and weak antilocalization, which is in contrast with those of undoped ribbons that show only weak antilocalization. We show that the observed magnetoconductances are governed by a one-dimensional localization theory that includes spin orbit coupling and magnetic impurity scattering, yielding various scattering and dephasing lengths for Bi(2)Se(3). The power law decay of the dephasing length on temperature also reflects one-dimensional localization regime in these narrow Bi(2)Se(3) nanoribbons. The emergence of weak localization in ferrocene-doped Bi(2)Se(3) nanoribbons presents ferrocene as an effective magnetic dopant source.     

Tem analyses of the local crystal and domain structures in (Na(1-x)K(x))0.5Bi(0.5)TiO3 perovskite ceramics

The local crystal and domain structures of the ((Na(1-x)K(x))(0.5)Bi(0.5)TiO(3) (NBT-KBT) solid solutions were studied because of their influence on the enhanced electromechanical properties of ceramics. Based on X-ray diffraction, the morphotropic phase boundary (MPB) was determined for the composition x = 0.20, in which the rhombohedral and the tetragonal structures were observed to coexist. However, detailed domain-structure analyses using transmission electron microscopy (TEM), performed on the NBT, KBT, and five NBT-KBT solid-solution compositions, revealed some structural changes at/near the MPB. In the samples on the tetragonal side of the MPB, the grains showed a lamellar domain structure with 90° orientations of the individual domains, separated by straight domain boundaries, i.e., (011)/(101) twin planes. The rhombohedral samples on the other side of the MPB showed a typical square-net pattern with needle-like or lamellar ~71°/109° rhombohedral domains with (001) and/or (110) twin planes separating the individual domains. The domain structure at the MPB showed well-defined lamellar domains. Based on the occurrence of the superstructure reflections in the SAED patterns of various crystallographic zones, on the characteristic splitting of the reflections, and on the domain morphology observations, the crystal structure in/near the boundary region was determined to be a tetragonal structure with an in-phase oxygen octahedral tilt system (probably a(0)a(0)c(+)). It is suggested that the tetragonal polar order is partly induced from the rhombohedral structure at the MPB as a result of mechanical loading during TEM sample preparation.     

Constructing anisotropic single-Dirac-cones in Bi(1-x)Sb(x) thin films

The electronic band structures of Bi(1-x)Sb(x) thin films can be varied as a function of temperature, pressure, stoichiometry, film thickness, and growth orientation. We here show how different anisotropic single-Dirac-cones can be constructed in a Bi(1-x)Sb(x) thin film for different applications or research purposes. For predicting anisotropic single-Dirac-cones, we have developed an iterative-two-dimensional-two-band model to get a consistent inverse-effective-mass-tensor and band gap, which can be used in a general two-dimensional system that has a nonparabolic dispersion relation as in the Bi(1-x)Sb(x) thin film system.