Thermoelectric properties of Bi2Se3/Bi2Te3/Bi2Se3 and Sb2Te3/Bi2Te3/Sb2Te3 quantum well systems

In this work, we develop a theory of thermoelectric transport properties in two-dimensional semiconducting quantum well structures. Calculations are performed for n-type 0.1 wt.% CuBr-doped Bi₂Se₃/Bi₂Te₃/Bi₂Se₃ and p-type 3 wt.% Te-doped Sb₂Te₃/Bi₂Te₃/Sb₂Te₃ quantum well systems in the temperature range 50–600 K. It is found that reducing the well thickness has a pronounced effect on enhancing the thermoelectric figure of merit (ZT). For the n-type Bi₂Se₃/Bi₂Te₃/Bi₂Se₃ with 7 nm well width, the maximum value of ZT is estimated to be 0.97 at 350 K and for the p-type Sb₂Te₃/Bi₂Te₃/Sb₂Te₃ with well width 10 nm the highest value of the ZT is found to be 1.945 at 440 K. An explanation is provided for the resulting higher ZT value of the p-type system compared to the n-type system.     

Density functional study of Bi2Te3 and Bi4Te6 molecules

Results of a density functional study for the molecules Bi₂Te₃ and Bi₄Te₆ are reported here. For Bi₂Te₃, calculations yield eight stable conformations. For Bi₄Te₆, eight stable isomers are identified. Equilibrium geometries, adiabatic ionisation potentials, atomisation energies, and vibrational bands are estimated. The lowest energy conformations in both cases are clusters of C_s symmetry with all Te atoms two-fold coordinated and all Bi atoms three-fold coordinated. The predicted low energy conformation for Bi₄Te₆ has alternating rows of Bi and Te atoms. This molecule seems a reasonable precursor to solid bismuth telluride, which has alternating Bi and Te layers.     

Interfacing 2D and 3D topological insulators: Bi(111) bilayer on Bi2Te3

We report the formation of a bilayer Bi(111) ultrathin film, which is theoretically predicted to be in a two-dimensional quantum spin Hall state, on a Bi(2)Te(3) substrate. From angle-resolved photoemission spectroscopy measurements and ab initio calculations, the electronic structure of the system can be understood as an overlap of the band dispersions of bilayer Bi and Bi(2)Te(3). Our results show that the Dirac cone is actually robust against nonmagnetic perturbations and imply a unique situation where the topologically protected one- and two-dimensional edge states are coexisting at the surface.     

Heterodiffusion of selenium in Bi2Te3

We present results for the anisotropic diffusion coefficient of selenium, isoelectronic impurity, in Bi₂Te₃ along the solidus. The experimental conditions permitted us to obtain a definite stoichiometric deviation, and diffusion profiles were obtained by SIMS.     

Temperature dependence of the Raman spectra of Bi2Te3 and Bi0.5Sb1.5Te3 thermoelectric films

The temperature dependence of the Raman spectra of Bi₂Te₃ and Bi_0.5Sb_1.5Te₃ thermoelectric films was investigated. The temperature coefficients of the E_g(2) peak positions were determined as –0.0137 cm^–1/°C and –0.0156 cm^–1/°C, respectively. The thermal expansion of the crystal caused a linear shift of the Raman peak induced by the temperature change. Based on the linear relation, a reliable and noninvasive micro‐Raman scattering method was shown to measure the thermal conductivity of the thermoelectric films. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

High pressure transport characteristics of Bi2Te3, Sb2Te3, and BiSbTe3

This paper presents ambient and high pressure measurements of transport properties of the Bi₂Te₃–Sb₂Te₃ series of materials. The electrical resistivity, thermal conductivity, and Seebeck coefficient have been measured on both end compounds and the direct solid solution of the two at pressure up to 10 GPa. An additional discussion involving the high pressure structure will be presented. From this, it was determined that these materials undergo at least two structural phase transitions between 0 and 20 GPa and a discussion is presented regarding this and the changes in the transport properties.     

Electron density redistribution in Sn-doped Bi2Te3

X-ray photoelectron spectroscopy is used to investigate the redistribution of the density of electronic states in the valence band, and of the binding energies and chemical shifts of core levels in bismuth telluride caused by introduction of impurity tin atoms. A substantial increase in the density of electronic states below the valence-band top at energies μ≈15–30 meV has been revealed. This feature in the energy spectrum accounts for the unusual behavior of the kinetic coefficients in p-Bi₂Te₃:Sn crystals. Fiz. Tverd. Tela (St. Petersburg) 41, 1969–1972 (November 1999)     

Energy formation of antisite defects in doped Sb2Te3 and Bi2Te3 crystals

The free carrier concentration of the Sb_2−xIn_xTe₃, Bi_2−xIn_xTe₃ and Bi₂Te_3−xS_x crystals has been determined from the values of the Hall constants and the free carrier concentration of the Sb_2−xTl_xTe₃ has been calculated from the plasma resonance frequency; with increasing value of x, the hole concentration decreases. As the incorporation of the elements In, Tl and S into the lattice Sb₂Te₃ or Bi₂Te₃, respectively, gives rise to the uncharged defects In^x_Sb, Tl^x_Sb, In^x_Bi and S^x_Te, the x causes the decrease of the antisite defects concentration. The proven effect is explained in the following way: the antisite defects can be created only in crystals whose atoms are bound by weakly polarized bonds. The incorporation of In, Tl and S atoms into the crystal lattice of Sb₂Te₃ or Bi₂Te₃ increases the bond polarity, the ionicity of ternary crystals increases. This unfavorably affects the increase of antisite defects whose concentration decreases. The change of the bond polarity is considered from the changes discovered in the formation energy of antisite defects of the above mentioned ternary crystals.     

Sensitive elements of pressure transducers made of layered intercalated InSe, GaSe, and Bi2Te3 crystals

It is shown that layered InSe, GaSe, and Bi₂Te₃ semiconductors are promising for sensitive elements of pressure transducers. Two ways for measuring pressure with layered crystals are suggested: from the pressure dependence of the intercalation parameter (current) and from the pressure dependence of the intercalate electromotive force.     

Structural defects in Cu-doped Bi2Te3 single crystals

The relation between the concentration of free charge carriers and the concentration of copper atoms in Bi₂Te₃ single crystals doped with copper over a wide range of concentrations has been investigated, with the aim of clarifying the existence of inactive Cu ions. Changes in the concentration of free charge carriers arising from Cu-doping of the melt with that induced by electrochemical intercalation of copper are compared. Models of possible defect structures are proposed for both doped and intercalated single crystals of Bi₂Te₃.     

Specific features of Bi2Te3 doping with Sn

The effect of doping bismuth telluride with tin, on its electrophysical properties, has been studied. It is shown that the main features in the transport coefficients of Bi₂Te₃:Sn can be explained by the existence of resonant Sn states within the valence band. The existence of resonant Sn states was confirmed by codoping Bi₂Te₃:Sn with the electroactive impurity I. Fiz. Tverd. Tela (St. Petersburg) 40, 1428–1432 (August 1998)     

Influence of grain size distribution on the lattice thermal conductivity of Bi2Te3-Sb2Te3-based nanostructured materials

The change of the lattice thermal conductivity of bulk nanostructured materials based on Bi₂Te₃-Sb₂Te₃ solid solutions with grain size distribution has been studied. These materials have a polycrystalline structure with grain sizes ranging from a few tens of a nanometer to a few micrometers. Large grains may contain inclusions or consist of several smaller parts which can be identified with coherent scattering regions seen in X-ray diffraction. The change of the lattice thermal conductivity mediated by additional scattering by inclusions and grain boundaries has been calculated. This calculation allows for the effect of nanoparticle size distribution. The calculated estimates are compared with the available experimental data.     

Effect of nanocrystallinity on lattice dynamics in Bi2Te3 based thermoelectrics

The lattice dynamics in as‐cast and nanocrystalline thermoelectric Bi₂Te₃ based p‐type and n‐type material were investigated using inelastic neutron scattering. Generalized densities of phonon states show substantial agreement between the lattice dynamics in as‐cast samples and previous studies. The lattice dynamics in the nanocrystalline materials differ significantly from its as‐cast counterparts in the acoustic phonon regime. In nanocrystalline p‐type and n‐type compounds, the average acoustic phonon group velocity was found to be reduced to 80(5)% and 95(2)% of the value in as‐cast material. It is argued that point‐defect and strain contrast scattering may play an important role for the understanding of lattice thermal conductivity in (nanocrystalline) Bi₂Te₃ based thermoelectrics beside the observed decrease of sound velocity. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Bi2Te3 基花状纳米片粉体的制备及热电性能研究

本文通过选用不同乙二胺四乙酸(EDTA) 的用量采用水热法制备出了具有花状纳米片形貌的碲化铋(Bi2Te3 ) 纳米粉体, 研究了不同 EDTA 的用量对 Bi2Te3 纳米粉体花状纳米片形貌的影响, 结果表明 EDTA 的用量对粉体的成相和形貌有很大的影响. EDTA 用量为0.2 g 是最佳值, 有助于形成纯相的 Bi2Te3 花状纳米片. 并以 Y元素掺杂为例, 研究了 EDTA 用量对元素掺杂 Bi2Te3 纳米粉体花状形貌的影响, 研究发现元素掺杂后对 Bi2Te3粉体的微观形貌有一定的影响. 为了保持元素掺杂 Bi2Te3 纳米粉体的花状形貌,EDTA 合适的用量至关重要. 以Y0 .2Bi1 .8Te3 为例, 研究了 EDTA 用量对样品的热电性能的影响. 结果表明, 花状纳米片形貌有利于在降低样品电阻率的同时有效降低样品的热导率, 从而大大增加了样品的ZT 值.     

Structural phase transition in Bi2Te3 under high pressure

Structural change in Bi₂Te₃ under high pressure up to 16.6 GPa has been studied by powder x-ray diffraction. We observed two times of phase transitions at room temperature at the pressures of 8 and 14 GPa, respectively. According to our preliminary result on electrical resistance, it is reasonable to suppose that superconducting transition with T _c =2.8 K at the pressures of 10.2 GPa is observed in phase II. On the other hand, we found anomalies of the pressure dependences of lattice parameters and volume at around 2 GPa, which probably means the change in electrical structure on the Fermi surface.     

Stiffness of reinforced and unreinforced Bi2Te3 single crystals

We investigate the thermodynamics of Ω- and V-shaped bending of Bi₂Te₃ single crystals. To change the shape of the bending from Ω to V, the sample is reinforced with thin steel rods, arranged perpendicular to the cleavage planes. We find that reinforcement increases the ultimate strength of the samples by more than twofold. We show that the transition Ω→V leads to a substantial change in the spectrum of energy dissipation in the straining of the sample. Some aspects of the practical application of the observed effects are considered. Zh. Tekh. Fiz. 67, 50–54 (February 1997)