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)     

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.     

Scanning tunneling microscope light emission spectra of polycrystalline Ge2Sb2Te5 and Sb2Te3

We have observed scanning tunneling microscope light emission (STM-LE) spectra of Ge₂Sb₂Te₅ and Sb₂Te₃. Although these chalcogenide alloys exhibit band gaps less than 0.5 eV, the STM-LE was observed with a narrow spectral width at a photon energy of 1.5 eV for both materials. By analyzing its bias voltage, polarity, and temperature dependencies combined with recently reported theoretical electronic structures, we concluded that the STM-LE is excited by electronic transitions taking place in the local electronic structure having a direct gap-like shape with a band gap of 1.5 eV, commonly found in the electronic structures of both materials.     

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.     

Point-contact tunneling spectroscopy between a Nb tip and an ideal topological insulator Sn-doped Bi1.1Sb0.9Te2S

We find that the quantum-classical correspondence in integrable systems is characterized by two time scales. One is the Ehrenfest time below which the system is classical; the other is the quantum revival time beyond which the system is fully quantum. In between, the quantum system can be well approximated by classical ensemble distribution in phase space. These results can be summarized in a diagram which we call Ehrenfest diagram. We derive an analytical expression for Ehrenfest time, which is proportional to h~(-1/2). According to our formula, the Ehrenfest time for the solar-earth system is about 10~(26) times of the age of the solar system. We also find an analytical expression for the quantum revival time, which is proportional to h~(-1). Both time scales involve ω(I), the classical frequency as a function of classical action. Our results are numerically illustrated with two simple integrable models. In addition, we show that similar results exist for Bose gases, where 1/N serves as an effective Planck constant.     

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.     

Structure and electrical properties of Sb2Te3 and Bi0.4Sb1.6Te3 metastable phases obtained by HPHT treatment

We have obtained the metastable phase of the thermoelectric alloy Bi_0.4Sb_1.6Te₃ with electron type conductivity for the first time using the method of quenching under pressure after treatment at P=4.0 GPa and T=400–850 °C. We have consequently performed comparative studies with the similar phase of Sb₂Te₃. The polycrystalline X-ray diffraction patterns of these phases are similar to the known monoclinic structure α-As₂Te₃ (C2/m) with less monoclinic distortion, β ≈ 92°. We have measured the electrical resistivity and the Hall coefficient in the temperature range of T=77?450 K and we have evaluated the Hall mobility and density of charge carriers. The negative Hall coefficient indicates the dominant electron type of carriers at temperatures up to 380 K in the metastable phase of Sb₂Te₃ and up to 440 K in the metastable state of Bi_0.4Sb_1.6Te₃. Above these temperatures, the p-type conductivity proper to the initial phases dominates.     

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.     

Infrared spectroscopy of Bi2Te2Se

The infrared reflection and transmission spectra of Bi₂Te₂Se single crystals grown by the modified Bridgeman method have been studied in the spectral range of 30–10000 cm^?1 at temperatures of 5–300 K. The bandgap and its temperature dependence, optical function spectra, and concentration of free charge carriers in Bi₂Te₂Se have been determined.     

Microstructure of thermoelectric (Bi0.15Sb0.85)2Te3 film

The film of thermoelectric ternary p-type (Bi_0.15Sb_0.85)₂Te₃ was deposited on polyimide foil substrate at 168 °C using direct-current magnetron sputtering. Microstructural investigations of the film were performed by electron microscopy techniques. SEM observations showed that the film surface consisted of large-sized particulates with small-sized particles and also mound-like crystal agglomerates in some areas. Chemical composition of the film was analyzed using energy-dispersive X-ray spectrometer (EDS). It has been observed that the EDS results were in an agreement with nominal composition for the film. Detailed microstructural investigations were carried out using transmission electron microscopy (TEM). TEM images and selected area electron diffraction patterns showed that the film has randomly oriented polycrystalline grain structure. High-resolution TEM images indicated that the microstructure of film also contained nano-crystal structure, smaller than 10 nm.     

Plasma screening of the fundamental absorption edge in crystals of Bi2Te3-Sb2Te3 solid solutions with more than 80 mol % Sb2Te3

The regularities of changes in the optical properties of crystals of Bi₂Te₃-Sb₂Te₃ solid solutions in the range of the effects caused by free-carrier plasma oscillations at a variation in the ratio of Bi₂Te₃ and Sb₂Te₃ components are investigated. It is established that, when the Sb₂Te₃ content in a solid solution exceeds 80 mol %, the fundamental absorption edge undergoes plasma screening. Doping a Sb₂Te₃ crystal with tin increases the free-carrier concentration and plasma frequencies, thus enhancing the screening effect, whereas introduction of 0.1 mol % selenium reduced the plasma frequency and thus removes the fundamental-absorption-edge screening.     

Electric quadrupole interaction of111Cd in the system Sb1−xMx (M=Ag,Cd,In,Sn), Sb2Te3, Bi2Te3 and In2Te3

We have performed TDPAC-measurements to investigate the static quadrupole interaction of₁₁₁Cd in the classic semimetal Sb. The coupling constant depends on the concentration of small amounts of metal admixtures. The only exception is the system Sb_1–xAg_x. The temperature dependence of the efg in the narrow gap semiconductors Sb₂Te₃ and Bi₂Te₃ is similar to that one in Te. In contrast to these results the efg in the III–VI-semiconductor In₂Te₃ is temperature independent.     

Thickness dependence of the forbidden energy gap in stibnite (Sb2S3) thin films

The increase in the energy gap with decreasing film thickness has been observed and studied in case of thin films of stibnite (Sb₂S₃). Such an increase in energy gap, which is inversely proportional to the square of the film thickness, is attributed to quantum effects. The effective mass of electrons has been estimated to be ((0.5±1.0) m₀ from this study.     

Characterization of Mn-doped Sb1.5Bi0.5Te3 single crystals

Abstract

Single crystals of (Sb_0.75Bi_0.25)_2-xMn_xTe₃ (x = 0.0–0.05) were characterized by X-ray diffraction, measurements of reflectance in the plasma resonance frequency region, Hall coefficient, electrical conductivity, and Seebeck coefficient. It was found that Mn atoms in the crystal structure of Sb_1.5Bi_0.5Te₃ behave like acceptors; the increase in the hole concentration is explained by the formation of substitutional defects of Mn'_Sb and Mn'_Bi in the crystal lattice of the studied crystals.     

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.     

Sb2Te3 纳米结构的制备与表征

以室温热电性能优异的传统热电材料Sb₂Te₃为研究对象,利用化学气相沉积法制备Sb₂Te₃单晶纳米结构,并研究其生长机理.实验结果表明,不加催化剂时Sb₂Te₃易生长成六方纳米盘,在金催化剂条件下定向生长成纳米线.Sb₂Te₃的形貌与其晶体结构和生长机理有关.Sb₂Te₃为三角结构,Sb和     

Growth behavior of Bi2Te3 and Sb2Te3 thin films on graphene substrate grown by plasma‐enhanced chemical vapor deposition

A comparative study of the substrate effect on the growth mechanism of chalcogenide Bi₂Te₃ and Sb₂Te₃ thin films was carried out. Obvious microstructural discrepancy in both the as‐deposited Bi₂Te₃ and Sb₂Te₃ thin films was observed when grown on graphene or SiO₂/Si substrate. Bi₂Te₃ and Sb₂Te₃ thin films deposited on the graphene substrate were observed to be grown epitaxially along c‐axis and show very smooth surface compared to that on SiO₂/Si substrate. Based on the experimental results of this study, the initial adsorption sites on graphene substrate during deposition process, which had been discussed theoretically, could be demonstrated empirically.