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.     

Hydrogen bubble-assisted growth of Pt3Te4 for electrochemical catalysts

Novel materials and structures with abundant active sites have been in continuous demand for electrochemical catalytic applications. In this study, we synthesized platinum telluride (Pt₃Te₄) nanocrystals on two-dimensional metallic molybdenum ditelluride (MoTe₂) using a dynamic hydrogen bubble template method in the hydrogen evolution reaction (HER). The local crystal structure and chemical state of the Pt₃Te₄ nanocrystals were investigated using X-ray nano-diffraction and X-ray nano-absorption spectroscopy. In our electrochemical deposition method, the morphology, and HER performance of the Pt₃Te₄ nanocrystals could be manipulated through the hydrogen bubble generation rate. Thus, the nanorod-shaped Pt₃Te₄ nanocrystals, fabricated by a high rate of hydrogen bubble generation, exhibited outstanding HER performance, which is in contrast with the HER performance of hemisphere-shaped Pt₃Te₄. Our study provides a facile and systematic way of synthesizing high-performance electrochemical catalysts using the hydrogen bubble-assisted growth method.     

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.     

Improving the thermoelectric properties of thick Sb2Te3 film via Cu doping and annealing deposited by DC magnetron sputtering using a mosaic target

Thick Cu-doped Sb₂Te₃ films were deposited on flexible substrate by DC magnetron sputtering from a mosaic Cu–Sb₂Te₃ target. The Cu-doped Sb₂Te₃ films were vacuum annealed to improve their thermoelectric properties. Density functional theory was used to clarify the internal mechanism of the Cu doped into the Sb₂Te₃ system. The results showed that Cu substitution on a Sb site induced electronic states or impurity peaks of Sb₂Te₃ at a valence band maximum. The carrier concentration of the Cu-doped Sb₂Te₃ films increased as the Cu-doped concentration increased. However, the crystallite size and Seebeck coefficient of the Cu-doped Sb₂Te₃ films decreased as the Cu-doped concentration increased. Post-annealing treatment improved the microstructure and thermoelectric properties of the Cu-doped Sb₂Te₃ films. The maximum electrical conductivity and power factor values of 754.20 S/cm at 50 °C and 1.56 10⁻³ W/mK² at 100 °C were obtained in the annealed film with a Cu-doped concentration of 3 at%.     

Effective phonon scattering and enhancement of thermoelectric performance in Ga-excess Bi0.4Sb1.6Te3 compounds

We investigate the thermoelectric properties on Ga-excess p-type Ga_xBi_0.4Sb_1.6Te₃ compounds. Even though the random distribution of Ga-doping increases electrical resistivity giving rise to the decrease of power factor, the significant decrease of lattice thermal conductivity by the excess Ga-doping induces significant enhancement of ZT value (1.13 at 350 K) for the Ga x = 0.03 doped compound. From the X-ray diffraction and elemental mapping by energy dispersive X-ray spectroscopy, we observed Sb and Ga phase separation leading to the phonon scattering. The Sb precipitation implies atomic defect in the matrix which can induce short wavelength phonon scattering. The synergetic phonon scatterings from various scattering sources such as point defect, alloy scattering, and grain boundary phonon scattering have an important role in the enhancement of thermoelectric performance.     

Thermoelectric properties of unoxidized graphene/Bi2Te2.7Se0.3 composites synthesized by exfoliation/re‐assembly method

Nanocomposites of n‐type thermoelectric Bi₂Te_2.7Se_0.3 (BTS) and unoxidized graphene (UG) were prepared from the exfoliated BTS and UG nanoplatelets. Polycrystalline BTS ingots were exfoliated into nanoscoll‐type crystals by chemical exfoliation, and were re‐assembled with UG nanoplatelets. The composites were chemically reduced by hydrazine hydrate and sintered by a spark‐plasma‐sintering method. The thermoelectric properties of the sintered composites were evaluated and exhibited decreased carrier concentration and increased thermal conductivity due to the embedded graphene. The peak ZT values for the UG/BTS‐US and UG/BTS‐EX composites were ～0.8 at the UG concentration of 0.05 wt%. (© 2014 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.     

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.     

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.     

Electronic properties of glassy As2Te3

Bulk samples of glassy As₂Te₃ and its alloys with Ge, Si or Tl were prepared by quenching of the melt and their electrical, photoelectrical, optical and thermodynamical properties were studied. The addition of Ge, Si or Tl produces three different types of glass from the thermodynamical point of view; the physical properties studied differ only quantitatively.Shortened version of this work was presented at the 4th International Conference on Amorphous and Liquid Semiconductors, Ann Arbor USA, August 1971.Cukrovarnická 10, Praha 6, Czechoslovakia.The authors wish to express their thanks to Mrs. J. Beránková for preparation of the As₂Te₃ glasses, to Mrs. M. ilhavá for her assistance with photoconductivity measurements and to Mr. S. Hotmar for his help with the conductivity measurements.     

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.     

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.     

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     

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.     

Infrared and Raman study of Si2Te3

Results of ir and Raman investigations on trigonal layer-structured Si₂Te₃ single crystals are reported. The ir reflection spectrum withEc exhibits seven reststrahl-like bands, whereas the corresponding spectrum withEc shows only one very small bump. Values for ₀ and are given. The Raman spectra are very rich in structure and can only be interpreted qualitatively as being a mixture of single phonon lines and one- and two-phonon density-of-states contributions. One-phonon density-of-states effects are disorder-induced owing to the statistical occurrence of the Si atoms within the regular hexagonal Te sublattice.     

A tunneling spectroscopy study of the temperature dependence of the forbidden band in Bi2Te3 and Sb2Te3

Tunneling measurements of dI/dV, d ² I/dV ², and d ³ I/dV ³ were formed along the C ₃ axis (normally to layers) for Bi₂Te₃ and Sb₂Te₃ layered semiconductors in the temperature range 4.2<T>29 5 K. Temperature dependences of the forbidden band energy E _g were obtained. The forbidden band energy in Bi₂Te₃ was 0.20 eV at room temperature and increased to 0.24 eV at T=4.2 K. The E _g value for Sb₂Te₃ was 0.25 eV at 295 K and 0.26 eV at 4.2 K. The distance between the top of the higher valence band of light holes and the top of the valence band of heavy holes situated lower was found to be ΔE _V≈19 meV in Bi₂Te₃; this distance was independent of temperature. The conduction bands of Bi₂Te₃ and Sb₂Te₃ each contain two extrema with distances between them of ΔE _c≈25 and 30 meV, respectively.     

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)