Solvothermal preparation and characterization of nanocrystalline Bi2Te3 powder with different morphology

Bi₂Te₃-based alloys are currently best-known, technological important thermoelectric materials near room temperature. In this paper, nanocrystalline Bi₂Te₃ with different morphologies was synthesized by a solvothermal process based on the reaction between BiCl₃, Te, and KBH₄ in N,N-dimethylformamide at 100-180 °C. KBH₄ was used as a reducing agent. The products were characterized by X-ray diffraction and transmission electron microscopy (TEM). The particle morphologies and size are dependent on the reaction temperature and time. A possible formation mechanism is proposed.     

Thermoelectric power and phase transition of polycrystalline As2Te3 under pressure

The pressure dependence of the thermoelectric power of monoclinic As₂Te₃ is measured up to 10 GPa using a Mao-Bell diamond anvil cell. The thermoelectric power never reaches an absolute value greater than the ambient pressure value of 242 μV/K. Evidence of a phase transition is present between 6 and 8 GPa where the thermoelectric power reaches an absolute value of 225 μV/K after passing through a minimum of S≈75 μV/K. X-ray diffraction experiments confirm that the resulting structure is β-As₂Te₃, which is isostructural with Bi₂Te₃ and Sb₂Te₃.     

Annealing temperature influence on electrical properties of ion beam sputtered Bi2Te3 thin films

Ion beam sputtering process was used to deposit n-type fine-grained Bi₂Te₃ thin films on BK7 glass substrates at room temperature. In order to enhance the thermoelectric properties, thin films are annealed at the temperatures ranging from 100 to 400 °C. X-ray diffraction (XRD) shows that the films have preferred orientations in the c-axis direction. It is confirmed that grain growth and crystallization along the c-axis are enhanced as the annealing temperature increased. However, broad impurity peaks related to some oxygen traces increase when the annealing temperature reached 400 °C. Thermoelectric properties of Bi₂Te₃ thin films were investigated at room temperature. The Bi₂Te₃ thin films, including as-deposited, exhibit the Seebeck coefficients of −90 to −168 μV K⁻¹ and the electrical conductivities of 3.92×10²-7.20×10² S cm⁻¹ after annealing. The Bi₂Te₃ film with a maximum power factor of 1.10×10⁻³ Wm⁻¹ K⁻² is achieved when annealed at 300 °C. As a result, both structural and transport properties have been found to be strongly affected by annealing treatment. It was considered that the annealing conditions reduce the number of potential scattering sites at grain boundaries and defects, thus improving the thermoelectric properties.     

Annealing effects on the structural and electrical transport properties of n-type Bi2Te2.7Se0.3 thin films deposited by flash evaporation

N-type Bi₂Te_2.7Se_0.3 thermoelectric thin films with thickness 800 nm have been deposited on glass substrates by flash evaporation method at 473 K. Annealing effects on the thermoelectric properties of Bi₂Te_2.7Se_0.3 thin films were examined in the temperature range 373-573 K. The structures, morphology and chemical composition of the thin films were characterized by X-ray diffraction, field emission scanning electron microscope and energy dispersive X-ray spectroscopy, respectively. Thermoelectric properties of the thin films have been evaluated by measurements of the electrical resistivity and Seebeck coefficient at 300 K. The Hall coefficients were measured at room temperature by the Van der Pauw method. The carrier concentration and mobility were calculated from the Hall coefficient. The films thickness of the annealed samples was measured by ellipsometer. When annealed at 473 K, the electrical resistivity and Seebeck coefficient are 2.7 mΩ cm and −180 μV/K, respectively. The maximum of thermoelectric power factor is enhanced to 12 μW/cm K².     

Optical and thermoelectric characterizations of electroplated n-Bi2(Te0.9Se0.1)3

Bismuth selenotelluride (Bi₂(Te_0.9Se_0.1)₃) films were electrodeposited at constant current density from acidic aqueous solutions with Arabic gum in order to produce thin films for miniaturized thermoelectric devices. X-ray fluorescence spectroscopy determined film compositions. X-ray diffraction pattern shows that the films as deposited are polycrystalline, isostructural to Bi₂Te₃ and covered by crystallites. Mueller-matrix analysis reveals that the electroplated layers are optically like an isotropic medium. Their pseudo-dielectric functions were determined using mid-infrared spectroscopic ellipsometry. Tauc-Lorentz combined with Drude dispersion relations were successfully used. The energy band gap E_g was found to be about 0.15 eV. Moreover, the fundamental absorption edge was described by an indirect optical band-to-band transition. From Seebeck coefficient measurement, films exhibit n-type charge carrier and the value of thermoelectric power is about −40 μV/K.     

Electrical transport and high thermoelectric properties of PbTe doped with Bi2Te3 prepared by HPHT

In this paper, n-type lead telluride (PbTe) compounds doped with Bi₂Te₃ have been successfully prepared by high pressure and high temperature (HPHT) technique. The composition-dependent thermoelectric properties of PbTe doped with Bi₂Te₃ have been studied at room temperature. The figure-of-merit, Z, for PbTe is very sentivite to the dopants, which could be improved largely although the doped content of Bi₂Te₃ is very small (<0.08 mol%). In addition, the maximum value reaches to 9.3×10⁻⁴ K⁻¹, which is about 20% higher than that of PbTe alloyed with Bi₂Te₃ sintered at ambient pressure (7.6×10⁻⁴ K⁻¹) and several times higher than that of small grain size PbTe containing other dopants. The improved thermoelectric performance in this study may be due to the effect of high pressure and the low lattice thermal conductivity resulting from Bi₂Te₃ as source of dopants.     

Electrochemical deposition of Bi2Te3-based thin films

The electrochemical reduction processes on stainless-steel substrates from an aqueous electrolyte composed of nitric acid, Bi³⁺, HTeO₂⁺, SbO⁺ and H₂SeO₃ systems were investigated using cyclic voltammetry. The thin films with a stoichiometry of Bi₂Te₃, Bi_0.5Sb_1.5Te₃ and Bi₂Te_2.7Se_0.3 have been prepared by electrochemical deposition at selected potentials. The structure, composition, and morphology of the films were studied by X-ray diffraction (XRD), environmental scanning electron microscopy (ESEM) and electron microprobe analysis (EMPA). The results showed that the films were single phase with the rhombohedral Bi₂Te₃ structure. The morphology and growth orientation of the films were dependent on the deposition potentials.     

Electrodeposition of n-type Bi2Te3−ySey thermoelectric thin films on stainless steel and gold substrates

Thermoelectric films of n-Bi₂Te_3−ySe_y were prepared by potentiostatic electrodeposition technique onto stainless steel and gold substrates at room temperature. These films were used for morphological, compositional and structural analysis by environment scanning electron microscope (ESEM), energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD). The effect of different substrates on the structure and morphology of Bi₂Te_3−ySe_y films and relation between Se content in the electrodepositing solutions and in the films were also investigated. These studies revealed that Bi, Te and Se could be co-deposited to form Bi₂Te_3−ySe_y semiconductor compound in the solution containing Bi³⁺, HTeO₂⁺ and H₂SeO₃. The morphology and structure of the films are sensitive to the substrate material. The doped content of Se element in the Bi₂Te_3−ySe_y compound can be controlled by adjusting the Se⁴⁺ concentration in the electrodepositing solution. X-ray diffraction analysis indicates that the films prepared at −40 mV versus saturated calomel electrode (SCE) exhibit strong (1 1 0) orientation with rhombohedral structure.     

Electronic structures and thermoelectric properties of La or Ce-doped Bi2Te3 alloys from first principles calculations

Thermoelectric properties of La or Ce-doped Bi₂Te₃ alloys were systematically investigated by ab initio calculations of electronic structures and Boltzmann transport equations. The Seebeck coefficient of p-type LaBi₇Te₁₂ and La₂Bi₆Te₁₂ was larger than that of Bi₂Te₃, because La doping increased the effective mass of carriers. On the other hand, the electrical conductivity of LaBi₇Te₁₂ and La₂Bi₆Te₁₂ decreased, which caused a reduction of power factor of these La-doped Bi₂Te₃ alloys in comparison with Bi₂Te₃. The influence of Ce doping on the band structure and thermoelectric properties of Bi₂Te₃ was similar to that of La doping. The theoretical calculation provided an insight into the transport properties of La or Ce-doped Bi₂Te₃-based thermoelectric materials.     

Electrodeposition and characterization of thermoelectric Bi2Te2Se/Te multilayer nanowire arrays

Multilayer nanowire arrays of a new heterogeneous thermoelectric material, Bi₂Te₂Se/Te, were successfully fabricated by a template-assisted pulsed electrodeposition method. The nanowires were characterized by X-ray diffraction, field emission scanning electron microscopy, transmission electron microscopy, high resolution transmission electron microscopy, and energy dispersive X-ray spectroscopy, respectively. The diameter of the nanowires is in the range of 60–85 nm. By adjusting the concentration of Te precursor, the length of the Te segment can be modulated. At sufficient low concentration of Te precursor, the phase composition of the other segment can even be changed from Bi₂Te₂Se to Bi₂Se₂Te. The formation and growth mechanisms of the nanowires were proposed.     

Fabrication of bismuth telluride nanotubes via a simple solvothermal process

The unique Bi₂Te₃ tubes were obtained via a simple solvothermal reaction in the presence of ethylenediaminetetraacetic acid disodium salt. The product was characterized by X-ray diffraction, scanning electron microscopy and transmission electron microscopy. Bi₂Te₃ nanosheets are vertically grown off the tube wall to form Bi₂Te₃ tubes. A possible formation mechanism is proposed.     

Low-temperature thermoelectric properties of Bi85Sb15−xNbx alloys prepared by high-press sintering

The nanocrystalline materials with the general formula Bi₈₅Sb_15−xNb_x (x=0, 0.5, 1, 2, 3) were prepared by mechanical alloying and subsequent high-pressure sintering. Their transport properties involving electrical conductivity, Seebeck coefficient and thermal conductivity have been investigated in the temperature range of 80-300 K. The absolute value of Seebeck coefficient of Bi₈₅Sb₁₃Nb₂ reaches a maximum of 161 μV/K at 105 K, which is 69% larger than that of Bi₈₅Sb₁₅ at the same temperature. The power factor and figure-of-merit are 4.45×10⁻³ WK⁻²m⁻¹ at 220 K and 1.79×10⁻³ K⁻¹ at 196 K, respectively. These results suggest that thermoelectric properties of Bi₈₅Sb₁₅ based material can be improved by Nb doping.     

Changes in the electronic properties of Bi2X3 (X: Se, Te) single crystals due to intercalation

Bi₂Se₃ and Bi₂Te₃ are layered semiconductors n-type and p-type, respectively, which belong to the family of thermoelectric materials. In this work we examine the insertion of Cu in Bi₂Te₃ and Bi₂Se₃ single crystals through an intercalation reaction. The inserted Cu acting as donor enhances the n-type character of Bi₂Se₃ while changing the native p-type character of Bi₂Te₃ to n-type. The spatial distribution of the intercalated species was monitoring by X-ray microanalysis and microscopic IR reflectivity measurements. Paper presented at the 4th Euroconference on Solid State Ionics, Renvyle, Galway, Ireland, Sept. 13–19, 1997     

Studies of the electrochemical reduction processes of Bi, HTeO2 and their mixtures

The reduction process of Bi³⁺, HTeO₂⁺ and their mixtures on Au electrode surface was studied by cyclic voltammetry, linear sweep voltammetry, electrochemical impedance spectroscopy and chronoamperometry. XRD and EDS methods were also used to measure the reductive products prepared under different potentials and provide the evidences of the reactions. The results indicate that the reduction of HTeO₂⁺ occurs at more positive potential than that of Bi³⁺, but its reduction rate is slower and adsorption phenomenon exists during its reduction process. Bi₂Te₃ compound can be obtained potentiostatically at a proper potential in all the mixed solutions with concentration ratio CHTe+O₂/CBi3+ in our research range (0.1-10). But pure Bi₂Te₃ compound can only be obtained at 42 mV in the solution with concentration ratio CHTe+O₂/CBi3+ equaling to 1. And the formation of Bi₂Te₃ compound is an inductive co-depositing process: (1) HTeO₂⁺ + 4e⁻ + 3H⁺ → Te⁰ + 2H₂O, (2) 3Te⁰ + 2Bi³⁺ + 6e⁻ → Bi₂Te₃.     

Electrodeposition of BixSb2−xTey thermoelectric thin films from nitric acid and hydrochloric acid systems

The electrochemical behaviors of Bi^III, Te^IV and Sb^III single ions and their mixtures were investigated in nitric acid and hydrochloric acid system separately. Based on which, Bi_xSb_2−xTe_y thermoelectric films were prepared by potentiostatic electrodeposition from the solutions with different concentrations of Bi^III, Te^IV and Sb^III in the two acid systems. The morphologies, compositions, structures, Seebeck coefficients and resistivities of the deposited thin films were characterized and compared by ESEM (or FESEM), EDS, XRD, Seebeck coefficient measurement system and four-probe resistivity measuring device respectively. The results show that although Bi_xSb_2−xTe_y thermoelectric thin film which structure is consistent with the standard pattern of Bi_0.5Sb_1.5Te₃ can be gained in both of the two acid solutions by adjusting the deposition potential, their morphologies and thermoelectric properties have big differences in different acid solutions.     

Superconductivity and non-metallicity induced by doping the topological insulators Bi2Se3 and Bi2Te3

We show that by Ca doping the Bi₂Se₃ topological insulator, the Fermi level can be fine tuned to fall inside the band gap and therefore suppresses the bulk conductivity. Non-metallic Bi₂Se₃ crystals are obtained. On the other hand, the Bi₂Se₃ topological insulator can also be induced to become a bulk superconductor, with T_c∼3.8 K, by copper intercalation in the van der Waals gaps between the Bi₂Se₃ layers. Likewise, an as-grown crystal of metallic Bi₂Te₃ can be turned into a non-metallic crystal by slight variation in the Te content. The Bi₂Te₃ topological insulator shows small amounts of superconductivity with T_c∼5.5 K when reacted with Pd to form materials of the type Pd_zBi₂Te₃.     

Structural characterization of thermally evaporated Bi2Te3 thin films

Thermally evaporated Bi₂Te₃ thin films were deposited on glass substrates. X-ray diffraction study confirmed that the growned films are polycrystalline in nature having hexagonal structure. The film exhibits preferential orientation along the [0 1 5] direction for the films of all thickness together with other abundant planes [0 1 1 1] and [1 1 0]. Various structural parameters such as lattice constants, crystallite size, strain, and dislocation density have been calculated and they are found to be thickness dependent. The lattice parameters are found to be a=4.38 Å and c=30.40 Å. The grain size of the films increases with thickness as the dislocation density and the microstrain decreases with thickness. The mean bond energy and the average coordination number of Bi₂Te₃ thin film are found to be 1.72 eV and 2.4, respectively.     

Characterization of PbSe1−xTex thin films

The lead salts and their alloys are extremely interesting semiconductors due to their technological importance. The fabrication of devices with alloys of these compounds possessing detecting and lasing capabilities has been an important recent technological development. The high quality polycrystalline thin films of PbSe_1−xTe_x with variable composition (0≤x≤1) have been deposited onto ultra clean glass substrates by vacuum evaporation technique. As deposited films were annealed in vacuum at 350 K. The optical, electrical and structural properties of PbSe_1−xTe_x thin films have been examined. The optical constants (absorption coefficient and bandgap) of the films were determined by absorbance measurements in the wavelength range 2500-5000 nm using Fourier transform infrared spectrophotometer. The dc conductivity and activation energy of the films were measured in the temperature range 300-380 K. The X-ray diffraction patterns were used to determine the sample quality, crystal structure and lattice parameter of the films.     

Switching phenomenon and optical properties of Se85Te10Bi5 films

Se₈₅Te₁₀Bi₅ films of different thicknesses ranging from 126 to 512 nm have been prepared. Energy-dispersive X-ray (EDX) spectroscopy technique showed that films are nearly stoichiometric. X-ray diffraction (XRD) measurements have showed that the Se₈₅Te₁₀Bi₅ films were amorphous. Electrical conduction activation energy (ΔE_σ) for the obtained films is found to be 0.662 eV independent of thickness in the investigated range. Investigation of the current voltage (I-V) characteristics in amorphous Se₈₅Te₁₀Bi₅ films reveals that it is typical for a memory switch. The switching voltage V_th increases with the increase of the thickness and decreases exponentially with temperature in the range from 298 to 383 K. The switching voltage activation energy (ε) calculated from the temperature dependence of V_th is found to be 0.325 eV. The switching phenomenon in amorphous Se₈₅Te₁₀Bi₅ films is explained according to an electrothermal model for the switching process. The optical constants, the refractive index (n) and the absorption index (k) have been determined from transmittance (T) and reflectance (R) of Se₈₅Te₁₀Bi₅ films. Allowed non-direct transitions with an optical energy gap (E_g^opt) of 1.33 eV have been obtained. ΔE_σ is almost half the obtained value of E_g^opt, which suggested band to band conduction as indicated by Davis and Mott.     

Electrodeposition of thermoelectric Bi2Te3 thin films with added surfactant

Bismuth telluride (Bi₂Te₃) thin films were electrodeposited at room temperature from nitric baths in the presence of a surfactant, cetyltrimethylammonium bromide (CTAB). Nearly stoichiometric Bi₂Te₃ thin films were obtained from electrolytes containing 7.5 mM Bi(NO₃)₃. The surface morphology and mechanical properties of the electrodeposited thin film were improved by the addition of CTAB to the electrolyte, while the electrical and thermoelectric properties were preserved. Post-deposition annealing in a reducing environment did not improve the electrical and thermoelectric properties, possibly because the change in the microstructure of the Bi₂Te₃ thin film was too small.