Fabrication and electrical properties of Bi2-xSbxTe3 ternary nanopillars array films

Highly oriented Bi_2-xSb_xTe₃ (x?=?0, 0.7, 1.1, 1.5, 2) ternary nanocrystalline films were fabricated using vacuum thermal evaporation method. Microstructures and morphologies indicate that Bi_2-xSb_xTe₃ films have pure rhombohedral phase with well-ordered nanopillars array. Bi, Sb and Te atoms uniformly distributed throughtout films with no precipitation. Electrical conductivity of Bi_2-xSb_xTe₃ films transforms from n-type to p-type when x?>?1.1. Metal-insulator transition was observed due to the incorporation of Sb in Bi₂Te₃. Bi_2-xSb_xTe₃ film with x?=?1.5 exhibits optimized electrical properties with maximum electrical conductivity σ of 2.95?×?10⁵ S?m^?1 at T?=?300?K, which is approximately ten times higher than that of the undoped Bi₂Te₃ film, and three times higher than previous report for Bi_0.5Sb_1.5Te₃ films and bulk materials. The maximum power factor PF of Bi_0.5Sb_1.5Te₃ nanopillars array film is about 3.83?μW?cm^?1 K^?2 at T?=?475?K. Highly oriented (Bi,Sb)₂Te₃ nanocrystalline films with tuned electronic transport properties have potentials in thermoelectric devices.     

Investigations on the electrodeposition behaviors of Bi0.5Sb1.5Te3 thin film from nitric acid baths

The electrochemical reduction process of Bi³⁺, HTeO₂⁺, Sb^III and their mixtures in nitric acid medium was investigated by means of cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) measurements. The reduction products electrodeposited at various potentials were examined using X-ray diffraction (XRD) and energy dispersive spectroscopy (EDS). The results show that cathodic process in the nitric acid solution containing Bi³⁺, HTeO₂⁺ and Sb^III involves the following reduction reactions in different polarizing potential ranges: In low polarizing potential ranges, Te⁰ is formed firstly on the electrode surface through the electrochemical reduction of HTeO₂⁺; with the negative shift of the cathodic polarizing potential, the reduction reaction of Bi³⁺ with Te⁰ to form Bi₂Te₃ takes place; when the cathodic polarizing potential is negative enough, Bi³⁺ and Sb^III react with Te⁰ to form Bi_0.5Sb_1.5Te₃. The results indicate that Bi_0.5Sb_1.5Te₃ films can be fabricated by controlling the electrodepositing potential in a proper high potential ranges.     

Electrodeposition of P-type BixSb2−xTey thermoelectric film from dimethyl sulfoxide solution

The electrochemical behaviors of Bi(III), Te(IV), Sb(III) and their mixtures in DMSO solutions were investigated using cyclic voltammetry and linear sweep voltammetry measurements. On this basis, Bi_xSb_2−xTe_y film thermoelectric materials were prepared by potentiodynamic electrodeposition technique from mixed DMSO solution, and the compositions, structures, morphologies as well as the thermoelectric properties of the deposited films were also analyzed. The results show that Bi_xSb_2−xTe_y compound can be prepared in a very wide potential range by potentiodynamic electrodeposition technique in the mixed DMSO solutions. After anneal treatment, the deposited film prepared in the potential range of −200 to −400 mV shows the highest Seebeck coefficient (185 μV/K), the lowest resistivity (3.34 × 10⁻⁵ Ω m), the smoothest surface, the most compact structure and processes the stoichiometry (Bi_0.49Sb_1.53Te_2.98) approaching to the Bi_0.5Sb_1.5Te₃ ideal material most. This Bi_0.49Sb_1.53Te_2.98 film is a kind of nanocrystalline material and (0 1 5) crystal plane is its preferred orientation.     

High thermoelectric performance achieved in Bi0.4Sb1.6Te3 films with high (00l) orientation via magnetron sputtering

To obtain p-type Bi–Sb–Te-based thin films with excellent thermoelectric performance, the Bi_0.4Sb_1.6Te₃ target is prepared by combining mechanical alloying with the spark plasma sintering technique. Afterward, Bi_0.4Sb_1.6Te₃ thin films are deposited via magnetron sputtering at variable working pressures. With an increasing working pressure, the frequency of collisions between the argon ions and sputtered atoms gradually increases, the preferred orientation of (00l) increases, and the sputtering rate decreases. The Seebeck coefficient increases from ∼140 μV/K to ∼220 μV/K as the carrier concentration decreases along with an increasing working pressure. Furthermore, the decrease in carrier concentration and acceleration of carrier mobility also affect the change in electrical conductivity. The maximum power factor of the p-type Bi_0.4Sb_1.6Te₃ thin film deposited at 4.0 Pa and at room temperature exceeds 20.0 μW/cm K² and is higher than that of most p-type Bi–Sb–Te-based films.     

Optimization of chemical and electrochemical parameters for the preparation of n-type Bi2Te2.7Se0.3 thin films by electrodeposition

A 2^3–1 fractional factorial design comprising four runs and three centre points was applied in order to optimize the electrodeposition process to find a compound with the best stoichiometry leading to a Bi₂Te_2.7Se_0.3 thin film suitable for thermoelectric applications. The key factors considered were the deposition potential, the percentage of bismuth and the percentage of selenium in the solution. The Bi^III, Se^IV, Te^IV electrolyte mixtures in 1 M HNO₃ (pH 0), allowed deposition of ternary alloys to be achieved at room temperature on stainless steel substrates. The deposition mechanism was investigated by linear voltammetry. The films were characterized by micropobe analysis, X-ray diffraction, scanning electron microscopy and atomic force microscopy. The XRD patterns of the film show that the as-deposited are polycrystalline and isostructural to Bi₂Te₃. The SEM study shows that the film is covered by crystallites while the AFM image reveals a low level of roughness.     

Thermoelectric properties of Cu-dispersed bi<Subscript>0.5</Subscript>sb<Subscript>1.5</Subscript>te<Subscript>3</Subscript>

A novel and simple approach was used to disperse Cu nanoparticles uniformly in the Bi_0.5Sb_1.5Te₃ matrix, and the thermoelectric properties were evaluated for the Cu-dispersed Bi_0.5Sb_1.5Te₃. Polycrystalline Bi_0.5Sb_1.5Te₃ powder prepared by encapsulated melting and grinding was dry-mixed with Cu(OAc)₂ powder. After Cu(OAc)₂ decomposition, the Cu-dispersed Bi_0.5Sb_1.5Te₃ was hot-pressed. Cu nanoparticles were well-dispersed in the Bi_0.5Sb_1.5Te₃ matrix and acted as effective phonon scattering centers. The electrical conductivity increased systematically with increasing level of Cu nanoparticle dispersion. All specimens had a positive Seebeck coefficient, which confirmed that the electrical charge was transported mainly by holes. The thermoelectric figure of merit was enhanced remarkably over a wide temperature range of 323-523 K.     

Galvanic displacement of BixTey thin films from sacrificial iron group thin films

Bi_xTe_y thin films synthesized by galvanic displacement were systematically investigated by observing open circuit potential (OCP), surface morphology, microstructure and film composition. Surface morphologies and crystal structures of synthesized Bi_xTe_y thin films were strongly depended on the type of the sacrificial materials (i.e., nickel (Ni), cobalt (Co) and iron (Fe)). Galvanically deposited Bi_xTe_y thin films from the sacrificial Ni and Co thin films exhibited Bi₂Te₃ intermetallic compounds and hierarchical structures with backbones and sub-branches. A linear relationship of deposited Bi content in Bi_xTe_y thin films as a function of [Bi³⁺]/[HTeO₂⁺] ratio (within a range of less than 0.8) in the electrolyte was also observed. Surface morphologies of Bi_xTe_y thin films were altered with the film composition.     

Laser ablation of Ga-Sb-Te thin films monitored with quadrupole ion trap time-of-flight mass spectrometry

Laser ablation of Ga-Sb-Te chalcogenide thin films prepared by radiofrequency magnetron co-sputtering was monitored with quadrupole ion trap time-of-flight mass spectrometry (QIT-TOF-MS). The mass spectra of 11 thin films of various compositions (Ga: 0–53.1, Sb: 0–52.0, and Te: 0–100.0 at. %) were recorded. Several series of unary (Ga_x, Sb_y, and Te_z) binary (Ga_xSb_y, Ga_xTe_z, and Sb_yTe_z), and ternary Ga_xSb_yTe_z clusters were identified in both positive and negative ion modes. Stoichiometry of observed clusters was determined. Up to 18 binary clusters (positively and negatively charged) were detected for thin film with low Sb content of 6.5 at. %. The highest number (4) of ternary clusters was observed for thin film with high Te content of 66.7 at. %. The number of generated clusters and their peaks intensity varied according to the chemical composition of thin films. Altogether, 41 clusters were detected. The laser ablation monitoring shows laser-induced fragmentation of thin film structure. The relation of clusters stoichiometries to the chemical composition of thin films is discussed. The fragmentation can be diminished by covering a surface of thin films with paraffin's, glycerol, or trehalose sugar thin layers. The stoichiometry of generated clusters shows partial structural characterization of thin films.     

Electrodeposition of Zn−Ni−Fe alloy in acidic chloride bath with separated anodes

The electrodeposition of ternary Zn–Ni–Fe alloy films was investigated in acidic chloride electrolyte. Electrodeposition was performed onto mild steel plates at pH 3 and 43°C. The influence of the chloride concentration (ZnCl₂, NiCl₂ and FeCl₂) on the surface appearance and deposit composition, as well as cathodic current efficiency, were investigated. Bright Zn–Ni–Fe alloy deposits were obtained in the electrolyte containing 0.4m of each of ZnCl₂ and NiCl₂ with 0.02 to 0.08m FeCl₂. The influence of current density, pH and temperature were also examined.     

Electrochemically deposited thermoelectric n-type Bi2Te3 thin films

Electrochemically deposited n-type BiTe alloy thin films were grown from nitric acid baths on sputtered Bi_xTe_y/SiO/Si substrates. The film compositions, which varied from 57 to 63 at.% Te were strongly dependent on the deposition conditions. Surface morphologies varied from needle-like to granular structures depending on deposited Te content. Electrical and thermoelectric properties of these electrodeposited Bi_xTe_y thin films were measured before and after annealing and compared to those of bulk Bi₂Te₃. Annealing at 250 °C in reducing H₂ atmosphere enhanced thermoelectric properties by reducing film defects. In-plane electrical resistivity was highly dependent on composition and microstructure. In-plane Hall mobility decreased with increasing carrier concentration, while the magnitude of the Seebeck coefficient increased with increasing electrical conductivity to a maximum of −188.5 μV/K. Overall, the thermoelectric properties of electrodeposited n-type BiTe thin films after annealing were comparable to those of bulk BiTe films.     

Comprehensive study of tellurium based glass ceramics for thermoelectric application

Tellurium based glasses have interesting thermoelectric characteristics. However, their high electrical resistivity is still an obstacle to considering them for thermoelectric applications. In this work, the (Te₈₅Se₁₅)_60???0.6xAs_40???0.4xCu_x glass system was studied. This revealed that Cu can act as glass former and increase both glass thermal stability and electrical conductivity. The best candidate, (Te₈₅Se₁₅)₄₅As₃₀Cu₂₅, was chosen to prepare composites with Bi_0.5Sb_1.5Te₃ using spark plasma sintering. These glass ceramic samples exhibited a much better thermoelectric performance. Glass ceramics with 50?mol. % of Bi_0.5Sb_1.5Te₃ show a maximum ZT value equal to 0.37 at 413?K. Meanwhile, the advantages of glass including low sintering temperature and high formability are well maintained.     

One-step electrochemical preparation of the ternary (BixSb1−x)2Te3 thin films on Au(1 1 1): Composition-dependent growth and characterization studies

This study reports on the synthesis of ternary semiconductor (Bi_xSb_1−x)₂Te₃ thin films on Au(1 1 1) using a practical electrochemical method, based on the simultaneous underpotential deposition (UPD) of Bi, Sb and Te from the same solution containing Bi³⁺, SbO⁺, and HTeO₂⁺ at a constant potential. The thin films are characterized by X-ray diffraction (XRD), atomic force microscopy (AFM), energy dispersive spectroscopy (EDS) and reflection absorption-FTIR (RA-FTIR) to determine structural, morphological, compositional and optic properties. The ternary thin films of (Bi_xSb_1−x)₂Te₃ with various compositions (0.0 ≤ x ≤ 1.0) are highly crystalline and have a kinetically preferred orientation at (0 1 5) for hexagonal crystal structure. AFM images show uniform morphology with hexagonal-shaped crystals deposited over the entire gold substrate. The structure and composition analyses reveal that the thin films are pure phase with corresponding atomic ratios. The optical studies show that the band gap of (Bi_xSb_1−x)₂Te₃ thin films could be tuned from 0.17 eV to 0.29 eV as a function of composition.     

Unique interface-driven crystallization mechanism and element-resolved structure imaging of ZnO-Ge2Sb2Te5 nanocomposites

Amorphous Ge₂Sb₂Te₅ thin films doped with ZnO were prepared and their crystallization behaviors were investigated. Our results showed that thermal annealing of amorphous ZnO-Ge₂Sb₂Te₅ nanocomposites produced face centered-cubic Ge₂Sb₂Te₅-nanocrystals embedded between interfaces. An increase in crystallization temperature and electrical resistance ratio were attributed to the increase of specific interfacial energies and inhomogeneous strain at the oxide/Ge₂Sb₂Te₅ interfaces. The formation of the interfaces not only accelerated crystallization, but also limited the grain growth due to the one-dimensional growth mode. We proposed a crystallization model to elucidate the derived kinetic mechanism of the nanocrystal growth in the Ge₂Sb₂Te₅ matrix. These experimental observations suggest that ZnO-Ge₂Sb₂Te₅ nanocomposites are a good candidate for the applications in phase change memory.     

Monatomic Sb thin films alloyed with Sb2S3 enables superior thermal stability and resistance drift by spontaneous self-decomposition

Monatomic Sb thin films can eliminate the risks of compositional partitioning, but it normally crystallizes instantly and fails to maintain amorphous state at room temperature. Here, we prepared Sb_x (Sb₂S₃)_100-x thin films. The materials consist of pure Sb with low resistance drift and chalcogenide Sb₂S₃ with high thermal stability. It is found that the Sb_64·8(Sb₂S₃)_35.2 thin film possesses the advantages of these two compounds. The thin film showed good phase-change ability with an ultralow resistance drift coefficient of 0.006, much lower than conventional Ge₂Sb₂Te₅ (0.076). Moreover, it also exhibited a better amorphous thermal stability. These improvements are closely related to the hybrid nanostructure of Sb crystals and Sb–S phase by spontaneous self-decomposition in the Sb-rich Sb–S material. Our work thus demonstrates that the binary Sb-rich Sb–S thin film can become a promising alternative to replace the conventional Ge₂Sb₂Te₅ thin film with potential for neuromorphic synaptic devices.     

Effect of Sintering Temperature on Thermoelectric Properties of p‐Bi2Te3 Alloys Produced by Gas Atomization

In this research, p‐type Bi₂Te₃–75% Sb₂Te₃ thermoelectric alloy powders were produced by gas atomization and subsequently sintered by hot pressing at different temperatures. The grain growth of the hot‐pressed samples was observed with increasing sintering temperature from 380°C to 460°C. The compressive strength increased with increasing hot‐pressing temperature due to the high relative density of bulk samples obtained at high temperatures. The effect of sintering temperature on thermoelectric (TE) properties was studied. The maximum power factor 3.48 mW/mK² was obtained for the sample hot pressed at 420°C due to the resulting high electrical conductivity and enhanced Seebeck coefficient values.     

Thermoelectric transport and magnetoresistance of electrochemical deposited Bi2Te3 films at micrometer thickness

Bismuth telluride (Bi₂Te₃) is so far the best thermoelectric material for applications near room temperature, and also exhibits large magnetoresistance. While the electrochemical deposition approach can achieve effective growth of the Bi₂Te₃ films at micrometer thickness, the magnetoresistance transportation behavior of the electrochemically deposited Bi₂Te₃ films is yet not clear. In this work, we demonstrate the thermoelectric and magnetoresistance behaviors of the micrometer thick Bi₂Te₃ films deposited via electrochemical deposition approach. The optimum thermoelectric power factor is observed in the Bi₂Te₃ sample with electrochemical deposition thickness of ~6 μm followed by rapid photon annealing treatment, reaching the magnitude of ~1 μWcm⁻¹K⁻² that is similar to the previous reports. In contrast to the single crystalline or vacuum deposited Bi₂Te₃ or Bi₂Se₃ films, the electronic transportations of the electrochemically deposited Bi₂Te₃ are more influenced by the carrier scatterings by the grain boundaries and lattice defect. As a result, their magnetoresistance (MR) shows a distinguished non-monotonic behavior when varying the magnetic field, while the magnitude of their MR exhibits a positive temperature dependence. These MR behaviors largely differ to the previously reported ones from the single crystalline or vacuum deposited Bi₂Te₃ or Bi₂Se₃, in which cases their MR monotonically increases with the magnetic field and exhibits negative temperature dependence. This work reveals the previously overlooked role of grain boundary that also regulates the transportation properties of bismuth chalcogenides in the presence of magnetic field.     

Recent progress in electrodeposition of thermoelectric thin films and nanostructures

Thermoelectric power generators and coolers have many advantages over conventional refrigerators and power generators such as solid-state operation, compact design, vast scalability, zero-emissions and long operating lifetime with no maintenance. However, the applications of thermoelectric devices are limited to where their unique advantages outweigh their low efficiency. Despite this practical confine, there has been a reinvigorated interest in the field of thermoelectrics through identification of classical and quantum mechanical size effects, which provide additional ways to enhance energy conversion efficiencies in nanostructured materials. Although, there are a few reports which demonstrated the improvement of efficiency through nanoengineering, the successful application of these nanostructures will be determined by a cost-effective and high through-put fabrication method. Electrodeposition is the method of choice to synthesize nanoengineered thermoelectric materials because of low operating and capital cost, high deposition rates, near room temperature operation, and the ability to tailor the properties of materials by adjusting deposition conditions. In this paper, we reviewed the recent progress of the electrodeposition of thermoelectric thin films and nanostructures including Bi, Bi_1−xSb_x, Bi₂Te₃, Sb₂Te₃, (Bi_1−xSb_x)₂Te₃, Bi₂Se₃, Bi₂Te_3−ySe_y, PbTe, PbSe, PbSe_1−xTe_x and CoSb₃.     

In situ analysis of bismuth telluride electrodeposition using combined spectroscopic ellipsometry and electrochemical quartz crystal microbalance

Electrodeposition of bismuth telluride (Bi₂Te₃) in an acidic medium with Arabic gum by galvanostatic polarization has been investigated. Simultaneous in situ spectroscopic ellipsometry and gravimetric measurements have been performed to study the morphological evolution of the compound. A progressive covering stage was demonstrated and revealed that a 40 nm thick film has already acquired morphological and optical behavior similar to that of thicker films. The optical thickness and electrochemical quartz crystal microbalance (EQCM) mass are coherent with a density of 7.06. Combined gravimetric and coulometric data confirm the formation of Bi₂Te₃ by determining the ratio m/z.     

Composition-dependent characterization and optimal control of electrodeposited Bi2Te3 films for thermoelectric application

A method to control composition of Bi₂Te₃ films by mass transfer manipulation has been developed. The film composition can be varied by a diffusion-controlled method, which is related to the change of Bi³⁺/HTeO₂⁺ ratios in a controlled diffusion layer. A homogeneous and dense film with precise chemical composition could thus be obtained under constant electrode polarization. Meanwhile, the solo dependence of film properties on composition change of both Te-rich and Bi-rich films were investigated. Firstly, the studies of XRD and FE-SEM showed that different Te contents in deposit would lead to different dimensions of unit cell and grain sizes. The Seebeck coefficient increased apparently when the Te content was over 60 at.% Te. Te-rich films had higher carrier concentration but slower mobility than Bi-rich films. Inverse relations were observed between carrier concentration and carrier mobility and between Seebeck coefficient and conductivity. Therefore, an optimal power factor of 7 × 10⁻⁴ W/m K² was realized near the stoichiometric Bi₂Te₃.     

Germanium-antimony-selenium-tellurium thin films: Clusters formation by laser ablation and comparison with clusters from mixtures of elements

Quaternary germanium-antimony-selenium-tellurium (Ge-Sb-Se-Te) thin films deposited from Ge_19.4Sb_16.7Se_63.9−xTe_x (x = 5, 10, 15, and 20) glass-ceramics targets by radio frequency magnetron sputtering were studied using laser ablation quadrupole ion trap time of flight mass spectrometry. Binary, ternary, and quaternary Ge_aSb_bSe_cTe_d clusters were formed and their stoichiometry was determined. By comparison of the clusters obtained from quaternary Ge-Sb-Se-Te thin films and those from ternary Ge-Sb-Te materials, we found that Ge-Te species are not detected from the quaternary system. Furthermore, Ge-Se and Se-Te species are missing in mass spectra generated from Ge-Sb-Se-Te thin films. From the Ge-Sb-Se-Te thin films, 16 clusters were detected while ternary Ge-Sb-Se glasses yielded 26 species. This might be considered as a signal of higher stability of Ge-Sb-Se-Te thin films which is increasing with a higher content of Te. The missing (Se₂⁺, Ge_aSb⁺ (a = 1–4), and GeSe_c⁺ (c = 1, 2)) and new (Ge⁺ and Sb_bTe⁺ (b = 1–3)) clusters may indicate that some of the structural features of the films (Ge₂Se_6/2 and Se₂Sb-SbSe₂) were replaced by (GeSe_4−xTe_x and SbSe_3−xTe_x) ones. In addition, when comparing the stoichiometry of clusters formed from Ge-Sb-Se-Te thin films with those from the mixtures of the elements, only Sb₃⁺ and SbSe⁺ were observed in both cases. The knowledge gained concerning clusters stoichiometry contributes to the elucidation of the processes proceeding during plasma formation used for the chalcogenide thin films deposition.