Undoped Bi2Te3 Research Articles

Influence of magnetic (Fe) and non-magnetic (In) doping on structural, magnetic, and weak anti-localization properties of Bi2Te3 topological insulator

In this article, we have studied the changes in structural, magnetic, and magneto-transport properties of Bi2Te3 topological insulator doped with magnetic (Fe) as well as non-magnetic (In) elements. The un-doped along with Fe, In-doped Bi2Te3 are grown using a melt growth technique. The Rietveld analysis of x-ray diffraction data expresses that both In and Fe-dopants substituted the Bi-position with a little bit of interstitial incorporation in host Bi2Te3. It is also noticed that In-doping is slightly more favorable for Bi-substitution than Fe. The magnetic characterization reveals a mixing of diamagnetic and Pauli paramagnetic behavior of un-doped and In-doped Bi2Te3, whereas, Fe-doping shows overall paramagnetism with local anti-ferromagnetic interactions among Fe-ions without long-range order. The electrical-transport study represents the metallic response of host Bi2Te3, which is well-maintained for In-doping; however, Fe-doping exhibits prominent anomalies in ρ xx –T profiles. Importantly, magneto-conductance research indicates a notable deviation of host quantum feature, weak anti-localization effect (WAL) upon magnetic (Fe) doping, whereas non-magnetic In-doping shows a comparatively weak deviation in WAL effect for high doping limit.

Influence of gallium doping on structural and thermoelectric properties of bismuth telluride

Pure and gallium-doped bismuth telluride (Bi2Te3) nanostructures were synthesized by solvothermal method at 200 °C for 24 h. The undoped Bi2Te3 nanostructures show flower-like morphology, on gallium-doping, the Bi2Te3 nanostructures show flake-like morphology. The thermoelectric measurements of nanostructured bulk sample show an enhancement in power factor in gallium-doped Bi2Te3 from around 60 μWm-1K−2 at 150 °C for undoped Bi2Te3 to around 960 μWm-1K−2 at 150 °C for 0.2 mmol gallium-doped Bi2Te3 and with the increase in gallium-doping to 0.3 mmol, the power factor was found to decrease to around 400 μWm-1K−2 at 150 °C. The figure of merit (ZT) value was found to be around 0.08 at 50 °C for undoped Bi2Te3 and for 0.2 mmol gallium-doped Bi2Te3 the ZT values was found around 1.1 at 50 °C. Further, increase in the ZT value shows a decreasing trend and for 0.3 mmol gallium-doped Bi2Te3ZT value was found to be around 0.8 at 50 °C.

Multiple Roles of Unconventional Heteroatom Dopants in Chalcogenide Thermoelectrics: The Influence of Nb on Transport and Defects in Bi2Te3.

Improvements in the thermoelectric performance of n-type Bi2Te3 materials to more closely match their p-type counterparts are critical to promote the continued development of bismuth telluride thermoelectric devices. Here the unconventional heteroatom dopant, niobium, has been employed as a donor in Bi2Te3. Nb substitutes for Bi in the rhombohedral Bi2Te3 structure and exhibits multiple roles in its modulation of electrical transport and defect-induced phonon scattering. The carrier concentration is significantly increased as electrons are afforded by aliovalent doping and formation of vacancies on the Te sites. In addition, incorporation of Nb in the pseudoternary Bi2-xNbxTe3-δ system increases the effective mass, m*, which is consistent with cases of "conventional" elemental doping in Bi2Te3. Lastly, inclusion of Nb induces both point and extended defects (tellurium vacancies and dislocations, respectively), enhancing phonon scattering and reducing the thermal conductivity. As a result, an optimum zT of 0.94 was achieved in n-type Bi0.92Nb0.08Te3 at 505 K, which is dramatically higher than an equivalent undoped Bi2Te3 sample. This study suggests not only that is Nb an exciting and novel electron dopant for the Bi2Te3 system but also that unconventional dopants might be utilized with similar effects in other chalcogenide thermoelectrics.

Bulk Cyclotron Resonance in the Topological Insulator Bi2Te3

We investigated magneto-optical response of undoped Bi2Te3 films in the terahertz frequency range (0.3–5.1 THz, 10–170 cm−1) in magnetic fields up to 10 T. The optical transmission, measured in the Faraday geometry, is dominated by a broad Lorentzian-shaped mode, whose central frequency linearly increases with applied field. In zero field, the Lorentzian is centered at zero frequency, representing hence the free-carrier Drude response. We interpret the mode as a cyclotron resonance (CR) of free carriers in Bi2Te3. Because the mode’s frequency position follows a linear magnetic-field dependence and because undoped Bi2Te3 is known to possess appreciable number of bulk carriers, we associate the mode with a bulk CR. In addition, the cyclotron mass obtained from our measurements fits well the literature data on the bulk effective mass in Bi2Te3. Interestingly, the width of the CR mode demonstrates a behavior non-monotonous in field. We propose that the CR width is defined by two competing factors: impurity scattering, which rate decreases in increasing field, and electron-phonon scattering, which exhibits the opposite behavior.

Attaining reduced lattice thermal conductivity and enhanced electrical conductivity in as-sintered pure n-type Bi2Te3 alloy

Undoped n-type Bi2Te3 bulks were prepared via the liquid state manipulation (LSM) with subsequent ball milling and spark plasma sintering processes. The sample with LSM obtains higher carrier concentration and larger effective mass compared with that without LSM, exhibiting favourable electrical transport properties. More importantly, a much reduced lattice thermal conductivity ~ 0.47 W m−1 K−1 (decreased by 43%) is obtained, due to the enhanced multiscale phonon scattering from hierarchical microstructures, including boundaries, nanograins and lattice dislocations. Additionally, due to the increased carrier concentration and enlarged band gap, the bipolar effect is effectively suppressed in sample BT-LSM. Consequently, zTmax ~ 0.66 is achieved in the sample with LSM at higher temperature of 475 K, almost 22% improvement compared with that of the contrast.

Improved thermoelectric properties of n-type Bi2Te3 alloy deriving from two-phased heterostructure by the reduction of CuI with Sn

In this report, CuI and Sn co-doped n-type Bi2Te3 samples have been prepared by a high-temperature solid-state reaction, and the effect of co-doping on the thermoelectric properties was investigated from room temperature to 525 K. Sn single-doped and undoped Bi2Te3 were prepared for comparison. Detailed charge transport data including electrical conductivity, Seebeck coefficient, Hall coefficient, and thermal conductivity are presented. Microscopic observation of CuI/Sn co-doped samples revealed that numerous distinctive microstructures such as nanoprecipitates of the Cu and SnI-rich phase were generated in the matrix. The lattice thermal conductivity of CuI/Sn co-doped Bi2Te3 was substantially reduced compared to those of undoped and single doped Bi2Te3. Benefiting from the improved electrical transport properties by doping and the reduced lattice thermal conductivity by numerous microstructures, the ZT value of the Bi2Te3 doped with 1 at.% CuI/Sn is distinctly enhanced to 1.24 at 425 K. The average ZT value (ZTave ~ 1.02) at 300–525 K was clearly higher than those of Sn-doped Bi2Te3 (ZTave ~ 0.54) and CuI-doped Bi2Te3 (ZTave ~ 0.98). This work indicates that the average ZT can be improved over a broad temperature range using a co-doping approach.

Synthesis of heavily Cu-doped Bi2Te3 nanoparticles and their thermoelectric properties

Heavily Cu-doped Bi2Te3 nanoparticles were prepared by intercalating copper metal into flower-like Bi2Te3 nanoparticles using the disproportionation redox reaction of Cu(I) salt. The phase, chemical composition, and morphology of the Bi2Te3 nanoflowers were analyzed by X-ray diffraction (XRD), energy-dispersive spectroscopy (EDS), and scanning electron microscopy (SEM). The synthesized Cu-doped Bi2Te3 nanopowders were consolidated by spark-plasma sintering into bulk pellets, and the effects of Cu-doping on the transport properties (electrical resistivity, Seebeck coefficient, and thermal conductivity) of these materials were investigated. Superstoichiometric amounts of Cu (up to ∼28 at%) can be incorporated into flower-like Bi2Te3 nanoparticles, which have large accessible surface area for diffusion of Cu ions. The flower-like morphologies did not change despite high Cu incorporation. Variation in carrier concentration was achieved by changing Cu precursor concentration. Cu-doping in Bi2Te3 can enhance the Seebeck coefficient due to a decrease in carrier concentration, thus the power factors increased compared with that of the un-doped sample. Furthermore, the thermal conductivity of Cu-doped Bi2Te3 is substantially reduced. As a result, Cu-doped Bi2Te3 sample with 15.6 at% Cu exhibited the best thermoelectric performance with a figure of merit of 0.67 at 415 K, which is more than two times higher than that of undoped Bi2Te3 nanopowder.

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

Highly oriented Bi2-xSbxTe3 (x = 0, 0.7, 1.1, 1.5, 2) ternary nanocrystalline films were fabricated using vacuum thermal evaporation method. Microstructures and morphologies indicate that Bi2-xSbxTe3 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 Bi2-xSbxTe3 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. Bi2-xSbxTe3 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 Bi0.5Sb1.5Te3 films and bulk materials. The maximum power factor PF of Bi0.5Sb1.5Te3 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.

Mechanisms of thermoelectric efficiency enhancement in Lu-doped Bi2Te3

The LuxBi2−xTe3 thermoelectrics with x = 0, 0.05, 0.1 and 0.2 have been prepared by microwave-solvothermal method and spark plasma sintering. All the compositions are semiconductors of n-type conductivity. It was found that electron concentration increases and electron mobility decreases with x increasing. The Lu doping results in remarkable increase of the thermoelectric figure-of-merit from ∼0.4 for undoped Bi2Te3 up to ∼0.9 for Bi1.9Lu0.1Te3. Enhancing the thermoelectric efficiency at the doping is originated from: (i) increase of the electron concentration since the Lu atoms behave as donors in the Bi2Te3 lattice that decreases the specific electrical resistance, (ii) increase of the Seebeck coefficient via increase of the density-of-states effective mass for conduction band, (iii) decrease of the total thermal conductivity via forming the point defects like antisite defects and Lu atoms substituting for the Bi sites. Formation of narrow non-parabolic impurity band lying near the Fermi energy with sharp density of states is believed to be responsible for increasing the density-of-states effective mass and decreasing the electron contribution to the total thermal conductivity.

Effects of Lu and Tm Doping on Thermoelectric Properties of Bi2Te3 Compound

The Bi2Te3, Bi1.9Lu0.1Te3 and Bi1.9Tm0.1Te3 thermoelectrics of n-type conductivity have been prepared by the microwave-solvothermal method and spark plasma sintering. These compounds behave as degenerate semiconductors from room temperature up to temperature T d ≈ 470 K. Within this temperature range the temperature behavior of the specific electrical resistivity is due to the temperature changes of electron mobility determined by acoustic and optical phonon scattering. Above T d, an onset of intrinsic conductivity takes place when electrons and holes are present. At the Lu and Tm doping, the Seebeck coefficient increases, while the specific electrical resistivity and total thermal conductivity decrease within the temperature 290–630 K range. The increase of the electrical resistivity is related to the increase of electron concentration since the Tm and Lu atoms are donor centres in the Bi2Te3 lattice. The increase of the density-of-state effective mass for conduction band can be responsible for the increase of the Seebeck coefficient. The decrease of the total thermal conductivity in doped Bi2Te3 is attributed to point defects like the antisite defects and Lu or Tm atoms substituting for the Bi sites. In addition, reducing the electron thermal conductivity due to forming a narrow impurity (Lu or Tm) band having high and sharp density-of-states near the Fermi level can effectively decrease the total thermal conductivity. The thermoelectric figure-of-merit is enhanced from ∼ 0.4 for undoped Bi2Te3 up to ∼ 0.7 for Bi1.9Tm0.1Te3 and ∼ 0.9 for Bi1.9Lu0.1Te3.

Thermoelectric Properties of Bi₂Te₃: CuI and the Effect of Its Doping with Pb Atoms.

In order to understand the effect of Pb-CuI co-doping on the thermoelectric performance of Bi2Te3, n-type Bi2Te3 co-doped with x at % CuI and 1/2x at % Pb (x = 0, 0.01, 0.03, 0.05, 0.07, and 0.10) were prepared via high temperature solid state reaction and consolidated using spark plasma sintering. Electron and thermal transport properties, i.e., electrical conductivity, carrier concentration, Hall mobility, Seebeck coefficient, and thermal conductivity, of CuI-Pb co-doped Bi2Te3 were measured in the temperature range from 300 K to 523 K, and compared to corresponding x% of CuI-doped Bi2Te3 and undoped Bi2Te3. The addition of a small amount of Pb significantly decreased the carrier concentration, which could be attributed to the holes from Pb atoms, thus the CuI-Pb co-doped samples show a lower electrical conductivity and a higher Seebeck coefficient when compared to CuI-doped samples with similar x values. The incorporation of Pb into CuI-doped Bi2Te3 rarely changed the power factor because of the trade-off relationship between the electrical conductivity and the Seebeck coefficient. The total thermal conductivity(κtot) of co-doped samples (κtot ~ 1.4 W/m∙K at 300 K) is slightly lower than that of 1% CuI-doped Bi2Te3 (κtot ~ 1.5 W/m∙K at 300 K) and undoped Bi2Te3 (κtot ~ 1.6 W/m∙K at 300 K) due to the alloy scattering. The 1% CuI-Pb co-doped Bi2Te3 sample shows the highest ZT value of 0.96 at 370 K. All data on electrical and thermal transport properties suggest that the thermoelectric properties of Bi2Te3 and its operating temperature can be controlled by co-doping.

Effect of doping with rare-earth elements (Eu, Tb, Dy) on the conductivity of Bi2Te3 layered single crystals

The temperature dependences of the resistivity in the directions parallel and perpendicular to the layer plane in the range of temperatures T = 5–300 K and the Hall and transverse magnetoresistance effects (magnetic fields <80 kOe, T = 5 K) are studied for doped and undoped Bi2Te3 layered single crystals. It is shown that, upon the doping of Bi2Te3 crystals with atoms of rare-earth elements (Eu, Tb, Dy), the resistivity in the directions parallel and perpendicular to the layer plane in Bi2Te3 increases. The increase in the resistivity is caused mainly by a decrease in the charge-carrier mobility because of an increased contribution of charge-carrier scattering at defects to scattering processes. The charge-carrier concentrations and mobilities as well as the Hall factor defined by the anisotropy of the effective masses and by the orientation of ellipsoids with respect to the crystallographic axes are estimated.

The p-n conduction type transition in Ge-incorporated Bi2Te3 thermoelectric materials

The capability of converting waste heat into electricity characterizes the technology of thermoelectricity as a feasible solution in dealing with the energy issue. The Bi2Te3-based alloys have long been in the spotlight as they show promising zTs and high thermal stability near the room-temperature region. Herein, the experimentally determined 523 K isothermal section of ternary Bi-Ge-Te system concludes that the four ternary compounds, the Ge3Bi2Te6, the Ge1.5Bi1.5Te5, the Ge1Bi2Te4 and the Ge1Bi4Te7, are thermally stabilized and the solubility of Ge in Bi2Te3 could reach ∼5.0 at.%Ge. The thermoelectric performance of various Ge-incorporated Bi2Te3 alloys with compositions of Bi40-yGexTe60-x+y (x = 0–12.5, y = 0–10.0) are evaluated within 300 K-650 K. The conduction type of those alloys transits from p-type, for the un-doped Bi2Te3, to n-type semiconducting as one of the following conditions are satisfied: (1) the concentration of Te drops to less than 60.0 at.%, or (2) the concentration of Te exceeds 60.0 at.% and the ratios of Ge/Bi is larger than 0.2. The zT peak values reach zT∼0.9 at 325 K for p-type alloy P2 (Bi39.0Ge1.0Te60.0), which features a Bi2Te3 single-phase microstructure, and zT∼0.45 at 525 K for n-type alloy N5 (Bi32.5Ge10Te57.5), which falls in a Bi2Te3+Ge1Bi2Te4+Te three-phase region, respectively.

Thermoelectric transport in surface- and antimony-doped bismuth telluride nanoplates

We report the in-plane thermoelectric properties of suspended (Bi1−xSbx)2Te3 nanoplates with x ranging from 0.07 to 0.95 and thicknesses ranging from 9 to 42 nm. The results presented here reveal a trend of increasing p-type behavior with increasing antimony concentration, and a maximum Seebeck coefficient and thermoelectric figure of merit at x ∼ 0.5. We additionally tuned extrinsic doping of the surface using a tetrafluoro-tetracyanoquinodimethane (F4-TCNQ) coating. The lattice thermal conductivity is found to be below that for undoped ultrathin Bi2Te3 nanoplates of comparable thickness and in the range of 0.2–0.7 W m−1 K−1 at room temperature.

Friction Consolidation Processing of n-Type Bismuth-Telluride Thermoelectric Material

Refined grain sizes and texture alignment have been shown to improve transport properties in bismuth-telluride (Bi2Te3) based thermoelectric materials. In this work we demonstrate a new approach, called friction consolidation processing (FCP), for consolidating Bi2Te3 thermoelectric powders into bulk form with a high degree of grain refinement and texture alignment. FCP is a solid-state process wherein a rotating tool is used to generate severe plastic deformation within the Bi2Te3 powder, resulting in a recrystallizing flow of material. Upon cooling, the far-from-equilibrium microstructure within the flow can be retained in the material. FCP was demonstrated on n-type Bi2Te3 feedstock powder having a −325 mesh size to form pucks with a diameter of 25.4 mm and thickness of 4.2 mm. Microstructural analysis confirmed that FCP can achieve highly textured bulk materials, with sub-micrometer grain size, directly from coarse feedstock powders in a single process. An average grain size of 0.8 μm was determined for regions of one sample and a multiple of uniform distribution (MUD) value of 15.49 was calculated for the (0001) pole figure of another sample. These results indicate that FCP can yield ultra-fine grains and textural alignment of the (0001) basal planes in Bi2Te3. ZT = 0.37 at 336 K was achieved for undoped stoichiometric Bi2Te3, which approximates literature values of ZT = 0.4–0.5. These results point toward the ability to fabricate bulk thermoelectric materials with refined microstructure and desirable texture using far-from-equilibrium FCP solid-state processing.

나노구조를 기반으로 하는 Bi2Te3소결과 그 시간에 따른 열전 특성

Thermoelectric materials have been the topic of intensive research due to their unique dual capability of directly converting heat into electricity or electrical power into cooling or heating. Bismuth telluride (Bi2Te3) is the best-known commercially used thermoelectric material in the bulk form for cooling and power generation applications In this work we focus on the large scale synthesis of nanostructured undoped bulk nanostructured Bi2Te3 materials by employing a novel bottom-up solution-based chemical approach. Spark plasma sintering has been employed for compaction and sintering of Bi2Te3 nanopowders, resulting in relative density of g⋅cm -3 while preserving the nanostructure. The average grain size of the final compacts was obtained as 200 nm after sintering. An improved NS bulk undoped Bi2Te3 is achieved with sintered at 400°C for 4 min holding time.

Paramagnetic Cu-doped Bi2Te3 nanoplates

Uniform Cu-doped Bi2Te3 hexagonal nanoplates with widths of ∼200 nm and thicknesses of ∼20 nm were synthesized using a solvothermal method. According to the structural characterization and compositional analysis, the Cu2+ ions were found to substitute Bi3+ ions in the lattice. High-level Cu doping induces a lattice distortion and decreases the crystal lattice by 1.17% in the a axis and 2.38% in the c axis. A paramagnetic state is observed in these nanoplates from 2 to 295 K, which is a significant difference from their diamagnetic un-doped Bi2Te3.

Thermopower Enhancement of Bi2Te3 Films by Doping I Ions

The thermoelectric properties of I-doped Bi2Te3 films grown by metal-organic chemical vapor deposition have been studied. I-doped epitaxial (00l) Bi2Te3 films were successfully grown on 4° tilted GaAs (001) substrates at 360 °C. I concentration in the Bi2Te3 films was easily controlled by the variation in a flow rate of H2 carrier gas for the delivery of an isopropyliodide precursor. As I ions in the as-grown Bi2Te3 films were not fully activated, they did not influence the carrier concentration and thermoelectric properties. However, a post-annealing process at 400 °C activated I ions as a donor, accompanied with an increase in the carrier concentration. Interestingly, the I-doped Bi2Te3 films after the post-annealing process also exhibited enhancement of the Seebeck coefficient at the same electron concentration compared to un-doped Bi2Te3 films. Through doping I ions into Bi2Te3, the thermopower was also enhanced in Bi2Te3, and a high power factor of 5 × 10−3 W K−2 m−1 was achieved.

Enhanced thermoelectric properties of Bi2Te3 doping Zn4Sb3 by combinatorial approach

Materials derived from Bi2Te3 compounds are known to have advantageous thermoelectric properties at room temperatures, with the potential to be used in either cooling or power generating applications. In this paper, the effect of doping Bi2Te3 thin films with Zn4Sb3 is studied using a combinatorial approach. Findings from this study demonstrate the structural and thermoelectric properties of Bi2Te3/(Zn4Sb3)x thin films grown with varying Zn4Sb3 concentrations ranging from x=1at.% to 16.5at.%. The crystalline phase and variations in the composition of the thin films were determined using scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). When doping Bi2Te3 with Zn4Sb3 concentrations of 5.1at.%, a dramatic change in electrical resistivity was observed. It was found that the electrical resistivity decreased from 6.68mΩ-cm to 1.63mΩ-cm when compared with undoped Bi2Te3, at room temperature. The power factor of Bi2Te3/(Zn4Sb3)x (x=5.1at.%) is 2.4μW/K2cm, which is higher than as-grown Bi2Te3 (1.4μW/K2cm), even though the Seebeck coefficient (−65.8μV/K) is slightly less than Bi2Te3 (−96.6μV/K). The observed effects on the thermoelectric properties of these materials are extremely important for understanding the fundamental characteristics of Zn4Sb3/Bi2Te3 embedded thermoelectric composites.

Synthesis, processing, and thermoelectric properties of bulk nanostructured bismuth telluride (Bi2Te3)

Bismuth telluride (Bi2Te3) is the best-known commercially used thermoelectric material in the bulk form for cooling and power generation applications at ambient temperature. However, its dimensionless figure-of-merit-ZT around 1 limits the large-scale industrial applications. Recent studies indicate that nanostructuring can enhance ZT while keeping the material form of bulk by employing an advanced synthetic process accompanied with novel consolidation techniques. Here, we report on bulk nanostructured (NS) undoped Bi2Te3 prepared via a promising chemical synthetic route. Spark plasma sintering has been employed for compaction and sintering of Bi2Te3 nanopowders, resulting in very high densification (>97%) while preserving the nanostructure. The average grain size of the final compacts was obtained as 90 ± 5 nm as calculated from electron micrographs. Evaluation of transport properties showed enhanced Seebeck coefficient (−120 μV K−1) and electrical conductivity compared to the literature state-of-the-art (30% enhanced power factor), especially in the low temperature range. An improved ZT for NS bulk undoped Bi2Te3 is achieved with a peak value of ∼1.1 at 340 K.