Bi2Te3 Nanowires Research Articles

The influence of a Te-depleted surface on the thermoelectric transport properties of Bi2Te3 nanowires

We report on thermoelectric transport measurements along the basal plane of several individual, single-crystalline Bi2Te3 nanowires (NWs) with different cross-sectional areas, grown by a vapor–liquid–solid method. Lithographically defined microdevices allowed us to determine the Seebeck coefficient S, electrical conductivity σ, and thermal conductivity κ of individual NWs. The NWs studied show near intrinsic transport properties with low electrical conductivities of around σ = (3.2 ± 0.9) × 104 Ω−1 m−1 at room temperature. We observe a transition of the Seebeck coefficient from positive to negative values (S = +133 μVK−1 to S = −87 μVK−1) with increasing surface-to-volume ratio at room temperature, which can be explained by the presence of an approximately 5 nm thick Te-depleted layer at the surface of the NWs. The thermal conductivities of our NWs are in the range of κ = (1.4 ± 0.4) Wm−1 K−1 at room temperature, which is lower than literature values for bulk Bi2Te3. We attribute this suppression in thermal conductivity to enhanced scattering of phonons at the surface of the NWs. Despite their reduced thermal conductivities, the NWs investigated only show a moderate figure of merit between 0.02 and 0.18 due to their near intrinsic transport properties.

Thermal Conductivity Measurements of Bi2Te3 Nanostructure By Raman Thermography

Single-crystal Bi2Te3 nanowire and nanoribbon were synthesized by using a vapor-liquid-solid method with Bi2Te3 powder. The composition and crystalline structure of nanowires were confirmed by energy-dispersive spectroscopy (EDS) and high resolution transmission electron microscopy (HRTEM), respectively. To investigate the thermal properties of Bi2Te3 nanostructure, a nondestructive technique based on Raman temperature mapping was carried out. The red shift of Raman peaks and peak broadening observed as temperature increased. In addition, the fraction of the laser power absorbed inside the Bi2Te3 nanostructure was estimated by optical simulation for calculation of thermal conductivity value (κ). The obtained thermal conductivity value of Bi2Te3 nanowire and nanoribbon is 2.67 Wm-1K-1 and 2.81 Wm-1K-1, respectively. The difference in thermal conductivity value between nanowire and nanoribbon is caused by the dimension difference.

An Electron Microscopic Investigation of (1/3) < 0ī11 > Dislocations in Bi2Te3 Nanowires: Defect Crystallography and Relationship to 7-layer Bi3Te4 Defects.

An Electron Microscopic Investigation of (1/3) < 0ī11 > Dislocations in Bi₂Te₃ Nanowires: Defect Crystallography and Relationship to 7-layer Bi₃Te₄ Defects.

Thermal transport along Bi2Te3 topological insulator nanowires

Using molecular dynamics simulations, we investigate the thermal conductivity and local heat flux distribution of Bi2Te3 nanowires. It is found that at room temperature, the converged length-independent thermal conductivity of Bi2Te3 nanowires is only 0.89 W/m K, which is about 2-fold lower than their bulk counterpart. Interestingly, the local heat flux density along the quintet boundary layer is only about 18% of that along the central layers due to different phonon edge scattering intensities. Our work demonstrates that topological insulator nanostructures are promising candidates for the development of high-performance thermoelectric devices for applications in nanoscale energy generation and temperature management.

Resolving the Dirac cone on the surface of Bi2Te3 topological insulator nanowires by field-effect measurements

We validate the linear dispersion relation (Dirac cone) on the surface of a single Bi2Te3 nanowire via a combination of field-effect and magnetoresistance measurements by which we unambiguously prove the topological insulator nature of the nanowire surface states. Moreover, we show that the experimentally determined carrier concentration, mobility, and cyclotron mass of the surface states are in excellent agreement with relativistic models. Our method provides a facile way to identify topological insulators that too small for angle-resolved photoemission spectroscopy.

Thermopower detection of single nanowire using a MEMS device

We present a MEMS-based device on a silicon nitride membrane in order to measure the thermoelectric properties of a single nanowire. A temperature gradient along a nanowire was generated by a nanoheater, and the temperature was measured by Pt thermometers. A thermal simulation using a finite element method was conducted to analyze the temperature distribution over the MEMS device. The validity of the MEMS device was established by testing the Pt nanowires which had different symmetry configurations. From the test results of Pt nanowires, a convincing temperature calibration method was proposed and applied to an actual case of Bi2Te3 nanowire. We measured a Seebeck coefficient of −53μV/K and electrical conductivity of 2.23×105S/m for a single Bi2Te3 nanowire with a diameter of 70nm at 300K. Our solid design for thermoelectric measurements based on a membrane structure enables the fast and high-yield characterization of one-dimensional nanostructures.

Dissociated $$ \frac{1}{3}\langle 0\bar{1}11\rangle $$ 1 3 〈 0 1 ¯ 11 〉 dislocations in Bi2Te3 and their relationship to seven-layer Bi3Te4 defects

We investigate the structure of $$ \frac{1}{3}\langle 0\bar{1}11\rangle $$ dislocations observed in Bi2Te3 nanowires. This particular type of dislocation is interesting because it has a large Burgers vector (b = 1.048 nm) with a component normal to the basal planes equal to the thickness of one full Bi2Te3 quintuple unit (i.e., c/3). Atomic-resolution high-angle annular dark-field scanning transmission electron microscopy observations show that the dislocations form with a complex dissociated core structure. This structure consists of two partial dislocations that separate a defected region consisting of a seven-plane-thick septuple unit, consistent with a local patch of Bi3Te4, rather than the normal Bi2Te3 quintuple layer structure. As we discuss, details of the core structure can be understood from an analysis of the crystallographic parameters of the observed partial dislocations. This analysis suggests a mechanism to accommodate the loss of tellurium through the heterogeneous nucleation and growth of seven-layer defects at $$ \frac{1}{3}\langle 0\bar{1}11\rangle $$ —type dislocations.

Large-scale solution-phase production of Bi₂Te₃ and PbTe nanowires using Te nanowire templates.

We report the first demonstration of large scale (>10 g) and high yield (>80%) production of ultrathin (<15 nm) PbTe and Bi₂Te₃ nanowires in a low-cost solution-based process using Te nanowire templates. The PbTe or Bi₂Te₃ nanowires can be compressed into high relative density disks with nanoscale grains.

Electrodeposition of Bi2Te3 Nanowire Arrays with Uniform Length and Controlled Composition in Anodic Aluminum Templates

Bismuth telluride (Bi2Te3) nanowire arrays with a uniform length and controlled composition were electrodeposited in anodic aluminum oxide (AAO) templates by controlling the applied potential and electrolytes concentration. The length and the contents of the electrodeposited nanowires were dependent on the ion ratio of Bi3+:HTeO2+ in the electrolyte as well as the applied potential. Te-rich BixTey nanowires could be obtained in a Te-rich electrolyte solution at relatively slow growth rates of 7.9–18.6 μm/h while both stoichiometric and Bi-rich deposits were prepared at high growth rates of 8.4–41.4 μm/h in Bi-rich electrolytes. Cyclic voltammetry (CV) was conducted for electrolytes with different ion ratios at voltages in the range of −0.7–0.8 V. Consequently, we found that slow nucleation and a steady growth rate in the underpotential deposition (UPD) region were needed to assure the uniformity and stoichiometry of as-deposited nanowires. The microstructure and crystallographic texture were examined by scanning electron microscopy and X-ray diffraction. This study can be extended to the electrodeposition of stoichiometric chalcogenide compounds derived by UPD with controlled length distribution.

Surface state dominated transport in topological insulator Bi2Te3 nanowires

We report on low temperature magnetoresistance measurements on single-crystalline Bi2Te3 nanowires synthesized via catalytic growth and post-annealing in a Te-rich atmosphere. The observation of Aharonov-Bohm oscillations indicates the presence of topological surface states. Analyses of Subnikov-de Haas oscillations in perpendicular magnetoresistance yield extremely low two-dimensional carrier concentrations and effective electron masses, and very high carrier mobilities. All our findings are in excellent agreement with theoretical predictions of massless Dirac fermions at the surfaces of topological insulators.

Thermal diffusivity measurements of templated nanocomposite using infrared thermography

Electrodeposited bismuth telluride (Bi2Te3) nanowires using anodic alumina (AAO) templates show anisotropic thermal properties in a direction parallel and perpendicular to the nanowire/nanochannel axis. The present study reports thermal diffusivity measurement of templated Bi2Te3 nanowires in a direction perpendicular to the nanowire axis, using an active infrared thermography based noncontact technique. Measurements were performed on empty AAO templates of varying pore dimensions, AAO/Bi2Te3 nanocomposites and a single crystal Bi2Te3 sample. A first order lower bound model estimation showed nearly five-fold reduction of thermal conductivity in 200nm Bi2Te3 nanowires as compared to the bulk values.

Dual evidence of surface Dirac states in thin cylindrical topological insulator Bi2Te3 nanowires

How the surface state (SS) develops and how the spin transport in a curved cylindrical topological insulator nanowire have attracted theoretical attention recently. However, experimental confirmation for the SS in such a real modeling system still remains insufficient. Here we carried out a systematic comparative study on the cylindrical single-crystal Bi2Te3 nanowires of various diameters, and report unambiguously dual evidence for the Dirac SS. Both the predicted anomalous Aharonov-Bohm (AB) quantum oscillations with a period of h/e in H// and the 1/2-shifted Shubnikov-de Haas (SdH) oscillations (i.e., γ = −1/2) in H⊥ were indentified below 1.4 K. In addition, Altshuler-Aronov-Spivak (AAS)-like oscillations with a period of h/2e and ordinary SdH oscillations with γ = 0 were also resolved. These data provide clear evidence of coexistence of the nontrivial topological Dirac state and trivial electron state on the surface of topological insulator nanowire.

Fabrication of Bi2Te3 nanowire arrays and thermal conductivity measurement by 3ω-scanning thermal microscopy

Bi2Te3 is well-known for its utility in thermoelectrical applications and more recently as topological insulator. Its nanostructuration has attracted plenty of attention because of its potential capacity to reduce thermal conductivity. Here, we have grown a composite sample made of a Bi2Te3 nanowires (NWs) array embedded in an alumina matrix. We have then performed scanning thermal microscopy (SThM) in a 3ω configuration to measure its equivalent thermal resistance. Using an effective medium model, we could then estimate the mean composite thermal conductivity as well as the thermal conductivity of the NWs to be, respectively, (λC) = (1.68 ± 0.20) W/mK and (λNW) = (1.37 ± 0.20) W/mK, showing a slight thermal conductivity reduction. Up to now, there have been two main techniques reported in literature to evaluate the thermal conductivity of nanostructures: the use of a thermal microchip to probe a single NW once its matrix has been dissolved or the probing of the whole NWs array embedded in a matrix, obtaining the thermal conductivity of the whole as an effective medium. However, the 3ω-SThM presented here is the only technique able to measure the thermal conductivity of single NWs embedded in a matrix as well as the thermal conductivity of the composite locally. This technique is more versatile and straightforward than other methods to obtain the thermal conductivity of nanostructures.

Bi2Te3纳米线的液相法合成及其结构表征

利用热电材料能够实现温差发电和热电制冷，所以可以有效地缓解日趋严重的能源危机，Bi2Te3目前是室温条件下热电性能最好的热电材料。材料的低维化能够很大程度地提升其自身的性能，Bi2Te3纳米线具有更好的热电转换效率。利用液相法能够很成功和方便地在实验室合成Bi2Te3纳米线，实验中先反应生成Te纳米线胶体溶液，然后再在其基础上合成Bi2Te3纳米线。热处理温度选定为170℃，研究发现加热反应的时间会显著影响所制备样品的结晶性。实验中按照不同的加热反应时间来分成三组进行对比，通过X射线衍射(XRD)、扫描电镜(SEM)和透射电镜(TEM)分析其结构和形貌，对比得知加热90 min所制备的Bi2Te3纳米线的结晶度和产率都比加热30 min和60 min的要好，表明液相法合成Bi2Te3纳米线需要一个较长的反应过程，适量延长加热反应时间有助于获得较高结晶质量的Bi2Te3纳米线。 Thermoelectric materials, which can covert heat to electric energy for power generation and conversely con- vert electric energy to heat energy for refrigeration, thereby alleviate the growing energy crisis effectively.Bi2Te3 is currently the best thermoelectric material for room temperature applications. Materials of lower dimensions can be used to obtain better properties, and therefore,Bi2Te3 nanowires have higher thermoelectric conversion efficiencies. In this work, we prepared Bi2Te3 nanowires very conveniently by using the liquid-phase synthesis method. Firstly, we syn- thesized Te nanowires colloidal solution, and then synthesized Bi2Te3 nanowires on the basis of it. The heating tem- perature was optimally set at 170˚C, while it was found that the heating reaction time affected greatly the crystallinity of the Bi2Te3 nanowires. Three sample groups were divided according to different heating reaction time. After synthesis, we analyzed systematically their structures and morphologies by XRD, SEM, and TEM. The crystallinity and yield of the third sample group, whose heating reaction time is 90 min, is much better than those of the other two groups, namely 30 min and 60 min, indicating that a long reaction process is required for the liquid-phase synthesis of Bi2Te3 nanowires and thus lengthening appropriately the heating reaction time is helpful in achieving highly crystalline Bi2Te3 nanowires.

Optimized thermoelectric performance of Bi2Te3 nanowires

The structural and electronic properties of a series of stoichiometric Bi2Te3 nanowires with two growth orientations [110] and [210] are studied by using density functional calculations. Our results indicate that the nanowires with [110] orientation are energetically more favorable than those with [210] orientation. All the investigated Bi2Te3 nanowires are found to be semiconducting and the band gaps of [110] nanowires monotonically increase with the decreasing cross-sectional width. For the [210] orientation, however, the band gaps exhibit an interesting width-dependent even–odd oscillation behavior. The electronic transport properties of these nanowires are then evaluated by using the semi-classical Boltzmann theory with the relaxation time approximation. For the phonon transport, the lattice thermal conductivity is predicted by using the non-equilibrium molecule dynamics simulations. Our theoretical calculations suggest that the thermoelectric performance of Bi2Te3 nanowires can be optimized at appropriate carrier concentration with particular orientation and cross-sectional size. The figure of merit (ZT value) can reach as high as 2.3 at 300 K and 2.5 at 350 K for the [210] nanowire with the width N = 5.

Phonon spectroscopy in a Bi2Te3 nanowire array

The lattice dynamics in an array of 56 nm diameter Bi2Te3 nanowires embedded in a self-ordered amorphous alumina membrane were investigated microscopically using (125)Te nuclear inelastic scattering. The element specific density of phonon states is measured on nanowires in two perpendicular orientations and the speed of sound is extracted. Combined high energy synchrotron radiation diffraction and transmission electron microscopy was carried out on the same sample and the crystallinity was investigated. The nanowires grow almost perpendicular to the c-axis, partly with twinning. The average speed of sound in the 56 nm diameter Bi2Te3 nanowires is ~7% smaller with respect to bulk Bi2Te3 and a decrease in the macroscopic lattice thermal conductivity by ~13% due to nanostructuration and to the reduced speed of sound is predicted.

Nanostructure, Excitations, and Thermoelectric Properties of Bi2Te3-Based Nanomaterials

The effect of dimensionality and nanostructure on thermoelectric properties in Bi2Te3-based nanomaterials is summarized. Stoichiometric, single-crystalline Bi2Te3 nanowires were prepared by potential-pulsed electrochemical deposition in a nanostructured Al2O3 matrix, yielding transport in the basal plane. Polycrystalline, textured Sb2Te3 and Bi2Te3 thin films were grown at room temperature using molecular beam epitaxy and subsequently annealed at 250°C. Sb2Te3 films revealed low charge carrier density of 2.6 × 1019 cm−3, large thermopower of 130 μV K−1, and large charge carrier mobility of 402 cm2 V−1 s−1. Bi2(Te0.91Se0.09)3 and (Bi0.26Sb0.74)2Te3 nanostructured bulk samples were prepared from as-cast materials by ball milling and subsequent spark plasma sintering, yielding grain sizes of 50 nm and thermal diffusivities reduced by 60%. Structure, chemical composition, as well as electronic and phononic excitations were investigated by x-ray and electron diffraction, nuclear resonance scattering, and analytical energy-filtered transmission electron microscopy. Ab initio calculations yielded point defect energies, excitation spectra, and band structure. Mechanisms limiting the thermoelectric figure of merit ZT for Bi2Te3 nanomaterials are discussed.

Design and fabrication of a thermoelectric nanowire characterization platform and nanowire assembly by utilizing dielectrophoresis

We demonstrate the fabrication of a novel micromachined Thermoelectric Nanowire Characterization Platform (TNCP) which will be utilized to characterize the thermoelectric properties of a single thermoelectric Bi2Te3 nanowire. The TNCP is composed of two adjacent, symmetric and suspended Si cantilevers with a length of 1000μm, a width of 100μm and a thickness of 10μm. Microheaters, temperature sensors and electrodes are fabricated on both cantilevers. Furthermore, a V-groove is integrated on the tip of each cantilever, in order to accommodate one single nanowire inside. The integration of the single nanowire onto the TNCP is conducted by means of dielectrophoresis (DEP) in combination with a controlled deposition and evaporation of a water droplet where the nanowires are dispersed. In addition to the DEP application, surface modifications are applied to form hydrophilic and hydrophobic surfaces on different parts of the cantilevers, to improve the water droplet deposition. In the end, one single nanowire is successfully integrated in the V-grooves bridging the cantilevers and ready for the further thermoelectric properties measurements. Within a series of 200 experiments, approximately 10% of them are successfully observed with one nanowire situated in the V-grooves as desired. The optimal applied voltage and frequency parameters for the DEP deposition process are in the range from 5V to 8V, and from 150kHz to 8MHz respectively.

Tuning the Geometrical and Crystallographic Characteristics of Bi2Te3 Nanowires by Electrodeposition in Ion-Track Membranes

We report the fabrication of Bi2Te3 nanowires with diameters as small as 15 nm, which is comparable to the size theoretically estimated for the onset of improvement of the thermoelectric figure of merit ZT by quantum-size effects. The versatility of the template-assisted growth, combining self-prepared ion-track etched membranes and electrochemical deposition, has been employed to synthesize Bi2Te3 nanowires with controlled diameters in 10, 30, and 60 μm thick membranes and with large aspect ratios (length over diameter) of up to 1000. SEM, HRTEM, and XRD investigations reveal how morphology, surface roughness, and crystalline orientation of the Bi2Te3 nanowires depend on deposition potential, temperature, and channel diameter.

Single-Crystalline, Stoichiometric Bi2Te3 Nanowires for Transport in the Basal Plane

Bi2Te3 nanowires were grown in a nanostructured Al2O3 matrix by potential-pulsed electrochemical deposition. The wires had diameters of 50 nm to 80 nm and length of about 56 μm. Cross-sections prepared for scanning electron microscopy revealed uniform growth with deviations in length of less than 10%. The wires were deposited with reduction potentials between −150 mV and −250 mV. A reduction potential of −200 mV yielded an average Te content of 63.2 at.% as determined by calibrated, high-accuracy energy-dispersive x-ray spectrometry in transmission electron microscopy (TEM). The chemical composition was uniform along the length of the nanowires but depended on their diameter. The Te content increased by about 1 at.% when the diameter of the nanowires decreased from 80 nm to 50 nm. Selected-area electron diffraction combined with dark-field TEM imaging revealed single-crystalline wires; no grain boundaries were detected. The nanowires grew along the [110] and [210] directions; the c axis was perpendicular to the wire axis, allowing basal plane transport unaffected by grain boundaries.