Bi2Te3 Nanowires Research Articles

Spin-dependent scattering induced negative magnetoresistance in topological insulator Bi2Te3 nanowires

Studies of negative magnetoresistance in novel materials have recently been in the forefront of spintronic research. Here, we report an experimental observation of the temperature dependent negative magnetoresistance in Bi2Te3 topological insulator (TI) nanowires at ultralow temperatures (20 mK). We find a crossover from negative to positive magnetoresistance while increasing temperature under longitudinal magnetic field. We observe a large negative magnetoresistance which reaches −22% at 8 T. The interplay between negative and positive magnetoresistance can be understood in terms of the competition between dephasing and spin-orbit scattering time scales. Based on the first-principles calculations within a density functional theory framework, we demonstrate that disorder (substitutional) by Ga+ ion milling process, which is used to fabricate nanowires, induces local magnetic moments in Bi2Te3 crystal that can lead to spin-dependent scattering of surface and bulk electrons. These experimental findings show a significant advance in the nanoscale spintronics applications based on longitudinal magnetoresistance in TIs. Our experimental results of large negative longitudinal magnetoresistance in 3D TIs further indicate that axial anomaly is a universal phenomenon in generic 3D metals.

Effect of deposition potential and concentration of electroactive substances on Bi2Te3 nanowires fabricated by electrochemical method

In this paper, Bi2Te3 nanowires were prepared in anodized aluminum oxide template by electrochemical deposition. The morphological microstructural and electrical resistance characteristics of the nanowires were discussed to reveal the effect of deposition potential and electroactive substance (HTeO2+) concentration. According to the electrode dynamics formula, it is found that the increase of electrode potential leads to the decrease of deposition current, so that deposition rate of nanowires decreases. At the same time, the deposition current controlled by diffusion in the mass transport process will have a maximum value with the increasing of deposition time. The deposition potential determines the favorable crystal plane for nanowires growth by the selection of proper surface energy. The temperature dependence of resistances in Bi2Te3 nanowires fabricated under different concentration of HTeO2+ reveals the transformation of the carriers’ main scattering mechanism. This study could provide a better understanding of the deposition process of Bi2Te3 nanowires.

Contributions to composite conductivity and Seebeck coefficient in commercial Bi2Te3—Conjugated polymer composites

We demonstrated the use of as-received conjugated polymer P3HT [poly (3-hexylthiophene-2,5 diyl)] doped with F4TCNQ (2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane) as a matrix for forming a composite with as-received, commercially available p-type Bi2Te3 powder. The optimized formulation exhibits a power factor of up to 5.3μWK−2m−1, about nine times higher than the highest power factor that we achieved from mixtures of only P3HT and F4TCNQ. Bi2Te3 was responsible for increases in both the Seebeck coefficient and the electrical conductivity. P3HT, with a higher hole mobility, was superior to PQT-12 [poly(bisdodecylquaterthiophene)], and F4TCNQ was at least as good as FeCl3, for matrix and dopant, respectively, for this purpose. The power factor obtained is about 40% of that reportedly obtained from synthesized Bi2Te3 nanowires in FeCl3-doped P3HT. We calculated the expected contributions of the bulk Bi2Te3 to the composite conductivity and then examined the resistance caused by interfaces on four different size distributions of Bi2Te3 particles, as well as a solid macroscopic ingot. A nonlinear I–V relationship was found for the doped P3HT-ingot bilayer. While our doped conjugated polymer system made only from commercial-grade components was shown to support the extraction of thermoelectric performance by a commonly used inorganic semiconductor, our results also suggest that an advantage of the smallest Bi2Te3 domains, including nanowires, may arise from their having less interfacial resistance than larger Bi2Te3 particles and pieces.

Phonon-electron coupling and nonlocal heat transport in Bi2Te3 nanowires

A hydrodynamical model of heat transfer with phonons and electrons is applied to infer the steady-state temperature profile in a Bi2Te3 nanowire. Our analysis is concerned with systems which allow for different temperatures of phonons and electrons. Along with the approach of kinetic theory, the phonons and the electrons are regarded as a gas-like mixture flowing through the crystal lattice. The coupling between phonons and electrons is taken into account by an additional term in the partial balances of energy of the components of the mixture. The compatibility of the model with the second law of thermodynamics is investigated in view of the requirement of positive entropy production, and of a nonlocal constitutive equation for the entropy flux.

Manipulating thermal and electronic transports in thermoelectric Bi2Te3 nanowires by porphyrin adsorption

Decoupling the electronic thermal and electrical conductivities is one of the limitations hindering a breakthrough in thermoelectric efficiency. After a conformal surface coating of bismuth telluride nanowires (Bi2Te3 NWs) by porphyrins, the thermal conductivity increases from 0.8 to 1.0 Wm-1K-1 at 300 K without any obvious change in electrical conductivity. Density Functional Theory (DFT) calculations assisted by Boltzmann Transport Equation (BTE) simulations of electronic transport properties indicate that the electronic thermal transport is enhanced by the depletion of surface charge carriers, which results in transition from metallic to semiconducting behavior. Thus, the adsorption of porphyrin onto the Bi2Te3 NWs layer suppresses the surface electronic conduction, resulting in thermal electronic conduction dictated by the bulk of the NW. The results mean that electronic thermal transport can be decoupled from the electrical conductivity by changing the density of surface states on Bi2Te3 NWs.

In-situ synthesis of flexible hybrid composite films for improved thermoelectric performance

Thermoelectric (TE) materials and devices, which enable direct conversion of thermal energy into electricity and vice versa, are a favorable technology for waste heat recovery and flexible energy generators and coolers. Here, we report the enhanced thermoelectric properties of flexible hybrid films composed of carbon nanotubes (CNTs) with good electrical and mechanical properties and inorganic nanowires with high Seebeck coefficient synthesized through an in-situ facile solution method. A two-step interface engineering process is applied to the flexible composite aiming to enhance thermoelectric properties: (1) Bi2Te3 nanowires are grown from the surface of uniformly dispersed CNTs by in-situ synthesis, and (2) a mechanical pressing and a post heat-treatment were employed to form tight interface bonding and adjust the nanowires close to stoichiometric composition. Impressively, the CNTs/Bi2Te3 nanowires composite films exhibited a peak TE power factor of approximately 0.74 mW m−1K−2 at room temperature along with admirable flexibility. The Seebeck coefficient and electrical conductivity were improved by a factor of 30 and 2.2, respectively, through mechanical pressing and post-annealing. This increase is attributed to the phase homogenization of inorganic nanowires to its stoichiometric phase and electron filtering effects due to enhanced interfaces both qualitatively and quantitatively. The flexibility and retention of the conductivity were evaluated via two different methods. This composite with high TE performance will be utilized to create flexible thermoelectric devices for energy harvesting. In addition, the findings from this study can be applied to other flexible thermoelectric materials systems to enhance thermoelectric properties.

Facile preparation and thermoelectric properties of PEDOT nanowires/Bi2Te3 nanocomposites

Poly (3,4-ethylenedioxythiophene) nanowires (PEDOT NWs) with high electric conductivity were synthesized by a facile self-assembled micellar soft-template method. And then, Bi2Te3 powders and Bi2Te3 nanowires (Bi2Te3 NWs) were added as inorganic filler to form the PEDOT NWs/inorganic nanocomposite films by a simple and convenient vacuum filtration method. The thermoelectric (TE) properties of the flexible films were characterized. PEDOT NWs film exhibited the high σ value of 249.5 S cm−1 and does not require any treatment at room temperature. By incorporating both Bi2Te3 powders and Bi2Te3 NWs into these PEDOT NWs, the power factor of the polymer/inorganic composite materials is enhanced. The resulting PEDOT NWs/Bi2Te3 powders nanocomposite film exhibited a high power factor of 7.49 µW m−1 K−2 compared to that of 2.54 µW m−1 K−2 in PEDOT NWs. A maximum power factor of 9.06 µW m−1 K−2 is obtained from the PEDOT NWs/Bi2Te3 NWs composite film containing 10 wt% Bi2Te3 NWs at room temperature, which is about 3 of times that of the pure PEDOT NWs film. These composites provide a promising route to flexible and high-performance thermoelectric materials.

Magnetotransport measurements on Bi2Te3 nanowires electrodeposited in etched ion-track membranes

We report on the magneto-transport properties of individual Bi2Te3 nanowires with diameters ranging from 25 nm to 120 nm fabricated by electrodeposition in etched ion-track membranes. The metallic behavior of all nanowires is typical for highly-degenerate semiconductors and the weak temperature dependence of the resistivity suggests a weak electron-phonon coupling, as expected for topological surface states with quantum confinement. The diameter dependence of the resistivity further confirms the quasi-ballistic nature of charge carriers, in agreement with the reduction of the number of surface conductance channels in disordered quantum wires with a stronger quantum confinement. Although the bulk conductance is not negligible, both the transport length of bulk carriers and their electrostatic screening length remain shorter than the nanowire diameter, probably leading to a small contribution to the resistivity change. Quantum transport measurements give further evidence of finite-size effects, as shown by the weak anti-localization correction to the conductance, below 15K, and reveal the coexistence of surface Aharonov-Bohm oscillations and bulk universal conductance fluctuations, below 5K.

Flexible n-type thermoelectric composite films with enhanced performance through interface engineering and post-treatment

Flexible thermoelectric (TE) materials, which are devices that convert thermal gradients to electrical energy, have attracted interest for practical energy-harvesting/recovery applications. However, as compared with p-type materials, the progress on the development of n-type TE flexible materials has been slow due to difficulties involved in n-type doping techniques. This study used high mobility carbon nanotubes (CNTs) to a uniformly mixed hybrid-composite, resulting in an enhanced power factor by increasing electrical conductivity. The energy filtering effect and stoichiometric composition of the material used, bismuth telluride (Bi2Te3) correlated to a significant enhancement in TE performance, with a power factor of 225.9 μW m−1K−2 at room temperature: a factor of 65 higher than as-fabricated composite film. This paper describes a simplified synthesis for the preparation of the composite film that eliminates time-intensive and cost-prohibitive processing, traditionally seen during extrusion and dicing inorganic manufacturing. The resulting post-annealed composite film consisting of Bi2Te3 nanowire and CNTs demonstrate a promising candidate for material that can be used for an n-type TE device that has improved energy conversion efficiency.

Ex Vivo Study of Telluride Nanowires in Minigut.

Compound semiconductor nanomaterials, such as telluride nanowires, nanorods, and nanoparticles, have many unique properties for wide range of potential applications. The interaction between organoids (a biological system) and telluride nanowires is a crucial research area for human health in terms of its safety concerns. In this study, we demonstrated a case study on Bi2Te3 nanowires. Through live/dead cell viability testing, bright-light image analysis, and surface area calculations, we showed that 50 μg/mL Bi2Te3 exerts minimum influence on shrinking crypts and disrupting lumen structure, which causes unhealthy growth. Within this optimal concentration, Bi2Te3 nanowires can stay as a stable and non-toxic material inside the intestine. Unlike the previous studies of the cytotoxicity of Telluride nanomaterials interacting with single type of cells, our research demonstrated the first study of the interactions of engineered Telluride nanomaterials with a real complex gastrointestinal tract system as our primary small intestinal crypts were directly isolated from mice and grew into a self-renewable system with various types of cells and different cell pathways, which has the capability to mimic a fully functional intestinal epithelium layer for a realistic study inside the gastrointestinal tract. Most importantly, we showed that Bi2Te3 nanowires, under infrared exposure, can act as a potential shield to stimulate cell viability and improve cell survivability.

High thermoelectric properties of (Sb, Bi)2Te3 nanowire arrays by tilt-structure engineering

In this paper, we present an innovative tilt-structure design concept for (Sb, Bi)2Te3 nanowire array assembled by high-quality nanowires with well oriented growth, utilizing a simple vacuum thermal evaporation technique. The unusual tilt-structure (Sb, Bi)2Te3 nanowire array with a tilted angle of 45° exhibits a high thermoelectric dimensionless figure-of-merit ZT = 1.72 at room temperature. The relatively high ZT value in contrast to that of previously reported (Sb, Bi)2Te3 materials and the vertical (Sb, Bi)2Te3 nanowire arrays evidently reveals the crucial role of the unique tilt-structure in favorably influencing carrier and phonon transport properties, resulting in a significantly improved ZT value. The transport mechanism of such tilt-structure is proposed and investigated. This method opens a new approach to optimize nano-structure in thin films for next-generation thermoelectric materials and devices.

Thermal conductivity anisotropy in nanostructures and nanostructured materials

Thermal conductivity anisotropy is a subject for both fundamental and application interests. The anisotropy can be induced either by van der Waals forces in bulk systems or by nanostructuration. Here, we will examine four cases in which thermal anisotropy has been observed: (i) Si/Ge superlattices which exhibit higher thermal anisotropy between in-plane and cross-plane directions for the case of smooth interfaces, (ii) amorphous/crystalline superlattices with much higher anisotropy than the crystalline/crystalline superlattices and which can reach a factor of six when the amorphous fraction increases, (iii) the impact of the density of edge and screw dislocations on the thermal anisotropy of defected GaN, and (iv) the influence of the growth direction of Bi2Te3 nanowires on thermal conductivity.

One-dimensional Bi_2Te_3 nanowire based broadband saturable absorber for passively Q-switched Yb-doped and Er-doped solid state lasers

The broad saturable absorption of topological insulator Bi2Te3 nanowires is presented for the first time, as well as the passively Q-switched pulse operations on both Yb-doped and Er-doped bulk laser with a bandwidth over about 1.7 μm. Uniform Bi2Te3 nanowires with a large aspect ratio have been synthesized using ultrathin Te nanowires as the template. The broad absorption spectrum from visible to middle infrared reveals a broad optical response. In a 1.0 μm Yb-doped laser, a pulse width of 303 ns with a pulse energy of 1.2 µJ is achieved. While at Er-doped mid-infrared 2.79 μm, the shortest pulse width and largest pulse energy are tested to be 444 ns and 5 µJ, respectively. Those results reveal the topological insulator Bi2Te3 nanowire to be a reliable broad optical switcher for solid state lasers in near- and mid-infrared bands. As far as we know, it is for the first time that TIs based solid state lasers at mid-infrared region are reported. Furthermore, this work can be extendable to explore TIs with other nanstructures for application as optoelectronic devices.

Robust broad spectral photodetection (UV-NIR) and ultra high responsivity investigated in nanosheets and nanowires of Bi2Te3 under harsh nano-milling conditions

Due to miniaturization of device dimensions, the next generation’s photodetector based devices are expected to be fabricated from robust nanostructured materials. Hence there is an utmost requirement of investigating exotic optoelectronic properties of nanodevices fabricated from new novel materials and testing their performances at harsh conditions. The recent advances on 2D layered materials indicate exciting progress on broad spectral photodetection (BSP) but still there is a great demand for fabricating ultra-high performance photodetectors made from single material sensing broad electromagnetic spectrum since the detection range 325 nm–1550 nm is not covered by the conventional Si or InGaAs photodetectors. Alternatively, Bi2Te3 is a layered material, possesses exciting optoelectronic, thermoelectric, plasmonics properties. Here we report robust photoconductivity measurements on Bi2Te3 nanosheets and nanowires demonstrating BSP from UV to NIR. The nanosheets of Bi2Te3 show the best ultra-high photoresponsivity (~74 A/W at 1550 nm). Further these nanosheets when transform into nanowires using harsh FIB milling conditions exhibit about one order enhancement in the photoresponsivity without affecting the performance of the device even after 4 months of storage at ambient conditions. An ultra-high photoresponsivity and BSP indicate exciting robust nature of topological insulator based nanodevices for optoelectronic applications.

Perfect quintuple layer Bi2Te3 nanowires: Growth and thermoelectric properties

Bi2Te3 nanowires are promising candidates for thermoelectric applications. Vapor-liquid-solid growth of these nanowires is straightforward, but the traditional Au-catalyzed method is expected to lead to Au contamination and subsequently crystal defects. Here, we present a comparison of the Au-catalyzed growth method with an alternative method using TiO2. We observe that the latter approach results in perfect quintuple layer nanowires, whilst using Au leads to mixed quintuple and septuple layer structures. Despite these differences, we surprisingly find only a negligible effect on their thermoelectric properties, namely conductivity and Seebeck coefficient. This result is relevant for the further optimization and engineering of thermoelectric nanomaterials for device applications.

Weakly-coupled quasi-1D helical modes in disordered 3D topological insulator quantum wires

Disorder remains a key limitation in the search for robust signatures of topological superconductivity in condensed matter. Whereas clean semiconducting quantum wires gave promising results discussed in terms of Majorana bound states, disorder makes the interpretation more complex. Quantum wires of 3D topological insulators offer a serious alternative due to their perfectly-transmitted mode. An important aspect to consider is the mixing of quasi-1D surface modes due to the strong degree of disorder typical for such materials. Here, we reveal that the energy broadening γ of such modes is much smaller than their energy spacing Δ, an unusual result for highly-disordered mesoscopic nanostructures. This is evidenced by non-universal conductance fluctuations in highly-doped and disordered Bi2Se3 and Bi2Te3 nanowires. Theory shows that such a unique behavior is specific to spin-helical Dirac fermions with strong quantum confinement, which retain ballistic properties over an unusually large energy scale due to their spin texture. Our result confirms their potential to investigate topological superconductivity without ambiguity despite strong disorder.

A simple thermoelectric device based on inorganic/organic composite thin film for energy harvesting

Thermoelectric materials have been widely used in the power generation and cooling. Tellurium (Te) nanomaterial has gained great attention due to its enhanced thermoelectric performance, however, the researches are lacking of practical applications for energy harvesting. Here, a simple thermoelectric device was built with n-type and p-type Te-based (Bi2Te3 and Te-PEDOT:PSS composite) nanowires (NWs) thin film as legs for the first time. Firstly, a facile solution method was used to synthesize Te and Bi2Te3 NWs. A composite thin film was composed of Te NWs and poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) to achieve a higher thermoelectric performance and a better environmental stability. The thermoelectric thin film device, consisting of Bi2Te3 NWs as n-type leg and Te-PEDOT:PSS as p-type leg, exhibits a stable output voltage of 56mV and high output power density value of 32μWcm−2 at temperature difference of 60K, which is only 20% lower than that composed of commercial Bi2Te3. However, Te-based thin film device can effectively decrease its costs and be further improved to achieve a larger output voltage in the future.

Thermal conductivity of Bi2Te3 nanowires: how size affects phonon scattering.

This work provides an in-depth study of how the thermal conductivity of stoichiometric [110] Bi2Te3 nanowires becomes affected when reducing its diameter from an experimental and theoretical point of view. The thermal conductivity was observed to decrease more than 70% (from 1.78 ± 0.46 W K-1 m-1 to 0.52 ± 0.35 W K-1 m-1) when the diameter of the nanowire was reduced one order of magnitude (from 300 nm to 25 nm). The Kinetic-Collective model was used to understand such a reduction, which can be explained by the impact that surface scattering has in acoustic phonons. The smaller the diameter of the nanowires is, the larger the alteration in the mean free path of the low-frequency phonons is. The model agrees well with the experimental data, and the reduction in the thermal conductivity of the nanowires can be explained in terms of an increment of phonon scattering.

Evolution of Microstructural Disorder in Annealed Bismuth Telluride Nanowires

Controlling the distribution of structural defects in nanostructures is important since such defects can strongly affect critical properties, including thermal and electronic transport. However, characterizing the defect arrangements in individual nanostructures is difficult because of the small length scales involved. Here, we investigate the evolution of microstructural disorder with annealing in electrochemically deposited Bi2Te3 nanowires, which are of interest for thermoelectrics. We combine Convergent Beam Electron Diffraction (CBED) and Scanning Transmission Electron Microscopy (STEM) to provide the necessary spatial and orientational resolution. We find that despite their large initial grain sizes and strong crystallographic texturing, the as-deposited nanowires still exhibit significant intragranular orientational disorder. Annealing drives both grain growth and a significant reduction in the intragranular disorder. The results are discussed in the context of the existing understanding of the initial microstructure of electrodeposited materials and the understanding of annealing microstructures in both electrochemically deposited and bulk-deformed materials. This analysis highlights the importance of assessing both the grain size and intragranular disorder in understanding the microstructural evolution of individual nanostructures.

Local Magnetic Suppression of Topological Surface States in Bi2Te3 Nanowires.

Locally induced, magnetic order on the surface of a topological insulator nanowire could enable room-temperature topological quantum devices. Here we report on the realization of selective magnetic control over topological surface states on a single facet of a rectangular Bi2Te3 nanowire via a magnetic insulating Fe3O4 substrate. Low-temperature magnetotransport studies provide evidence for local time-reversal symmetry breaking and for enhanced gapping of the interfacial 1D energy spectrum by perpendicular magnetic-field components, leaving the remaining nanowire facets unaffected. Our results open up great opportunities for development of dissipation-less electronics and spintronics.