Epitaxial Growth of Ternary Topological Insulator Bi2Te2Se 2D Crystals on Mica

Nanostructures of ternary topological insulator (TI) Bi₂Te₂Se are, in principle, advantageous to the manifestation of topologically nontrivial surface states, due to significantly enhanced surface‐to‐volume ratio compared with its bulk crystals counterparts. Herein, the synthesis of 2D Bi₂Te₂Se crystals on mica via the van der Waals epitaxy method is explored and systematically the growth behaviors during the synthesis process are investigated. Accordingly, 2D Bi₂Te₂Se crystals with domain size up to 50 µm large and thickness down to 2 nm are obtained. A pronounced weak antilocalization effect is clearly observed in the 2D Bi₂Te₂Se crystals at 2 K. The method for epitaxial growth of 2D ternary Bi₂Te₂Se crystals may inspire materials engineering toward enhanced manifestation of the subtle surface states of TIs and thereby facilitate their potential applications in next‐generation spintronics.     

Electric conductivity of a bulk composite based on Bi<Subscript>2</Subscript>Te<Subscript>3</Subscript>/SiO<Subscript>2</Subscript> core-shell nanoparticles

Composite Bi₂Te₃/SiO₂ nanoparticles of the core-shell type have been synthesized for the first time with a view to creating bulk composites possessing high thermoelectric figure of merit (conversion efficiency). It is suggested that bulk composited based on Bi₂Te₃/SiO₂ nanoparticles will provide a combination of low lattice heat conduction due to SiO₂ insulator and rather high electric conduction due to charge-carrier tunneling via dielectric spacers between adjacent Bi₂Te₃ semiconductor grains. The electric resistance of the composite increases with increasing temperature in the range of 130–300 K. This temperature dependence can be described in terms of a tunneling conduction model.     

Terahertz Emission from Hybrid Perovskites Driven by Ultrafast Charge Separation and Strong Electron–Phonon Coupling

Unusual photophysical properties of organic–inorganic hybrid perovskites have not only enabled exceptional performance in optoelectronic devices, but also led to debates on the nature of charge carriers in these materials. This study makes the first observation of intense terahertz (THz) emission from the hybrid perovskite methylammonium lead iodide (CH₃NH₃PbI₃) following photoexcitation, enabling an ultrafast probe of charge separation, hot‐carrier transport, and carrier–lattice coupling under 1‐sun‐equivalent illumination conditions. Using this approach, the initial charge separation/transport in the hybrid perovskites is shown to be driven by diffusion and not by surface fields or intrinsic ferroelectricity. Diffusivities of the hot and band‐edge carriers along the surface normal direction are calculated by analyzing the emitted THz transients, with direct implications for hot‐carrier device applications. Furthermore, photogenerated carriers are found to drive coherent terahertz‐frequency lattice distortions, associated with reorganizations of the lead‐iodide octahedra as well as coupled vibrations of the organic and inorganic sublattices. This strong and coherent carrier–lattice coupling is resolved on femtosecond timescales and found to be important both for resonant and far‐above‐gap photoexcitation. This study indicates that ultrafast lattice distortions play a key role in the initial processes associated with charge transport.     

Physical properties of Bi2 (Te,Se)3 and Bi2Se1.2Te1.8 prepared using solid-state microwave synthesis

Bi₂(Te, Se)₃ and Bi₂Se_1.2Te_1.8 bulk products were synthesised using standard solid-state microwave synthesis. The Bi₂(Te, Se)₃ and Bi₂Se_1.2Te_1.8 were then deposited thermally onto glass substrates at a pressure of 10^? 6 Torr. The structure of the samples was analysed using X-ray diffraction (XRD), and the powders and thin films were observed to be polycrystalline and rhombohedral in structure. The surface morphology of the samples was determined using scanning electron microscopy (SEM). From the measurements of optical properties, the energy gap values for the Bi₂Te₃, Bi₂Se₃, and Bi₂Se_1.2Te_1.8 thin films were 0.43, 0.73, and 0.65 eV, respectively.     

Influence of sintering temperature on thermoelectric properties of Bi<Subscript>2</Subscript>Te<Subscript>3</Subscript>/Polythiophene composite materials

Bi₂Te₃/Polythiophene (PTH) thermoelectric bulk composite materials were prepared by a two-step method. Firstly, Bi₂Te₃ and PTH nanopowders were prepared by hydrothermal synthesis and chemical oxidative polymerization, respectively. Secondly, the mixture of the Bi₂Te₃ and PTH nanopowders (50:50 wt) was pressed under vacuum at 80 MPa and 298, 473, or 623 K. For comparison, Bi₂Te₃ powders were hot pressed at 623 K. The bulk materials were analyzed by conventional methods, such as X-ray diffraction (XRD), Fourier transform infrared spectroscopy, thermogravimetric analysis (TGA) and field emission scanning electron microscopy equipped with electron dispersive X-ray spectroscopy. The XRD and TGA results showed that the PTH decomposed when the hot pressing temperature exceeded 473 K, and Bi₂Te₂S phase was formed. The thermoelectric properties of the bulk composite materials were investigated. The composite pressed at 623 K showed a higher power factor, ~2.54 μ Wm⁻¹ K⁻² at 473 K, which is as ~20 times as that of the composite pressed at 473 K, although, it is still much lower than that of the pressed Bi₂Te₃ material (~1,266 μ Wm⁻¹ K⁻² at 348 K).     

High performance Bi2Te3 nanocomposites prepared by single-element-melt-spinning spark-plasma sintering

The last decade has witnessed nanocomposites becoming a new paradigm in the field of thermoelectric (TE) research. At its core is to prepare high performance TE nanocomposites, both p- and n-type, in a time and energy efficient way. To this end, we in this article summarize our recent effort and results on both p- and n-type Bi₂Te₃-based nanocomposites prepared by a unique single-element-melt-spinning spark-plasma sintering procedure. The results of transport measurements, scanning and transmission electronic microscopy, and small angle neutron scattering have proved essential in order to establish the correlation between the nanostructures and the TE performance of the materials. Interestingly, we find that in situ formed nanocrystals with coherent boundaries are the key nanostructures responsible for the significantly improved TE performance of p-type Bi₂Te₃ nanocomposites whereas similar nanostructures turn out to be less effective for n-type Bi₂Te₃ nanocomposites. We also discuss the alternative strategies to further improve the TE performance of n-type Bi₂Te₃ materials via nanostructuring processes.     

Flexible and High‐Performance All‐2D Photodetector for Wearable Devices

Emerging novel applications at the forefront of innovation horizon raise new requirements including good flexibility and unprecedented properties for the photoelectronic industry. On account of diversity in transport and photoelectric properties, 2D layered materials have proven as competent building blocks toward next‐generation photodetectors. Herein, an all‐2D Bi₂Te₃‐SnS‐Bi₂Te₃ photodetector is fabricated with pulsed‐laser deposition. It is sensitive to broadband wavelength from ultraviolet (370 nm) to near‐infrared (808 nm). In addition, it exhibits great durability to bend, with intact photoresponse after 100 bend cycles. Upon 370 nm illumination, it achieves a high responsivity of 115 A W^?1, a large external quantum efficiency of 3.9 × 10⁴%, and a superior detectivity of 4.1 × 10¹¹ Jones. They are among the best figures‐of‐merit of state‐of‐the‐art 2D photodetectors. The synergistic effect of SnS's strong light–matter interaction, efficient carrier separation of Bi₂Te₃–SnS interface, expedite carrier injection across Bi₂Te₃–SnS interface, and excellent carrier collection of Bi₂Te₃ topological insulator electrodes accounts for the superior photodetection properties. In summary, this work depicts a facile all‐in‐one fabrication strategy toward a Bi₂Te₃‐SnS‐Bi₂Te₃ photodetector. More importantly, it reveals a novel all‐2D concept for construction of flexible, broadband, and high‐performance photoelectronic devices by integrating 2D layered metallic electrodes and 2D layered semiconducting channels.     

Percolation effect in copper- and nickel-doped Bi<Subscript>2</Subscript>Te<Subscript>3</Subscript> crystals

The (0001) surface morphology of Bi₂Te₃〈M〉 (M = Cu, Ni) layered crystals has been studied using atomic force microscopy (AFM) and scanning electron microscopy. Two- and three-dimensional AFM images reveal charge transport paths through inhomogeneities created by nanostructured elements (5–20 nm) in the Te(1)-Te(1) interlayer spaces. The nanoparticle distribution in the (0001) plane is similar to the arrangement of model two-dimensional percolation clusters on a square lattice. The carrier mobility in Bi₂Te₃〈0.5 wt % Ni〉 crystals varies anomalously between 80 and 120 K.     

Melt‐Centrifuged (Bi,Sb)2Te3: Engineering Microstructure toward High Thermoelectric Efficiency

Microstructure engineering is an effective strategy to reduce lattice thermal conductivity (κ_l) and enhance the thermoelectric figure of merit (zT). Through a new process based on melt‐centrifugation to squeeze out excess eutectic liquid, microstructure modulation is realized to manipulate the formation of dislocations and clean grain boundaries, resulting in a porous network with a platelet structure. In this way, phonon transport is strongly disrupted by a combination of porosity, pore surfaces/junctions, grain boundaries, and lattice dislocations. These collectively result in a ≈60% reduction of κ_l compared to zone melted ingot, while the charge carriers remain relatively mobile across the liquid‐fused grains. This porous material displays a zT value of 1.2, which is higher than fully dense conventional zone melted ingots and hot pressed (Bi,Sb)₂Te₃ alloys. A segmented leg of melt‐centrifuged Bi_0.5Sb_1.5Te₃ and Bi_0.3Sb_1.7Te₃ could produce a high device ZT exceeding 1.0 over the whole temperature range of 323–523 K and an efficiency up to 9%. The present work demonstrates a method for synthesizing high‐efficiency porous thermoelectric materials through an unconventional melt‐centrifugation technique.     

The synthesis of Bi<Subscript>2</Subscript>Te<Subscript>3</Subscript> nanobelts by vapor–liquid–solid method and their electrical transport properties

Bi₂Te₃ nanobelts were synthesized on quartz substrates by gold-mediated vapor–liquid–solid (VLS) growth through a thermal evaporation process. The structure and morphology were characterized by using scanning electron microscopy (SEM) and transmission electron microscopy (TEM).The temperature dependence of the conductivity of Bi₂Te₃ single crystal nanobelt shows a semiconductor behavior, and the activation energy was calculated as about 25 meV, indicating that the thermal activation of carriers from the impurity level dominates the transport property.     

Coexistence of Bulk and Surface Shubnikov-de Haas Oscillations in Bi2Se3

Our Hall bar shape device based on Bi₂Se₃ nano-plate was fabricated and studied at a dilution temperature with a tilted magnetic field up to 45 Tesla. It is found that, three types of carriers, one of three-dimensional (3D) and two of two-dimensional (2D), were identified by analyzing the angular dependence of Shubnikov-de Haas (SdH) oscillations, which confirmed the coexistence of bulk carriers and band bending induced two-dimensional electron gas in our transport experiment. The co-contributions to quantum oscillations indicated the independence of these states, without smearing out by scattering between each other, which may lay the foundations for detecting topological surface states (TSS) with residual bulk carriers in Bi₂Se₃.     

Selective Dissolution-Derived Nanoporous Design of Impurity-Free Bi2Te3 Alloys with High Thermoelectric Performance

Thermoelectric technology, which has been receiving attention as a sustainable energy source, has limited applications because of its relatively low conversion efficiency. To broaden their application scope, thermoelectric materials require a high dimensionless figure of merit (ZT). Porous structuring of a thermoelectric material is a promising approach to enhance ZT by reducing its thermal conductivity. However, nanopores do not form in thermoelectric materials in a straightforward manner; impurities are also likely to be present in thermoelectric materials. Here, a simple but effective way to synthesize impurity-free nanoporous Bi_0.4Sb_1.6Te₃ via the use of nanoporous raw powder, which is scalably formed by the selective dissolution of KCl after collision between Bi_0.4Sb_1.6Te₃ and KCl powders, is proposed. This approach creates abundant nanopores, which effectively scatter phonons, thereby reducing the lattice thermal conductivity by 33% from 0.55 to 0.37 W m⁻¹ K⁻¹. Benefitting from the optimized porous structure, porous Bi_0.4Sb_1.6Te₃ achieves a high ZT of 1.41 in the temperature range of 333–373 K, and an excellent average ZT of 1.34 over a wide temperature range of 298–473 K. This study provides a facile and scalable method for developing high thermoelectric performance Bi₂Te₃-based alloys that can be further applied to other thermoelectric materials.     

Morphological evolution in single-crystalline Bi2Te3 nanoparticles, nanosheets and nanotubes with different synthesis temperatures

A general surfactant-assisted wet chemical route has been developed for the synthesis of a variety of bismuth telluride (Bi₂Te₃) single-crystalline nanostructures with varied morphologies at different temperatures in which hydrazine hydrate plays as an important solvent. Bi₂Te₃ sheet grown nanoparticles, nanosheets and nanotubes have been synthesized by a simplest wet chemical route at 50, 70 and 100 °C within 4 h. Bi₂Te₃ sheet grown nanoparticles are obtained in agglomerate state and they are found with many wrinkles. Various types of Bi₂Te₃ nanotubes are also found which are tapered with one end open and the other closed. X-ray diffraction (XRD), transmission electron microscopy (TEM), selected area electron diffraction (SAED) pattern and energy dispersive X-ray (EDX) spectroscopy were employed to characterize the powder product. It is found that all nanoparticles, nanosheets and nanotubes are well-crystallized nanocrystals and morphologies of the powder products are greatly affected by different synthesis temperatures. The formation mechanisms of bismuth telluride nanostructures are also discussed.     

Hikami-Larkin-Nagaoka (HLN) Treatment of the Magneto-Conductivity of Bi<Subscript>2</Subscript>Te<Subscript>3</Subscript> Topological Insulator

We report the magneto-conductivity analysis at different temperatures under a magnetic field of up to 5 T of a well- characterized Bi₂Te₃ crystal. Details of crystal growth and various physical properties including high linear magneto-resistance are already reported by some of us. To elaborate upon the transport properties of Bi₂Te₃ crystal, the magneto-conductivity is fitted to the known Hikami-Larkin-Nagaoka (HLN) equation and it is found that the conduction mechanism is dominated by both surface-driven weak anti-localization (WAL) and the bulk weak localization (WL) states. The value of HLN equation coefficient (α) signifying the type of localization (WL, WAL or both WL and WAL) falls within the range of ??0.5 to ??1.5. In our case, the low-field (±?0.25 T) fitting of studied crystal exhibited a value of α close to ??0.86 for studied temperatures of up to 50 K, indicating both WAL and WL contributions. The phase coherence length (l_φ) is found to decrease from 98.266 to 40.314 nm with increasing temperature. Summarily, the short letter reports the fact that bulk Bi₂Te₃ follows the HLN equation and quantitative analysis of the same facilitates to know the quality of studied crystal in terms of WAL to WL contributions and thus the surface to bulk conduction ratio.     

Barrier/bonding layers on bismuth telluride (Bi<Subscript>2</Subscript>Te<Subscript>3</Subscript>) for high temperature thermoelectric modules

In this research, a fundamental study is conducted to identify the materials and develop the processes for producing barrier/bonding composite on Bi₂Te₃ for high temperature thermoelectric applications. The composite must meet four basic requirements: (a) prevent inter-diffusion between the electrode material, for our design, silver(Ag) and Bi₂Te₃, (b) bond well to Bi₂Te₃, (c) bond well to Ag electrode, and (d) do not themselves diffuse into Bi₂Te₃. The composites investigated include palladium (Pd), nickel/gold (Ni/Au), Ag, and titanium/gold (Ti/Au). After annealing at 250 °C for 200 h, only the Ti/Au design meets all four requirements. The thickness of Ti and Au, respectively, is only 100 nm. Other than meeting these four requirements, the Ti/Au layers exhibit excellent step coverage on the rough Bi₂Te₃ surface even after the annealing process.     

Understanding the Structure and Properties of Sesqui‐Chalcogenides (i.e., V2VI3 or Pn2Ch3 (Pn = Pnictogen,Ch = Chalcogen) Compounds) from a Bonding Perspective

A number of sesqui‐chalcogenides show remarkable properties, which make them attractive for applications as thermoelectrics, topological insulators, and phase‐change materials. To see if these properties can be related to a special bonding mechanism, seven sesqui‐chalcogenides (Bi₂Te₃, Bi₂Se₃, Bi₂S₃, Sb₂Te₃, Sb₂Se₃, Sb₂S₃, and β‐As₂Te₃) and GaSe are investigated. Atom probe tomography studies reveal that four of the seven sesqui‐chalcogenides (Bi₂Te₃, Bi₂Se₃, Sb₂Te₃, and β‐As₂Te₃) show an unconventional bond‐breaking mechanism. The same four compounds evidence a remarkable property portfolio in density functional theory calculations including large Born effective charges, high optical dielectric constants, low Debye temperatures and an almost metal‐like electrical conductivity. These results are indicative for unconventional bonding leading to physical properties distinctively different from those caused by covalent, metallic, or ionic bonding. The experiments reveal that this bonding mechanism prevails in four sesqui‐chalcogenides, characterized by rather short interlayer distances at the van der Waals like gaps, suggestive of significant interlayer coupling. These conclusions are further supported by a subsequent quantum‐chemistry‐based bonding analysis employing charge partitioning, which reveals that the four sesqui‐chalcogenides with unconventional properties are characterized by modest levels of charge transfer and sharing of about one electron between adjacent atoms. Finally, the 3D maps for different properties reveal discernible property trends and enable material design.     

Facile synthesis and thermoelectric studies of n-type bismuth telluride nanorods with cathodic stripping Te electrode

Bismuth telluride (Bi₂Te₃) nanorods (NRs) of n-type thermoelectric materials were prepared using an electrogenerated precursor of tellurium electrode in the presence of Bi³⁺ and mercapto protecting agent in aqueous solution under atmosphere condition. The optimal preparation conditions were obtained with ratio of Bi³⁺ to mercapto group and Te coulomb by photoluminescence spectra. The mechanism for generation of Bi₂Te₃ precursor was investigated via the cyclic voltammetry. The highly crystalline rhombohedral structure of as-prepared Bi₂Te₃ NRs with the shell of Bi₂S₃ was evaluated with high resolution transmission electron microscopy (HRTEM) and powder X-ray diffraction (XRD) spectroscopy. The near-infrared absorption of synthetic Bi₂Te₃ NRs was characterized with spectrophotometer to obtain information of electron at interband transition. The thermoelectric performance of Bi₂Te₃ NRs was assessed with the result of electrical resistivity, Seebeck coefficient, thermal conductivity, and the figure of merit ZT parameters, indicating that thermoelectric performance of as-prepared Bi₂Te₃ nanocrystals was improved by reducing thermal conductivity while maintaining the power factor.     

Indentation fracture toughness of single-crystal Bi<Subscript>2</Subscript>Te<Subscript>3</Subscript> topological insulators

Bismuth telluride (Bi₂Te₃) is one of the most important commercial thermoelectric materials. In recent years, the discovery of topologically protected surface states in Bi chalcogenides has paved the way for their application in nanoelectronics. Determination of the fracture toughness plays a crucial role for the potential application of topological insulators in flexible electronics and nanoelectromechanical devices. Using depth-sensing nanoindentation tests, we investigated for the first time the fracture toughness of bulk single crystals of Bi₂Te₃ topological insulators, grown using the Bridgman-Stockbarger method. Our results highlight one of the possible pitfalls of the technology based on topological insulators.

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Synthesis of polycrystalline nanotubular Bi2Te3

Polycrystalline nanotubular Bi₂Te₃ could be prepared via a high-temperature solution process using nanoscale tellurium, decomposed from trioctylphosphine oxide (TOPO) extracted tellurium species (Te-TOPO), as sacrificial template. The formation of such tubular structure is believed to be the result of outward diffusion of Te during the alloying process. The electrical properties (Seebeck coefficient and electrical conductivity) of the polycrystalline nanotubular Bi₂Te₃ have been studied and the experimental results show that the electrical conductivity is approximately three orders of magnitude smaller than bulk bismuth telluride materials mainly due to the much larger resistance brought by the insufficient contact between the nanotubular structures.     

Microstructural features and deformation-induced lattice defects in hot-extruded Bi2Te3 thermoelectric compound

Bismuth telluride (Bi₂Te₃) bulk material was prepared by a powder hot-extrusion technique. The microstructure, texture evolution, and deformation-induced lattice defects, as well as the effect of heat treatment on its microstructure and thermoelectric properties were investigated. As-extruded Bi₂Te₃ compound was found to be characterized by a dynamically and statically recrystallized microstructure with fine-equiaxed grains and low dislocation density. The majority of boundaries in the extruded specimens were large-angle boundaries with average misorientation angles of more than 30°. There was no significant change in dislocation configuration, even after heat treatment at a temperature higher than the extrusion temperature. Moreover, the Seebeck coefficient and electrical resistivity of the extruded specimens could not be improved by annealing. Microstructural examinations indicated the existence of a deformation-induced stacking disorder with local atomic shift, which we term “phase disorder,” on the basal plane in the extruded Bi₂Te₃ compound. The presence of this structural defect may be one of the main reasons for the slightly lower values of thermoelectric properties for the hot-extruded Bi₂Te₃ compound, compared with a unidirectionally solidified specimen.