Bi2Te3 Single Crystals Research Articles

Green Synthesis and Morphological Evolution for Bi2Te3 Nanosystems via a PVP-Assisted Hydrothermal Method.

Bi2Te3 has been extensively used because of its excellent thermoelectric properties at room temperature. Here, 230-420 nm of Bi2Te3 hexagonal nanosheets has been successfully synthesized via a "green" method by using ethylene glycol solution and applying polyvinyl pyrrolidone (PVP) as a surfactant. In addition, factors influencing morphological evolution are discussed in detail in this study. Among these parameters, the reaction temperature, molar mass of NaOH, different surfactants, and reaction duration are considered as the most essential. The results show that the existence of PVP is vital to the formation of a plate-like morphology. The reaction temperature and alkaline surroundings played essential roles in the formation of Bi2Te3 single crystals. By spark plasma sintering, the Bi2Te3 hexagonal nanosheets were hot pressed into solid-state samples. We also studied the transport properties of solid-state samples. The electrical conductivity σ was 18.5 × 103 Sm-1 to 28.69 × 103 Sm-1, and the Seebeck coefficient S was -90.4 to -113.3 µVK-1 over a temperature range of 300-550 K. In conclusion, the observation above could serve as a catalyst for future exploration into photocatalysis, solar cells, nonlinear optics, thermoelectric generators, and ultraviolet selective photodetectors of Bi2Te3 nanosheet-based photodetectors.

Preparation of Bi2Te3 Based on Saturable Absorption System and Its Application in Fiber Lasers

Fiber laser is a fundamental component of laser systems and is of great significance for development of laser technology. Its pulse output can be divided into Q-switched and mode-locked. Achieving ultrashort pulse with narrower pulse duration and higher power is the focus of current research on mode-locked lasers. As an important component of fiber laser systems, saturable absorber (SA) can modulate losses in the optical cavity and generate pulses, enabling the laser system to achieve pulse output under long-term normal operating conditions better. Therefore, expanding the selection range of materials with better saturable absorption properties to improve the quality of pulse output is an important topic in current research. Here, the second generation topological insulator Bi2Te3 single crystal is prepared, and a ring fiber laser system is built with the Bi2Te3 SA. The mode-locked pulse with a pulse duration of 288 fs and a signal-to-noise ratio of 80.202 dB is realized. This result verifies that Bi2Te3, as a member of topological insulator, has good saturable absorption characteristics, and has broad prospects for the application research in lasers.

Electronic Structure and Transport Properties of Bi2Te3 and Bi2Se3 Single Crystals.

The electrical resistivity and the Hall effect of topological insulator Bi2Te3 and Bi2Se3 single crystals were studied in the temperature range from 4.2 to 300 K and in magnetic fields up to 10 T. Theoretical calculations of the electronic structure of these compounds were carried out in density functional approach, taking into account spin-orbit coupling and crystal structure data for temperatures of 5, 50 and 300 K. A clear correlation was found between the density of electronic states at the Fermi level and the current carrier concentration. In the case of Bi2Te3, the density of states at the Fermi level and the current carrier concentration increase with increasing temperature, from 0.296 states eV-1 cell-1 (5 K) to 0.307 states eV-1 cell-1 (300 K) and from 0.9 × 1019 cm-3 (5 K) to 2.6 × 1019 cm-3 (300 K), respectively. On the contrary, in the case of Bi2Se3, the density of states decreases with increasing temperature, from 0.201 states eV-1 cell-1 (5 K) to 0.198 states eV-1 cell-1 (300 K), and, as a consequence, the charge carrier concentration also decreases from 2.94 × 1019 cm-3 (5 K) to 2.81 × 1019 cm-3 (300 K).

Crossover from n- to p-Type Charge Carriers in Cr-Substituted Bi2Te3 Single Crystal

Crossover from n- to p-Type Charge Carriers in Cr-Substituted Bi2Te3 Single Crystal

Deep‐Subwavelength Resonant Meta‐Optics Enabled by Ultra‐High Index Topological Insulators

AbstractIn nanophotonics, small mode volumes, high‐quality factor resonances, and large field enhancements without metals fundamentally scale with the refractive index and are key for many implementations involving light‐matter interactions. Topological insulators (TIs) are a class of insulating materials that host topologically protected surface states, some of which exhibit extraordinarily high permittivity values. Here, the optical properties of TI bismuth telluride (Bi2Te3) single crystals are studied. It is found that both the bulk and surface states contribute to the extremely large optical constants, with the real part of the refractive index peaking at n ≈ 11. Utilizing these ultra‐high index values, it is demonstrated that Bi2Te3 metasurfaces are capable of squeezing light in deep‐subwavelength structures, with the fundamental magnetic dipole (MD) resonance confined in unit cell sizes smaller than λ/10. It is further shown that dense ultrathin metasurface arrays can simultaneously provide large magnetic and electric field enhancements arising from the high index of the bulk and the surface metallic states. These findings demonstrate the potential of chalcogenide TIs as a platform leveraging the unique combination of ultra‐high‐index dielectric response with surface metallic states for metamaterial design and nanophotonic applications in sensing, non‐linear generation, and quantum information.

Role of defects on carrier dynamics and transport mechanism in Bi2Te3 single crystals

Defects play an important role in determining the type of carriers as well as in tunning the physical properties of layered materials. In this study, we have demonstrated that by varying the growth kinetics one can control the defects and can achieve electrons or holes dominated Bi2Te3 single crystals using the modified Bridgman method. The correlation between structural defects and the type of dominant charge carriers in crystals is discussed using X-ray diffraction and Hall resistivity. Electrons are found to be originating from Te vacancy type defects, while holes are manifested from predominant structural defects viz. BiTe antisite defects or interstitial Te atoms. We observe that the alteration of charge carriers from electrons to holes has enhanced magnetoresistance from 103% to 224%. The enhancement in magnetoresistance emerges from the 2D multichannel quantum coherent conduction mechanism.

Femtosecond Laser‐Induced Periodic Surface Structuring of the Topological Insulator Bismuth Telluride

AbstractSurface structuring of topological insulator Bi2Te3 single crystals with femtosecond laser by varying pulses N and energy E is reported. Interesting effects related to laser‐induced periodic surface structures formation in this class of materials are evidenced. At low pulse energy, a clear formation of periodic, subwavelength ripples oriented orthogonally to the laser polarization is observed; those are restricted to an annular region surrounding a featureless central disk as the laser energy progressively increases. The structural analysis shows that some degree of crystallinity is preserved in the rippled area, but the central disk is amorphous resembling what is observed for germanium (Ge) and is associated with the hindering of surface structure formation due to a thick melted surface layer. Interestingly, at larger fluence or number of pulses, a transition to suprawavelength grooves occurs within the annular region covered by surface structures. The findings demonstrate a clear incubation behavior, suggesting that the formation of laser‐induced periodic surface structures is coherent with the general features of the process already reported for other materials. However, the disappearance of these structures in the central area, possibly resulting from the influence of the depth of the melt layer, indicates a mixed behavior for Bi2Te3.

Нернстовская подвижность дырок в Bi2Te3

This paper presents the results of a study of the electrical conductivity coefficient, the Hall coefficient, the Seebeck coefficient, the transverse Nernst-Ettingshausen coefficient and their anisotropy in a Bi2Te3 single crystal with a hole concentration p =1 × 1019 cm–3 at temperatures 77–350 К. It is established that hole scattering occurs mainly on long-wave acoustic phonons. Despite the fact thatthe chemical potential level is located near the top of the additional extreme of the valence band, no interband scattering was detected. The complex structure of the valence band is confirmed.

Effect of Sr Doping on Structural and Transport Properties of Bi2Te3.

Search for doped superconducting topological insulators is of prime importance for new quantum technologies. We report on fabrication of Sr-doped BiTe single crystals. We found that Bridgman grown samples have p-type conductivity in the low 10 cm, high mobility of 4000 cmVs, crystal structure independent on nominal dopant content, and no signs of superconductivity. We also studied molecular beam epitaxy grown SrBiTe films on lattice matched (1 1 1) BaF polar surface. Contrary to the bulk crystals thin films have n-type conductivity. Carrier concentration, mobility and c-lattice constant demonstrate pronounced dependence on Sr concentration x. Variation of the parameters did not lead to superconductivity. We revealed, that transport and structural parameters are governed by Sr dopants incorporation in randomly inserted Bi bilayers into the parent matrix. Thus, our data shed light on the structural position of dopant in BiTe and should be helpful for further design of topological insulator-based superconductors.

Improved electrical conductivity and power factor in Sn and Se co-doped melt-grown Bi2Te3 single crystal

In the current work, growth and thermoelectric characterization of tin and selenium co-doped single crystal bismuth telluride have been carried out in the range of temperature 10–400 K. The crystals show hexagonal crystal structure with Roverline{3 }m space group. The direction of growth, quality of the single crystals, the density of dislocation, and dopants effect on the inner plane structure of the crystals have been analyzed through high-resolution X-ray diffraction study. Energy dispersive analysis of X-rays approves the elemental composition, and field emission scanning electron microscopy shows uniform growth with micro precipitates on the surface of the crystals. Quasi degenerate and non-degenerate electrical resistivity is observed in the pristine and doped samples, respectively. Temperature-dependent Seebeck coefficient measurements confirm the n-type semiconducting nature of the pristine as well as doped samples. Temperature-dependent power factor of (Bi0.96Sn0.04)2Te2.7Se0.3 is found to increase by 1.1 times, and electrical resistivity reduced by 3.3 times as compared to pristine Bi2Te3.

Comprehensive analysis of Terahertz frequency response of Bi2Se3 and Bi2Te3 single crystals using Terahertz time-domain spectroscopy

We report the Terahertz time-domain spectroscopy (THz-TDS) dielectric response of the large single crystal of Bi2Se3 and Bi2Te3, which are grown in-house through a solid-state reaction route via the self-flux method technique. We fit the obtained experimental complex dielectric constant to the Lorentz oscillator model and obtain various resonating mode strengths in both materials. The strength of dielectric constants is an order of magnitude different while comparing the Terahertz response of both the crystals. However, the dielectric relaxation times are similar for stronger modes, depicting the similarity between the two crystals.

Comprehensive analysis for the high field magneto-conductivity of Bi2Te3 single crystal

Here, we report the magneto-conductivity (up to 14 T and down to 5 K) analysis of Bi2Te3 single-crystal. A sharp magneto-conductivity (MC) rise (inverted v-type cusp) is observed near H = 0 due to the weak antilocalization (WAL) effect, while a linear curve is observed at higher fields. We account for magneto-conductivity (MC) over the entire range of applied magnetic fields of up to 14 T and temperatures from 100 K to 5 K in a modified HLN modelling (addition of quadratic (βH2) through quantum and classical components involvement. The additional term βH2 reveals a gradual change of a (HLN parameter) from −0.421(6) to −0.216(1) as the temperature increases from 5 K to 100 K. The phase coherence length Lϕ obtained from both conventional and modified modelling decreased with increasing temperature but remains more protracted than the mean free path (L) of electrons. The analysis shows that the quantum phase coherence effect dominates at high temperatures.

Origin of anomalous high-rate Na-ion electrochemistry in layered bismuth telluride anodes

Summary van der Waals layered metal chalcogenide Bi2Te3 has shown exceptional capacity and rate capability in alkali-ion batteries but the underlying reaction mechanism with Li+, Na+, and K+ remains undiscovered. It is unexpected that Na+ electrochemistry outperforms Li+ and K+ at high current densities. Here, in situ transmission electron microscopy is used to uncover nanoscale transformations during lithiation, sodiation, and potassiation, which follows two-step conversion and alloying reactions with Li+ and Na+, and three-step intercalation-conversion-alloying reactions with K+. Counterintuitively, sodiation exhibits the highest reaction kinetics, and its origin can be elucidated by first-principles and finite-element simulations in two aspects. The lower interfacial strain accommodation energy between Bi2Te3 and its Na-conversion products allows more facile sodiation phase transformation than Li- and K-ion reactions. The higher electrochemo-mechanical stress concentration at the concave-shaped sodiation reaction front facilitates continued Na-ion diffusion and reaction propagation. These fundamental insights are essential for fast-charging alkali-ion batteries.

Large positive magnetoresistance with high Hall mobility and intercalation of Fe dopants in the quenched Bi2Te3 single crystals

A study of Fe-ion intercalation in Bi2Te3 probed by the x-ray diffraction, magnetization, Hall measurement, and spectra of synchrotron x-ray photoemission spectroscopy is reported, in which a large positive magnetoresistance (MR) and a high carrier mobility for applications have been achieved and probed in the quenched FexBi2Te3 single crystals with x = 0.01 and 0.15. Large positive MR of ∼470% with a Hall mobility of ∼44000 cm2/Vs at 5 K and 6 T has been observed on a quenched Fe0.01Bi2Te3 sample with a quenching temperature Tq of 400 °C, where the electrical parameters can be tuned by Tq. Larger c-axis lattice constants and electron-dominant carriers in the quenched samples lead to the inference that the doped Fe atoms are mostly located at the intercalated van der Waals gap positions and exist as a metallic state of Fe0, which is confirmed by the synchrotron x-ray photoemission spectroscopy (XPS) spectra. Combining the x-ray diffraction, magnetization, and Hall-coefficient studies, here we pave a simple way for probing into the location of Fe dopants in Bi2Te3. In addition, novel magnetotransports of Fe-doped Bi2Te3 associated with the absence of ferromagnetism have been presented, and satisfy the scenario that the topological surface state persists without the loss of topological insulating characteristics of the linear band dispersion, and the large MR may originate from the surface magnetism, even in the presence of Fe impurities. This study also discusses the huge enhancement of Hall mobility arising from the surface Dirac electrons that are accompanied with a modified band structure in quenched Fe0.01Bi2Te3.

Effect of band bending on topological surface transport of Bi2Te3 single crystal

Understanding the effect of surface to bulk coupling on topological surface states is important for harnessing the topological insulators for low dissipation electronics and quantum technologies. Here we investigate this effect on a low bulk carrier density Bi2Te3 single crystal using magnetoresistance, Hall resistivity, and Shubnikov–de Haas oscillations. Our results show the presence of high mobility surface bands and low mobility bulk bands. The surface states exhibit ambipolar transport without any gating. The mobility of surface states strongly depend on the nature of band bending, the upward band bending with holes as surface charge carrier exhibit large mobility while the downward band bending with electrons as surface charge carriers exhibit low surface mobility. The large mobility of surface Dirac holes is related to low surface defect density and small cyclotron mass. We also observe large magnetoresistance ∼285% due to multichannel quantum coherent transport in the bulk.

Crossover from weak-antilocalization transport to quantum magnetoresistance of Dirac states in quenched Fe0.01Bi2Te3 single crystals with large magnetoresistance and high Hall mobility

Magnetotransport properties with a large positive magnetoresistance (MR) and a high carrier mobility for applications have been achieved and probed for quenched Fe0.01Bi2Te3 single crystals. Large positive MR of ∼470% with a Hall mobility of ∼44 000 cm2 V−1 s−1 at 5 K and 6 T has been observed on a quenched Fe0.01Bi2Te3 sample, in which the electrical parameters can be tuned by the quenching temperature Tq. The MR behaviors for the quenched samples show a crossover from a weak antilocalization-dominant MR to a linear and non-saturating MR at temperatures of T* ≈ 58−100 K, where the large MR at low temperatures possibly originates from the mechanism of topologically protected backscattering. On the contrary, the MR behaviors for the strain-released sample do not show such a distinct crossover, where only linear-like and non-saturating MR behaviors can be observed. Different electrical transports between the quenched and strain-released samples indicate that the band structure, as well as the surface Dirac electrons in Fe0.01Bi2Te3, can be modified by the lattice strain. Furthermore, it is found that the low-temperature magnetoconductivity can be well described by the weak-antilocalization transport formula, while the high-field linear-like MR at T > T* can be explained in terms of Abrikosov’s quantum transport of Dirac-cone states in quenched Fe0.01Bi2Te3 single crystals.

Observation of multichannel quantum coherent transport and electron-electron interaction in Bi2Te3 single crystal

The bulk of topological insulators is relatively unexplored, due to the excess contribution of conduction from native defects. Here, we investigate the bulk conduction in a Bi2Te3 crystal having a reduced number of defect states. Our results reveal the presence of three transport regimes which are dominated by thermal activation across the bulk band gap, defect state charge conduction, and quantum coherent transport. The low temperature conductance and magnetoconductance reveal the presence of multichannel two-dimensional quantum coherent transport in the bulk. The number of channels is of the order of quintuple layers, signifying each quintuple layer as a single transport channel. These transport channels exhibit a two-dimensional electron-electron interaction effect causing electron dephasing, whereas the conduction from defect states exhibits a three-dimensional electron-electron interaction effect.

NMR studies of 3D topological insulators over a large temperature range

Results of NMR measurements of the Bi2Te3 and Bi2Se3 3D topological insulators within the temperature range 11-293 K are presented. Two lines were demonstrated in the 125Te and 77Se spectra over the whole temperature range for single crystals in the c^B0 and c||B0 orientations as well as for Bi2Te3 and Bi2Se3 powders. The observed lines correlated to two non-equivalent positions of chalcogen atoms in the crystalline structure of investigated bismuth chalcogenides. The thermal activated character of charge carriers was found for the Bi2Te3 single crystal and powder. The activation energy was evaluated. It was shown for the Bi2Se3 powder that thermally activated charge carriers did not play the main role in conductivity. The nontrivial behaviour of the 125Te and 77Se NMR spectra for Bi2Te3 and Bi2Se3 single crystals in the orientation c||B0 was found at low temperatures which could not be explained by the orientation dependence of the line shapes and shifts in the topologically trivial semiconductors without structural phase transitions.

Anharmonicity of Lattice Vibrations in Bi2Se3 Single Crystals

The temperature dependences of the Raman-active frequencies $$E_{g}^{2}$$ , $$A_{{1g}}^{2}$$ in layered Bi2Se3 single crystals are studied. The contribution of the thermal expansion to a temperature variation in the frequencies is determined. The decay of the anisotropy of the elastic properties in layered Bi2Se3 single crystals due to strong spin–orbit coupling is shown. The Gruneisen-mode parameters for phonons $$E_{g}^{2}$$ and $$A_{{1g}}^{2}$$ are calculated. Systematic features in the dependences of vibrational frequencies on the atomic masses in layered Bi2Te3, Bi2Se3, and Sb2Te3 single crystals are determined.

Bi2Te3 single crystals with high room-temperature thermoelectric performance enhanced by manipulating point defects based on first-principles calculation.

Intrinsic Bi2Te3 is a representative thermoelectric (TE) material with high performance at low temperature, which enables applications for electronic cooling. However, antisite defects easily form in p-type Bi2Te3, resulting in the difficulty of further property enhancement. In this work, the formation energy of native point defects in Bi2Te3 supercells and the electronic structure of Bi2Te3 primitive unit cell were calculated using first-principles. The antisite defect Bi_Te1 has a lower formation energy (0.68 eV) under the Te-lack condition for p-type Bi2Te3. The effects of point defects on TE properties were investigated via a series of p-type Bi2Te3−x (x = 0, 0.02, 0.04, 0.06, 0.08) single crystals prepared by the temperature gradient growth method (TGGM). Apart from the increased power factor (PF∥) which originates from the increased carrier concentration (n∥) and m*, the thermal conductivity (κ∥) was also cut down by the increased point defects. Benefitting from the high PF∥ of 4.09 mW m−1 K−2 and the low κ∥ of 1.77 W m−1 K−1, the highest ZT∥ of 0.70 was obtained for x = 0.06 composition at 300 K, which is 30% higher than that (0.54) of the intrinsic Bi2Te3.