Bi2Se3 Crystals Research Articles

Improvement in thermoelectric figure of merit of Bi2Se3 crystal with Sulfur substitution

Bismuth Selenide has gained a great deal of attention as a thermoelectric material owing to its lower toxicity compared to Bi2Te3 based materials. In this work, Sulfur substitution in Bi2Se3 is carried out by the vertical Bridgman method and the influence of Sulfur on the structural, electrical and thermal properties of Bi2Se3 was studied. The thermoelectric figure of merit (ZT) was calculated from electrical conductivity, Seebeck coefficient and thermal conductivity, which were measured from 303 K to 773 K. Sulfur incorporation did not improve the electrical conductivity of Bi2Se3 but proved to be beneficial in improving the Seebeck coefficient and reducing thermal conductivity. Hence, the calculated figure of merit of Bi2Se2.7S0.3 crystal comes out to be 0.72 at 543 K, which is higher than that of pure Bi2Se3 crystal, which is 0.31 at 533 K.

Coexistence of Kondo effect and Weak anti-localization in Topological insulator/Ferromagnetic heterostructure

The presence of a magnetic impurity in a topological insulator weakens the time-reversal symmetry by creating a limited gap at the Dirac point, and could result in the creation of new quantum states. On the other hand, causing magnetization in a topological insulator through the interlayer proximity effect to a magnetically ordered system is a better choice, as magnetic doping may have negative effects. In this report, the magnetic and magneto-transport properties of FeSe intercalated Bi2Se3 crystals and FeSe/Bi2Se3/FeSe hetero-structure have been investigated. The XPS depth profile was studied to confirm the layered structure of the hetero-structure. The weak anti-localization state in the multilayer system has been seen to coexist with the Kondo effect, which may have caused by the interfacial magnetic domains modifying the electronic characteristics at interfaces. Such magnetic spin-induced inter-layer coupling can be a pathway to room-temperature spintronic applications.

Characterization of the Edge States in Colloidal Bi2Se3 Platelets.

The remarkable development of colloidal nanocrystals with controlled dimensions and surface chemistry has resulted in vast optoelectronic applications. But can they also form a platform for quantum materials, in which electronic coherence is key? Here, we use colloidal, two-dimensional Bi2Se3 crystals, with precise and uniform thickness and finite lateral dimensions in the 100 nm range, to study the evolution of a topological insulator from three to two dimensions. For a thickness of 4-6 quintuple layers, scanning tunneling spectroscopy shows an 8 nm wide, nonscattering state encircling the platelet. We discuss the nature of this edge state with a low-energy continuum model and ab initio GW-Tight Binding theory. Our results also provide an indication of the maximum density of such states on a device.

Intrinsic free carrier absorption limited THz generation from Bi2Te3 and Bi2Se3 topological insulators

We investigate the influence of intrinsic free charge carriers on the ultrashort THz pulse generation efficiency of three-dimensional topological insulators (3dTIs). Wavelength dependence of the optical penetration depth of the femtosecond excitation pulses is exploited to vary the excitation volume within Bi2Te3 and Bi2Se3 crystals and accordingly, the free carrier population, to contribute to THz attenuation. The standard free carrier absorption (FCA) formalism is inadequate to explain wavelength-dependent enhancement in the THz emission as observed in the experiments. Within a modified framework, the THz attenuation by FCA is accounted for accurately, which consistently explains the experimental results for samples having different carrier density and mobility. We conclude that the THz generation efficiency of 3dTIs can be enhanced by engineering samples with high carrier mobility and low intrinsic carrier density and by using excitation wavelengths of minimal penetration depth.

The Raman Spectra of Co-, Fe-, and Mn-doped Bi2Se3 Single Crystals

In this study, single crystals of TrxBi2Se3 (Tr = Co, Fe, and Mn) were produced via a melt-grown reaction. All crystals are c-axis oriented with an excellent single crystalline phase. The smaller ionic radius of the transition metal elements makes the QLs of Bi2Se3 thinner. Their abundant magnetism will provide spin–phonon coupling channels when the phonons are scattering in the system. Both Stokes and anti-Stokes Raman spectroscopy were performed for the three optical phonon modes of the Bi2Se3 crystals. These peaks were centered at 74.5 cm−1, 133.4 cm−1, and 175.2 cm−1 and were attributable to the A1g(1), Eg(2), and A1g(2) modes, respectively. With an increase in the doping concentration, the magnetic and compressive strains induced by impurities dominate the phonon dynamics of materials. The results provide an effective scheme to regulate the thermoelectric properties of a Bi2Se3 system.

Evidence for nitrogen binding to surface defects for topological insulator Bi2Se3

Using scanning tunneling spectroscopy and theoretical simulations we have studied the effects of nitrogen gas exposure on the electronic density of states of Bi2Se3, a well-studied topological insulator. In carefully controlled measurements, Bi2Se3 crystals were initially cleaved in a helium gas environment and then exposed to a 22 SCFH flow of ultra-high purity N2 gas. We observe a resulting change in the spectral curves, with the exposure effect saturating after approximately 50 min, ultimately bringing the Dirac point about 50 meV closer to the Fermi level. These results are compared to density functional theory calculations, which support a picture of N2 molecules physisorbing near Se vacancies. Furthermore, ab initio molecular dynamics simulations aided by a Blue Moon ensemble method reveal the dissociative adsorption of N2 molecules which then bind strongly to Se vacancies at the surface. In this scenario, the binding of the N atom to a Se vacancy site removes the surface defect state created by the vacancy and changes the position of the Fermi energy with respect to the Dirac point.

Detecting surface-state-dominated electron-phonon coupling in Bi2TexSe3−x crystals at low temperatures by femtosecond transient optical spectroscopy

The structure, transport properties, and ΔR/R ultrafast spectra of Bi2TexSe3-x crystals are studied in this work. The lattice parameter c and saturation temperature of resistance increase with increasing Te concentration. There are two electron-phonon coupling processes in Bi2TexSe3-x crystals corresponding to the interactions between two topological surface states and phonons. The electron-phonon coupling of one surface state is dominant at low temperatures (T ≤ 40 K), and the coupling of the second surface state and phonon is significant. With increasing temperature, electron-phonon coupling in bulk Bi2TexSe3−x is enhanced, which weakens electron-phonon coupling of its surface states. A small amount of Te has little effect on the electron-phonon coupling for Bi2Se3 crystals.

Effect of Potassium Adsorption on the Surface Properties of Bi2Se3 and Bi2Te3 Crystals

Effect of Potassium Adsorption on the Surface Properties of Bi2Se3 and Bi2Te3 Crystals

Superconductivity and electron–phonon interaction in Sr x Bi2Se3 under pressure

Pressure-induced superconductivity has been widely explored and observed in Bi2Se3-based topological materials to hunt for topological superconductors. Although their triggered superconductivity has a close connection to their pressure-induced structural phase transitions, the quest for the electron pairing mechanism of these superconducting semiconductors in both their initial rhombohedral and high-pressure phases remains unknown. In this work, we present a systematic study of the pressure effect on superconducting properties and lattice dynamics using a combination of electrical transport, Raman-scattering, and synchrotron x-ray diffraction measurements using diamond anvil cells. One key finding is our observation of a cooperative connection between the strength of the electron–phonon interaction (EPI) generated by optical branches and the pressure-tunable superconductivity in rhombohedral Sr x Bi2Se3 crystal. The underlying suppression mechanism of the T c by pressure is ascribed to the weakening of the electrons’ interaction with the optical phonon modes in the rhombohedral phase. In the intermediate monoclinic phase, the T c value underwent a sharp increase with carrier density accumulation accompanying the concurrent enhanced EPI. This is intuitively unusual since it is expected that the EPI shall be weakened by inducing more conducting carriers in a normal metal. In the tetragonal phase, the superconductivity is interpreted within BCS theory, since it is fully metallized and obeys the adiabatic Born–Oppenheimer approximation well. Our findings are important to fully understand unconventional superconductivity and the unusual pairing mechanism in the layered rhombohedral Bi2Se3-based superconductors.

Supercritically exfoliated Bi2Se3 nanosheets for enhanced photocatalytic hydrogen production by topological surface states over TiO2

Owing to the unique electronic properties of layered materials, topological insulators have interestingly grabbed much attention in the field of photocatalytic water splitting. Nowadays, 2D layered materials were composited with semiconductor photocatalysts, encourage much as it provides enormous active sites and also significantly prevent photogenerated charge recombination. Especially, Bi2Se3 possesses exceptional properties like topologically preserved conducting surface states with bulk insulating behavior and high surface area, which provides unconventional electron dynamics, resulting in facile electron transport and effective charge separation to photocatalyst. So far, several methods have been attempted to synthesize few-layered Bi2Se3 nanosheets from its bulk crystals. Here, a unique attempt is made and succeeded to exfoliate bulk Bi2Se3 to few layered nanosheets via surfactant free supercritical fluid processing using N-Methyl-2-pyrrolidone (NMP) as an exfoliating agent, with a short reaction time of 15 min. The exfoliation of Bi2Se3 crystal was confirmed by several characterization techniques, such as XRD, SEM, Raman, and HR-TEM. Furthermore, different weight percentages of exfoliated Bi2Se3 sheets/anatase TiO2 nanoparticles were prepared and examined the photocatalytic activity using glycerol as a hole scavenger. Among them, 15 wt.% Bi2Se3 coupled TiO2 nanocomposite showed enormous hydrogen evolution rate of 84.9 mmol h-1g-1cat, which is 80 times higher than that of TiO2 nanoparticles. In addition, the photostability of the nanocomposite was also verified, where it retains 94% of activity even after 4 cycles of continuous experiments. The improved rate of H2 production was understood by theoretical calculations that topologically preserved conducting surface states of co-catalyst, Bi2Se3 nanosheets is supported to high mobile and scatter free electrons. It mediates the transport of electrons with TiO2 nanoparticles that helped the effective charge separation. Thus, it proves a promising candidate for photocatalytic hydrogen production.

Coexistence of magnetic states and metamagnetism in the Bi2-x CrxSe3topological insulators

The dichalcogenide Bi2Te3 and Bi2Se3 crystals belong to the 3D topological insulator family. Doping the crystals with magnetic ions can break the time-reversal symmetry and open an energy gap at surface Dirac points. This provides the opportunity of manipulating the surface transport and observing the anomalous quantum Hall effect. We studied magnetic properties of three single-crystalline BiCr x Se3 samples with , 0.03, and 0.06 within a temperature range from 2 to 300 K. The dc magnetization revealed the coexistence of antiferromagnetic and ferromagnetic ordering and paramagnetism. Their significance depends on the chromium content. The ferromagnetic phase transition was suppressed by high enough magnetic field. The antiferromagnetic transition near 80 K did not shift visibly up to 50 kOe. The Curie-Weiss law approximation gave the effective magnetic moment close to , which corresponds to divalent chromium ions. Metamagnetic phenomena were found for crystals with and 0.06.

Copper ion chemistry in a new rechargeable all-solid-state copper-ion battery

In this study, a safe and environmental friendly new all-solid-state copper-ion battery composed of Bi2Se3 anode, RbCu4Cl3I2 electrolyte and copper cathode was constructed for the first time. The electrochemical reaction processes during the charging and discharging were revealed. There are two electrochemical reaction processes in this new Cu ion battery. At the scanning rate of 0.5 ​mVs−1, Cu+ reversibly intercalated as an interstitial into van der wales layer of two-dimensional Bi2Se3 crystal when applying a reversible maximum voltage of 0.5 ​V. With further increase the maximum voltage up to 0.8 ​V, Cu+ undergoes a conversion reaction with Bi2Se3, reducing Bi2Se3 into Bi metal concomitantly forming Cu2Se. The irreversible conversion reaction is predominant capacitive process, while the reversible intercalation process is a diffusion-controlled process. The charge and discharge of this all-solid-state copper ion battery demonstrates that it can achieve repeated charge and discharge behavior, which shows perspective as a new all-solid-state rechargeable battery especially for the application in some special field.

High-performance mid-infrared photodetection based on Bi2Se3 maze and free-standing nanoplates

The pursuit of optoelectronic devices operating in mid-infrared regime is driven by both fundamental interests and commercial applications. The narrow bandgap (0.3 eV) of layered Bi2Se3 makes it a promising material for mid-infrared photodetection. However, the weak absorption of mid-infrared optical power and high dark current level restrict its performance. Here, a supply-control technique is applied to modulate the growth mode of Bi2Se3 crystal, and Bi2Se3 crystals with various morphologies are obtained. The nanoplates pattern transits from maze to freestanding when source mass was tuned. Due to the strong infrared absorption and photoelectric conversion efficiency of vertical Bi2Se3 nanoplates, the as-prepared vertical Bi2Se3 nanoplates/Si heterojunction shows excellent photoresponse and extremely low dark current. Among these devices based on different Bi2Se3 morphologies, freestanding nanoplates show the optimal mid-infrared characteristics, namely a photo-to-dark ratio of 2.0 × 104, a dark current of 0.21 pA, a response time of 23 ms, a specific detectivity of 6.1 × 1010 Jones (calculated) and 1.2 × 1010 Jones (measured) under 2.7 μm illumination and at room temperature. Notably, the specific detectivity of our devices are comparable to commercial InGaAs photodetectors. With the tunable- morphology growing technique and excellent photoresponding characteristics, Bi2Se3 nanomaterials are worth attention in optoelectronic field.

Modification of Shubnikov–de Haas oscillation and quantum Hall effect for Bi2Se3 crystals by Fe

Shubnikov–de Haas (SdH) oscillations and the quantum Hall effect (QHE) can give insights into quantum oscillations of charge carriers. We report the SdH oscillations and QHE of magnetically doped topological insulator FexBi2−xSe3 crystals, in which the lattice constant c decreases with the increasing dopant concentration but increases when x is greater than 0.08. From the fitting of low-temperature magnetoresistance data to the Lifshitz–Kosevich formula, the effective mass of the carriers and the Dingle temperature of the samples were extracted and show a similar trend of change as the lattice constant. A constant step size of 1/Rxy observed during Hall measurements at low temperatures implies two-dimensional-like transport behavior associated with the layered structure of the doped Bi2Se3 samples. Two-dimensional and three-dimensional charge carrier densities were calculated, and the latter had a similar trend of change as the lattice constant. A systematic study of the changes in all these properties with increasing dopant concentration suggests that they are highly correlated.

Pressure-induced electronic anomaly and multiband superconductivity in the doped topological insulator NbxBi2Se3

The tunability of the electronic topological transition is fundamentally important to unveil new quantum matters. Here, we report the observation of pressure-induced electronic anomaly below 12.0 GPa in Nb0.25Bi2Se3 using a combination of electrical transport, synchrotron x-ray diffraction, Raman-scattering spectroscopy measurements, and first-principles calculations. At ambient pressure, the band-structure calculations demonstrate the Nb0.25Bi2Se3 is a heavily electron-doped semiconductor with multiple band structure. With applying pressure, it is shown that the pressure can induce Fermi-surface reconstruction at a time-reversal invariant point. We further present evidence of multiband superconductivity characterized by an upward curvature in the upper critical field in the pressurized Nb0.25Bi2Se3 crystal. The superconducting critical parameters of pressurized Nb0.25Bi2Se3 crystal are obtained. Furthermore, the superconducting phase diagram under high pressure is discussed within BCS theory. These findings highlight the critical role of the multiple-band structure induced by strong hybridization between the Nb-4d and Bi/Se-p orbitals in accessing quantum states.

Laser Ablation Generation of Bismuth Selenide Clusters from Mixtures of Elements, Crystalline Bi2Se3, or Thin Films: Laser Desorption Ionization (LDI) and Surface Assisted LDI Time-of-Flight Mass Spectrometry using Graphene and Nanodiamonds.

A bismuth-selenium system from mixtures of the powdered elements in various molar ratios and from Bi2Se3 crystals and/or thin films was studied using laser desorption ionization and surface assisted laser desorption ionization. The BimSen clusters were observed in both positive and negative ion modes, but the mass spectra were more intense, and also a higher number of clusters was formed in the positive ion mode than in the negative mode. The BiSen+ (n = 1-8), Bi2Sen+ (n = 1-5), and Bi3Sen+ (n = 1-6) clusters were detected. Similarly, in the negative ion mode, BiSen- (n = 2-9) and Bi2Sen- (n = 1-2) clusters were observed. In addition, the formation of Bim+ (m = 1-5), Sen+ (n = 1-8), and Sen- (n = 1-7) clusters was also observed. In total, 33 clusters were generated, and 4 new bismuth selenide clusters that have not been reported before (namely, BiSe7+/-, BiSe8+/-, BiSe9-, and Bi2Se5+) were detected. The formation of similar clusters was also observed from bismuth-selenium mixtures and from crystalline Bi2Se3. Furthermore, the Bi2Se3 thin films prepared from a magnetron sputtering technique were also examined via laser desorption ionization. The generation of clusters from the surface of graphene and nanodiamonds was also studied, but no remarkable difference with comparison to the metal surface was observed.

Anharmonicity of Bi2Se3 revealed by fs transient optical spectroscopy

We investigate the anharmonic effects in Bi2Se3 crystals using femtosecond transient optical spectroscopy at 5–280 K. The reflectivity time series consist of exponential decay due to hot carriers and decaying oscillations due to the A1g1 phonon vibration. Vibration frequency and dephasing time of this optical phonon mode are obtained as a function of temperature, decreasing with increasing temperature; both the red shift in frequency and the increased dephasing rate induced by heating can be well described using the anharmonicity model including lattice thermal expansion and phonon-phonon coupling.

Tunable and Multichannel Terahertz Perfect Absorber Due to Tamm Plasmons with Topological Insulators

In this paper, Tamm plasmons with topological insulators in a composite structure consisting of Bi2Se3, spacer layer, and one-dimensional photonic crystal (1DPC) have been demonstrated theoretically. The perfect absorption has been realized in the terahertz regime because of the optical Tamm states (OTSs) excited at the interface between Bi2Se3 and 1DPC. The perfect absorption can be realized for both TE and TM waves, and it is noted that the perfect absorption can be obtained at any incident angle by simultaneously changing the wavelength of incident light for TE-polarizations. Moreover, the perfect absorption can be realized at different wavelengths with the change of the chemical potential and the thickness of Bi2Se3. The thickness and the dielectric constant of the spacer layer will also play a vital role in the performance of the perfect absorber. Especially, the multichannel perfect absorption phenomenon can be achieved by choosing the appropriate thickness of the spacer layer. This tunable and multichannel terahertz perfect absorber has great application potential in the solar energy, photodetection, and THz biosensor.

High power factor and mobility of single crystals of Bi2Se3 induced by Mo doping

Few binary compounds attract as much attention as Bi2Se3. In addition to its use as thermoelectric, it has become one of the prototype 3D topological insulators, and it has been demonstrated that doping in appropriate levels can induce magnetism and even superconductivity in bulk Bi2Se3. In this article, we continue the examination of doping with sixth group transition elements, namely, Mo. The solubility of Mo in the host matrix is very low; however, it induces remarkable changes in the material properties. For low doping levels, Mo changes the intrinsic defect structure of Bi2Se3 featuring the Se vacancies toward a structure characterized by embedded Bi2 double layers. Counterintuitively, Mo doping markedly enhances the mobility of free charge carriers in Bi2Se3 and drives its thermoelectric power factor to its maximum value over a broad temperature range. We provide evidence that the rather disordered structure must not be necessarily detrimental to charge transport in Bi2Se3.

Signatures of Quantum Transport Steps in Bi2Se3 Single Crystal

We report signatures of quantum magneto transport steps in a Bi2Se3 single crystal. The Bi2Se3 crystal is grown by self-flux method through solid-state reaction. Details of its structural analysis along with physical properties in terms of low temperature and high field transport, thermal (heat capacity), Raman, X-ray photo electron and angle resolved photo electron spectroscopy were reported earlier (J. Sup. Novel. Mag. 30, 2031 2017). In a present short letter, we show the details of its 2 K and up to 14 Tesla magneto transport, exhibiting quantum transport (QT) like steps above say 11 Tesla field. The possible QT steps cannot be modelled with the SdH (Shubnikov-de Haas) or other possible quantum oscillations. The observed QT like steps are possibly the result of the mixed out-of-plane Rxy(H) and in-plane Rxx(H) contributions from the misaligned [00l] planes of the studied crystal and this may happen with other studied topological insulator (TI) crystals as well. Our careful assessment of crystal XRD data shows the same.