Bi Se Research Articles

Distinct effects of Cr bulk doping and surface deposition on the chemical environment and electronic structure of the topological insulator Bi2Se3

In this report, it is shown that Cr doped into the bulk and Cr deposited on the surface of Bi2Se3 films produced by molecular beam epitaxy (MBE) have strikingly different effects on both the electronic structure and chemical environment. Angle resolved photoemission spectroscopy (ARPES) shows that Cr doped into the bulk opens a surface state energy gap which can be seen at room temperature; much higher than the measured ferromagnetic transition temperature of ≈10K. On the other hand, similar ARPES measurements show that the surface states remain gapless down to 15K for films with Cr surface deposition. In addition, core-level photoemission spectroscopy of the Bi 5d, Se 3d, and Cr 3p core levels show distinct differences in the chemical environment for the two methods of Cr introduction. Surface deposition of Cr results in the formation of shoulders on the lower binding energy side for the Bi 5d peaks and two distinct Cr 3p peaks indicative of two Cr sites. These striking differences suggests an interesting possibility that better control of doping at only near surface region may offer a path to quantum anomalous Hall states at higher temperatures than reported in the literature.

Crystal Growth and Magneto-transport of Bi2Se3 Single Crystals

In this letter, we report on the growth and characterization of bulk Bi 2Se 3 single crystals. The studied Bi 2Se 3 crystals are grown by the self-flux method through the solid-state reaction from high-temperature (950 °C) melt of constituent elements and slow cooling (2 ℃/h). The resultant crystals are shiny and grown in the [00l] direction, as evidenced from surface XRD. Detailed Reitveld analysis of powder X-ray diffraction (PXRD) of the crystals showed that these are crystallized in the rhombohedral crystal structure with a space group of R3m (D5), and the lattice parameters are a = 4.14 (2), b = 4.14 (2), and c = 28.7010 (7) A. Temperature versus resistivity (ρ−T) plots revealed metallic conduction down to 2 K, with typical room temperature resistivity (ρ 300 K) of around 0.53 m Ω-cm and residual resistivity (ρ 0 K) of 0.12 m Ω-cm. Resistivity under magnetic field [ ρ(T)H] measurements exhibited large + ve magneto-resistance right from 2 to 200 K. Isothermal magneto-resistance [ ρH] measurements at 2, 100, and 200 K exhibited magneto-resistance (MR) of up to 240 %, 130 %, and 60 %, respectively, at 14 T. Further, the MR plots are nonsaturating and linear with the field at all temperatures. At 2 K, the MR plots showed clear quantum oscillations at above say 10 T applied field. Also, the Kohler plots, i.e., Δρ/ ρ oversus B/ ρ, were seen consolidating on one plot. Interestingly, the studied Bi 2Se 3 single crystal exhibited the Shubnikov-de Haas (SdH) oscillations at 2 K under different applied magnetic fields ranging from 4 to 14 T.

Coherent phonons in Bi2Se3 induced by a powerful THz pulse

Using the electro-optic detection method we have detected coherent phonons of Eg 1 , Eu 1 , Eg 2 , and A 1g 1 symmetry generated in a crystalline film of Bi2 Se3 by a powerful single-cycle THz pulse. Coherent excitation of Raman active modes is interpreted as a result of three-and four-phonon interactions in the anharmonic crystal lattice of Bi2 Se3 .

On the berry phase in quantum oscillations observed in 3D topological insulators

It is demonstrated that the copper-doped high-quality Bi Se single crystals with a high density of bulk charge carriers are certainly 3D topological insulators. The analysis of quantum Shubnikov−de Haas (SdH) oscillations reveals that these materials simultaneously exhibit two types of such oscillations determined by the Landau levels related to both the 3D and 2D Fermi surfaces.

Phase equilibria and thermodynamic properties of saturated solid solutions based on the compounds BiSeI, Bi19Se27I3, and BiI3 in the Ag–Bi–Se–I system

We have studied phase relations in the AgI–BiI3–Se–Bi2Se3 region of the Ag–Bi–Se–I system for T ≤ 530 K and derived equations of overall potential-forming reactions involving the phases BiSeI, Bi19Se27I3, and BiI3. The reactions were brought about in electrochemical cells of the form C |Ag| Ag3GeS3I glass |D| C (where C stands for an inert (graphite) electrode, Ag and D denote the electrodes of the cell, D is a mechanical mixture of four phases in the system under investigation, and Ag3GeS3I glass is a membrane with purely ionic (Ag+) conductivity). The linear temperature dependences of EMF for the cells in the range 485–510 K were used to calculate the standard thermodynamic functions of the compounds BiSeI, Bi19Se27I3, and BiI3.

Metabolizable Ultrathin Bi2 Se3 Nanosheets in Imaging-Guided Photothermal Therapy.

Poly(vinylpyrrolidone)-encapsulated Bi2 Se3 nanosheets with a thickness of 1.7 nm and diameter of 31.4 nm are prepared by a solution method. Possessing an extinction coefficient of 11.5 L g(-1) cm(-1) at 808 nm, the ultrathin Bi2 Se3 nanosheets boast a high photothermal conversion efficiency of 34.6% and excellent photoacoustic performance. After systemic administration, the Bi2 Se3 nanosheets with the proper size and surface properties accumulate passively in tumors enabling efficient photoacoustic imaging of the entire tumors to facilitate photothermal cancer therapy. In vivo biodistribution studies reveal that they are expelled from the body efficiently after 30 d. The ultrathin Bi2 Se3 nanosheets have large clinical potential as metabolizable near-infrared-triggered theranostic agents.

Grundmannite, CuBiSe2, the Se-analogue of emplectite, a new mineral from the El Dragón mine, Potosí, Bolivia

Grundmannite, ideally CuBiSe 2 , is a new mineral species from the El Dragon mine, Department of Potosi, Bolivia. It either fills small shrinkage cracks or interstices in brecciated krut’aite – penroseite solid solutions or forms independent grains in the matrix. Grain size of the anhedral to subhedral crystals is usually in the range 50–150 μm, but may approach 250 μm. Grundmannite is usually intergrown with watkinsonite and clausthalite; other minerals occasionally being in grain-boundary contact comprise quartz, dolomite, native gold, eldragonite, eskebornite, umangite, klockmannite, Co-rich penroseite, and three unnamed phases of the Cu–Hg–Pb–Bi–Se system, among which is an as-yet uncharacterized species with the ideal composition Cu 4 Pb 2 HgBi 4 Se 11 . Grundmannite is non-fluorescent, black and opaque with a metallic lustre and black streak. It is brittle, with an irregular fracture and no obvious parting. Some grains display a perfect {001} cleavage. The VHN 20 values range from 45 to 61 (mean 53) kg mm −2 , which equates to a Mohs hardness of 2 to 2½. In plane-polarized incident light, grundmannite is weakly bireflectant and weakly pleochroic, from cream to light grey, and shows no internal reflections. Between crossed polarisers, grundmannite is distinctly anisotropic, with light-brown to brown rotation tints. The reflectance values in air for the COM standard wavelengths are: 41.0–43.4 (470 nm), 41.8–45.1 (546 nm), 42.1–45.7 (589 nm), and 42.5–46.2 (650 nm). Electron probe micro-analyses yielded a mean composition Cu 14.88, Pb 1.23, Hg 0.07, Ni 0.05, Bi 44.90, Se 38.92, total 100.05 wt.%. The mean empirical formula, normalized to 4 atoms per formula unit ( apfu ), is Cu 0.99 (Bi 0.91 Pb 0.02 ) ∑0.93 Se 2.08 ( n = 19). The ideal formula is CuBiSe 2 , which requires (in wt.%) Cu 14.76, Bi 48.55, Se 36.69, sum 100.00. Grundmannite is orthorhombic, space group Pnma , with a = 6.6362(5), b = 4.2581(3), c = 15.3691(9) A, V = 434.29(5) A 3 , and Z = 4. Density, calculated on the basis of the mean chemical composition and unit-cell parameters derived from the single-crystal X-ray study, is 6.582 g cm −3 . The five strongest X-ray powder-diffraction lines [ d in A ( I/I 0 ) ( hkl )] are: 3.4901 (50) (111), 3.3180 (70) (200), 3.2746 (100) (013), 2.4923 (45) (015), and 2.3307 (50) (213). The crystal structure of grundmannite is topologically identical to that of emplectite, CuBiS 2 , with the two independent sulfur positions occupied by Se, thus being the Se-isotype of emplectite. In the structure, Bi forms BiSe 3 trigonal pyramids (with two additional longer distances) and Cu nearly regular CuSe 4 tetrahedra. Grundmannite is a primary mineral, deposited from an oxidizing low- temperature hydrothermal fluid at the waning stage of selenide-mineral formation at El Dragon, at a f Se 2 / f S 2 ratio greater than unity and in the presence of hematite, conditions typically prevailing during the formation of telethermal vein-type selenide deposits. The mineral was named after Gunter Grundmann, in recognition of his pioneering work on the El Dragon mine.

Surface Tension Components Based Selection of Cosolvents for Efficient Liquid Phase Exfoliation of 2D Materials.

A proper design of direct liquid phase exfoliation (LPE) for 2D materials as graphene, MoS2 , WS2 , h-BN, Bi2 Se3 , MoSe2 , SnS2 , and TaS2 with common cosolvents is carried out based on considering the polar and dispersive components of surface tensions of various cosolvents and 2D materials. It has been found that the exfoliation efficiency is enhanced by matching the ratio of surface tension components of cosolvents to that of the targeted 2D materials, based on which common cosolvents composed of IPA/water, THF/water, and acetone/water can be designed for sufficient LPE process. In this context, the library of low-toxic and low-cost solvents with low boiling points for LPE is infinitely enlarged when extending to common cosolvents. Polymer-based composites reinforced with a series of different 2D materials are compared with each other. It is demonstrated that the incorporation of cosolvents-exfoliated 2D materials can substantially improve the mechanical and thermal properties of polymer matrices. Typically, with the addition of 0.5 wt% of such 2D material as MoS2 nanosheets, the tensile strength and Young's modulus increased up to 74.85% and 136.97%, respectively. The different enhancement effect of 2D materials is corresponded to the intrinsic properties and LPE capacity of 2D materials.

Conversion Reactions of Solids: From a Surprising Three-Step Mechanism towards Directed Product Formation.

Directed conversion reactions from binary to multinary compounds are discovered from the reaction of Bi2 S3 and Bi2 Se3 with NiCl2 ⋅6 H2 O in polyol media under basic conditions. Control of the synthesis conditions allows the preparation of NiBiSe and superconducting Ni3 Bi2 S2 and Ni3 Bi2 Se2 . The formation of Ni3 Bi2 S2 from Bi2 S3 is found from an unexpected three-step reaction path with Bi and NiBi as intermediates. In the more complex Ni/Bi/Se system, the mechanism found can be used to selectively direct the reaction between the competing ternaries and to suppress side-product formation. Contrary to solid-state reactions (500-900 °C) control of product formation is reached at reaction temperatures and times between 166-300 °C and 0.5-10 h, respectively. The formation of different phases is discussed from results of DFT calculations.

Kinetic limitation of chemical ordering in Bi2Te3−xSex layers grown by molecular beam epitaxy

We study the chemical ordering in Bi2Te3−xSex grown by molecular beam epitaxy on Si substrates. We produce films in the full composition range from x = 0 to 3, and determine their material properties using energy dispersive x-ray spectroscopy, x-ray diffraction and Raman spectroscopy. By fitting the parameters of a kinetic growth model to these results, we obtain a consistent description of growth at a microscopic level. Our main finding is that despite the incorporation of Se in the central layer being much more probable than that of Te, the formation of a fully ordered Te–Bi–Se–Bi–Te layer is prevented by kinetic of the growth process. Indeed, the Se concentration in the central layer of Bi2Te2Se1 reaches a maximum of only ≈75% even under ideal growth conditions. A second finding of our work is that the intensity ratio of the 0 0 12 and 0 0 6 x-ray reflections serves as an experimentally accessible quantitative measure of the degree of ordering in these films.

Perfluorocarbon-Loaded Hollow Bi2Se3 Nanoparticles for Timely Supply of Oxygen under Near-Infrared Light to Enhance the Radiotherapy of Cancer.

Hollow Bi2 Se3 nanoparticles prepared by a cation exchange method are loaded with perfluorocarbon as an oxygen carrier. With these nanoparticles, a promising concept is demonstrated to enhance radiotherapy by not only using their X-ray-absorbing ability to locally concentrate radiation energy in the tumor, but also employing near-infrared light to trigger burst release of oxygen from the nanoparticles to overcome hypoxia-associated radio-resistance.

Shape and stoichiometry control of bismuth selenide nanocrystals in colloidal synthesis

Temperature-dependent synthesis of hexagonal, disk-like and octahedron-like Bi–Se nanocrystals.

Two-dimensional bismuth-rich nanosheets through the evaporative thinning of Se-doped Bi2Te3

High bulk conductance obscures the behavior of surface states in the prototypical topological insulators Bi2Te3 and Bi2Se3. However, ternary phases of Bi2Te3−ySey with balanced donor and acceptor levels may lead to large bulk resistivity, allowing for the observation of the surface states. Additionally, the contribution of the bulk conductance may be further suppressed by nanostructuring, increasing the surface-to-volume ratio. Herein we report the synthesis of a ternary tetradymite newly confined to two dimensions. Ultra-thin large-area stable nanosheets were fabricated via evaporative thinning of a Bi2Te2.9Se0.1 original phase. Owing to vapor pressure differences, a compositional shift to a final Bi-rich phase is observed. The Se/Te ratio of the nanosheet increases tenfold, due to the higher stability of the Bi–Se bonds. Hexagonal crystal symmetry is maintained despite dramatic changes in thickness and stoichiometry. Given that small variations in stoichiometry of this ternary system can incur large changes in carrier concentration and switch majority carrier type, the large compositional shifts found in this case imply that compositional analysis of similar CVD and PVD grown materials is critical to correctly interpret topological insulator performance. Further, the characterization techniques deployed, including STEM-EDS and ToF-SIMS, serve as a case study in determining such compositional shifts in two-dimensional form.

Laser ablation ICPMS study of trace element chemistry in molybdenite coupled with scanning electron microscopy (SEM) — An important tool for identification of different types of mineralization

Using the example of four different types of associations from the Bohemian Massif (greisen-, gold-, base metal-, and “barren granite”-related) and of one porphyry–Cu–Mo(Au) deposit from Uzbekistan, it is shown that molybdenite from a specific type of mineralization possesses a distinct trace-element geochemical composition, which is locally influenced by the presence of relevant mineral phases occurring in the form of sub-nano-, nano- to micron-scale impurities/inclusions. It is documented that, besides typically lattice-bound elements (Re, and W) and isostructural concentrations of Se and Te (substituting for S), molybdenite can concentrate other metals (e.g., Ag, As, Au, Bi, Cu, Nb, Pb, Zn, Zr) which mostly form inclusions or impurities. Typical lattice-bound elements (W, Se, Te) can locally occur as impurities or micro-inclusions. The distribution of Re in molybdenite indicates crustal sources for the group of greisen-, base-metal-, and a few of the granite-related deposits, compared to a mixed mantle/crustal source for the Au-related and majority of granite-related deposits. Greisen-related molybdenites show the highest mean values of As (40ppm), Cu (58ppm), and Zn (45ppm), but the lowest mean Re value (0.4ppm). They contain abundant inclusions of native bismuth, which are accompanied, depending on the individual deposit, by variable quantities of Bi(Se)-, Bi(Pb)-, Pb(Bi)- and Pb–Bi–Cu-bearing phases and also bismuthinite (Bi2S3), wolframite and scheelite. Gold-related molybdenite typically has the highest mean values of Ag (464ppm), Au (24ppm), Bi (1188ppm), Sb (39ppm), and Te (112ppm) and also the highest median values of these elements. Gold mostly occurs as inclusions of native Au or in association with Bi (maldonite) and/or as Bi–Au–Te phases whereas silver is commonly present in the form of Au(Ag), and less commonly as Bi–Te–Ag, Ag–Se and Ag–Te phases. Selenium and Te are either isomorphously bound in the molybdenite structure (similarly to Re and W) and/or form micro-inclusions of different tellurides and selenides. Molybdenite from base metal associations is characterized by the highest mean Pb (3223ppm) and the highest median Ag (12ppm). These metals are present in the form of abundant micro-inclusions of galena locally accompanied by an Ag–S phase and anglesite [PbSO4] with indications of other chalcophile element sub-micro-inclusions. “Barren granite”-related molybdenites, dispersed in the rock matrix, in quartz and/or pegmatite veins and/or located in fractures, have the highest mean and median Ni values (16ppm). These molybdenites are either inclusion-free or contain micro-inclusions and also indications of sub-nano-impurities depending on the locality and a type of mineralization processes reported from the region. For example, inclusions of native bismuth, galena and anglesite occur in molybdenite from Harrachov and Bohutín where base-metal mineralization was exploited in the past. Molybdenite from the Kalmakyr porphyry–Cu–Mo(Au) deposit in Uzbekistan is characterized by the highest mean Re value (236ppm). Rhenium, Se and W have a homogeneous distribution in molybdenite from this deposit. Only galena and anglesite inclusions were identified visually in molybdenite from this type of deposit, while the presence of most likely sub-nano impurities of Cu, Zn and Ag is indicated by LA-ICPMS time-resolved depth profiles.

Traveling heater method에 의해 성장된 Bi2Te2.7Se0.3의 열전특성

Bi Te -Bi Se alloy which is typical n-type thermoelectric material were grown by traveling heater method (THM) technique. We investigate the effect of the composition of 100-x(Bi Te ) - x(Bi Se ) and doping of n-type dopants such as SbI and CdCl . Maximum figure of merit of Bi Te -Bi Se alloy was observed with CdCl 0.1 wt% (Z: 2.73 × 10 / K) and SbI 0.05 wt% (Z: 2.29 × 10 /K). Deviation along the length of Bi Te -Bi Se ingot grown by THM method is low, which indicates that the ingot is very homogenized. Also we observed the close relationship of between anisotropy ratio and dopant in the 90(Bi Te ) - 10(Bi Se ) alloys. And we confirmed the fact that anisotropy ratio exerts thermoelectric performance in Bi Te based n-type thermoelectric material.

High power L-band mode-locked fiber laser based on topological insulator saturable absorber

We demonstrate a passive mode-locked Er:Yb doped double-clad fiber laser using a microfiber-based topological insulator (Bi(2)Se(3)) saturable absorber (TISA). By optimizing the cavity loss and output coupling ratio, the mode-locked fiber laser can operate in L-band with high average output power. With the highest pump power of 5 W, 91st harmonic mode locking of soliton bunches with average output power of 308 mW was obtained. This is the first report that the TISA based erbium-doped fiber laser operating above 1.6 μm and is also the highest output power yet reported in TISA based passive mode-locked fiber laser.

Topological Surface States Originated Spin-Orbit Torques in Bi(2)Se(3).

The three dimensional topological insulator bismuth selenide (Bi(2)Se(3)) is expected to possess strong spin-orbit coupling and spin-textured topological surface states and, thus, exhibit a high charge to spin current conversion efficiency. We evaluate spin-orbit torques in Bi(2)Se(3)/Co(40)Fe(40)B(20) devices at different temperatures by spin torque ferromagnetic resonance measurements. As the temperature decreases, the spin-orbit torque ratio increases from ∼0.047 at 300 K to ∼0.42 below 50 K. Moreover, we observe a significant out-of-plane torque at low temperatures. Detailed analysis indicates that the origin of the observed spin-orbit torques is topological surface states in Bi(2)Se(3). Our results suggest that topological insulators with strong spin-orbit coupling could be promising candidates as highly efficient spin current sources for exploring the next generation of spintronic applications.

2D Hybrid Nanostructured Dirac Materials for Broadband Transparent Electrodes.

Broadband transparent electrodes based on 2D hybrid nanostructured Dirac materials between Bi2 Se3 and graphene are synthesized using a chemical vapor deposition (CVD) method. Bi2 Se3 nanoplates are preferentially grown along graphene grain boundaries as "smart" conductive patches to bridge the graphene boundary. These hybrid films increase by one- to threefold in conductivity while remaining highly transparent over broadband wavelength. They also display outstanding chemical stability and mechanical flexibility.

Momentum-resolved spin dynamics of bulk and surface excited States in the topological insulator Bi(2)Se(3).

The prospect of optically inducing and controlling a spin-polarized current in spintronic devices has generated wide interest in the out-of-equilibrium electronic and spin structure of topological insulators. In this Letter we show that only measuring the spin intensity signal over several orders of magnitude by spin-, time-, and angle-resolved photoemission spectroscopy can provide a comprehensive description of the optically excited electronic states in Bi_{2}Se_{3}. Our experiments reveal the existence of a surface resonance state in the second bulk band gap that is benchmarked by fully relativistic abinitio spin-resolved photoemission calculations. We propose that the newly reported state plays a major role in the ultrafast dynamics of the system, acting as a bottleneck for the interaction between the topologically protected surface state and the bulk conduction band. In fact, the spin-polarization dynamics in momentum space show that these states display macroscopically different temperatures and, more importantly, different cooling rates over several picoseconds.

A Roadmap for Controlled Production of Topological Insulator Nanostructures and Thin Films.

The group V-VI chalcogenide semiconductors (Bi2 Se3 , Bi2 Te3 , and Sb2 Te3 ) have long been known as thermoelectric materials. Recently, they have been once more generating interest because Bi2 Se3 , Bi2 Te3 and Sb2 Te3 have been crowned as 3D topological insulators (TIs), which have insulating bulk gaps and metallic Dirac surface states. One big challenge in the study of TIs is the lack of high-quality materials with few defects and insulating bulk states. To manifest the topological surface states, it is critical to suppress the contribution from the bulk carriers. Controlled production of TI nanostructures that have a large surface-to-volume ratio is an efficient way to reduce the bulk conductance and to significantly enhance the topological surface conduction. In this review article, the recent progress on the preparation of TI nanostructures is highlighted. Basic production methods for TI nanostructures are introduced in detail. Furthermore, several specific production approaches to reduce the residual bulk carriers from defects are summarized. Finally, the progress and the prospects of the production of TI-based heterostructures, which hold promise in both fundamental study and novel applications are discussed.