Bulk Bi2Se3 Research Articles

Electrical, Thermal and Spectroscopic Characterization of Bulk Bi2Se3 Topological Insulator

We report the electrical (angular magnetoresistance and Hall), thermal (heat capacity) and spectroscopic (Raman, X-ray photoelectron, angle-resolved photoelectron) characterization of a bulk Bi2Se3 topological insulator, which was grown by self-flux method through solid-state reaction from high-temperature (950. degrees C) melt and slow cooling (2. degrees C/h) of constituent elements. Bi2Se3 exhibited metallic behaviour down to 5 K. Magnetotransport measurements revealed linear up to 400 and 30% magneto-resistance (MR) at 5 K under a 14-T field in perpendicular and parallel field directions, respectively. We noticed that the MR of Bi2Se3 is very sensitive to the angle of the applied field. The MR is maximum when the field is normal to the sample surface, while it is minimum when the field is parallel. The Hall coefficient (RH) is seen nearly invariant with a negative carrier sign down to 5 K albeit having near-periodic oscillations above 100 K. The heat capacity (C-p) versus temperature plot is seen without any phase transitions down to 5 K and is well fitted (Cp =gamma T + beta T-3) at low temperature with a calculated Debye temperature (theta(D)) value of 105.5 K. Clear Raman peaks are seen at 72, 131 and 177 cm(-1) corresponding to A(1g)(1), E-g(2) and A(1g)(2), respectively. Though two distinct asymmetric characteristic peak shapes are seen for Bi 4f(7/2) and Bi 4f(5/2), the Se 3d region is found to be broad, displaying the overlapping of spin-orbit components of the same. Angle-resolved photoemission spectroscopy (ARPES) data of Bi2Se3 revealed distinctly the bulk conduction bands (BCB), surface state (SS), Dirac point (DP) and bulk valence bands (BVB), and 3D bulk conduction signatures are clearly seen. Summarily, a host of physical properties for the as-grown Bi2Se3 crystal are reported here.

Tight-binding theory of NMR shifts in topological insulatorsBi2Se3andBi2Te3

Motivated by recent nuclear magnetic resonance (NMR) experiments, we present a microscopic sp3 tight-binding model calculation of the NMR shifts in bulk Bi2Se3, and Bi2Te3. We compute the contact, dipolar, orbital and core polarization contributions to the carrier-density-dependent part of the NMR shifts in Bi209, Te125 and Se77. The spin-orbit coupling and the layered crystal structure result in a contact Knight shift with strong uniaxial anisotropy. Likewise, because of spin-orbit coupling, dipolar interactions make a significant contribution to the isotropic part of the NMR shift. The contact interaction dominates the isotropic Knight shift in Bi209 NMR, even though the electronic states at the Fermi level have a rather weak s-orbital character. In contrast, the contribution from the contact hyperfine interaction to the NMR shift of Se77 and Te125 is weak compared to the dipolar and orbital shifts therein. In all cases, the orbital shift is at least comparable to the contact and dipolar shifts, while the shift due to core polarization is subdominant (except for Te nuclei located at the inversion centers). By artificially varying the strength of spin-orbit coupling, we evaluate the evolution of the NMR shift across a band inversion but find no clear signature of the topological transition.

Band structure and spin texture ofBi2Se3 3dferromagnetic metal interface

The spin-helical surface states in three-dimensional topological insulator (TI), such as Bi2Se3, are predicted to have superior efficiency in converting charge current into spin polarization. This property is said to be responsible for the giant spin-orbit torques observed in ferromagnetic metal/TI structures. In this work, using first-principles and model tight-binding calculations, we investigate the interface between the topological insulator Bi2Se3 and 3d-transition ferromagnetic metals Ni and Co. We find that the difference in the work functions of the topological insulator and the ferromagnetic metals shift the topological surface states down about 0.5 eV below the Fermi energy where the hybridization of these surface states with the metal bands destroys their helical spin structure. The band alignment of Bi2Se3 and Ni (Co) places the Fermi energy far in the conduction band of bulk Bi2Se3, where the spin of the carriers is aligned with the magnetization in the metal. Our results indicate that the topological surface states are unlikely to be responsible for the huge spin-orbit torque effect observed experimentally in these systems.

Topological modification of the electronic structure by Bi-bilayers lying deep inside bulk Bi2Se3

We observe the modified surface states of an epitaxial thin film of a homologous series of (Bi2)m(Bi2Se3)n, as a topological insulator (TI), by angle-resolved photoemission spectroscopy measurements. A thin film with m : n = 1 : 3 (Bi8Se9) has been grown with Bi2 bilayers embedded every other three quintuple layers (QLs) of Bi2Se3. Despite the reduced dimension of continuous QLs due to the Bi2 heterolayers, we find that the topological surface states stem from the inverted Bi and Se states and the topologically nontrivial structures are mainly based on the prototype of 3D TI Bi2Se3 without affecting the overall topological order.

Growth of the Bi2Se3 Surface Oxide for Metal–Semiconductor–Metal Device Applications

The effect of the surface structure of Bi2Se3 on its interior properties has been well studied recently, but the interfacial structure and electrical properties of the oxidized Bi2Se3 surface are little known. In contrast to the self-limited formation of native oxide on Bi2Se3, the degree of oxidation on the Bi2Se3 surface in oxygen plasma is enhanced. Results of transmission electron microscopy and X-ray photoelectron spectroscopy show that the surface of the oxidized Bi2Se3 is composed of a layer of amorphous bismuth oxide (BiOx), and the thickness of the BiOx layer can be controlled by the length of the plasma process. Electrical measurements of this structure present the Schottky-type transport property at the interface between the oxidized layer and the bulk Bi2Se3 crystal, and the turn-on voltage depends on the thickness of the surface BiOx layer. This study of the structure, formation mechanism, and electrical properties of the surface oxide of Bi2Se3 formed in oxygen plasma provides useful informa...

Electrodynamics and quantum capacity: The case of Bi2Se3 topological insulator

Layered Bi-chalcogenide topological insulators are among the most available energy conversion (thermoelectric) and storage (battery) materials. Motivated by this applied aspect of fundamental importance and the good agreement between theory and key experiments probing spectroscopy and dc transport, we undertake a detailed study of electrodynamic responses of bulk Bi2Se3 topological insulator. In particular, we confirm that the interplay between spin-orbit and electron correlations underpins its bulk metallicity. We show the implications of our proposal for the multi-channel nature of galvanostatic, voltage-capacity profiles intrinsic to LixBi2Se3 battery material. Supported by a microscopic description of quantum capacity, we predict that layered Bi-based topological insulators are promising candidates for future high-voltage solid-state batteries.

Magneto-Optics of Massive Dirac Fermions in BulkBi2Se3

We report on magneto-optical studies of Bi2Se3, a representative member of the 3D topological insulator family. Its electronic states in bulk are shown to be well described by a simple Dirac-type Hamiltonian for massive particles with only two parameters: the fundamental band gap and the band velocity. In a magnetic field, this model implies a unique property-spin splitting equal to twice the cyclotron energy: Es=2Ec. This explains the extensive magnetotransport studies concluding a fortuitous degeneracy of the spin and orbital split Landau levels in this material. The Es=2Ec match differentiates the massive Dirac electrons in bulk Bi2Se3 from those in quantum electrodynamics, for which Es=Ec always holds.

Topological Insulator Bi2Se3 Nanowire Field Effect Transistors

Topological insulator is characterized as a new class of materials which have an insulating band gap in the bulk and gapless surface state which is protected by its intrinsic time-reversal symmetry. Here, we report high-performance topological insulator nanowire field effect transistors (FETs) with sharp turn-on, clear cutoff current, large On/Off current ratio (~ 108) and well-saturated, strong inversion-mode output current-voltage characteristics. The metallic electron transport at surface, with effective mobility of 200 cm2V-1s-1 ~ 1000 cm2V-1s-1, and bulk conduction, with activation energy close to band gap of bulk Bi2Se3 can be effectively separated and tuned by gated field effect. We also report the first experimental observation of anomalous Aharonov-Bohm oscillation in Bi2Se3 nanowire FET under strong surface disorder. The magnetotransport properties demonstrated here of topological insulator nanostructure provide effective approaches of surface states modulations, and can enable more spintronics device applications and sensing applications.

Topological Insulator Bi2Se3 Nanowire Field Effect Transistors

Topological insulators (TI) have insulating band gaps in the bulk but gapless surface states topologically protected by time-reversal symmetry [1, 2]. Bi2Se3, a well-known semiconductor and thermoelectric material, is a TI with a bulk band gap of 0.35 eV and a single Dirac cone on the surface [2, 3]. Most current experimental research focus on the surface states of thin TI samples exfoliated from bulk material [4]. A few groups have reported modification of the surface conduction of such exfoliated TI samples by doping, applying a vertical electric field or polarized light. However, high-performance microelectronic devices based on topological insulators such as the analog of metal oxide semiconductor field effect transistors (MOSFETs) have not yet been reported.MOSFETs are the basic building blocks for digital and analog circuits. In this work, we fabricated and measured surrounding-gate Bi2Se3 nanowire (NW) field-effect transistors. The current-voltage (I-V) characteristics were measured at different temperatures, exhibiting excellent performance: well-saturated output I-V curves and large on/off ratio. The surface metallic conduction can be separated from the bulk conduction with gate electric field at different temperatures. We found that the surface conduction increases as temperature decreases, while the bulk conductance decreases with decreasing temperature. This indicates that the carriers at the surface and the carriers inside the bulk Bi2Se3 body have different properties. In addition, the Bi2Se3 FET exhibits I-V characteristics different to conventional MOSFETs in the weak inversion region. The drain current (IDS) increases significantly beyond 3фt (фt is thermal potential). This trend indicates that the Bi2Se3 nanowire FET current does not follow the same diffusion current model as that describes conventional MOSFETs. We believe IDSin the weak/moderate conductive regions is also dominated by drift current.To further study the topological insulator properties, we have performed magneto-transport measurements and observed Aharonov-Bohm (AB) oscillation that arise from the surface conduction channel that surrounds the Bi2Se3 NW in these FETs. The AB oscillations are obtained at low temperatures with a magnetic field oriented along the channel direction. The devices exhibit the anomalous AB effect [5] while maintaining high FET performance. The AB oscillations have a period of h/e and can be significantly tuned by the top surrounding gate. The resistance has a peak at B = 0 and decreases as the magnetic field increases. This phenomenon indicates that the surface states may be bound together by their spin in the circle and activated for conduction by the magnetic field along the axial direction of the circle.In summary, we have fabricated and measured Bi2Se3nanowire field effect transistors with excellent I-V characteristics. The devices exhibit anomalous AB oscillation, which may lead to new discovery of spin coupling in topological insulators.

Preparation of few-layer bismuth selenide by liquid-phase-exfoliation and its optical absorption properties.

Bismuth selenide (Bi2Se3), a new topological insulator, has attracted much attention in recent years owing to its relatively simple band structure and large bulk band gap. Compared to bulk, few-layer Bi2Se3 is recently considered as a highly promising material. Here, we use a liquid-phase exfoliation method to prepare few-layer Bi2Se3 in N-methyl-2-pyrrolidone or chitosan acetic solution. The resulted few-layer Bi2Se3 dispersion demonstrates an interesting absorption in the visible light region, which is different from bulk Bi2Se3 without any absorption in this region. The absorption spectrum of few-layer Bi2Se3 depends on its size and layer number. At the same time, the nonlinear and saturable absorption of few-layer Bi2Se3 thin film in near infrared is also characterized well and further exploited to generate laser pulses by a passive Q-switching technique. Stable Q-switched operation is achieved with a lower pump threshold of 9.3 mW at 974 nm, pulse energy of 39.8 nJ and a wide range of pulse-repetition-rate from 6.2 to 40.1 kHz. Therefore, the few-layer Bi2Se3 may excite a potential applications in laser photonics and optoelectronic devices.

Stacking effects in topological insulator Bi2Se3：a first-principles study

By using first-principles method, we study the stacking effects on the electronic structure, topological phase and spin splitting in the bulk and film of topological insulator Bi2Se3. It is found that the different stackings can lead to different interlayer interactions and change the centrosymmetry of Bi2Se3. The centrosymmetric ABC and AAA stackings in bulk Bi2Se3 have similar band structures. ABA stacking breaks the centrosymmetry, giving rise to considerable changes of the band structure and large spin splitting. We further study the stacking effects in the film of Bi2Se3 and find that the non-centrosymmetric ABA stacking can induce large spin splitting in Bi2Se3 film. It is proposed and illustrated that the strain can tune the spin splitting effectively.

Ultrafast carrier dynamics in thin-films of the topological insulator Bi2Se3

Transient reflectivity measurements of thin films, ranging from 6 to 40 nm in thickness, of the topological insulator Bi2Se3 reveal a strong dependence of the carrier relaxation time on the film thickness. For thicker films, the relaxation dynamics are similar to those of bulk Bi2Se3, where the contribution of the bulk insulating phase dominates over that of the surface metallic phase. The carrier relaxation time shortens with decreasing film thickness, reaching values comparable to those of noble metals. This effect may result from the hybridization of Dirac cone states at the opposite surfaces for the thinnest films.

Theoretical study of influencing factors on the dispersion of bulk band-gap edges and the surface states in topological insulators Bi2Te3 and Bi2Se3

The dispersion of the band-gap edge states in bulk topological insulators Bi2Te3 and Bi2Se3 is considered within density functional theory. The dependences of this dispersion both on the approximation used for an exchange-correlation functional at fixed unit cell parameters and atomic positions and on these parameters and positions that are obtained upon structural relaxation performed using a certain approximated functional are analyzed. The relative position of the Dirac point of topologically protected surface states and the valence band maximum in the surface electronic structure of the topological insulators is discussed.

Topological Insulator Bi2Se3 Nanowire High Performance Field-Effect Transistors

Topological insulators are unique electronic materials with insulating interiors and robust metallic surfaces. Device applications exploiting their remarkable properties have so far been hampered by the difficulty to electrically tune the Fermi levels of both bulk and thin film samples. Here we show experimentally that single-crystal nanowires of the topological insulator Bi2Se3 can be used as the conduction channel in high-performance field effect transistor (FET), a basic circuit building block. Its current-voltage characteristics are superior to many of those reported for semiconductor nanowire transistors, including sharp turn-on, nearly zero cutoff current, very large On/Off current ratio, and well-saturated output current. The metallic electron transport at the surface with good FET effective mobility can be effectively separated from the conduction of bulk Bi2Se3 and adjusted by field effect at a small gate voltage. This opens up a suite of potential applications in nanoelectronics and spintronics.

First-principles investigations of the atomic, electronic, and thermoelectric properties of equilibrium and strained Bi2Se3and Bi2Te3including van der Waals interactions

Bi2Se3 and Bi2Te3 are layered compounds of technological importance, being excellent thermoelectric materials as well as topological insulators. We report density functional theory calculations of the atomic, electronic and thermoelectric properties of strained bulk and thin film Bi2Se3 and Bi2Te3, focusing on an appropriate description of van der Waals interactions. The calculations show that the van der Waals Density functional with Cooper's exchange can reproduce closely the experimental interlayer distances in unstrained Bi2Se3 and Bi2Te3. Interestingly, we predict atomic structures that are in much better agreement with the experimentally determined structure from Nakajima than that obtained from Wyckoff, especially for Bi2Se3 where the difference in atomic structures qualitatively changes the electronic band structure. The band structure obtained using the Nakajima structure, and the theoretically optimized structure, are in much better agreement with previous reports of photoemission measurements, than that obtained using the Wyckoff structure. Fully optimizing atomic structures of bulk and thin film Bi2Se3 and Bi2Te3 under different in-plane and uniaxial strains, we predict that the electronic band gap of both the bulk materials and thin films decreases with tensile in-plane strain and increases with compressive in-plane strain. We also predict, using the semiclassical Boltzmann approach, that the magnitude of the n-type Seebeck coefficient of Bi2Te3 can be increased by the compressive in-plane strain, while that of Bi2Se3 can be increased with tensile in-plane strain. Further, the in-plane power factor of n-doped Bi2Se3 can be increased with compressive uniaxial strain, while that of n-doped Bi2Te3 can be increased by compressive in-plane strain. ...

Erratum: Crossover of the three-dimensional topological insulator Bi2Se3 to the two-dimensional limit

Bi2Se3 is theoretically predicted1 2and experimentally observed2,3 to be a three dimensional topological insulator. For possible applications, it is important to understand the electronic structure of the planar device. In this work, thickness dependent band structure of molecular beam epitaxy grown ultrathin films of Bi2Se3 is investigated by in situ angle-resolved photoemission spectroscopy. An energy gap is observed for the first time in the topologically protected metallic surface states of bulk Bi2Se3 below the thickness of six quintuple layers, due to the coupling between the surface states from two opposite surfaces of the Bi2Se3 film. The gapped surface states exhibit sizable Rashba-type spin-orbit splitting, due to breaking of structural inversion symmetry induced by SiC substrate. The spin-splitting can be controlled by tuning the potential difference between the two surfaces.

Topological Insulator Nanowires and Nanoribbons

Recent theoretical calculations and photoemission spectroscopy measurements on the bulk Bi(2)Se(3) material show that it is a three-dimensional topological insulator possessing conductive surface states with nondegenerate spins, attractive for dissipationless electronics and spintronics applications. Nanoscale topological insulator materials have a large surface-to-volume ratio that can manifest the conductive surface states and are promising candidates for devices. Here we report the synthesis and characterization of high quality single crystalline Bi(2)Se(3) nanomaterials with a variety of morphologies. The synthesis of Bi(2)Se(3) nanowires and nanoribbons employs Au-catalyzed vapor-liquid-solid (VLS) mechanism. Nanowires, which exhibit rough surfaces, are formed by stacking nanoplatelets along the axial direction of the wires. Nanoribbons are grown along [1120] direction with a rectangular cross-section and have diverse morphologies, including quasi-one-dimensional, sheetlike, zigzag and sawtooth shapes. Scanning tunneling microscopy (STM) studies on nanoribbons show atomically smooth surfaces with approximately 1 nm step edges, indicating single Se-Bi-Se-Bi-Se quintuple layers. STM measurements reveal a honeycomb atomic lattice, suggesting that the STM tip couples not only to the top Se atomic layer, but also to the Bi atomic layer underneath, which opens up the possibility to investigate the contribution of different atomic orbitals to the topological surface states. Transport measurements of a single nanoribbon device (four terminal resistance and Hall resistance) show great promise for nanoribbons as candidates to study topological surface states.

Preparation, Characterization, and Electrophysical Properties of Nanostructured BiPO4 and Bi2Se3 Derived from a Structurally Characterized, Single-Source Precursor Bi[Se2P(OiPr)2]3

A Tris-chelated diselenophosphato complex of bismuth, Bi[Se2P(OiPr)2]3, was successfully prepared and used to obtain two separate, uniform deposits of nanostructured metal phosphate, BiPO4, and metal chalcogenide, Bi2Se3, in a one-step metal−organic chemical vapor deposition process. This work expands the applications of single-source precursors in a new dimension by producing two separate, uniform products from decomposition of a single-source precursor in one step. The inclusion of oxygen and phosphorus elements in the precursor molecule makes possible the simultaneous production of BiPO4 nanowires and Bi2Se3 nanoplates from the single-source precursor. The resulting BiPO4 nanowires and Bi2Se3 nanoplates show promising field emission properties, comparable to the more popular oxide semiconductor nanowires. The Bi2Se3 nanoplates also exhibit a superior thermoelectric property over bulk Bi2Se3.