Topological Insulator Bi2Se3 Research Articles

Lattice thermal conductivity of topological insulator Bi2Se3 nanocrystals: comparison from theoretical and experimental.

Lattice thermal conductivity (κL) calculations using the Wiedemann-Franz law involve electrical conductivity, which introduces an error in the actual value of κL. We have adopted a non-contact measurement technique and calculated the κL from the temperature and power-dependent Raman spectra of the Bi2Se3 nanocrystals with truncated hexagon plate morphology stabilized in a hexagonal crystal structure. The hexagon plates of Bi2Se3 are 37 to 55 nm thick with lateral dimensions around 550 nm. These Bi2Se3 nanocrystals show three Raman lines, which agree with the theoretical prediction of A11g, E2g and A21g modes. Although the first-order thermal coefficient (-0.016) of Bi2Se3 nanocrystals is quite low, the room temperature κL ∼1.72 W m-1 K-1 is close to the value obtained from the simulation adopting a three-phonon process. The phonon lifetime of Bi2Se3 nanocrystals observed between ∼0.2 ps and 2 ps confirmed carrier-carrier thermalization with a small contribution from electron-electron and intraband electron-longitudinal-optical-phonon relaxation. The variations of phonon lifetime, Gruneisen parameter and κL of the mode frequencies outline the crucial role of the anharmonicity and acoustic-optical phonon scattering in reducing the κL of Bi2Se3. The non-contact measurements and relevant thermal property parameters open up exciting opportunities to address the anharmonic effects in other thermoelectric materials for obtaining a high figure of merit.

Sensitive biosensors based on topological insulator Bi2Se3 and peptide

In this work, we designed a facile and label-free electrochemical biosensor based on intrinsic topological insulator (TI) Bi2Se3 and peptide for the detection of immune checkpoint molecules. With topological protection, Bi2Se3 could have robust surface states with low electronic noise, which was beneficial for the stable and sensitive electron transport between electrode and electrolyte interface. The peptides are easily synthesized and chemically modified, and have good biocompatibility and bioavailability, which is a suitable candidate as the recognition units for immune checkpoint molecules. Therefore, the peptide/Bi2Se3 was developed as a suitable working electrode for the electrochemical biosensor. The basic performance of the designed peptide/Bi2Se3 biosensor was investigated to determine the Anti-HA Tag Antibody and PD-L1 molecules. The linear detection range was from 3.6 × 10−10 mg mL−1 to 3.6 × 10−5 mg mL−1, and the detection limit was 1.07 × 10−11 mg mL−1. Moreover, the biosensor also displayed good selectivity and stability.

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.

Milliwatt terahertz harmonic generation from topological insulator metamaterials

Achieving efficient, high-power harmonic generation in the terahertz spectral domain has technological applications, for example, in sixth generation (6G) communication networks. Massless Dirac fermions possess extremely large terahertz nonlinear susceptibilities and harmonic conversion efficiencies. However, the observed maximum generated harmonic power is limited, because of saturation effects at increasing incident powers, as shown recently for graphene. Here, we demonstrate room-temperature terahertz harmonic generation in a Bi2Se3 topological insulator and topological-insulator-grating metamaterial structures with surface-selective terahertz field enhancement. We obtain a third-harmonic power approaching the milliwatt range for an incident power of 75 mW—an improvement by two orders of magnitude compared to a benchmarked graphene sample. We establish a framework in which this exceptional performance is the result of thermodynamic harmonic generation by the massless topological surface states, benefiting from ultrafast dissipation of electronic heat via surface-bulk Coulomb interactions. These results are an important step towards on-chip terahertz (opto)electronic applications.

Observation of unconventional proximity induced superconducting effects in Bi2Se3 flakes

Topological superconductors (TSCs) are the materials with superconducting gap in bulk and exotic quasiparticles form gapless edge state. These quasiparticles can be employed as topological quantum bits for execution of quantum computation. Intrinsic TSCs are very rare in nature but artificial TSCs can be engineered by combining topological insulators with s-wave conventional superconductors. Hence recently hybrid structure composed of topological insulator and superconductor has been subject of research in condensed matter physics. In this paper, we use Bi2Se3 topological insulator thin flakes and tungsten (W) superconducting electrodes are deposited by using FIB based gas injection system (GIS). Fabricated Bi2Se3 flake/W hybrid structures were used to investigate induced superconductivity in Bi2Se3 flake via electric transport measurements. The resistance of the flake shows a drastic and significant decrease (8.4%) when the electrodes become superconducting. The superconducting transition (8.4%) is observed even though the length of Bi2Se3 flake between the superconducting contacts was 1.38 μm (≫ superconducting coherence length). Surprisingly, we find that resistance of Bi2Se3 flake shows an abrupt increase just before the transition temperature of superconducting electrodes (T < Tc) due to presence of electron-electron interaction. We also observe that the magnetoresistance of the flake is negative below transition temperature (T < Tc), but above transition temperature (T > Tc) magnetoresistance becomes positive due to the effects of superconducting electrodes which we discussed here in detail and it may help in constructing the topological superconductor.

A sample-position-autocorrection system with precision better than 1 µm in angle-resolved photoemission experiments

We present the development of a high-precision sample-position-autocorrection system for photoemission experiments. A binocular vision method based on image pattern matching calculations was realized to track the sample position with an accuracy better than 1 µm, which was much smaller than the spot size of the incident laser. We illustrate the performance of the sample-position-autocorrection system with representative photoemission data on the topological insulator Bi2Se3 and an optimally doped cuprate superconductor Bi2Sr2CaCu2O8+δ. Our method provides new possibilities for studying the temperature-dependent electronic structures in quantum materials using laser-based or spatially resolved photoemission systems with high precision and efficiency.

Topological insulator nanoparticles for strong light-matter interaction in the terahertz regime.

We study the spontaneous emission (SPEM) for a quantum emitter (QUEM) near a topological insulator Bi2Se3 nanosphere. We calculate numerically the QUEM Purcell factor near nanospheres of radii between 40 nm and 100 nm, with and without taking into account the topologically protected delocalized states at the surface of the nanosphere. We find exceptionally large Purcell factors up to 1010 at distances between the QUEM and the nanosphere as large as half its radius in the terahertz regime. By computing the SPEM dynamics for a QUEM with transition frequencies in the terahertz and free-space decay rates in the nanosecond to millisecond range, we observe intense reversible dynamics, as well as population trapping effects. This work demonstrates that a Bi2Se3 nanosphere provides the conditions for strong light-matter interaction at the nanoscale in the terahertz regime.

Two-photon IR pumped UV–Vis transient absorption spectroscopy of Dirac fermions in the topological insulator Bi2Se3

It is often taken for granted that in pump-probe experiments on the topological insulator (TI) Bi2Se3 using IR pumping with a commercial Ti:sapphire laser [∼800 nm (1.55 eV photon energy)], the electrons are excited in the one-photon absorption regime, even when pumped with absorbed fluences in the mJ cm−2 range. Here, using UV–Vis transient absorption (TA) spectroscopy, we show that even at low-power Infrared (IR) pumping with absorbed fluences in the μJ cm−2 range, the TA spectra of the TI Bi2Se3 extend across a part of the UV and the entire visible region. This observation suggests unambiguously that the two-photon pumping regime accompanies the usual one-photon pumping regime even at low laser powers applied. We attribute the high efficiency of two-photon pumping to the giant nonlinearity of Dirac fermions in the Dirac surface states (SS). On the contrary, one-photon pumping is associated with the excitation of bound valence electrons in the bulk into the conduction band. Two mechanisms of absorption bleaching were also revealed since they manifest themselves in different spectral regions of probing and cause the appearance of three different relaxation dynamics. These two mechanisms were attributed to the filling of the phase-space in the Dirac SS and bulk states, followed by the corresponding Pauli blocking.

Effect of gallium doping on structural and transport properties of the topological insulator Bi2Se3 grown by molecular beam epitaxy

Topological insulators possess non-conductive bulk and present surface states, henceforth, they are electrically conductive along their boundaries. Bismuth selenide (Bi2Se3) is one of the most promising topological insulators. However, a major drawback is its n-type nature arising from its natural doping, which makes the transport in the bulk dominant. This effect can be overcome by shifting the chemical potential into the bandgap, turning the transport of the surface states to be more pronounced than the bulk counterpart. In this work, Bi2Se3 was grown by molecular beam epitaxy and doped with 0.8, 2, 7, and 14 at. % of Ga, with the aim of shifting the chemical potential into the bandgap. The structural, morphological, and electronic properties of the Ga doped Bi2Se3 are studied. Raman and x-ray diffraction measurements confirmed the incorporation of the dopants into the crystal structure. Transport and magnetoresistance measurements in the temperature range of 1.5–300 K show that Ga-doped Bi2Se3 is n-type with a bulk charge carrier concentration of 1019 cm−3. Remarkably, magnetotransport of the weak antilocalization effect measurements confirms the existence of surface states up to a doping percentage of 2 at. % of Ga and coherence length values between 50 and 800 nm, which envisages the possibility of topological superconductivity in this material.

Electronic transport studies of Ag-doped Bi2Se3 topological insulator

The structural, magnetotransport, and angle-resolved photoemission spectroscopy (ARPES) of Ag-doped Bi2Se3 single crystals are presented. Temperature dependent resistivity exhibits metallic behavior with a slope change above 200 K for Ag-doped Bi2Se3. The magnetoresistance shows positive quadratic dependence at low fields satisfying Kohler's rule. Hall resistivity measurement shows that electrons are dominant charge carriers. Furthermore, these results agree well with the ARPES spectra observed at T = 20 K, where the Fermi level lies inside the bulk conduction band. The Dirac point of the topological surface states is shifted toward higher binding energy (∼0.12 eV) for Ag-doped samples as compared to pristine Bi2Se3.

Manipulating the magneto-resistance of Bi2Se3 thin films by strontium doping

We systematically studied the magneto-resistance of strontium (Sr) doped topological insulator (TI) Bi2Se3 films. For Bi2Se3 films with relatively large classic magneto-resistance, we found that slightly Sr doping can completely suppress the classic magneto-resistance and make the weak antilocalization effect dominate in the weak-magnetic-field region. Consequently, the value of the magneto-resistance is enhanced by more than 2.7 times at 0.2 T in slightly doped samples. In contrast, in the strong-magnetic-field region, the magneto-resistance is strongly reduced by doping but exhibits linear magneto-resistance behavior. Our results suggest that the linear magneto-resistance behavior originates from the mobility fluctuation induced by disorder described by the Parish–Littlewood model. Our findings not only gain insights into the doping effect on the topological insulator but also provide an effective way to manipulate the magneto-transport properties of TI for potential applications in future.

Electrical Transport and Magnetic Properties of High-Quality Sr x Bi2Se3 Single Crystals Obtained by Annealing

Topological insulator Bi2Se3 becomes topological superconductors by doping Cu or Sr. These materials attract much interest because of the nematic state showing two-fold symmetry when the magnetic field is applied parallel to ab-plane. In this study, single crystals of Sr x Bi2Se3 (0.1 ≤ x ≤ 0.3) were grown by the self-flux method followed by annealing process, which improved superconducting properties. Using these high-quality samples, in-plane magnetic-field-angle dependence of resistivity was measured. The result showed two-fold symmetry and indicated the existence of nematic domains. In addition, magnetoresistance as a function of in-plane field direction was measured and is discussed with regard to crystal inhomogeneity. These results help us to perform future experiments of direct visualization of nematic domains.

Electrochemical biosensor based on topological insulator Bi2Se3 tape electrode for HIV-1 DNA detection.

A large-size Bi2Se3 tape electrode (BTE) was prepared by peeling off a 2 × 1 × 0.5cm high-quality single crystal. The feasibility of using the flexible BTE as an efficient bioplatform to load Au nanoparticles and probe DNA for HIV-1 DNA electrochemical sensing was explored. Differential pulse voltammetry (DPV) and electrochemical impedance spectroscopy (EIS) show that the resultant biosensor has a wide linear range from 0.1 fM to 1pM, a low detection limit of 50 aM, excellent selectivity, reproducibility and stability, and is superior to the pM DNA detection level of Pt-Au, graphene-AuNPs hybrid biosensors. This outstanding performance is attributed to the intrinsic surface state of Bi2Se3 topological insulator in facilitating electron transfer. Therefore, BTE electrochemical biosensor platform has great potential in the application for sensitive detection of DNA biomarkers.

First-principles calculations of the surface states of doped and alloyed topological materials via band unfolding method

One of the most remarkable characteristics of topological materials is that they have special surface states, which are determined by the topological properties of their bulk materials. Angle resolved photoemission spectroscopy (ARPES) is a powerful tool to explore surface states, which allows to further investigate the topological phase transitions. However, it is very difficult to compare the first-principles calculated band structures to the ARPES results, when the systems are doped or alloyed, because the band structures are heavily folded. We develop an efficient band unfolding method based on numerical atomic orbitals (NAOs). We apply this method to study the surface states of the non-magnetically and magnetically doped topological insulators Bi2Se3 and the topological crystalline insulators Pb1−xSnxTe.

Ultrafast interfacial carrier dynamics and persistent topological surface states of Bi2Se3 in heterojunctions with VSe2

Vanadium diselenide (VSe2) has recently been highlighted as an efficient 2D electrode owing to its extra-high conductivity, thickness controllability, and van der Waals contact. However, as the electrode, applications of VSe2 to various materials are still lacking. Here, by employing ultrafast time-resolved spectroscopy, we study VSe2-thickness-dependent interfacial effects in heterostructures with topological insulator Bi2Se3 that is severely affected by contact with conventional 3D electrodes. Our results show unaltered Dirac surface state of Bi2Se3 against forming junctions with VSe2, efficient ultrafast hot electron transfer from VSe2 to Bi2Se3 across the interface, shortened metastable carrier lifetimes in Bi2Se3 due to dipole interactions enabling efficient current flow, and the electronic level shift (~tens meV) of bulk states of Bi2Se3 by interfacial interactions, which is ~10 times lower compared to conventional electrodes, implying weak Fermi level pinning. Our observations confirm VSe2 as an ideal electrode for efficient Bi2Se3-based-applications with full utilization of topological insulator characteristics.

Temperature dependence of nanoscale friction on topological insulator Bi2Se3 surfaces

The topological insulator Bi2Se3 is an insulator in the bulk and has unusual metallic surface states that consist of spin-polarized Dirac fermions. Although the electronic properties of topological insulators have been extensively studied, the friction characteristic that is a key factor for further applications is barely known. In this study, conductive friction force microscopy (c-FFM) in ultrahigh vacuum (UHV) conditions was used to probe the nanoscale friction on freshly cleaved Bi2Se3 planes (0001) as a function of surface temperature from 105.0 K to 300.0 K. The experimental results demonstrate a non-monotonic enhancement of dry friction, with distinct friction peaks emerging at K and K. While the widths of the friction peaks are nearly independent of the normal force, the relative peak height increases and the peak temperature decreases with normal load. We reveal that this anomalous friction behavior originates from the formation and rupture of multiple thermally activated sub-contacts, which is further confirmed by velocity dependence measurements. These results for multiple thermally activated sub-contact formation and rupture provide a deep insight into the frictional behaviors of Bi2Se3.

A new type of cyclotron resonance from charge-impurity scattering in the bulk-insulating Bi2Se3 thin films

This work focuses on the low frequency Drude response of bulk-insulating topological insulator (TI) Bi2Se3 films. The frequency and field dependence of the mobility and carrier density are measured simultaneously via time-domain terahertz spectroscopy. These films are grown on buffer layers, capped by Se, and have been exposed in air for months. Under a magnetic field up to 7 Tesla, we observe prominent cyclotron resonances (CRs). We attribute the sharp CR to two different topological surface states from both surfaces of the films. The CR sharpens at high fields due to an electron-impurity scattering. By using magneto-terahertz spectroscopy, we confirm that these films are bulk-insulating, which paves the way to use intrinsic TIs without bulk carriers for applications including topological spintronics and quantum computing.

Signature of weak-antilocalization in sputtered topological insulator Bi2Se3 thin films with varying thickness

We report a low-temperature magneto transport study of Bi2Se3 thin films of different thicknesses (40, 80 and 160 nm), deposited on sapphire (0001) substrates, using radio frequency magnetron sputtering technique. The high-resolution x-ray diffraction measurements revealed the growth of rhombohedral c-axis {0003n} oriented Bi2Se3 films on sapphire (0001). Vibrational modes of Bi2Se3 thin films were obtained in the low wavenumber region using Raman spectroscopy. The surface roughness of sputtered Bi2Se3 thin films on sapphire (0001) substrates were obtained to be ~ 2.26–6.45 nm. The chemical and electronic state of the deposited Bi2Se3 was confirmed by X-ray photoelectron spectroscopy and it showed the formation of Bi2Se3 compound. Resistivity versus temperature measurements show the metallic nature of Bi2Se3 films and a slight up-turn transition in resistivity at lower temperatures < 25 K. The positive magneto-resistance value of Bi2Se3 films measured at low temperatures (2–100 K) confirmed the gapless topological surface states in Bi2Se3 thin films. The quantum correction to the magnetoconductivity of thin films in low magnetic field is done by employing Hikami–Larkin–Nagaoka theory and the calculated value of coefficient ‘α’ (defining number of conduction channels) was found to be 0.65, 0.83 and 1.56 for film thickness of 40, 80 and 160 nm, respectively. These observations indicate that the top and bottom surface states are coupled with the bulk states and the conduction mechanism in Bi2Se3 thin films varied with the film thicknesses.

Proximity-Induced Magnetism in a Topological Insulator/Half-Metallic Ferromagnetic Thin Film Heterostructure

Topological insulator (TI) Bi2Se3 thin films were prepared on half-metallic ferromagnetic La0.7Sr0.3MnO3 thin film by magnetron sputtering, forming a TI/FM heterostructure. The conductivity of Bi2Se3was modified by La0.7Sr0.3MnO3 at high- and low-temperature regions via different mechanisms, which could be explained by the short-range interactions and long-range interaction between ferromagnetic insulator and Bi2Se3 due to the proximity effect. Magnetic and transport measurements prove that the ferromagnetic phase and extra magnetic moment are induced in Bi2Se3 films. The weak anti-localized (WAL) effect was suppressed in Bi2Se3 films, accounting for the magnetism of La0.7Sr0.3MnO3 layers. This work clarifies the special behavior in Bi2Se3/La0.7Sr0.3MnO3 heterojunctions, which provides an effective way to study the magnetic proximity effect of the ferromagnetic phase in topological insulators.

Probing the growth quality of molecular beam epitaxy-grown Bi2Se3 films via in-situ spectroscopic ellipsometry

Multilayer heterostructures of the topological insulator Bi2Se3 and conventional semiconductor In2Se3, as well as solid solution layers of (Bi1-xInx)2Se3 layers were grown by molecular beam epitaxy and analyzed in-operando using spectroscopic ellipsometry (SE). SE spectra were obtained after the deposition of each layer to determine the respective dielectric functions and thicknesses of each layer. In contrast to ex-situ SE, where uncertainty in the dielectric function and thicknesses of individual layer impose limitations to extract a correct model for the dielectric function of such multilayer heterostructures from a single set of SE data, the step-by-step in-situ SE data recover more precise dielectric functions for Bi2Se3, In2Se3, (Bi0.7In 0.3)2Se3 and the cap-layer. The optical models developed for multilayer structures can decipher minute perturbations in layers as the growth progresses. Our models show that a ∼7 nm Bi2Se3 layer grown next to a sapphire substrate seems to disappear as the structure is annealed at 600 °C. Finally, when the dielectric functions were represented as a collection of Kramers-Kronig-consistent oscillators, in-situ SE predicted the quality of films; the weighted-average broadening parameter for oscillators used for Bi2Se3 films grown on (Bi0.7In 0.3)2Se3 layer is ∼20% smaller compared to Bi2Se3 films directly grown on sapphire, confirming that the former film is of better quality, and providing a direct metric to quantify film quality and defect concentration. These conclusions were corroborated by transport data.