Nanoimprinted substrates for high-yield production of topological insulator nanoribbons

We present a growth process mediated by nanoimprinted nanostructures specifically for producing bismuth selenide (Bi₂Se₃) topological insulator nanoribbons with a high yield. In this process, topological insulator nanostructures are grown on nanoimprinted gratings by using a nanoparticle-catalyzed vapor–liquid–solid mechanism. In comparison with the growth processes performed on flat and randomly rough substrates, such a nanograting-mediated growth method produces topological insulator nanoribbons with a higher yield (～15?000 nanoribbons/mm²), a narrower average ribbon width (w _avg<60 nm), and a higher uniformity in ribbon width (σ<30 nm); effectively suppresses the formation of other unwanted morphologies; and also results in the axial growth of nanoribbons along specific in-plane directions relative to pre-structured gratings. Such technical merits of nanograting-mediated growth are attributed to the preferential nucleation of Bi₂Se₃ crystal seeds and the concomitant pinning of catalytic nanoparticles at ordered grating edges. Finally, Aharonov–Bohm interference oscillations in the magnetoresistance were observed and demonstrated the coherent transport of electrons through topological surface states of Bi₂Se₃ nanoribbons. This growth process in combination with large-area nanoimprint lithography could serve as an important foundation for nanomanufacturing topological insulator nanoribbons with controllable feature size, large-area uniformity, and ordering, suitable for applications in future low-dissipation nanoelectronics.     

Superconductivity and non-metallicity induced by doping the topological insulators Bi2Se3 and Bi2Te3

We show that by Ca doping the Bi₂Se₃ topological insulator, the Fermi level can be fine tuned to fall inside the band gap and therefore suppresses the bulk conductivity. Non-metallic Bi₂Se₃ crystals are obtained. On the other hand, the Bi₂Se₃ topological insulator can also be induced to become a bulk superconductor, with T_c∼3.8 K, by copper intercalation in the van der Waals gaps between the Bi₂Se₃ layers. Likewise, an as-grown crystal of metallic Bi₂Te₃ can be turned into a non-metallic crystal by slight variation in the Te content. The Bi₂Te₃ topological insulator shows small amounts of superconductivity with T_c∼5.5 K when reacted with Pd to form materials of the type Pd_zBi₂Te₃.     

Impact of Ultrathin Pb Films on the Topological Surface and Quantum-Well States of Bi<Subscript>2</Subscript>Se<Subscript>3</Subscript> and Sb<Subscript>2</Subscript>Te<Subscript>3</Subscript> Topological Insulators

The effect of an ultrathin Pb film deposited on the surface of Bi₂Se₃ and Sb₂Te₃ compounds on the electronic state structure of topological insulators is studied experimentally by the angle-resolved photoemission spectroscopy (ARPES) technique. The following features are revealed: formation of two-dimensional quantum-well states in the near-surface region, an increase in the binding energy of the Dirac cone and the core levels, and a simultaneous electronic states intensity redistribution in the system in photoemission spectra. The results obtained show that topological states may coexist at the interface between studied materials and a superconductor, which seems to be promising for application in quantum computers.     

Specific features of the electronic,spin, and atomic structures of a topological insulator Bi<Subscript>2</Subscript>Te<Subscript>2.4</Subscript>Se<Subscript>0.6</Subscript>

The specific features of the electronic and spin structures of a triple topological insulator Bi₂Te_2.4Se_0.6, which is characterized by high-efficiency thermoelectric properties, have been studied with the use of angular- and spin-resolved photoelectron spectroscopy and compared with theoretical calculations in the framework of the density functional theory. It has been shown that the Fermi level for Bi₂Te_2.4Se_0.6 falls outside the band gap and traverses the topological surface state (the Dirac cone). Theoretical calculations of the electronic structure of the surface have demonstrated that the character of distribution of Se atoms on the Te–Se sublattice practically does not influence the dispersion of the surface topological electronic state. The spin structure of this state is characterized by helical spin polarization. Analysis of the Bi₂Te_2.4Se_0.6 surface by scanning tunnel microscopy has revealed atomic smoothness of the surface of a sample cleaved in an ultrahigh vacuum, with a lattice constant of ~4.23 Å. Stability of the Dirac cone of the Bi₂Te_2.4Se_0.6 compound to deposition of a Pt monolayer on the surface is shown.     

Three- and two-dimensional topological insulators in Pb2Sb2Te5, Pb2Bi2Te5, and Pb2Bi2Se5 layered compounds

The electronic structure of ternary compounds Pb₂Sb₂Te₅, Pb₂Bi₂Te₅, and Pb₂Bi₂Se₅, which have a layered structure that consists of nine-layer atomic blocks separated by van der Waals gaps, has been theoretically studied. It has been shown that all studied compounds are three-dimensional topological insulators. The possibility of the existence of a two-dimensional topological insulator has been found in ultrathin (0001) Pb₂Sb₂Te₅ and Pb₂Bi₂Te₅ films. Oscillations of the ℤ₂ topological invariant with an increase in the film thickness have been observed in the latter compound.     

One-pot synthesis in liquid paraffin of ternary alloy CdSexS1−x nanocrystals with fluorescence emission covering entire visible region

We report the synthesis and characterization of layer-structure Bi₂Se₃ nanomaterials. Bi₂Se₃ nanomaterial has attracted many researchers, because it has a unique three-dimensional topological insulator property characterized by a metallic surface which coexists with an insulating interior. This can be achieved by having large surface-to-volume ratio in the nanomaterial. We synthesized highly single-crystalline topological insulator Bi₂Se₃ nanomaterials with various morphologies, including straight nanowires, zig-zag nanowires, and nanobelts, by adjusting experimental parameters such as the growth temperature, pressure, and carrier gas flow rate. The results show that the width and length of Bi₂Se₃ nanowires increase significantly with increasing values of each parameter. Furthermore, we studied the growth mechanism of individual morphologies based on the layered structure of Bi₂Se₃.     

Superconductivity of topological matters induced via pressure

By applying pressure on the topological insulator Bi₂Te₃ single crystal, superconducting phase was found without a crystal structure phase transition. The new superconducting phase is under the pressure range of 3 GPa to 6 GPa. The high pressure Hall effect measurements indicated that the superconductivity caused by bulk hole pockets. The high pressure structure investigations with synchrotron X-ray diffraction indicated that the superconducting phase is of similar structure to that of ambient phase structure with only slight change with lattice parameter and internal atomic position. The topological band structures indicate the superconducting phase under high pressure remained topologically nontrivial. The results suggested that topological superconductivity can be realized in Bi₂Te₃ due to the proximity effect between superconducting bulk states and Diractype surface states. We also discussed the possibility that the bulk state could be a topological superconductor.     

On the origin of two-dimensional electron gas states at the surface of topological insulators

The ab initio calculations of the electronic structure in the bulk and at the (0001) surface of narrow-band Bi₂Se₃, Sb₂Te₃, Sb₂STe₃, and Sb₂SeTe₂ semiconductors have been performed. It has been shown that ternary compounds Sb₂STe₂ and Sb₂SeTe₂, as well as the previously known compounds Bi₂Se₃ and Sb₂Te₃, are three-dimensional topological insulators. The influence of the subsurface van der Waals gap expansion on the surface electronic structure of these compounds has been analyzed. It has been shown that this expansion leads to the formation of new (trivial) surface states, namely a parabolic state in the conduction band and an M-shaped state in the valence band. These results explain the phenomena discovered recently in photoemission experiments and reveal the nature of new states that are caused by the adsorption of atoms on the surfaces of the layered topological insulators.     

The heterostructure and electrical properties of Sb2Se3/Bi2Se3 grown by molecular beam epitaxy

We report the growth and characterization of Sb₂Se₃/Bi₂Se₃ bilayer films fabricated by molecular beam epitaxy. High quality heterostructures are obtained as evidenced from the X-ray diffraction (XRD), atomic force microscopy and high-resolution transmission electron microscopic (HRTEM) analysis. Interestingly, Sb₂Se₃ grows as a (120) hexacrystal film in orthorhombic phase on rhombohedral Bi₂Se₃ (0001) plane, as verified by the out-of-plane and in-plane XRD scans. The cross-sectional TEM studies indicate a sharp interface between Sb₂Se₃ and Bi₂Se₃, which is important for the protection of surface states Bi₂Se₃. The ultraviolet photoelectron spectroscopy indicates that the Fermi level located 0.95 eV above the valence band maximum in Sb₂Se₃. The insulating nature of Sb₂Se₃ is confirmed by the nonlinear current-voltage curve via the vertical junction electrical measurement. By four point probe measurements, we confirm the charge transfer effect from Sb₂Se₃ into Bi₂Se₃, and such effect can be reduced in the Sb₂Se₃/(Bi_0.7Sb_0.3)₂Se₃ bilayer. This work opens a new avenue for the synthesis of multilayers consisting of topological insulators and ordinary insulator, which is important for harvesting of the multiple surface states for advanced electronic and spintronic applications.     

Weak antilocalization in thin films of the Bi<Subscript>2</Subscript>Te<Subscript>2.7</Subscript>Se<Subscript>0.3</Subscript> solid solution

A technology has been developed for the preparation of thin films of the Bi₂Te_2.7Se_0.3 solid solution through the thermal evaporation in a vacuum using the “hot-wall” method. The high quality of the thin films thus prepared has been confirmed by the X-ray diffraction and Raman scattering data. The electron transport has been investigated over wide ranges of temperatures (1.4–300 K) and magnetic fields (up to 8 T). It has been assumed that the observed weak antilocalization is associated with the dominant contribution from the surface states of a topological insulator. The dephasing length has been estimated.     

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.     

Effect of heat treatment on the structure and the thermoelectric properties of Sb<Subscript>0.9</Subscript>Bi<Subscript>1.1</Subscript>Te<Subscript>2.9</Subscript>Se<Subscript>0.1</Subscript> thin films and composites based on them

This work considers the effect of vacuum annealing on the thermoelectric properties of Sb_0.9Bi_1.1Te_2.9Se_0.1 thin film and Sb_0.9Bi_1.1Te_2.9Se_0.1–C composites with various carbon contents produced by ion-beam deposition in an argon atmosphere. The electrical resistivity and the thermopower of Sb_0.9Bi_1.1Te_2.9Se_0.1–C nanocomposites are found to be dependent on not only the carbon concentration but also the type and the concentration of intrinsic point defects of the Sb_0.9Bi_1.1Te_2.9Se_0.1 solid solution, which determine the type of conductivity of Sb_0.9Bi_1.1Te_2.9Se_0.1 granules. The power factors are estimated for films of Sb_0.9Bi_1.1Te_2.9Se_0.1 solid solution and films of Sb_0.9Bi_1.1Te_2.9Se_0.1–C composites and found to have values comparable with the values for nanostructured materials on the basis of (Bi,Sb)₂(Te,Se)₃ solid solutions.     

Multiple surface conduction channels via topological insulator and amorphous insulator thin film multi-stacks

Multi-channel Bi₂Se₃ thin films were grown by combining molecular beam epitaxy and atomic layer deposition. High-resolution transmission electron microscope images showed that c-axis oriented Bi₂Se₃ grew on amorphous Al₂O₃ even after multiple stacking. While the surface morphology degraded for the upper layers, each layer was electrically similar. The electrical transport measurements showed that the weak anti-localization effect was quantitatively enhanced upon increasing the number of Bi₂Se₃ channels. Our results provide a promising approach to exploit diverse combinations of layered topological insulator films vertically stacked with amorphous insulator films.     

A new topological insulator built from quasi one‐dimensional atomic ribbons

A novel topological insulator with orthorhombic crystal structure is demonstrated. It is characterized by quasi one‐dimensional, conducting atomic chains instead of the layered, two‐dimensional sheets known from the established Bi₂(Se,Te)₃ system. The Sb‐doped Bi₂Se₃ nanowires are grown in a TiO₂‐catalyzed process by chemical vapor deposition. The binary Bi₂Se₃ is transformed from rhombohedral to orthorhombic by substituting Sb on ～38% of the Bi sites. Pure Sb₂Se₃ is a topologically trivial band insulator with an orthorhombic crystal structure at ambient conditions, and it is known to transform into a topological insulator at high pressure. Angle‐resolved photoemission spectroscopy shows a topological surface state, while Sb doping also tunes the Fermi level to reside in the bandgap. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Changes in the electronic properties of Bi2X3 (X: Se, Te) single crystals due to intercalation

Bi₂Se₃ and Bi₂Te₃ are layered semiconductors n-type and p-type, respectively, which belong to the family of thermoelectric materials. In this work we examine the insertion of Cu in Bi₂Te₃ and Bi₂Se₃ single crystals through an intercalation reaction. The inserted Cu acting as donor enhances the n-type character of Bi₂Se₃ while changing the native p-type character of Bi₂Te₃ to n-type. The spatial distribution of the intercalated species was monitoring by X-ray microanalysis and microscopic IR reflectivity measurements. Paper presented at the 4th Euroconference on Solid State Ionics, Renvyle, Galway, Ireland, Sept. 13–19, 1997     

Bound states induced by a ferromagnetic delta-layer inserted into a three-dimensional topological insulator

We report on theoretical study of the bound electron states induced by a ferromagnetic delta-layer embedded into a narrow-band-gap semiconductor of the Bi₂Se₃-type which is a three-dimensional topological insulator with large spin-orbit coupling. We make use of an effective Hamiltonian taking into account the inverted band structure of the semiconductor host at the ?? point and describe the properties of the in-gap bound states: energy spectrum, characteristic length and spin polarization. We highlight a role of these states for a magnetic proximity effect in digital magnetic heterostructures based on the Bi₂Se₃-type semiconductors.     

Topological surface states of Bi2Te2Se are robust against surface chemical modification

The robustness of the Dirac‐like electronic states on the surfaces of topological insulators (TIs) during materials process‐ing is a prerequisite for their eventual device application. Here, the (001) cleavage surfaces of crystals of the topological insulator Bi₂Te₂Se (BTS) were subjected to several surface chemical modification procedures that are common for electronic materials. Through measurement of Shubnikov–de Hass (SdH) oscillations, which are the most sensitive measure of their quality, the surface states of the treated surfaces were compared to those of pristine BTS that had been exposed to ambient conditions. In each case – surface oxidation, deposition of thin layers of Ti or Zr oxides, or chemical modification of the surface oxides – the robustness of the topological surface electronic states was demonstrated by noting only very small changes in the frequency and amplitude of the SdH oscillations. (© 2014 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Multiple andreev reflections in topological insulator nanoribbons

Superconducting proximity junctions made of topological insulator (TI) nanoribbons (NRs) provide a useful platform for studying topological superconductivity. We report on the fabrication and measurement of Josephson junctions (JJs) using Sb-doped Bi₂Se₃ NRs in contact with Al electrodes. Aharonov–Bohm and Altshuler–Aronov–Spivak oscillations of the axial magneto-conductance of TI NR were observed, indicating the existence of metallic surface states along the circumference of the TI NR. We observed the supercurrent in the TI NR JJ and subharmonic gap structures of the differential conductance due to multiple Andreev reflections. The interface transparency of the TI NR JJs estimated based on the excess current reaches τ = 0.83, which is among the highest values reported for TI JJs. The temperature dependence of critical current is consistent with the short and ballistic junction model confirming the formation of highly transparent superconducting contacts on the TI NR. Our observations would be useful for exploring topological Josephson effects in TI NRs.     

Effect of thermal treatment on the thermoelectric properties of Sb<Subscript>0.9</Subscript>Bi<Subscript>1.1</Subscript>Te<Subscript>2.9</Subscript>Se<Subscript>0.1</Subscript> solid solution thin films

The effect thermal treatment in a vacuum has on the thermoelectric properties of Sb_0.9Bi_1.1Te_2.9Se_0.1 solid solution thin films obtained via ion-beam sputtering in an argon atmosphere is considered. It is established that the specific resistance and thermopower are determined by the type and concentration of intrinsic point defects of the Sb_0.9Bi_1.1Te_2.9Se_0.1 solid solution. The power factor values are found to be comparable to those of nanostructured materials based on (Bi,Sb)₂(Te,Se)₃ solid solutions.