Band Bending Inversion in Bi2Se3 Nanostructures

Semiconductor-metal transition in Bi2Se3 caused by impurity doping

Doping a typical topological insulator, Bi2Se3, with Ag impurity causes a semiconductor-metal (S-M) transition at 35 K. To deepen the understanding of this phenomenon, structural and transport properties of Ag-doped Bi2Se3 were studied. Single-crystal X-ray diffraction (SC-XRD) showed no structural transitions but slight shrinkage of the lattice, indicating no structural origin of the transition. To better understand electronic properties of Ag-doped Bi2Se3, extended analyses of Hall effect and electric-field effect were carried out. Hall effect measurements revealed that the reduction of resistance was accompanied by increases in not only carrier density but carrier mobility. The field-effect mobility is different for positive and negative gate voltages, indicating that the EF is located at around the bottom of the bulk conduction band (BCB) and that the carrier mobility in the bulk is larger than that at the bottom surface at all temperatures. The pinning of the EF at the BCB is found to be a key issue to induce the S-M transition, because the transition can be caused by depinning of the EF or the crossover between the bulk and the top surface transport.

Magnetotransport Studies of Encapsulated Topological Insulator Bi2Se3 Nanoribbons

The majority of proposed exotic applications employing 3D topological insulators require high-quality materials with reduced dimensions. Catalyst-free, PVD-grown Bi₂Se₃ nanoribbons are particularly promising for these applications due to the extraordinarily high mobility of their surface Dirac states, and low bulk carrier densities. However, these materials are prone to the formation of surface accumulation layers; therefore, the implementation of surface encapsulation layers and the choice of appropriate dielectrics for building gate-tunable devices are important. In this work, all-around ZnO-encapsulated nanoribbons are investigated. Gate-dependent magnetotransport measurements show improved charge transport characteristics as reduced nanoribbon/substrate interface carrier densities compared to the values obtained for the as-grown nanoribbons on SiO₂ substrates.

High-Yield Growth and Tunable Morphology of Bi2Se3 Nanoribbons Synthesized on Thermally Dewetted Au

The yield and morphology (length, width, thickness) of stoichiometric Bi₂Se₃ nanoribbons grown by physical vapor deposition is studied as a function of the diameters and areal number density of the Au catalyst nanoparticles of mean diameters 8–150 nm formed by dewetting Au layers of thicknesses 1.5–16 nm. The highest yield of the Bi₂Se₃ nanoribbons is reached when synthesized on dewetted 3 nm thick Au layer (mean diameter of Au nanoparticles ~10 nm) and exceeds the nanoribbon yield obtained in catalyst-free synthesis by almost 50 times. The mean lengths and thicknesses of the Bi₂Se₃ nanoribbons are directly proportional to the mean diameters of Au catalyst nanoparticles. In contrast, the mean widths of the Bi₂Se₃ nanoribbons do not show a direct correlation with the Au nanoparticle size as they depend on the contribution ratio of two main growth mechanisms—catalyst-free and vapor–liquid–solid deposition. The Bi₂Se₃ nanoribbon growth mechanisms in relation to the Au catalyst nanoparticle size and areal number density are discussed. Determined charge transport characteristics confirm the high quality of the synthesized Bi₂Se₃ nanoribbons, which, together with the high yield and tunable morphology, makes these suitable for application in a variety of nanoscale devices.

Surface structure promoted high-yield growth and magnetotransport properties of Bi2Se3 nanoribbons

In the present work, a catalyst-free physical vapour deposition method is used to synthesize high yield of Bi2Se3 nanoribbons. By replacing standard glass or quartz substrates with aluminium covered with ultrathin porous anodized aluminium oxide (AAO), the number of synthesized nanoribbons per unit area can be increased by 20-100 times. The mechanisms of formation and yield of the nanoribbons synthesized on AAO substrates having different arrangement and size of pores are analysed and discussed. It is shown that the yield and average length of the nanoribbons can base tuned by adjustment of the synthesis parameters. Analysis of magnetotransport measurements for the individual Bi2Se3 nanoribbons transferred on a Si/SiO2 substrate show the presence of three different populations of charge carriers, originating from the Dirac surface states, bulk carriers and carriers from a trivial 2DEG from an accumulation layer at the Bi2Se3 nanoribbon interface with the substrate.

Bulk-free topological insulator Bi2Se3 nanoribbons with magnetotransport signatures of Dirac surface states

Many applications of topological insulators (TIs) as well as new phenomena require devices with reduced dimensions. While much progress has been made to realize thin films of TIs with low bulk carrier densities, nanostructures have not yet been reported with similar properties, despite the fact that reduced dimensions should help diminish the contributions from bulk carriers. Here we demonstrate that Bi₂Se₃ nanoribbons, grown by a simple catalyst-free physical-vapour deposition, have inherently low bulk carrier densities, and can be further made bulk-free by thickness reduction, thus revealing the high mobility topological surface states. Magnetotransport and Hall conductance measurements, in single nanoribbons, show that at thicknesses below 30 nm, the bulk transport is completely suppressed which is supported by self-consistent band-bending calculations. The results highlight the importance of material growth and geometrical confinement to properly exploit the unique properties of topological surface states.

Scanning tunneling microscopy investigations of unoccupied surface states in two-dimensional semiconducting β-√3 × √3-Bi/Si(111) surface

Two-dimensional surface structures often host a surface state in the bulk gap, which plays a crucial role in the surface electron transport. The diversity of in-gap surface states extends the category of two-dimensional systems and gives us more choices in material applications. In this article, we investigated the surface states of β-√3 × √3-Bi/Si(111) surface by scanning tunneling microscopy. Two nearly free electron states in the bulk gap of silicon were found in the unoccupied states. Combined with first-principles calculations, these two states were verified to be the Bi-contributed surface states and electron-accumulation-induced quantum well states. Due to the spin-orbit coupling of Bi atoms, Bi-contributed surface states exhibit free-electron Rashba splitting. The in-gap surface states with spin splitting can possibly be used for spin polarized electronics applications.

Determination of the energy band gap of Bi2Se3

Despite intensive investigations of Bi₂Se₃ in past few years, the size and nature of the bulk energy band gap of this well-known 3D topological insulator still remain unclear. Here we report on a combined magneto-transport, photoluminescence and infrared transmission study of Bi₂Se₃, which unambiguously shows that the energy band gap of this material is direct and reaches E_g = (220 ± 5) meV at low temperatures.

Weakly-coupled quasi-1D helical modes in disordered 3D topological insulator quantum wires

Disorder remains a key limitation in the search for robust signatures of topological superconductivity in condensed matter. Whereas clean semiconducting quantum wires gave promising results discussed in terms of Majorana bound states, disorder makes the interpretation more complex. Quantum wires of 3D topological insulators offer a serious alternative due to their perfectly-transmitted mode. An important aspect to consider is the mixing of quasi-1D surface modes due to the strong degree of disorder typical for such materials. Here, we reveal that the energy broadening γ of such modes is much smaller than their energy spacing Δ, an unusual result for highly-disordered mesoscopic nanostructures. This is evidenced by non-universal conductance fluctuations in highly-doped and disordered Bi2Se3 and Bi₂Te₃ nanowires. Theory shows that such a unique behavior is specific to spin-helical Dirac fermions with strong quantum confinement, which retain ballistic properties over an unusually large energy scale due to their spin texture. Our result confirms their potential to investigate topological superconductivity without ambiguity despite strong disorder.

Assembly of g-C3N4-based type II and Z-scheme heterojunction anodes with improved charge separation for photoelectrojunction water oxidation

g-C₃N₄, studied as a metal-free photocatalyst, can lead to excellent results but the recombination of photogenerated charge carriers can substantially limit its performance.

In-plane topological p-n junction in the three-dimensional topological insulator Bi2-xSbxTe3-ySey

A topological p-n junction (TPNJ) is an important concept to control spin and charge transport on a surface of three-dimensional topological insulators (3D-TIs). Here we report successful fabrication of such TPNJ on a surface of 3D-TI Bi_2−xSb_xTe_3−ySe_y thin films and experimental observation of the electrical transport. By tuning the chemical potential of n-type topological Dirac surface of Bi_2−xSb_xTe_3−ySe_y on its top half by using tetrafluoro-7,7,8,8-tetracyanoquinodimethane as an organic acceptor molecule, a half surface can be converted to p-type with leaving the other half side as the opposite n-type, and consequently TPNJ can be created. By sweeping the back-gate voltage in the field effect transistor structure, the TPNJ was controlled both on the bottom and the top surfaces. A dramatic change in electrical transport observed at the TPNJ on 3D-TI thin films promises novel spin and charge transport of 3D-TIs for future spintronics.

Dirac cone surface states rectify an ultralow dissipative spin and charge current, but it is yet to be confirmed in devices. Here, Tu et al. observe p-type electrical transport on one half surface and n-type on the other in Bi_2−xSb_xTe_3−ySe_y thin films, realizing a topological p-n junction.

Enhanced Mobility of Spin-Helical Dirac Fermions in Disordered 3D Topological Insulators

The transport length l_tr and the mean free path l_e are determined for bulk and surface states in a Bi₂Se₃ nanoribbon by quantum transport and transconductance measurements. We show that the anisotropic scattering of spin-helical Dirac fermions results in a strong enhancement of l_tr (≈ 200 nm) and of the related mobility μ_tr (≈ 4000 cm² V^-1 s^-1), which confirms theoretical predictions.1 Despite strong disorder, the long-range nature of the scattering potential gives a large ratio l_tr/l_e ≈ 8, likely limited by bulk/surface coupling. This suggests that the spin-flip length l_sf ≈ l_tr could reach the micron size in materials with a reduced bulk doping and paves the way for building functionalized spintronic and ballistic electronic devices out of disordered 3D topological insulators.

Quantum Transport and Nano Angle-resolved Photoemission Spectroscopy on the Topological Surface States of Single Sb2Te3 Nanowires

We report on low-temperature transport and electronic band structure of p-type Sb2Te3 nanowires, grown by chemical vapor deposition. Magnetoresistance measurements unravel quantum interference phenomena, which depend on the cross-sectional dimensions of the nanowires. The observation of periodic Aharonov-Bohm-type oscillations is attributed to transport in topologically protected surface states in the Sb2Te3 nanowires. The study of universal conductance fluctuations demonstrates coherent transport along the Aharonov-Bohm paths encircling the rectangular cross-section of the nanowires. We use nanoscale angle-resolved photoemission spectroscopy on single nanowires (nano-ARPES) to provide direct experimental evidence on the nontrivial topological character of those surface states. The compiled study of the bandstructure and the magnetotransport response unambiguosly points out the presence of topologically protected surface states in the nanowires and their substantial contribution to the quantum transport effects, as well as the hole doping and Fermi velocity among other key issues. The results are consistent with the theoretical description of quantum transport in intrinsically doped quasi-one-dimensional topological insulator nanowires.