Individual Bi Research Articles

Surface superconductivity in thin cylindrical Bi nanowire.

The physical origin and the nature of superconductivity in nanostructured Bi remains puzzling. Here, we report transport measurements of individual cylindrical single-crystal Bi nanowires, 20 and 32 nm in diameter. In contrast to nonsuperconducting Bi nanoribbons with two flat surfaces, cylindrical Bi nanowires show superconductivity below 1.3 K. However, their superconducting critical magnetic fields decrease with their diameter, which is the opposite of the expected behavior for thin superconducting wires. Quasiperiodic oscillations of magnetoresistance were observed in perpendicular fields but were not seen in the parallel orientation. These results can be understood by a model of surface superconductivity with an enhanced surface-to-bulk volume in small diameter wires, where the superconductivity originates from the strained surface states of the nanowires due to the surface curvature-induced stress.

Diameter-dependent thermoelectric figure of merit in single-crystalline Bi nanowires.

The diameter-dependent thermoelectric properties of individual single-crystalline Bi nanowires grown by the on-film formation of nanowires method have been investigated. The electrical resistivity, Seebeck coefficient, and thermal conductivity were measured as functions of the nanowire diameter using an individual nanowire device. The thermoelectric figure of merit (ZT) calculated from the measured thermoelectric properties shows an increase from the bulk value to a maximum value of 0.28 at 109 nm-diameter, followed by a decrease upon further decreasing the diameter. This non-monotonic diameter dependence of ZT in Bi nanowires reveals simultaneous positive and negative contributions to the thermoelectric efficiency, driven by the change in intrinsic properties, which originates from the diameter-dependent classical and quantum size effects.

Photocatalytic detoxification of Acid Red 18 by modified ZnO catalyst under sunlight irradiation

In this work, hybrid structured Bi–Au–ZnO composite was prepared by precipitation–decomposition method. This method is mild, economical and efficient. Bi–Au–ZnO was characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), transmission electron microscopy (TEM), energy dispersive spectrum (EDS), diffuse reflectance spectra (DRS), photoluminescence spectra (PL) and BET surface area measurements. Photocatalytic activity of Bi–Au–ZnO was evaluated by irradiating the Acid Red 18 (AR 18) dye solution under sun light. Heterostructured Bi–Au–ZnO photocatalyst showed higher photocatalytic activity than those of individual Bi–ZnO, Au–ZnO, bare ZnO, and TiO2-P25 at pH 11. The effects of operational parameters such as the amount of catalyst dosage, dye concentration, initial pH on photo mineralization of AR 18 dye have been analyzed. The mineralization of AR 18 has been confirmed by chemical oxygen demand (COD) measurements. A possible mechanism is proposed for the degradation of AR 18 under sun light. Finally, Bi–Au–ZnO heterojunction photocatalyst was more stable and could be easily recycled several times opening a new avenue for potential industrial applications.

Bismuth Growth on Ge(111): Evolution of Morphological Changes From Nanocrystals to Films

The growth of ultra-thin bismuth films up to 15 atomic layers on the atomically clean Ge(111)-c(2×8) substrate at 300 K is investigated before and after the annealing at 450 K by means of ultra-high vacuum scanning tunneling microscopy. The whole range of morphologies is observed for the Bi adsorbate such as individual atoms, two-dimensional nanoislands, three-dimensional nanocrystals and microcrystals, as well as an atomically flat film. The 3D nanostructuring is achieved through the Vollmer–Weber growth mode up to 10 atomic layers, followed by film’s flattening from 10 to 15 atomic layers. Annealed submonolayer films display individual Bi atoms incorporated into the uppermost atomic layer of Ge(111), as well as two-dimensional nanoislands of the first layer (2D nanostructuring). Thicker films display the coarsening after the annealing, compared to the as-deposited specimen, containing a diversity from nano- to microcrystals of bismuth.

Temperature and magnetic dependence of the resistance and thermopower in the topological insulator Bi1−xSbx nanowires

We report on the electrical transport and thermoelectric properties of Bi1−xSbx nanowires in the semiconductor region made of Bi1−xSbx. Such alloys are classified as topological insulators. The individual Bi–17 at%Sb wires in a glass capillary with diameters ranging from 100 to 1000 nm were prepared by high‐frequency liquid‐phase casting in an argon atmosphere. They were cylindrical single crystals with (1011) orientation along the wire axis. For large‐diameter wires we observed that the temperature‐dependent resistance, R(T), displays the temperature‐activated dependence that is expected of semiconductors. We also found that small‐diameter wires at low temperatures show a sharp deviation from the behavior of the resistance R(T), characteristic of semiconductors. The contrasting behavior of wires of different diameters can be interpreted in terms of the conductance of the surface states in BiSb where the surface states arise through a spin‐orbital Rashba interaction in the surface of BiSb. The thermopower remains negative over the entire temperature range, but it is strongly temperature dependent. The longitudinal magnetoresistance R(H) at low temperature shows quantum Shubnikov–de Haas oscillations only in thin (d < 200 nm) Bi–17 at%Sb nanowires. Only high‐mobility carriers display SdH oscillations. Since we expect the electronic transport in the 200‐nm semiconducting Bi1−xSbx nanowires to be dominated by the surface, this indicates that the surface states have sufficiently high mobility to display SdH.

Weak antilocalization and conductance fluctuation in a single crystalline Bi nanowire

We present the low temperature transport properties of an individual single-crystalline Bi nanowire grown by the on-film formation of nanowire method. The temperature dependent resistance and magnetoresistance of Bi nanowires were investigated. The phase coherence length was obtained from the fluctuation pattern of the magnetoresistance below 40 K using universal conductance fluctuation theory. The obtained temperature dependence of phase coherence length and the fluctuation amplitude indicates that the transport of electrons shows 2-dimensional characteristics originating from the surface states. The temperature dependence of the coherence length derived from the weak antilocalization effect using the Hikami–Larkin–Nagaoka model is consistent with that from the universal conductance fluctuations theory.

Strain induced peculiarities in transport properties of Bi nanowires

We report results on the effect of strain on the thermopower and electrical resistance of glass-coated individual Bi nanowires. Here, we show that there is a critical diameter of wires below which the contribution of holes to the charge transport in pure Bi nanowires is more significant than that of electrons. The properties of Bi nanowires are examined in the light of a strain induced electronic topological transition. At low temperatures, the thermopower dependences on strain exhibit a non-monotonic behavior inherent in thinner wires, where the thermopower is dominated by the diffusion transport mechanism of holes. The hole-dominated transport can be transformed into electron-dominated transport through a smooth manipulation with the phonon spectrum and Fermi surface by applying a uniaxial strain. A fairly high value of the thermoelectric power factor (S2/ρ = 89 μW cm−1 K−2) was found in the temperature range of 80–300 K, where the dominant mechanism contributing to the thermopower is diffusive thermoelectric generation with electrons as the majority carrier.

Proton irradiation effects on thermal transport in individual single-crystalline Bi nanowires

We investigated the proton irradiation effect of thermal conductivities for individual single-crystalline Bi nanowires grown by the on-film formation of nanowires (ON–OFF). The thermal conductivities of Bi nanowires with diameter of 154 and 112 nm were measured using suspended devices before and after proton irradiation, respectively. It was founded thermal conductivities of Bi nanowires appreciably decrease after proton irradiation, which was caused by the destruction of single-crystallinity due to the high-energy proton impingement. This result indicates the defects of Bi nanowires created by proton drastically limit the mean free paths of phonons, resulting in the change of thermal transport of Bi nanowires.

Development of 2-D Bi-SQUID Arrays With High Linearity

We develop a two-dimensional (2D) Superconducting Quantum Interference Filter (SQIF) array based on recently introduced high-linearity tri-junction bi-SQUIDs. Our bi-SQUID SQIF array design is based on a tight integration of individual bi- SQUID cells sharing inductances with adjacent cells. We provide extensive computer simulations, analysis and experimental measurements, in which we explore the phase dynamics and linearity of the array voltage response. The non-uniformity in inductances of the bi-SQUIDs produces a pronounced zero-field single antipeak in the voltage response. The anti-peak linearity and size can be optimized by varying the critical current of the additional junction of each bi-SQUID. The layout implementation of the tight 2D array integration leads to a distinct geometrical diamond shape formed by the merged dual bi-SQUID cells. Different size 2D arrays are fabricated using standard HYPRES niobium 4.5 kA/cm2 fabrication process. The measured linearity, power gain, and noise properties will be analyzed for different array sizes and the results will be compared with circuit simulations. We will discuss a design approach for the electrically small magnetic field antenna and low-noise amplifiers with high bandwidth based on these 2D bi-SQUID SQIF arrays. The results from this work will be used to design chips densely and completely covered in bi-SQUIDs that has optimized parameters such as linearity and power gain.

The Conformity of EU-ACP Trade Partnerships with Sustainable Development and Good Governance Norms or Do (I)EPAs Live Up to Their Standards?

Is trade cooperation between the European Union (EU) and the African-Caribbean-Pacific (ACP) countries an engine for sustainable growth?As the basis for individual bi- or multilateral (interim) Economic Partnership Agreements ((I)EPAs) of the EU with ACP countries, the current Cotonou Agreement advocates the globally acknowledged conditions and crucial policy norms for the pursuit of sustainable development. Therefore, one would expect that the (I)EPAs translate the sustainable development aims into precise legal provisions, i.e. that they create bi- or multilateral obligations and concrete commitments between the signatory states which support the achievement of the relevant MDGs and the consolidation of good governance practices. With that premise in mind, this legal Master thesis verifies whether the individual (I)EPAs live up to the standards set for the EU-ACP partnerships in the current Cotonou Agreement as foundation stone for the strategic implementation of sustainable development policies. In accordance with the revised text of the Agreement itself as well as in light of the development cooperation and trade for development objective of the (I)EPAs, its focus is limited to the policy norms of the most relevant Millenium Development Goals (MDGs), being poverty reduction (MDG#1), environmental sustainability (MDG#7) and global partnerships (MDG#8) as well as the sine qua non requirement of good governance practices.

Observation of Anisotropy in Thermal Conductivity of Individual Single-Crystalline Bismuth Nanowires

The thermal conductivity of individual single-crystalline Bi nanowires grown by the on-film formation of nanowires (ON-OFF) has been investigated. We observed that the thermal conductivity of single-crystalline Bi nanowires is highly anisotropic. Thermal conductivity of nanowires (diameter ∼100 nm) in the off-axis [102] and [110] directions exhibits a difference of ∼7.0 W/m·K. The thermal conductivity in both growth directions is diameter-dependent, which indicates that thermal transport through the individual Bi nanowires is limited by boundary scattering of both electrons and phonons. This huge anisotropy in thermal conductivities of Bi nanowires suggests the importance of direction-dependent characterization of charge, thermal transport, and thermoelectric properties of Bi nanowires.

Direct observation of the semimetal-to-semiconductor transition of individual single-crystalbismuth nanowires grown by on-film formation of nanowires

We have systematically investigated the semimetal-to-semiconductor transition ofindividual single-crystalline Bi nanowires. For this work, we developed a technique toreduce the diameter of Bi nanowires grown by our unique on-film formation ofnanowires (OFF-ON) method. Cooling down the substrate temperature during Bifilm deposition by use of liquid nitrogen, film structures with small-sized grainswere obtained. Through thermal annealing of these fine-granular Bi films,single-crystalline Bi nanowires can be produced with minimum diameter of ∼ 20 nm. Elaborative nanofabrication techniques were employed to shape state-of-the-artfour-probe devices based on the individual small diameter Bi nanowires.Diameter-dependent transport measurements on the individual Bi nanowiresrevealed that the semimetal-to-semiconductor transition really occurred at aboutdw = 63 nm. Moreover, band structure calculations supported this occurrence of thesemimetal-to-semiconductor transition.

Weak-antilocalization signatures in the magnetotransport properties of individual electrodeposited Bi Nanowires

We study the electrical resistivity of individual Bi nanowires of diameter 100 nm fabricated by electrodeposition using a four-probe method in the temperature range 5–300 K with magnetic fields up to 90 kOe. Low-resistance Ohmic contacts to individual Bi nanowires are achieved using a focused ion beam to deposit W-based nanocontacts. Magnetoresistance measurements show evidence for weak antilocalization at temperatures below 10 K, with a phase-breaking length of 100 nm.

Twinning mediated growth of ZnSe tri- and bi-crystal nanobelts with single crystalline wurtzite nanobelts as building blocks

A variety of ZnSe nanostructures, including the single-, bi- and tri-crystal nanobelts, are successfully fabricated through the thermal evaporation method and comprehensively investigated by a combination of various transmission electron microscopy (TEM) techniques. All nanostructures are of metastable wurtzite phase and the bi- and tri-crystal nanobelts are both mediated by the {013} twinning, with the single crystal nanobelts as building blocks. Benefiting from the commonly observed morphological evolutions, not only the crystallographic relations for different variants in an individual bi- or tri-crystal nanobelt, but also the intrinsic connections between different morphologies are fully understood by selected area electron diffraction, convergent beam electron diffraction and high-resolution TEM techniques, which reveals that the accompanying formation of tri-crystal nanobelt, the {013} relationship, though maintained still, is not as strict as that in a bi-crystal nanobelt and holds a ∼0.75° deviation for a case study; the angles between adjacent twinning variants/planes are compressed from ∼125.0° in bi-crystal nanobelts to ∼120°, and finally form an asymmetric twinning plane distributions. The study helps deepen the understanding of tri-crystal and related complex semiconductor nanostructures.

Oxide and halide nanoclusters on ionic substrates: heterofilm formation and lattice mismatch

We examine the form of clusters and islands formed by oxides and halides on ionic substrates where there is an appreciable lattice mismatch between substrate and adsorbate. Structures are optimised in the static limit using both periodic density functional theory and atomistic simulation techniques. Results are presented for three examples: (i) CaO on the (100) surface of BaO, (ii) CaF2 on the (110) and (111) surfaces of BaF2 ,(iii) Bi2O3 on the SrO-terminated (100) surface of SrTiO3. In none of these do we observe direct overlay of groups of ions over substrate ions, accompanied by marked changes in bond lengths in the adsorbate cluster or layer. Rather, islands have a critical maximum domain size because larger islands would result in unfavourable electrostatic interactions between adsorbate and substrate. Instead loops or gaps or other defect regions form between individual islands. For Bi2O3 we propose the low temperature stabilization of the superionic δ-phase of Bi2O3 on deposition on (100) SrTiO3 is not a consequence of epitaxial matching of Bi⋯Bi with Sr⋯Sr, as previously suggested, but rather due to the mismatch between the Bi–O and Sr–O bond lengths. This gives rise to individual Bi–O islands, which promote the formation of a disordered surface phase. We find no evidence for the stabilization of a symmetric cubic structure. The formation of the disordered Bi2O3 ultrathin film is also associated with a decrease in the band gap.

Shubnikov–de Haas oscillations in an individual single-crystalline bismuth nanowire grown by on-film formation of nanowires

Shubnikov–de Haas (SdH) oscillations have been investigated in an individual Bi nanowire grown by on-film formation of nanowires that is a growth method producing extremely high-quality single-crystalline nanowires. The variation of observed SdH oscillations with transverse and longitudinal magnetic fields to the axis of the Bi nanowire is qualitatively consistent with the geometry of the highly anisotropic Fermi surfaces of Bi, and in turn, reveals the growth direction of the nanowires and demonstrates the high crystal quality. Our results demonstrate the vast potential of high-quality single-crystalline Bi nanowires for a variety of device applications and for fundamental investigations such as quantum transport.

Effect of Strip Width on Low-Field Peak Effect of Critical Current Density in Bi2Sr2CaCu2O8+δ Superconducting Strips

We investigate the dependence of the critical current density J c along the crystal a -axis on the magnetic field along the c -axis at high temperatures below the superconducting transition temperature in individual Bi 2 Sr 2 CaCu 2 O 8+δ (Bi-2212) whiskers with surface barriers by electrical transport measurements. J c shows a low-field peak effect; J c increases with the magnetic field in a low-field region and shows a peak at the threshold field H peak below 100 Oe. A comparison of the low-field peak effects in whiskers of different widths shows that H peak depends on the sample width (i.e., the lateral dimension); the lateral axis is perpendicular to the magnetic field and the direction of current flow. A decrease in the sample width w shifts H peak toward a higher field. This systematic variation in H peak with w implies that the low-field peak effect observed in the Bi-2212 whiskers is associated with the effect of the finite w on dilute vortices.

Heterojunctioned BiOCl/Bi 2O 3, a new visible light photocatalyst

For the first time we report novel BiOCl/Bi 2O 3 photocatalyst functional under visible light irradiation. Even though both the individual BiOCl and Bi 2O 3 show very low photocatalytic efficiency under visible light irradiation, their heterojunctions provide unexpectedly high efficiency in decomposing organic compounds. Furthermore, the BiOCl/Bi 2O 3 can induce complete mineralization without formation of intermediate species by utilizing the holes generated in the VB of BiOCl. Compared to the Degussa P25, it demonstrates 5.7 times of efficiency in evolving CO 2 from gaseous 2-propanol (IP), and 10.5 times of efficiency in removing aqueous 1,4-terephthalic acid (TA) under visible light ( λ ⩾ 420 nm ) irradiation. In this BiOCl/Bi 2O 3 system the BiOCl seems to work as main photocatalyst, while the role of Bi 2O 3 is a sensitizer absorbing visible light. The photocatalytic mechanism has been proposed based on the relative band position of these two semiconductors.

On-Film Formation of Bi Nanowires with Extraordinary Electron Mobility

A novel stress-induced method to grow semimetallic Bi nanowires along with an analysis of their transport properties is presented. Single crystalline Bi nanowires were found to grow on as-sputtered films after thermal annealing at 260-270 degrees C. This was facilitated by relaxation of stress between the film and the thermally oxidized Si substrate that originated from a mismatch of the thermal expansion. The diameter-tunable Bi nanowires can be produced by controlling the mean grain size of the film, which is dependent upon the thickness of the film. Four-terminal devices based on individual Bi nanowires were found to exhibit very large transverse and longitudinal ordinary magnetoresistance, indicating high-quality, single crystalline Bi nanowires. Unusual transport properties, including a mobility value of 76900 cm(2)/(V s) and a mean free path of 1.35 mum in a 120 nm Bi nanowire, were observed at room temperature.

Magnetotransport properties of an individual single-crystalline Bi nanowire grown by a stress induced method

The magnetotransport properties of an individual crystalline Bi nanowire have been investigated in the range of 2–300 K using four-point measurements. I-V measurements show that the contacts were Ohmic at both 2 and 300 K, corresponding to resistivities of 1.0×10−4 and 8.2×10−5 Ω cm, respectively. The transverse magnetoresistance (MR) (2496% at 110 K) and longitudinal MR (−38% at 2 K) for the Bi nanowire were found to be larger than any values reported in the literature, demonstrating that the Bi nanowires grown by a stress induced method are high-quality single crystalline. The observed transverse and longitudinal MR behaviors in the Bi nanowire are consistent with variations in carrier concentrations as well as electronic structures, such as Fermi level and band overlap, based on simple two band model.