Bi Nanowires Research Articles

Bi-Te Core/Shell Nanowires Synthesis Based on On-Film Formation of Nanowires Method for Thermoelectric Applications

For an enhanced thermoelectric performance, one-dimensional heterostructure nanowires were created that consisted of aBi core and Te shell. The structure was fabricated by depositing Te in-situ onto a Bi nanowire grown by our unique OFF-ON (on-film formation of nanowires) method. After examining a cross-sectional TEM image, it was found that diffusive interface was formed between Bi and Te. Selected area electron diffraction revealed that the crystallinity of the Te shell was some what lower compared to the highly single-crystalline Bi core. The Bi-Te core/shell nanowires can be a smart structure that suppresses phonon transport by several scattering mechanisms, making the OFF-ON method the simplest way to realize that structure. (Received October 29, 2009)

Self-assembled Bi interconnections produced by on-film formation of nanowires for in situdevice fabrication

We fabricated Bi nanowire interconnections between two pre-patterned electrodes using acombination of on-film formation of nanowires (OFF–ON) and self-assembly. Binanowires were found to grow laterally from a multilayer structure with a Cr (orSiO2) overlayer on top of a Bi thin film through thermal annealing to relievevertically stored compressive stress. A Bi nanobridge with a diameter of 192 nmwas formed between two Cr electrodes and was highly ohmic according toI–V measurements. A high transverse magnetoresistance of 123% was also observed at 300 K.Our results indicate that self-assembled lateral nanowire growth can be utilized as an easymeans for fabricating a variety of nanowire devices without the use of catalysts or complexpatterning processes.

Dynamics of coherent acoustic and optical phonon oscillations in nanostructures studied by a femtosecond pump-probe technique

Coherent phononic oscillations in Bi and Ag nanowires were studied with a femtosecond pump-probe technique. In Bi nanowires laser pulses of 50 fs excited simultaneously acoustic oscillations at a frequency of about 9.5 GHz and optical phonons in the THz range. The transmission of nanowires on a glass substrate and the light scattered from free standing nanowires were measured. In Ag nanowires laser-induced acoustic oscillations at different excitation levels were studied. The observed reduction of the oscillation frequency at higher pump energy was related to a transient softening of the material. This was directly confirmed for optical phonons by experiments with a pre-pump pulse of variable energy, producing different excitation densities.

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.

Photocatalytic Activity and Electron Field Emission of Necked ZnO:Bi Nanowires

Bismuth-doped ZnO nanowires were synthesized by a vapor transfer process at 600°C. Raman spectra revealed that a slight blueshift occurred in ZnO:Bi (438 cm -1 ) nanowires in contrast to ZnO (436 cm -1 ) nanowires. The X-ray photoelectron spectrum showed that the ZnO:Bi nanowires contained the Bi dopant as Bi° and Bi 3+ . The turn-on fields of the ZnO:Bi and ZnO nanowires were ∼4.19 and 6.57 V/μm, respectively. Because the ZnO:Bi nanowires exhibited a redshift of absorbance spectrum with a high ratio in the UV range, the photocatalytic activities of ZnO:Bi nanowires were considered superior to those of ZnO nanowires.

Stenciled conducting bismuth nanowires

Stencil lithography is used here for the fabrication of bismuth nanowires using thermal evaporation. This technique provides good electrical contact resistance by having the nanowire structure and the contact pads deposited at the same time. It has also the advantage of modulating nanowires’ height as a function of their width. As the evaporated material deposits on the stencil mask, the apertures shrink in size until they are fully clogged and no more material can pass through. Thus, the authors obtain variable-height (from 27 to 95 nm) nanowires in the same evaporation. Upon their morphological (scanning electron microscopy and atomic force microscopy) and electrical characterizations, the authors obtain their resistivity, which is independent of the nanowire size and is the lowest reported for physical vapor deposition of Bi nanowires (1.2×10−3 Ω cm), only an order of magnitude higher than that of bulk bismuth.

Magnetic Quantum Oscillations in Single Bi Nanowires

Our study presents the experimental measurements of Aharonov-Bohm (AB) oscillations on single Bi nanowires with diameter d<80 nm. The single nanowire samples prepared by the Ulitovsky technique were cylindrical single crystals with (10 $\bar{1}$ 1) orientation along the wire axis. Due to semimetal-to-semiconductor transformation and high density of surface states, Bi nanowire (d∼50 nm) effectively turns into a conducting tube. The oscillations of longitudinal magnetoresistance (MR) of Bi nanowires with two periods Δ B 1∼Φ 0 and Δ B 2∼Φ 0/2 were observed, where Φ 0=h/e is the flux quantum. From B∼8 T down to B=0 the extremums of Φ 0/2 oscillations are shifted up to 3π at B=0 which is the manifestation of the Berry phase shift due to electron moving in nonuniform magnetic field. The derivative of MR for a 55 nm bismuth nanowire was measured at various inclined angles. An interpretation of the MR oscillations is presented.

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.

Observation of the Semiconductor-Semimetal and Semimetal-Semiconductor Transitions in Bi Quantum Wires Induced by Anisotropic Deformation and Magnetic Field

The semimetal-semiconductor transition is observed in glass-coated quantum single-crystal bismuth wires with diameters less than 70 nm due to the quantum size effect. It is found that elastic deformation of Bi nanowires (10\(\bar{1}\)1) oriented along the wire axis with the semiconductor dependence R(T) leads to the approaching of L and T bands and to the semiconductor-semimetal transition; as a result, Shubnikov-de Haas oscillations appear on the magnetoresistance dependences R(H). It is shown that strong magnetic field and elastic deformation are the tools to control gap size in quantum bismuth wires, which is principal for their practical use in particular in thermoelectricity.

Quantum Interference and Surface States Effects in Bismuth Nanowires

We report the observation of Aharonov-Bohm (AB) oscillations for single Bi nanowires with diameter d< 80 nm. The single nanowire samples with glass coating were prepared by the Ulitovsky technique; they were cylindrical single crystals with (10 \(\bar{1}\) 1) orientation along the wire axis. The surface of Bi nanowire supports surface states which give rise to a significant population of charge carriers with high effective mass that form a highly conducting tube around the nanowire. The oscillations of longitudinal magnetoresistance (MR) of Bi nanowires with two periods ΔB1 and ΔB2 proportional to Φ0 and Φ0/2 were observed, where Φ0=h/e is the flux quantum. From B∼ 8 T down to B = 0 the extremums of Φ0/2 oscillations are shifted up to 3π at B = 0 which is the manifestation of Berry phase shift due to electron moving in a nonuniform magnetic field. A derivative of MR was measured at various inclined angles. The observed angle variation of the periods is not in agreement with the theoretical dependence Δ(α)=Δ(0)/cos α of the size effect oscillations of the “flux quantization” type. Moreover, the equidistant oscillations of MR exist in transverse magnetic fields under certain rotation angles. An interpretation of the MR oscillations is presented.

Superconductivity and Quantum Oscillations in Crystalline Bi Nanowire

While bulk bismuth (Bi) is a semimetal, we have found clear evidence of superconductivity in 72 nm diameter crystalline Bi nanowire below 1.3 K. In a parallel magnetic field (H), the residual resistance of the nanowire below T(c) displays periodic oscillations with H, and the period corresponds to a superconducting flux quantum. This result indicates that the superconductivity originates from the cylindrical shell between Bi inner core and the surface oxide layer. Under a perpendicular H, the resistance in the superconducting state shows Shubnikov-de Haas (SdH)-like oscillations, a signature of a normal metal. These results suggest a novel coexistence of superconducting and metallic states in the temperatures well below T(c).

Growth of well-aligned Bi nanowire on Ag(1 1 1)

We report the fabrication of one-dimensional (1D) Bi nanowires grown on Ag(1 1 1) with average lateral width from 9 to 20 nm by physical vapor deposition in ultra high vacuum conditions. In situ low-temperature scanning tunneling microscopy analyses reveal that the preferred growth of 1D Bi nanostructures is driven by the highly anisotropic bonding in the crystallographic structure of the Bi(1 1 0) plane. The Bi nanowires grow along Bi [ 1 1 ¯ 0 ] direction and align with the [ 1 1 ¯ 0 ] directions on Ag(1 1 1). The growth of the Bi nanowires proceeds in a bilayer growth mode resulting from the layer pairing in Bi(1 1 0) which saturates the dangling bonds and lowers the total energy.

Structures and Electronic Properties of the Bi−Sb Superlattice Nanowires and Core−Shell Structural Bi/Sb Nanowires

We systematically study the structural and electronic properties of the Bi−Sb superlattice nanowires and core−shell structural Bi/Sb nanowires by first-principles calculations. The relaxed structures of these heterostructural nanowires are first obtained and found to be similar to those of homogeneous Bi nanowires. For the Bi−Sb superlattice nanowires, band lineups of Bi and Sb zones result in multiple quantum wells, where specific states at the band edges and in band continua are confined. The confined electrons (holes) become more localized if the width of the barrier is larger. For the core−shell structural Bi/Sb nanowires, the electronic properties show dependence on the size, the atom type of core, and the chemical composition. Meanwhile, the valence bands are less affected by the chemical composition, while the conduction bands depend on it. These findings might have important implications for understanding the structural and electronic properties of the heterostructural nanowires and further utiliz...

Observation of three-dimensional behavior in surface states of bismuth nanowires and the evidence for bulk-Bi surface quasiparticles

We studied trigonal Bi nanowires $(30\text{ }\text{nm}&lt;\text{diameter}&lt;200\text{ }\text{nm})$ via low-temperature magnetotransport for fields up to 14 T in order to investigate the role of their surfaces. A two-dimensional behavior was expected; we found instead a three-dimensional behavior, with a rich spectrum of Landau levels in a nearly spherical Fermi surface. We show that recent observations of sharp peaks in the bulk-Bi Nernst thermopower near the 9 T quantum limit attributed to charge fractionalization, can be more plausibly interpreted in terms of surface states. Bismuth has a true quantum limit at around 70 T.

Indirect L to T point optical transition in bismuth nanowires

An indirect electronic transition from the L point valence band to the T point valence band has been previously observed in Bi nanowires oriented along the $[01\overline{1}2]$ crystalline direction (used by Black et al. and by Reppert et al.) but not in $[11\overline{2}0]$-oriented nanowires (used by Cornelius et al.) or in bulk bismuth. Here we measure the Bi nanowire samples from each of these prior experimental studies on the same Fourier transform infrared apparatus, confirming that the differences are indeed physical and are not associated with the experimental setup. We develop an analytical model for the threshold energy of the indirect L to T point valence-band transition that takes as parameters the nanowire diameter and crystalline orientation. Our model shows good agreement with experimental results, and demonstrates that the nonparabolic nature of the L point bands is essential for calculating the energy of this transition. Finally, we propose a mechanism based on symmetry lowering to explain why this indirect transition is observed for $[01\overline{1}2]$-oriented but not for $[11\overline{2}0]$-oriented nanowires.

Low temperature magnetoresistance measurements on bismuth nanowire arrays

We present low temperature resistanceR(T) and magnetoresistance measurements for Bi nanowires with diameters between100 and 500 nm, which are close to being single-crystalline. The nanowires werefabricated by electrochemical deposition in pores of polycarbonate membranes.R(T) varies asT2 in the lowtemperature range 1.5 K<T<10 K and exhibits a maximum which shifts from 140 to 250 K withdecreasing wire diameter. An unexpected effect is observed inR(T) when a magnetic field is present. It can be related to the temperature dependence of themagnetoresistance. The transverse magnetoresistance of all samples shows a clearB1.5 variation. Its size depends strongly on the diameter of the wires but only weakly ontemperature. Finally, a steplike increase in the magnetoresistance of our sample with a wirediameter of 100 nm was found and this might be attributed to a transition fromone-dimensional to three-dimensional localization.

Size Effects in Quantum Single Crystal Bismuth Wires in Glass Cover

The present paper describes the results of researches in the size effects (classical and quantum ones) in single-crystal bismuth nanowires of strictly cylindrical form in a glass isolation, obtained by liquid phase casting with a minimum diameter of 45 nm. The results of the complex study of temperature and thickness dependences of the resistance, thermopower, longitudinal (LMR) and transverse (TMR) magnetoresistance in magnetic fields up to 14 T, in the temperature range 1.5-300 K and under the influence of anisotropic elastic deformation are discussed. A number of new effects in Bi nanowires observed for the first time, such as semiconductor type of conductivity, thermopower sign inversion, and formation of a maximum in the positive range, depending on the wire diameter d; disappearance of ShdH oscillations at d < 80 nm; presence of negative magnetoresistance in the perpendicular magnetic field (H⊥I) are interpreted in terms of manifestation of the quantum size effect with regard for theoretical justification.

Aharonov-Bohm Oscillations in Single Crystal Bi Nanowires

The Aharonov-Bohm effect (AB) exists in cylindrical wires as the magnetoresistance (MR) oscillations with a period ΔB that is proportional to Φ 0 /S, where Φ 0 = h/e is the quantum and S is the wire cross section. The AB-type longitudinal MR oscillations flux quantization type with period ΔB = Φ 0 /S caused by electrons undergoing continuous grazing incidence at the wire wall have been studied previously at 4.2 K in single bismuth nanowires with diameter 0.2 < d < 0.8 μm. Recently the new type of AB oscillations with period ΔB = h/e and ΔB= h/2e on single Bi nanowires with diameter d = 45-73 nm have been observed. The single nanowire samples were prepared by improved Ulitovsky technique and represented cylindrical single crystals with (1011) orientation along the wire axis. Here we present a review of experimental investigations of size-effect oscillations, equidistant in the direct magnetic field of the magnetoresistance of Bi thin cylindrical single crystals obtained by Ulitovsky technique with diameter 45 nm < d < 800 nm.

Berry's phase manifestation in Aharonov-Bohm oscillations in single Bi nanowires

Here we report on Aharonov-Bohm oscillations of magnetoresistance (MR) of the single Bi nanowires with diameter d<80 nm. The samples were prepared by Ulitovsky technique and represented cylindrical single crystals with the 1011 orientation along the wire axis. Due to semimetal-to-semiconductor transformation and big density of surface states with strong spin-orbit interactions Bi nanowire should effectively become a conducting tube. The equidistant oscillations of the MR have been observed in a wide range of magnetic fields up to 14 T at various temperatures (1.5 K< T< 4.2 K) and angles θ (0< θ < 90°) of the sample orientation relative to the magnetic field. We have obtained longitudinal MR oscillations with periods ΔB1=Φ0/S and ΔB2=Φ0/2S, where Φ0=h/e is the flux quantum and S is the wire cross section. From B ≈ 8 T down to B=0 the extremums of Φ0/2S oscillations are shifted up to 3π at B=0 which is the manifestation of Berry phase shift due to carriers moving in inhomogeneous magnetic field. An interpretation of the MR oscillations in terms of a subband structure in the surface state band caused by quantum interference is presented.

Bi nanowires: Magnetism and the semimetal-to-semiconductor transition

Cantilever magnetometry experiments were performed on arrays of Bi nanowires of various diameters both on the semimetal and semiconductor side of the semimetal-to-semiconductor transition (SMSC) that is predicted to occur for diameters of approximately 50 nm. Our 270 nm nanowire samples, which are on the semimetal side of the SMSC transition, exhibit strong dHvA oscillations that can be interpreted by considering crystalline orientation and size effects and a large magnetization. Arrays of wires with diameters near or below the SMSC (60 and 36 nm) have dramatically reduced magnetizations per volume, and do not exhibit dHvA oscillations, consistent with a reduction in free carriers due to the SMSC.