Individual Bi Nanowires Research Articles

Proton irradiation effects on the thermoelectric properties in single-crystalline Bi nanowires

The effects of proton irradiation on the thermoelectric properties of Bi nanowires (Bi-NWs) were investigated. Single crystalline Bi-NWs were grown by the on-film formation of nanowires method. The devices based on individual Bi-NWs were irradiated with protons at different energies. The total number of displaced atoms was estimated using the Kinchin-Pease displacement model. The electric conductivity and Seebeck coefficient in the Bi-NW devices were investigated before and after proton irradiation at different temperatures. Although the Seebeck coefficient remained stable at various irradiation energies, the electrical conductivity significantly declined with increasing proton energy up to 40 MeV.

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.

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.

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.

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.

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.

Diameter-dependent thermopower of bismuth nanowires

We present a study of electronic transport in individual Bi nanowires of large diameter relative to the Fermi wavelength. Measurements of the resistance and thermopower of intrinsic and Sn-doped Bi wires with various wire diameters, ranging from $150\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}480\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$, have been carried out over a wide range of temperatures $(4--300\phantom{\rule{0.3em}{0ex}}\mathrm{K})$ and magnetic fields $(0--14\phantom{\rule{0.3em}{0ex}}\mathrm{T})$. We find that the thermopower of intrinsic Bi wires in this diameter range is positive (type $p$) below about $150\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, displaying a peak at around $40\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. In comparison, intrinsic bulk Bi is type $n$. Magnetothermopower effects due to the decrease of surface scattering when the cyclotron diameter is less than the wire diameter are demonstrated. The measurements are interpreted in terms of a model of diffusive thermopower, where the mobility limitations posed by hole-boundary scattering are much less severe than those due to electron-boundary scattering.

Size‐Dependent Transport and Thermoelectric Properties of Individual Polycrystalline Bismuth Nanowires

Nanofabrication methods and a device architecture are combined to allow for four-point electric, thermoelectric, and electric field-effect measurements on individual Bi nanowires. The figure shows a false-color SEM image of a typical device used in this study, with the 11 Bi nanowires shown as a red → white gradient. The temperature dependence of the various transport properties as a function of nanowire width is investigated and discussed.

Making electrical contacts to nanowires with a thick oxide coating

Techniques are presented for making ohmic contacts to nanowires with a thickoxide coating. Although experiments were carried out on Bi nanowires, thetechniques described in this paper are generally applicable to other nanowiresystems. Metal electrodes are patterned to individual Bi nanowires using electronbeam lithography. Imaging the chemical reaction on the atomic scale with in situhigh-resolution transmission electron microscopy shows that annealing in H2or NH3 can reduce the nanowires’ oxide coating completely. The hightemperatures required for this annealing, however, are not compatible with thelithographic techniques. Low-resistance ohmic contacts to individual bismuthnanowires are achieved using a focused ion beam (FIB) to first sputteraway the oxide layer and then deposit Pt contacts. By combining electronbeam lithography and FIB techniques, ohmic contacts stable from 2 to400 K are successfully made to the nanowires. A method for preventingthe burnout of nanowires from electrostatic discharge is also developed.

4-Point Resistance Measurements of Individual Bi Nanowires

AbstractWe have synthesized single crystal bismuth nanowires by pressure injecting molten Bi into anodic alumina templates. By varying the template fabrication conditions, nanowires with diameters ranging from 10 to 200nm and lengths of ~50[.proportional]m can be produced. We present a scheme for measuring the resistance of a single Bi nanowire using a 4-point measurement technique. The nanowires are found to have a 7nm thick oxide layer which causes very high contact resistance when electrodes are patterned on top of the nanowires. The oxide is found to be resilient to acid etching, but can be successfully reduced in high temperature hydrogen and ammonia environments. The reformation time of the oxide in air is found to be less than 1 minute. Focused ion beam milling is attempted as an alternate solution to oxide removal.

Processing and Characterization of High-conductance Bismuth Wire Array Composites

We fabricated Bi nanowire array composites with wire diameters from 30 to 200 nm by high-pressure injection (HPI) of Bi melt into porous anodic alumina templates. The composites were dense, with Bi volume fraction in excess of 50%. The parallel Bi nanowires, whose length appeared to be limited only by the thickness of the host template (up to 55 μm), terminated at both sides of the composite in the Bi bulk. The individual Bi nanowire crystal structure was rhombohedral, with the same lattice parameters as that of bulk Bi; the wires in the array were predominantly oriented with the trigonal axis along the wire length. Low contact resistance was achieved by bonding the composite to copper electrodes.

Processing and Characterization of Single-Crystalline Ultrafine Bismuth Nanowires

By pressure injecting Bi liquid melt into the nanochannels of an anodic alumina template, we have successfully fabricated Bi nanowire arrays with ultrafine wire diameters and extremely high wire packing densities. Free-standing Bi nanowires with controlled wire diameters and large aspect ratios (length/diameter) were also obtained by subsequent etching of the alumina template. Various techniques such as SEM, TEM, AFM, EFM, HREM, and XRD have been used to investigate the physical characteristics of these nanowires. The Bi nanowires were found to be dense and continuous and had a uniform diameter throughout the length of the wires. Individual Bi nanowires were shown to be single crystals, and all the wires in an array were highly oriented. An interesting metastable phase of Bi was also observed, which can be attributed to a lattice stress-induced high-pressure phase of Bi formed inside the porous anodic alumina template.

Transport Measurements of Individual Bi Nanowires

ABSTRACTTransport properties are reported for Bi nanowires, prepared by the filling of an alumina template with molten Bi. The temperature dependence of the resistance is presented for such arrays of Bi nanowires with diameters in the 40 to 200nm range. The data are understood qualitatively on the basis of a model for a quantum-confined system. Finally, a 4-point measurement is performed on an individual Bi nanowire prepared by using an electron beam lithography technique. Techniques for handling the practical issues of non-ohmic contacts and wire burn-out are given. The physical significance of the final results of the measurements are discussed in light of various scattering mechanisms in the nanowire.