Pure Nanowires Research Articles

Electro- and magneto-transport properties of a single CoNi nanowire

The temperature-dependent electro- and magneto-transport properties of a single nickel-rich CoNi alloynanowire (∼200 nm in diameter) bridging microfabricated gold electrodes were investigated. Thetemperature coefficient of resistivity (TCR) for a single nickel-rich CoNi nanowire is low incomparison with pure Ni nanowires, and this might be attributed to a higher residualresistivity. The magneto-resistance (MR) of a single nickel-rich CoNi nanowireshows typical anisotropic behaviour, where the magneto-resistance ratio decreaseswith increasing temperature and, as expected, the value is much greater than forpure Ni nanowires. The angular dependence of the magnetization switching field(HSW) indicated that the magnetization reversal mechanism for CoNi nanowiresat 300 K changed from curling to coherent rotation when the anglebetween the nanowire axis and the magnetic field direction is greater than75°.

FIB induced growth of antimony nanowires

The impact of high energy Ga ion beams focused to diameters below 100 nm on antimony (Sb) substrates offers a new approach for the formation of pure Sb nanowires. In contrast to several well-known processes for bottom–up fabrication of one-dimensional nanostructures, for this process neither additional temperature treatment nor any additional material component is needed. Initially, the resulting nanostructures are completely amorphous and show very homogeneous diameters in the range of 20 nm. Lengths up to several microns can be achieved. Annealing at temperatures around 150 °C leads to re-crystallization of these nanowires.

Quantitative EELS Analysis of AlGaN Nanowires Grown by Ni Promoted MBE on Sapphire Substrate

AbstractQuantitative analysis of the elemental distributions within AlGaN has been investigated using electron energy loss spectroscopy in a scanning transmission electron microscopy. The nanowires were grown on c-sapphire by radio frequency plasma assisted molecular beam epitaxy. Crystallographic and compositional analyses of the nickel seeds used to promote the nanowire type growth yielded values of lattice spacings within the seeds remaining at the growth tip which were attributed to either (002) NiO, (111) Ni3Ga or (111) Ni-Ga solid solution. The seeds structures exhibited a metallic core encompassed by an oxide shell. The relative gallium and nickel concentrations were quantified by EELS analyses and were found to be consistent with the equilibrium phase α' of Ni3Ga or Ni-Ga solid solution. No nitrogen was observed within the seeds, which is predicted thermodynamically due to the instability of Ni-N compounds at the NW growth temperature used in this study. No aluminium was detected at the tip of nanowires. These measurements are compared with previous studies made concerning pure GaN nanowires. The Al distribution along the nanowire length was measured and is discussed in respect of a possible Al incorporation mechanism.

Electrochemical fabrication and magnetic properties of highly ordered silver–nickel core-shell nanowires

Ordered silver–nickel core-shell nanowire arrays were successfully fabricated by electrodeposition. The ordered silver nanowire arrays embedded in a porous alumina template were first fabricated from an aqueous solution of Ag(NO 3) 2 and Ac(NH 3). After removing out the template, the obtained silver nanowire arrays were subsequently electrodeposited with nickel at 1.6–2.6 V and 60 °C using the electrolyte composed of NiSO 4, NiCl 2 and H 3BO 3. Transmission electron microscopy observation revealed that a ∼15 nm thick nickel film was coated on the surface of the silver nanowires with about 200 nm in diameter. It was found that the silver nanowires with nickel coating showed enhanced magnetic properties in comparison to that of pure silver nanowires. The magnetic force microscope image of silver–nickel core-shell nanowires showed magnetic domain state. In addition, the hysteresis loops of the silver–nickel nanowire arrays showed a coercive field of 180 Oe, almost independent of the applied magnetic field. However, it was observed that a larger magnetic domain was found in parallel direction than that in perpendicular direction.

Direct synthesis of SiC nanowires by multiple reaction VS growth

β-SiC nanowires were directly synthesized by heating single-crystal silicon wafer and graphite without metal catalysts. The diameter of SiC nanowires is in the range of 10–30 nm, and the length is up to a few millimeters. Two kinds of SiC nanowires, namely pure SiC nanowires and SiC/SiO 2 composite nanowires, formed at higher temperature (the holding stage) and lower temperature (the cooling stage), respectively. A multiple-reaction model was proposed to explain the formation of SiC nanowires.

Hierarchical Saw-like ZnO Nanobelt/ZnS Nanowire Heterostructures Induced by Polar Surfaces

Saw-like nanostructures composed of single-crystalline ZnO nanobelts and single-crystalline ZnS nanowires have been successfully synthesized by a vapor-solid process. Several techniques, including scanning electron microscope, transmission electron microscopy, and photoluminescence spectroscopy, were used to investigate the structures, morphology, and photoluminescence properties of the products. Due to the similar crystal habits of wurtzite ZnO and ZnS with chemically active Zn-terminated (0001) and chemically inactive O-terminated (or S-terminated) (000) polar surfaces, hierarchical saw-like nanostructures were considered to be formed by the initiation of a chemically active Zn-terminated ZnO (0001) polar surface. Photoluminescence properties of the heterostructures, different from pure ZnO nanobelts or ZnS nanowires, were also studied at room temperature.

Fabrication of Cu/Cu 2O composite nanowire arrays on Si via AAO template-mediated electrodeposition

Cu/Cu 2O composite nanowire arrays on Si substrate with an Au interlayer have been prepared by anodic aluminum oxide (AAO) template-mediated electrochemical deposition. The pure Cu 2O nanowires are obtained at low current density, 0.05 mA/cm 2, while pure Cu nanowires appear at high current density, 15 mA/cm 2. Between these two current densities, the Cu/Cu 2O composite nanowires are produced and the content of Cu increase as the applied current density is raised. The transmission electron microscopy (TEM) observation indicates that the composite nanowires are composed of isolated Cu and Cu 2O grains, which contradicts to the layered structure observed in the composite films. The spontaneous potential oscillation observed in preparing Cu/Cu 2O composite films is also recorded during the formation of composite nanowires. The periods of spontaneous oscillation are a function of the applied current density. The mechanism of the variation of local pH can account the occurrence of spontaneous potential oscillations in the present system.

Fabrication of CoPd alloy nanowire arrays on an anodic aluminum oxide/Ti/Si substrate and their enhanced magnetic properties

An anodic aluminum oxide/Ti/Si substrate was successfully synthesized by the anodization of an aluminum film on a Ti/Si substrate and then used as a template to grow 10 nm diameter CoPd alloy nanowires. X-ray diffraction and energy-dispersed X-ray patterns indicated that Co0.97Pd0.03 nanowire arrays with a preferential orientation of (002) were formed during electrodeposition. High coercivity (about 1700 Oe) and squareness (about 0.85) were obtained in the samples when the magnetic field was applied parallel to the axis of the nanowires; these values are much larger than those of pure Co nanowire arrays with the same diameters. (c) 2006 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Fabrication of amorphous Co and Co-P nanometer array with different shapes in alumina template by AC electrodeposition

Using the porous alumina film as a template, the arrays of Co and Co-P with different shapes have been fabricated by AC electrodeposition. The as-obtained samples have been characterized by AFM, TEM and EDS. The results indicate that a small amount of non-metallic phosphor added in the electrolyte has a notable influence on the morphologies of electrodeposited nanostructural materials. An array of pure cobalt nanowires can be obtained, however, a mixed array of Co-P nanowires and nanotubes doped 2.0 at.% phosphor can be produced when the electrolytic solution contains 5.0 g/L sodium hypophosphite (NaH 2PO 2–H 2O) during the process of electrodeposition. Furthermore, only a Co-P nanoparticle array can be synthesized when the sodium hypophosphite (NaH 2PO 2–H 2O,) was raised to 10.0 g/L in the electrolytic solution. Also, the Co and Co-P nanowires, nanotubes and nanoparticles obtained display identical amorphous structure.

Controlled Synthesis of Ag/TiO2 Core−Shell Nanowires with Smooth and Bristled Surfaces via a One-Step Solution Route

Ag/TiO2 core-shell nanowires were synthesized via a one-step solution method without using a template. Interestingly, the shell morphologies can be controlled to be smooth or bristled by altering the reaction temperature. Moreover, the TiO2 shell thickness and bristle length can be tuned by changing the AgNO3 concentration. Scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), selected-area electron diffraction (SAED), energy-dispersive X-ray analysis (EDS), X-ray powder diffraction (XRD), and X-ray photoelectron spectroscopy (XPS) were used to characterize the resultant Ag/TiO2 core-shell nanowires. Moreover, the absorption peaks of our samples are significantly red-shifted compared with those of the uncoated pure silver nanowires, indicating that interaction between the core and shell occurred. On the basis of the experimental results, we proposed a template-induced Oswald ripening mechanism to explain the formation of the Ag/TiO2 core-shell nanowires.

Computer simulations in the study of gold nanowires: the effect of impurities

Suspended gold nanowires have recently been made in an ultra-high vacuum ambient and were imaged by electron microscopy. Two puzzles were presented: one atom thick wires are produced and some of the atomic distances between these atoms before their breaking were too large. Simulations using realistic molecular dynamics method were able to unveil some processes to explain the mechanisms of formation, evolution, and breaking of these atomically thin Au nanowires under stress. The calculations showed how defects induce the formation of constrictions that eventually will form the one-atom chains. Atomically thin chains, five atoms long were obtained, before breaking. The results were in excellent agreement with experimental results except for the large Au-Au distances. In fact no theoretical calculation of pure gold nanowires have been able to produce such large distances. Light impurities that cannot be imaged in these experiments may be responsible for these large Au-Au distances. Using ab initio total energy calculations based on the density functional theory, we have studied the effect of H, C, O, N, B, S, CH, CH2, and H2 impurities on the nanowire’s electronic and structural properties, in particular how they affect the rupture of the nanowire. We find that the impurities tend to locally increase the nanowire’s strength, in such a way that its rupture always occurs at an Au-Au bond and never at an Au-X bond (X being an impurity). In particular, oxygen seems to form very stable bonds that may be used to pull longer Au chains. Regarding the observed large Au-Au bond lengths, it was found, based on quasi-static calculations, that the best candidate to explain the large distances is H. However, some particular experimental conditions may lead to different results.

Influence of thickness and cap layer on the switching behavior of single Co nanowires

We have measured the low-temperature magnetoresistance (MR) of single cobalt nanowires of various lengths, thicknesses, and widths prepared by electron-beam lithography. The MR measurements exhibit peaks at the coercive fields Hc where Hc increases with decreasing wire width. From the shape of the peaks different switching mechanisms can be distinguished. While the resistance jumps for platinum-covered Co nanowires are sharp, pure Co nanowires exhibit broadened peaks due to domain-wall pinning at the Co/CoO interface. Covering the Co nanowires with insulating carbon yields a sharp switching behavior while simultaneously offering the possibility to investigate the resistance behavior of pure cobalt. By reducing the thickness of the wires the sharpness of the switching behavior continuously degenerates.

Electronic and structural properties of silicon carbide nanowires

We present density-functional calculations of the geometrical and electronic structure of nanometer-thick silicon carbide nanowires grown along the ⟨100⟩ axis. We discuss first hydrogen-passivated wires and show that the quantum confinement results in a broadening of the band gap. Second, we study pure nanowires. In this case the facets' dangling bonds strongly reconstruct and the surface states that form turn the system into metallic. Both the gap broadening and the surface reconstruction driven metalization are analyzed in the case of C- and Si-rich wires.

Photoelectrochemistry of Pure and Core/Sheath Nanowire Arrays of Cu2 S Directly Grown on Copper Electrodes

Photoelectrochemistry (PEC) of straight and isolated nanowires arrayed on a copper foil is studied. A cathodic photocurrent is observed, which increases with the increasing negative bias of the film electrode, conforming to the p-type semiconducting nature of PEC studies on the core/sheath nanowires of nanowires coated with other nanoparticles such as CdS, polypyrrole, and Au) have revealed distinctly different characteristics. nanowires exhibit a slightly higher photocurrent response due to the charge transfer between the p-type core and n-type CdS sheath, whereas the photocurrent response obtained for nanowires is generally an order of magnitude smaller than that for pure nanowires under the same bias voltage. For nanowires, however, Au nanoparticles can readily accept the photogenerated electrons from the conduction band of this redirects the electron transfer pathway and results in the anodic photocurrent response. On the basis of these findings, different photocurrent generation mechanisms are proposed. © 2005 The Electrochemical Society. All rights reserved.

In situ synthesis and characterization of pure SiC nanowires on silicon wafer

A simple template/catalyst-free chemical vapor growth process was developed for growing SiC nanowire directly on silicon wafers. The nanowires were identified as single crystalline β-phase SiC growing along <1 1 1> direction. The nanowires possess Si–C chemistry. The length and thickness of the nanowires are generally from several tens to over 100 μm and ∼80 nm, respectively. The process also demonstrated the possibility of in situ deposition of thin graphite coatings on the SiC nanowires. A contribution of present work to the applications of SiC nanowires, especially as reinforcement materials in ceramic nanocomposites, is expected.

Structure and magnetic properties of Co xPb 1− x nanowire arrays

Ordered Co x Pb 1− x nanowire arrays embedded in the porous anodic aluminum oxide (AAO) template have been fabricated by electrodeposition. XRD experiments prove that neither hexagonal-close-packed (hcp) nor face-centered-cubic (fcc) Co peaks are detected when the Co component ( x) is below 0.91. The coercivity ( H c) and squareness ( M r/ M s) are found to increase with ferromagnetic Co component and the maximum value is at the position x=1 (pure Co nanowires). Annealing effects cause H c and M r/ M s increase, which surpasses the pure Co nanowires in the 0.2< x<0.6 at the annealing temperature of 700 °C. Microstructure change during annealing process is proposed to explain the magnetic properties change of samples.

Ordered Co48Pb52 Nanowire Arrays Electrodeposited in the Porous Anodic Alumina Oxide Template with Enhanced Coercivity.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Effect of impurities on the breaking of Au nanowires

Using ab initio density functional theory total energy calculations, we study the influence of H, B, C, N, O, and S in the rupture of a gold nanowire. In particular, using an as realistic as possible model for a suspended gold nanowire under stress, we observe that the Au wire always breaks at an Au–Au bond, with a maximum bond length between 3.0 and 3.1 Å. Therefore, the experimentally observed large Au–Au bonds before the rupture of the nanowire (≈3.6 Å) are probably due to the presence of light impurities (X) forming Au–X–Au bonds. We obtain that the maximum Au–Au distance, for X equals C or N, is of the order of 3.9 Å, whereas for B and O it is of the order of 4.1 Å. On the other hand, for H this maximum distance before the rupture of the wire is approximately 3.6 Å, being the best candidate to explain the experimental results. For both C and H impurities, we present a detailed analysis of the neck atoms electronic structures, and compare them with similar results for the pure nanowire.

Ordered Co48Pb52 Nanowire Arrays Electrodeposited in the Porous Anodic Alumina Oxide Template with Enhanced Coercivity

Co-Pb nanowire arrays with an average diameter of 20 nm and lengths up to several micrometers were grown in ordered anodic alumina templates using AC electrodeposition. The as-deposited samples were annealed at different temperatures. The structure and magnetic properties of Co-Pb nanowires were analyzed by X-ray diffraction and a vibrating sample magnetometer. We found that the sample annealed at 700 °C had large perpendicular coercivity He (as high as 2660 Oe) and high squareness (M R /M s ) (about 0.97), which was much larger than that of pure Co nanowires annealed at 700 °C (2170 Oe, 0.9). The change in coercivity and squareness associated with the microstructure was discussed.

Molecular dynamics study of the lattice thermal conductivity of Kr/Ar superlattice nanowires

The nonequilibrium molecular dynamics (NEMD) method has been used to calculate the lattice thermal conductivities of Ar and Kr/Ar nanostructures in order to study the effects of interface scattering, boundary scattering, and elastic strain on lattice thermal conductivity. Results show that interface scattering poses significant resistance to phonon transport in superlattices and superlattice nanowires. The thermal conductivity of the Kr/Ar superlattice nanowire is only about 1 3 of that for pure Ar nanowires with the same cross-sectional area and total length due to the additional interfacial thermal resistance. It is found that nanowire boundary scattering provides significant resistance to phonon transport. As the cross-sectional area increases, the nanowire boundary scattering decreases, which leads to increased nanowire thermal conductivity. The ratio of the interfacial thermal resistance to the total effective thermal resistance increases from 30% for the superlattice nanowire to 42% for the superlattice film. Period length is another important factor affecting the effective thermal conductivity of the nanostructures. Increasing the period length will lead to increased acoustic mismatch between the adjacent layers, and hence increased interfacial thermal resistance. However, if the total length of the superlattice nanowire is fixed, reducing the period length will lead to decreased effective thermal conductivity due to the increased number of interfaces. Finally, it is found that the interfacial thermal resistance decreases as the reference temperature increases, which might be due to the inelastic interface scattering.