True nanocable assemblies with insulating BN nanotube sheaths and conducting Cu nanowire cores

Nanocable models comprised of BN nanotubes filled with close-packed Cu nanowires were investigated by gradient-corrected density functional theory (DFT) computations. The optimal distance between the sidewall of BN nanotubes and the atoms in a copper nanowire is about 0.35 nm, with a weak insertion energy (ca. -0.04 eV per Cu atom). Hence, such nanocables are assembled by weaker van der Waals (vdW) forces, rather than by chemical bonding interactions. The electronic band structures of the BN/Cu hybrid systems are superposition of those of the separate components, the BN nanotubes, and the Cu nanowires. Since charge density analyses show that the conduction electrons are distributed only on the copper atoms, charge transport will occur only in these inner nanowires, which are effectively insulated by the outer BN nanotubes. On the basis of these computational results, BN/Cu hybrid structures should be ideal nanocables.     

From Al4B2O9 nanowires to BN-coated Al18B4O33 nanowires

A novel route was proposed to completely coat aluminum borate nanowires by in situ providing the precursor for BN coating. Uniformly BN-coated Al18B4O33 nanowires could be obtained by the reaction of Al4B2O9 nanowires with ammonia at high temperature. The high-temperature unstable Al4B2O9 nanowires were converted into Al18B4O33 nanowires, simultaneously evaporated boron oxide. The reaction between the in situ generated vapors and ammonia ensures that the BN layers are attached tightly on the surface of the as-formed Al18B4O33 nanowires.     

Density functional theory study of geometrical structures and electronic properties of silica nanowires

Silica nanowires are expected to possess structural diversity like bulk silica. We modeled three silica nanowires based on the side-shared two-membered rings, spiro-united two-membered rings, and three-membered rings, respectively. By performing density functional theory calculations, we studied their geometrical structures and electronic properties with and without the presence of external electric field. It is found that the stability of silica nanowires increases with length and diameter. As indicated by calculated large HOMO-LUMO gaps, silica nanowires are expected to be good insulating materials. The energy gaps, however, gradually decrease with applied electronic field and finally close, resulting in the breakdown of the insulating nanowires. Moreover, it is shown that the breakdown threshold remarkably increases with the nanowire diameter. These significant findings from the present calculations for the simplest silica nanowires will provide relevant insight into the structures and properties of much more complicated real silica nanowires.     

Fluorination and electrical conductivity of BN nanotubes

Fluorination of BN nanotubes has been performed using a catalytic growth method, which leads to the appearance of markedly curved fluorine-doped BN sheets and converts originally insulating BN nanotubes to semiconductors, as confirmed by the comparative electron transport four-probe measurements on doped and undoped individual BN nanotubes.     

铁纳米线-氮化硼纳米管复合结构的形成及其压缩性质的模拟研究

通过分子动力学理论计算方法对铁纳米线(FeNW)在氮化硼纳米管(BNNT)内的形成及其复合结构(FeNW@BNNT)的压缩性质进行了模拟研究。通过对充以铁原子的BN(5,5)和BN(8,8)纳米管的进行结构优化可以发现,在BN(5,5)纳米管轴线上能生成稳定的一维FeNW,而BN(8,8)纳米管内形成呈螺旋状的三束绞缠的FeNW。其径向分布函数表明在BN(5,5)内生成的FeNW具有良好的一维性且原子分布均匀等特征。通过对BN(5,5)与FeNW@BN(5,5)轴向压缩及其能量分析,可以发现它们虽具有相同屈曲应变,但屈曲前FeNW@BN(5,5)的弹性系数稍大于BN(5,5),且FeNW@BN(5,5)抗压屈曲能力也明显较强。     

β-Ga2O3 nanowires sheathed with boron nitrogen

A new synthetic route was developed to obtain β-Ga₂O₃ nanowires by heating mixed B and Ga₂O₃ powder precursor at 1600 °C under Ar atmosphere. The nanowires have widths of 30–60 nm and perfect crystallinity. When the heating of B and Ga₂O₃ powder precursor was carried out in N₂ atmosphere, β-Ga₂O₃ nanowires sheathed with BN layers (3–5 nm) were yielded. The structure and composition of the products were characterized. The growth scenario for the β-Ga₂O₃ nanowires as well as the formation of outer BN sheaths was also discussed.     

Synthesis of tubular gold and silver nanoshells using silica nanowire core templates

We report the synthesis of tubular gold and silver nanoshells on silica nanowire core templates in solution. Silica nanowires were synthesized and characterized with optical and NMR methods. Gold nanoparticle seeds (2 to 3 nm) with weak repulsive surfactants such as tetrakis-hydroxymethyl-phosphonium chloride (THPC) were conjugated to the surface of these nanowires. A regrowth process was initiated from these nanoparticles on the surface of the silica nanowires dispersed in gold or silver stock solutions in the presence of reducing agents. Micrometers-long gold and silver tubular nanoshells (80-150 nm o.d.) were made, fully covering the silica nanowires.     

Synthesis of nickel nanowires via electroless nanowire deposition on micropatterned substrates

Electroless nanowire deposition on micropatterned substrates (ENDOM) is a promising new technique by which to direct the synthesis and precise placement of metallic nanowires. ENDOM is generally applicable to the preparation of metallic, semiconducting, and even insulating nanowires on technologically relevant substrates, is inexpensive, and can achieve high growth rates. The deposited nanowires are ultralong (centimeters) and can be patterned in arbitrary shapes. We demonstrate ENDOM using the growth of nickel nanowires. By controlling the deposition time, the width of the nanowires can be varied from 200 to 1000 nm and the height can be varied from 7 to 20 nm.     

Polyaniline nanowires on Si surfaces fabricated with DNA templates

It is essential to put individual, free-standing nanowires onto insulating substrates and integrate them to useful devices. Here we report a strategy for fabrication of conducting polymer nanowires on thermally oxidized Si surfaces by use of DNA as templates. The direct use of stretched and immobilized DNA strands as templates avoids the agglomeration of DNA caused by shielding of charges on DNA when polyaniline/DNA complexes formed in solution. Most importantly, the oriented DNA strands immobilized on the Si surface predetermine the position and the orientation of the nanowires. The approach described here is the first step toward uniting the programmable-assembly ability of DNA with the unique electronic properties of conducting polymers for high-density functional nanodevices. The conductivity of the nanowires is very sensitive to the proton doping-undoping process, suggesting that the nanowires hold great promise for sensitive chemical sensor applications.     

Effect of 2-D BN on enhanced corona-resistant characteristic of PI/AlN+BN composite investigated by quasi-in-situ technology

Incorporating two-dimensional (2-D) boron nitride (BN) into polyimide/aluminum nitride (PI/AlN) renders a simple strategy in producing new ternary composite films with longer corona aging life and higher breakdown strength compared with PI/AlN. During corona aging process, the effects of 2-D BN on adjusting the microstructure evolution of ternary composites were investigated using quasi-in-situ technology systematically as follow: (SEM), (AFM), (FTIR), (XRD) and (SAXS) at different corona time. Quasi-in-situ characterization results show that the corona aging modes of the two composite films are quite different, the introduction of BN nanosheets could protect the C–N–C and C–O–C bonds from breaking as well as accelerate the fillers expose via autoxidation reaction. In addition, BN nanosheets could induce the distance between PI molecular chains to shorten during the corona process, resulting in more dense structures coordinating with AlN and PI. Different corona aging mechanism compared with that of the PI/AlN composite films was proposed. This work demonstrated the effect of 2-D BN nanosheets in the PI/AlN+BN ternary composites, which can be used to expand their application in insulating polymer based composites.     

Structural and electronic properties of fluorinated boron nitride nanotubes

The effects of F doping on the structural and electronic properties of the (5, 5) single-walled boron nitride nanotube (BNNT) are investigated by using the density functional theory method. The chemiadsorption of F maintains the hexagonal BN network, increases the lattice constant, and introduces acceptor impurity states. On the other hand, substitutional doping of F destroys the hexagonal BN network, decreases the lattice constant, but does not alter the insulating feature of the BNNT. The observed insulator-to-semiconducting transition, a lattice contraction, and a highly disordered atom arrangement in the sidewall of BNNTs upon F doping appear to be most reasonably attributed to a codoping of dominating substitutional F over chemiabsorbed F, which can induce deep donor impurity states, a lattice contraction, and a destruction of the hexagonal BN network simultaneously.     

Synthesis of CoFe2O4 nanowire in carbon nanotubes. A new use of the confinement effect

Cobalt ferrite nanowires with an average diameter of 50 nm and lengths up to several micrometers were synthesized inside carbon nanotubes under mild reaction conditions using the confinement effect provided by the carbon tubular template.     

Seamless Rim‐Functionalization of h‐BN with Silica—Experiment and Theoretical Modeling

Boron nitride contains six‐ring layers, which are isostructural to graphene, and it exhibits similar extraordinary mechanical strength. Unlike graphene, hexagonal boron nitride (h‐BN) is an insulator and has some polar features that make it a perfect material for those applications graphene is not suitable for, for example, purely ionic conductors, insulating membranes, transparent coatings, composite ceramics, high oxidation resistance materials. We report here a selective rim‐functionalization of h‐BN with SiO₂ by using the Stöber process. A closed, protruding ring of SiO₂ is formed covering all edges perpendicular to the [001] zones of the h‐BN stacks and thus shield the most reactive centers of BN layers. SEM and HAADF‐STEM images, X‐ray spectroscopy, and atomic force microscopy confirm the rim‐functionalization by SiO₂. XRD demonstrates the absence of any intercalation phenomenon of BN and reveals the glassy nature of the SiO₂ rims. Selected variations of synthesis and theoretical modeling both confirm that rim activation by water prior to the Stöber condensation is crucial. First‐principles calculations also confirm that dangling bonds of clean BN edges merge to give interlayer bonds that make further functionalization much more difficult. The reported reaction pathway should allow for other new functionalizations of pure BN and of the rimmed SiO₂/h‐BN composites.     

In Situ Observation of Dynamic Galvanic Replacement Reactions in Twinned Metallic Nanowires by Liquid Cell Transmission Electron Microscopy

Galvanic replacement is a versatile approach to prepare hollow nanostructures with controllable morphology and elemental composition. The primary issue is to identify its fundamental mechanism. In this study, in situ liquid cell transmission electron microscopy was employed to monitor the dynamic reaction process and to explore the mechanism of galvanic replacement. The detailed reaction process was revealed based on in situ experiments in which small Au particles first appeared around Ag nanowires; they coalesced, grew, and adhered to Ag nanowires. After that, small pits grew from the edge of Ag nanowires to form tubular structures, and then extended along the Ag nanowires to obtain hollowed structures. All of our experimental observations from the viewpoint of electron microscopy, combined with DFT calculations, contribute towards an in‐depth understanding of the galvanic replacement reaction process and the design of new materials with hollow structures.     

In-depth profile analysis of filled alumina and titania nanostructured templates by radiofrequency glow discharge coupled to optical emission spectrometry

The development of highly ordered and self-assembled magnetic nanostructures such as arrays of Fe or Ni nanowires and their alloys is arousing increasing interest due to the peculiar magnetic properties of such materials at the nanoscale. These nanostructures can be fabricated using nanoporous anodic alumina membranes or self-assembled nanotubular titanium dioxide as templates. The chemical characterization of the nanostructured layers is of great importance to assist the optimization of the filling procedure or to determine their manufacturing quality. Radiofrequency glow discharge (RF-GD) coupled to optical emission spectrometry (OES) is a powerful tool for the direct analysis of either conducting or insulating materials and to carry out depth profile analysis of thin layers by multi-matrix calibration procedures. Thus, the capability of RF-GD-OES is investigated here for the in-depth quantitative analysis of self-aligned titania nanotubes and self-ordered nanoporous alumina filled with arrays of metallic and magnetic nanowires obtained using the template-assisted filling method. The samples analysed in this work consisted of arrays of Ni nanowires with different lengths (from 1.2 up to 5 μm) and multilayer nanowires of alternating layers with different thicknesses (of 1–2 μm) of Ni and Au, or Au and FeNi alloy, deposited inside the alumina and titania membranes. Results, compared with other techniques such as scanning electron microscopy and energy-dispersive X-ray spectroscopy, show that the RF-GD-OES surface analysis technique proves to be adequate and promising for this challenging application.     

Dielectrophoretic alignment of metal and metal oxide nanowires and nanotubes: a universal set of parameters for bridging prepatterned microelectrodes

Nanowires and nanotubes were synthesized from metals and metal oxides using templated cathodic electrodeposition. With templated electrodeposition, small structures are electrodeposited using a template that is the inverse of the final desired shape. Dielectrophoresis was used for the alignment of the as-formed nanowires and nanotubes between prepatterned electrodes. For reproducible nanowire alignment, a universal set of dielectrophoresis parameters to align any arbitrary nanowire material was determined. The parameters include peak-to-peak potential and frequency, thickness of the silicon oxide layer, grounding of the silicon substrate, and nature of the solvent medium used. It involves applying a field with a frequency >10(5) Hz, an insulating silicon oxide layer with a thickness of 2.5 μm or more, grounding of the underlying silicon substrate, and the use of a solvent medium with a low dielectric constant. In our experiments, we obtained good results by using a peak-to-peak potential of 2.1 V at a frequency of 1.2 × 10(5) Hz. Furthermore, an indirect alignment technique is proposed that prevents short circuiting of nanowires after contacting both electrodes. After alignment, a considerably lower resistivity was found for ZnO nanowires made by templated electrodeposition (2.2-3.4 × 10(-3) Ωm) compared to ZnO nanorods synthesized by electrodeposition (10 Ωm) or molecular beam epitaxy (MBE) (500 Ωm).     

Synthesis and synchrotron light-induced luminescence of ZnO nanostructures: nanowires, nanoneedles, nanoflowers, and tubular whiskers

ZnO nanostructures, including single-crystal nanowires, nanoneedles, nanoflowers, and tubular whiskers, have been fabricated at a modestly low temperature of 550 degrees C via the oxidation of metallic Zn powder without a metal catalyst. Specific ZnO nanostructures can be obtained at a specific temperature zone in the furnace depending on the temperature and the pressure of oxygen. Scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), and X-ray diffraction (XRD) studies show that ZnO nanostructures thus prepared are single crystals with a wurtzite structure. X-ray excited optical luminescence (XEOL) from the ZnO nanostructures show noticeable morphology-dependent luminescence. Specifically, ZnO nanowires of around 15 nm in diameter emit the strongest green light. The morphology of these nanostructures, their XEOL, and the implication of the results will be discussed.     

Faradaic electrochemistry at microcylinder,band, and tubular band electrodes

Current-time relationships of faradaic processes at microcylinder, band, and tubular band electrodes have been evaluated. Microcylinder electrodes were fabricated from platinum wires (5 μm radius) sealed in glass capillaries. Band and tubular electrodes were constructed with platinum sheets (～ 20 μm width) or thin pieces of graphite (～ 5 μm width) sealed between insulating mateials. The temporal response of the current at a microcylinder electrode for the reduction of ferricyanide in aqueous potassium chloride solutions is in excellent agreement with that predicted by equations derived for heat flux to a cylinder. An estimation of the magnitude and temporal properties of the measured current at a band electrode can be obtained when a hemicylinder geometry is assumed. The current respone is identical at band and tubular band electrodes even for the smallest tubular radius investigated, 0.54 mm. Cyclic voltammograms at electrodes of all three geometries show significant contributions from radial diffusion at slow scan rates (< 20 mV s^?1). The current at a graphite tubular band electrode was found to be independent of flow of solution through the electrode at flow rates up to 3 ml min^?1.     

A novel synthesis method to produce silver-doped CeO2 nanotubes based on Ag nanowire templates

Silver nanowires were used as templates to synthesize silver-doped CeO(2) (Ag-CeO(2)) nanotubes by the precipitation method. The precipitated solid was dried at 100 °C for 24 h and calcined at 500 °C for 5 h. A TEM, HRTEM, LV-SEM and XRD study was carried out to determine the micro and nanostructural characteristics of the samples. LV-SEM analysis allowed us to observe microtubular empty structures constituted by Ce, O and Ag as indicated by EDS. These tubular structures, with an external diameter from 120 to 280 nm and an internal diameter from 40 to 80 nm, were mainly composed of 11 nm ceria nanoparticles. This kind of structures was obtained when CeO(2) nanoparticles covered the Ag nanowires during the synthesis. Due to the presence of ammonium hydroxide used during the synthesis, a fraction of the silver nanowire reacts and Ag atoms begin to migrate outside the ceria microtube. When the sintering process is applied, the Kirkendall effect can occur. So, out-diffusion of the remnant Ag through the interface is faster than the in-diffusion of the shell material (CeO(2)), which eventually results in a coaxial nanotube on completion of the non-equilibrium interdiffusion, leaving the central core completely empty, driving the formation of hollow tubular Ag-CeO(2) structures as a result.     

Formation,structure, and structural properties of a new filamentary tubular form: hollow conical-helix of graphitic boron nitride

A novel tubular form of graphitic boron nitride (BN) displaying a hollow conical-helix was discovered. It was generated via wrapping a single beltlike filament according to the geometry of an Archimedes spiral. Cone apex angles of helical-conical nanotubes (HCNTs) were found to exhibit specific values, each of which refers to a certain coincidence site lattice. A unique structural property of HCNTs was observed, displaying the transformation of apex angles during the annealing process. The observed apex angles were reduced with decreasing annealing temperature, which is in accordance with an estimated HCNT strain energy decrease for a given tubular radius. It is suggested that the curvature and apex angle of a HCNT are determined by a sole dynamic element, that is, enthalpy (DeltaH), whereas the HCNT disclination configuration changes through helical sliding of the filament.