Nanowire Bunches Research Articles

Fabrication and characterization of amorphous silica nanostructures

Large-scale amorphous silica nanostructures, including nanowires, nanotubes and flowerlike nanowire bunches depending on the position, have been fabricated on silicon wafer through a cheap route under the assistance of gold and germanium. Accompanying the observation of blue–green light emission, comprehensive micro-structural characterization reveals that the growth of nanostructures is catalyzed only by gold whereas the final morphology of nanostructures depends on the location to germanium ball. Au 2Si, a compound of gold and silicon, is also disclosed as an intermediate state during the catalysis. Correspondingly, a growth scheme is proposed based on the experimental results and the vapor-liquid-solid mechanism.

Template-Free Preparation of Bunches of Aligned Boehmite Nanowires

A simple method based on a hydrothermal process using alkali salts as mineralizers is proposed for the synthesis of aligned bunches of boehmite (gamma-AlOOH) nanowires without a template's assistance. Most bunches of aligned boehmite nanowires are constructed by two separated shorter bundles with widths of 700 to approximately 800 nm and lengths of about 1 microm. XRD patterns, FTIR spectra, and SEM and TEM images were used to characterize the products. The specific surface area and pore-size distribution of the obtained product as determined by gas-sorption measurements show that the boehmite bundles possess a high BET surface area and porosity properties. The importance of adding Na2B4O7 salts for the formation of bundle morphologies has been discussed.

GaN nanowires: CVD synthesis and properties

The growth of GaN nanowires from Ga and NH 3 sources in the flow of Ar carrier gas using a chemical vapor deposition (CVD) system was systematically studied. The substrates used were Si(111) and Si(100). Fabricated nanowires were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDX). We investigated the influence of growth temperature, catalyst used, Ga amount, and the ratio of Ar and NH 3 flow rates on the morphology and properties of GaN nanowires. We found that the best results were obtained for a growth temperature of 950 °C. Optimal catalysts were Au and metallic Ni, while the use of nickel nitrate was found to lead to formation of SiO x nanowire bunches in addition to GaN nanowires. For the optimal temperature and catalyst used, the influence of the Ga to N ratio on the nanowire growth was studied. It was found that different types of nanostructures are observed in relatively Ga-rich and in relatively N-rich conditions. Growth mechanisms of different types of nanowires, including the stacked-cone nanowires and the microscale structures formed by lateral growth under N-rich conditions, are discussed.

Influence of annealing on electrochromic performance of template assisted, electrochemically grown, nanostructured assembly of tungsten oxide

Nanostructured tungsten oxide (WO 3) thin films have been electrochemically grown from a self-assembly of sodium dodecyl sulfate–tungsten oxide aggregates at the electrode–electrolyte interface. The nanoparticulates embedded in a quasi-layered structure of the as-deposited WO 3 film with a single-crystalline hexagonal phase transform to a hybrid microstructure encapsulating both nanoscaled particles and rods with a tetragonal modification at 100 °C, reverting again to orthorhombic single-crystalline nanoplatelets and bunches of tangled nanowires at 250 °C and finally coarse nanoparticles at 500 °C. The WO 3 film annealed at 100 °C shows the fastest color-bleach kinetics, highest optical modulation and coloration efficiency, which is a repercussion of the pentagonal channels equipped with the tetragonal phase, the mesopore structure and the channeled porosity. In spite of perpendicularly oriented hexagonal channels, which facilitate ion insertion, the switching performance of the as-deposited films comes a close second owing to extremely random pore morphology. Poor color-bleach rates observed for the films annealed at 250 and 500 °C are attributed to pore shrinkage, high density and crystallinity.

Gallium-Catalyzed Silicon Oxide Nanowire Growth

Silicon oxide nanowires tend to assemble into various complex morphologies through a metal-catalyzed vapor-liquid-solid (VLS) growth process. This article summarizes our recent efforts in the controlled growth of silicon oxide nanowire assemblies by using molten gallium as the catalyst and silicon wafer, SiO powder, or silane (SiH 4) as the silicon sources. Silicon oxide nanowire assemblies with morphologies of carrotlike, cometlike, gourdlike, spindlelike, badmintonlike, sandwichlike, etc. were obtained. Although the morphologies of the nanowire assemblies are temperature- and silicon source-dependent, they share similar structural and compositional features: all the assemblies contain a microscale spherical liquid Ga ball and a highly aligned, closely packed amorphous silicon oxide nanowire bunch. The Ga-catalyzed silicon oxide nanowire growth reveals several interesting new nanowire growth phenomena that expand our knowledge of the conventional VLS nanowire growth mechanism.

Selective growth of silica nanowires on nickel nanostructures created by atomic forcemicroscopy nanomachining

We report the selective growth of amorphous silica nanowires on nickel nanostructurescreated by an atomic force microscopy nanomachining and lift-off process. Successfulgrowth of patterned nanowire bunches or single nanowires on Si substrates has beenrealized by thermal annealing. The growth mechanism is found to be a combination ofvapour–liquid–solid and solid–liquid–solid modes. In addition, the relationshipbetween the nanowire length and the growth time has a nonlinear dependence.

Magnetic properties of iron nanoparticles in mesoporous silica matrix

Anisotropic iron nanoparticles were successfully synthesized within the pores of mesoporous silica matrix by intercalation of hydrophobic iron complex (Fe(CO) 5) into the hydrophobic part of the mesoporous silica-surfactant composite. It was shown that the particles are uniform and well ordered in the matrix and their shape and size are in good agreement with that of the pores. The samples demonstrate spin-glass-like behavior at low temperatures, which transforms to the superparamagnetic one with temperature increases. Nanoscale-related magnetic properties were investigated by polarized SANS. It was shown that hysteretic and the spin-glass-like behavior may be attributed to bunches of the iron nanowires with average radius of 4.7 nm.

Growth of SiOx nanowire bunches cocatalyzed with Ga and Ni

Si O x nanowire bunches were fabricated on Ni(NO3)2*6H2O solution-coated Si(111) substrates in a chemical vapor deposition system in the presence of Ga and under the flow of Ar and NH3 gases. The roles of nickel nitrate hydrate, gallium, and ammonia in the formation of SiOx nanowire bunches were investigated. It was found that Ni and Ga act as catalysts for the growth, while nickel nitrate hydrate also serves as a source of oxygen. The growth mechanisms of different nanowire structures obtained by varying the fabrication conditions are discussed.

Template-free preparation of bunches of aligned manganese oxide nanowires

A simple method based on a conventional solid-state process is proposed for synthesis of manganese oxide nanowires. This method provides an opportunity for bunching the nanowires synthesized, since the nanowires form in one direction. Thus, aligned bunches (e.g. >100 µm long and about 50 µm in diameter) consisting of individual nanowires (diameters ranging between 50 and 200 nm) can be prepared simply. The phenomenon of simultaneous alignment of the nanowires in one direction to form ordered bunches is indeed an interesting development in the realm of nanotechnology. It can be considered as an alternative to conventional template-based methods.

Magnetic properties of iron nanoparticles in mesoporous silica

Magnetic properties of nanocomposites based on mesoporous silica SiO 2 with embedded iron particles are studied by small angle polarized neutron scattering at temperatures from 8 to 300 K and magnetic fields from 1 to 350 mT. This mesoporous silica forms highly regular hexagonal structures of nanotubes. A diffraction peak in SANS at q c=1.55 nm −1 is observed, corresponding to a hexagonal structure with periodicity a=4.6 nm. The scattering of the pure matrix Si0 2 is well fitted by a sum of two contributions: Porod scattering from large particles (1/ q 4) plus the scattering on a regular structure of pores (diffraction peak). Additional scattering is observed from samples with iron in the pores. The latter is fitted as scattering from bunches of nanowires with an average radius of R b=4.68 nm and a spread of Δ R b=0.9 nm. The nuclear-magnetic interference in the scattering of polarized neutrons is studied. We observe no interference scattering on the periodic structure that would be consistent with a hexagonal structure of nanotubes/nanowires. Instead, appreciable interference scattering is detected which is directly related with bunches of nanowires created during crystallization. The temperature and magnetic field dependences of this interference scattering demonstrate a transition from superparamagnetic to dipole–glass state at T = 80 K caused by an interplay of magnetic field, dipole–dipole, and thermal interactions.

Preparation and characterization of oriented silica nanowires

Large-scale of oriented closely packed silica nanowire bunches have been synthesized by using large size (1–10 μm in diameter), low melting point tin droplets as catalyst on silicon wafers at 980 °C. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) analyses show that the amorphous silica nanowires have lengths of 50–100 μm and diameters of 100–200 nm. Unlike any previous observed results using high melting point metal (such as gold and iron) as catalyst, the Sn catalyst growth exhibits many interesting phenomena. Each Sn ball can simultaneously catalyze the growth of many silica nanowires, which is quite different from the conventional vapor–liquid–solid process.

Temperature Dependence of Morphologies of Aligned Silicon Oxide Nanowire Assemblies Catalyzed by Molten Gallium

Silicon oxide nanowire assemblies with fishbonelike, gourdlike, spindlelike, badmintonlike, and octopuslike morphologies were synthesized by the chemical vapor deposition of silane at 1150 °C with molten gallium as the catalyst via a vapor−liquid−solid process. The morphologies of the nanowire assemblies were temperature-dependent so that within a specific temperature range nanowire assemblies with a specific morphology were formed. Although the nanowire assemblies formed in different temperature ranges have different morphologies, they all are composed of a spherical liquid-gallium ball (3 to 5 μm in diameter) and a silicon oxide nanowire bunch that grows out from the lower-hemisphere surface of the gallium ball. Branching-growth and batch-growth phenomena were observed in the samples and were believed to be responsible for the formation of the unique morphologies described here. The growth mechanism of the nanowire assemblies is discussed.

Molten gallium as a catalyst for the large-scale growth of highly aligned silica nanowires.

The vapor-liquid-solid (VLS) process is a fundamental mechanism for the growth of nanowires, in which a small size (5-100 nm in diameter), high melting point metal (such as gold and iron) catalyst particle directs the nanowire's growth direction and defines the diameter of the crystalline nanowire. In this article, we show that the large size (5-50 microm in diameter), low melting point gallium droplets can be used as an effective catalyst for the large-scale growth of highly aligned, closely packed silica nanowire bunches. Unlike any previously observed results using gold or iron as catalyst, the gallium-catalyzed VLS growth exhibits many amazing growth phenomena. The silica nanowires tend to grow batch by batch. For each batch, numerous nanowires simultaneously nucleate, grow at nearly the same rate and direction, and simultaneously stop growing. The force between the batches periodically lifts the gallium catalyst upward, forming two different kinds of products on a silicon wafer and alumina substrate. On the silicon wafer, carrot-shaped tubes whose walls are composed of highly aligned silica nanowires with diameters of 15-30 nm and length of 10-40 microm were obtained. On the alumina substrate, cometlike structures composed of highly oriented silica nanowires with diameters of 50-100 nm and length of 10-50 microm were formed. A growth model was proposed. The experimental results expand the VLS mechanism to a broader range.