Silica Nanowires Research Articles

Gold nanoparticle-modulated conductivity in gold peapodded silica nanowires

We report the enhanced electrical conductivity properties of single gold-peapodded amorphous silica nanowires synthesized using microwave plasma enhanced chemical vapor deposition. Dark conductivity of the gold-peapodded silica nanowires can be adjusted by controlling the number of incorporated metal nanoparticles. The temperature-dependent conductivity measurement reveals that the band tail hopping mechanism dominates the electron transport in the gold-peapodded silica nanowires. The high conductivity in the nano-peapodded nanowires with more embedded gold-nanoparticles can be explained by the higher density of hopping states and shorter hopping distance. These Au-embedded amorphous silica nanowires have provided a new approach to enhance not only the electron conduction, but also the chemical-sensor response/sensitivity.

Biotemplated Silica and Titania Nanowires: Synthesis, Characterization and Potential Applications

A simple biotemplating method for the synthesis of silica (SiO2) and titania (TiO2) nanowires was designed on a fibrillar protein (alpha-synuclein) template. The diameter of SiO2 and TiO2 nanowires could be varied, between 20-100 nm, by varying the processing conditions. The nanowires were characterized by energy dispersive spectroscopy (EDS) and electron energy loss spectroscopy (EELS). Due to their high surface area and porosity, the nanowires were tested for potential applications in enzymatic biosensor design.

Silica nanowires synthesized from gas by-product of SiC synthesis from alkoxide precursors

We synthesised silica nanowires (NWs) without catalysts from the otherwise waste SiO gas by-product of SiC synthesis from alkoxide precursors (phenyltrimethoxysilane and tetraethyl orthosilicate). Separate temperature regions were locally defined in a reactor tube for conversion of alkoxide (1500 °C) and deposition of NWs (1130 °C). Thermodynamic calculations prove that SiO gas abruptly condenses and deposits as silica NWs in the range 1160 to 1000 °C. The factors determining the growth of the NWs in this temperature range were fully elucidated. The photoluminescence spectra revealed differences in oxygen defects in the NWs synthesized under growth conditions with different stability. We expect that this strategy could be extended to synthesize various silica nanostructures from the waste silicon- and oxygen-containing gas by-products of other syntheses.

The Mechanical and Photochemical Properties of Titania Coated Silica Nanowires

Dense layers of silica nanowires, grown directly on Au-coated Si substrates via simple active oxidation processes, were coated with a Ti-alkoxide solution and annealed at different temperatures. Mechanical measurements of the resulting anatase and rutile titania coated silica nanowire samples showed that although the volume fraction of nanowires increased with increasing indentation load, a comparative global effective elastic modulus and hardness could be calculated. The rutile coated samples were found to be more robust under low indentation loads, but more readily suffered from brittle failure at higher loads. The photochemical activity of the coated layers, measured by the photodissociation of deuterium oxide vapor, was found to increase with ultra-violet exposure time, most likely due to the desorption of atmospheric N2 or alkoxide residues. This increase in photoactivity was found to be more pronounced for the anatase coated nanowire samples, consistent with the anatase phase having a greater effective reactive surface.

Towards white luminophores: developing luminescent silica on the nanoscale

Nanoscale luminescent probes are useful in a number of applications, from optics to biomedicine. The phenomenon of bulk luminescent silica which emits in the UV-visible range prepared without the use of a metallorganic emitter or organic dye host is prevalent in the literature, although the mechanism of luminescence is still a source of controversy. Nanomaterials displaying similar intrinsic or defect emission would be highly advantageous, particularly as imaging probes. The preparation of nano-sized emitters of this type is less well documented however, mainly due to the challenges of translating bulk preparative procedures to the nanoscale. In this work we report the preparation of silica nanowires and nanoparticles based on the aqueous sol–gel technique which display broad white emission in the visible range of the spectrum without the need for traditional dyes. The source of emission is from the formation of hydrocarbon-based defects during synthesis. We also describe for the first time white-light emitting silica nanoparticles prepared by the Stöber technique.

Growth Characteristics of Amorphous Silicon Oxide Nanowires Synthesized via Annealing of Ni/SiO2/Si Substrates

mixed gas. A pre-oxidized silicon waferand a nickel film are used as the substrate and catalyst, respectively. We propose two distinctive growth modesfor the silicon oxide nanowires that both act as a unique solid-liquid-solid growth process. We named the twogrowth mechanisms “grounded-growth” and “branched-growth” modes to characterize their unique solid-liquid-solid growth behavior. The two growth modes were classified by the generation site of the nanowires.The grounded-growth mode in which the grown nanowires are generated from the substrate and the branched-growth mode where the nanowires are grown from the side of the previously grown nanowires or at the metalcatalyst drop attached at the tip of the nanowire stem.Key Words : Silicon oxide, Nanowires, Solid-Liquid-Solid, Vapor-Liquid-Solid, Growth behaviorIntroductionIn the past decade, the one-dimensional nanostructure hasreceived much attention not only because of its nanoscalesize but also its novel properties, variable morphologies, andextraordinary applications.

Growth of highly bright-white silica nanowires as diffusive reflection coating in LED lighting

Large quantities of silica nanowires were synthesized through thermal treatment of silicon wafer in the atmosphere of N(2)/H(2)(5%) under 1200 °C with Cu as catalyst. These nanowires grew to form a natural bright-white mat, which showed highly diffusive reflectivity over the UV-visible range, with more than 60% at the whole range and up to 88% at 350 nm. The utilization of silica nanowires in diffusive coating on the reflector cup of LED is demonstrated, which shows greatly improved light distribution comparing with the specular reflector cup. It is expected that these nanowires can be promising coating material for optoelectronic applications.

Optimized growth and integration of silica nanowires into interdigitated microelectrode structures for biosensing

Abstract Single step methodology has been developed and optimized for the integration of silica nanowires in interdigitated microelectrodes network. Silica nanowires were grown directly via Vapor-Liquid-Solid (VLS) process into interdigitated microelectrodes gap. Growth optimization was performed to determine an appropriate temperature and annealing time that would allow the dense nanowire growth between interdigitated structures and not compromise the physical/electrical structure of the microelectrode devices. The results indicate the optimum integration for growth at 1050 °C and 60 min. Finally, electrochemical impedance spectroscopy was used to determine the viability of using these nano–micro hybrid devices for biosensing. Electrochemical impedance spectroscopy results indicate a noticeable change in charge transfer (12,609 Ω) for chips with nanowires in comparison to chips without nanowires.

Mesoporous silica nanowires synthesized by electrodeposition in AAO

Mesoporous silica nanowires were prepared by electrodeposition of a prehydrolysed silica sol with surfactant (CTAB) onto the cylindrical pores of AAO template, followed by calcinations to remove the surfactant and removing aluminum oxide membranes with phosphoric acid. The samples show ordered hexagonal mesostructure, which were demonstrated by small angle X-ray diffraction (XRD) patterns and the morphologies of mesostructured silica were demonstrated by scanning electron microscopy (SEM). The results of the SEM indicate that the mesoporous silica nanowires were highly infiltrated into the porous alumina and the sample has a narrow distribution in the range of 2.5–3.8nm from the BET results.

Preparation of Amorphous Silicon Oxide Nanowires by the Thermal Heating of Ni or Au-Coated Si Substrates

Silicon oxide (SiOx) nanowires may have many applications due to their electrical, mechanical and optical properties. Many methods have been reported for the synthesis of SiOx nanowires, including laser ablation, sol–gel, thermal evaporation, carbothermal reduction, physical evaporation, rapid thermal annealing, chemical vapor deposition and thermal oxidation route, oxide assisted growth and thermal sublimation. In this paper, we reported SiOx nanowires fabricated by simple thermal heating process of catalyst thin film-coated Si substrates with various parameters, such as synthesis temperature, synthesis gases, catalysts, and buffer layer (SiO2 layer). Synthesized silicon oxide nanowires were amorphous crystalline. The best synthesis condition of prepared SiOx nanowire is slightly varied with catalysts and buffer layer. The flow rate of synthesis gas affected diameter of silicon oxide nanowires.

Carbonization-assisted integration of silica nanowires to photoresist-derived three-dimensionalcarbon microelectrode arrays

We propose a novel technique of integrating silica nanowires to carbon microelectrode arrayson silicon substrates. The silica nanowires were grown on photoresist-derived three-dimensionalcarbon microelectrode arrays during carbonization of patterned photoresist in a tube furnace at1000 °C under a gaseousenvironment of N2 and H2 in the presence of Cu catalyst, sputtered initially as a thin layer on the structuresurface. Carbonization-assisted nucleation and growth are proposed to extendthe Cu-catalyzed vapor–liquid–solid mechanism for the nanowire integrationbehaviour. The growth of silica nanowires exploits Si from the etched siliconsubstrate under the Cu particles. It is found that the thickness of the initial Cucoating layer plays an important role as catalyst on the morphology and on theamount of grown silica nanowires. These nanowires have lengths of up to 100 µm and diameters ranging from 50 to 200 nm, with 30 nm Cu film sputtered initially. The studyalso reveals that the nanowire-integrated microelectrodes significantly enhance theelectrochemical performance compared to blank ones. A specific capacitance increase ofover 13 times is demonstrated in the electrochemical experiment. The platform canbe used to develop large-scale miniaturized devices and systems with increasedefficiency for applications in electrochemical, biological and energy-related fields.

Photoconduction and the electronic structure of silica nanowires embedded with gold nanoparticles

Silica nanowires (SiO${}_{x}$-NWs) embedded with Au peapods have been studied by energy-filtered scanning transmission electron microscopy (EFTEM), O $K$- and Au ${L}_{3}$-edge x-ray absorption near-edge structure (XANES), and extended x-ray absorption fine structure (EXAFS), x-ray emission spectroscopy (XES) and scanning photoelectron microscopy. XANES and XES data show that band gaps of Au-peapod-embedded and pure SiO${}_{x}$-NWs were 6.8 eV. In additional, XANES and EXAFS results indicate illumination-induced electron transfer from Au peapod to SiO${}_{x}$-NWs and does not show any feature attributable to the formation of Au-Si bonding in the Au peapod embedded in SiO${}_{x}$-NWs with or without illumination. Photoresponse and EFTEM measurements show that green light has more significant enhancement of photoconductivity than red and blue light due to surface plasmon resonance and suggest that transport of electrons across SiO${}_{x}$-NWs is via Mott-variable-range hopping mechanism through localized or defect states.

Stress evolution during the oxidation of silicon nanowires in the sub-10 nm diameter regime

Using a reactive molecular dynamics simulation, the oxidation of Si nanowires (Si-NWs) with diameters of 5, 10, and 20 nm was investigated. The compressive stress at the interface between the oxide and the Si core decreased with increasing curvature in the sub-10 nm regime of the diameter, in contrast to the theory of self-limiting oxidation where rigid mechanical constraint of the Si core was assumed. The Si core of the thinner Si-NW was deformed more with surface oxidation, resulting in a lower compressive stress at the interface. These results explain the experimental observation of full oxidation of very thin Si-NWs.

Preparation of silica nanowires using porous silicon as Si source

This very paper is focusing on the preparation of silica nano-wires via annealing porous silicon wafer at 1200°C in H2 atmosphere and without the assistant metal catalysts. X-ray diffraction, X-ray energy dispersion spectroscopy, scanning electron microscopy, high-resolution transmission electron microscopy and selected area diffraction technology have been employed for characterizing the structures, the morphology and the chemical components of the nano-wires prepared, respectively. It is found that the diameter and the length of the nano-wires were about 100nm and tens micron, respectively. Meanwhile, it is also necessary to be pointed out that silica NWs only formed in the cracks of porous wafers, where the stress induced both by the electro-chemical etching procedure for the porous silicon preparation and nanowires growth procedure is believed to be lower than that at the center of the island. Therefore, a stress-driven mechanism for the NWs growth model is proposed to explain these findings.

Titanium-assisted growth of silica nanowires: from surface-matched to free-standingmorphologies

We report on an oxide-assisted growth technique for silica nanowires which allows tuningthe growth from surface-matched nanowires to free-standing morphologies based on growthcontrol by Ti in the role of a catalyst and surfactant. Using an adjustable Ticoncentration, we grew silica nanowires with lengths ranging from 100 nm up to severalmillimetres whose defect chemistry was analysed by electron microscopy tools,monochromatic cathodoluminescence imaging and time resolved photoluminescencespectroscopy. The knowledge of the luminescence properties and the related defectoccurrence along with their spatial distribution is pivotal for advancing silicananowire growth in order to realize successful device designs based on self-assembledSi/SiOx nanostructures. We demonstrate a core–shell structure of the grown nanowires with ahighly luminescent 150 nm thick shell and outstandingly fast decaying dynamics (≈1 ns) for glass-like materials. The conjunction of the observed efficient and stableluminescences with their attributed decaying behaviours suggests applications forsilica nanowires such as active and passive optical interconnectors and white lightphosphors. The identification of a time domain difference for the spectral regimefrom 2.3 to 3.3 eV, within the confined spatial dimensions of a single nanowire, isvery promising for future, e.g. data transmission applications, employing silicananowires which exhibit achievable compatibility with commonly applied silicon-basedelectronics. A qualitative growth model based on silica particle diffusion andTi-assisted seed formation is developed for the various types of segregated silicananowires which extends commonly assumed oxide-assisted growth mechanisms.

SiO<sub>x</sub>/GeO<sub>x</sub> Nanowires Grown via the Active Oxidation of Si/Ge Substrates

Silica nanowires have recently been grown via the vapor-liquid-solid growth mechanism where the vapor precursor is obtained directly from the substrate via active oxidation processes. In this study, we extend this technique to the Ge-O system and show that Au coated Ge substrates can be used as a volatile GeO source, resulting in germania nanowire formation above 550°C. The process is highly dependent on Au and native oxide thickness’, the partial pressure of O2and annealing temperature. If the oxide layer is too thick, the bare wafer is protected from the active oxidation process. However, if the oxide layer is too thin, it will be readily decomposed leaving no stable surface for nanowires to grow and only an etched surface is observed. In this study we show that a native Ge oxide is unstable and that a thicker oxide is required as a buffer layer, separating active oxidation and nanowire nucleation processes. We also show that nanowires can be grown on stable oxide particles present on the Ge wafer surface.

Photo-conductivity and electronic structures of Au-nanoparticle embedded silica nanowires

Photo-conductivity and electronic structures of Au-nanoparticle embedded silica nanowires

Synthesis and Photoluminescence of Silica Nanowires Grown on Si Substrate

A new simple method was developed to synthesize silica nanowires by physical thermal evaporation of a mixture of silica particles and iron trichloride. The silica nanowires have diameters from 30 to 50 nm, and lengths of ~10 μm, with iron particles at their tips. The growth mechanism of the amorphous silica nanowires follows vapor–liquid–solid model. A broad blue–green photoluminescence band with defined maximum peak energy of ~2.45 eV is observed at room temperature revealing that the silica nanowires could have potential applications in optoelectronic devices.

Microstructure formation on exposure of silicon carbide surfaces to the partial oxidation of methane

Gas-phase silicon-based species, produced on exposure of silicon carbide (SiC) surfaces to the partial oxidation of methane, can be transported away from the site of corrosion to form surface nano- and microstructures. These structures can be divided into two groups, silicon-based structures and silicon/carbon-based structures. Silicon oxide nanowires are the most prevalent of the silicon-based structures, and form on SiC surfaces downstream of the combustion zone. The silicon/carbon-based structures, formed towards the end of the combustion zone, are core–shell heterostructured fibres and take the form of either conical fibres or cross-linked fibre lattices. Low reactive gas concentrations, high temperatures and low methane/oxygen ratios enhance microstructure formation. The effect of gas-phase chemistry and fluid flow on surface behavior is discussed.

Formation Characteristics of Silica Nanowires Grown by Annealing Double Layers of ZnO/SiOx without Precursors

Formation Characteristics of Silica Nanowires Grown by Annealing Double Layers of ZnO/SiOx without Precursors