SiO2 Shell Layer Research Articles

One step growth of GaN/SiO2 core/shell nanowire in vapor-liquid-solid route by chemical vapor deposition technique

GaN/SiO2 core/shell nanowires are grown by cobalt phthalocyanine catalyst assisted vapor-liquid-solid route, in which Si wafer coated with a mixture of gallium and indium is used as the source for Ga and Si and ammonia is used as the precursor for nitrogen and hydrogen. Gallium in the presence of indium and hydrogen, which results from the dissociation of ammonia, forms Si-Ga-In alloy at the growth temperature ∼910°C. This alloy acts as the source of Si, Ga and In. A detailed study using a variety of characterization tools reveals that these wires, which are several tens of micron long, has a diameter distribution of the core ranging from 20 to 50nm, while the thickness of the amorphous SiO2 shell layer is about 10nm. These wires grow along [101¯0] direction. It has also been observed that the average diameter of these wires decreases, while their density increases as the gallium proportion in the Ga-In mixture is increased.

Synthesis and up-conversion luminescence properties of BaFBr-Er3+@SiO2 core/shell heterostructures

BaFBr:Er3+@SiO2 core-shell composites were synthesized starting from BaFBr:Er3+ phosphor core grains (BaFBr doped with 1at% Er3+) prepared by coprecipitation method. A sol-gel process was used to obtain the outer SiO2 layer shell from silica sol precursor. The polycrystalline BaFBr:Er3+@SiO2 heterostructures were formed following calcination at 700°C, in air. The resulting material was characterized in terms of microstructure, photoluminescence and up-conversion luminescence properties. The results indicate that the as-obtained composite has a core-shell structure and shows good luminescence properties. The BaFBr:Er3+@SiO2 heterostructure is made of BaFBr:Er3+ square sheets encased in a SiO2 shell layer with thickness values between 100 and 400nm. Under 810nm laser light pumping, the core-shell heterostructure exhibits Er3+ green up-conversion luminescence bands ((2H11/2, 4S3/2) → 4I15/2) at 525 and 545nm, ascribed to a two-photon process.

Synthesis of Si/SiO2 core–shell nanowire arrays and broadband anti-reflection effects in diluted Si nanowire arrays by adjusting dielectric shell thickness

A low filling ratio and enhanced absorption is needed to enable the full potential of Si nanowire (NW) arrays for optoelectronic applications. In this paper, we report a versatile, scalable fabrication technique that uses nanosphere lithography (NSL) patterning for the synthesis of vertically aligned Si and Si/SiO2 NW arrays. The optical reflection of the NW arrays can be substantially suppressed by the addition of the transparent shell. Meanwhile, by the finite-difference time-domain (FDTD) simulation, we find that the absorption enhancement in the core Si NW can be obtained by adding the transparent shell. The special absorption enhancement of the Si NW arrays with a core–shell structure can be theoretically understood by modal analysis. The absorption in such Si NW array structures is very sensitive to the thickness of transparent coating. By the addition of a SiO2 shell layer, the absorption in the inner Si NW array can be substantially enhanced. Furthermore, significant absorption enhancement and broadband anti-reflection effects can be achieved by the diluted Si NWs combined with the single dielectric shell.

Synthesis, Magnetic and Electromagnetic Absorption Properties of Amorphous Core-Shell Fe-B@SiO<sub>2</sub> Submicrospheres

Amorphous core-shell Fe-B@SiO2 submicrospheres with the SiO2 shell layer of about 18 nm were fabricated via a two-step process. Fe-B submicrospheres were first obtained, and core-shell Fe-B@SiO2 submicrospheres were subsequently fabricated using TEOs as a Si source through a modified Stöber method. The measurements of the magnetic properties demonstrated that the amorphous core-shell Fe-B@SiO2 submicrospheres exhibit ferromagnetic behavior at room temperature. The maximum reflection loss reaches −24.8 dB at 11.76 GHz for the absorber with thickness of 2.2 mm. The absorption bandwidth with the reflection loss below −10 dB is up to 14.76 GHz for the absorber with a thickness of 1.5-6 mm and the absorption bandwidth with the reflection loss below −20 dB is up to 6.6 GHz for the absorber with a thickness of 1.8-3.1 mm. Our results demonstrates that the amorphous core-shell Fe-B@SiO2 submicrospheres obtained in this work are attractive candidate materials for the magnetic and EM wave absorption applications.

Architecture of Directed‐Channel Mesoporous Silica/Titania Shell on Bi‐Alkaline‐Earth Carbonate Particles for Core‐Shell Structure

AbstractA generalized, single‐step synthetic method is outlined for the formation of directed‐channel mesoporous SiO2 or TiO2 shells on bi‐alkaline‐earth carbonate particles to create a core‐shell structure. Cetyltrimethylammonium bromide (CTAB) is used both as the structure‐directing template for the SiO2 and TiO2 shell layer during TEOS and TEOT hydrolysis/condensation and as the mesopore‐channel template in the shell. The resulting core‐shell particles display high properties for the specific surface area and pore volume. In addition, the reaction parameters for the formation of the SiO2 and TiO2 shell during the sol‐gel process are broadly investigated. Consequently, this directed mesoporous core‐shell structure can serve as a platform for further surface modifications to facilitate the rapid translation of particles to a wide range of end applications.

Preparation of SiC/SiO2 core–shell nanowires via molten salt mediated carbothermal reduction route

The growth of silicon carbide (SiC) crystal generally requires a high temperature, especially when low quality industrial wastes are used as the starting raw materials. In this work, SiC/SiO2 core–shell nanowires (NWs) were synthesized from low cost silica fume and sucrose via a molten salt mediated carbothermal reduction (CR) route. The molten salt was found to be effective in promoting the SiC growth and lowering the synthesis temperature. The resultant NWs exhibited a heterostructure composed of a 3C–SiC core of 100nm in diameter and a 5–10nm thick amorphous SiO2 shell layer. The photoluminescence spectrum of the achieved SiC NWs displayed a significant blue shift (a dominant luminescence at round 422nm), which suggested that they were high quality and could be a promising candidate material for future optoelectronic applications.

Effect of Reaction Temperature on Particle Size of Iron Oxide Nanoparticles via Heating of Platelet <inline-formula> <tex-math notation="LaTeX">${\alpha }$ </tex-math></inline-formula>-FeOOH in Tetraethylene Glycol

We investigated the effect of reaction temperature on particle size of spinel-structured ferromagnetic iron oxide particles obtained by heating the platelet $\alpha $ -FeOOH particles in tetraethylene glycol (TEG). This method is advantageous as it enables the omission of outer SiO2 shell layer and makes the surface modification easier. Oxide samples were reduced from $\alpha $ -FeOOH particles using TEG. The Fe3O4 particles were obtained through the formation of $\alpha $ -Fe2O3. The particles with single-phase Fe3O4 were obtained by heating above the temperature of 290 °C and confirmed by X-ray diffraction and Mossbauer spectroscopy. Among these single-phase Fe3O4 particles, a large proportion of relatively small-sized particles was obtained by increasing the reaction temperature to 310 °C. Fe3O4 particles sized 30–80 nm were finally obtained after heating at 310 °C for 60 min with the saturation magnetization of 83 Am2/kg and a coercive force of 13 kA/m.

Highly reproducible SERS substrate based on polarization-free Ag nanoparticles decorated SiO2/Si core-shell nanowires array

SiO2/Si core-shell nanowires array coated with gap-rich silver nanoparticles were demonstrated as a highly reproducible surface-enhanced Raman scattering (SERS) substrate. SERS detection of a relative standard deviation of 8% for 10−4 M R6G with a spot size of ∼2 μm and 900 spots over an area of 150 × 150 μm2 was reported. The high reproducibility is ascribed to the polarization-independent electrical field distribution among three-dimensional nanowire structure with an optimized thickness of SiO2 shell layer.

Preparation, characterization and application of soluble TiO2@SiO2 nanospheres by a simple modified sol–gel procedure

In this study, we presented a simple approach for preparing TiO2@SiO2 nanospheres via ambient temperature sol–gel procedure. Due to the difference of reaction rate, amorphous SiO2 was stabilized in the surface of TiO2 nanospheres. The core–shell structure of the synthesized product were confirmed by FT-IR, XRD, SEM, and TEM analysis. The size of TiO2@SiO2 nanospheres was about 100–140 nm via microscopy analysis, and the thickness of SiO2 shell layer was in the range of 10–20 nm. In addition, the TiO2@SiO2 nanospheres displayed good dispersion in organic solvent, and soluble TiO2@SiO2 nanocomposites were presented in approximate transparent solution. Besides, the prepared PMMA/TiO2@SiO2 nanocomposite films had a well UV-shielding effect with almost blocking the damage of entire UV range, and the nanocomposite films based on the additives of TiO2@SiO2 possessed better UV aging-resistant effect than using the additives of TiO2. Hence, such facile synthesized route can be considered as a simple common method for preparing multifunctional nanocomposites.

Vertical Arrays of SiO2 Micro/Nanotubes Templated from Si Pillars by Chemical Oxidation for High Loading Capacity Buoyant Aquatic Devices

A simple and facile method to fabricate SiO2 micro- or nanotubes has been demonstrated based on room temperature wet chemical oxidation of a porous layer of Si pillar templates that have been prepared by metal-assisted chemical etching (MaCE). Under typical conditions, Si pillars produced by the MaCE have been found to be covered with a thin nanoporous Si layer. The porous Si skin layer has been chemically oxidized by simple dipping in AgNO3 solution at room temperature, which has led to seamless SiO2 shell layer thanks to the accompanying volume expansion during the wet oxidation. Following wet removal of core Si by KOH yields the SiO2 micro- or nanotubes, either in test tube shape or in open shape at both ends, depending on processing method. The vertical arrays of the SiO2 tube on the Si substrate, after hydrophobic siloxane oligomer printing, has been found to have very large loading capacity on water, due to extremely high porosity (>90%) and good enough mechanical stability. The novel method to fabricate SiO2 tubes can shed new light in design of novel aquatic devices, other than simple mimicking the leg of a water strider. Also, the method may be very helpful in various applications of SiO2 nanotubes.

Fabrication, structure, and property of epoxy-based composites with metal–insulator core–shell structure fillers

Abstract

<I>β</I>-MnO<SUB>2</SUB>/SiO<SUB>2</SUB> Core–Shell Nanorods: Synthesis and Dielectric Properties

Large-scale beta-MnO2/SiO2 core-shell nanorods were synthesized by hydrolysis process. The product was characterized by XRD, EDS, SEM and TEM. The thickness of the SiO2 shell layer is about 3 nm approximately 5 nm, which can be tuned by changing the amount of tetraethyl orthosilicate (TEOS) and the reaction time. The dielectric properties of the synthesized core-shell nanorods at the temperature range from 373 K to 773 K in X-band were investigated in detail and the mechanism of the dielectric response was discussed. The dielectric loss of the SiO2-coated MnO2 nanorods at 773 K was about twice than that at 373 K. The high dielectric loss is mainly attributed to the interfacial polarization and the electromagnetic impedance match between the SiO2 shell layer and MnO2 core layer. The quantitative formula between the permittivity of beta-MnO2/SiO2 core-shell nanorods and the thickness of the SiO2 shell is established, which can be used to tune the dielectric properties of the core-shell nanorods through controlling the thickness of the SiO2 shell layer.

Enhanced Stokes and Anti-Stokes Photoluminescence Emission from LaAlO<SUB>3</SUB>:Nd<SUP>+3</SUP> Nanosized Powder Coated with a SiO<SUB>2</SUB> Shell Layer

The research on and development of materials in the field of rare-earth-ion doped nanocrystals for downconversion and up-conversion emission has been recognized to hold tremendous potential in the areas of photonic and biophotonic applications. In the present manuscript, the comprehensive results of the investigation of Stokes and anti-Stokes photoluminescence emission from the prepared LaAlO3:Nd+3 nanoparticles synthesized via the Pechini-type sol-gel method are presented and explained. The XRD diffraction peaks of the LaAlO3:Nd+3 nanoparticles can be easily assigned to the cubic-perovskite LaAlO3 structure. The FESEM image confirms the formation of approximately spherical particles within the range of 80 +/- 30 nm. The PL results showed that the LaAlO3 doped with 5% of Nd+3 ions shows the strongest emission. The core-shell structures obviously enhanced the photoluminescence intensity by suppressing the non-radiative emission and surface defects. As they yield the best PL measurement results, the LaAlO3:0.05Nd+3 nanoparticles were coated with a SiO2 shell layer, to enhance the photoluminescence emission. The mechanisms that are responsible for the photoluminescence emission process observed in the samples are discussed herein, with the help of the Nd+3 ion Dieke energy level diagram. Power dependence slope measurements were performed to identify the processes involved in the LaAlO3:Nd+3 up-conversion photoluminescence.

Photochemical Shape Control of Cadmium Sulfide Nanorods Coated with an Amorphous Silica Thin Layer

The surface of cadmium sulfide nanorods was modified by 3-mercaptopropyltrimethoxysilane, followed by the hydrolysis of trimethoxysilyl groups to form a silica shell structure (SiO2/CdS[rod]). Size-selective photoetching was applied to SiO2/CdS[rod] to modify the size of the CdS rod core. The absorption spectra were blue-shifted by irradiation of monochromatic light, and finally absorption onset agreed with the wavelength of irradiation light. These facts indicated that CdS rod particles were photoetched to smaller ones until the irradiated photons were no longer absorbed by the photoetched particles and that the SiO2 shell layer surrounding the CdS rod core prevented coalescence between photoetched particles. Changes in the wavelength of irradiation light from 488 to 436 nm caused a decrease in rod width from 3.5 to 2.3 nm along with remarkable decrease in the length of rod from 14 to 4.2 nm, suggesting that the photoetching rate was dependent on the kind of crystal faces and that the photocorrosion reactions at the tips of the CdS rod, that is, on (001) and/or (001) faces, were faster than those on other faces that appeared on the sides of the rod. This technique enabled control of CdS rod shape by selecting the wavelength of irradiation light.

Photochemical Fine-Tuning of Luminescent Color of Cadmium Selenide Nanoparticles: Fabricating a Single-Source Multicolor Luminophore

Size-selective photoetching was applied to silica-coated cadmium selenide (SiO2/CdSe) nanoparticles to precisely control their photoluminescence properties. The absorption spectra of CdSe was blue-shifted by irradiation of monochromatic light, and finally, the absorption onset agreed with the wavelength of irradiation light, indicating that CdSe particles were photoetched to smaller ones until the irradiated photons were not absorbed by the photoetched particles and that the SiO2 shell layer surrounding the CdSe core prevented coalescence between the photoetched particles. Although as-prepared SiO2/CdSe did not exhibit photoluminescence, the application of size-selective photoetching to SiO2/CdSe resulted in the development of the band gap emission, with the degree being enhanced with progress of the photoetching. The peak wavelength of photoluminescence decreased with a decrease in the wavelength used for the photoetching, so that the luminescence color could be tuned between red and blue. Partial photoetching of SiO2/CdSe nanoparticle films produced intense band gap emission of CdSe at the photoetched area, while the remainder of the SiO2/CdSe films did not exhibit detectable photoluminescence, resulting in the formation of a clear photoluminescence image under UV irradiation. This technique makes it possible to produce a multicolored photoluminescence image by irradiation with monochromatic lights having various wavelengths using a single source material.

Optical absorption behaviour of platinum core–silica shell nanoparticle layer and itsinfluence on the reflection spectra of a multi-layer coating system in the visible spectrumrange

Platinum core–silica shell nanoparticles(Pt@SiO2) have been applied to the outermost layer of a three-layer film structure to yield aPt@SiO2/SiO2/ITO (indium tin oxide) coating on a glass substrate. Optical properties of the three-layer film havebeen investigated in the visible spectrum regime. The absorption of visible light by the Ptcore–SiO2 shell layer was higher in low- and high-wavelength regimes while it was minimal at amid-visible light range, about 550 nm. This characteristic absorption of the core–shell layerresulted in a broad-band anti-reflectance behaviour of the multi-layer coating system in theentire visible light regime. Transmittance of the three-layer coating–glass system was in therange between 80% and 85% and thus the application of the platinum core–silica shellnanoparticle layer with such absorption characteristics is shown to provide flexibility ofways to achieve a broad-band anti-reflectance and transmittance of a multi-layer coatingsystem.