Showing papers on "Silicon dioxide published in 2005"

Field-effect electroluminescence in silicon nanocrystals.

Abstract: There is currently worldwide interest in developing silicon-based active optical components in order to leverage the infrastructure of silicon microelectronics technology for the fabrication of optoelectronic devices. Light emission in bulk silicon-based devices is constrained in wavelength to infrared emission, and in efficiency by the indirect bandgap of silicon. One promising strategy for overcoming these challenges is to make use of quantum-confined excitonic emission in silicon nanocrystals. A critical challenge for silicon nanocrystal devices based on nanocrystals embedded in silicon dioxide has been the development of a method for efficient electrical carrier injection. We report here a scheme for electrically pumping dense silicon nanocrystal arrays by a field-effect electroluminescence mechanism. In this excitation process, electrons and holes are both injected from the same semiconductor channel across a tunnelling barrier in a sequential programming process, in contrast to simultaneous carrier injection in conventional pn-junction light-emitting-diode structures. Light emission is strongly correlated with the injection of a second carrier into a nanocrystal that has been previously programmed with a charge of the opposite sign.

Cross-linking Amine-Modified Silica Aerogels with Epoxies: Mechanically Strong Lightweight Porous Materials

Abstract: The mesoporous surfaces of TMOS-derived silica aerogels have been modified with amines by copolymerization of TMOS with APTES. The amine sites have become anchors for cross-linking the nanoparticles of the skeletal backbone of the aerogel by attachment of di-, tri-, and tetra-functional epoxies. The resulting conformal coatings increase the density of the native aerogels by a factor of 2−3 but the strength of the resulting materials may increase by more than 2 orders of magnitude. Processing variables such as the amount of APTES used to make the gels, the epoxy type and concentration used for cross-linking, and the cross-linking temperature and time were varied according to a multivariable design-of-experiments (DOE) model. It was found that while elastic modulus follows a similar trend with density, maximum strength is attained neither at the maximum density nor at the highest concentration of −NH2 groups, suggesting surface saturation effects. Aerogels cross-linked with the trifunctional epoxide always ...

Stöber synthesis of monodispersed luminescent silica nanoparticles for bioanalytical assays.

Abstract: We have developed a simple method to prepare bright and photostable luminescent silica nanoparticles of different sizes and narrow size distribution in high yield. The method is based on the use of Stober synthesis in the presence of a fluorophore to form bright silica nanoparticles. Unlike micro-emulsion-based methods often used to prepare luminescent silica particles, the Stober method is a one-pot synthesis that is carried out at room temperature under alkaline conditions in ethanol:water mixtures and avoids the use of potentially toxic organic solvents and surfactants. Our luminescent particles contained the transition metal complex tris(1,10-phenanthroline) ruthenium(II) chloride, [Ru(phen)3]Cl2. They showed higher photostability and a longer fluorescence lifetime compared to free Ru(phen)3 solutions. Leakage of dye molecules from the silica particles was negligible, which was attributed to strong electrostatic attractions between the positively charged ruthenium complex and the negatively charged silica. To demonstrate the utility of the highly luminescent silica nanoparticles in bioassays, we further modified their surface with streptavidin and demonstrated their binding to biotinylated glass slides. The study showed that digital counting of the luminescent nanoparticles could be used as an attractive alternative to detection techniques involving analogue luminescence detection in bioanalytical assays.

Defects in oxide glasses

Abstract: An insight into the present understanding of point defects in the simplest and the most radiation-resistant oxide glass, glassy silicon dioxide (silica) is presented. The defects and their generation processes in glassy and α-quartz forms of silicon dioxide are significantly different. The only defect, confirmed to be similar in both materials, is oxygen vacancy. In silica, additional defects of dangling bond type are generated from precursor sites formed by strained Si-O bonds, and by modifier ions. The optical absorption spectra of silica are dominated by paramagnetic dangling bond type defects: silicon dangling bond (“E′-center”) and oxygen dangling bond (“non-bridging oxygen hole center, NBOHC”). Radiation-induced interstitial oxygen atoms exist in peroxy linkage (Si-O-O-Si) form, they can react with oxygen dangling bonds to create peroxy radicals or dimerize into interstitial O2 molecules. Hydrogen doping helps to reduce the defect concentration, however, creates new precursors in the form of hydroxyl groups and may stimulate O vacancy generation. Doping by fluorine reduces the number of strained Si-O bonds and results in glass, which has higher vacuum ultraviolet transparency and higher resistance to excimer laser light than pure silica. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

DNA multilayer films on planar and colloidal supports: sequential assembly of like-charged polyelectrolytes.

Abstract: Multilayer films comprising solely negatively charged polyelectrolytes were sequentially assembled based on DNA hybridization. Films prepared from alternating layers of two-block homopolymeric nucleotides (polyA20G20/polyT20C20) grew linearly with increasing layer number, as verified by quartz crystal microgravimetry, UV−vis spectrophotometry and optical microscopy. Urea treatment of the films induced morphological changes, while exposure to low ionic strength solutions resulted in film disassembly. DNA multilayer films were also formed on silica particles, and DNA hollow capsules were obtained following dissolution of the template core.

Biomimetic silica formation: Analysis of the phosphate-induced self-assembly of polyamines

Abstract: The highly siliceous cell walls of diatoms are probably the most outstanding examples of nanostructured materials in nature. Previous in vitro experiments have shown that the biomolecules found in the cell walls of diatoms, namely polyamines and silaffins, are capable of catalysing the formation of silica nanospheres from silicic/oligosilicic acid solutions. In a previous publication, silica precipitation was found to be strictly correlated with a phosphate-induced microscopic phase separation of the polyamines. The present contribution further characterises the phase separation behaviour of polyamines in aqueous solutions. In particular, a pronounced pH-dependence of the average particle diameter is found. It is, furthermore, shown that the ability of phosphate ions to form polyamine aggregates in aqueous solutions cannot be a purely electrostatic effect. Instead, a defined hydrogen-bonded network stabilised by properly balanced electrostatic interactions should be considered. Finally, solid-state 31P NMR studies on phase-separated polyamines, synthetic silica precipitates, and diatom cell walls from the species Coscinodicus granii support the assumption of a phosphate-induced phase separation process taking place during cell wall formation.

Synthesis, characterization, and controlled nitric oxide release from S‐nitrosothiol‐derivatized fumed silica polymer filler particles

Abstract: A new type of nitric oxide (NO)-releasing material is described that utilizes S-nitrosothiols anchored to tiny fumed silica (FS) particles as the NO donor system. The synthetic procedures suitable for tethering three different thiol species (cysteine, N-acetylcysteine, and N-acetylpenicillamine) to the surface of FS polymer filler particles are detailed. The thiol-derivatized particles are converted to their corresponding S-nitrosothiols by reaction with t-butylnitrite. The total NO loading on the resulting particles range from 21-138 nmol/mg for the three different thiol-derivatized materials [S-nitrosocysteine-(NO-Cys)-FS, S-nitroso-N-acetylcysteine (SNAC)-FS, and S-nitroso-N-acetylpenicillamine (SNAP)-FS], with SNAP-FS yielding the highest NO loading. NO can be generated from these particles when suspended in solution via the addition of copper(II) ions, ascorbate, or irradiation with visible light. The SNAC-FS and SNAP-FS particles can be blended in polyurethane and silicone rubber matrixes to create films that release NO at controlled rates. Polyurethane films containing SNAC-FS submerged in phosphate-buffered saline (pH 7.4) generate NO surface fluxes approximately 0.1-0.7x10(-10) mol cm-2 min-1 and SNAP-FS films generate NO fluxes of approximately 0-7.5x10(-10) mol cm-2 min-1 upon addition of increasing amounts of copper ions. Silicone rubber films containing SNAC-FS or SNAP-FS do not liberate NO upon exposure to copper ions or ascorbate in phosphate-buffered saline solution. However, such films are shown to release NO at rates proportional to increasing intensities of visible light impinging on the films. Such photoinitiated NO release from these composite materials offers the first NO-releasing hydrophobic polymers with an external on/off trigger to control NO generation.

Chelating sorbents based on silica gel and their application in atomic spectrometry

Abstract: This mini-review covers chelating sorbents anchored to silica gel and their analytical applications for the preconcentration, separation and determination of trace metal ions, focussing mainly on the last 20 years. The article summarizes also the experience gathered by our research group in the synthesis and characterization of new modified silica gels “via silanization”, and their affinity toward selective extraction and separation of trace elements. The introduction of 1,5-bis(di-2-pyridyl)methylene thiocarbohydrazide silica gel (DPTH-gel) and methylthiosalicylate silica gel (TS-gel) chelating sorbents in trace and ultratrace analysis provide vital breakthroughs in preconcentration methods. These home-made materials allow certain analytes to be selectively extracted from complex matrices without matrix interference and good detection limits. The advantages of these new chelating sorbents in comparison with 8-hydroxyquinoline chelating sorbent immobilized on silica gel are discussed.

Porous Silicon Nitride Ceramics Prepared by Reduction–Nitridation of Silica

Abstract: A new method for preparing porous silicon nitride ceramics with high porosity had been developed by carbothermal reduction of die-pressed green bodies composed of silicon dioxide, carbon, sintering additives, and seeds. The resultant porous silicon nitride ceramics showed fine microstructure and uniform pore structure. The influence of SiO2 particle size and sintering process (sintering temperature and retaining time) on the microstructure of sintering bodies was analyzed. X-ray diffractometry demonstrated the formation of single-phase β-Si3N4 via the reaction between silicon dioxide and carbon at high temperature. SEM analysis showed that pores were formed by the banding up of rod-like β-Si3N4 grains. Porous Si3N4 ceramics with a porosity of 70–75%, and a strength of 5–8 MPa, were obtained.

Fabrication of colloidal crystals with tubular-like packings

Abstract: Methods are reported for the fabrication of colloidal crystal wires with tubular packings. Both free and silica-encased wires have been prepared. Porous silicon membranes are infiltrated with silica spheres, treated with silane, and annealed. After removal of the silicon template, short annealing times were found to result in colloidal crystal wires with varied packing geometries, while repeated annealing cycles produced a thin translucent silica sheath around the wires. Packing in the wires varies with the channel diameter of the Si membrane. The channels used in this study typically produce colloidal crystal wires with six strands, though wires with four to seven strands have been observed. Both chiral and achiral packings are also possible.

Sol-gel processing of drug delivery materials and release kinetics

Abstract: Silica, calcium (5 mol%) silicate and silica/polycaprolactone hybrid inorganic/organic amorphous materials, all mixed with sodium ampicillin, a broad-spectrum antibiotic, have been synthesized by sol-gel method. The amorphous nature of the gels was ascertained by X-ray diffraction analysis. Release kinetics in a simulate body fluid (SBF) have been subsequently investigated. The amount of sodium ampicillin released has been detected by UV-Vis spectroscopy and SEM. The release kinetics seems to occur in more than one stage. Finally FTIR measurements and SEM micrograph showed the formation of a hydroxyapatite layer on the surface of the samples soaked in SBF. All data showed that these materials could be used as drug delivery bioactive systems.

Oxidation Conditions for Octadecyl Trichlorosilane Monolayers on Silicon: A Detailed Atomic Force Microscopy Study of the Effects of Pulse Height and Duration on the Oxidation of the Monolayer and the Underlying Si Substrate

Abstract: In current scanning-probe nanolithography research, substrates consisting of octadecyl trichlorosilane monolayers on silicon are often used. On one hand, the presence of an organic monolayer can be used as a passive resist, influencing the formation of silicon dioxide on the substrate, whereas in other cases the monolayer itself is patterned, creating local chemical functionality. In this study we investigate the time scales involved in either process. By looking at friction and height images of lines oxidized at different bias voltages and different pulse durations, we have determined the parameter space in which the formation of silicon dioxide is dominant as well as the region in which the oxidation of the monolayer itself is dominant.

Light emission and charge trapping in Er-doped silicon dioxide films containing silicon nanocrystals

Abstract: The processes of electro- (EL) and photoluminescence (PL) and charge trapping in Er-implanted SiO2 containing silicon nanoclusters have been studied. It is shown that in Er-doped SiO2 with an excess of silicon nanoclusters of 10 at. %, a strong energy transfer from silicon nanoclusters results in a ten-fold increase of the PL peak at 1540 nm from Er luminescent centers, whereas the EL is strongly quenched by the excess silicon nanoclusters. It is further shown that the implantation of Er creates in the oxide positive charge traps with a giant cross section (σh0>10−13cm2). Introducing subsequent silicon nanocrystals in the oxide leads to the formation of negative charge traps of a giant cross section (σe0>10−13cm2). The possible reason for the EL quenching in the Er-doped SiO2 by silicon nanoclusters is discussed.

Method for producing dislocation-free strained crystalline films

Abstract: A method for forming dislocation-free strained silicon thin film includes the step of providing two curved silicon substrates. One substrate is curved by the presence of silicon dioxide on a back surface. The other substrate is curved by the presence of a silicon nitride layer. One of the substrates is subject to hydrogen implantation and the two substrates are bonded to one another in an annealing process. The two substrates are separated, thereby leaving a layer of strained silicon on a front side of one of the substrates. A back side layer of silicon dioxide or silicon nitride is then removed to restore the substrate to a substantially planar state. The method may be employed to form dislocation-free strained silicon thin films. The films may be under tensile or compressive strain.

Interfacial effects on vitrification of confined glass-forming liquids

Abstract: Mesoporous silica phases, with uniform pores of dimensions in the 2-30 nm range, offer a uniquely well-defined environment for the study of the effects of two-dimensional spatial confinement on the properties of glass-forming liquids. We report observations by differential scanning calorimetry of the vitrification of o-terphenyl (OTP), salol, and glycerol in hexagonal mesoporous silica (MCM-41 and SBA-15) in a wide range of pore sizes from 2.6 to 26.4 nm. In agreement with previous studies, where a controlled porous glass is used as a solid matrix, the glass transition temperature for o-terphenyl diminishes with decreasing pore size. In contrast to OTP, glycerol shows a gradual increase in glass transition temperature, while in salol a slight reduction of glass transition temperature is observed, followed by an increase, which results in glass transition temperature indistinguishable from that of the bulk for the smallest pores. These results are discussed in terms of liquid-surface interactions in an interfacial layer, monitored by Fourier-transformed infrared spectroscopy in the study. The hydrogen bonding with silica surface silanols dominates the glass transition trends observed in salol and glycerol.

Interfacial Reactivity Between Ceria and Silicon Dioxide and Silicon Nitride Surfaces Organic Additive Effects

Abstract: The relative and material removal rates during chemical mechanical planarization were varied by three orders of magnitude as a function of water soluble organic additives in aqueous ceria suspensions The removal requires a hydrolysis step which is influenced by the additive acid-base characteristics The additives are postulated to stabilize the surface via site blocking or surface amine deprotonation Molecules which can adsorb to either silica or ceria surfaces will also decrease removal rates The results highlight the importance of additive functionality in mitigating ceria reactivity with surface silanol groups

Controlled silica synthesis inspired by diatom silicon biomineralization

Abstract: Silica becomes increasingly used in chemical, pharmaceutical, and (nano)technological processes, resulting in an increased demand for well-defined silicas and silica-based materials. The production of highly structured silica from cheap starting materials and under ambient conditions, which is a target for many researchers, is already realized in the formation of diatom biosilica, producing highly hierarchical ordered meso- and macropores silica structures. This notion formed the starting point in our integrative biomolecular and biomimetic study on diatom silicon biomineralization in which we have analyzed silica transformations and structure-direction in polymer-mediated silica syntheses using a combination of (ultra)small-angle X-ray scattering and (cryo)electron microscopy. Using bio-analogous reaction conditions and reagents, such as waterglass and (combinations of) polyethylene oxide (PEO) based polymers, we demonstrate in this review the synthesis of tailor-made mesoporous silicas in which we can, as in biosilica synthesis, control the morphological features of the resulting materials on the nanometer level as well as on the micrometer level.

The dynamics of water in nanoporous silica studied by dielectric spectroscopy

Abstract: Broad-band dielectric spectroscopy is used to investigate the dynamics of hydration water on the surface of the cylindrical pores of a nanostructured silica material (MCM-41, with pore diameter of 3.2 nm) at various hydrations, in the temperature range 250-150 K. We focus our attention on orientational relaxations that shift from 0.5 MHz at 250 K to less than 1 Hz at 150 K. The measurements distinguish the relaxation of the hydroxyl groups at the surface of silica from the orientational dynamics of hydration water which strongly depends on the degree of hydration. Although it is significantly faster than the dynamics of water in ice, the orientational relaxation of "non-freezing" water has an activation energy comparable to that in ice when the hydration layer is complete and approximately two-molecule thick.

Modeling Radiative Properties of Silicon with Coatings and Comparison with Reflectance Measurements

Abstract: Achieving high-accuracy temperature measurements in rapid thermal processing using radiation thermometry requires knowledge of the optical properties of silicon and related materials, such as silicon dioxide, silicon nitride, and polysilicon. However, available optical property models lack consistency and are not fully validated by experiments at the wavelength and temperature ranges critical to radiation thermometry. A critical survey is given of the existing optical models, with emphasis on the need for extrapolation and validation. Also described is an algorithm for calculating the radiative properties of lightly doped silicon with coatings. The effect of coatings covering one or both sides of a smooth silicon wafer is theoretically studied at room temperature, as well as at elevated temperatures. A spectrophotometer was used to measure the reflectance for selected samples in the wavelength region from (1.5 to 1 μm at room temperature. The measurements agree well with the predicted reflectance for bare silicon, a silicon wafer with a nitride coating, and wafers with an oxide coating of different thicknesses, whereas a larger deviation of as much as twice the measurement uncertainty is observed for a silicon wafer coated with polysilicon and oxide films.

Surface-Modified Non-Metal/Metal Oxides Coated With Silicon Dioxide

Abstract: Surface-modified metal oxide particles coated with silicon dioxide and having a low structure are produced by adding a base dissolved in water, with stirring, to a dispersion consisting of a metal oxide, at least one compound of the type XnSi(OR)4-n and water, separating off, optionally washing with water, drying and surface-modifying the reaction product. The surface-modified metal oxide particles coated with silicon dioxide can be used in sunscreens and in CMP applications.

Control of carboxylic acid and ester groups on chromium (VI) binding to functionalized silica/water interfaces studied by second harmonic generation.

Abstract: Resonantly enhanced surface second harmonic generation (SHG) measurements carried out at pH 7 and room temperature were performed to study how surface-bound carboxylic acid and methyl ester functional groups control the interaction of chromate ions with fused silica/water interfaces. These functional groups were chosen because of their high abundance in humic and fulvic acids and related biopolymers commonly found in soils. They were anchored to the silica surface using organosilane chemistry to avoid competing complexation processes in the aqueous solution as well as competitive adsorption of the organic compounds and chromate. The SHG experiments were carried out at room temperature and pH 7 while using environmentally representative chromate concentrations ranging from 1 ×10-6 to 2 × 10-4 M. Chromate is found to bind to the acid- and ester-functionalized silica/water interfaces in a reversible fashion. In contrast to the plain silica/water interface, chromate binding studies performed on the functional...

Silicon dioxide dispersion comprising polyol

Abstract: Stable, pourable silicon dioxide dispersion, in which the average, number-related aggregate diameter of the silicon dioxide particles in dispersion is less than 200 nm, and which comprises at least 35 wt.% of a silicon dioxide powder, 3 to 35 wt.% of at least one polyol, 20 to 60 wt.% of water, 0 to 10 wt.% of an additive and a substance having an alkaline action in an amount such that a pH of 10 < pH < 12 is established. It can be prepared by a procedure in which silicon dioxide powder is introduced into water and a polyol in a rotor/stator machine, the pH being less than 5, and the mixture is dispersed until the current uptake of the rotor/stator machine is largely constant, and a substance having an alkaline action is subsequently added in an amount such that a pH of the dispersion of 10 < pH < 12 results, the substance having an alkaline reaction being added so rapidly that no gel formation takes place. It can be used as a component of a flame-retardant filling of hollow spaces between building components, in particular for insulating glass arrangements.