Electronic spectrum of small silicon clusters in silicon dioxide

Institute of Theoretical and Applied Mechanics, Siberian Branch, Academy of Sciences of the USSR. Translated from Zhurnal Strukturnoi Khimii, Vol. 30, No. 4, pp. 23–26, July–August, 1989.     

Dielectric properties of pyrogenic silicon dioxide

The dielectric characteristics of pyrogenic silicon dioxide were measured, and a theoretical analysis of the change of the dielectric properties of pyrogenic silicon dioxide was carried out with due regard for the surface conduction of water on its particles. A model of a two-layer particle was chosen as the theoretical model. It was shown that the choice of such a model makes it possible to explain experimentally the relaxation maximums of the dielectric losses observed in the frequency range 10⁵–10⁷ Hz.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 24, No. 5, pp. 619–622, September–October, 1988.     

Band distortions and inversions in FTIR spectra of silicon dioxide on silicon

We report the FTIR spectra as a function of incident angle and polarization of a 307 Å layer of silicon dioxide on silicon substrate. Bands normally obscured in transmission spectra are revealed and can dominate the reflection spectra at incident angles away from normal. Strong band distortions were observed as the incident angle and polarization were varied. Our experimental results are in excellent agreement with calculations based on published optical constants. Thus, quantitative interpretations of IR spectra of silicon dioxide on silicon should be made with caution.     

Photoassisted etching of silicon dioxide films

Theoretical approaches to resolving the problem of photoassisted etching of silicon dioxide as the most widely used protective layer in microelectronics are presented. A model of donor-acceptor interaction providing for the desolvation of the F^? ion involved in SiO₂ photoetching is proposed. The etchant compositions were optimized, and the effect of the most important factors on the etching process was examined. It has been shown that the maximal photoetching rate is 0.42 μm/min and the chemical contribution to etching is small, being about 0.02 μm/min.     

J-aggregation in alpha-sexithiophene submonolayer films on silicon dioxide

We report on the observation of J-aggregates in submonolayer films of alpha-sexithiophene grown on silicon dioxide. Photoluminescence spectroscopy reveals that submonolayers are formed by molecules lying flat on the substrate with a head to tail configuration. Excitation energy dependence of photoluminescence shows a red-shifted absorption with respect to isolated molecules and a negligible Stokes shift between absorption and emission. The pronounced structural order of J-aggregates is reflected in the fwhm of the emission bands. From time-resolved and low-temperature photoluminescence experiments, we infer a quantum yield of the J-aggregate between 0.6 and 1. The demonstration of spontaneous formation of J-aggregates of pi-conjugated systems on amorphous silicon-based substrates can be relevant for the development of organic-inorganic hybrid photonic devices.     

X-ray photoelectron spectroscopy of thermally grown silicon dioxide films on silicon

At various stages of in situ thermal oxidation of Si(111) monocrystals, X-ray photoelectron spectroscopy (XPS or ESCA) reveals a shift in the silicon core-level binding energies which varies continuously from 2.4 to 4.2 eV. From the oxygen and silicon ESCA peak intensities, these films can be said to have the silicon dioxide composition with an excess in oxygen concentration. By correlating the silicon 2p or 2s binding-energy shifts with oxygen KLL Auger energy and oxygen 1s binding-energy shifts, it is shown that a Fermi level shift and differential extra-atomic relaxation energy in the interfacial region must be invoked, in addition to chemical structure considerations, to interpret these data.     

On the characteristics and mechanism of the radiation-induced chain reaction between sulfur dioxide and molecular oxygen in acid solutions

Characteristics of the -induced chain reaction between sulfur dioxide and molecular oxygen in perchloric and sulfuric acid media in the presence of Ce(III) ions have been studied. The concentration effects of dissolved oxygen (0.2·10^–3–9.4·10^–3 mol/dm³, sulfur dioxide (0.3·10^–1–2.0·10^–1 mol/dm³ and Ce(III) (0.2·10^–3–4.8·10^–3 mol/dm³) and dose rate (0.26·10¹⁹–1.0·10¹⁹ eV/dm³·s) on the radiation — chemical yield of oxygen consumption G(–O₂) and accumulation of sulfate G(HSO ₄ ^– ), have been investigated. The reaction proceeds with G(–O₂) reaching 10²–10³ molecule/100eV in a catalytic regime. The reaction rate in perchloric acid medium is 3–4 times lower than that in the sulfuric acid medium and depends on the SO₂, O₂ and Ce(III) concentrations, the reaction order varying from 1.0 to 0 and/or in the reverse direction. The mechanism of the process involves chain propagation with 3 stages and 3 intermediates: SO₃H, HSO₅ and Ce(IV). The catalytic effect is caused by the interaction of HSO₄ with Ce(IV) ions followed by their reduction when interacting with SO₂, yielding SO₃H radicals. Chain termination may be due to one or two of the three intermediates or due to all three particles, the kinetics depending on this. Kinetic equations describing the experimental data have been obtained.     

Chemical modification of a porous silicon surface induced by nitrogen dioxide adsorption

The effect of gaseous and liquid nitrogen dioxide on the composition and electronic properties of porous silicon (PS) is investigated by means of optical spectroscopy and electron paramagnetic resonance. It is detected that the interaction process is weak and strong forms of chemisorption on the PS surface, and the process may be regarded as an actual chemical reaction between PS and NO(2). It is found that NO(2) adsorption consists in forming different surface nitrogen-containing molecular groups and dangling bonds of Si atoms (P(b)-centers) as well as in oxidizing and hydrating the PS surface. Also observed are the formation of ionic complexes of P(b)-centers with NO(2) molecules and the generation of free charge carriers (holes) in the volume of silicon nanocrystals forming PS.     

Amorphous silicon dioxide from waste of Ferroally production

The optimum conditions for the formation of amorphous silicon dioxide from wastes formed in ferroalloy production containing more than 99% of the main substance were determined and its physicochemical properties were studied.     

Physicochemical and photocatalytic properties of nanosized titanium dioxide deposited on silicon dioxide microspheres

The synthesis of SiO₂ core-TiO₂ shell composites from a titanium dioxide sol and a suspension of microspherical silicon dioxide is described. The main factors ensuring the formation of a composite with a preset morphology are the size and charge of the TiO₂ sol particles (10–45 nm) and silicon dioxide core particles (300–700 nm), the pH values of the suspensions of the starting components and the resulting composite, and the proportions and way of mixing of the siliconand titanium-containing components. The SiO₂ core-TiO₂ shell composites show high photocatalytic activity in the degradation of Rhodamine FL-BM dye (rate constant of k = 0.0813 min⁻¹) and are much more active than precipitated TiO₂ powder (k = 0.0022 min⁻¹). The activity of the composite is determined by the calcination temperature (700–800°C), by the proportion and accessibility of the active component (TiO₂), and by the presence of a dopant (P₂O₅).     

Chemically assisted ion beam etching of silicon and silicon dioxide using SF6

A chemical flux of sulfur hexafluoride (SF₆) in conjunction with low-energy Ar-ion bombardment has been used for chemically assisted ion beam etching (CAIBE) of silicon and silicon dioxide. The study has shown a large degree of independent control over the selectivity and anisotropy in dry etching. The total etch rate could be controlled by varying either the Ar-ion milling parameters or the chemical flux of SF₆. Etch rate enhancement of 7–8 for silicon and 3–4 for silicon dioxide have been obtained over pure physical etching.     

二氧化硅脱除羟基的动力学研究

醇盐水解法制备的纳米二氧化硅粉体中含有大量羟基,结合差热分析,采用Doyle-Ozawa法和Kissinger法研究二氧化硅粉体脱除羟基的动力学,得到反应表观活化能为134.78kJ.mol-1;并采用Coats-Redfern法进行验证,结果为140.22 kJ.mol-1.采用SEM和XRD对样品进行检测,表明非晶态二氧化硅样品颗粒大小没有变化,晶态没有改变.     

Ab initio calculations on CO4 centers in silicon dioxide

Ab initio SCF-MO Hartree–Fock calculations were performed using the STO-3G, 6-31G, and 6-31G* basis sets to model hypothetical substitutional carbon impurities in silicon dioxide. We utilized nine-atom clusters, [C(OH)₄]^qt, with charge number q_t = 0 and + 1. The positions of the C and O atoms were varied to achieve minimum total energies, while the fixed protons served to simulate the rigid crystal surroundings. In the optimized configuration of the neutral cluster, the C? O bond lengths are appreciably longer than typical C? O bonds, indicating relatively weak bonds for a carbon impurity at a silicon site. For comparison, the relative positions of all nine atoms in the [C(OH)₄]⁰ model were allowed to vary. This unconstrained model yielded more normal bond lengths and was lower in energy than the fixed-proton model by 6.80 eV with the 6-31G* basis set. The free-H model compared favorably with the x-ray diffraction data for an analogous orthocarbonate. Our results are in concert with the lack of reports of any substitutional carbon impurity in α-quartz. In the fixed-H models, the twofold local symmetry was found to be retained when q_t is 0 but not when q_t is + 1. For the latter ion, the unrestricted H-F calculations indicate that this paramagnetic center has its spin population almost entirely on one oxygen ion and is high in energy (5.31 eV with 6-31G) compared to the diamagnetic neutral one. Conclusions reached with the nine-atom clusters were confirmed by a series of calculations on the extended model [C(OSiH₃)₄]⁰.     

Determination of chlorine distribution in silicon dioxide by neutron activation

A neutron activation method for the determination of chlorine distribution in silicon dioxide on the surface of Si wafers has been developed. After irradiation of the sample with a standard, successive layers are removed from the oxide surface by chemical etching and the chlorine content in the solutions obtained is determined gamma-spectrometrically. Using a Ge(Li) detector, a lower limit of determination of 1.0·10⁻⁷ g was obtained. This limit can be improved by using a NaI(Tl) detector. The error of chlorine determination is discussed.     

Radiation changes in the spectral characteristics of silicon dioxide modifications

The results of a comparative study on the modification of the properties of glassy and crystalline silicon dioxide under the radiation induced structural rearrangement are presented. The patterns of a non-linear change in the spectral parameters of bending vibrations of the Si—O—Si bonds in the function of fast neutron flux are revealed. The radiation kinetics of the peak value of the reflection coefficient and the respective frequency modes in the silicon dioxide modifications are compared. The graphs are found to demonstrate a dose dependence of the spectral parameters of special points. They are considered to be related to a radiation-induced rearrangement of the original structure accompanied by a deformation of the bridging bond angles.     

Microchip-based immunoassays with application of silicon dioxide nanoparticle film

Highly sensitive detection of proteins offers the possibility of early and rapid diagnosis of various diseases. Microchip-based immunoassay integrates the benefits from both immunoassays (high specificity of target sample) and microfluidics (fast analysis and low sample consumption). However, direct capture of proteins on bare microchannel surface suffers from low sensitivity due to the low capacity of microsystem. In this study, we demonstrated a microchip-based heterogeneous immunoassay using functionalized SiO(2) nanoparticles which were covalently assembled on the surface of microchannels via a liquid-phase deposition technique. The formation of covalent bonds between SiO(2) nanoparticles and polydimethylsiloxane substrate offered sufficient stability of the microfluidic surface, and furthermore, substantially enhanced the protein capturing capability, mainly due to the increased surface-area-to-volume ratio. IgG antigen and FITC-labeled anti-IgG antibody conjugates were adopted to compare protein-enrichment effect, and the fluorescence signals were increased by ~75-fold after introduction of functionalized SiO(2) nanoparticles film. Finally, a proof-of-concept experiment was performed by highly efficient capture and detection of inactivated H1N1 influenza virus using a microfluidic chip comprising highly ordered SiO(2) nanoparticles coated micropillars array. The detection limit of H1N1 virus antigen was 0.5 ng mL(-1), with a linear range from 20 to 1,000 ng mL(-1) and mean coefficient of variance of 4.71%.     

Physicochemical properties of amorphous silicon dioxide produced from nepheline

The properties of silicon dioxide precipitates produced by acid processing of nepheline have been investigated. Regardless of the production method, silicon dioxide takes an intermediate position between commercial silica gel and white soot in terms of the structure and some physicochemical properties, this position determining possible applications of this product.     

Electronic spectroscopy for aromatic-hydrocarbon complexes adsorbed on silicon dioxide

The range of adsorption states determines the electronic spectra for organic fluorophors dispersed in molecular form on porous or nonporous silicon dioxide, including effects from electron and proton transfer, molecular association, and energy transfer. The photonic effects are dependent on the type of molecule, the carrier activation conditions, the uptake, the excitation conditions, and any coadsorbates. These fluorophors can be used to probe surfaces.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 25, No. 2, pp. 203–212, March–April, 1989.