Si-O Stretch Research Articles

Identification of Large Isotope Anomalies in Quartz by Infrared Spectroscopy.

We report an infrared (IR) spectroscopic technique to detect quartz grains with large isotope anomalies. We synthesized isotopically doped quartz and used Fourier transform infrared spectroscopy (FT-IR) in two different instruments: a traditional far-field instrument and a neaSpec nanoFT-IR, to quantify the shift in the peak of the Si-O stretch near 780 cm-1 as a function of isotope composition, and the uncertainty in this shift. From these measurements, we estimated the minimum detectable isotope anomaly using FT-IR. The described technique can be used to nondestructively detect very small (30 nm) presolar grains. In particular, supernova grains, which can have very large isotope anomalies, are detectable by this method.

The rotational spectrum and potential energy surface of the Ar-SiO complex.

The rotational spectra of five isotopic species of the Ar-SiO complex have been observed at high-spectral resolution between 8 and 18 GHz using chirped Fourier transform microwave spectroscopy and a discharge nozzle source; follow-up cavity measurements have extended these measurements to as high as 35 GHz. The spectrum of the normal species is dominated by an intense progression of a-type rotational transitions arising from increasing quanta in the Si-O stretch, in which lines up to = 12 (∼14 500 cm-1) were identified. A structural determination by isotopic substitution and a hyperfine analysis of the Ar-Si17O spectrum both suggest that the complex is a highly fluxional prolate symmetric rotor with a vibrationally averaged structure between T-shaped and collinear in which the oxygen atom lies closer to argon than the silicon atom, much like Ar-CO. To complement the experimental studies, a full dimensional potential and a series of effective vibrationally averaged, two-dimensional potential energy surfaces of Ar + SiO have been computed at the CCSD(T)-F12b/CBS level of theory. The equilibrium structure of Ar-SiO is predicted to be T-shaped with a well depth of 152 cm-1, but the linear geometry is also a minimum, and the potential energy surface has a long, flat channel between 140 and 180°. Because the barrier between the two wells is calculated to be small (of order 5 cm-1) and well below the zero-point energy, the vibrationally averaged wavefunction is delocalized over nearly 100° of angular freedom. For this reason, Ar-SiO should exhibit large amplitude zero-point motion, in which the vibrationally excited states can be viewed as resonances with long lifetimes. Calculations of the rovibrational level pattern agree to within 2% with the transition frequencies of normal and isotopic ground state Ar-SiO, and the putative K a = ±1 levels for Ar-28SiO, suggesting that the present theoretical treatment well reproduces the salient properties of the intramolecular potential.

Application of visible and infrared spectroscopy for the evaluation of evolved glauconite

The Oligocene Maniyara Fort Formation in western India exhibits two distinct glauconite types with different maturation states, which are characterized by their spectral response in the visible to infrared spectrum of electromagnetic radiation. Spectral signatures of Maniyara Fort glauconites display absorption features at approximately 0.77, 1.08, 1.9, 2.3μm in the visible-short-wave infrared (SWIR) and 2.8 and 10μm in the mid-infrared (MIR) region which vary with K2O content of glauconite. The spectra of glauconite varies significantly as a function of its cationic contents and substitution in different sites. The maturity is found to increase in tandem with the metal–metal charge transfer (CT) and the Fe2+ dd absorption band respectively at 1.08 and 0.77μm. H2O and OH− signatures at the NIR region reflect differences in the sensitivity of glauconites with different molecular H2O content. In the MIR region, a gradual shift of the SiO stretch at 10μm towards lower wavelengths indicates the dominance of smectite layers in glauconites. This study demonstrates a strong correlation between the proportion of expandable layers in the glauconite structure with variations in characteristic band position, depth and symmetry in reflectance and emissivity.

Organophilic treatments of bentonite increase the adsorption of aflatoxin B1 and protect stem cells against cellular damage

Bentonite clays exhibit high adsorptive capacity for contaminants, including aflatoxin B1 (AFB1), a mycotoxin responsible for causing severe toxicity in several species including pigs, poultry and man. Organophilic treatments is known to increase the adsorption capacity of bentonites, and the primary aim of this study was to evaluate the ability of Brazilian bentonite and two organic salts - benzalkonium chloride (BAC) and cetyltrimethylammonium bromide (CTAB) to adsorb AFB1. For this end, 22 factorial designs were used in order to analyze if BAC or CTAB was able to increase AFB1 adsorption when submitted in different temperature and concentration. Both BAC and CTAB treatment (at 30°C and 2% of salt concentration) were found to increase the adsorption of AFB1 significantly compared with untreated bentonite. After organophilic bentonite treatments with BAC or CTAB, a vibration of CH stretch (2850 and 2920cm−1) were detected. A frequency of the SiO stretch (1020 and 1090cm−1) was changed by intercalation of organic cation. Furthermore, the interlayer spacing of bentonite increases to 1.23nm (d001 reflection at 2θ=7.16) and 1.22 (d001 reflection at 2θ=7.22) after the addition of BAC and CTAB, respectively. Another aim of the study was to observe the effects of these two bentonite salts in neural crest stem cell cultures. The two materials that were created by organophilic treatments were not found to be toxic to stem cells. Furthermore the results indicate that the two materials tested may protect the neural crest stem cells against damage caused by AFB1.

Surface vibrational structure of colloidal silica and its direct correlation with surface charge density.

We show that attenuated total reflection Fourier-transform infrared (ATR-FTIR) spectroscopy can be used to determine the surface charge density (SCD) of colloidal silica nanoparticles (NPs) in aqueous solution. We identify the Si-O stretch vibrations of neutral surface bound silanol, ≡Si-OH, and of the deprotonated group, ≡Si-O(-). The position of the Si-(OH) stretch vibration is shown to directly correlate with the NPs SCD as determined by traditional potentiometric titrations, shifting to lower wavenumber (cm(-1)) with increasing density of ≡Si-O(-). The origin of this shift is discussed in terms of inductive effects that reduce the ionic character of the Si-(OH) bond after delocalization of the negative charge left on a terminal ≡Si-O(-) group across the atoms within ∼1 nm of the charged site. Using this new methodology, we quantitatively determine the SCD of 9, 14, and 25 nm diameter colloidal silica in varying concentrations of NaCl electrolyte at different bulk pH. This novel spectroscopic approach to investigate SCDs provides several opportunities for in situ coupling, for example, in microfluidic channels or with liquid microjets, and requires only very little sample—all potential advantages over a traditional potentiometric titration.

Synthesis and characterization of low-k films for large area imaging applications

An organic liquid polymer based on methyltriethoxysilane has been synthesized. The material;when spin-coated onto wafers and thermally treated;lead to dielectric films of low permittivity;which can be used in hydrogenated amorphous silicon (a-Si:H) thin film transistor (TFT) based large area imaging arrays as an inter-level dielectric between the TFT and pixel levels. Fourier Transform Infrared Spectroscopy (FTIR) shows prominent peaks of Si-CH3 stretch and Si-O stretch modes. The dielectric constant (k) of the film was found to decrease with increasing curing temperature (Tcure). However;k values were<3 for Tcure<260^oC;which is the upper limit for our a-Si:H process. Thermogravimetric analysis showed that major weight loss takes place for up to 200^oC;and the film stabilizes thereafter. The stress in the films was found to be compressive;and increased from 50 to 220MPa when Tcure was increased from 250 to 450^oC. The low stress for low Tcure is a desirable property for film integration.

5–14 μmSpitzerspectra of Themis family asteroids

Context. The Themis collisional family is one of the largest and best established families in the main belt. Composed of primitive asteroids, there is evidence that water is likely present in a large fraction of its members, either in aqueously altered silicates or in water ice reservoirs. The study of the abundance of water in the outer asteroid belt is important as it may be linked to the origin of Earth’s water. Studying the Themis family can also help to constrain the compositional and thermal environment in the region of the solar nebula where these asteroids formed. Aims. Our aim is to constrain the composition and thermal properties of the surfaces of several Themis family asteroids. Methods. We present 5–14 μm spectra of 8 Themis family asteroids observed with Spitzer: (222) Lucia, (223) Rosa, (316) Goberta, (383) Janina, (468) Lina, (492) Gismonda, (515) Athalia, and (526) Jena. We determine their diameters, geometric albedos and beaming parameters using the near-Earth asteroid thermal model. Their emissivity spectra are studied in order to determine if they exhibit an emission plateau from 9 to 12 μm which has been observed in other primitive asteroids and attributed to fine-grained silicates (the Si-O stretch fundamental). Results. The derived mean albedo of our sample of Themis family asteroids is ¯ pV = 0.07 ± 0.02, and the mean beaming parameter is ¯ η = 1.05 ± 0.10. The derived ¯ η value is close to unity, which implies that the infrared beaming is not significant, there is likely little night-side emission from the asteroids, and the thermal inertia is probably low. The emissivity spectra of at least 5 of our 8 asteroids show a 9–12 μm emission plateau with spectral contrast of ∼2–4%, similar but smaller than that observed in the spectra of Trojan asteroids and cometary dust. The plateau may be due to the surfaces having either small silicate grains embedded in a relatively transparent matrix, or from a very under-dense (fairy-castle) surface structure. Conclusions. The surfaces of a large fraction of Themis family asteroids with D ∼ 50 km are covered by a fine grained silicate mantle as observed on Trojan asteroids of similar or larger size. The lower amplitude of the silicate emission in Themis family asteroids spectra (2–4%) with respect to that of Trojan asteroids (10–15%) could be attributed to larger dust particles, a slightly denser structure, or a lower silicate dust fraction.

Polarized ATR-FTIR investigation of Fe reduction in the Uley nontronites

AbstractReduction of structural Fe in smectites affects the surface chemical behavior of the clay, but the underlying mechanism and changes in clay structure are still in need of investigation, particularly with respect to changes in the tetrahedral sheet. The purpose of this study was to probe changes in the tetrahedral sheet that occur when structural Fe is reduced in the Uley nontronites, NAu-1 and NAu-2, using polarized attenuated total internal reflection Fourier-transform infrared spectroscopy. Despite the differences in their structures — NAu-2 has tetrahedral Fe3+ while NAu-1 does not — the changes observed in the Si-O stretching region were quite similar. Reduction results in a shift of the in-plane Si-O stretching modes to lower frequencies, while the out-of-plane Si-O stretch shifts to higher frequencies. The magnitude of these shifts is greater in NAu-2 than in NAu-1, but the crystallinity of the tetrahedral silicate sheet of NAu-2 is preserved upon reduction. In both nontronites, the orientation of the out-of-plane Si-O bond changes and becomes completely perpendicular to the basal (001) surface of the clay, indicating the formation of trioctahedral domains wherein the individual tetrahedra reorient relative to the plane of the clay layer.

Secondary oxidation product on Si(111)-(7×7) characterized by isotope-labeled vibrational spectroscopy

The reaction of O(2) with Si(111)-(7 x 7) has been studied by electron energy-loss spectroscopy at 82 K. In addition to the losses due to Si-O-Si configurations, we observed two Si-O stretch modes depending on the coverage. A 146-meV peak appears at the initial reaction stage and was ascribed to a metastable product with one oxygen atom bonding on top of Si adatom and the other inserted into the backbond. The initial product is further oxidized to produce the second Si-O stretch peak at 150 meV. The secondary product was partially substituted with isotopes and analyzed with a simple model of coupled oscillators. The vibrational spectra reflect dynamical couplings between the isotopes, which is consistent with those predicted from the tetrahedral SiO(4) structure with one on top and three inserted oxygen atoms.

Effects of Annealing Atmosphere on the Characteristics and Optical Properties of SiON Films Prepared Plasma Enhanced Chemical Vapor Deposition

AbstractSiON thin films were deposited by plasma-enhanced chemical vapor deposition method at 350 °C using N2O/SiH4 gas mixtures as precursors. As-deposited SiON films were annealed in different gas atmospheres (air, N2, and O2) and at different annealing temperatures (800 oC ∼ 1100 oC). Effects of annealing atmosphere on the Si-O, Si-N, Si-H, and N-H bonding characteristics in SiON films and their structural and optical properties have been investigated. Cross-sectional and planar microstructures were characterized by scanning electron microscopy and atomic force microscopy, and crystallinity was investigated by X-ray diffraction. Chemical bonding characteristics and optical properties SiON films were studied using fourier transform infrared spectroscopy and prism coupler. Xray diffractions showed no evidence of any crystals in all SiON films. The deposition rate strongly depended on the processing parameters such as radio frequency (rf) power, N2O/SiH4 flow ratio, and SiH4 flow rate. Deposition rate increased as N2O/SiH4 flow ratio increased and SiH4 flow rate increased. It was possible to obtain SiON films with surface roughness of about 1 nm and a high deposition rate of about 4 μm/h when the processing parameters were optimized as rf power of 200 W, N2O/SiH4 flow ratio of 3, SiH4 flow rate of 100 sccm. It was observed that the intensity and the shift of the Si-O stretch and Si-N peaks depended on the annealing atmosphere as well as the annealing temperature. The intensity of Si-O peaks increased in the samples annealed in oxygen atmosphere, but it decreased in the samples annealed in nitrogen atmosphere. The intensity of Si-N peak decreased in the samples annealed in oxygen atmosphere, but it increased in the samples annealed in nitrogen atmosphere. The position of Si-O peaks shifted from 1030 nm to 1140 nm in the samples annealed both in oxygen and in nitrogen atmosphere. It was also observed that the intensities of Si-H (∼2250 cm−1) and N-H (∼3550 cm−1) peaks decreased apparently as the annealing temperature increased in all annealed samples.

Synthesis and Characterization of Methyltriethoxysilane Based Low Permittivity (Low-k) Polymeric Dielectrics

AbstractA low-permittivity (low-k) polymeric material has been synthesized using methyltriethoxysilane as base material. The films were reproduceably deposited, by spin-coating on Si wafers with a uniform thickness in the range of 0.3-0.5μm depending on speed. The parameters for spin coating (spin speed and spin time etc.,) have been optimized. The effects of various curing conditions on the structural and dielectric properties have been studied. Fourier Transform Infrared Spectroscopy (FTIR) shows prominent peaks of Si-CH3 stretch and Si-O stretch modes. Test structures on silicon were fabricated to measure the dielectric constant (k) of the material. The values of k were found to be in the range of 2.6-1.2 for annealing temperatures 150°C-450°C. The dielectric constant decreases as the curing temperature increases.

Ablation of dielectric materials during laser irradiation involving strong vibrational coupling

Photon energy can be deposited efficiently into an exceedingly small volume by using a laser tuned to a vibrational resonance of the material. Both calcite and silica have absorption depths of a few tenths micron at the asymmetric carbonate stretch (7.1 μm) and the Si-O stretch (9.6 μm), respectively. With the pulse energy available from a tunable mid-infrared free-electron laser, the small absorption volume is rapidly superheated. The result is a very clean ablation crater with little or no evidence of melted material or other debris at the rim or outside of the ablation crater. We propose that this clean, fracture-free, highly efficient ablation of dielectric materials is the result of the rapid heating of the absorption volume, in which the heat is confined by the low thermal conductivity of the material, to a critical temperature Tc where explosive homogenous nucleation occurs.

Silicon Nanoparticles: Source of Extended Red Emission?

We have reviewed the characteristics of the extended red emission (ERE) as observed in many dusty astronomical environments, in particular, the diffuse interstellar medium of the Galaxy. The spectral nature and the photon conversion efficiency of the ERE identify the underlying process as highly efficient photoluminescence by an abundant component of interstellar dust. We have compared the photoluminescence properties of a variety of carbon- and silicon-based materials proposed as sources for the ERE with the observationally established constraints. We found that silicon nanoparticles provide the best match to the spectrum and the efficiency requirement of the ERE. If present in interstellar space with an abundance sufficient to explain the intensity of the ERE, silicon nanoparticles will also contribute to the interstellar 9.7 micron Si-O stretch feature in absorption, to the near- and mid-IR nonequilibrium thermal background radiation, and to the continuum extinction in the near- and far-UV. About 36% of the interstellar silicon depleted into the dust phase would be needed in the form of silicon nanoparticles, amounting to less than 5% of the interstellar dust mass. We propose that silicon nanoparticles form through the nucleation of SiO in oxygen-rich stellar mass outflows and that they represent an important small-grain component of the interstellar dust spectrum.

Suggested Correlation between the Visible Photoluminescence and the Fourier Transform Infrared Spectrum of a Porous Silicon Surface

The FTIR spectrum for a heavily oxidized porous silicon (PS) surface is considered. It is suggested that a portion of this spectrum be reassigned to the SiO stretch frequency of a surface bound-silanone-based silicon oxyhydride consistent also with the 18O2/16O2 isotopic labeling of the FTIR spectrum and with matrix isolation infrared spectra for silanone, silanoic acid, and silicic acid. Within the framework of this assignment the FTIR spectrum, the photoluminescence excitation spectrum, and the visible photoluminescence spectrum of PS correlate well with a surface-bound silanone-based silicon oxyhydride fluorophor.

Reactions of water and methanol at hydrogen-terminated silicon surfaces studied by transmission FTIR

A simple method for studying the reactivity of hydrogen-terminated silicon surfaces via transmission FTIR spectroscopy is described. Abraded (100) single crystals of silicon, after etching in ammonium fluoride, show features in the IR spectrum that are assigned to Si–H stretching vibrations. Confirmation of the utility of this approach is demonstrated in the reactions of the Si–H surfaces with water and methanol. Native-oxide growth in the former case is evidenced by the appearance of two bands in the Si–O stretch region accompanied by a shift to higher frequency of the longitudinal polar Si–O vibrational mode with increased exposure, consistent with an island-growth model. Similarly, the reaction with methanol generates two bands in the Si–O stretch region. However, the frequency and behaviour of these bands on further exposure to methanol are consistent with a chemisorbed methoxy species with little oxidation taking place, contrary to previous reports.

Preparation and Characterization of Reduced-Charge Hectorites

AbstractA series of reduced-charge (RC) hectorites were prepared by multiple heat (250°C) treatments of Mg-saturated hectorites (SHCa-1 ). Cation exchange capacity (CEC) measurements and alkylammonium exchange indicated that a decrease in layer charge occurred with each Mg-250 treatment. Chemical analyses showed that decreases in structural Li and increases in structural Mg contents coincided with charge reduction. Fluorescence measurements of adsorbed quinoline indicated that the hectorite surface was acidified during charge reduction; hydroxyl group deprotonation is a possible source for the acidity. Fourier transform infrared spectra (FTIR) indicated that the Mg-250 treatment induced the loss of structural Li and shifted the SiO stretch band to a position similar to that in talc. The relative intensities of the OH and SiO stretch bands in FTIR spectra suggest that some of the hydroxyl groups in hectorite were lost, possibly by deprotonation. However, thermogravimetric data (TG) reveal no significant difference in the hydroxyl contents of the hectorites.The FTIR spectra, CEC, layer charge, chemical, and TG data all supported the view that Mg substitution for octahedral Li occurred which resulted in a more “talc-like” structure. Charge reduction in smectites is evidently a general phenomenon and can be induced by heat treatment with the proper exchangeable cation. The ability to reduce the charge of hectorites makes it possible to prepare a series of clays which vary in charge but lack structural Fe. Such RC smectites should be suitable for expandable clay mineral studies which utilize spectroscopic techniques that are sensitive to Fe content.

Raman spectroscopy of silicate melts at magmatic temperatures: Na2O-SiO2, K2O-SiO2 and Li2O-SiO2 binary compositions in the temperature range 25–1475°C

High-quality Raman spectra of silicate glasses, supercooled liquids and liquids have been obtained in situ to temperatures of 1475°C. The success of the spectroscopic technique is fundamentally dependent on the ability to avoid spectra degradation caused by black-body radiation from the furnace. This can be accomplished by focussing the diameter of the exciting laser beam to ∼ 1 mm, and to control the depth of focus in the sample to 6–40-μm depth. The samples can be viewed visually through a microscope throughout the process, which ensures that the distance between the sources of blackbody radiation (furnace wall and bubbles in glass and liquid) and focus is optimized.With this technique, more than 100 spectra of glasses, supercooled melts and melts in the systems Li2O-SiO2, Na2-SiO2 and K2O-SiO2 have been recorded at 25–1475°C in the frequency range most sensitive to the overall anionic structure of amorphous silicates (800–1300 cm−1). The coexisting structural units generally are SiO32− (Q2), Si2O52− (O3) and SiO2 (Q4) in all glasses and melts in the temperature range investigated. The abundance of the structural units appear sensitive to temperature. The temperature dependence is qualitatively consistent with a shift to the right of the reaction: Si2O52−(2Q3)⇌SiO32−(Q2)+SiO2(Q4 with increasing temperature. From intensity variations with temperature of relevant Si-O stretch bands, the free energy for this reaction probably is sensitive to both bulk melt NBO/Si and to the electronic properties of the alkali metal.

Calculation of the scattering properties of a computer-relaxed 614 atom bell and dean SiO2 network

Using a Keating potential with intratetrahedral SiO and OO stretch forces and intertetrahedral SiOSi bend forces, we have relaxed the Bell and Dean continuous random network model of SiO2. The resulting pair distribution, G(r), is considerably sharpened and the density is close to the experimental value. Partial S(Q)'s were generated for each pair and combined into a total S(Q) to compare with neutron scattering data. A comparable fit was also made to the X-ray data. The first sharp diffraction peak appears in each of the partial S(Q)'s to a varying extent and the model agrees rather well with experimental neutron data but less well with the X-ray data. Problems with the SiOSi bond angle distribution and SiO pair distribution function lead us to suggest an additional term in the Keating potential correlating the dihedral angles with rotations of the tetrahedra about their bridging vertices

Ab initio calculation of interatomic force constants in H6Si2O7 and the bulk modulus of ? quartz and ? cristobalite

Ab initio force constants calculated for Si-O stretch and Si...Si non-bonded interactions in H6Si2O7 are found comparable with experimental values derived from the lattice dynamics of α quartz. The bulk moduli of α quartz and α cristobalite are calculated using the molecular Si...Si force constant and assuming rigid regular SiO4 tetrahedra. In the case (α quartz) where data are available the calculation agrees well with experiment.

Ordering in synthetic aluminous serpentines; infrared spectra and cell dimensions

Infrared and X-ray diffraction studies were made on synthetic serpentines (Si4 to Si2Al2 tetrahedral compositions). Changes in cell dimensions and variations in infrared spectra indicate that ordering can occur in the octahedrally coordinated site and possibly the tetrahedral site. Octahedra order into an Mg3 and/or Mg2Al configuration depending upon bulk mineral composition. In the latter cases an Al ion preferentially fills only one of the large M2 sites found in dioctahedral minerals and is thus characteristic of neither di-nor trioctahedral minerals. Ordering in octahedral and possibly tetrahedral sites does not appear to affect the basic higher frequency vibrations (OH stretch, Si-O stretch) by creating new modes but definite band splitting is seen for vibrations of lower frequency.