Fourier transform infrared absorption spectroscopy has been utilized to characterize the structure of porous silica gel films, both deposited on c‐Si substrates and free standing. The films were either dried at room temperature or subjected to partial densification at 400–450 °C. The spectra of the gel films are compared to those of thermal SiO2 grown on c‐Si and to Kramers–Kronig analysis of the reflection spectra of bulk SiO2 gels and v‐SiO2. The gel films show small frequency shifts compared to the latter spectra and they also exhibit new bands due to the presence of OH groups, although very little molecular water or residual organic species were found. The results are interpreted in terms of the gel structure. Compared to the thermal oxide, the sharp peak near 1070 cm−1 is narrower for the gels and the spread in intertetrahedral angles is estimated at 24° and 27° for room temperature dried and partially densified gels, respectively, compared to 33° for the thermal oxide. This is in agreement with a st...

Structural investigation of silica gel films by infrared spectroscopy

Two techniques, infrared (IR) and X-ray induced photoelectron spectroscopies (XPS) were used to study a wide range of glasses in the system Na2O–CaO–P2O5–K2O–MgO–B2O3–SiO2 that have been shown elsewhere to be bioactive. The IR frequency shifting of the bridging oxygen stretching vibration (Si–O–Si) and the binding energy shifting of the O1s and Si2p photoelectron peaks have been observed and evaluated. Both effects are originated by the incorporation of the alkali ions into the vitreous silica network, which provokes a modification of the local symmetry and of the electronic density around the different atoms. The presence of non-bridging silicon–oxygen (Si–O–NBO) groups in the glass have been identified, and important correlations between the concentration of Si–O–NBO groups and the content of network modifiers were found. This information is very useful for the development and tailoring of new bioactive glasses with an optimum bioactivity. This study leads to a better knowledge of the structure and bonding configuration of the bioactive glasses and, consequently, can contribute to clarify the role of the network modifiers in the bioactive process. � 2003 Elsevier B.V. All rights reserved.

FTIR and XPS studies of bioactive silica based glasses

The present state of research on the structure of amorphous silicon monoxide (SiO) is reviewed. The black, coal-like modification of bulk SiO is studied by a combination of diffraction, microscopy, spectroscopy, and magnetometry methods. Partial radial distribution functions of SiO are obtained by X-ray, neutron and electron diffraction. Disproportionation of SiO into Si and SiO2 is verified. High resolution TEM gives an upper limit of less than 2 nm for the typical Si cluster size. The Si K-edge electron energy-loss near-edge structure (ELNES) data of SiO are interpreted in terms of the oxidation states Si4+ and Si0. X-ray photoelectron spectroscopy gives first details about possible stoichiometric inhomogeneities related to internal interfaces. The wipe-out effect in the 29Si MAS NMR signal of SiO is confirmed experimentally. The new estimation of the wipe-out radius is about 1.1 nm. First-time W-band, Q-band, and X-band ESR and SQUID measurements indicate an interfacial defect structure. Frequency distributions of atomic nearest-neighbours are derived. The interface clusters mixture model (ICM model) suggested here describes the SiO structure as a disproportionation in the initial state. The model implies clusters of silicon dioxide and clusters of silicon surrounded by a sub-oxide matrix that is comparable to the well-known thin Si/SiO2 interface and significant in the volume because of small cluster sizes.

An interface clusters mixture model for the structure of amorphous silicon monoxide (SiO)

Ultrathin silicon dioxide (SiO2) layers with excellent electrical characteristics can be formed using the nitric acid oxidation of Si (NAOS) method, i.e., by immersion of Si in nitric acid (HNO3) solutions. The SiO2 layer formed with 61 wt % HNO3 at its boiling temperature of 113 °C has a 1.3 nm thickness with a considerably high density leakage current. When the SiO2 layer is formed in 68 wt % HNO3 (i.e., azeotropic mixture with water), on the other hand, the leakage current density (e.g., 1.5 A/cm2 at the forward gate bias, VG, of 1 V) becomes as low as that of thermally grown SiO2 layers, in spite of the nearly identical SiO2 thickness of 1.4 nm. Due to the relatively low leakage current density of the NAOS oxide layer, capacitance–voltage (C–V) curves can be measured in spite of the ultrathin oxide thickness. However, a hump is present in the C–V curve, indicating the presence of high-density interface states. Fourier transformed infrared absorption measurements show that the atomic density of the SiO...

Nitric acid oxidation of Si to form ultrathin silicon dioxide layers with a low leakage current density

Ion‐assisted pulsed laser deposition has been used to produce films containing ≳85% sp3‐bonded cubic boron nitride (c‐BN) By ablating from a target of hexagonal boron nitride (h‐BN), BN films have been deposited on heated (50–800 °C) Si(100) surfaces The growing films are irradiated with ions from a broad beam ion source operated with Ar and N2 source gasses Successful c‐BN synthesis has been confirmed by Fourier transform infrared (FTIR) spectroscopy, high‐resolution transmission electron microscopy (TEM), selected‐area electron diffraction, electron energy‐loss spectroscopy, and x‐ray diffraction The films are polycrystalline and show grain sizes up to 300 A In addition, Rutherford backscattering, elastic recoil detection, and Auger electron spectroscopies have been used to further characterize the samples The effects of varying ion current density, substrate growth temperature, growth time, and ion energy have been investigated It is found that stoichiometric films with a high c‐BN percentage ca

Ion-assisted pulsed laser deposition of cubic boron nitride films

Until now there exists no calculation of the phonon density of states of amorphous SiO2 and related materials, which could interpret the experimentally detected splittings between longitudinal and transverse modes in their optical phonon spectra. This problem is solved by taking into account the influence of the vibrationally induced polarization on the motion of atoms. A simple model for the description of this polarization in non-crystalline SiO2 is developed and used for the calculation of the density of states of LO and TO phonons in the framework of a coherent potential approximation for structurally disordered systems. This new method leads also to the determination of the IR response function and to simple analytical expressions for the microscopic parameters of the model, i.e. the bond angle at the oxygen atom and transverse dynamic effective charges. The values of these parameters are determined from IR transmission spectra of thin SiO2 layers prepared with the help of different methods. The calculated density of states and IR response function are in good agreement with corresponding experimental results.

Optical Phonons in Amorphous Silicon Oxides. I. Calculation of the Density of States and Interpretation of LoTo Splittings of Amorphous Sio2

Basing on the theoretical investigations of Part I of this work a simple model for the calculation of the density of vibrational modes and of the IR response function of the geometrically and chemically disordered material SiOx (0 < x ≦ 2) is developed. This model leads for the first time to a theoretical interpretation of LO—TO splittings and of chemical shifts of the minima of IR transmission spectra of thin SiOx films prepared by reactive sputtering.

L. Schumann

Papers

Optical Phonons in Amorphous Silicon Oxides. I. Calculation of the Density of States and Interpretation of LoTo Splittings of Amorphous Sio2

Optical Phonons in Amorphous Silicon Oxides. II. Calculation of Phonon Spectra and Interpretation of the IR Transmission of SiOx

Infrared Studies of Reactively Sputtered SiOx Films in the Composition Range 0.2 ≦ x ≦ 1.9

Detection of LO and TO Phonons in Amorphous SiO2 Films by Oblique Incidence of IR Light

LO‐TO Mode Splittings and Dynamic Effective Charges of Oxygen in Reactively Sputtered SiOx Determined by IR Spectroscopy