Photoluminescence in as-drawn and irradiated silica optical fibers: an assessment of the role of non-bridging oxygen defect centers

The spectral and temporal characteristics of the photoluminescence in as-drawn and irradiated silica and doped silica fiber-optic waveguides have been investigated. The extended pathlength available with a fiber-optic geometry has offered the opportunity to make high sensitivity emission measurements on high silica glasses under both steady state and pulsed laser excitation. The analyses of the fiber data coupled with emission studies on selectivity doped bulk glasses suggest that the dominant emission band centered near 650 nm is intrinsic to defects in the SiO network, specifically, dangling non-bridging oxygens ions which can be generated by irradiation, fiber drawing or by the introduction of network modifying ions such as alkali.     

New intrinsic oxygen related defect bands in oxygen implanted silica

Type III silica samples were implanted with O using a multi-energy process that produced a layer of constant concentration to within ± 5% beginning ~ 80 nm from the surface and extending to ~ 640 nm below the surfaces of samples. The concentrations in the layer ranged from 0.035 atomic percent to ~ 2.1 atomic percent. The optical absorption was measured from 2 to 6.5 eV. The absorption due to the implanted ions was analyzed using the difference spectra of the various samples. These difference spectra showed that optical bands at ~ 4.7 eV, 5.35 eV, and ~ 6.2 eV were a primary result of O implantation. Based on these data, we attribute these bands to O-related centers. Comparison of the difference spectra of the O samples with those of Si samples implanted to approximately the same concentrations showed that bands at the three energies in the spectra of the O samples were not observed in the spectra of the Si samples. Based on the spin concentrations of peroxy radicals, spin concentrations of E′_γ, and optical absorption coefficients at 4.8-5.0 eV and at 5.83 eV, the oscillator strengths of the peroxy band (~ 4.8 eV) was estimated, from the oscillator strength of the absorption band at 5.83 eV to be < 0.014.     

Cathodoluminescence of Ge, Si, and O implanted SiO2 layers and the role of mobile oxygen in defect transformations

Thermally grown SiO₂ layers of thickness d=500 nm have been implanted by Ge⁺, Si⁺, and O⁺ ions of energy 350, 150, and 100 keV, respectively, and a uniform implantation dose of D_i=5×10¹⁶ ions/cm². Thus the implantation profiles are expected with a concentration maximum of nearly 4 at.% at the half-depth d_m≅250 nm of the SiO₂ layers. After thermal annealing to 900 °C for 1 h in dry nitrogen or vacuum the typical violet luminescence band (λ=400 nm) of the Ge⁺ implanted centers is increased more than 200-fold and the Ge luminescent center depth profile is shifted from about 250 to 170 nm towards the surface as determined by cathodoluminescence (CL) depth profiling. Implanting oxygen increases the red band (λ=650 nm) but does not affect the blue band (λ=460 nm). Silicon surplus increases the amplitude of the blue (B) luminescence, but reduces the amplitude of the red (R) one. Studying the irradiation dose dependence of these blue and red bands we have established defect kinetics in SiO₂ including six main defects and precursors, including the non-bridging oxygen hole center for the red luminescence, the twofold-coordinated silicon as the oxygen deficient center ODC(2) for the blue luminescence and the mobile oxygen as the main transmitter between precursors and the radiation induced defects. The kinetics are described by a set of eight differential equations which predict the dose dependence of the CL.     

Doubly positively charged oxygen vacancies in silica glass

The geometric and electronic structures of doubly positively charged oxygen vacancies in a-SiO₂ are calculated. We have found four types of relaxed configurations in the amorphous matrix, corresponding to puckered and unpuckerd configurations of Si atoms of the vacancy. The predicted optical absorption strongly depends on the atomic configuration of particular center and transition energies are distributed in the range from 4.5 to 6.5 eV.     

Optical activity of Sn-variants of oxygen deficient centers in fluorine-modified silica

Photoluminescence in fluorine-modified Sn-doped silica has been analyzed by means of synchrotron radiation in the UV and vacuum-UV, from 120 to 330 nm, looking at the optical activity of oxygen-deficient-centers ODC(II) in Sn-substituted cationic sites. The comparison between F-modified Sn-doped samples and previous data on F-free Sn-doped material evidences differences in the intensity of the 3.2 eV emission band excited at 3.7 eV, and in the thermal dependence of the intensity of this emission excited via intersystem crossing. The role of fluorine in modifying the optical activity of ODC(II) and in the SnO₂ clustering is discussed, showing that an efficient excitation transfer may be activated from SnO₂ to the Sn-variant of ODC(II).     

Radiation induced generation of non-bridging oxygen hole center in silica: Intrinsic and extrinsic processes

The generation of non-bridging oxygen hole center (Si–O) was investigated in a wide variety of natural (fused quartz) and synthetic silica samples exposed to different γ- and β-irradiation doses by looking at its optical bands. We distinguish two different generation processes: intrinsic associated with the cleavage of Si–O bond and characterized by a sublinear law and extrinsic due to the conversion of OH precursor characterized by a growth curve with a saturating tendency. The interplay between the two processes and the role of H are discussed.     

Luminescence properties of nonbridging oxygen hole centers at the silica surface

Two variants of the surface-nonbridging oxygen hole center, (Si–O)₃Si–O^? and (Si–O)₂(H–O)Si–O^?, stabilized in porous films of silica nano-particles were investigated by time resolved luminescence excited in the visible and UV spectral range by a tunable laser system. Both defects emit a photoluminescence around 2.0 eV with an excitation spectrum evidencing two maxima at 2.0 and 4.8 eV, this emission decreases by a factor ～2 on increasing the temperature from 8 up to 290 K. However, the different local structure influences the emission lineshape, the quantum yield and the decay lifetime. Such peculiarities are discussed on the basis of the symmetry properties of these defects.     

Cathodoluminescence of SiO2 films

Cathodoluminescence on wet and dry thermal SiO₂ films shows significant time, temperature, and current dependence in the intensities of the observed emission bands at 1.9, 2.7 and 4.3 eV. The low temperature behavior of the 2.7 and 4.3 eV lines suggests a relation between the corresponding luminescence centers. The 1.9 eV line is not found to behave similarly.     

The role of the main silica precursor and the additive in the preparation of low-density xerogels

The incorporation of an additive during sol-gel synthesis reduces shinkage during ambient drying. The following additives have been studied: 3-(2-aminoethylamino)propyltrimethoxysilane (EDAS), 3-aminopropyltriethoxysilane (AES) and 3-(2-aminoethylamino)propyltriethoxysilane (EDAES) and the main silica precursors were tetraethylorthosilicate (TEOS) and tetrapropylorthosilicate (TPOS). When the additive contains methoxy groups (EDAS), it acts as a nucleation agent of the silica particles and exactly the same properties (pore volume, specific surface area, particle and aggregate size) are obtained whether the main reagent is TEOS or TPOS. The nucleation mechanism is based on the difference in reactivity between additive and main reagent. In case of nucleation by the additive, the nucleation agent fixes the properties whatever the main silica precursor is. When both the additive and the main reagent contain ethoxy groups (series AES-TEOS and EDAES-TEOS), there is no nucleation mechanism by the additive, and the silica particle size remains nearly constant. With less reactive main reagent (series AES-TPOS and EDAES-TPOS), pore volumes up to 17 cm³/g have been obtained with pore sizes up to nearly 10 μm and very big particles (∼100 nm). The absence of nucleation by the additive for the couples AES-TPOS and EDAES-TPOS could be due to the fact that the difference in reactivity between ethoxy groups and propoxy groups is not sufficient to initiate the nucleation mechanism by the additive. In the absence of nucleation by the additive, the main reagent plays a role: highly porous materials with very large pores are prepared with TPOS.     

Generation of oxygen deficient point defects in silica by γ and β irradiation

We report an experimental study of the effects of γ and β irradiation on the generation of a point defect known as ODC(II) in various types of commercial silica (a-SiO₂). The ODC(II) has been detected by means of photoluminescence (PL) spectroscopy measuring the PL band centered at 4.4 eV and excited at 5.0 eV associated to this defect. Our experiments show that ODC(II) are induced in all the investigated materials after irradiation at doses higher than 5 × 10² kGy. A good agreement is observed between the efficiencies of generation of ODC(II) under γ and β irradiation, enabling a comprehensive study up to the dose of 5 × 10⁶ kGy. Two different growth rates, one in the low and one in the high dose range, can be distinguished in all the samples examined, suggesting that the efficiency of generation of the ODC(II) depends on the dose but not on the kind of irradiation and on the dose rate. Furthermore a nonlinear dependence of the photoluminescence band amplitude on the dose D, through a power law of the kind D^α with α < 1, has been observed in the low dose range in all the materials examined.     

The interstitial structure of vitreous silica

A careful analysis of gas solubility data provides a unique tool for characterizing glass network geometry. The density of interstitial (solubility) sites in vitreous silica and its crystalline analog, high cristobalite, was reviewed. Close inspection of high cristobalite indicates that there are eight solubility sites per unit cell giving a bulk density of 2.34 × 10²² sites/cm³. A statistical thermodynamic analysis of experimental solubility data for helium and neon in high cristobalite correlates with this calculated value. Correcting this value for the slightly lower bulk density of vitreous silica gives an estimated density of solubility sites in vitreous silica of 2.22 × 10²² sites/cm³. Literature data for solubility site density (obtained from the same statistical thermodynamic analysis) depend strongly on the dissolved gas species. This implies a statistical distribution of effective site diameters. The probability distribution function is adequately represented as log-normal.     

Solid phase epitaxy of Na-ion irradiated α-quartz: Cathodoluminescence,kinetics and surface morphology

Even at low fluences doping of quartz by ion implantation results in amorphization. Here we report on the measurement of cathodoluminescence and surface structures during solid phase epitaxy in Na-implanted α-quartz annealed in ¹⁸O₂ atmosphere. Complete epitaxy was achieved under appropriate conditions of ion fluence, annealing temperature and time. The crystalline structure of the samples was studied by Rutherford backscattering channelling spectroscopy and the ¹⁸O–¹⁶O exchange between the matrix and ¹⁸O₂ annealing gas by elastic recoil detection analysis. In the cathodoluminescence spectra taken at room temperature, five bands were identified and assigned to various defect centres. Two of the bands in the violet region at 3.25 and 3.65 eV strongly vary in intensity at 843–1173 K annealing temperature and appear to be intimately correlated with the presence of Na-ions in the implanted region of the matrix. Finally, atomic force microscopy enabled, for the first time, the observation of the correlation of surface structures and epitaxy.     

Cathodoluminescence of epitaxial GaN and ZnO thin films for scintillator applications

A comparative study of cathodoluminescence ultraviolet photon yields and decay times of large area GaN and zinc oxide (ZnO) layers grown for scintillator applications by metalorganic vapor phase epitaxy is presented. Silicon-doped GaN and non-intentionally-doped ZnO yield up to 1.4±0.2 photons/kVe⁻ and 1.3±0.2 photons/kVe^– at room-temperature, respectively. For GaN the decay times scatter between 0.4 and 0.9 ns, and for ZnO between 2.5 and 3.0 ns. The GaN and the ZnO absorption coefficients, α, internal efficiencies, η_i, and radiative constants, B, are determined. The characteristics of thin-film scintillators based on these materials are compared with commercially available granular scintillators.     

The optical constants of quartz,vitreous silica and neutron-irradiated vitreous silica (I)

The infrared reflectivity of three forms of silica, α-quartz, vitreous silica and neutron-irradiated vitreous silica (≈ 2.7 × 10²⁰ neutrons cm^?2) has beenmeasured from 400–2000 cm^?1. These data have been analysed by a Kramers-Kronig transform to give the real and imaginary parts of the complex dielectric constant. Considerable care has been taken to identify and minimize errors arising in the measurements of the reflectivity spectra and in the subsequent analysis. Data are presented for the optical constants, oscillator frequencies, band strengths and halfwidths of each band. The spectra for vitreous silica and neutron-irradiated silica show two regions of absorption which are not present in the crystalline form — a strong band is observed near 950 cm^?1 and a broad band from 600–800 cm^?1. A difference spectrum obtained by subtracting the spectrum of the imaginary part of the dielectric contrast for vitreous silica from the corresponding data for neutron-irradiated silica reveals more detailed structure in the form of a weak but sharp band at 620 cm^?1. Interpretation of these results is contained in a companion paper.     

Phase segregation of non-stoichiometric aluminosilicate gels characterized by Al and Si MAS-NMR

Polymeric aluminosilicate gels with Al₂O₃/SiO₂ molar ratios of 1.3/2, 2.5/2, 3/2 and 4.7/2 were prepared by gelling a mixture of tetraethoxysilane and ethyl acetoacetate aluminium diisopropoxide. Mullite, without any other crystalline phase, directly crystallizes from the gel matrix at about 1000 °C for all investigated samples. The Al₂O₃ or SiO₂ crystalline phase can only be detected at relatively high temperatures accompanying the modification of mullite in the lattice structure. ²⁷Al and ²⁹Si MAS-NMR studies indicate that segregation of Al and Si atoms occurs in all samples below 900 °C in the amorphous state, regardless of the gel composition. However, the species of segregated units are very different and strongly dependent on the composition of the starting gels. This segregation may not cause the exothermic effect in differential thermal analysis examination, but it appears to be responsible for the composition change behavior of crystalline mullite with different heat treatment.     

The role of silica and alkaline earth metals with biomolecules in the biomineralization processes: the eggshell's formation and the crystallization in vivo for x-ray crystallography

This contribution is a scientific journey divided into three parts. In the first part, we review the role that silica biomorphs of alkaline earth metals have played in the formation of complex structures as a reminiscence of the chemistry of the primitive life on Earth. These biomorphs, and their variety of forms synthesized by simple chemical reactions, can nowadays be experimentally used to explain some mechanisms of biomineralization in living organisms. In the second part, we review the role of calcium carbonates in the formation of eggshells in avian. The mechanism of the mineral eggshell´s formation of the biogenic calcite deposited on an organic matrix is revised. The competitive crystal growth mechanism of the mineralized part orientates these crystals preserving the semispherical shape of the egg. We are using these eggshell formations as a second model to understand the biomineralization processes in Nature. The third and final part is about the importance that biomineralization concepts have to produce hybrid materials for the future. This has allowed us to obtain tailored size control of complex morphologies by synthetic chemical procedures that give rise to these new materials’ specific forms and ad hoc properties. We conclude this part with the advantage of knowing the biological mechanisms, based on molecular biology concepts, to obtain protein crystals in vivo and in cellulo techniques. Both methods use the cellular machinery of growing biocrystals in specialized cells that have evolved through millions of years. This new way of producing protein crystals has been trending topic for modern crystallography when using the facilities of the X-ray free-electron lasers (four generation of synchrotrons) for megahertz serial crystallography.     

The study of removing hydroxyl from silica glass

The removal of hydroxyl from silica glass produced by melting quartz powder under an atmosphere containing hydrogen was investigated. After heat-treatment at the temperature range (700–1200 °C) in nitrogen atmosphere, the effective hydrogen diffusion coefficients were evaluated based on the law of nonsteady-state diffusion. The activation energy obtained is 254 kJ mol⁻¹ for the dehydroxylation process in the heat-treatment temperature range of 700–900 °C, and a different activation energy calculated is 32 kJ mol⁻¹ in the temperature range of 900–1200 °C. The activation energies for the dehydroxylation process at the temperature (700–900 °C) and the higher temperature (900–1200 °C) correspond to the binding energy of SiO–H bond and the activation energy for the diffusion of hydrogen in silica glass respectively, which indicate there is a change of mechanism for dehydroxylation with heat-treatment temperature.     

Crystal structure of non-stoichiometric copper selenides studied by neutron scattering and X-ray diffraction

Structural characteristics of non-stoichiometric copper selenides were studied by the elastic neutron and X-ray scattering techniques. Rietveld analysis was used to refine the structure of the high-temperature β-phase of the Cu_1.75Se, Cu_1.78Se, and Cu_1.83Se samples. The homogeneity ranges of the cubic phase were determined. The modification of the crystal structure accompanying the β-α phase transition was studied for Cu_1.75Se and Cu_1.98Se compounds within the 443-10 K temperature range. It was shown that the phase transition is accompanied by distortions of the fcc lattice and the ordering of copper ions.