Search results for "Composite"
showing 10 items of 4584 documents
Photoluminescence activity in natural silica excited in the vacuum-UV range
1999
Abstract We report an experimental study on the optical absorption and photoluminescence detected in samples of natural silica. Our results show that the two emission bands, β (∼3.1 eV) and α E (∼4.3 eV), have an excitation profile in the vacuum ultraviolet region with a maximum at ∼7.5 eV. This excitation profile indicates that, in terms of energy levels of the luminescent defect, there is a transition from a ground state, S 0 , to a second excited state, S 2 , able to excite PL emission, in addition to the well known transition corresponding to the optical absorption band, B 2β . Our data are in a quantitative agreement with `ab initio' calculations carried out for a two-fold coordinated …
Formation of optically active oxygen deficient centers in Ge-doped SiO2 by γ- and β-ray irradiation
2010
Abstract We report an experimental study on the comparison between the γ- or β-ray induced Ge related point defects in Ge-doped silica. Silica samples doped with ∼2.2 1017 Ge atoms/cm3 produced with the sol–gel technique have been irradiated with γ-ray or with β-ray. The effects of the irradiation have been investigated by optical absorption, photoluminescence and electron paramagnetic resonance spectroscopy in order to evaluate the generation and the dependence on dose of the Ge(1), E’Ge, GLPC (Germanium lone pair center) and H(II) point defects. No relevant differences between the concentrations of γ- or β-ray induced Ge(1) and E’Ge point defects have been observed and, in addition, it ha…
Absorption and luminescence in amorphous silica synthesized by low-pressure plasmachemical technology
2007
A comparison study of as-deposited by the hydrogen-free SPCVD process silicon dioxide with and without fluorine as well as the one in the form of fused materials is performed to define whether glass forming processing affects its optical properties. Raman scattering, optical absorption in the vacuum ultraviolet region as well as luminescence at different temperatures and various excitation conditions are measured. A difference in Urbach rule parameters and intrinsic defect concentrations for different samples is revealed. No significant impact on the structure responsible for Raman scattering is observed.
Luminescence properties of nonbridging oxygen hole centers at the silica surface
2009
Abstract 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.
Spontaneous oxygen loading into SiO2 glass by thermal anneal
2004
The interstitial oxygen molecules (O 2 ) in SiO 2 glass were detected down to ∼10 15 cm -3 by photoluminescence of O 2 at 1272nm excited at 765nm by a continuous-wave titanium sapphire laser. It was evidenced that SiO 2 glass thermally annealed in air between 800 and 1100°C spontaneously absorbs ∼10 16 cm -3 of O 2 from the ambient atmosphere. The time-dependent concentration change of the interstitial O 2 allows the determination of both the diffusion coefficient and the solubility of the interstitial O 2 .
Concentration growth and thermal stability of gamma-ray induced germanium lone pair center in Ge-doped sol–gel a-SiO2
2009
Abstract We report an experimental study of the concentration growth by γ-ray irradiation of germanium lone pair center (GLPC) in 10 4 part per million molar Ge-doped sol–gel silica. The data show that γ-ray induced GLPC concentration increases linearly up to ∼5 MGy and then it seems to reach a limit value. In addition to the dose dependence, we have studied the thermal stability of the radiation induced GLPC in ambient atmosphere up to 415 °C. We found that the concentration of this latter GLPC starts to decrease at ∼300 °C, at variance to native GLPC, suggesting that the annealing is related to irradiation products. After the thermal treatments the photoluminescence (PL) activity of the γ…
The landscape of the excitation profiles of the αE and β emission bands in silica
1999
Abstract We report data about the relevance of the conformational heterogeneity in determining the optical properties of oxygen deficiency point defects in natural silica samples. The spectral profiles of the photoluminescence emissions at about 4.2 eV (α E band) and at about 3.15 eV (β band), and the efficiency of the intersystem-crossing mechanism connecting them appear modified by a fine tuning of the excitation energy within the B 2β absorption band. Moreover, the relative excitation optically spectra indicate the presence of optically distinguishable contributions to the emission profile. The reported data are attributed to a distribution of centers that maps into a spectral inhomogene…
The origin of the intrinsic 1.9 eV luminescence band in glassy SiO2
1994
Abstract The current controversy over the nature of the centers giving rise to the 1.9 eV photoluminescence (PL) band (the R-band), the suggested defect models and the relevant experimental data are briefly reviewed. The luminescence emission, excitation and polarization spectra of neutron-irradiated synthetic silica were studied between 6 and 300 K using site-selective dye-laser and Ar ion laser excitation. Resonant zero-phonon lines (ZPL) were observed below 80 K both in luminescence emission and excitation spectra in the 1.9–2.1 eV region. A vibration line in emission spectra 890 cm−1 below the ZPL energy is attributed to the symmetric stretching vibration of the silicon-non-bridging oxy…
Oxygen-excess amorphous SiO2 with 18O-labeled interstitial oxygen molecules
2011
Abstract Exchange between oxygen molecules embedded in amorphous SiO 2 (interstitial O 2 ) and oxygen atoms in the a -SiO 2 network is found to be remarkably slow at 500 °C. Thermal loading of 18 O 2 at this temperature yields a -SiO 2 containing 18 O-labeled interstitial O 2 whose 18 O fraction is as high as ~ 90%. The 18 O fraction of interstitial O 2 in this sample is quickly decreased by thermal annealing at or above 700 °C because of the oxygen exchange accompanied by the release of 16 O from the a -SiO 2 network. This finding indicates that the oxygen exchange starts at much lower temperatures than indicated by previous works, based on monitoring of the isotopic composition of oxygen …
Luminescence of fluorine-doped and non-doped silica glass excited with an ArF laser
2004
Abstract The role of fluorine doping on silica has been studied through comparison of the luminescence of fluorine doped and fluorine-free samples made by melting in on SiF4 atmosphere and excited by an ArF laser (6.4 eV) in the temperature range 10–290 K. The fluorine doped sample possesses a very weak absorption band at 7.6 eV on the level of 0.1 cm−1 and there the photoluminescence of so-called oxygen-deficient centers in the blue (2.7 eV) and UV bands (4.4 eV) could be excited. The same luminescence bands are observable in the fluorine-free sample, which contains an absorption band at 7.6 eV on the level of 20 cm−1. In the fluorine-doped sample the UV band prevails over the blue band. T…