Search results for "composites"
showing 10 items of 1905 documents
α- and β-AgVO3 polymorphs as photoluminescent materials: an example of temperature-driven synthesis
2018
Abstract Controlling the synthesis of a given polymorph of an inorganic material is a further step in the design of property and function. In this letter, we report for the first time a simple procedure to effectively control the reversible transformation between the crystalline polymorphs α-AgVO3 and β-AgVO3. Photoluminescence emission (PL) performance is analyzed; at low temperatures (up to 35 °C) when α-AgVO3 is formed the PL emission is red, while at temperatures larger than 45 °C when β-AgVO3 is obtained the color of emission PL emission goes from green to blue. The findings highlight the ability of temperature to dramatically alter the nature of phase transformation at the atomic leve…
Intra-center and recombination luminescence of bismuth defects in fused and unfused amorphous silica fabricated by SPCVD
2013
Abstract Photoluminescence (PL) of bismuth doped silicon dioxide excited by UV excimer lasers (ArF — 193 nm, KrF — 248 nm) and a green light laser diode (532 nm) is studied in a wide spectral band at temperatures ranging from 12 to 750 K. Two types of samples are investigated: unfused, 100 μm in thickness amorphous layer immediately deposited on the inner surface of silica substrate tube, and the same material after profusion resulted from tube collapsing to a rod by external heating. PL bands centered at 620–650 nm, 820 nm and 1400 nm wavelengths are observed in both fused and unfused samples. Under excitation by the green laser diode decay time constants for 650 nm (orange) and 1400 nm (N…
Study of the germanium luminescence in silica: from non-controlled impurity to germano-silicate core of telecommunication fiber preforms
2003
Abstract We have studed luminescence properties of doped silica with different concentrations of germanium. The basic luminescence parameters such as spectral dependencies, decay kinetics and polarization at different temperatures were measured. Three spectral ranges 3.5–5.5 eV(I), 5.5–7 eV(II), 7–8 eV(III) in the optical transparency range of silica could be chosen from these data. Range I possesses a weak variation of basic parameters of luminescence of the germanium related oxygen deficient center with the change of luminescence center concentration from extremely low in pure silica to the germano-silica core of optical telecommunication fiber preforms. The temperature dependence of lumi…
Luminescence and absorption spectroscopy of Sn-related impurity centers in silica
2006
We report an experimental study on the absorption and luminescence spectra of oxygen deficient point defects in Sn-doped silica. The absorption band at 4.9 eV (B2β band) and the two related photoluminescence bands at ∼4.2 eV (singlet-singlet emission, S1 → S0) and at ∼3.2 eV (triplet-singlet emission, T1 → S0), linked by a thermally activated T1 → S1 inter-system crossing process (ISC), are studied as a function of temperature from 300 to 20 K. This approach allows us to investigate the dynamics properties of the matrix in the surroundings of the point defects and the effects of local disorder on the two relaxation processes from S1: the radiative channel to S0 and the ISC process to T1. We…
Luminescence of fluorine doped silica glass
2003
Abstract The role of fluorine doping on silica properties was studied by luminescence methods. Non-doped samples of the same preparation technology possess an absorption band at 7.6 eV on the level of 2 cm−1. A trace of this band in the fluorine-doped sample is on the level of 0.1 cm−1. In both samples 7.6 eV photons as well as ionizing irradiation (X-ray, electron beam) excite photoluminescence of so-called oxygen deficient centers with a blue (2.7 eV) and a UV band (4.4 eV). The luminescence of the fluorine doped sample increases with dose many times from the initial low level for the same excitation. Also, thermally stimulated luminescence appears after irradiation. The energetic yield u…
Luminescence activity of surface and interior Ge-oxygen deficient centers in silica
2005
We report a comparative study on the optical activity of surface and interior Ge–oxygen deficient centers in pressed porous and sol–gel Ge-doped silica, respectively. The experimental approach is based on the temperature dependence of the two photoluminescence bands at 4.2 (singlet–singlet emission, S1! S0) and 3.1 eV (triplet–singlet emission, T1! S0), excited within the absorption band at about 5 eV. Our data show that the phonon assisted intersystem crossing process, linking the two excited electronic states, more effective for surface than for interior centers in the temperature range 5–300 K. For both centers, a distribution of the activation energies of the process is found. Based on th…
Generation of oxygen deficient point defects in silica by γ and β irradiation
2007
We report an experimental study of the effects of y and β irradiation on the generation of a point defect known as ODC(II) in various types of commercial silica (a-SiO 2 ). 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 x 10 2 kGy. A good agreement is observed between the efficiencies of generation of ODC(II) under y and β irradiation, enabling a comprehensive study up to the dose of 5 x 10 6 kGy. Two different growth rates, one in the low and one in th…
Recombination luminescence of oxygen-deficient centers in silica
2008
Abstract The luminescence of silica glass, prepared by plasma chemical vapor deposition (PCVD) and quartz glass of type IV (trade mark KS-4V) methods, were studied while irradiated with pulses of ArF laser (193 nm) light in the range of sample temperatures between 10 and 300 K. The samples contain less than 0.1 ppm metallic and hydroxyl impurities. The samples synthesized by PCVD were of two kinds. The first one (amorphous) was as-deposited from plasma at a substrate tube temperature of ∼1200 °C. The second one (fused) was prepared from the first by the tube collapsing with an external burner. In this process, a section of the substrate tube with the deposited glass was installed in a lathe…
Mechanistic studies on the photogeneration of o- and p-xylylenes from α,α′-dichloroxylenes
1998
Two-colour two-laser techniques have unambiguously proved that photolysis of the o-/p-(chloromethyl)benzyl radical leads to the sequential two-photon generation of o-/pxylylene from α,α'-dichloro-o-/p-xylene. Perez Prieto, Julia, Julia.Perez@uv.es
Post UV irradiation annealing of E’ centers in silica controlled by H2 diffusion
2004
Abstract We investigate the isothermal annealing of E′ centers generated by UV photons (266 nm) of a pulsed Nd:YAG laser in two natural silica types differing for their OH content. Electron spin resonance and absorption spectra recorded at room temperature at different delays from the laser exposure evidenced a partial reduction of E′ centers, more pronounced in the wet silica. These post irradiation kinetics complete within 10 5 s, regardless the silica type, and they are consistent with a diffusion limited reaction between the E′ centers and the molecular hydrogen H 2 . Analysis of our data is done by theoretical fits using the Waite's equation and compared with the H 2 diffusion paramete…