F. M. Gelardi

ESR and PL centers induced by gamma rays in silica

We have studied the point defects created by γ irradiation in various types of commercial silica glasses, including both natural and synthetic samples, with different OH content, in the low dose regime (0.05–100 Mrad). We found that the growth rate of E′ centers depends strongly on the silica type, ranging from 2 × 1015 cm−3 Mrad−1 to 6 × 1017 cm−3 Mrad−1. Samples of natural silica are rather susceptible to γ ray exposure, as E′ concentration saturates (typically 5 × 1017 cm−3) for doses as low as a few Mrads. For both synthetic and natural samples, the radiation hardness is higher in wet than in dry silica. Moreover, we found a strict correlation between the concentration of E′ centers and the γ-induced absorption band at 5.8 eV. Finally, exposure to γ rays generates in all the samples a photoluminescence band at 4.4 eV, whose excitation spectrum has a maximum at 4.95 eV. This band exhibits a sublinear growth kinetics in all the investigated samples.

Atomic force microscopy and Raman investigation on the sintering process of amorphous SiO2 nanoparticles

We report an experimental investigation on the sintering process induced in fumed silica powders by isochronal thermal treatments at T=1270 K. Three types of fumed silica are considered, consisting of amorphous SiO2 (a-SiO2) particles with mean diameters 7, 14, and 40 nm. The study is performed by atomic force microscopy (AFM), to follow the morphological changes, and by Raman scattering, to obtain information on the concomitant structural modifications. The former method indicates that the sintering process proceeds by aggregation of single particles into larger grains, whose sizes increase with the thermal treatment duration. Furthermore, for each fumed silica type considered, the quantitative analysis of the AFM images shows that the grain growth process takes place approximately at constant rate for thermal treatment durations up to 290 h. Nevertheless, the value of the grain growth rate is sensitive to the system properties. In fact, it is found to increase with decreasing the particle mean diameter,...

Experimental evidence of the composite nature of the 3.1 eV luminescence in natural silica

Abstract The 3.1 eV photoluminescence of silica has been investigated on several samples of commercial origin, by measuring both time-resolved and stationary spectra. This structure, known as the β band, displayed the well known peculiarities: 3.1 eV energy position, 0.4 eV full width at half maximum, excitation peak at 5.1 eV and 100 μs decay time. However, an accurate analysis of the experimental data has shown the double nature of the β band in all natural silica samples here studied. The characteristic values of the two components β1 and β2 are: peak energy 3.04 and 3.20 eV, full width at half maximum of 0.44 and 0.35 eV and decay time of 115 and 97 μs respectively. In addition the intensity ratio between the luminescent emissions at 3.1 and 4.2 eV was found to be constant in the whole set of samples. The occurrence of two β components with so close spectral and time peculiarities might be ascribed to fluorescence processes involving the same oxygen deficient centers surrounded by two slightly different environments.

Chapter 1 – Optical absorption, luminescence, and ESR spectral properties of point defects in silica

Publisher Summary This chapter is divided into two parts: (1) In the introductory part, it describes the problems of point defects in a-SiO2, and (2) in the second part it discusses the experimental results. This chapter focuses on the Oxygen-Deficient Centers (ODCs) species in silica. This chapter investigates the ODC defects in a-SiO2 through their optical absorption, photoluminescence, and electron spin resonance activities. The effects of γ-ray irradiation are also investigated to evidence their ability to generate or transform structural defects. The aim of this chapter is to understand the optical activity of such defects to help in the characterization of their structure. The properties of point defects in a wide variety of both natural and synthetic silica types of commercial origin are investigated in the chapter. This chapter outlines the role of structural and dynamic properties of the vitreous matrix in determining the observed spectral properties of different centers. It deals with the theoretical aspects of the mechanism that are able to influence the fine structure of the spectral band profiles of point defects in interaction with the glassy matrix.

Fluorescent nitrogen-rich carbon nanodots with an unexpected β-C3N4 nanocrystalline structure

Carbon nanodots are a class of nanoparticles with variable structures and compositions which exhibit a range of useful optical and photochemical properties. Since nitrogen doping is commonly used to enhance the fluorescence properties of carbon nanodots, understanding how nitrogen affects their structure, electronic properties and fluorescence mechanism is important to fully unravel their potential. Here we use a multi-technique approach to study heavily nitrogen-doped carbon dots synthesized by a simple bottom-up approach and capable of bright and color-tunable fluorescence in the visible region. These experiments reveal a new variant of optically active carbonaceous dots, that is a nanocrystal of beta carbon nitride (β-C3N4) capped by a disordered surface shell hosting a variety of polar functional groups. Because β-C3N4 is a network of sp3 carbon and sp2 nitrogen atoms, such a structure markedly contrast with the prevailing view of carbon nanodots as sp2-carbon materials. The fluorescence mechanism of these nanoparticles is thoroughly analyzed and attributed to electronic transitions within a manifold of surface states associated with nitrogen-related groups. The sizeable bandgap of the β-C3N4 nanocrystalline core has an indirect, albeit important role in favoring an efficient emission. These results have deep implications on our current understanding of optically active carbon-based nanoparticles and reveal the role of nitrogen in controlling their properties.

Weak hyperfine interaction of E′ centers in gamma and beta irradiated silica

We report on the effects of photon (γ) and electron (β) irradiation in a dose range extending from 100 to 5×109 Gy in a variety of silica samples studied by electron paramagnetic resonance. The E′ centers and a weak intensity satellite signal of their resonance line were generated both in γ- and in β-irradiated samples. We investigated the dependence of their intensity on the irradiation dose. Evidence of the existence of a common generation mechanism for the related paramagnetic point defects is found. These defects are induced mainly through the conversion of precursors except at very high doses, where the direct activation from the unperturbed matrix is concurrent. Our data support the model attributing the satellite signal to the weak hyperfine structure of the E′ center arising from interaction with a second nearest neighbor nuclear spin.

X-ray irradiation effects on fluorine-doped germanosilicate optical fibers

We report an experimental investigation on the effects of fluorine codoping on the radiation response of Ge-doped Optical Fibers (OFs) obtained by three different drawing conditions. The OFs were irradiated with 10 keV X-rays up to 300 Mrad and studied by online Radiation-Induced-Attenuation (RIA) measurements. Confocal Micro- Luminescence (CML) and Electron Paramagnetic Resonance (EPR) were also employed to investigate the permanent radiation-induced-defects. The variation of the Germanium-Lone-Pair-Center (GLPC) and Non-Bridging- Oxygen-Hole-Centers (NBOHC) concentration with the radiation dose is investigated by CML, whereas the ones of the induced Ge(1), Ge(2) and Eʹ centers by EPR. No relevant differences are found in the RIA of the three fibers, as well as in the induced concentrations of Ge(1) and Ge(2) and in the decrease of the GLPC, showing minor relevance of changing the drawing conditions. We found that fluorine codoping does not affect the RIA and that, unexpectedly, the fluorine co-doped zones of the OFs show an enhanced photoluminescence of the radiation induced NBOHC enabling to suggest the presence of both Si and Ge variants. Moreover, an overall increase of the radiation induced Eʹ(Ge) centers is registered in relation to the presence of fluorine showing that this codopant has relevant effects.

Twofold coordinated Ge defects induced by gamma-ray irradiation in Ge-doped SiO 2

We report an experimental study by photoluminescence, optical absorption and Electron Paramagnetic Resonance measurements on the effects of exposure of Ge-doped amorphous SiO2 to γ ray radiation at room temperature. We have evidenced that irradiation at doses of the order of 1 MGy is able to generate Ge-related defects, recognizable from their optical properties as twofold coordinated Ge centers. Until now, such centers, responsible for photosensitivity of Ge-doped SiO2, have been induced only in synthesis procedures of materials. The found result evidences a role played by γ radiation in generating photosensitive defects and could furnish a novel basis for photosensitive pattern writing through ionizing radiation.

Photoluminescence band at 4.4 eV in oxygen-deficient silica: temperature effects

We report experimental results on the spectral properties and time behaviour of the 4.4 eV photoluminescence (PL) band in oxygen-deficient silica [Formula: see text]. Our measurements, performed both at T = 300 K and T = 10 K, show that at room temperature the PL features are independent of the particular excitation energy (5.0 eV, 6.8 eV and 7.6 eV) whereas at low temperature, upon excitation at 7.6 eV, the decay of the PL emission is faster than for lower excitation energies. This shortening of the PL lifetime is consistent with previously reported data, which were explained by hypothesizing an interconversion mechanism between two structural configurations of the same oxygen defect. Nevertheless, our results do not support the proposed mechanism and we tentatively suggest a different interpretation of the experimental data.

Ultraviolet emission lifetime in Si and Ge oxygen deficient centers in silica

We have measured the temperature dependence, in the range 10–295 K, of the lifetimes of the ultraviolet emissions associated with twofold coordinated Si and Ge, in pure and Ge-doped silica under excitation by synchrotron radiation. The Si-related fluorescence, centered at 4.4 eV, has a single exponential decay and its lifetime decreases from 4.0 ± 0.2 to 3.5 ± 0.2 ns on increasing the temperature. The luminescence at 4.2 eV, associated with the Ge defect, decays with a single exponential dependence below 150 K and with a more complex dependence at higher temperatures, its lifetime reducing from 7.8 ± 0.2 to 3.4 ± 0.2 ns. These features give evidence of the effectiveness of the thermally activated intersystem crossing in the relaxation of the excited singlet states, which is larger in the Ge-related center.