Structure of Amorphous SiO2 Nanoparticles Probed through the E'_gamma Centers

We report an experimental investigation by electron paramagnetic resonance (EPR) spectroscopy on the properties of the E'γ centers induced by β-ray irradiation in nanoparticles of amorphous SiO2 (fumed silica) with mean diameters from 7 up to 40 nm. We found that the E'γ centers are induced in all the fumed silica types in the dose range 4-400 kGy. They are characterized by an EPR line shape similar to that observed in common bulk silica materials independently on the particle diameter. Moreover, the E'γ center concentration decreases on decreasing of the particle size for each given dose. Our findings are interpreted in terms of a shell-like model of nanoparticles in which it is assumed that stable E'γ centers can be mainly induced in the inner part of the particles, whereas those induced in the surface shell are supposed to be essentially unstable and rapidly disappear after irradiation. Furthermore, we have found that the same shell-like model naturally explains the actual radiation resistance observed for nanoparticles which has been previously tentatively attributed to the high O2 content of the materials.