The structural disorder of a silica network probed by site selective luminescence of the nonbridging oxygen hole centre

We studied the inhomogeneous distribution of the luminescence band associated with the nonbridging oxygen hole centre in silica through site selective excitation/detection of the zero phonon line by a tunable laser source. Defects induced in the bulk of synthetic samples by γ and β exposure exhibit an increase of the inhomogeneous width from 0.071 to 0.086 eV on increasing the irradiation dose from 2 × 10^6 to 5 × 10^9 Gy. We also investigated two defect variants stabilized at the surface of the silica nanoparticles, (≡Si–O)3Si–O• and (≡Si–O)2(H–O)Si–O•, whose inhomogeneous width was measured to be 0.042 eV and 0.060 eV, respectively. These results can be accounted for by the structural disorder around the defect, and specific causes of its variation can be pointed out, such as the network modification induced by irradiation or the structural alteration near the SiO4 coordination sphere.