Vibrational properties of the surface-nonbridging oxygen in silica nanoparticles

By studying the site-selective luminescence spectra of oxidized silica nanoparticles we identify the electronic and the vibrational lines associated with the surface nonbridging oxygen, Si-O•. This defect emits a zero-phonon line inhomogeneously distributed around 2.0 eV with full width at half maximum of 0.04 eV, weakly coupled with the local Si-O• stretching mode whose frequency is measured to be 920 cm−1. These findings are different from those of the well-characterized defect in the bulk silica thus evidencing structural peculiarities of the surface defect that is characterized by a nearly unperturbed nonbridging oxygen and of the nanoparticles that induce a narrower inhomogeneous broadening of the electronic transition.