Bright Visible Luminescence in Silica Nanoparticles

We demonstrate that a porous film of silica nanoparticles emits a bright visible luminescence associated with defects stabilized by oxygen chemisorption at oxygen-deficient center sites. Time-resolved spectra excited by a tunable laser allow us to distinguish the luminescence at 1.99 eV, characteristic of the nonbridging oxygen hole center (NBOHC) (equivalent to-Si-O)(3)Si-O center dot, and a fast and a slow emission: the first (lifetime tau approximate to 25 ns) is peaked at 2.27 eV with an excitation spectrum centered at 5.5 eV; the second (tau approximate to 7.5 mu s) is peaked at 2.41 eV and is excited around 3.2 and 5.2 eV. Reaction in an air atmosphere leads to the disappearance of the NBOHC luminescence and of the fast band, whereas the slow one remains stable. On the basis of the comparison with previous experimental and computational works, we discuss the role of the silanone Si=O and of the dioxasilyrane Si(O-2) as the emitting defects.