Near-Infrared Emission of O2 Embedded in Amorphous SiO2 Nanoparticles

We report an experimental study on the emission properties of O-2 molecules loaded by a thermal diffusion process at 200 degrees C into high-purity silica nanoparticles with mean diameters of 7 and 40 nm. The embedded O-2 features a singlet to triplet emission band peaked at 1272 nm in agreement with the band observed for bulk silica materials. The photoluminescence excitation spectra have been determined in the visible and in the infrared range and are characterized by narrow bands peaked at 691, 764, and 1069 nm, respectively. By comparison of the transition energies, the vibrational quanta have been determined for the ground and for both the excited states; the values found are lower than the corresponding energies reported for the O-2 gaseous molecule. The singlet to triplet emission lifetime has been also evaluated and shows a value lower than that typically reported for bulk silica. Our results show that even if the main spectroscopic properties of O-2 embedded in nanometric silica are analogous to those of free O-2 and O-2 embedded in bulk silica, the nanometric host imposes differences related to the dynamics of the electronic states.