Visible Photoluminescence from Photoinduced Molecular Species in Nanometer-Sized Oxides: Crystalline Al2O3 and Amorphous SiO2 Nanoparticles

Photoluminescence (PL) characteristics of two structurally and compositionally different oxide nanoparticles, i.e., crystalline gamma-alumina and amorphous silica nanoparticles, are investigated. Time-resolved PL measurements have shown that both of these nanoparticles exhibit blue PL emissions characterized by fast (several tens of nanoseconds) and slow (a few seconds) decay constants. The fast PL component is observed over a wide temperature range up to similar to 400 K, whereas the slow PL component is observed only at temperatures below similar to 200 K. The close similarity between the PL characteristics of gamma-alumina and amorphous silica nanoparticles indicates that common molecular-like species are responsible for the observed PL emissions. From the detailed comparison of the present experimental observations with the reported data, it can be concluded that the fast and slow PL components result from O-2 molecules and OH radicals, respectively, both of which are photoinduced transient molecular species generated at the surface of these oxide nanoparticles.