Two-photon-excited luminescence and defect formation in SiO2 nanoparticles induced by 6.4-eV ArF laser light

The photoluminescence (PL) from 7- and 15-nm silica (SiO2) nanoparticles induced both by ArF laser light [lambda(exc)=193nm (6.4eV), tau(L)=15ns] and by Nd:YAG (yttrium-aluminum-garnet) laser light [lambda(exc) = 266 nm (4.66 eV), tau(L)= 8 ns] was studied. The laser light intensity dependencies of the PL yields reveal the two-photon (TP) process of the PL excitation in the case of ArF laser light. The PL results from the radiative relaxation of self-trapped excitons (STE- the blue band), also from the surface hydrogen-related species (the green band), and the bulk nonbridging oxygen hole centers (NBOHC's- the red band) excited by a radiationless relaxation of TP-produced free excitons (FE's). The main point is focused on the effect of the nanoparticle surface condition on the FE dynamics, The dynamics includes either an elastic scattering or quenching by the nanoparticle boundary, the laser heating of FE's up to energies in excess of the STE barrier, the FE energy transfer to the surface and bulk NBOHC's and hydrogen-related centers, the saturation of the FE density, and the biexciton process in the formation of Frenkel defects with their subsequent transformation into NBOHC's.