Photoluminescence spectroscopy of silica-based mesoporous materials

The photoluminescence (PL) at 300 and 90 K from the mesoporous silicas (MSs) [ordered (MCM-41) and disordered (DMSs)] of variously sized pores induced either by ArF laser light (lambda(exc) = 193 nm) or by Nd: YAG (yttrium-aluminum-garnet) laser Light (lambda(exc) = 266 nm) was studied. The spectra were measured in both conventional and time-resolved detection modes. It has been shown that PL induced by 266-nm light results from a direct one-photon excitation of hydrogen-related species (the green band) situated on the internal pore surfaces and nonbridging oxygen hole centers (NBOHCs, the red bands) located both on the surface and inside similar to 1-nm sized bulk fragments (pore walls). Alternatively, an indirect excitation of the light emitters due to the energy transfer through two-photon produced free excitons (FEs) is responsible for PL in the case of 193-nm excitation. In addition, the self-trapping of FEs followed by radiative relaxation results in the blue band, which can only be induced by 193-nm light. The PL properties of MSs in comparison with those of silica nanoparticles and bulk silica are discussed.