Time-resolved photoluminescence study of silica nanoparticles as compared to bulk type-III fused silica

Photoluminescence (PL) from composites of 7- and 15-nm silica (amorphous ${\mathrm{SiO}}_{2})$ nanoparticles and bulk type-III fused silica induced by two-photon band-to-band excitation with 193-nm (6.4-eV) ArF laser light has been measured in time-resolved detection mode. The PL spectra taken for 15-nm particles allow one to identify three PL bands peaked in the red (\ensuremath{\sim}1.9 eV), green (\ensuremath{\sim}2.35 eV), and blue (\ensuremath{\sim}2.85 eV) spectral ranges. The green and blue bands are normally overlapped in conventional scan measurements, giving no way for determining their exact peak positions. Similar red and green PL bands were observed for 7-nm particles, whereas the blue band extends toward the higher-energy range and is peaked at \ensuremath{\sim}3.25 eV. The aforementioned red, green, and blue PL bands are assigned to nonbridging oxygen hole centers, hydrogen-related species, and self-trapped excitons (STE's), respectively. The red and green PL bands for bulk type-III fused silica are peaked at practically the same spectral positions as those for nanoscale silicas, indicating the similarity of light-emitter types. However, the blue band for bulk silica used is peaked at \ensuremath{\sim}2.75 eV, that is, at typical position for STE's in crystalline ${\mathrm{SiO}}_{2}.$ The blueshift of STE (PL) (STEPL) band with decreasing nanoparticle size is consistent with the previously proposed model of phonon-assisted radiative relaxation of STE's [Yu. D. Glinka et al., Phys. Rev. B 64, 085421 (2001)]. As a result of time-resolved measurements, the extremely broad PL band peaked at \ensuremath{\sim}2.35 eV, which is typically observed for bulk silicas and initially assigned to STE's, is found to consist of two bands peaked at \ensuremath{\sim}2.75 and \ensuremath{\sim}2.35 eV. We suggest that these bands are due to the radiative deexcitation of STE's and hydrogen-related centers, respectively. We thus conclude that the STEPL band is peaked at \ensuremath{\sim}2.75 eV for both bulk amorphous and crystalline ${\mathrm{SiO}}_{2}.$

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