Luminescence of closed shell molecular complex centers in nanoporous sol-gel SiO₂ glasses

Sol-gel silica glasses doped with transition metal ions (M = Ti⁴⁺, V⁵⁺, Cr⁶⁺, Zr⁴⁺, Nb⁵⁺, Mo⁶⁺, Hf⁴⁺, Ta⁵⁺, W⁶⁺) were prepared to investigate luminescence properties of nanoporous phosphors by measurements of luminescence spectra and lifetimes in the temperature range between 10 and 300 K. The glasses show broad emission bands at around 18,000 cm^-1 under a N₂ laser excitation. Luminescence is due to the ligand-to-metal charge transfer (³LMCT) transition of ions with closed-shell electronic structures, based on tetrahedral molecular complex [MO₄] centers. These ions reveal vibronic structures depending on surface conditions. The presence of oxygen-coordinated octahedral [MO₆] centers must also be considered to understand the spectral band at higher energy observed in Ti, Nb, Ta, and W doped glasses. Enhancement of quantum efficiency up to 10% is found in hybrid glasses doped with Al and V ions.