Visible and ultraviolet emission of porous silica excited by synchrotron radiation

The photoluminescence properties of sol-gel prepared porous silica in visible and ultraviolet range were studied exciting in the 4-8 eV range by means of synchrotron radiation. Two main emissions at 2.8 and 3.7 eV were detected and studied as a function of the temperature from 8 up to 300 K. The two emission bands displayed different excitation channels: at 5.6 and 6.2 eV for the 3.7 eV band and at 4.8, 5.2 and 6.0 eV for the 2.8 eV band. By increasing the temperature the amplitude of the 2.8 eV emission decreases with respect to the contribution of the 3.7 eV band when excited at 5.6 eV. The time decays of the two bands excited at 5.6 eV were fitted with two exponential decays with mean life times of about 2.6 and in the range 20-60 ns for the 2.8 eV emission and 2.6 and 21.8 ns for the 3.7 eV emission. The analysis of the spectroscopic features suggests the attribution to different emitting surface centers and the presence of at least two contributions for both the emissions. We assign the 3.7 eV band to two different species of interacting silanols.