Figure 1.

Photoluminescence spectra of large and small Si NPs before and after incorporation into aerogels. Photoluminescence spectra of free-standing PSi NPs of type LH (red dash-dot, second to top) and SO silicon nanospheres (blue dash-dot, third from top) and the silica aerogel without any particles (green dashed, bottom). The black, solid lines show PL spectra of aerogel-NP composites for a series of concentrations of LH particles increasing upwards (the initial concentrations in the gel preparation increase by factors of approximately 2 from 0.005 to 0.167 mg/ml). The arrow (bottom right) indicates the excitation energy of 3.81 eV (325 nm). All spectra are normalised to unit peak height. Top spectrum: un-normalised PL spectra of free-standing PSi NPs of type LH in vacuum (solid line) and in the presence of oxygen (dashed line); the arrow shows how the intensity drops on introduction of oxygen. No such change is observed for the same particles once incorporated in aerogel.