lifetime of ∼ 1 ns, which was independent of the excitation energy. Furthermore, the appearance of blue fluorescence was correlated to the presence of silicon oxide. Silicon-hydrogen bonds were absent. 

Harris and co-workers prepared blue light-emitting silicon samples by elec- trochemical etching. They found that the samples rapidly degraded, but were able to measure photoluminescence spectra at a sample temperature of 120 K; both red- and blue-emitting components were observed. Fluorescence decay of the blue fluorescence was measured at 77 K as having a time-constant of 0.86 ns. The decay was mono-exponential (Harris et al., 1994). 

By using different post-processing chemical treatments, porous silicon can be prepared to emit either blue or red fluorescence. Zidek and co-workers used such methods to measure the luminescence decay time separately for the two different wavelength ranges. They found that both types of fluorescence have distinct characteristics in their ultra-fast decay time. The blue fluorescent band is attributed to several underlying bands which vary further, depending on sam- ple preparation ( ˇ Z´ıdek et al., 2011). 

Light emission in the blue spectral range is a known phenomenon for col- loidal suspensions of silicon nanoparticles (Kimura, 1999; Belomoin et al., 2000; Valenta et al., 2008) and silicon nanocrystal films (Loni et al., 1995; Canham et al., 1996; de Boer et al., 2010; Ondiˇ c et al., 2014). At the present time, there seems to be a consensus that the vast majority of reported fluorescent bands in the blue spectral range are due to localised transitions, rather than being caused by quantum confinement (Dasog et al., 2013, 2016). The precise nature of this fluorescence is nevertheless debated, though it is nevertheless reasonable to attribute it to a range of different transitions which have similar transition energies. 

To elucidate the nature of the blue fluorescence, various sample preparation 

