CHAPTER 1. FLUORESCENT SILICON CLUSTERS 24 Figure 1.5: Schematic of experimental set-up used by (von Haeften et al., 2009; Brewer and von Haeften, 2009; Torricelli et al., 2011). isopropanol, the fluorescence wavelength shifted to 365 and 380 nm, respec- tively (Galinis et al., 2012b). The fluorescence lifetime was measured using monochromatic synchrotron radiation. For an excitation wavelength at 195 nm and fluorescence at 300 nm, a lifetime of 3.7 ns was determined (Yazdanfar et al., 2012). Time-correlated fluorescence spectroscopy showed that the blue fluorescence consisted of at least two components: the spectral component with a long fluo- rescent lifetime that peaked at 2.7 eV (Yazdanfar et al., 2012), which is a perfect match with the fluorescence of defect-rich silica (Skuja et al., 1984), and a sec- ond short-lived, and much more intense, component, however, peaked at 3.0 eV (Yazdanfar et al., 2012). The good energy match and the long lifetime of the first of these bands suggested it arose due to the spin-forbidden T$_{1}$ → S$_{0}$ tran- sition observed for two-fold coordinated Si in SiO$_{2}$ (O – Si – O) (Skuja et al., 1984; Skuja, 1992; Nishikawa et al., 1992; Fitting et al., 2004). Short lifetimes have frequently been reported for the blue fluorescence of nanoscale silicon (Kovalev et al., 1994). Tsybeskov and co-workers investigated the lifetime of blue fluorescence emitted from thermally and chemically oxidised porous silicon (Tsybeskov et al., 1994). The decay was multi-exponential with a