CHAPTER 1. FLUORESCENT SILICON CLUSTERS 27 vibrational spectrum. The interaction with water has been found to chemi- cally modify silicon nanoparticles, and as a consequence its fluorescence activity has been seen to ’degrade’ (Li and Ruckenstein, 2004; Erogbogbo and Swihart, 2007; Erogbogbo et al., 2008). However, blue fluorescence from nanoscale sili- con has been frequently observed, and specifically in connection with treatment with water ( ˇ Svrˇ cek et al., 2009a,b; Intartaglia et al., 2012b,a; Alkis et al., 2012; ˇ Svrˇ cek et al., 2016; Rodio et al., 2016). Hou and co-workers treated light-emitting porous silicon with boiling water (Hou et al., 1993) and observed a large blue shift in the fluorescence towards the green-blue spectral range. Infrared spectroscopy was performed with the specific goal to assess whether the formation of silicon monohydride (2080 cm − $^{1}$) and silicon dihydride (2120 cm − $^{1}$) was correlated with the appearance of blue fluorescence. Prior to treatment with water, both lines were present in the spectrum, and decreased in intensity after boiling water was added. Instead a band appeared at 1105 cm − $^{1}$, showing that treatment with water had caused oxidation (Hou et al., 1993). Koyama and co-workers annealed oxidised porous silicon in water vapour and observed a drastic enhancement in the blue fluorescence intensity (Koyama et al., 1998). Infrared absorption spectroscopy indicated that this annealing increased the absorption peaks related to OH vibrations except for those of free silanol, which disappeared completely. No traces of carbon-related signals were observed, contrary to the previously suspected involvement of carbon (Kon- tkiewicz et al., 1994; Canham et al., 1996). Many authors report that the emission of blue fluorescence is correlated with very small structures, perhaps only a few nanometres in size. Akcakir and co-workers etched p-type boron-doped silicon using H$_{2}$O$_{2}$ and HF (Yamani et al., 1997; Akcakir et al., 2000). The combined effect of the two chemicals