further, and to date numerous examples of nanostructured forms of fluorescent silicon have been reported (Takagi et al., 1990; Brus et al., 1995; Hirschman et al., 1996; Borsella et al., 1997; Ehbrecht et al., 1997; Cullis et al., 1997; Huisken et al., 1999; Pavesi et al., 2000; Belomoin et al., 2000, 2002; Mangolini et al., 2005; Falconieri et al., 2005; Brewer and von Haeften, 2009; Vincent et al., 2010; He et al., 2011; Dasog et al., 2014; Li et al., 2016). Hence, we have a rich set of data available on electronic and structural properties that unpin our understanding of the fluorescence of silicon clusters and nanoparticles. 

The strong appeal of fluorescent silicon clusters and nanoparticles arises due to a veritable multitude of applications, for example in electronic circuits (Pavesi, 2003; ˇ Svrˇ cek et al., 2004; Stupca et al., 2007) and biomedicine (Nel et al., 2009; Chinnathambi et al., 2014). Silicon is the most frequently used semiconductor material in electronics. It has been suggested that by combin- ing electric and optical signal transmission, higher performance can be achieved (Canham, 2000; Pavesi, 2003). Issues directly related to the decreasing size of electronic units, such as signal delay caused by increasingly longer intercon- nects, can be addressed by optically transmitted refresh pulses (Pavesi, 2003). Fluorescent silicon clusters and nanoparticles are also attractive in biomedical applications because nanoscale silicon and silicon dioxide are considered to be non-toxic, or at least considerably less harmful than fluorescent nanoparticles of other materials (Warner et al., 2005; Erogbogbo and Swihart, 2007; Erogbogbo et al., 2008; Choi et al., 2009; Erogbogbo et al., 2010; Cheng et al., 2014; Peng et al., 2014; McVey and Tilley, 2014) as well as biodegradable (Park et al., 2009). Fluorescent silicon nanoparticles play an important role as biological markers (Nel et al., 2009; Montalti et al., 2015). 

In this chapter, the foundations of silicon cluster experiments and cluster production will be discussed. The underlying principles behind the fluorescence 

