and compared to ab initio calculations (Vogel et al., 2012). 

## 1.4 Fluorescent silicon clusters 

The electronic level structure associated with dense packing, suggesting very small band gap energies for small- and medium-sized silicon clusters, and even the absence of band gaps, is unfavourable towards fluorescence. Indeed, fluores- cence from free silicon clusters in traps or in molecular beams has not yet been reported in the literature. The work on neutral and cationic silver clusters in argon droplets and solid matrices (Felix et al., 2001; Sieber et al., 2004; Conus et al., 2006; Harbich et al., 2007) suggests that fluorescence might be possi- ble if silicon clusters are deposited and embedded in rare gas matrices. While such deposition experiments have been carried out (Honea et al., 1993b, 1999), attempts to observe fluorescence with this setup are not known to the author. 

It appears that fluorescence reported for nanoscale silicon can be attributed to the effects of quantum confinement, passivation, and the presence of defects. To achieve confinement, passivation or defects due to other materials, molecules or atoms are actively introduced. In the vast majority of bottom-up methods used to produce fluorescent silicon nanoparticles, chemical methods are em- ployed. Exceptions are laser vaporisation of silicon targets in liquids ( ˇ Svrˇ cek et al., 2009a,b; Intartaglia et al., 2012a; Alkis et al., 2012; ˇ Svrˇ cek et al., 2016; Rodio et al., 2016), pyrolysis of silane in gas-flow reactors (Ehbrecht et al., 1997) and silicon cluster co-deposition with water vapour (von Haeften et al., 2009). 

## 1.5 Red-orange luminescence 

Silicon clusters produced by CO$_{2}$ laser-induced decomposition of silane were found to show red photoluminescence after they were deposited onto LiF or 

