Chapter 1 Fluorescent Silicon Clusters and Nanoparticles Klaus von Haeften 1.1 Overview Clusters, consisting of a small number of atoms, have been in the focus of physical and chemical research for several decades. They often show dramatic size effects. The addition of a single atoms can change their properties rather abruptly because of, for example, the discreteness of shell filling (Knight et al., 1984) or sphere-packing effects (Echt et al., 1981). When clusters become larger and reach the nanometre scale, other effects are observed, such as quantum confinement; The intense red fluorescence observed for nanostructured silicon (Canham, 1990; Cullis and Canham, 1991; Wilson et al., 1993; Lockwood, 1994; Cullis et al., 1997) is a popular and frequently cited example of this effect. The discovery of fluorescent nanoscale silicon at room temperature by Canham (Canham, 1990) increased the already quite intense research into silicon clusters 2