CHAPTER 1. FLUORESCENT SILICON CLUSTERS 12 possibility be suppressed. In broadest terms, the passivation of dangling bonds at a surface with atoms such as hydrogen (hydrides), halogens (halides), oxy- gen (oxides), or molecular groups such as alkenes, amines or polymers, can be understood to confine the electronically excited electron to the core region of the cluster. The distinctly higher fluorescence intensities that arise from this confinement can be observed. This was confirmed by Seraphin and coworkers (Seraphin et al., 1996) who investigated silicon nanoparticles produced by laser ablation with and without passivation in vacuo. They found that passivation in- creased fluorescence intensity, but left the fluorescence energy itself unchanged. Embedding clusters and nanocrystals within a lattice-matching host of a larger band gap energy fulfils the same purpose. Also, in the ideal case of a lattice- matching interface, large amplitude vibrations would be successfully suppressed purely as, though not necessarily limited to, a matter of sterics. Passivated or core-shell clusters embedded in a solvent can also effectively transfer and absorb excess vibrational energy. Clusters are understood as being particles consisting of as few as two, three, or four, or as having as many as a few thousand, atoms. The term ’cluster’ is used alongside the term ’nanoparticles’, but clusters are commonly understood to represent smaller particles. The study of clusters is motivated by the desire to understand the often dramatic changes of material properties that accompany changes in size. Material properties also depend on structure and dimensionality, and similarly accompany changes in size. All such characteristics are relevant in cluster science research. 1.3 Clusters