The field is interdisciplinary to condensed matter physics, molecular physics, materials science, and physical chemistry. The central issue in cluster science is the evolution of properties as the particles grow in size. The accompanying changes are often pronounced, as small systems feature a discrete number of vibrational and electronic states, while condensed matter is more appropriately characterized by its phonon spectrum and electronic band structure. For example, most of the optical and electronic features of metals or semiconductors develop only if a large number of atoms are held together in close proximity. Fullerenes are hollow carbon clusters comprising several dozens of atoms.
Endohedral fullerenes contain one or more atoms in the cage.