The figure shows a Si45 cluster. Silicon is a semiconductor, which crystallizes in the diamond crystal structure. The atoms are tetra-coordinated. This is different in a cluster. The geometric structure is disordered and there are tetra- and penta-coordinated atoms. The particle has no band gap but is metallic. (Figure taken from J. Bai et al., J. Phys. Chem. A 2006, 110, 908.)

So much for the theory. It is more difficult to actually measure the geometric and electronic structure. Because properties can change with every atom, experiments need to be carried out on mass-separated clusters. This is the area of expertise of our group.