Figure 1 Energy levels in a semiconductor quantum dot. The levels have a shell structure formed of s and p states, and substructure arising from the spin of the electrons. Electrons (filled triangles) and holes (empty triangles) are shown. a, One or more electrons can be injected into the quantum dots, which fill up the states in a way that minimizes the energy of interactions (Hund's rules of atomic physics). Furthermore, one or more electron–hole pairs (excitons) can be photoexcited. Other possible systems are: b, a single electron–hole pair created in a quantum dot with a single extra electron; c, two electron–hole pairs. In the experiments of Bayer et al.² and Warburton et al.³, luminescence spectroscopy is used to find the energies (actually the energy differences between states) of these multi-electron quantum dots. In this way, analogues to the periodic table and Hund's rules in quantum dots can be explored.