Identification of atomic-like electronic states in indium arsenide nanocrystal quantum dots

Semiconductor quantum dots, due to their small size, mark the transition between molecular and solid-state regimes, and are often described as 'artificial atoms' (refs 1-3). This analogy originates from the early work on quantum confinement effects in semiconductor nanocrystals, where the electronic wavefunctions are predicted(4) to exhibit atomic-like symmetries, for example 's' and 'p'. Spectroscopic studies of quantum dots have demonstrated discrete energy level structures and narrow transition linewidths(5-9), but the symmetry of the discrete states could be inferred only indirectly. Here we use cryogenic scanning tunnelling spectroscopy to identify directly atomic-like electronic states with s and p character in a series of indium arsenide nanocrystals. These states are manifest in tunnelling current-voltage measurements as two- and six-fold single-electron-charging multiplets respectively, and they follow an atom-like Aufbau principle of sequential energy level occupation(10).