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

Semiconductor quantum dots, due to their small size, mark the transition be tween molecular and solid-state regimes, and are often described as 'artifi cial atoms' (refs 1-3). This analogy originates from the early work on quan tum confinement effects in semiconductor nanocrystals, where the electronic wavefunctions are predicted(4) to exhibit atomic-like symmetries, for exam ple 's' and 'p'. Spectroscopic studies of quantum dots have demonstrated di screte energy level structures and narrow transition linewidths(5-9), but t he symmetry of the discrete states could be inferred only indirectly. Here we use cryogenic scanning tunnelling spectroscopy to identify directly atom ic-like electronic states with s and p character in a series of indium arse nide nanocrystals. These states are manifest in tunnelling current-voltage measurements as two- and six-fold single-electron-charging multiplets respe ctively, and they follow an atom-like Aufbau principle of sequential energy level occupation(10).