1. Department of Physical Chemistry and the Farkas Center for Light Induced Processes,
2. Racah Institute of Physics, The Hebrew University, Jerusalem 91904, Israel

Correspondence to: Oded Millo² Correspondence and requests for materials should be addressed to O.M. (Email: milode@vms.huji.ac.il).

Abstract

Semiconductor quantum dots, due to their small size, mark the transition between molecular and solid-state regimes, and are often described as 'artificial atoms' (ref 1-3). This analogy originates from the early work on quantum confinement effects in semiconductor nanocrystals, where the electronic wavefunctions are predicted⁴ 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, 6, 7, 8, 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 occupation¹⁰.