Size-dependent tunneling and optical spectroscopy of InAs nanocrystal quantum dots

Scanning tunneling spectroscopy is used to investigate single InAs nanocrystals, 2 to 8 nm in diameter. The tunneling current-voltage characteristics exhibit a pronounced size dependence of both single electron charging energy and quantum-confined electronic states. These are well described using the "Orthodox Model" for single electron tunneling through a quantum dot with a discrete level structure. Quantum confined states having atomic-like S and P symmetries are directly identified in the tunneling spectra as two and six-fold single electron charging multiplets, respectively. The tunneling spectra acquired on un-passivated nanocrystals show clear evidence of sub-gap surface states. Surprisingly, excellent agreement is found between the strongly allowed optical transitions and spacing of levels detected in the tunneling experiment.