Size-dependent Tunneling and Optical Spectroscopy of InAs Nanocrystal Quantum Dots

Scanning tunneling spectroscopy is used to investigate singleInAs nanocrystals, 2 to 8 nm in diameter. The tunnelingcurrent-voltage characteristics exhibit a pronounced sizedependence of both single electron charging energy andquantum-confined electronic states. These are well describedusing the Orthodox Model for single electron tunnelingthrough a quantum dot with a discrete level structure. Quantumconfined states having atomic-like S and P symmetries aredirectly identified in the tunneling spectra as two andsixfold single electron charging multiplets, respectively. Thetunneling spectra acquired on un-passivated nanocrystals showclear evidence of sub-gap surface states. Surprisingly,excellent agreement is found between the strongly allowedoptical transitions and spacing of levels detected in thetunneling experiment.