Tunneling and optical spectroscopy of InAs and InAs/ZnSe core/shell nanocrystalline quantum dots

We combine scanning tunneling spectroscopy and photoluminescence excitation spectroscopy (PLE) to study the electronic level structure and single electron charging effects of InAs and novel InAs/ZnSe core/shell nanocrystal quantum dots. The two techniques provide complementary information on the electronic structure of these systems. In the tunneling spectra of core InAs nanocrystals grown by colloidal chemistry, 2-7 nm in diameter, we directly identify atomic-like electronic quantum dot states with s and p character as two- and sixfold charging multiplets. These measurements are correlated with the low temperature PLE data and, surprisingly, excellent agreement was observed between spacings of levels detected by the two techniques, yielding new information on the quantum dot level structure. The combined tunneling and optical spectroscopy approach was also applied to the study of InAs/ZnSe core/shell nanocrystals, which have a high fluorescence quantum yield in the near IR range. The tunneling spectroscopy shows changes in their electronic structure compared to the cores with the s-p gap closing in thicker shells. This observation is supported by PLE spectroscopy, establishing the effectiveness of the combined optical-tunneling spectroscopy approach in the study of quantum dots.