A tunnelling spectroscopy study on the single-particle energy levels and electron-electron interactions in CdSe quantum dots

We performed scanning tunnelling spectroscopy on insulating colloidal CdSe quantum dots attached to gold with a rigid self-assembled monolayer. By varying the tip-dot distance we change the relative rate of tunnelling into versus tunnelling out of the dot over a wide range. If the tip is retracted relatively far from the dot, tunnelling in is much slower than tunnelling out of the dot; electrons tunnel one at a time through the dot. The resonances in the conductance spectrum correspond to the single-particle energy levels of the CdSe quantum dot. When the tip is brought closer to the dot, tunnelling in can become as fast as tunnelling out of the dot. Up to three electrons can be present in the particle; the electron-electron Coulomb interactions lead to a more complex conductance spectrum.