Electron Counting Spectroscopy of CdSe Quantum Dots

We report on the electronic properties of semiconducting CdSe quantum dots that can be filled or emptied with many electrons. To accomplish that, we employ a device layout where the investigated quantum dot is attached to only one electrode, a carbon nanotube. Measurements consist of detecting individual electrons transferred onto the quantum dot (by monitoring the nanotube resistance) while sweeping the electrochemical potential of the dot with a gate. This technique allows us to detect the energy gap of the semiconducting quantum dot and to access many electronic levels. We exploit the latter finding to study the statistical aspects of the spectrum of the quantum dot. The measured spectrum distribution approaches the bimodal Wigner distribution, which is the most basic prediction of the random matrix theory applied to quantum dots.