We have developed a technique to create ultra-clean carbon nanotube quantum dots. The nanotubes are grown as the last step of the fabrication process over a trench in the substrate and contact the source and drain electrodes. Then using scanning gate microscopy, we have explored the charge states of these quantum dots

A voltage is applied to the microscope tip to act as a moveable gate electrode. Then by monitoring the conductance through the nanotube as a function of tip position, we obtain a map of the quantum dot. Bright spots correspond to locations where the number of electrons on the quantum dot changes by one while dark areas have a fixed charge. This work has been posted on arXiv