  Coulomb blockade is observed in a graphene nanoribbon device with a top gate.
When two pn junctions are formed via the back gate and the local top gate,
electrons are confined between the pn junctions which act as the barriers. When
no pn junctions are induced by the gate voltages, electrons are still confined,
as a result of strong disorder, but in a larger area. Measurements on five
other devices with different dimensions yield consistent results.
