Energy and transport gaps in etched graphene nanoribbons

We report transport measurements on etched graphene nanoribbons. We show that two distinct voltage (i.e. energy) scales can be experimentally extracted for characterizing the parameter regions of suppressed conductance at low charge density in graphene nanoribbons. The energy scales are related to the charging energy of localized states and to the strength of the disorder potential. We discuss the scaling behaviour of these two energy scales as a function of the minimum width omega for a number of different devices. Finally, we present a model based on Coulomb blockade, due to quantum dots forming inside the nanoribbon, explaining the observed energy scales.