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We study the transport properties of graphene nanoribbons of standardized 30 nm width and varying lengths. We find that the extent of the gap observed in transport as a function of Fermi energy in these ribbons (the ``transport gap'') does not have a strong dependence on ribbon length, while the extent of the gap as a function of source-drain voltage (the ``source-drain gap'') increases with increasing ribbon length. We anneal the ribbons to reduce the amplitude of the disorder potential and find that the transport gap both shrinks and moves closer to zero gate voltage. In contrast, annealing does not systematically affect the source-drain gap. We conclude that the transport gap reflects the overall strength of the background disorder potential, while the source-drain gap is sensitively dependent on its details. Our results support the model that transport in graphene nanoribbons occurs through quantum dots forming along the ribbon due to a disorder potential induced by charged impurities.
Gallagher et al. (Fri,) studied this question.