  We have fabricated suspended few layer (1-3 layers) graphene nanoribbon field
effect transistors from unzipped multiwall carbon nanotubes. Electrical
transport measurements show that current-annealing effectively removes the
impurities on the suspended graphene nanoribbons, uncovering the intrinsic
ambipolar transfer characteristic of graphene. Further increasing the annealing
current creates a narrow constriction in the ribbon, leading to the formation
of a large band-gap and subsequent high on/off ratio (which can exceed 104).
Such fabricated devices are thermally and mechanically stable: repeated thermal
cycling has little effect on their electrical properties. This work shows for
the first time that ambipolar field effect characteristics and high on/off
ratios at room temperature can be achieved in relatively wide graphene
nanoribbon (15 nm ~50 nm) by controlled current annealing.
