  The transmission properties of armchair graphene nanoribbon junctions between
graphene electrodes are investigated by means of first-principles quantum
transport calculations. First the dependence of the transmission function on
the size of the nanoribbon has been studied. Two regimes are highlighted: for
small applied bias transport takes place via tunneling and the length of the
ribbon is the key parameter that determines the junction conductance; at higher
applied bias resonant transport through HOMO and LUMO starts to play a more
determinant role, and the transport properties depend on the details of the
geometry (width and length) of the carbon nanoribbon. In the case of the
thinnest ribbon it has been verified that a tilted geometry of the central
phenyl ring is the most stable configuration. As a consequence of this rotation
the conductance decreases due to the misalignment of the $pi$ orbitals between
the phenyl ring and the remaining part of the junction. All the computed
transmission functions have shown a negligible dependence on different
saturations and reconstructions of the edges of the graphene leads, suggesting
a general validity of the reported results.
