Conductance through the armchair graphene nanoribbons 9-AGNR: Strong dependence on contact to leads

Abstract

The successful use of graphene nanoribbons (GNRs) in a variety of applications in nanoelectronics depends not only on reliable control of their forbidden gaps, but also on the understanding of the effects that contacts to leads may have on their conductance $G$. By combining Landauer's formalism and a simplified version of the embedded cluster method, $G$ through suspended 9-AGNR has been calculated as a function of energy (sample bias) and the strength of the contact between the ribbon and leads attached to both zigzag edges. Green's functions of contacted ribbons have been derived from HF nonspin polarized solutions of the Pariser-Parr-Pople Hamiltonian. It is shown that the G associated with the two quasidegenerate states around the Fermi level, which are strongly localized at the zigzag edges, equals a conductance quantum ${G}_{0}$ for very weak leads-ribbon coupling, decreasing to zero as that coupling increases. At the Fermi level $G$ is zero for small coupling, increasing up to ${G}_{0}$ for a value of coupling that depends on the GNR length, and, finally, decreasing to zero for large coupling. Conductance through other energies, starting at $G=0$ for no coupling, increases with coupling to the electrodes up to near one quantum at a pace that may appreciably depend on the particular molecular orbital. These results illustrate the difficulties that may be found in exploring practical uses of GNRs.