Dual response of graphene-based ultra-small molecular junctions to defect engineering

It has been reported that N and B doping induce a quasi-bound state that suppresses the conduction in graphene nanoribbon (GNR)-based junctions, while an H defect or a pyridine-like N-atom (PN) substitution at the edge of the GNR does not affect the transmission close to the Fermi energy. However, these results may vary when the size of the functional unit of the GNR junction decreases to a molecular level. In this study, a defect is introduced to a test-bed architecture consisting of a polyacene bridging two zigzag GNR electrodes, which changes the molecular state alignment and coupling to the electrode states, and varies the equivalence between two eigen-channels at the Fermi level. It is revealed that B and N atom substitution, and H defects play a dual role in the molecular conductance, whereas the PN substitution acts as an ineffective dopant. The results obtained from density functional theory combined with the non-equilibrium Green's function method aid in determining the optimal design for the GNR-based ultra-small molecular devices via defect engineering.