We present first principles calculations of current-voltage characteristics (IVC) and conductance of Au(111):S-2-cumulene-S-2:Au(111) molecular wire junctions with realistic contacts. The transport properties are calculated using full self-consistent ab initio nonequilibrium Green's function density-functional theory methods under external bias. The conductance of the cumulene wires shows oscillatory behavior depending on the number of carbon atoms (double bonds). Among all conjugated oligomers, we find that cumulene wires with odd number of carbon atoms yield the highest conductance with metalliclike ballistic transport behavior. The reason is the high density of states in broad lowest unoccupied molecular orbital levels spanning the Fermi level of the electrodes. The transmission spectrum and the conductance depend only weakly on applied bias, and the IVC is nearly linear over a bias region of +/- 1 V. Cumulene wires are therefore potential candidates for metallic connections in nanoelectronic applications.

The cumulenes bridging two-dimensional electrodes provide a model for interconnecting molecular electronics circuit with one of the most conductive molecular wires known. In recent experiments cumulene molecules bridging graphene sheets were observed [PRL 102, 205501 (2009)], thus demonstrating the mechanical way of producing cumulenes. Appearance of carbon wires: cumulenes and polynes, is also feasible between graphene sheets or carbon nanotubes (CNTs). In this work, we study structure and conductance of these wires  suspended between CNTs of different chirality (zigzag and armchair), and graphene sheets (infinite radii CNTs) and corresponding conductance variation upon stretching. We find the geometrical structures of the carbon wire bridging CNT similar to the experimentally observed in the carbon wires obtained between graphene electrodes. We show a capability to modulate the conductance by changing bridging sites between the carbon wire and CNT without breaking the wire. Observed current modulation via cumulene wire stretching/elongation together with CNT stability makes it a promising candidate for mechano-switching device in molecular nanoelectronics.

We investigate the electrical transport properties of two hydrogen tautomer configurations of phthalocyanine (H₂Pc) connected to cumulene and gold leads. Hydrogen tautomerization affects the electronic state of H₂Pc by switching the character of molecular orbitals with the same symmetry close to the Fermi level. The near degeneracy between the HOMO and HOMO-1 leads to pronounced interference effects, causing a large change in current for the two tautomer configurations, especially in the low-bias regime. Two types of planar junctions are considered: cumulene-H₂Pc-cumulene and gold-H₂Pc-gold. Both demonstrate a prominent difference in molecular conductance between ON and OFF states. In addition, junctions with gold leads show pronounced negative differential resistance (NDR) at high bias voltage, as well as weak NDR at intermediate bias.

The fabrication of nanopores in atomically thin graphene has recently been achieved, and translocation of DNA has been demonstrated. Taken together with an earlier proposal to use graphene nanogaps for the purpose of DNA sequencing, this approach can resolve the technical problem of achieving single-base resolution in electronic nucleobase detection. We have theoretically evaluated the performance of a graphene nanogap setup for the purpose of whole-genome sequencing, by employing density functional theory and the nonequilibrium Green's function method to investigate the transverse conductance properties of nucleotides inside the gap. In particular, we determined the electrical tunneling current variation at finite bias due to changes in the nucleotides orientation and lateral position. Although the resulting tunneling current is found to fluctuate over several orders of magnitude, a distinction between the four DNA bases appears possible, thus ranking the approach promising for rapid whole-genome sequencing applications.

Graphene nanogaps and nanopores show potential for the purpose of electrical DNA sequencing, in particular because single-base resolution appears to be readily achievable. Here, we evaluated from first principles the advantages of a nanogap setup with functionalized graphene edges. To this end, we employed density functional theory and the non-equilibrium Green's function method to investigate the transverse conductance properties of the four nucleotides occurring in DNA when located between opposing functionalized graphene electrodes. In particular, we determined the electrical tunneling current variation as a function of the applied bias and analyzed the associated differential conductance at a voltage which appears suitable to distinguish between the four nucleotides. Intriguingly, we predict for one of the nucleotides (deoxyguanosine monophosphate) a negative differential resistance effect.