Quantum conductance of 4,4-bipyridine molecular junctions: Role of electrode work function and local $d$ band

We present density-functional theory calculations for the geometry and conductance of 4,4-bipyridine (BPD) nanojunctions with Au and Pt electrodes. The fact that transport takes place via bipyridine’s lowest unoccupied molecular orbital (LUMO) suggests that the Au-BPD junction should have larger conductance than the Pt-BPD junction due to the smaller work function of Au as compared to Pt. On the other hand, coupling to the local $d$ band is stronger in the case of Pt and this broadens the LUMO resonance. We find that these effects largely outbalance each other leading to conductances of $0.01G_0$ and $0.02G_0$ for the Au and Pt contacts, respectively ($G_0=2e^2/h$ is the conductance quantum). The effect of coupling to the electrodes is investigated by means of the group orbital which makes precise the concept of the local band. The construction allows us to explain and rationalize the first-principles results within a simple single-level model.

Figure 1

(Color online) Supercell used for the metal-bipyridine-metal molecular junction.

Figure 2

(Color online) The transmission function for the Au-bipyridine junction (dashed line) and the Pt-bipyridine junction (full line). The inset shows a close-up of the region around the Fermi level. In this region, the transmission function can be completely accounted for by the LUMO orbital of the bipyridine.

Figure 3

(Color online) Isosurfaces of the LUMO (above) and the corresponding group orbitals (below) for the Pt-BPD junction. The group orbitals are localized at the molecule-electrode interface, and their symmetry follows from the $\pi$ symmetry of the LUMO.

Figure 4

(Color online) Projected density of states for the group orbital of the LUMO (full red line) and an atomic $d$ orbital of the contacting Au atom (black dashed line). Also shown is a semielliptical approximation to the group orbital PDOS (full black line) and the resulting PDOS of the LUMO (full blue line) as calculated from Eq. (7) when the semielliptical band is used together with the coupling strength, $V$, and level position ${\epsilon }_a$ extracted from the DFT calculation. Notice that the distortion of the LUMO resonance tail in the case of Pt-BPD junction is due to the Pt $d$ band.