Stretch dependent electronic structure and vibrational energy of the bipyridine single molecule junction.

We have studied the stretch dependence of the electronic structure and vibrational energy for the 4,4'-bipyridine (BPY) single molecule junction, which was fabricated by the mechanically controllable break junction (MCBJ) using the highly stable nano MCBJ electrodes. The electronic structure and vibrational energy of the single molecule junction were studied by the current-voltage (I-V) curve and surface enhanced Raman scattering (SERS), respectively. The simultaneous SERS and I-V curve measurements revealed the lowest unoccupied molecular orbital (LUMO) and vibrational energy of the C-C stretching mode decreased with an increase in the metal-molecule distance. The molecular orbital energy shift and vibrational energy shift can be explained by the change in the degree of the hybridization of molecular and metal orbitals.