Investigation of the transition voltage spectra of molecular junctions considering frontier molecular orbitals and the asymmetric coupling effect

This study experimentally and theoretically investigated the characteristics of transition voltage spectroscopy (TVS) with different asymmetric coupling in molecular junctions that were formed by several alkyl- and oligoacene-based molecules. On the basis of the analysis of a statistically significant number of device data (a total of ∼300 devices and ∼880 TVS data), we confirmed that the minimum point appearing in the Fowler-Nordheim (F-N) plot of the current-voltage characteristics depended linearly on the energy offset between the Fermi energy of the electrodes and the highest-occupied molecular orbital (HOMO) energy level of the molecular channel. We also observed that the slope of the log(I)-log(V) plot in molecular junctions changed sequentially. In addition, we extended a coherent molecular model by considering both the HOMO and the lowest-unoccupied molecular orbital (LUMO) levels of molecules and developed more comprehensive explanations for the TVS characteristics that are influenced by the asymmetric coupling, molecular length, and molecular orbital as well as for the sequential change in the slope of the log(I)-log(V) plot in molecular junctions.