Molecule/Electrode Interface Energetics in Molecular Junction: A Transition Voltage Spectroscopy Study

We assess the performances of the transition voltage spectroscopy (TVS) method to determine the energies of the molecular orbitals involved in the electronic transport through molecular junctions. A large number of various molecular junctions made with alkyl chains but with different chemical structure of the electrode/molecule interfaces are studied. In the case of molecular junctions with clean, unoxidized electrode/molecule interfaces, that is, alkylthiols and alkenes directly grafted on Au and hydrogenated Si, respectively, we measure transition voltages in the range 0.9-1.4 V. We conclude that the TVS method allows estimating the onset of the tail of the LUMO density of states, at energy located 1.0-1.2 eV above the electrode Fermi energy. For oxidized interfaces (e.g., the same monolayer measured with Hg or eGaIn drops, or monolayers formed on a slightly oxidized silicon substrate), lower transition voltages (0.1-0.6 V) are systematically measured. These values are explained by the presence of oxide-related density of states at energies lower than the HOMO/LUMO of the molecules. As such, the TVS method is a useful technique to assess the quality of the molecule/electrode interfaces in molecular junctions.