Molecule-Electrode Interface Energetics in Molecular Junction: a Transition Voltage Spectroscopy Study

TL;DR

This study evaluates the transition voltage spectroscopy (TVS) method for determining molecular orbital energies in various molecular junctions, revealing its effectiveness in assessing interface quality and electronic states.

Contribution

The paper demonstrates the capability of TVS to estimate the LUMO tail onset and distinguish interface oxidation states in molecular junctions, providing a practical interface assessment tool.

Findings

TVS estimates LUMO tail onset at 1.0-1.2 eV above Fermi energy.

Oxidized interfaces show lower transition voltages (0.1-0.6 V).

TVS effectively assesses interface quality in molecular junctions.

Abstract

We assess the performances of the transition voltage spectroscopy (TVS) method to determine the energies of the molecular orbitals involved in the electronic transport though 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, i.e. 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.