Determination of Energy Level Alignment in Molecular Tunnel Junctions by Transport and Spectroscopy

Seminars
23.05.2019
14:30
David Wang Auditorium, 3rd floor, Dalia Maydan Bldg.
Dr. Zuoti Xie

Dr. Zuoti Xie

Research Associate, University of Minnesota

One of the central challenges of molecular electronics is to establish clear connections between molecular structure, the ensuing electronic structure, and the current-voltage (I-V) characteristics of molecular junctions. In particular, the offset ε of the Fermi level relative to the appropriate frontier molecular orbital (HOMO in this case) and the electrode-molecule coupling strength Γ are recognized as two main factors that determine the electrical properties of a typical molecular junction. The seminar will show that a compact analytical model derived from the Landauer formalism provides a quantitative fit to the I-V data and yields values of ε and Γ that vary systematically with molecular structure and choice of electrode materials. Furthermore, Ultraviolet photoelectron spectroscopy (UPS) was employed to determine the ε to be compared with the values predicted by the model from transport data. This provides a consistency check and substantiation of the proposed transport-electronic structure relationship. Finally, I will show the correspondence of ε derived from transport and spectroscopic measurements and how these findings provide important insight into the mechanisms of energy level lineup in molecular junctions, specifically the roles of image charge effects versus metal-molecule chemical bonding on the magnitude of ε.