Relating Energy Level Alignment and Amine-Linked Single Molecule Junction Conductance

TL;DR

This study links the energy level alignment at metal-molecule interfaces with single-molecule conductance, using spectroscopy, experimental measurements, and first-principles calculations to reveal correlations and shifts in HOMO levels.

Contribution

It provides the first direct comparison between spectroscopic energy level measurements and single-molecule junction conductance data.

Findings

HOMO level positions correlate with conductance changes.

HOMO levels shift further from Fermi level on Au(110).

Results agree with quasiparticle energy calculations.

Abstract

Using photoemission spectroscopy, we determine the relationship between electronic energy level alignment at a metal-molecule interface and single-molecule junction transport data. We measure the position of the highest occupied molecular orbital (HOMO) relative to the Au metal Fermi level for three 1,4-benzenediamine derivatives on Au(111) and Au(110) with ultraviolet and resonant x-ray photoemission spectroscopy. We compare these results to scanning tunnelling microscope based break-junction measurements of single molecule conductance and to first-principles calculations. We find that the energy difference between the HOMO and Fermi level for the three molecules adsorbed on Au(111) correlate well with changes in conductance, and agree well with quasiparticle energies computed from first-principles calculations incorporating self-energy corrections. On the Au(110) which present Au atoms with lower-coordination, critical in break-junction conductance measurements, we see that the HOMO level shifts further from the Fermi level. These results provide the first direct comparison of spectroscopic energy level alignment measurements with single molecule junction transport data.