ELETTRA is a multidisciplinary Synchrotron Light Laboratory in AREA Science Park, open to researchers in diverse basic and applied fields. The laboratory is equipped with ultra-bright light sources in the spectral range from UV to X-rays and offers a stimulating and competitive environment to researchers from all over the world.

Understanding the electronic structure of individual molecules bonded between metal electrodes is critical to progress in molecular electronics. Typically, the electronic structure of molecular junctions is inferred from low-bias conductance measurements. However, transport measurements are at best indirect probes of the alignment of the molecular orbitals to the metal Fermi level, due to additional physical effects such as charge transfer and rearrangement, hybridization, and electrode polarization. We overcome these limitations by combining the break-junction measurements of single molecule conductance to the photoemission spectroscopy (XPS) from molecular monolayer and sub-monolayer grown on metal surfaces. By means of XPS, 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. The data correlate well with changes in conductance and agree well with quasiparticle energies computed from firstprinciples calculations incorporating self-energy corrections. The results we present provide the first direct comparison of spectroscopic energy level alignment measurements with single molecule junction transport data.

Nano Letters, 2010, 10, 2470–2474