Structural Characterization of a Molecular Junction by X-ray Reflectivity

In the field of molecular electronics, the nature and pathways of charge transfer through molecules is among the most intensely studied open questions. Experimental studies, employing both single molecules and self-assembled monolayers attached to the electrodes, have demonstrated that the electronic characteristic of these junctions is difficult to reproduce. We report x-ray reflectivity studies of the structure of organic mono- and bi-layers self-assembled between two conducting electrodes: silicon and mercury. At high molecular coverage, the Angstrom resolution high-energy synchrotron x-ray measurements reveal densely-packed layers of roughly interface-normal molecules. The interface normal structure is stable and relatively insensitive to electric fields when a voltage is applied across the junction. Furthermore, our x-ray studies reveal that variation of the molecular coverage of the electrodes influences the structure and quality of the molecular junctions.