Correlating electrical properties and molecular structure of SAMs organized between two metal surfaces

We describe the results obtained on a series of junctions that are formed by mercury electrodes. The junctions comprise self-assembled monolayers (SAMs) sandwiched between two metal electrodes, i.e. metal (mercury)-SAM//SAM-metal (mercury, gold or silver) junctions. We describe three different variations on this type of Hg-based junction. The first junction, formed by bringing into contact two mercury drops covered by the same type of SAM, is a prototype system that provided useful information on the structure and electrical properties of the Hg-based junctions. The second junction consists of a Hg drop covered by one SAM (Hg-SAM(1)) in contact with a second SAM supported on a silver film (Ag-SAM(2)) -- that is, a Hg-SAM(1)//SAM(2)-Ag junction. This junction (for constant SAM(1)) allowed systematic measurements of the current that flowed across SAM(2) as a function of its chemical structure. The same type of junction, when comprising a transparent solid metal electrode, allows to irradiate through the transparent surface photoactive units organized in a SAM and to measure the current photoresponse. The third type of junction, Hg-SAM//R//SAM-Hg (or Hg-SAM//SAM-Hg for redox-active SAMs), is an electrochemical junction that can i) trap redox-active molecules (R) in the interfacial region between the SAMs, and ii) control the potential of the electrodes with respect to the redox potential of R using an external reference electrode. This junction becomes conductive when the electrode potentials are adjusted to the formal potential of the redox centers, and it shows diode- and transistor-like characteristics analogous to those of solid-state devices.