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

Abstract 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))—i.e., 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.