Ionic Liquid Based Approach for Single-Molecule Electronics with Cobalt Contacts

An electrochemical method is presented for fabricating cobalt thin films for single-molecule electrical transport measurements. These films are electroplated in an aqueous electrolyte, but the crucial stages of electrochemical reduction to remove surface oxide and adsorption of alkane­(di)­thiol target molecules under electrochemical control to form self-assembled monolayers which protect the oxide-free cobalt surface are carried out in an ionic liquid. This approach yields monolayers on Co that are of comparable quality to those formed on Au by standard self-assembly protocols, as assessed by electrochemical methods and surface infrared spectroscopy. Using an adapted scanning tunneling microscopy (STM) method, we have determined the single-molecule conductance of cobalt/1,8-octanedithiol/cobalt junctions by employing a monolayer on cobalt and a cobalt STM tip in an ionic liquid environment and have compared the results with those of experiments using gold electrodes as a control. These cobalt substrates could therefore have future application in organic spintronic devices such as magnetic tunnel junctions

Single-Molecule Electrochemical Gating in Ionic Liquids

The single-molecular conductance of a redox active molecular bridge has been studied in an electrochemical single-molecule transistor configuration in a room-temperature ionic liquid (RTIL). The redox active pyrrolo-tetrathiafulvalene (pTTF) moiety was attached to gold contacts at both ends through −(CH₂)₆S– groups, and gating of the redox state was achieved with the electrochemical potential. The water-free, room-temperature, ionic liquid environment enabled both the monocationic and the previously inaccessible dicationic redox states of the pTTF moiety to be studied in the in situ scanning tunneling microscopy (STM) molecular break junction configuration. As the electrode potential is swept to positive potentials through both redox transitions, an ideal switching behavior is observed in which the conductance increases and then decreases as the first redox wave is passed, and then increases and decreases again as the second redox process is passed. This is described as an “off–on–off–on–off” conductance switching behavior. This molecular conductance vs electrochemical potential relation could be modeled well as a sequential two-step charge transfer process with full or partial vibrational relaxation. Using this view, reorganization energies of ∼1.2 eV have been estimated for both the first and second redox transitions for the pTTF bridge in the 1-butyl-3-methylimidazolium trifluoromethanesulfonate (BMIOTf) ionic liquid environment. By contrast, in aqueous environments, a much smaller reorganization energy of ∼0.4 eV has been obtained for the same molecular bridge. These differences are attributed to the large, outer-sphere reorganization energy for charge transfer across the molecular junction in the RTIL

Single-Molecule Conductance Behavior of Molecular Bundles

Controlling the orientation of complex molecules in molecular junctions is crucial to their development into functional devices. To date, this has been achieved through the use of multipodal compounds (i.e., containing more than two anchoring groups), resulting in the formation of tri/tetrapodal compounds. While such compounds have greatly improved orientation control, this comes at the cost of lower surface coverage. In this study, we examine an alternative approach for generating multimodal compounds by binding multiple independent molecular wires together through metal coordination to form a molecular bundle. This was achieved by coordinating iron(II) and cobalt(II) to 5,5′-bis(methylthio)-2,2′-bipyridine (L1) and (methylenebis(4,1-phenylene))bis(1-(5-(methylthio)pyridin-2-yl)methanimine) (L2) to give two monometallic complexes, Fe-1 and Co-1, and two bimetallic helicates, Fe-2 and Co-2. Using XPS, all of the complexes were shown to bind to a gold surface in a fac fashion through three thiomethyl groups. Using single-molecule conductance and DFT calculations, each of the ligands was shown to conduct as an independent wire with no impact from the rest of the complex. These results suggest that this is a useful approach for controlling the geometry of junction formation without altering the conductance behavior of the individual molecular wires