Controlling the Electronic Contact at the Terpyridine/Metal Interface

Terpyridine derivatives reveal rich coordination chemistry and are frequently used to construct reliable metallo-supramolecular wires, which are promising candidates for optoelectronic or nanoelectronic devices. Here, we examine especially the terpyridine/electrode interface, which is a critical point in these organic/inorganic hybrid architectures and of utmost importance with respect to the device performance. We use the approach to assemble nanodevices by immobilization of single terpyridine-functionalized gold nanoparticles with a diameter of 13 nm in between nanoelectrodes with a separation of about 10 nm. Conductance measurements on the formed double-barrier tunnel junctions reveal several discrete conductance values in the range of 10–9–10–7 S. They can be attributed to distinct terpyridine/electrode contact geometries by comparison with conductance values estimated based on the Landauer formula. We could clearly deduce that the respective terpyridine/metal contact determines the length of the tunneling path through the molecule and thus the measured device conductance. Furthermore, the formation of a distinct terpyridine/electrode contact geometry correlates with the chemical pretreatment of the terpyridine ligand shell of the gold nanoparticles with an alkaline solution. By applying infrared reflection absorption spectroscopy, we found that only a chemical treatment with a concentrated ammonia solution results in effective deprotonation of the terpyridine anchor group. This enables the electrical contact to the middle pyridyl ring and thus a short tunneling path through the molecule corresponding to a high conductance value. These findings indicate a way to control the contact geometry at the terpyridine/metal interface, which is a prerequisite for reliable nanodevices based on this class of molecules.