In molecular electronic devices, a critical step is making the connection between the active molecules and a pair of electrical terminals. For vertical devices, a connection can be made by depositing a layer of metal on top of the molecular layer, but the quality of the interface is unpredictable, and short circuits often occur. An alternative is to build a lateral device, but here the problem is that the smallest achievable electrode separation is about 6 nm, which is much larger than the length of the typical molecules of interest.
Lin and Kagan solve the lateral problem by using a layer-by-layer deposition process to grow a metal-metal paddlewheel complex based on ruthenium. This sort of complex has been shown to undergo multiple redox processes, with extensive coupling between the metal centers, and can be tuned by changing the ligand chemistry. The authors demonstrate that the complexes can be layered onto two gold electrodes until the intervening gap of 60 to 80 nm closes. Measurement of the current-voltage characteristics showed a voltage-controlled negative differential resistance (a rise and then a fall of current as voltage was increased). However, this was observed only during the first cycling of the device, possibly due to irreversible oxidation of the ruthenium units in the solid state or to the use of symmetric electrodes. -- MSL
J. Am. Chem. Soc. 10.1021/ja028653y (2002).
