"This provides a novel method to do reproducible and reliable electrical measurements on a single molecular layer," Bert de Boer of the University of Groningen told nanotechweb.org. "Incorporation into state-of-the-art electronic circuits is still far away, although the processing techniques used – photolithography, RIE [reactive ion etching], spin coating, vapour deposition – are fully compatible with industrial standards and could be incorporated today."
Boer and colleagues made the junctions by applying a layer of photoresist to a gold electrode. They patterned holes into the photoresist before filling them with a self-assembled monolayer (SAM) of an alkane dithiol. The next step was to coat the SAM with a 90 nm thick layer of the conductive polymer PEDOT:PSS (poly(3,4-ethylene-dioxythiophene stabilized with poly(4-styrenesulphonic acid)). Finally, the researchers deposited a top gold electrode.
Two crucial factors behind the success of the technique are the incorporation of the monolayer into vertical interconnects (or via holes), which prevents parasitic currents and interaction of the monolayer with the environment, and the addition of the conducting polymer layer between the SAM and the top gold electrode, which stops electrical shorts.
"Because we process our devices in via holes, the active layer – the self-assembled monolayer – is shielded from its surroundings," said de Boer. "When leaving our samples in air for 75 days, the current density versus voltage (J-V) characteristics are identical to the characteristics directly after fabrication."
According to de Boer, the scientists had to use the simplest type of molecules – alkane dithiols – in order to test the technique. "In such a system the electron transport mechanism is known and well established," he said. "We will continue our research in the direction of more interesting molecules, such as semiconducting molecules."
The researchers reported their work in Nature.