MCBJs contain a nanosized electrode gap that can be used to measure single-molecule electron transport in solution. Researchers have also recently started to use the devices to identify biomolecules like nucleotides too and have found that the four different types of nucleobases in DNA can be clearly distinguished thanks to the fact that they generate distinct tunnelling currents as they flow through the electrode gap.

However, there is a problem because these molecules produce relatively large ionic currents at the electrode surface that appear as a slowly changing background current. Although this ionic contribution to the overall tunnelling current can be subtracted after collecting data, it would be much better to supress it during the actual measurements. Such a strategy would not only make interpreting the electrical signals from single molecules more straightforward, but it would also lead to less noise in the signal being analysed – something that is crucial for when it comes to detecting the tiny tunnelling currents generated by single molecules.

The Osaka team, led by Masateru Taniguchi, made its MCBJ by first forming a polyimide layer on a phosphor bronze substrate. The researchers then covered the polyimide layer with a SiO2 film 25 nm thick that had been produced by chemical vapour deposition. “The fabrication technique is almost the same as that used to make conventional lithographed BJs but the main difference is that we cover the polyimide-coated metal substrate with SiO2,” explained Taniguchi. “Conventional BJs are usually formed directly on the polyimide layer.”

The researchers then coated the break junction with a Cr/Au/Cr layer and covered this layer with SiO2 too so that the entire surface of the device is fully protected by the insulator.

Fresh fracture surfaces

The team made the actual BJ by simply bending the phosphor bronze substrate and breaking the junction at the narrowest constriction. “As a result, we obtain fresh fracture surfaces of Au that are just nanometres across and that can be used to measure tunnelling currents in solution,” said Taniguchi. “The fact that the rest of the Au surface is covered with SiO2 minimizes the ionic contribution to the overall current measured.”

The device could be ideal for measuring tunnelling currents from single biomolecules such as DNA and RNA. “And since our SiO2 covering mitigates the ion current during these measurements, it could thus also help improve signal-to-noise ratios when identifying the molecules,” he told nanotechweb.org.

The MCBJ is detailed in the Journal of Applied Physics.