Nervous energy

The emerging technology of plastic electronic circuits has been boosted by the construction of the first molecular diodes.

The diodes, which have high production yields, are just one molecule thick and while similar structures appear in nature they have always been beyond the capabilities of any man-made machine or process — until now.

Scientists hope molecular electronics will enable them to build a working circuit in a single layer of molecules and embed it in a host material. They appear in nature as self-organised structures for conducting an electrical charge, as in mammals’ nerve systems and plant photosynthesis.

If artificial processes mimicking nature’s designs could be perfected there would be a ready market for the technology, particularly now that plastic products such as flexible and roll-up displays, are being commercialised. An added attraction is that they could be fabricated at low temperatures.

But making them has not been easy. Layers of single molecules are prone to flaws — with gaps forming spontaneously as the material organises itself. Well-defined diodes made through molecular electronics can only be created when the molecules are sandwiched between two metallic electrodes. The trouble is that the flaws lead to short circuits, with the current easily crossing the 1-2nm thick gap that opens up between the electrodes. So scientists at Philips Research and the University of Groningen, in the Netherlands, came up with a new strategy.

They coated one surface of a gold electrode with a photoresist polymer film and made holes in it of a defined size. The molecular diode material, alkanethiol, was put in the hole and the fact that it was confined by the surrounding polymer prevented any gaps from forming. Then a second gold electrode was put on top. The alkanethiol was shown to work flawlessly.

‘Based on a molecular self-assembly process we have developed a reliable way to fabricate well-defined molecular diodes,’ said Groningen‘s Dr Bert de Boer. ‘It will enable us, for the first time, to do reliable and reproducible measurements on molecular junctions, which is essential for the exploration of molecular electronics applications.’

The next step is to replace the alkanethiol with a functional material. ‘It was used to prove the process,’ said a Philips spokesman. ‘It has no specific electronic function and we will use something which has a specific electronic capability so that we can create a real rectifying diode.’

It has taken two years to prove the new process and plastic electronics is only now emerging after 10 years in development, so molecular electronics will not be commercialised for some time.

‘Molecular electronics will not compete with current silicon-based integrated circuit technologies,’ said Philips’ Dago de Leeuw. ‘They could be an interesting option for manufacturing plastic electronics, which themselves are very promising where low temperature or low cost in-line processing techniques are required.’