Molecular diodes bring single molecule devices a little closer.

Molecular electronic devices are a step closer, thanks to the efforts of chemists from Cranfield University, UK.

Interest in molecular scale electronics is widespread, with many researchers pursuing the holy grail of miniaturising integrated circuits down to the molecular level.

The diode is a basic electronic component: a device with asymmetric current-voltage (I-V) characteristics - it is harder for an electric current to travel through in one direction than the other. Diodes are used to 'rectify' current, changing alternating current into direct current. Conventional diodes are usually semiconductors, but molecular rectification, using organic molecular diodes, has recently become the subject of intense theoretical and experimental attention.

With this in mind, Geoff Ashwell and his team have been investigating the requirements for molecular rectification. Molecular diodes comprise a donor-(electron bridge)-acceptor sequence, contacted by non-oxidisable electrodes. When an appropriate voltage is applied, electrons tunnel from electrode to acceptor at one end of the device, and from donor to electrode at the opposite end.

Ashwell has described the I-V characteristics of self-assembled monolayers of two dyes, where the donor is connected to the acceptor by a pi-electron bridge: the only difference between the dyes was that one had a substituted methyl group. He discovered that only the sterically hindered methyl-substituted derivative exhibits rectification, and attributes this to the dye's altered molecular geometry.

More specifically, Ashwell believes the methyl group causes out-of-plane rotations of the acceptor and donor portions, suppressing through-molecule orbital overlap. Further experiments showed that the rectifying behaviour can be reversibly switched through exposure to acidic and basic media, and that altering the linking group does not affect the rectification. These findings contradict recent theoretical predictions by researchers from the University of Alabama, US, and are sure to spark further studies.

Despite these developments, there are still many challenges ahead before the dream of single molecule circuit components becomes a reality. Ashwell admits: 'It is necessary to improve the current rectification ratio. However, self-assembled monolayers of donor-(electron bridge)-acceptor molecules appear to provide a link to the ultimate challenge in device miniaturisation.'

Philip Earis