Molecular electronics boost miniaturisation

As electronic devices are being scaled smaller and smaller, scientists are beginning to hit the limits of what silicon-based transistors can achieve. The field of molecular electronics offers a way around this problem, by using single molecules, each of the exact same chemical make up, to construct electronic devices.

While this can be demonstrated in the lab, it’s not practical on larger scale, primarily because of the difficulty in making reliable electrical contacts between molecules and conducting metals to create circuits. Essentially, the components exist, but it’s the “wiring” of the system that causes a bottleneck.

Publishing in the journal Nature, a collaborative team of IBM scientists and researchers from the Universities of Basel and Zurich, Switzerland and Macquarie University in Australia have developed a technique for fabricating molecular electronics that appears to solve this problem.

Manipulating individual molecules is an increasingly possible, yet still daunting task for chemists, and currently impossible on a commercial scale. The Swiss-led team have addressed this problem using a property possessed by some molecules called self-assembly.

Molecular self-assembly is a process where molecules can arrange themselves into complex and well defined arrangements, without the control of an outside force. A good example of this is lipid bi-layers, in which the fats that make up the walls of biological cells will assemble into two-molecule-thick layers when placed in water.

The research team, led by Basel’s Gabriel Puebla-Hellmann, used a similar type of self-assembly to deposit a single-molecule layer onto a platinum surface. The molecules comprised chains of carbon atoms, with a sulfur group on each end.

The chains self-assembled onto the metal surface, standing upright, with one sulfur atom attached to the platinum and the other exposed. The effect was a little like a dense clump of reeds, albeit at a much, much smaller scale.

With this single-molecule layer complete, the researchers could then create a second metal layer on top, by passing a solution of gold nanoparticles over it. The nanoparticles adhered to the exposed sulfur atoms, creating a new metallic layer. This created a sandwich-like metal-metal structure, separated by only a film of molecules.

The team went on to fabricate miniature electronic devices by coating a surface of platinum with an insulating layer, and then etching tiny pores only 60 nanometres wide into the insulator, exposing the metal.

After depositing the single-molecule layer and gold nanoparticles inside the pores, the gold was further coated with conducting metal by a conventional technique called vapour deposition.

The work may open the way for molecular electronic devices such as ultra-miniaturised transistors, and also lead to artificial neurons able to exploit quantum effects.

“Molecular electronics hasn’t previously lived up to expectations, but we’ve seen a renaissance of the field in the last five to six years,” says Koushik Venkatesan, from Macquarie, one of the authors of the study.

“The device platform is the missing link. We hope work like ours will accelerate this type of technology.”