Logic Functions Within A Single Molecule

By Roland Piquepaille

Molecular computing is a fascinating subject that I already covered several times (check here or there for example). Now, researchers from France and England are going further computing. They want to make logic circuits by linking individual molecules. They even think that they can build these logic circuits within a single one, according to this article from Technology Research News. But challenges remain and practical applications will not appear before at least ten years.

Researchers from the French National Center for Scientific Research (CNRS) and University College London in England have devised a scheme for designing logic circuits within individual molecules.

The scheme could eventually be used to produce small, fast computers and to store large amounts of data in very small spaces. The method could also be modified to make sensors for detecting individual molecules.

Here are some details about their approach.

The researchers' plan calls for connecting a pair of benzene molecules to two gold electrodes. The molecules contain nitrogen-oxygen side groups whose rotational positions can represent the 1s and 0s of computer information. The researchers' simulations show that the set-up would allow for simple two-input logic gates like AND and XOR, said Robert Stadler, an assistant professor of physics and astronomy at University College London.

An AND gate contains two inputs and one output. If the inputs are the same -- either a pair of 0s or a pair of 1s -- the gate returns an output of 1. If the inputs are different, the output is 0. An XOR gate returns a 1 if either or both of the inputs are 1.

This diagram shows a benzene molecule configured as an XOR logic gate. Each sphere represents an atom. The red and dark blue spheres represent the molecule's side groups, which can be rotated to change the molecule's electrical conductance. The green shapes are electrodes (Credit: University College London).

But even the researchers admit they face numerous challenges before being able to deliver a product.

The first challenge is bringing a large number of electrodes within nanometers of each other. A nanometer is one millionth of a millimeter. "Gap sizes of about five nanometers between two electrodes are now possible," said Stadler. "But for more than two electrodes this of course becomes increasingly difficult."

A second challenge will be positioning the molecules on electrodes, said Stadler. "In situ manipulation of the molecules when they are anchored on electrodes is the next big hurdle," he said.

Finally, to construct a working information processing or storage system, molecules must be interconnected. "This raises a large number of architectural and manufacturing issues," said Stadler.

So when will we see some real products based on these findings? Not soon.

Practical applications for molecular electronics are more than a decade away, said Stadler. They "should not be expected before 2015," he said.

Even further down the road, molecular electronics could be coaxed to interact with a chemical environment, said Stadler. "Prospects for medical applications, where molecular devices could be linked to bio-chemical processes would be very exciting," he said. These possibilities won't be realized anytime soon, he added.

The research paper has been published by Nanotechnology in its April 2004 issue under the name "Integrating logic functions inside a single molecule." You can read the abstract here.

Source: Eric Smalley, Technology Research News, March 24/31, 2004; and various websites