Molecular logic proposed

By Eric Smalley, Technology Research News

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.

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 both 1s the gate returns an output of 1. If the inputs are different or both 0s, the output is 0. An XOR gate returns a 1 if either but not both of the inputs are 1.

The molecular circuits also have the potential to perform more complex functions, said Stadler. "We envision [a] move towards complexities where a far larger number of inputs can be processed through a single molecule," he said.

The researchers hit upon the idea while they were trying to build another molecular logic design and encountered problems with parts of molecules interfering with each other. The previous attempt copied the structure of larger, diode-based logic circuits, said Stadler. "There we encountered problems with quantum interferences," he said. "So we decided to try to use those interferences for information processing rather than trying to avoid them."

The logic scheme works because a molecule guides electrons passing through it as quantum waves rather than as particles passing through larger electrical circuits. "By changing the chemical side-groups, the geometry of the waveguide can be tuned," said Stadler. Depending on how the side-groups are rotated, they are coupled or decoupled from the cloud of electrons associated with the benzene molecule. The side-group positions represent inputs and two side groups provide the requisite number of inputs for the two basic logic gates.

Changing the electronic structure of the molecule by rotating the side groups changes the interference pattern of the different paths the electron waves can travel through the molecule, which changes the electrical conductance of the molecule. The conductance levels represent logic gate's output.

There are several technical challenges to implementing even a simple version of the molecular logic, said Stadler. 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," he said. "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.

At this early stage, it is difficult to predict exactly how well such molecular devices will work. In addition to the performance of a single molecular system, interconnections among the molecules and a means of connection to the larger world must be taken into consideration, said Stadler. "These interconnections and the detailed computer architecture of the whole system will be [the] limiting factors for performance in density rather than properties of the molecules themselves," he said.

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.

Stadler's research colleagues were S. Ami and Christian Joachim at CNRS in France and Michael Forshaw at University College London. The work appeared in the January 23, 2004 issue of Nanotechnology. The research was funded by the European Community and the Consortium for Hamiltonian Intramolecular Computing.

Timeline:   10 years
Funding:   Government
TRN Categories:  Biological, Chemical, DNA and Molecular Computing
Story Type:   News
Related Elements:  Technical paper, "Integrating Logic Functions inside a Single Molecule," Nanotechnology, January 23, 2004


March 24/31, 2004

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