http://www.nature.com/nsu/010607/010607-7.html

physics: Tiny minds
PHILIP BALL

Researchers have developed prototype computer memories in which information is recorded, read and erased by molecular switches. Computer memories that store information in single molecules could be far more powerful than those of today's machines.

Mark Reed of Yale University in New Haven, Connecticut, and colleagues built molecular switches consisting of rod-like organic (carbon-based) molecules that carry a current between two gold electrodes. The molecules are like tiny wires, each more than a thousand times smaller than the miniaturized transistors used as switches in silicon chips.

Initially the molecules don't conduct electricity very well. But by applying a voltage pulse to one of the electrodes, the molecules can be kicked into another state in which their electrons are arranged differently. This new state is a better conductor.

These high- and low-conductivity states are like the 'on' and 'off' positions of a switch. An array of molecules can store information by using the on and off states to represent the 1's and 0's used by computers to store information.

Subsequent voltage pulses can read the stored information by determining whether a molecule is on or off. Information can be erased by applying a negative voltage pulse, which transforms the molecules back to their low-conductivity (off) state. So the device can function like a random-access memory (RAM), which can be repeatedly written and erased.

At present, Reed and colleagues make their memory devices by sandwiching the molecules between a flat upper electrode and a dome-shaped lower electrode. These electrodes make contact over a roughly circular area 30–50 millionths of a millimetre wide, which contains about a thousand molecules. All of the molecules are switched together by the voltage pulses applied to the electrodes. So each bit of information is stored in a thousand molecules.

This already makes for a smaller memory device than a transistor. But the researchers say that, if molecules could be wired up individually rather than in groups, there is no reason why each could not be switched independently, so that each molecule encodes a bit. To make these memories practical, however, the team will also have to worry about issues such as switching speed and long-term stability.

Reed, M. A., Chen, J., Rawlett, A. M., Price, D. W. & Tour, J. M. Molecular random access memory cell. Applied Physics Letters 78, 3735–3737 (2001).

© Macmillan Magazines Ltd 2001 - NATURE NEWS SERVICE

http://www.nature.com/nsu/010607/010607-2.html

technology: You’ve got mail
PHILIP BALL

If Sir Lancelot were eight inches high, it would suit him to a tee. But the miniature chain-mail made by researchers at Harvard University in Cambridge, Massachusetts, is not intended to set a new fashion among pint-sized knights. Instead, it demonstrates the researchers’ scheme for making complex three-dimensional ‘microstructures’, with component parts on a scale of millimetres or less.

Carving out tiny shapes such as hoops is no problem for today’s engineers, who have an array of delicate methods to assist intricate feats of construction. Using ‘lithographic’ procedures, masking part of a surface with a patterned coating and dissolving the rest away, microelectronic engineers routinely make circuit components just a few hundred millionths of a millimetre wide.

The difficulty arises when the components have to cross over or link with one another. There is a world of a difference, for example, between carving a flat Greek phi (F) sign and making a similar structure in which the bar passes under one side of the hoop and over the other.

Chain-mail is an extreme version of a sheet-like structure that is actually three-dimensional. All the hoops are separate entities, held together by being woven under and over their neighbours. This is why chain-mail can crumple and fold like fabric.

George Whitesides and colleagues produced the interwoven structure of chain-mail using standard lithographic methods to make two-dimensional structures on different surfaces, and then joining them together1. Their links, each as thin as a hair, are not circular but stadium-shaped, with straight sides and round ends.

Wu, H., Whitesides, S. & Whitesides, G. Fabrication of micro-chain mail by simultaneous, patterned electrodeposition on a plane and multiple cylinders. Angewandte Chemie International Edition 40, 2059–2060 (2001).

© Macmillan Magazines Ltd 2001 - NATURE NEWS SERVICE