Plastic memory promises cheap, dense storage

[Wise Young]

http://www.newscientist.com/news/news.jsp?id=ns99994379

Plastic memory promises cheap, dense storage
12:25Â*13Â*NovemberÂ*03

NewScientist.com news service

A conducting plastic has been used to create a new memory technology with the potential to store a megabit of data in a millimetre-square device - 10 times denser than current magnetic memories. The device should also be cheap and fast, but cannot be rewritten, so would only be suitable for permanent storage.

The device sandwiches a blob of a conducting polymer called PEDOT and a silicon diode between two perpendicular wires. Substantial research effort has focused on polymer-based transistors, which could form cheap, flexible circuits, but polymer-based memory has received relatively little attention.

The key to the new technology was the discovery by researchers from Princeton University, New Jersey, and Hewlett-Packard Laboratories in Palo Alto, California, that passing a high current through PEDOT turns it into an insulator, rather like blowing a fuse. The polymers two possible states, conductor or insulator, then form the one and zero necessary to store digital data.

"The beauty of the device is that it combines the best of silicon technology - diodes - with the capability to form a fuse, which does not exist in silicon," says Vladimir Bulovic, who works on organic electronics at the Massachusetts Institute of Technology.

However, turning the polymer INTO an insulator involves a permanent chemical change, meaning the memory can only be written to once. Its creators say this makes it ideal for archiving images and other data directly from a digital camera, cellphone or PDA, like an electronic version of film negatives.

Ion snatch

PEDOT's ability to conduct electricity means it is already used widely as the anti-static coating on camera film. But until now, no one suspected that it could be converted into an insulator.

The material is a blend of a negatively-charged polymer called PSS- and a positively-charged one called PEDT+. Having distinct, charged components allows it to conduct electricity and means that it is water soluble.

The team is not sure why it stops conducting when high currents pass through. But Princeton researcher Stephen Forrest suspects that the heat produced by a high current gives the PSS- layer sufficient energy to snatch a positively-charged hydrogen ion from any water that has dissolved on its surface, forming a neutral PSSH.

Without the negatively-charged PSS- to stabilise it, PED+ in turn grabs on to an extra electron and also becomes neutral, converting PEDOT into an insulating polymer.

To store the memory, the researchers use the wires and the diode surrounding the PEDOT blob to run either a high or a low current through it. This either creates an insulator or leaves it as a conductor.

To read the memory, they run current through the top wire and measure the current in the bottom wire. No current means the bit is a zero, and vice versa.

In their paper in Nature, the researchers describe just one such junction. But for a memory application, the device will need many more. So the Hewlett-Packard team is now working on building a grid of intersecting wires. In this way, they can read and write multiple bits to one device. A grid system is commonly used in display screens to switch individual pixels.

Polymer devices can sprayed or printed, and are therefore much cheaper than silicon devices, which must be etched.

Celeste Biever