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Ultradense Molecular Memory

Researchers develop a large-scale array of nanoscale memory circuits.
January 24, 2007

Researchers at Caltech and University of California, Los Angeles, have built memory chips that pack more than 50 times more bits into a given area than those currently in production. The work is a strong demonstration that molecular electronics using molecules and nanowires can be employed to make large arrays of memory bits.

A colored micrograph of the largest-ever array of memory bits made of molecular switches. The switches are addressed by two sets of nanowires that cross in the light-blue square. (The individual nanowires are too small to see here.) Spoke-like test electrodes converge on the array.

Researchers have already assembled simple memory devices that use molecular switches, but these previous circuits incorporated at most a few thousand bits. (See “Molecular Computing.”)

The California researchers made molecular memory that integrates 160,000 devices for storing bits of data. That’s still just 20 kilobytes–small by today’s memory standards. But the bits are packed much more densely than in today’s memory technology. According to the International Technology Roadmap for Semiconductors, memory chips in 2006 had 1.79 gigabits per square centimeter. The new work reaches 100 gigabits per square centimeter, a density that the Roadmap forecasts chips won’t reach until sometime after 2020.

“We thought that if we weren’t able to make something at this scale, people would say that this is just an academic exercise,” says James Heath, professor of chemistry at Caltech and one of the researchers on the project. “There are problems still. We’re not talking about technology that you would expect to come out tomorrow. We’re talking about hitting the benchmark that is twenty years off or so. So you’ve got time.”

The work is part of a growing effort to find nano-based alternatives to conventional silicon electronics, as chip makers pack more and more devices, such as transistors, onto chips. Current silicon-based technology is facing limits in terms of how much more it can be miniaturized. So researchers are turning to new approaches, including chemical and physical techniques that assemble regular patterns of nanowires with atomic-level control. They’ve also developed new chemical synthesis techniques to make molecules that act as switches. (See “Molecular Memory.”)

In the new chips, a layer of molecules is sandwiched between two layers of 400 nanowires each. The nanowires run perpendicular to each other, forming a grid. Where two wires intersect, they deliver electronic signals that read or write information to the molecular switches.

The chip uses molecular switches developed by J. Fraser Stoddart, the head of nanosystems science at UCLA, to store data. The switch molecules, called rotaxane, are barbell-shaped, with a ring of atoms that moves between two stations on the bar, its position depending on the voltage applied. The conductivity of the molecule changes according to the location of the ring, and these two distinct states represent the ones and zeros that make up memory.

Heath and his colleagues at Caltech then developed techniques for incorporating these molecules into a memory chip. Taking advantage of the fact that one end of the molecules is attracted to water while the other end is repelled by it, the researchers were able to arrange the molecules so that they were all oriented in the same direction in a layer just one molecule thick.

Much work remains to be done before the method can be useful for practical electronics. Of the bits tested in the device, nearly 75 percent didn’t work for a variety of different reasons. Furthermore, those that did work could only be switched a few times before failing.

Still, experts say the current work is already an important step forward for molecular electronics. Although molecules and nanowires still can’t compete with the manufacturability of silicon chips, the new work “takes molecular electronics to the next level,” says James Tour, professor of chemistry and computer science at Rice University. To achieve such densities in the memory devices, he adds, is “impressive.”

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