Molecule makes mini memory

By Chhavi Sachdev, Technology Research News

If you’re reading this on a computer, these words are stored in memory that is made of transistors and capacitors, and grouped into chips that measure an inch or so. The most common type of memory, dynamic random access memory (DRAM), needs to be refreshed thousands of times per second to prevent the words from fading away.

Researchers at Pennsylvania State University and Rice University are working on computer memory that looks and works very differently. They have demonstrated that they can change the electrical conductance of a single molecule to use it as a switch to perform the same function as a transistor.

“We have demonstrated that single molecules can switch. So switching and memory could be scaled to the single molecule level - one million times smaller than the smallest transistor,” said James Tour, a professor of chemistry at Rice University.

Memory made from molecular switches would require little power and hold information for hours at a stretch, according to the researchers.

In the past, scientists have demonstrated switching in bundles of thousands of molecules. The Penn State and Rice researchers were able to attain switching in a single phenylene ethynylene oligomer molecule by changing the spatial arrangement of its atoms and thereby its conductance, said Paul Weiss, an assistant professor of chemistry at Pennsylvania State University. “The specific conformational changes are not known,” he said. “It might be [a change in] tilt or an internal motion.”

The molecules, which are two nanometers long and half a nanometer across, can retain the change for as long as 26 hours. "If we made memory out of these switches, the persistence time determines how often we need to refresh memory," said Weiss. “The persistence time in a state prior to switching ranges from fractions of seconds up to tens of hours-depending largely upon the tightness of the environment around them,” he said.

The researchers demonstrated the molecules’ switching ability by anchoring them in a matrix of a alkanethiol layer on a gold substrate, then passing a current through them. “[We] add an electron by applying a voltage, and the switch turns on - it is in a conductive state,” said Tour. Removing the electron makes the molecule non-conductive, which is the off position.

Like soldiers standing at attention in a crowd, when the switches are on they appear to become taller and straighter than their neighbors in the matrix. When the switches are off they are at ease.

The researchers found that molecules packed in a tight matrix stayed in the on position longer than those in a loosely packed, poorly ordered matrix. “Conformation can affect the switch hold time in the on state,” he said. Think of the molecules as marbles. If you don’t confine them, the slightest thing makes them wobble around. Put them in a box, they settle in and don’t move.

The researchers got the idea to use the molecules as switches from an experiment five years ago that involved positioning a related series of molecules under a Scanning Tunneling Microscope (STM), said Weiss. Just as in the later experiment, one end of the oligomers was bound to a gold electrode through a sulfur atom. “They are prevented from lying flat on the gold by the alkanethiolate matrix,” he said. The molecules stand off the surface in a tilted, but ordered way, he said.

“Our idea was to hook up metal crystal as one electrode and the tip of our STM as the other electrode, [but] it turned out to be a little more complicated,” said Weiss. “When we mixed those two molecules together on the surface, not only did the molecules we wanted to study not stand up, but the other molecules [did not remain] in any sort of ordered array,” he said. To rectify the mess, one of the researchers used defects in the matrix as places to insert and anchor the molecules.

“In the intervening five years, we’ve gotten very good at controlling the type of defect, the size of the defect and the number of defects,” said Weiss. By controlling the defects, the researchers can manipulate the number of molecules that will be in the field of view of the STM, he said.

A central issue is the precise gap between the tip of the STM and the molecule, said Weiss. “We’ve learned how to interpret the contribution of that gap quantitatively [by observing] how the current decreases when we pull the tip away,” he said Weiss. “We were able to tell the difference between supplying an electron to them, by which I mean running a current, or simply applying an electric field.” By backing the STM tip off just enough, the researchers have been able to “apply a voltage between the tip and the underlying metal electrode and show that just with that electric field we are able to make the molecules switch,” he said.

The researchers are not yet sure exactly how the switching process works. It could be that the changes within the molecule affect how the molecule conducts current. Electron paths, or orbitals, might no longer extend over the full length of the molecule as they did, but become localized in its middle instead, said Weiss.

Another possibility has to do with contacts. A working hypothesis is that a tilt of the entire molecule would change how the electrons of the molecule connect with the underlying metal to cause the large conductance change, said Weiss. “The molecule is not [completely] static… It is able to rotate internal bonds freely on the time scale on which we see switching,” said Weiss.

The process is reversible. “The molecules can go back and forth many times between different states, which is important in having memory which you can write over… and using these things for logic,” said Weiss.

The work is clever, said Mike Ward, Professor of Inorganic Chemistry at the University of Bristol in England. “The combination of demonstrating a remarkable switching [and] memory effect, … demonstrating what causes it, and [showing] that in principle it is feasible on the single-molecule scale make this a piece of science…at the forefront of current work into molecular electronics,” he said.

The work shows clearly that the switching process is associated with some sort of structural or orientational change of the molecules, Ward said. The alternative possibility that the switching process is related to the gain or loss of an electron can be ruled out, he said. “This effect would not be influenced by the tightness of the packing around the… molecules.”

The researchers are currently making variations of the molecules in order to pinpoint what features drive the switching. "We are looking at molecules with different backbones, …changing the environment around the molecules, and developing a careful way of turning them on,” said Weiss.

The researchers' main goal is making primitive logic and addressable devices like memory from these molecules, said Tour. Eventually, they would like to make devices that meld silicon and molecules, said Weiss.

"This is not a technology that will one day overturn silicon dominance. It's more likely that some of these molecules will be used in some hybrid system where you take advantage of integrating molecular functions with some other properties” for activities like sensing, said Weiss. Biological and chemical sensors could, for instance, pick up changes in current and resistance.

Weiss and Tour’s colleagues were Zachary Donhauser, Brent Mantooth, Kevin Kelly, Lloyd Bumm, Jason Monnell, Josh Stapleton, and David Allara at Penn State and David Price and Adam Rawlett at Rice University. The research was funded by the Army Research Office, the Defense Advanced Research Projects Agency (DARPA), the National Science Foundation (NSF), the Office of Naval Research (ONR), and Zyvex.

Timeline:   2 to 3 years
Funding:  Corporate; Government
TRN Categories:  Biological, Chemical, DNA and Molecular Computing
Story Type:   News
Related Elements:  Technical paper, "Conductance Switching in Single Molecules Through Conformational Changes," Science, June 22, 2001.


August 15, 2001

Page One

Atom lasers made easy

Molecular makes mini memory

Does heavy volume smooth Net traffic?

Mind game smooths streaming audio

Quantum effect for chipmaking confirmed


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