makes mini memory
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,
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
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
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
Story Type: News
Related Elements: Technical paper, "Conductance Switching
in Single Molecules Through Conformational Changes," Science, June 22,
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