Molecule toggle makes nano logic
Technology Research News
A popular trend in technology research
is copying nature, and another source of inspiration is the world of everyday
Researchers at Hewlett-Packard Laboratories have proposed a series
of molecules that work like ordinary light switches.
Toggle switches, which open or close a circuit, "gave me the idea
of a molecular-scale... toggle switch," said Pavel Kornilovitch, a theoretical
materials scientist at Hewlett-Packard Laboratories.
Molecule-size switches have several potential uses, including
as memory cells in ultrahigh- capacity computer memory. The 1s and 0s
of computing can be represented by the on and off positions of the switch.
If each bit of information were represented by just one molecule, molecular
memory devices could hold as much as 1½ terabits per square inch, said
One and a half terabits is about 185 gigabytes, or 40 times the
capacity of a DVD. The microscopically thin layers could also be stacked
up to increase this capacity dramatically, said Kornilovitch.
Networks of molecular switches could also be used to make reconfigurable
electronic circuits. "Such networks could be used to create adaptive computer
logic that would react [to] damage, or artificial brains where reconfiguration
would facilitate the process of learning," said Kornilovitch.
The switches could also be used to form logic and memory components
in microscopic machines like microbe-size computers or sensors, Kornilovitch
The researchers' molecular switch design has two components, a
stator and a rotor. The oblong stator is fixed between two electrodes.
The knob-like rotor is attached to the side of the stator by a single
atom and is free to rotate around this bond. The stator could be as simple
as a row of three benzene rings. Benzene is a ring of six carbon atoms.
The rotor could be as simple as a hydrogen, carbon and oxygen atom, with
the carbon atom attached to the stator.
Key to the design is an electric charge that guides the rotor's
position. "The key design feature is a large electric dipole moment of
the rotor," said Kornilovitch. "That means that one end of the rotor carries
an excess of positive charge and the other end carries an excess of negative
The dipole moment acts like a magnet, forcing the rotor to orient
toward one end of the stator or the other. Putting electric current through
the stator's electrodes flips the rotor from one orientation to the other,
toggling its position between a 1 and a 0.
The position, or state, of the switch can be read by measuring
the molecule's conductivity. In one position, the rotor increases the
electrical resistance of the stator and in the other position it decreases
Applying a sufficient voltage to the molecule flips the rotor
to write a bit. Applying a lower voltage measures the molecule's conductivity,
which reads the bit.
Other researchers have made molecules that can be flipped between
two electronic states, but the HP design is simpler -- two electrodes
rather than three, said Kornilovitch.
Another molecular switch, demonstrated by University of California
at Los Angeles researchers, switches by changing shape. That molecule
is a rod surrounded by a ring, and moving the ring from one end of the
rod to the other changes the molecule's electrical resistance.
The UCLA ring is relatively heavy, however, which leads to data
writing times on the order of milliseconds, said Kornilovitch. "In our
design, switching is achieved through direct interaction of the rotor's
dipole moment with the external electric field. This is a very fast process,
measured in picoseconds," he said. A millisecond is one thousandth of
a second, and a picosecond is one trillionth of a second. A picosecond
is to a millisecond as a second is to 31.7 years.
There's a lot of work to be done before the HP molecular switch
can even be considered for technological applications. "The biggest fundamental
challenge is to achieve the right balance between the temperature stability
and switchability of the molecule," said Kornilovitch. There is a narrow
window between keeping the energy required to flip the switch low enough
to work in practical devices but high enough to remain stable at ambient
Another major challenge is keeping the connections between nanowire
electrodes and the molecules perfectly uniform, said Kornilovitch. "Theoretical
modeling predicts that [the] shift of just one wire atom could lead to
an order of magnitude change in resistance. [This] means that the arrangement
of atoms in the wires has to be controlled with single-atom precision,"
Making devices from the switches also presents major challenges,
including how to position the trillions of molecules involved, how to
direct electrical signals to each molecule, and how to deal with the inevitable
defects, said Kornilovitch.
The researchers' next step is to synthesize the molecules and
test them experimentally, said Kornilovitch. "We are hoping to have the
first molecules within the next six months," he said. "Still, there will
be another two years or so until we know whether the very idea works or
Practical application of molecular switches will take 15 to 20
years, said Kornilovitch.
Kornilovitch's research colleagues were A. M. Bratkovsky and R.
Stanley Williams. The work appeared in the December 15, 2002 issue of
Physical Review B. The research was funded by Hewlett-Packard and the
Defense Advanced Research Projects Agency (DARPA).
Timeline: 15-20 years
Funding: Corporate, Government
TRN Categories: Biological, Chemical, DNA and Molecular
Computing; Nanotechnology; Chemistry
Story Type: News
Related Elements: Technical paper, "Bistable molecular conductors
with a field-switchable dipole group," Physical Review B, December 15,
March 26/April 2, 2003
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