Molecule toggle makes nano logic

By Eric Smalley, Technology Research News

A popular trend in technology research is copying nature, and another source of inspiration is the world of everyday objects.

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 1 terabits per square inch, said Kornilovitch.

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 said.

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 charge."

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 the resistance.

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 temperatures.

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," he said.

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 not."

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, 2002


March 26/April 2, 2003

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Molecule toggle makes nano logic

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