Tuesday, September 04, 2007

A New Type of Molecular Switch

IBM researchers report a potential breakthrough in molecular electronics.

By Duncan Graham-Rowe

Molecular switch: The tip of a scanning tunneling microscope (shown in silver) probes a cross-shaped molecular switch to turn on and off a neighboring molecule. By inducing voltages, the probe causes two hydrogen atoms within the naphthalocyanine molecule to flip from one orientation to another. Credit: IBM Zurich Research Labs

Multimedia •  Watch IBM's molecular switch in action.

IBM scientists have created a novel molecular switch that is able to turn on and off without altering its shape. While such a switch is still years from being used in working devices, the scientists suggest that it does show a potential way to link together such molecular switches to form molecular logic gates for future computers.

Researchers during the past decade have been working to use individual molecules as electronic switches in the hope that they will eventually help make electronic devices even smaller and more powerful. (See "Molecular Computing.") But so far, such efforts have involved molecular processes that in some way deform the geometric shape of the molecule, says Peter Liljeroth, a researcher at IBM Zurich Research Laboratory, in Switzerland.

The problem is that changing the molecule's shape makes it difficult to link them together as switches. If a researcher wants to make something more complicated than just a molecular switch, such as a logic gate, then he or she has to be able to couple them together, says Liljeroth. "Having a single molecular switch is not really going to be useful for anything."

Liljeroth and his colleagues exploit atomic changes that take place at the center of a molecular cage, which does not alter the molecule's overall structure. In the latest issue of the journal Science, the group shows how its molecule can be electrically switched on and off. The researchers also demonstrate how three of these molecules can be made to work together when placed next to one another. "Injecting a current in one molecule will switch the state of another," says Liljeroth.

"The report constitutes an outstanding and remarkable piece of fundamental science," says Fraser Stoddart, director of the California Nanosystems Institute at the University of California, Los Angeles, who also works on molecular switching.

The IBM molecule is a naphthalocyanine, a class of compounds used in paints and organic optical electronics because of their intense bluish-purple color. The structure of IBM's molecule forms a cross shape that contains two opposing hydrogen atoms on either side of a central square void.

When the researchers placed the molecule on an ultrathin substrate, these opposing hydrogen atoms were found to flip from the sides of this quadrant to the top and bottom, or vice versa, when a sufficient voltage was applied. Yet regardless of which of these two states it's in, the geometry of the molecule remains constant.

When a lower voltage is applied, it's possible to read the state of the switch by measuring the current flowing through it. "A low voltage does not switch it, so we can read the state of the molecule," says Liljeroth.

"It's beautiful science," says Mark Reed, a physicist at Yale University, in New Haven, CT, who studies molecular devices. "The fact that they have this reversible change of the structure is very nice."

IBM's discovery was made by accident. "What we were actually investigating was the molecular vibration caused by adding electrons to the molecule," says Liljeroth. But in doing so, the researchers noticed this flipping of hydrogen atoms, a molecular reaction known as tautomerization.