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.
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.
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.
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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.”
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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.
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To switch the molecule, the group used a scanning tunneling microscope (STM) operating at extremely low temperatures and in a vacuum. However, the reaction is driven electrically, albeit at picoamps, so the STM is not necessary for this reaction to take place, says Liljeroth. But the low temperature could be a major obstacle to making the process practical.
For this particular molecule, the temperature had to be maintained at just five degrees kelvin in order for the reaction to occur in a controlled way. “The reaction still occurs at room temperature,” says Liljeroth. “But at room temperature, it would happen spontaneously.” Nevertheless, he says, the potential is there to find new molecules that exhibit this behavior at higher temperatures in the hope of eventually building logic devices.
Demonstrating that one molecular switch can be turned on and off by applying a current to a neighboring molecule is a first step toward such logic. “The ability to apply a voltage to one molecule and cause tautomerization of a neighboring one has interesting implications for logic devices,” says Stoddart. But, he says, the temperature constraint remains a huge challenge.
Stoddart also rejects the IBM group’s dismissal of molecular switches that change shape; he argues that such molecules are at a much more advanced stage and can operate at room temperature. “I find it galling that scientists in the field of molecular electronics continue to be unfairly dismissive of research by others that is much more technologically advanced than their own, and yet also has a very sound theoretical and experimental basis to it.”
Yale’s Reed is also skeptical about the practical implications of the IBM finding. Any talk of turning this reaction into a device amounts to “excessive hyperbole” at this stage, he says. “It’s like saying we have discovered silicon semiconductors, therefore we can make a Pentium.”
