Focus: A Molecule-Sized Light Switch
Electronic circuits built from single atoms and molecules will be the ultimate in miniaturization. These circuits will require molecular switches, and now a team reports in the 31 December Physical Review Letters that they have made one using the single chemical bond between a gold atom and an organic molecule. The team applied voltages to reversibly switch the complex between a bonded configuration that would carry a high current and a nonbonded, low-current configuration. Using two types of microscope, along with computer simulations, the researchers revealed with unprecedented detail the nanoscale dance involved in their switch, which they believe could be the basis for a practical electronic device.
A molecular switch must allow current to flow or be shut off when desired, which requires a physical change between an “on” and an “off” configuration of a molecule. To find good candidates, researchers have employed scanning tunneling microscopy (STM) and atomic force microscopy (AFM), both of which use an ultrasharp probe “tip” to image and manipulate atoms and molecules on a surface.
With these tools, researchers have previously used the controlled formation and breaking of a single bond to generate the two states, but these systems have not been practical for working devices, says Fabian Mohn of IBM Research-Zurich. One problem was that they required a complex sequence of steps to position an STM tip, control the voltage pulse, and control current injection. In addition, switching between “on” and “off” states often wasn’t especially clear-cut or reliable, he says.
To make a better switch, Mohn and his colleagues first scattered gold atoms and an organic molecule called PTCDA onto a thin film of sodium chloride on top of a copper surface. Next they used an STM tip to move a gold atom–which had become a negatively-charged ion–close to a negatively charged PTCDA molecule. Applying the right voltage between the tip hovering above and the surface caused an electron to vacate the molecule, temporarily neutralizing it. This change reduced the repulsion between the molecule and the gold ion enough to allow a covalent bond to form. The new bond altered the electronic structure of the molecule, so that a much higher current could flow from the tip, through the molecular complex, to the surface. This state was the switch’s “on” position.
Changing the STM’s voltage allowed the PTCDA to take on an extra electron, which repelled the gold ion and turned the switch “off.” The team could reliably repeat the cycle, and they found that the “on” state allowed about a hundred times more current than the “off” state. In a real circuit, the voltage applied by the STM would come from another nanoscale circuit element.
The team compared AFM images with computer simulations to understand the details of atomic and molecular motion involved in the switching process. Observed with an AFM, the PTCDA’s planar arrangement of five carbon rings is clearly visible. In the “on” position, the molecule arches away from the surface like a dome, and the gold atom slides in under an edge as it bonds. In the “off” position, the dome flattens out, and the gold is pushed away, breaking the bond. Mohn says that it appears to be the relatively simple bonding and unbonding mechanism, involving two different charge states of the complex, that makes the switch so reliable and effective.
The new results are “astonishing,” says Oscar Custance of the National Institute for Materials Science in Tsukuba, Japan. Despite other examples of atom-molecule switches, “this is the first time in which both STM and AFM are simultaneously applied to characterize an atom-molecule complex, achieving an unprecedented spatial resolution,” says Custance. Mark Ratner of Northwestern University in Evanston, Illinois, says that previous single-molecule switches have not been very practical for devices, but this new system provides rapid and stable switching. “Building a molecular electronics toolkit requires more than switches,” he adds, “but this repeatable, robust, single-molecule switch is a significant step towards constructing that toolkit.”
28 December update: The description of the voltages applied between the surface and the STM tip has been corrected.
David Harris is a freelance science writer in Palo Alto, California.