Mini Mover and Shaker: Single-Molecule "Engine" Vibrates Macro Object

The random motion of a hydrogen molecule can drive the oscillation of a much larger structure

By John Matson

 A single hydrogen molecule just might be a real-life little engine that could, according to a new study. It would be hard to imagine an engine much tinier.

Physicists in Germany and Spain have demonstrated that a hydrogen molecule dancing between two possible positions can induce regular vibration of a nearby cantileverأ¢â‚¬â€‌essentially a miniature tuning fork made of quartz.

The molecule's effect is no small feat, given the tiny size of a hydrogen molecule (H2) in relation to the cantilever. "A single molecule, actually the smallest molecule we have, is capable of exciting the motion of something that is macroscopic," says physicist Jose Ignacio Pascual, now at the CIC nanoGUNE Consolider research center in San Sebastián, Spain, who led the research while working at the Free University of Berlin. "If the molecule is a person, like me, the cantilever would be something like Mount Everest." Pascual and his Berlin colleagues reported their experimental results in the November 9 issue of Science.

Although the hydrogen molecule switches position at random times, resonances between the hydrogen molecule's own vibrations and the tuning fork's natural vibration rate convert that randomness into regular oscillations in the cantilever, essentially harvesting energy from noise. (This is no perpetual-motion machine, howeverأ¢â‚¬â€‌the physicists applied an electric current to spur the hydrogen molecule's fluctuations.)

The type of cantilever in the new demonstration is well known for its orderly oscillations. "It's actually the same tuning fork that you have in your quartz watch," Pascual says. He and his colleagues affixed the quartz beam to the end of an atomic force microscopeأ¢â‚¬â€‌a scanning probe with a sensor tip akin to a tiny phonograph needleأ¢â‚¬â€‌that also doubles as an electrode to deliver current into the hydrogen molecule. The hydrogen molecule's current-induced fluctuation between its two states alternately attracted and repelled the tip of the microscope, driving the cantilever up and down. But the researchers found that the cantilever's motion could in turn influence the fluctuations of the hydrogen molecule, which depends on the proximity of the atomic force microscope tip. The feedback between the two systemsأ¢â‚¬â€‌molecule and tuning forkأ¢â‚¬â€‌can be tuned to resonances that drive the cantilever to surprisingly large-amplitude vibrations.

The new leveraging of molecular motion exemplifies a phenomenon called stochastic resonance, which bridges the worlds of the random and of the orderly. A stochastic resonance is "one way of coupling something that is periodic with something that is noisy," Pascual says. "In this case it turns out that the coupling is very effective." Such resonances, he adds, could be useful in building nanoscale motors driven by single molecules or other tiny objects, whose haphazard fluctuations could be harnessed to produce coordinated motion.