Multiple Motions: Applied electrons make molecules vibrate and move

Chemical reactions involve billions of individual molecules enacting a complicated dance of bond breaking and formation. In an elegant technical accomplishment that could help researchers better understand these complex interactions, scientists have now choreographed individual molecules to vibrate, break bonds, and move on a surface in specified ways.

In the May 29 Nature, an international team of researchers describes how it used the electron-emitting tip of a scanning tunneling microscope (STM) to make individual ammonia molecules move in either of two ways.

Since the advent of STMs in the 1980s, scientists have used the instruments to produce striking images of surfaces at atomic scales and more recently to push or pull individual atoms and molecules around surfaces. In other experiments, researchers have bombarded molecules with electrons possessing enough energy to detach the molecules from an underlying surface.

In the current study, Jose Pascual of the Fritz-Haber-Institut der Max-Planck-Gesellschaft in Berlin and the Institut de Ciència de Materials de Barcelona-CSIC in Bellaterra, Spain, and his colleagues selectively tweaked the vibrations of individual molecules to produce two different movements. To do this, the team used STM electrons with precise energies that excited only one of two types of molecular vibration. That, in turn, primed individual molecules to react in specific ways.

The researchers demonstrated the technique with ammonia molecules, which are each made of a nitrogen atom linked to three hydrogen atoms in a pyramidal geometry. When ammonia rests on a copper surface, nitrogen chemically bonds to the copper and the hydrogens stick up. But when Pascual and his colleagues adjust their STM’s tip to emit electrons of about 270 millielectronvolts, ammonia molecules that are hit invert like an umbrella in the wind. Their hydrogen atoms flip toward the surface, the copper-nitrogen bond breaks, and the molecule pops off the surface.

When the researchers used electrons of about 400 millielectronvolts, however, the hydrogen atoms repeatedly stretched apart and pulled back together. This vibration weakened the nitrogen-copper bond, permitting the molecule to slide across the copper surface.

This is a pioneering experiment, says Dennis Jacobs of the University of Notre Dame in Indiana. Pascual’s group has shown that it’s possible to use STMs with “surgical precision” to dictate the behavior of individual molecules on a surface, he says.

Ammonia molecules and copper surfaces don’t have special significance, notes Jacobs. Nor are the demonstrated behaviors–desorption from a surface and lateral movement–necessarily useful ones. Rather, he says, the exciting achievement is the technique itself, which may now be applied to a variety of molecules and motions.

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