Electrons rock and roll in nanotubes

The rolled sheets of carbon atoms known as nanotubes have attracted attention for their potential to usher in a new generation of molecule-based electronics. On such a scale, researchers say, electrons coursing along nanotubes should reveal their quantum mechanical nature. No one had actually imaged such effects, however.

Because of its wave nature, an electron will more likely appear at the white disks than elsewhere on the regular atomic lattice (outlined in white) of a carbon nanotube. Bright green spots show measured electron peaks. The scale bar at top represents 0.5 nanometer. Lemay et al./Nature

Now, Serge G. Lemay of Delft University of Technology in The Netherlands and his colleagues have used a scanning tunneling microscope (STM) to take snapshots of wavelike electron behavior in nanotubes. In doing so, the scientists have shown that a single electron trapped on the surface of a snippet of nanotube sloshes back and forth, much the way a tone reverberates in an organ pipe.

Earlier, Wenjie Liang and his colleagues at Harvard University inferred that wave pattern by making a nanotube transistor that exploited wave interference, but the team didn’t image the effect.

Meanwhile, another group at Harvard has recently used an STM to observe how a defect in a nanotube can affect electrons’ wavelike behavior. The team, led by Charles M. Lieber, observed that an irregularity in the otherwise uniform chicken wire pattern of a nanotube’s carbon atoms disturbs the flow of electrons, much as a rock in a shallow stream disrupts the flow of water.

These wavelike aspects of electrons in nanotubes make possible electronic components that take advantage of such quantum properties, Lemay says. In today’s silicon chips, quantum effects play little or no explicit role in the devices’ operation because the components are relatively large and the current flows, high.

To tease out the wave behavior of electrons, scientists at both Delft and Harvard scanned the surfaces of nanotubes with the extremely sharp STM tips.

Researchers can often detect invisible topography–sometimes in atomic detail–by monitoring changes in the subtle electronic flow between an STM’s tip and an underlying sample (SN: 10/24/98, p. 268: https://www.sciencenews.org/sn_arc98/10_24_98/Bob2.htm.). They can also eke out information about the wavelike distribution of individual electrons within the sample’s surface.

To detect the minute variations typical of such interactions, researchers must keep their instruments steady enough to “sit in one spot and move less than an atom in 1 day,” Lemay says. To achieve such extreme stability, the teams need to shield their instruments from the smallest vibrations while also chilling them to temperatures near absolute zero.

From these painstaking scans, the Delft team detected a fixed pattern of undulations–a so-called standing wave–along nanotubes. Such a pattern indicates that rebounding electron waves are adding together at some locations along the nanotubes while canceling each other out at others. That’s just what theorists had predicted for short, open-ended nanotubes like the ones imaged, Lemay says.

The Delft researchers, whose collaborators at Rice University in Houston provided the nanotubes, report their results in the Aug. 9 Nature.

Although Lieber’s team also studied an open-ended nanotube, it saw a localized pattern of oscillations that died off with distance. That result was caused by an impurity or an out-of-place atom, the group says. It posted the unpublished study on July 27 on the Internet physics preprint archive (http://xxx.aps.org/abs/cond-mat/0107580).

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