OCTOBER 2009 | VOLUME 12 | NUMBER 1010

RESEARCH NEWS

When it comes to charges, molecules of hydrogen are

just too symmetrical. It makes them impossible to

control with electric fields because their lack of polarity

means that thermal motions always outweigh any

applied electrical force. Water and other asymmetric

molecules, by contrast, can be pulled to and fro by an

electric field. Remember the electrostatic trick with a hair

comb and a dribbling bathroom tap from childhood?

In the 1980s, it was suggested that hydrogen molecules

could be rendered polar by flinging one of its electrons

into a high-energy orbital, so disrupting the molecule’s

symmetry. The electron would then feel the pull of an

electric field to a different degree at one end of the

molecule and so drag it along like a puppet on a string.

Until now, no one had become puppet master of the

hydrogen molecule.

Now, researchers at ETH Zurich [Hogan et al. Phys.

Rev. Lett., (2009) 103, 123001] have found a way to

dangle hydrogen molecules, H2, on an electron string

without the excited electron simply dropping back to

the ground state before the researchers can achieve

anything useful with their molecular puppet.

Stephen Hogan, Christian Seiler, and Frederic Merkt

looked at several of hydrogen’s excited molecular

orbitals in detail and picked out the one that had

the potential for longevity. They then used circularly

polarized laser light to “cool” the hydrogen molecules,

slowing them to speeds between static and 500 metres

per second, which essentially brings them close to

absolute zero in terms of thermal energy. In a three-

dimensional electrostatic trap they could then take on

the role of puppet master of the excited state for some

50 microseconds; ample time for a detailed study.

Indeed, in this cold and controlled state, the researchers

suggest that it should be possible to study molecular

collisions at very low energies. They add that carrying

out precision spectroscopic measurements should

also be possible on this the apparently simplest of

molecules. The study also paves the way for exploring

the properties of molecular gases held in so-called

quantum degeneracy, where each molecule is in the

exact same energy state as its neighbours.

Merkt and colleagues point out that all atoms and

molecules have the potential to be controlled in this

way. They conclude that their method might therefore

be used to prepare cold, stationary samples of a wide

range of molecules.

David Bradley

Molecular puppetsCHARACTERIZATION

Nanotubes and nanowires are not as amenable to manipulation as macroscopic commodities, however, their promise as building blocks for future electronics, sensors, and electromechanical devices, means that researchers are keen to find ways to handle these tiny entities easily.Now, an international team has measured the different frictional forces experienced by carbon nanotubes as they slide across a surface both in the direction of their long axis or perpendicular to it. [Lucas et al. Nature Mater. (2009) DOI: 10.1038/NMAT2529]. The study not only explains the so-called soft lateral distortions that nanotubes can undergo but could offer a practical solution to controlling and assembling nanotubes into devices. At the fundamental level, studying these forces also reveals information about the handedness, or chirality, of the nanotubes, which cannot be obtained easily using other techniques. Marcel Lucas of the Georgia Institute of Technology and colleagues there and in Italy and Germany used an atomic force microscope (AFM) tip to scan transversely across a multi-

walled carbon nanotube deposited on a flat silicon substrate as well as molecular dynamics calculations to simulate these scans. The nanotubes are held stationary on the surface by van der Waal’s forces. The team then compared the forces measured with a transverse scan with the results of a longitudinal AFM scan.They found that, surprisingly, the transverse friction is twice the magnitude of the friction

seen with a longitudinal scan. This, they explain, is due to “hindered rolling” as the nanotube has a tendency to roll as the AFM tip strokes across it rather than along its length and this distorts its cross section.This study provides the first detailed information about the frictional forces at work when an AFM tip interacts with a nanotube. The significant difference in energy needed to move a nanotube with an AFM tip, suggests a possible way to control the assembly

of carbon nanotubes for nanoelectronics, sensors and other applications. .The computer models also suggested that it might be possible to discern chiral as opposed to non-chiral nanotubes, whether the nanotube has a clockwise or anticlockwise thread depending on the forces experienced by the AFM tip as it scans in different directions. This could allow researchers to develop a way to sort chiral and non-chiral nanotubes as well as controlling the large-scale self-assembly of these entities into sophisticated composite materials and architectures.

David Bradley

Rock ‘n’ Roll nanotubesNANOTECHNOLOGY

An AFM tip slides along (left) and across a carbon nanotube. Deformation is detectable in the transverse slide leading to a dimple caused by the “hindered rolling” of the nanotube.

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