Trapped molecules forced to reveal their movements

CHARACTERIZATION

June 13 , 2008

From top: Experimental setup, a nanotube containing metallofullerenes, its low-temperature topography image, and the corresponding energy loss image. (Courtesy of Makoto Ashino.)

Scientists have obtained atomically resolved force data for individual metallofullerene molecules confined within single-walled carbon nanotubes (SWNTs). [Ashino et al., Nature Nanotechnol. (2008) doi:10.1038/nnano.2008.126]

Small molecules can be trapped inside the hollow cores of SWNTs and images of the molecules’ movement along the nanotubes have been recorded. So far, however, researchers have been unable to control the moving molecules or measure the forces required for motion.

Now, an international collaboration, led by Roland Wiesendanger has gone one step further. The team use an atomic force microscope (AFM) operated in dynamic mode to probe and measure the response of individual metallocene atoms, trapped inside SWNTs.

“Almost friction-free motion, suggested by previous experimental and theoretical evidence, has, for the first time, been observed,” explains Makoto Ashino. “In our study, such motion has been activated ‘atom-by-atom’ by the atomically sharp tip of an AFM, and its driving force has been measured.

The metallofullerenes are inserted into SWNTs using thermal treatments and the structures
obtained are deposited onto SiO2 surfaces. The elastic and inelastic behaviors of these molecules are studied by measuring attractive forces and energy losses.

While scanning tunneling microscopy has already been used to obtain images of molecules confined in SWNTs, the protrusions visible in the images can only be related to electronic activity caused by the molecules. Dynamic force spectroscopy, on the other hand, can detect atomic structures independently of their electronic properties.

“The site-specific control of individual dynamic motions of inner molecules inside carbon nanotubes can be important for the future development and precise control of nanomolecular machines and nanotransporters, as well as for ultra-sensitive molecular sensors,”

Ashino tells Materials Today. “Currently, we are measuring the dynamic motions of individual molecules inside a variety of SWNTs to evaluate dependencies of their motions on the outer tube diameters,” he continues. “This is actually very important for the next step, that is, the manipulation of individual molecular motions inside SWNTs and the transportation of molecules along the nanotube axes.”

Katerina Busuttil