Buckyballs inside Nanotubes

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Buckyballs inside Nanotubes

December 19, 2000• Phys. Rev. Focus 6, 27

Carbon nanotubes’ electrical resistance is changed when ‘buckyball’ cages of carbon atoms–with a single gadolinium atom in each–are placed inside the tubes.

Figure caption — **Nanoscopic peapod.** “Buckyball” cages of carbon atoms with a single gadolinium atom in each can be placed inside carbon nanotubes, as shown in the schematic and electron micrograph. This changes the tubes’ electrical resistance, a small step toward the control that will be required for applications.

A miniature materials science boom is occurring as researchers scramble to poke, prod, and zap carbon nanotubes in every way possible. These pure carbon tubes with diameters as small as a nanometer have incredible strength and promising electrical properties, so enthusiasts talk of new types of nanodevices, chemical synthesis techniques, and electron beam sources, among many other ideas. A Japanese team reports in the 18 December PRL that they have filled nanotubes with metallofullerenes–pure carbon spheres enclosing metal atoms–hoping for a new way to control nanotube properties. Their paper gives the first evidence that putting molecules inside a nanotube can alter its electrical properties.

In 1998 David Luzzi and his colleagues at the University of Pennsylvania published images of “nanoscopic peapods,” nanotubes containing tightly packed chains of the spherical-cage-like “buckyball” carbon molecules. This work demonstrated a new way to exploit the open space in the tubes and possibly gain more control over their properties, but ordinary $C_{60}$ buckyballs were not expected to have much of an effect.

To look for an effect, Sumio Iijima of Meijo University and NEC Research Corp., Hisanori Shinohara of Nagoya University, and their colleagues, put $C_{82}$ buckyballs containing gadolinium atoms inside nanotubes. Gadolinium and other metal atoms were known to change the electronic structure of buckyballs, so they might also have an effect on nanotubes, the team reasoned.

They made single-walled carbon nanotubes (SWNTs) and gadolinium-filled buckyballs (written as ” $G d @ C_{82}$ ”) using standard techniques, and then exposed the nanotubes to dry air at 420 degrees C to open the capped ends of the tubes. A sample of $G d @ C_{82}$ heated to a vapor easily penetrated and filled the spaghetti-like sample of nanotubes, as the researchers confirmed with electron microscopy. Their images show fullerenes lined up inside the nanotubes at about 1 nm intervals. The team describes these as one dimensional crystals.

Iijima and his colleagues probed the material with electrons (EELS technique) and showed that the gadolinium atoms had each transferred three electrons to their surroundings, just as they normally do in $G d @ C_{82}$ . Finally, Iijima and his colleagues found that the electrical resistance vs. temperature curves of their $(G d @ C_{82})_{n} @ S W N T$ and SWNTs containing empty buckyballs were steeper than those of empty SWNTs.

No one has previously shown that molecules placed inside nanotubes can change the SWNTs’ electrical properties, says Luzzi. “This is the first signature that something is happening.” But Luzzi adds that, because of the disoriented state of the nanotubes, the researchers were unable to conclude anything very specific, such as whether the filled nanotubes were more or less conducting than the empty ones. “Nobody’s willing to bet salary” on this important question.