Carbon Nanotube 'Species' Decoded with New Method

Carbon nanotubes are almost incomprehensibly small in size, possessing a diameter that's just one ten-thousandth the thickness of a human hair. Yet these nanotubes can come in a plethora of different "species," each with its own structure and unique combination of electronic and optical properties. Now, scientists have managed to characterize the structure and properties of an individual carbon nanotube, paving the way for their use in future applications.

"Using a novel high-contrast polarization-based optical microscopy set-up, we've demonstrated video-rate imaging and in-situ spectroscopy of individual carbon nanotubes on various substrates and in functional devices," said Feng Wang, one of the researchers, in a news release. "For the first time, we can take images and spectra of individual nanotubes in a general environment, including on substrates or in functional devices, which should be a great tool for advancing nanotube technology."

The physical structure and electronic properties of each individual species of single-walled carbon nanotubes are governed by chirality. This means their structure has a distinct left/right orientation or "handedness," which cannot be superimposed on a mirror image. This, in turn, means that achieving chirality-controlled growth of the nanotubes and understanding the physics between chirality-dependent devices are two big challenges in carbon nanotube research.

 "The key to our success was the realization that light illumination and light collection can be controlled separately," said Wang in a news release. "We used a large NA objective for light collection to obtain high spatial resolution, but were able to create an effectively small NA objective for illumination to maintain high polarization purity."

More specifically, the researchers collected nanotube-scattered polarized light with a 0.8 NA objective, but used a much narrower incident beam to create illumination light from a supercontinuum laser with a much smaller NA. The result was polarization an order of magnitude higher than what has been achieved with conventional polarized microscopy, which allowed them to obtain complete chirality profiles of carbon nanotubes.

The findings could be huge for future carbon nanotube research. More specifically, it could take the guesswork of characterizing the structure and properties of an individual carbon nanotube.

The findings are published in the journal Nature Nanotechnology.