A SWCNT is a sheet of carbon just one atom thick that has been rolled up into a tube with a diameter of about 1–nm. The atoms in the sheet are arranged in a hexagonal lattice and the relative orientation of the lattice to the axis of the tube (its chirality) determines whether the tube is a metal or a semiconductor. SWCNTs are ideal for use in a host of applications, such as sensors and transistors, thanks to their extremely high surface area and excellent charge transport properties, but even the best techniques to fabricate them invariably produce a mix of semiconducting and metallic structures.

The best way to distinguish between the different chiralities is to use polarization-based optical microscopy and spectroscopy, explains team member Kaihui Liu of the University of California at Berkeley. This is because polarized light is extremely sensitive to optical anisotropy in a system. However, the problem here is that individual nanotubes produce an extremely small signal when illuminated with polarized light and there is an awful lot of environmental background noise, which makes it difficult to pick up this signal.

Light control

These difficulties arise, because, for any optical microscope, a large numerical aperture (NA) objective is needed to characterize material structures in any detail. But when polarized light passes through such a large objective, it strongly depolarizes. A team led by Feng Wang has now managed to overcome this problem by separately controlling the light used to illuminate an object and the light collected by the microscope once it has been reflected by the sample in question.

“We employed a large NA objective for light collection to obtain high spatial resolution but were able to create an effectively small NA objective to illuminate the sample and so make sure that the light remains highly polarized,” said Wang.

Chirality profiles of hundreds of as-grown SWCNTs

The experiments consisted of collecting light that had been scattered by the nanotubes using a “0.8” NA objective, which is relatively large. However, the researchers used light from a supercontinuum laser that had a significantly smaller NA to produce a much narrower incident light beam. This approach allowed them to obtain chirality profiles of hundreds of as-grown SWCNTs and to monitor, in situ, the electronic structures of individual tubes in real, functioning devices, such as field-effect transistors.

As well as characterizing individual SWCNTs, the California team says that its technique might come in handy for enhancing the optical contrast of other anisotropic nanomaterials that are “invisible” to conventional microscopes. Such materials include graphene nanoribbons, semiconducting nanowires and nanorods, and even nanobiomaterials like actin filaments.

“We would also like use our technique to visualize various physical and electronic processes in carbon nanotubes – such as electron and phonon ultrafast dynamics,” Liu told nanotechweb.org.

The current work is reported in Nature Nanotechnology doi:10.1038/nnano.2013.227.

Further reading

Shedding more light on graphene (Mar 2011)
Surfactants separate CNTs (Dec 2010)
DNA sorts carbon nanotubes (Jul 2009)