March 2006

EEEL Researchers Demonstrate Rapid, Inexpensive Identification of Bulk Carbon Nanotubes

EEEL researchers Katherine Gilbert and John Lehman, along with collaborators Anne Dillon and Jeff Blackburn from the National Renewable Energy Laboratory, have estimated the metal-to-semiconductor ratio of bulk carbon nanotubes using an effective medium approximation and the measured spectral responsivity of a LiTaO3 pyroelectric detector as a fixed platform for single-wall carbon nanotubes (SWNTs). This is the first such demonstration of a simple quantitative method for determining the metal-to-semiconductor ratio for solid-state forms of SWNTs. Carbon nanotubes are revolutionary materials, exhibiting properties that are vastly different than any bulk form of carbon. They have valuable electrical, optical, mechanical, and thermal characteristics, due to their unique quasi-one-dimensional electron confinement. At least 50 different SWNT species have been identified, over half of which are semiconducting. Current manufacturing processes do not produce a single type of SWNT. Instead, these processes produce an unknown mixture of metallic and semiconducting SWNT species. As a result, technologists confront SWNT quality issues at every level, ranging from composite manufacturers integrating SWNTs into high strength structures, to scientists who dream of the next generation of optical sources, detectors, and displays. Advanced, cost effective analytical techniques are needed so that carbon nanotube manufacturers, product developers, and regulatory agencies can truly "see" what they have.

Despite the visibly black appearance of bulk SWNTs, their dielectric properties yield variations in the absorption coefficient. This is due to features such as chirality, diameter, purity, and bulk topology. By measuring the detector responsivity, the team takes advantage of a relatively large specular absorptance at normal incidence, which is easily modeled from simplified Fresnel equations for normal incidence. Further analysis reveals that a change in responsivity as small as 4 % is manifested by a change in ratio of metallic-to-semiconducting SWNTs of approximately 10 %. The relative expanded uncertainty of their measured responsivity is 1.24 %. The fixed platform spectral responsivity method has an advantage over other quantitative methods which require samples of SWNTs in solution. Arguably, evaluation of SWNTs is less repeatable over time if the SWNTs fall out of suspension. SWNTs on a fixed platform such as a pyroelectric detector are repeatable over time and are compatible with other qualitative measurement techniques such as Raman spectroscopy.

The team's purified SWNTs produced by laser vaporization and applied to a pyroelectric detector have sufficient length and lack of defects to exhibit a spectral character in the wavelength range 600-2000 nm that reveal interband transitions that are characteristic of either metallic or semiconducting SWNTs. Applying an effective medium approximation to spectral responsivity measurements on these purified SWNT samples indicates that the samples have a proportion of SWNT material content that is 20 % metallic and 80 % semiconducting.

Visit the Laser Radiometry Project web page or contact John Lehman at 303-497-3654 for more information about upcoming display courses.