Down the tubes we glow

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  • Published: Jan 1, 2004
  • Author: David Bradley
  • Channels: UV/Vis Spectroscopy
thumbnail image: Down the tubes we glow

A new way to test nanotubes has been devised by US chemists. The method is based on their fluorescence and is much simpler and faster than existing approaches.

In the summer of 2002, researchers led by Rick Smalley and Bruce Weisman at Rice University in Texas discovered that carbon nanotubes - the strait-laced cousins of the soccerball molecule buckminsterfullerene - are themselves fluorescent. Smalley and Weisman found that nanotubes absorbed and give off light in the near-infrared spectrum, which they suggested could be useful in biomedical and nanoelectronics applications. (Science 2002, 297, 593)

Now, the same team led by Bruce Weisman has identified the optical signatures of some 33 different "species" of nanotube. (Science - online November 29, 2002) "Light emission from our samples provides a way to characterize excited state properties," explains Weisman, "so we had been attempting to see nanotube emission for some time before we succeeded." He told Spectral Lines that, "It finally became detectable when we obtained samples in which the tubes did not aggregate with each other preventing fluorescence emission). Before that there weren't really any hints, just hopes based on the emission behaviour of molecular samples.

The cylindrical carbon nanotube has been seen as an astounding breakthrough in materials science and researchers are only just beginning to reveal their potential. The nanotubes, as their name would suggest measure about 1 nm across but can vary in length to micrometres. Applications as diverse as spacecraft coatings to molecular wiring have been envisioned for nanotubes. They might even be fashioned into single-molecule transistors, the building blocks of new types of flat-screen displays, molecular gears and storage units, bearings and telescopic springs - all are being considered potential applications of carbon nanotubes.

However, the synthesis of these materials always results in a spread of different diameters and wrapping angles, giving a wide range of electronic and optical properties. Indeed, there are two main classes of carbon nanotubes - some have metallic properties, while others with very similar structures are semiconductors. Since any conventional preparation yields a pick-and-mix array of nanotubes sorting and separating them will be critical for applications.

Standard approaches for testing the size, shape and uniformity of nanotubes have relied on time-consuming microscopic imaging and analysis. Weisman and his colleagues figured that the newly discovered fluorescence of these technologically important materials might be exploited in assaying nanotubes. An optical test would likely be much faster and simpler.

"Optical nanotube spectroscopy is an important enabling tool for nanotechnology research, because it reveals the composition of nanotube samples through simple measurements," explains Weisman.

"Chemists and biochemists commonly use optical instruments that can characterize samples within a matter of seconds. With refinement, similar methodologies can be applied to nanotube analysis."

The potential for quality control of nanotube production is obvious. However, theoreticians will also be able to make use of the spectroscopic results to help them refine their models of how nanotubes form and why they have the particular physical, mechanical, structural and electrical properties they do.

Indeed, Weisman's group have now reported experimental data that differ significantly from earlier theoretical predictions. A better understanding of the factors that contribute to theformation of particular classes of carbon nanotube will in turn allow improved synthetic schemes to be devised.

"Some electronic companies are hoping to make flat panel displays using nanotubes as field emitters," Weisman adds, "This might be the first application to be realized." On a longer timescale, he suggests that nano-electronic fabrication may emerge based on nanotubes. "I also think that the unique optical properties of carbon nanotubes will lead to biomedical uses, with research applications perhaps not that far away and clinical uses coming later after more development and the normal approval processes needed in the medical realm."

Light-emission intensity of carbon nanotubes
This three-dimensional plot of light-emission intensity of carbon nanotubes shows a peak for each "species" of light-emitting nanotube, indicating that each "species" has a unique optical signature. Variations in signature are due to slight differences in nanotube structure and diameter. Emission intensity is plotted as a function of excitation wavelength and emission wavelength.

Fluorescence intensity data
Fluorescence intensity data as a function of the excitation and emission wavelengths. Includes a contour plot on top, and photos of the sample cell being excited and luminescing at the bottom.

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