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Control over the average length of single-walled carbon nanotubes (SWNTs) in suspension is of critical importance to characterizing and developing flexible, transparent thin films that use percolative transport to achieve reproducibility in electronic properties. This paper demonstrates how the average length of SWNTs in aqueous suspensions can be controlled by the conditions used to form (sonication) and purify (low-G centrifugation) the dispersions. The effect of ultrasonic probe sonication, which was used to disperse SWNT bundles into suspension, on the length and extent of defects on the nanotubes was investigated via atomic force microscopy (AFM) and confocal Raman spectroscopy, respectively. Quantitative information about the suspension concentration and the effect of sonication power on unbundling the SWNTs was obtained via UV−vis and near-IR spectroscopy, respectively. To obtain a clear understanding of the effect of sonication power on SWNT suspensions, repeated low-G centrifugation cycles were used to remove impurities such as bundles of SWNTs, amorphous carbon, and catalyst nanoparticles. This nonoxidizing purification method, which was performed prior to all analyses, allows direct determination of the effect of sonication power on defect formation.

Single-walled carbon nanotubes (SWNTs) have had significant impact on the development of gas sensors in the last decade. However, useful applications of SWNTs are limited by the lack of manufacturable routes to device formation. This Highlight article chronicles recent progress in this area and demonstrates the great promise of a new room temperature deposition method for SWNT networks in gas sensing applications. This liquid deposition technique allows the deposition of pre-treated, highly aligned SWNT networks on a wide variety of substrates. A significant advantage of SWNT-network sensors is that fluctuations in the electrical response of individual SWNTs become less important as the size of the network increases. Therefore, device properties can be controlled by the overall density of the network rather than the physical properties of any individual SWNT. At densities where semiconducting pathways dominate, highly sensitive thin-film chemoresistive sensors can be fabricated. Such devices also have higher signal-to-noise ratios and are easier to fabricate than devices based on a single SWNT.

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A mild route to the purification of single-walled carbon nanotube (SWNT) soot has been investigated. The removal of all forms of impurities commonly found in bulk SWNT material (metal catalyst nanoparticles, amorphous carbon, and bundles of SWNTs) has been obtained. This technique, which involves purification based on centrifugation at low centrifugal force, results in deposits that are composed of individual SWNTs that do not have the defects that result from harsh oxidative treatments. While ultracentrifugation has been used to separate and purify raw SWNT soot, this method removes all but the shortest nanotubes. Therefore, it is not practical for many electronic applications. The method presented in this manuscript removes carbonaceous and metallic impurities, while leaving long undamaged SWNTs in suspension. A liquid deposition method under development in this group was used to form electrically continuous two-dimensional networks of SWNTs. Atomic force microscopy (AFM) then is used to verify the efficacy of centrifugation cycles for purification of SWNT suspensions. AFM images of the deposits indicate significant improvement of deposit morphology, with long individual SWNTs that are observed without the presence of impurities. SWNT networks formed from these purified deposits behaved as metallic conductors at high density and as semiconductive thin films at low density. This further indicates that bundles of SWNTs were removed, along with other undesirable materials.