ABTGLES Sorting carbon nanotubes by electronic structure using density differentiation MICHAEL S. ARNOLD, ALEXANDER A GREEN. JAMES F. HULVAT SAMUEL L STUPP AND MARK C HERSAM* Department of Materials Science and Engineering, Northwestern University, Evanston, Illinois 60208-3108, USA Re-mail: m-hersam@northwestern. edu Published online: 4 October 2006: doi: 10. 1038/nano. 2006.52 The heterogeneity of as-synthesized single-walled carbon nanotubes (SWNTs) electronics, optics and sensing. We report on the sorting of carbon nanotubes by andgap and electronic type using structure-discriminating surfactants to engineer subtle differences in their buoyant Using the scalable technique of density-gradient ultracentrifugation, we have isolated narrow distributions of Sw h >97% are within a 0.02-nm- diameter range. Furthermore, using competing mixtures of surfactants, we hav bulk quantities of SwNTs of characterized a single electronic type. These materials were used to fabricate thin-film electrical devices of networked SWNTs d by either metallic or semiconducting behaviour Carbon nanotubes have recently received extensive attention' due to Other methods o-l8 for sorting SWNTs have been reported their nanoscale dimensions and outstanding materials properties recently; however, none of these techniques has demonstrated such as ballistic electronic conduction, immunity from the simultaneous sensitivity, scalability and effectiveness that we electromigration effects at high current densities, and transparent report here. In addition, we have previously exploited density- conduction,. However, as-synthesized carbon nanotubes vary in gradient ultracentrifugation to enrich DNA-wrapped SWNTs by their diameter and chiral angle, and these physical variations result diameter and bandgap". However, there are critical drawbacks in striking changes in their electronic and optical behaviours A. For to using DNA for carbon nanotube functionalization. First, xample, about one-third of all possible SWNTs exhibit metallic DNA-wrapped SWNTs have limited stability in aqueous density es and the remaining two-thirds act as semiconductors. gradients and thus are not amenable to the refinements in Moreover, the bandgap of semiconducting SWNTS scales inversely enrichment gained from repeated centrifugation. Furthermore, with tube diameter. For instance, semiconducting SWNTs produced complete removal of the DNA wrapping after enrichment has by the laser-ablation method range from 11 to 16 A in diameter and not been demonstrated, and sensitivity to electronic type has have optical bandgaps that vary from 0.65 to 0.95 ev(ref 5). The not been observed. Finally, the availability and cost of specific SWNTs prevents their widespread applicationty of as-synthesized custom oligomers of single-stranded DNA are prohibitive field-effect transistors, optoelectronic near-infrared emitters/ surfactant encapsulating agents in place of DNA and have detectors, chemical sensors, materials for interconnects in integrated discovered that bile salts and their mixtures with other surfactants circuits and conductive additives in composites. Accordingly, their enable the separation of SWNTs by diameter, bandgap and /or use will be limited until large quantities of these nanomaterials can electronic type. Using surfactants, the isolation of specific chiralitie be produced that are monodisperse in their structure and properties. of SWNTs can be significantly refined by separating in multi To address the problem of heterogeneity and enable future successive density gradients. Furthermore, the adsorption of SWNT-based technologies, we have developed a general approach surfactants to SWNTs is reversible and compatible with a wide range for sorting carbon nanotubes by diameter, bandgap and electronic of tube diameters. For example, we demonstrate here the sorting of type(metallic versus semiconducting), using the technique of SWNTs over the diameter range 7-16 A. Most importantly, the density-gradient ultracentrifugation. This scalable approach structure-density relationship for SWNTs can be easily controlled exploits differences in the buoyant densities(mass per volume) by varying the surfactant itself. For instance, by using mixtures of among SWNTs of different structures and has been adapted from two surfactants that competitively adsorb to the SWNT surface, we similar techniques used in biochemistry. for the purification of biological macromolecules, such as nucleic acids and proteins. In nis technique, purification is induced by ultracentrif density gradient. In response to the resulting centripetal force, RESULTS particles sediment toward their respective buoyant densities and oMPARISONLnE ENCAPSILATINGAGENTS spatially separate in the gradient. This approach differs from The buoyant density of SWNTs in aqueous solution will subtly previously reported work on the ultracentrifugation of SWNTs.o, depend on multiple factors, including the mass and volume of the in which density gradients were not used. carbon nanotube itself, its surface functionalization andBlack plate (60,1) Sorting carbon nanotubes by electronic structure using density differentiation MICHAEL S. ARNOLD, ALEXANDER A. GREEN, JAMES F. HULVAT, SAMUEL I. STUPP AND MARK C. HERSAM* Department of Materials Science and Engineering, Northwestern University, Evanston, Illinois 60208-3108, USA *e-mail: m-hersam@northwestern.edu Published online: 4 October 2006; doi:10.1038/nnano.2006.52 The heterogeneity of as-synthesized single-walled carbon nanotubes (SWNTs) precludes their widespread application in electronics, optics and sensing. We report on the sorting of carbon nanotubes by diameter, bandgap and electronic type using structure-discriminating surfactants to engineer subtle differences in their buoyant densities. Using the scalable technique of density-gradient ultracentrifugation, we have isolated narrow distributions of SWNTs in which >97% are within a 0.02-nm￾diameter range. Furthermore, using competing mixtures of surfactants, we have produced bulk quantities of SWNTs of predominantly a single electronic type. These materials were used to fabricate thin-film electrical devices of networked SWNTs characterized by either metallic or semiconducting behaviour. Carbon nanotubes have recently received extensive attention1 due to their nanoscale dimensions and outstanding materials properties such as ballistic electronic conduction2 , immunity from electromigration effects at high current densities1 , and transparent conduction3 . However, as-synthesized carbon nanotubes vary in their diameter and chiral angle, and these physical variations result in striking changes in their electronic and optical behaviours1,4. For example, about one-third of all possible SWNTs exhibit metallic properties and the remaining two-thirds act as semiconductors. Moreover, the bandgap of semiconducting SWNTs scales inversely with tube diameter. For instance, semiconducting SWNTs produced by the laser-ablation method range from 11 to 16 A˚ in diameter and have optical bandgaps that vary from 0.65 to 0.95 eV (ref. 5). The currently unavoidable structural heterogeneity of as-synthesized SWNTs prevents their widespread application as high-performance field-effect transistors, optoelectronic near-infrared emitters/ detectors, chemical sensors, materials for interconnects in integrated circuits and conductive additives in composites1 . Accordingly, their use will be limited until large quantities of these nanomaterials can be produced that are monodisperse in their structure and properties. To address the problem of heterogeneity and enable future SWNT-based technologies, we have developed a general approach for sorting carbon nanotubes by diameter, bandgap and electronic type (metallic versus semiconducting), using the technique of density-gradient ultracentrifugation. This scalable approach exploits differences in the buoyant densities (mass per volume) among SWNTs of different structures and has been adapted from similar techniques used in biochemistry6,7 for the purification of biological macromolecules, such as nucleic acids and proteins. In this technique, purification is induced by ultracentrifugation in a density gradient. In response to the resulting centripetal force, particles sediment toward their respective buoyant densities and spatially separate in the gradient. This approach differs from previously reported work on the ultracentrifugation of SWNTs9,10, in which density gradients were not used. Other methods8,10–18 for sorting SWNTs have been reported recently; however, none of these techniques has demonstrated the simultaneous sensitivity, scalability and effectiveness that we report here. In addition, we have previously exploited density￾gradient ultracentrifugation to enrich DNA-wrapped SWNTs by diameter and bandgap8 . However, there are critical drawbacks to using DNA for carbon nanotube functionalization. First, DNA-wrapped SWNTs have limited stability in aqueous density gradients and thus are not amenable to the refinements in enrichment gained from repeated centrifugation. Furthermore, complete removal of the DNA wrapping after enrichment has not been demonstrated, and sensitivity to electronic type has not been observed. Finally, the availability and cost of specific, custom oligomers of single-stranded DNA are prohibitive. To overcome these obstacles, we have recently explored surfactant encapsulating agents in place of DNA and have discovered that bile salts and their mixtures with other surfactants enable the separation of SWNTs by diameter, bandgap and/or electronic type. Using surfactants, the isolation of specific chiralities of SWNTs can be significantly refined by separating in multiple successive density gradients. Furthermore, the adsorption of surfactants to SWNTs is reversible and compatible with a wide range of tube diameters. For example, we demonstrate here the sorting of SWNTs over the diameter range 7–16 A˚. Most importantly, the structure–density relationship for SWNTs can be easily controlled by varying the surfactant itself. For instance, by using mixtures of two surfactants that competitively adsorb to the SWNT surface, we have achieved optimal metal–semiconductor separation. RESULTS COMPARISON OF ENCAPSULATING AGENTS The buoyant density of SWNTs in aqueous solution will subtly depend on multiple factors, including the mass and volume of the carbon nanotube itself, its surface functionalization and ARTICLES 60 nature nanotechnology | VOL 1 | OCTOBER 2006 | www.nature.com/naturenanotechnology ©2006 NaturePublishingGroup