ARTCLES d e 005 S33M11 006008001,0001,2 Wavelength (nm) Figure 1 Sorting of SWNTs by diameter, bandgap and electronic type using density gradient ultracentrifugation. a, Schematic of surfactant encapsulation and sorting, where p is density b-g, Photographs and optical absorbance(1 cm path length) spectra after separation using density gradient ultracentrifugation. A rich tructure-density relationship is observed for SC-encapsulated SWNTs, enabling their separation by diameter, bandgap and electronic type. In contrast, no separation observed for SDBS-encapsulated SWNTs. b, c, SC encapsulated, CoMoCAT-grown SWNTs(7-11 A). Visually the separation is made evident by the formation of coloured bands(b) of isolated SWNTs sorted by diameter and bandgap. Bundles, aggregates and insoluble material sediment to lower in the gradient. The spectra indicate SWNTs of increasing diameter are more concentrated at larger densities. Three diameter ranges of semiconducting SWNTs are maximized in the third, sixth and seventh fractions(highlighted by the pink, green and light brown bands). These have chiralities of (6, 5),(7, 5 )and (9, 5)/(8. 7), and diameters of 7.6, 8.3 and 9.8/10.3 A respectively. d, e, SDBS-encapsulated CoMoCAT-grown SWNTs(7-11 A). In contrast, all of the SWNTs have converged to a narrow black band(d) and diameter or bandgap separation is not indicated (e ). f,g, SC-encapsulated, laser-ablation-grown SWNTs(11-16 A). Both enrichment by diameter and electronic type are observed. Visually, coloured bands of SWNTs(f) are apparent, suggesting separation by electronic structure. In the optical absorbance spectra, the second-and third-order semiconducting (highlighted pink) and first-order metallic (highlighted blue)optical transitions are labelled S22, S33 and M11, respectively 5. 2. The purple highlighted regions show where the semiconducting and metallic transitions overlap. The diameter separation is indicated by a red shift in the S22 band for fracti of increasing density. Additionally, the metallic SWNTS (M11)are depleted in the most buoyant fractions. Ap from top to bottom fraction, and p for the top fraction for c, e and g are 0.022, 0.096 and 0.026 g cm-3 and 1.08, 1. 11 and 1.08+0.02 g cm-3, respectively. pH= 7 for all parts. SWNTs before sorting are depicted as a dashed grey line in c and g electrostatically bound hydration layers. To into the were sodium cholate (SC), sodium deoxycholate and sodium structure-density relationship SWNTS sle of taurodeoxycholate. The bile salts are more molecularly rigid and apsulating agents, we first compared two families planar amphiphiles with a charged fa sing a hydrophobe of surfactants-anionic-alkyl amphiphiles and bile salts. one, which is expected to interact with the SWNT surface(Fig. la Specifically, we used two amphiphiles with anionic head groups and Initially, we explored the sorting of SWNTs in the 7-11A flexible alkyl tails: sodium dodecyl sulphate (SDS)and sodium diameter range synthesized by the CoMoCAT method, using SO dodecylbenzene sulphonate( SDBS). The three bile salts used and SDBS encapsulations, as depicted in Fig. 1. For the case of naturenanotechnologyivol1ioCtoBer2006www.nature.com/naturenanotechnology @2006 Nature Publishing GroupBlack plate (61,1) electrostatically bound hydration layers. To gain insight into the structure–density relationship for SWNTs and the role of encapsulating agents, we first compared two different families of surfactants—anionic-alkyl amphiphiles19 and bile salts20. Specifically, we used two amphiphiles with anionic head groups and flexible alkyl tails: sodium dodecyl sulphate (SDS) and sodium dodecylbenzene sulphonate (SDBS). The three bile salts used were sodium cholate (SC), sodium deoxycholate and sodium taurodeoxycholate. The bile salts are more molecularly rigid and planar amphiphiles with a charged face opposing a hydrophobic one21, which is expected to interact with the SWNT surface (Fig. 1a). Initially, we explored the sorting of SWNTs in the 7–11 A˚ diameter range synthesized by the CoMoCAT method, using SC and SDBS encapsulations, as depicted in Fig. 1. For the case of a b c d e f g ρ 900 1,100 Wavelength (nm) 1,300 900 1,100 Wavelength (nm) 1,300 400 600 S33 S22 M11 Wavelength (nm) 800 1,000 1,200 0.5 0.4 0.1 0.0 0.6 0.5 0.1 Absorbance 0.0 0.20 0.15 Absorbance Absorbance 0.00 0.05 0.3 0.2 0.10 0.2 0.3 0.4 Figure 1 Sorting of SWNTs by diameter, bandgap and electronic type using density gradient ultracentrifugation. a, Schematic of surfactant encapsulation and sorting, where r is density. b–g, Photographs and optical absorbance (1 cm path length) spectra after separation using density gradient ultracentrifugation. A rich structure–density relationship is observed for SC-encapsulated SWNTs, enabling their separation by diameter, bandgap and electronic type. In contrast, no separation is observed for SDBS-encapsulated SWNTs. b,c, SC encapsulated, CoMoCAT-grown SWNTs (7–11 A˚ ). Visually, the separation is made evident by the formation of coloured bands (b) of isolated SWNTs sorted by diameter and bandgap. Bundles, aggregates and insoluble material sediment to lower in the gradient. The spectra indicate SWNTs of increasing diameter are more concentrated at larger densities. Three diameter ranges of semiconducting SWNTs are maximized in the third, sixth and seventh fractions (highlighted by the pink, green and light brown bands). These have chiralities of (6,5), (7,5) and (9,5)/(8,7), and diameters of 7.6, 8.3 and 9.8/10.3 A˚ respectively. d,e, SDBS-encapsulated CoMoCAT-grown SWNTs (7–11 A˚ ). In contrast, all of the SWNTs have converged to a narrow black band (d) and diameter or bandgap separation is not indicated (e). f,g, SC-encapsulated, laser-ablation-grown SWNTs (11–16 A˚ ). Both enrichment by diameter and electronic type are observed. Visually, coloured bands of SWNTs (f) are apparent, suggesting separation by electronic structure. In the optical absorbance spectra, the second- and third-order semiconducting (highlighted pink) and first-order metallic (highlighted blue) optical transitions are labelled S22, S33 and M11, respectively5,22. The purple highlighted regions show where the semiconducting and metallic transitions overlap. The diameter separation is indicated by a red shift in the S22 band for fractions of increasing density. Additionally, the metallic SWNTs (M11) are depleted in the most buoyant fractions. Dr from top to bottom fraction, and r for the top fraction for c, e and g are 0.022, 0.096 and 0.026 g cm23 and 1.08, 1.11 and 1.08+0.02 g cm23 , respectively. pH ¼ 7 for all parts. SWNTs before sorting are depicted as a dashed grey line in c and g. ARTICLES nature nanotechnology |VOL 1 | OCTOBER 2006 | www.nature.com/naturenanotechnology 61 ©2006 NaturePublishingGroup