复旦大学：《纳米线材料和功能器件》课外阅读内容_碳纳米管与石墨烯_2_Hersam_NatNano06_Sorting CNTs by density differentiation

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 and

Black 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

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 Group

Black 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

ABTGLES ensity (a u) 9001,0001,1001200 Emission wavelength(nm) Figure 2 Refinement by repeated centrifugation in density gradients By successively separating SC-encapsulated SWNTS, the isolation of specific, targeted chiralities improves. Plotted are photoluminescence intensities as a function of excitation and emission wavelengths. here, the isolation of the(6, 5)and (7, 5) chiralities(circled red and green in the left-most plot of SwNs grown by the CoMoCAT-method before sorting, is improved (in the top and bottom panels, respectively) by successively repeating density gradient centrifugation for three iterations(from left to right). After three iterations of enriching the(6, 5)chirality (7.6 A), a narrow diameter distribution is achieved in which >97% of semiconducting SWNTs are within 0.2 A of the mean diameter. Alternatively, refined isolation of the(7, 5) chirality can be realized(bottom). In this case, after three iterations of sorting, the(7, 5) chirality (8.3 A), initially substantially less concentrated than the (6, 5)chirality, becomes dominant. Further improvements may be with additional centrifugation cycles SC encapsulation, multiple regions of separated SWNTs are BEPFATED REEINEn SORTING visible throughout the density gradient(Fig. 1b). The most Using density-gradient centrifugation with bile salts such as SC, buoyant region is characterized by SWNTs that have been sorted it is clear that we can enrich SWNTs by both their structure and into bands of various colours, corresponding to the different electronic properties. However, the degree of isolation achieved bandgaps of the semiconducting tubes. In contrast, for the case after a single step of the technique is limited by the diffusion of of SDBS-encapsulated SWNTs, all of the SWNTs are compressed SWNTs during ultracentrifugation, mixing during fractionation, into a narrow black band(Fig. ld) and statistical fluctuations in surfactant encapsulation. To from the centrifuge tubes, layer by layer, using established centrifugation process can be t mprove the sorting process,the techniques for fractionation, and each layer can be optically example, an enriched fraction of SWNTs sorted in a density characterized to determine quantitatively the mode and quality gradient can be further enriched in a second density gradient. of separation(see Supplementary Information, Figs SI and S2, This enables the optimal isolation of a targeted electronic type or nd Methods ). For the case of SC-encapsulated SWNTS, the a specific chirality of SWNT. To demonstrate the approach, plitudes of optical absorbance for different transitions in the targeted the enrichment of the (6, 5)and (7, 5) chiralities of indicate separation by diameter and bandgap. More specifically, In Fig. 2, photoluminescence emission-excitation matrices depict the spectra illustrate that SWNTs of increasingly larger diameters the photoluminescence intensity of semiconducting SWNTs as a are enhanced at increasingly larger densities. A similar correlation function of excitation and emission wavelengths for SwNTs between diameter and density was also observed for the cases of before and after each of three iterations of density-gradient for the case of SDBS(Fig. le) and SDS(see Supplementary of the corresponding absorbance spectra). After each iteration, Information,Fig S3)encapsulations, separation as a function of the relative concentrations of the (6, 5)and (7, 5)chiralities of diameter was absent semiconducting SWNTs increase (Fig. 2). After enriching the This trend of increasing density with increasing diameter also (6,5)chirality(7. 6 A)three times, we achieved bulk solutions of ds to SC-encapsulat 6a diameter the SWNTs in which >97% are within 0.2 A of the mean range that were synthesized by laser ablation( Fig. If). In the diameter (see Supplementary Information, Fig. S5, and optical spectra, separation by diameter is observed as a red shift Methods ). Further improvements in the isolation of individual in the second-order optical absorbance transitions for chiralities of SWNTs may be possible with additional cycles semiconducting SWNTs, 800-1,075 nm (ref 5), with increasing density(Fig. 1g). Moreover, an enrichment of these SWNTs by INING OF THE STRLICTURE-DENSITYRELATIONSHIP electronic type is also detected In the most buoyant fractions, we Although the separation of SWNTs can be significantly enhanced observe an enhancement in concentration of semiconducting via multiple cycles of ultracentrifugation, further improvements SWNTs with respect to metallic SWNTs, which have first-order can be realized by optimizing the effectiveness of a single cycle optical transitions ranging from 525 to 750 nm(ref. 22) hrough tuning of the structure-density relationship for SWNTs. naturenanotechnologyVol1octoBer2006www.nature.com/naturenanotechnology

Black plate (62,1) SC encapsulation, multiple regions of separated SWNTs are visible throughout the density gradient (Fig. 1b). The most buoyant region is characterized by SWNTs that have been sorted into bands of various colours, corresponding to the different bandgaps of the semiconducting tubes. In contrast, for the case of SDBS-encapsulated SWNTs, all of the SWNTs are compressed into a narrow black band (Fig. 1d). After centrifugation, the separated SWNTs can be removed from the centrifuge tubes, layer by layer, using established techniques for fractionation, and each layer can be optically characterized to determine quantitatively the mode and quality of separation (see Supplementary Information, Figs S1 and S2, and Methods). For the case of SC-encapsulated SWNTs, the amplitudes of optical absorbance for different transitions in the 900–1,340 nm range (first-order semiconducting transitions) indicate separation by diameter and bandgap. More specifically, the spectra illustrate that SWNTs of increasingly larger diameters are enhanced at increasingly larger densities. A similar correlation between diameter and density was also observed for the cases of sodium deoxycholate and sodium taurodeoxycholate. However, for the case of SDBS (Fig. 1e) and SDS (see Supplementary Information, Fig. S3) encapsulations, separation as a function of diameter was absent. This trend of increasing density with increasing diameter also extends to SC-encapsulated SWNTs in the 11–16 A˚ diameter range that were synthesized by laser ablation (Fig. 1f ). In the optical spectra, separation by diameter is observed as a red shift in the second-order optical absorbance transitions for semiconducting SWNTs, 800 –1,075 nm (ref. 5), with increasing density (Fig. 1g). Moreover, an enrichment of these SWNTs by electronic type is also detected. In the most buoyant fractions, we observe an enhancement in concentration of semiconducting SWNTs with respect to metallic SWNTs, which have first-order optical transitions ranging from 525 to 750 nm (ref. 22). REPEATED, REFINED SORTING Using density-gradient centrifugation with bile salts such as SC, it is clear that we can enrich SWNTs by both their structure and electronic properties. However, the degree of isolation achieved after a single step of the technique is limited by the diffusion of SWNTs during ultracentrifugation, mixing during fractionation, and statistical fluctuations in surfactant encapsulation. To overcome these limitations and improve the sorting process, the centrifugation process can be repeated for multiple cycles. For example, an enriched fraction of SWNTs sorted in a density gradient can be further enriched in a second density gradient. This enables the optimal isolation of a targeted electronic type or a specific chirality of SWNT. To demonstrate the approach, we targeted the enrichment of the (6,5) and (7,5) chiralities of semiconducting SWNTs (7.6 and 8.3 A˚ in diameter, respectively). In Fig. 2, photoluminescence emission –excitation matrices depict the photoluminescence intensity of semiconducting SWNTs as a function of excitation and emission wavelengths for SWNTs before and after each of three iterations of density-gradient centrifugation (see Supplementary Information, Fig. S4, for plots of the corresponding absorbance spectra). After each iteration, the relative concentrations of the (6,5) and (7,5) chiralities of semiconducting SWNTs increase (Fig. 2). After enriching the (6,5) chirality (7.6 A˚) three times, we achieved bulk solutions of the SWNTs in which .97% are within 0.2 A˚ of the mean diameter (see Supplementary Information, Fig. S5, and Methods). Further improvements in the isolation of individual chiralities of SWNTs may be possible with additional cycles. TUNING OF THE STRUCTURE –DENSITY RELATIONSHIP Although the separation of SWNTs can be significantly enhanced via multiple cycles of ultracentrifugation, further improvements can be realized by optimizing the effectiveness of a single cycle through tuning of the structure–density relationship for SWNTs. Photoluminescence intensity (a.u.) (6, 5) 0.0Excitation wavelength (nm) 750 650 700 600 550 900 Emission wavelength (nm) 1,000 1,100 1,200 0.5 1.0 (7, 5) Figure 2 Refinement by repeated centrifugation in density gradients. By successively separating SC-encapsulated SWNTs, the isolation of specific, targeted chiralities improves. Plotted are photoluminescence intensities as a function of excitation and emission wavelengths. Here, the isolation of the (6,5) and (7,5) chiralities (circled red and green in the left-most plot) of SWNTs grown by the CoMoCAT-method before sorting, is improved (in the top and bottom panels, respectively) by successively repeating density gradient centrifugation for three iterations (from left to right). After three iterations of enriching the (6,5) chirality (7.6 A˚ ), a narrow diameter distribution is achieved in which .97% of semiconducting SWNTs are within 0.2 A˚ of the mean diameter. Alternatively, refined isolation of the (7,5) chirality can be realized (bottom). In this case, after three iterations of sorting, the (7,5) chirality (8.3 A˚ ), initially substantially less concentrated than the (6,5) chirality, becomes dominant. Further improvements may be possible with additional centrifugation cycles. ARTICLES 62 nature nanotechnology | VOL 1 | OCTOBER 2006 | www.nature.com/naturenanotechnology ©2006 NaturePublishingGroup

ARTCLES Metallic Semiconducting 是00 b Semiconducting Metallic Wavelength (nm) Figure 3 Tuning the structure-density relationship for optimal separation by diameter and bandgap or electronic type(metal-semiconductor). a-c, Optimization of separation by diameter and bandgap. The concentration of the(6, 5),(7, 5)and(9, 5)/(8, 7) chiralities of CoMoCAT-grown SWNTs(coloured red, green and blue; diameters (ref.5)of 7.6, 8.3 and 9.8/10.3A, respectively) are plotted against Ap. Concentrations were determined from absorbance spectra(Fig. 1c and Supplementary Fig S1).Th nditions were SC, no buffer, pH 7. 4 (a), SC, 20 mM Tris buffer, pH 8.5, enhanced isolation of the larger diameter SWNTS, (9, 5)/(8. 7)(b), SC the addition of SDS as a co-surfactant (1: 4 ratio by weight, SDS/SC), enhanced isolation of the smaller diameter SWNTs, (6, 5), pH 7.4 (c). p for the fractions with the highest (6,5 chirality relative concentration in a-c are all 1.08+ 0.02 g cm. Arrows mark shifts with respect to a. d, e, Optimization of separation by electronic type d Photograph of laser-ablation-grown SWNTs separated in a Co-surfactant solution( 1: 4 SDS /SC). The top band (orange)corresponds to predominantly semiconducting SWNTS (absorbance spectra plotted in red in e) and the band just below it (green)is highly enriched in metallIc SWNTS, although some semiconducting SWNTs remain(absorbance spectra plotted in Supplementary Fig S6). Ap between the two bands and p for the top band are 0.006 g cm-3 and 1. 12+ 0.02 g cm-, respectively. Further tuning of the structure-density relationship (3: 2 ratio by weight SDS/SC)results in the isolation of predominantly metallic SWNTs(absorbance spectra plotted in blue in e, heterogeneous mixture before sorting plotted with a dashed grey line ).(S33, M11, S22 highlighted as in Fig. 1g) For example, by adjusting pH or by adding competing co- Co-surfactant populations have an even greater effect on the surfactants to a gradient, the isolation of a specific diameter optimization of metal-semiconductor separation for SWNTs in range or electronic type can be targeted the 11-16 A diameter regime. By adding one part SDS for every In Fig. 3a-c we demonstrate diameter tunability. The relative four parts SC(by weight, 2% by weight overall) to a gradient, concentration of several different diameters(7.6, 8.3 and much more distinct metal-semiconductor separation is made 9.8/10.3 A) of SWNTs is plotted against density for the cases of evident. For SWNTs separated in 1: 4 SDS/ SC mixtures, only SC-encapsulated SWNTS at PH 7.4, SC-encapsulated SWNTs at three bands are observed(Fig. 3d). We can deduce from PH 8.5, and for a mixture of 1: 4 SDS/SC(by weight) at pH 7.4. measured optical absorbance spectra that the top band(orange By tuning the structure-density relationship, the differences in hue)consists of predominantly semiconducting SWNTs(Fig. 3d) density among SWNTs of these diameters can be modified. and that the band just below the top band (green hue)is highly For example, by increasing the pH to 8.5, the SWNTs near 8.3 A enriched in metallic SWNTs(see Supplementary Information, in diameter shift to more buoyant densities, enabling optimal Fig. S6, for a plot of the absorbance spectrum). Further tuning separation of SWNTS in the 9.8/10.3A range (Fig. 3b). of the co-surfactant mixture to a 3: 2 SDS/SC ratio permits Alternatively, by adding SDS to compete with the SC for non- significantly improved isolation of metallic SWNTs. In Fig. 3e, covalent binding to the nanotube surface, the SWNTs in the 8.3 spectra corresponding to primarily metallic(3: 2 SDS/SC)and and 9.8/10. A diameter regime shift to significantly larger primarily semiconducting(1: 4 SDS/SC)SWNTs are shown optimal separation of SwNTs near 7.6A in diameter(Fig. 3c). At the highest densities, the FIELD-EFFECT TRANSISTORS relative concentration of SWNTs can appear anomalously high in To demonstrate the applicability of SWNTs sorted in density some cases( for example, the blue curve of Fig. 3a)due to gradients and to confirm their separation by electronic type, field- contributions to the absorbance spectrum from high-density effect transistors were fabricated(see Supplementary Information, SWNT aggregates. Methods) consisting of per naturenanotechnologyivol1ioCtoBer2006www.nature.com/naturenanotechnolo

Black plate (63,1) For example, by adjusting pH or by adding competing co￾surfactants to a gradient, the isolation of a specific diameter range or electronic type can be targeted. In Fig. 3a–c we demonstrate diameter tunability. The relative concentration of several different diameters (7.6, 8.3 and 9.8/10.3 A˚) of SWNTs is plotted against density for the cases of SC-encapsulated SWNTs at pH 7.4, SC-encapsulated SWNTs at pH 8.5, and for a mixture of 1:4 SDS/SC (by weight) at pH 7.4. By tuning the structure–density relationship, the differences in density among SWNTs of these diameters can be modified. For example, by increasing the pH to 8.5, the SWNTs near 8.3 A˚ in diameter shift to more buoyant densities, enabling optimal separation of SWNTs in the 9.8/10.3 A˚ range (Fig. 3b). Alternatively, by adding SDS to compete with the SC for non￾covalent binding to the nanotube surface, the SWNTs in the 8.3 and 9.8/10.3 A˚ diameter regime shift to significantly larger buoyant densities, enabling optimal separation of SWNTs near 7.6 A˚ in diameter (Fig. 3c). At the highest densities, the relative concentration of SWNTs can appear anomalously high in some cases (for example, the blue curve of Fig. 3a) due to contributions to the absorbance spectrum from high-density SWNT aggregates. Co-surfactant populations have an even greater effect on the optimization of metal – semiconductor separation for SWNTs in the 11–16 A˚ diameter regime. By adding one part SDS for every four parts SC (by weight, 2% by weight overall) to a gradient, much more distinct metal– semiconductor separation is made evident. For SWNTs separated in 1:4 SDS/SC mixtures, only three bands are observed (Fig. 3d). We can deduce from measured optical absorbance spectra that the top band (orange hue) consists of predominantly semiconducting SWNTs (Fig. 3d) and that the band just below the top band (green hue) is highly enriched in metallic SWNTs (see Supplementary Information, Fig. S6, for a plot of the absorbance spectrum). Further tuning of the co-surfactant mixture to a 3:2 SDS/SC ratio permits significantly improved isolation of metallic SWNTs. In Fig. 3e, spectra corresponding to primarily metallic (3:2 SDS/SC) and primarily semiconducting (1:4 SDS/SC) SWNTs are shown. FIELD-EFFECT TRANSISTORS To demonstrate the applicability of SWNTs sorted in density gradients and to confirm their separation by electronic type, field￾effect transistors were fabricated (see Supplementary Information, Methods) consisting of percolating networks of thousands of a b c d e 1.0 0.8 0.6 0.4 0.2 Relative concentration (a.u.) 0.0 1.0 0.8 0.6 0.4 0.2 Relative concentration (a.u.) 0.0 1.0 0.8 0.6 0.4 0.2 Relative concentration (a.u.) 0.0 –10 0 10 20 30 40 50 ∆ρ (mg cm–3) –4 0 4 8 12 16 20 –4 0 4 8 12 16 20 Semiconducting Metallic 400 Normalized absorbance (a.u.) 1.0 0.8 0.6 0.4 0.2 600 800 Wavelength (nm) S33 M11 S22 Semiconducting Metallic 1,000 Figure 3 Tuning the structure–density relationship for optimal separation by diameter and bandgap or electronic type (metal–semiconductor). a–c, Optimization of separation by diameter and bandgap. The concentration of the (6,5), (7,5) and (9,5)/(8,7) chiralities of CoMoCAT-grown SWNTs (coloured red, green and blue; diameters (ref. 5) of 7.6, 8.3 and 9.8/10.3 A˚ , respectively) are plotted against Dr. Concentrations were determined from absorbance spectra (Fig. 1c and Supplementary Fig. S1). The encapsulation agents and conditions were SC, no buffer, pH 7.4 (a), SC, 20 mM Tris buffer, pH 8.5, enhanced isolation of the larger diameter SWNTs, (9,5)/(8,7) (b), SC with the addition of SDS as a co-surfactant (1:4 ratio by weight, SDS/SC), enhanced isolation of the smaller diameter SWNTs, (6,5), pH 7.4 (c). r for the fractions with the highest (6,5) chirality relative concentration in a–c are all 1.08+0.02 g cm23 . Arrows mark shifts with respect to a. d,e, Optimization of separation by electronic type. d, Photograph of laser-ablation-grown SWNTs separated in a co-surfactant solution (1:4 SDS/SC). The top band (orange) corresponds to predominantly semiconducting SWNTs (absorbance spectra plotted in red in e) and the band just below it (green) is highly enriched in metallic SWNTs, although some semiconducting SWNTs remain (absorbance spectra plotted in Supplementary Fig. S6). Dr between the two bands and r for the top band are 0.006 g cm23 and 1.12+0.02 g cm23 , respectively. Further tuning of the structure–density relationship (3:2 ratio by weight SDS/SC) results in the isolation of predominantly metallic SWNTs (absorbance spectra plotted in blue in e; heterogeneous mixture before sorting plotted with a dashed grey line). (S33, M11, S22 highlighted as in Fig. 1g.) ARTICLES nature nanotechnology |VOL 1 | OCTOBER 2006 | www.nature.com/naturenanotechnology 63 ©2006 NaturePublishingGroup

ABTGLES characterized by on/off ratios of less than two. The two distinct behaviours of the semiconducting and metallic films independently confirm the separation by electronic type initially observed by optical absorption spectroscopy of the sorted materials presented in Fig. 3e. Additionally, the two films establish the applicability of this method in producing usable quantities of sorted, functional material. For example, a single fraction of semiconducting SWNTs 150 ul)contains enough SWNTs for 20 cm- of a thin-film network similar to that demonstrated in Fig. 4, corresponding to >10 SWNTs(see Supplementary Information, Fig. S7). Such thin-film networks have possible applications flexible and transparent semiconductors and conductors. DISCUSSION We believe that surfactant-based gradient ultracentrifugation is largel how surfactants organize around SWNTs of di res and electronic types. The energetic balance amor surfactant, water surfactant and surfactant -surfactant interactions as well as the packing density, orientation, ionization and the resulting affecting buoyant density and the quality of sorting. Additionally, the capacity of a surfactant to disperse SWNTs solution should also play a role in determining the degree of separation. However, this characteristic alone is not a good predictor. For example, sorting of SDBS-encapsulated SWNTs as not observed here, despite the fact that Wenseleers and co-workers20 have demonstrated that both SDBS and SC disperse SWNTs equally well in aqueous solution. Differences in the organization of anionic-alkyl surfactants and bile salts around carbon nanotubes are expected based on previous is the closest analogue to an SWNT, anionic-alkyl surfactants organize into hemicylindrical micelles with liquid-like hydrophobic cores, whereas bile salts form well-structured monolayers with their less-polar sides facing the hydrophob Figure 4 Electrical devices of semiconducting and metallic SWNTS. surface Bile salts also order to form well-defined host-guest a, Periodic array of source and drain electrodes(single device highlighted in red, structures around small hydrophobic molecules212. Accordingly, scale bar 40 um, gap 20 um). b, Representative AFM image of thin-film, the rigidity and planarity of bile salts, in contrast with anionic percolating SWNT network scale bar= 1 um). The density of SwNTs per unit alkyl surfactants, are expected to result in encapsulation layers that area is >10 times the percolation limit (Supplementary Fig S7).G, Field-effect are sensitive to subtle changes in the underlying SwNT. transistor geometry (s= source: g=gate; d= drain). The SWNT networks were Furthermore, the observed metal-semiconductor selectivity formed on a 100-nm, thermally grown Sio, layer, which served as the gate indicates a coupling of the surfactant and or its hydration with the dielectric. d, Inverse of sheet resistance as a function of gate bias for electronic nature of the underlying SWNT Lu and co-workers have semiconducting(red, triangles) and metallic(blue, squares) swNTs sorted in suggested that metallic SWNTs interact more strongly with co-surfactant density gradients(characterized in Fig. 3e). The metallic SWNTs adsorbates via T interactions than semiconducting SWNTs, due to did not significantly switch with gate bias(2 x 104(Error bars density of the surfactants and their hydration are likely to b described in Supplementary Information, Methods. )The inset shows a sensitive to electrostatic screening by the underlying SWNT. Oth semiconducting device plotted on a linear scale(red curve, same units). A lower effects, such as partial charge transfer between metallic SWNTs bound for mobility in the semiconducting SWNTs is estimated (from the grey fit and surfactants induced by CH- interactions could also to be 20 cm2v-s(see Supplementary Information, Methods), comparable to be important previously reported mobilities for thin films of as-synthesized mixtures of Density-gradient ultracentrifugation provides a scalable metallic and semiconducting SWNTs near their percolation threshold2. approach for sorting carbon nanotubes by diameter, bandgap and electronic type. This strategy has been demonstrated for SWNTs encapsulated by bile salts and mixtures of bile salts with anionic alkyl surfactants for SWNTs between 7 and 16 A in diameter. By etallic or semiconducting SWNTS(Fig. 4). At negative gate successive iterations of ultracentrifugation, sharp diamet biases, both networks exhibited similar sheet resistances of distributions have been achieved in which more than 97% of 500 kn per square. However, by varying the voltage applied 山b semiconducting SWNTs are within 0.2 A of the mean diameter. the gate dielectric capacitor(100 nm Sio,), the resistivity of the Furthermore, the structure-density relationship for SWNTs has semiconducting network was increased by over four orders of been engineered to achieve exceptional metal-semiconductor magnitude (on/off ratio >20,000). In contrast, the metallic separation by using mixtures of competing co-surfactants, thus networks were significantly less sensitive to the applied gate bias enabling the isolation of bulk quantities of SWNTs that are

Black plate (64,1) metallic or semiconducting SWNTs (Fig. 4). At negative gate biases, both networks exhibited similar sheet resistances of about 500 kV per square. However, by varying the voltage applied across the gate dielectric capacitor (100 nm SiO2), the resistivity of the semiconducting network was increased by over four orders of magnitude (on/off ratio .20,000). In contrast, the metallic networks were significantly less sensitive to the applied gate bias characterized by on/off ratios of less than two. The two distinct behaviours of the semiconducting and metallic films independently confirm the separation by electronic type initially observed by optical absorption spectroscopy of the sorted materials presented in Fig. 3e. Additionally, the two films establish the applicability of this method in producing usable quantities of sorted, functional material. For example, a single fraction of semiconducting SWNTs (150 ml) contains enough SWNTs for 20 cm2 of a thin-film network similar to that demonstrated in Fig. 4, corresponding to .1011 SWNTs (see Supplementary Information, Fig. S7). Such thin-film networks have possible applications as flexible and transparent semiconductors and conductors. DISCUSSION We believe that surfactant-based separation using density￾gradient ultracentrifugation is largely driven by how surfactants organize around SWNTs of different structures and electronic types. The energetic balance among nanotube – surfactant, water– surfactant and surfactant – surfactant interactions as well as the packing density, orientation, ionization and the resulting hydration of the surfactants should all be critical parameters affecting buoyant density and the quality of sorting. Additionally, the capacity of a surfactant to disperse SWNTs in aqueous solution should also play a role in determining the degree of separation. However, this characteristic alone is not a good predictor. For example, sorting of SDBS-encapsulated SWNTs was not observed here, despite the fact that Wenseleers and co-workers20 have demonstrated that both SDBS and SC disperse SWNTs equally well in aqueous solution. Differences in the organization of anionic-alkyl surfactants and bile salts around carbon nanotubes are expected based on previous studies of these amphiphiles in other systems. On graphene, which is the closest analogue to an SWNT, anionic-alkyl surfactants organize into hemicylindrical micelles with liquid-like hydrophobic cores19,23, whereas bile salts form well-structured monolayers with their less-polar sides facing the hydrophobic surface24. Bile salts also order to form well-defined host–guest structures around small hydrophobic molecules21,25. Accordingly, the rigidity and planarity of bile salts, in contrast with anionic￾alkyl surfactants, are expected to result in encapsulation layers that are sensitive to subtle changes in the underlying SWNT. Furthermore, the observed metal– semiconductor selectivity indicates a coupling of the surfactant and/or its hydration with the electronic nature of the underlying SWNT. Lu and co-workers have suggested that metallic SWNTs interact more strongly with adsorbates via p interactions than semiconducting SWNTs, due to their larger electronic polarizability26. Additionally, the packing density of the surfactants and their hydration are likely to be sensitive to electrostatic screening by the underlying SWNT. Other effects, such as partial charge transfer between metallic SWNTs and surfactants induced by CH–p interactions could also be important. Density-gradient ultracentrifugation provides a scalable approach for sorting carbon nanotubes by diameter, bandgap and electronic type. This strategy has been demonstrated for SWNTs encapsulated by bile salts and mixtures of bile salts with anionic￾alkyl surfactants for SWNTs between 7 and 16 A˚ in diameter. By successive iterations of ultracentrifugation, sharp diameter distributions have been achieved in which more than 97% of semiconducting SWNTs are within 0.2 A˚ of the mean diameter. Furthermore, the structure –density relationship for SWNTs has been engineered to achieve exceptional metal– semiconductor separation by using mixtures of competing co-surfactants, thus enabling the isolation of bulk quantities of SWNTs that are a b c 10–5 10–6 10–7 10–8 10–9 (Sheet resistance)–1 (Ω per square)–1 –60 –40 –20 0 20 40 60 80 Gate bias (V) 20 15 10 5 0 50 d s g –50 0 d Figure 4 Electrical devices of semiconducting and metallic SWNTs. a, Periodic array of source and drain electrodes (single device highlighted in red, scale bar 40 mm, gap 20 mm). b, Representative AFM image of thin-film, percolating SWNT network (scale bar ¼ 1 mm). The density of SWNTs per unit area is .10 times the percolation limit (Supplementary Fig. S7). c, Field-effect transistor geometry (s ¼ source; g ¼ gate; d ¼ drain). The SWNT networks were formed on a 100-nm, thermally grown SiO2 layer, which served as the gate dielectric. d, Inverse of sheet resistance as a function of gate bias for semiconducting (red, triangles) and metallic (blue, squares) SWNTs sorted in co-surfactant density gradients (characterized in Fig. 3e). The metallic SWNTs did not significantly switch with gate bias (,2), in contrast with the semiconducting SWNTs, which switched by a factor of .2
104 . (Error bars described in Supplementary Information, Methods.) The inset shows a semiconducting device plotted on a linear scale (red curve, same units). A lower bound for mobility in the semiconducting SWNTs is estimated (from the grey fit) to be 20 cm2 V21 s21 (see Supplementary Information, Methods), comparable to previously reported mobilities for thin films of as-synthesized mixtures of metallic and semiconducting SWNTs near their percolation threshold29. ARTICLES 64 nature nanotechnology | VOL 1 | OCTOBER 2006 | www.nature.com/naturenanotechnology ©2006 NaturePublishingGroup

ARTCLES predominantly a single electronic type(typical yields are shown in 6 Graham, L M Biological Centrifugation(BIOS Scientitic, Mlton Park, 2000) SWNTs sorted by this method are highly compatible with 8. Arnold, M S, Stupp S L& Hersam, M C Enrichnent of single-walled carbon nanotubes by ubsequent processing techniques and can be integrated into 9. o' Connel, M L et aL Band gap fluorescence from individual single-walled carbon nanotubes. devices, it is expected that density-gradient ultracentrifugation will Science 297, 593-596(2002 require SWNTs with monodisperse structure andprpPplications that1m乙且;业B业账如 metallic or semiconducting single-walled carbon directly impact the large number of technological operties 11. Haddon, R C Sippel, L-, Rinzler, A G. Pa akopoulos, E Purification and separation 12.Krupke, R. Hennrich, E, Kappes, M. M.& Lohneysen, H. V Surface conductance induced METIHODS dielectrophoresis of semiconducting single-walled carbon nanotubes. Nano Lett. 4, 1395-1399(2004) SRFACTANT ENCAPSULATION 13. Maeda, Y. et al. Large-scale separation of metallic and semiconducting single-walled carbon Soc.127,10287-10290(2005) perse SWNTs in solutions of bile salts or other surfactants, I mg ml olume(w/v)surfactant Ia sy lt, al breakdown. Science 292, 706-709(200 arbon nanotubes and nanotube circuits via ultrasonication(see Supplementary Information, Methods, for more ecylamine-assisted bulk separation of details). In co-surfactant experiments, SWNTs were initially dispersed in a epi phry. Lin. s, 10 6-lc08 g 004). arbon nanotubes by resonance Raman spectroscopy. 2%w/v SC surfactant solution and then diluted into 2%w/v co-surfactant 6. Strano, control of single-walled carbon nanotube solution. For example, in a 1: 4 SDS/SC co-surfactant experiment, one part functionalization. Science 301, 1519-1522(2003) 2%w/v co-surfactant was added to four parts of a 2% w/v SC solution 17. Zheng, M. et al Structure-based carbon nanotube sorting by sequence-dependent DNA assembly. ded ontaining dispersed SWNTs. Furthermore, density gradients were load 18. Peng. H. Q, Alvarez, N. T, Kittrell, C-, Hauge, R. H Schmidt, H. K Dielectrophoresis hel omogeneously from top to bottom with these surfactants at the same ratios. single-walled carbon anotubes. J. Am. Chen. Sac. 128, 8396-8397(200 Thus, no gradients in surfactant concentrations were present. 19.Islam, M. F, Rojas, E Bergey, D. M. Johnson, A. T& Yodh, A Gi. High weight fraction nanotubes in water. Nano Letf. 3, 269-273(2003) DENSIY GRADIENIS 20. Wenseieers, W. er aL Effcient isolation and solubilization of pristine single-walled nanotubes in Density gradients were formed from aqueous solutions of a non-ionic density 21. Mukhopadhyay, S. Maitra, U. Chemistry and biology of bile acids. Curr. Sai. 87, gradient medium, iodixanol. 27.28, purchased as OptiPrep 60% w/v iodixanol 1.32 g cm-3(Sigma-Aldrich). Gradients were created directly in centrifuge 22 Strano, M. S. Probing chiral selective reactions using a revised Kataura plot for the interpretation tubes by one of two methods: by layering of discrete steps and subsequent 23. Wanless, E I.& Ducker, w.A. sodium dodecyl sulfate at the graphite-solution to linea Supplementary Information, Figs. S9-SIl, and Methods). of elective hydrophobicity of bile acids. Colloids Surf. B 5, 241-247(1995) CENIRIEUIGATION AND FRACIONATION 5. Tamminen, I.& Kolehmainen, E. Bile acids as building blocks of supramolecular hosts. Molecules 6,21-46(2001) Centrifugation was carried out in two different rotors, a fixed angle TLA1003 26. Lu, I et al Selective interaction of large or charge-transfer aromatic molecules with metallic tor and a swing bucket Sw4l rotor(Beckman-Coulter), at 22C and at bes: Critical role of the molecular size and orientation. /.Arm. Che. 000 r.P. m and 41,000 r.P. m, respectively, for 9-24 h, depending on the spatial extent and initial slope of the gradient. At the average radii(37.9 mm and 110 mm, respectively), these rotational velocities result in centripetal 28. Davies, 1. Graham, I M. The use of ol for the purification of accelerations of 174,000 g and 207, 000 8, respectively. To fractionate TLA1003 ubes, a modified Beckman Fractionation System(Beckman-Coulter)was Snow, E S. Novak, I. P, Campbell, P. M. Park, D. Random networks of carbon nanotubes as utilized in an upward displacement mode using Fluorinert FC-40(Sigm an electronic material. Appl. Phys. Lett. 82, 2145-2147(2003) Aldrich)as a dense chase media. 25 ul fractions were collected. To fractionate Sw4l centrifuge tubes, a Piston Gradient Fractionator system was utilized Acknowledgements ents, Canada). Fractions of 0.5-3.0 mm were collected This work was supported by (70-420 ul in volume). In both cases, fractions were diluted to I ml Graduate Student Fellowship(MSA ) a Natur 2%w/v surfactant solution for optical characterizatio measurement of optical absorbance spectra(1.S, X.D. ) and evaporation of Au electrodes(MD ) w Received 24 July 2006: accepted 25 August 2006: published 4 October 2006. thank widom and the Keck Biophysics Facility for use of their ultracentrifuges, L Chen i d requests for materials should be addressed to M. CH References SupplementaryiNformationaccompaniesthispaperonwww.nature.comnaturenanotechnology. Baughman, R H Zakhidow, A. A& de Heer, w. A Carbon nanotubes-the route toward Author contributions 2 Javey, A Guo, J, Wang, Q- Lundstrom, M& Dai, H J Ballistic carbon nanotube feld-effect A authors conceived and designed the experiments: M.S.A. and AAG performed the experiments: 4. Charlier, I. C& Issi, L P. Electronic structure and quantum transport in carbon nanotubes. Appl Competing financial interests 5. Weisman, R. B. Bachilo, S M. Dependence of optical transition energies on structure for The authors declare that they have no competing financial interests. le-walled carbon nanotubes in aqueous suspension: An empirical Kataura plot. Nano Lett.3. 1235-1238(200 Reprintsandpermissoninformationisavailableonlineathttp://npg.nature.com/reprintsandpermissions/ naturenanotechnologyivol1ioCtoBer2006www.nature.com/naturenanotechnology @2006 Nature P

Black plate (65,1) predominantly a single electronic type (typical yields are shown in the Supplementary Information, Fig. S8, and Methods). Because SWNTs sorted by this method are highly compatible with subsequent processing techniques and can be integrated into devices, it is expected that density-gradient ultracentrifugation will directly impact the large number of technological applications that require SWNTs with monodisperse structure and properties. METHODS SURFACTANT ENCAPSULATION To disperse SWNTs in solutions of bile salts or other surfactants, 1 mg ml21 SWNTs were dispersed in solutions of 2% weight per volume (w/v) surfactant via ultrasonication (see Supplementary Information, Methods, for more details). In co-surfactant experiments, SWNTs were initially dispersed in a 2% w/v SC surfactant solution and then diluted into 2% w/v co-surfactant solution. For example, in a 1:4 SDS/SC co-surfactant experiment, one part 2% w/v co-surfactant was added to four parts of a 2% w/v SC solution containing dispersed SWNTs. Furthermore, density gradients were loaded homogeneously from top to bottom with these surfactants at the same ratios. Thus, no gradients in surfactant concentrations were present. DENSITY GRADIENTS Density gradients were formed from aqueous solutions of a non-ionic density gradient medium, iodixanol6,8,27,28, purchased as OptiPrep 60% w/v iodixanol, 1.32 g cm23 (Sigma-Aldrich). Gradients were created directly in centrifuge tubes by one of two methods: by layering of discrete steps and subsequent diffusion into linear gradients or by using a linear gradient maker (see Supplementary Information, Figs. S9 –S11, and Methods). CENTRIFUGATION AND FRACTIONATION Centrifugation was carried out in two different rotors, a fixed angle TLA100.3 rotor and a swing bucket SW41 rotor (Beckman-Coulter), at 22 8C and at 64,000 r.p.m. and 41,000 r.p.m., respectively, for 9–24 h, depending on the spatial extent and initial slope of the gradient. At the average radii (37.9 mm and 110 mm, respectively), these rotational velocities result in centripetal accelerations of 174,000 g and 207,000 g, respectively. To fractionate TLA100.3 tubes, a modified Beckman Fractionation System (Beckman-Coulter) was utilized in an upward displacement mode using Fluorinert FC-40 (Sigma￾Aldrich) as a dense chase media. 25 ml fractions were collected. To fractionate SW41 centrifuge tubes, a Piston Gradient Fractionator system was utilized (Biocomp Instruments, Canada). Fractions of 0.5–3.0 mm were collected (70 –420 ml in volume). In both cases, fractions were diluted to 1 ml in 2% w/v surfactant solution for optical characterization. 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Acknowledgements This work was supported by the US Army Telemedicine and Advanced Technology Research Center, the National Science Foundation and the Department of Energy. A National Science Foundation Graduate Student Fellowship (M.S.A.), a Natural Sciences and Engineering Research Council of Canada Postgraduate Scholarship (A.A.G.), and an Alfred P. Sloan Research Fellowship (M.C.H.) are also acknowledged. Furthermore, J. Suntivich, X. Du and M. Disabb are gratefully recognized for measurement of optical absorbance spectra (J.S., X.D.) and evaporation of Au electrodes (M.D.). We thank J. Widom and the Keck Biophysics Facility for use of their ultracentrifuges, J. Chen for providing laser-ablation-grown SWNTs, and L. Palmer and Ph. Avouris for useful discussions. Correspondence and requests for materials should be addressed to M.C.H. Supplementary Information accompanies this paper on www.nature.com/naturenanotechnology. Author contributions All authors conceived and designed the experiments; M.S.A. and A.A.G. performed the experiments; J.F.H. measured and analysed the optical spectra; and all authors co-wrote the paper. Competing financial interests The authors declare that they have no competing financial interests. Reprints and permission information is available online at http://npg.nature.com/reprintsandpermissions/ ARTICLES nature nanotechnology |VOL 1 | OCTOBER 2006 | www.nature.com/naturenanotechnology 65 ©2006 NaturePublishingGroup