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), in contrast with the heir larger electronic polarizability b. Additionally, the packing semiconducting SWNTs, which switched by a factor of >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 areBlack 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