Semiconductor Separation of Single-wall

Apr 29, 2010 - The separation of semiconducting and metallic single-wall carbon ..... the expense of the purity of M-SWCNTs with use of agarose gel me...
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Diameter-Selective Metal/Semiconductor Separation of Single-wall Carbon Nanotubes by Agarose Gel Huaping Liu,†,‡ Ye Feng,†,‡,§ Takeshi Tanaka,† Yasuko Urabe,† and Hiromichi Kataura*,†,‡ Nanosystem Research Institute, National Institute of AdVanced Industrial Science and Technology (AIST), Tsukuba, Ibaraki 305-8562, Japan, Japan Science and Technology Agency, CREST, Kawaguchi, Saitama 330-0012, Japan, and Institute of Materials Science, UniVersity of Tsukuba, Ibaraki 305-8573, Japan ReceiVed: February 25, 2010; ReVised Manuscript ReceiVed: April 20, 2010

The separation of semiconducting and metallic single-wall carbon nanotubes (SWCNTs) by agarose gel chromatography was investigated. SWCNTs dispersed in sodium dodecyl sulfate (SDS) solution were applied to the top of the gel column. Metallic SWCNTs were eluted with SDS solution. Subsequently, the semiconducting SWCNTs that remained in the gel column were collected with sodium deoxycholate (DOC) solution as the eluant. By the successive addition of DOC solutions with concentrations ranging from 0.05 to 2 wt % and fractional collection at each concentration, we found that smaller-diameter enriched S-SWCNTs were eluted first with the lower concentration DOC solutions and then larger-diameter enriched S-SWCNTs were eluted with the higher DOC concentration solutions. Thus, diameter-selective enrichment of semiconducting SWCNTs was achieved. Diameter-selective enrichment of metallic SWCNTs was also demonstrated by adding a series of SDS solutions with different concentrations. These results demonstrate that agarose gel can be used to simultaneously separate metallic and semiconducting SWCNTs and to perform diameter separation of these SWCNTs. 1. Introduction Single-wall carbon nanotubes (SWCNTs) are promising for device fabrication because of their excellent mechanical and electronic properties.1 The electronic properties (such as metallic (M) or semiconducting (S)) of SWCNTs are determined by their diameters and chiral angles (which are characterized by the chiral indices (n, m)). For S-SWCNTs, the bandgap varies inversely with diameter. For both M- and S-SWCNTs, the dominant optical transitions vary with diameter and chiral vector. SWCNTs are usually grown in complex mixtures containing many different structure types. The structural heterogeneity of as-grown SWCNTs gives rise to striking differences in their electronic behaviors and often results in low on-off ratios, low effective field-effect mobilities, and low-yield high-performance electronic devices.2,3 Consequently, one of the critical tasks to realize applications based on SWCNT films is to obtain SWCNTs with well-defined structures and electronic properties. Although several research groups have made some breakthroughs showing that preferential growth of SWCNTs with a narrow chirality distribution is possible,4-6 controlled growth of SWCNTs with preselected structures and electronic properties still remains a challenge. Thus, the postgrowth separation of SWCNTs is expected to play an important role in achieving monodisperse structures and properties. So far, various postgrowth methods have been developed to separate SWCNTs such as enrichment by selective functionalization,7,8 eletrophoretic separation,9 selective oxidation,10 electrical breakdown,11 ultracentrifugation,12,13 and chromatography.14-17 However, the * To whom correspondence should be addressed. E-mail: h-kataura@ aist.go.jp. Tel: +81-29-861-2551. Fax: 81-29-861-2786. † Nanosystem Research Institute, National Institute of Advanced Industrial Science and Technology. ‡ Japan Science and Technology Agency. § Institute of Materials Science, University of Tsukuba.

problems in yield, purity, cost, etc. for industrial separation of SWCNTs are present in these methods.18 Most recently, Tanaka et al. have developed a novel and simple method for large-scale separation of M- and S-SWCNTs that uses agarose gel.19,20 On the basis of this method, a chromatographic method has been developed to further simplify the process of separating M- and S-SWCNTs.21,22 Simply, SWCNTs dispersed in sodium dodecyl sulfate (SDS) solution are applied to a column filled with agarose gel. M-SWCNTs pass through the column, whereas S-SWCNTs are trapped in the gel. S-SWCNTs are then eluted with sodium deoxycholate (DOC) solution. Although Tanaka et al. obtained high-purity M- and S-SWCNTs by this method, they did not report diameter-selective enrichment of SWCNTs. Column-gel filtration chromatography is extensively used for purifying and performing length separation of SWCNTs, but it has not been used for separating M- and S-SWCNTs.23-28 Interestingly, Heller et al. reported length-dependent diameter separation of SWCNTs using a gel column, but they were unable to separate M- and S-SWCNTs.28 On the basis of these studies, realizing simultaneous M- and S-SWCNT separation and diameter separation by gel chromatography is an interesting issue. In the present report, we investigated the separation of Mand S-SWCNTs by gel chromatography. We found that after elution of M-SWCNTs with SDS solutions, smaller-diameter enriched S-SWCNTs in the gel were eluted first with lowerconcentration DOC solutions and then larger-diameter enriched S-SWCNTs were eluted with higher-concentration DOC solutions, when we employed a series of DOC solutions with concentrations ranging from 0.05 to 2 wt % as the eluants. Similar results were obtained for M-SWCNTs when SDS solutions with concentrations ranging from 0.1 to 2 wt % were employed as the eluants in sequence. Therefore, by fractional collection, M- and S-SWCNT separation and diameter separation of SWCNTs could be simultaneously achieved by gel

10.1021/jp1017136  2010 American Chemical Society Published on Web 04/29/2010

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Figure 1. (a) Optical absorption spectra (normalized at 620 nm) and (b-d) normalized photoluminescence maps of the pristine HiPco SWCNTs and corresponding separated S- and M-SWCNTs.

chromatography. Atomic force microscopy (AFM) analysis revealed no length differences for the diameter-separated SWCNTs, indicating that the diameter enrichment in our column chromatography method was unrelated to the length of SWCNTs. On the basis of these findings, the diameter separation mechanism of SWCNTs is proposed. 2. Experimental Section SWCNTs produced by high-pressure catalytic CO decomposition (HiPco, RL500, Carbon Nanotechnology, 1.0 ( 0.3 nm) were used as the starting material. A 100 mg HiPco SWCNTs sample was dispersed for 20 h in 100 mL of purified water with 2 wt % sodium dodecyl sulfate (SDS, 99%, SigmaAldrich) using an ultrasonic homogenizer (Sonifire 450D, Branson) equipped with a 0.5-in. flat tip with a power density of 20 W/cm2. During sonication, the bottle containing the sample solution was immersed in a bath of cold water to prevent heating. The sample was centrifuged at 197 000 g for 15 min with a swing bucket rotor (S52ST, Hitachi Koki) to remove bundles and insoluble materials. SWCNT separation was performed by a continuous separation method based on column chromatography.21 A plastic column (8 cm in length and 1.5 cm in inner-diameter) was filled with Sepharose 2B gel (a bead-formed agarose-based gel matrix, GE Healthcare, bead size range 60-200 µm). After filling the column with the gel, the gel column was compressed slightly to a final height of ∼3.5 cm. For M- and S-SWCNT separation, 5 mL of SWCNT/SDS dispersion was applied to the top of the column and the eluant of 2 wt % SDS aqueous solution was pushed through the column. M-SWCNTs were collected in the eluant. After the addition of ∼50 mL of SDS solution, the collected eluant no longer contained nanotubes and the gel medium became green, indicating that most of the M-SWCNTs were washed away from the gel column. The S-SWCNTs bounded to the medium were collected by adding sodium deoxycholate (DOC, Wako Pure Chemical Industries) solutions

with concentrations of 0.05, 0.1, 0.25, 0.5, and 2 wt % in series. Fractions were collected in 2-3 mL portions at every DOC concentration. A lower-concentration DOC eluant was replaced by a higher-concentration eluant when no nanotubes were detected in the collected fractions. Optical absorption data were recorded with an UV-vis-NIR spectrophotometer (Shimadzu, SolidSpec-3700DUV), using a quartz cell with a path length of 5 mm. The chirality distribution of the collected S-SWCNTs was characterized by photoluminescence (PL) spectroscopy. PL spectra were measured with a spectrofluorometer (Horiba, Nanolog) equipped with a liquidnitrogen-cooled InGaAs near-IR alloy detector. The slit widths were 7 nm for both excitation and emission. The raw data were corrected for instrumental factors. The samples for AFM characterization were prepared by casting a SWCNT dispersion onto a SiO2/Si substrate coated with a self-assembled monolayer of 3-aminopropyltriethoxysilane (Sigma-Aldrich) and rinsing it with water. AFM images were recorded by dynamic force mode AFM (SII, SPI3800). Raman spectra were measured with a triple monochromator (Photon Design, PDPT3-640S) equipped with a charge-coupled device detector. The sample was excited by a helium-neon laser beam (wavelength: 633 nm; power: 10 mW) or an argon-ion laser beam (wavelength: 488 nm; power: 20 mW). 3. Results and Discussion To demonstrate effective MS separation of HiPco SWCNTs by gel column chromatography, M- and S-SWCNTs were collected with use of 2 wt % SDS and 2 wt % DOC solutions as eluants in series. Figure 1 shows optical absorption spectra and PL maps of the pristine HiPco SWCNTs and the corresponding separated M- and S-SWCNTs. Due to the quasi 1-D structure of the nanotubes, diameter-dependent van Hove maxima appear in the density of states. This causes the SWCNT absorption spectra to exhibit sharp interband transitions associated with van Hove singularities E11 (v1 f c1), E22, and E33 of

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S- and M-SWCNTs. For HiPco SWCNTs, the absorption peaks around 900-1350, 550-900, and 300-450 nm derive from first, second, and third optical transitions of S-SWCNTs, and were designated as S11, S22, and S33, respectively. The absorbance peak around 450-650 nm represented the first optical transition of M-SWCNTs (M11). The optical absorption spectra in Figure 1a reveal successful separation of HiPco M- and S-SWCNTs by agarose gel chromatography.19-21 S-SWCNTs also display nearinfrared fluorescence features in the 800-1600 nm range that are consistent with first van Hove (E11) transitions.29 The chirality distribution of the S-SWCNTs can be easily determined from the PL map, in which the emission wavelength is a function of the excitation wavelength. Miyata et al. demonstrated that the chirality-assigned PL intensity reflected the relative abundances of semiconducting nanotubes with different diameters.30 Comparing the PL maps reveals that the collected S-SWCNTs have similar diameter distributions as the pristine HiPco SWCNTs, except that the relative abundance of (8, 6) increased. The PL map of the collected M-SWCNTs in Figure 1d shows that some S-SWCNTs still remain in the M-SWCNT solution, in which larger-diameter S-SWCNTs dominate. Therefore, the slight change in PL peak relative intensities of several species in the collected S-SWCNTs is possibly related to the incomplete separation of S-SWCNTs from M-SWCNTs. On the other hand, the separation of M-SWCNTs from S-SWCNTs perhaps has removed a nonradiative recombination pathway and also affected PL peak relative intensities of S-SWCNTs. In previous studies,19-21 the separation of M- and S-SWCNTs by a gel is explained by selective adsorption of surfactants on M- and S-SWCNTs due to their different structures. Since HiPco S-SWCNTs have a wide diameter distribution,17 selective adsorption of the DOC surfactant on S-SWCNTs with different diameters is possible. If so, diameter separation of S-SWCNTs could be realized by fractional collection during the elution of S-SWCNTs with the DOC surfactants. To clarify this hypothesis, we fractionally collected S-SWCNTs in 2-3 mL portions with an eluant of 2 wt % DOC. A white gel was recovered after elution of S-SWCNTs with DOC solution, indicating that most of the S-SWCNTs had been removed from the gel. However, diameter-selective enrichment was not detected in the collected portions, possibly because the high concentration of DOC eluant used does not generate clear selectivity in the adsorption of the surfactants on nanotubes with different diameters. For this, we employed a 0.05 wt % DOC solution as the eluant and 12 fractions of 2-3 mL were collected. The color of the column gel was still green. These results imply that many S-SWCNTs still remained in the gel. The absorption spectra of fractions 1, 3, 6, and 9 at the DOC concentration of 0.05 wt % are presented in Figure 2, which respectively correspond to spectra a, b, c, and d. Comparison of these spectra with that of the collected S-SWCNTs with the DOC eluant of 2 wt % (Figure 1a) reveals that the relative intensities of the absorbance peaks at the higher wavelengths (1150-1300 nm) in the S11 band are much lower. At the same time, spectra a-d in Figure 2 show that the relative intensities of the absorption peaks changed with increasing elution time. These results indicate that the diameter-selective enrichment occurs in these collected fractions. To remove the residual S-SWCNTs from the column gel, we continuously employed DOC solutions with concentrations of 0.1, 0.25, 0.5, and 2 wt % as the eluants in series. Similarly, factional collection in 2-3 mL portions was performed at every DOC concentration. For these higher-concentration DOC eluants, the optical absorption spectra of the collected SWCNTs

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Figure 2. Optical absorption spectra of the selectively enriched S-SWCNTs fractions by fractional collection with DOC eluants of different concentrations: (a) DOC 0.05 wt %, fraction 1, (b) DOC 0.05 wt %, fraction 3, (c) DOC 0.05 wt %, fraction 6, (d) DOC 0.05 wt %, fraction 9, (e) DOC 0.1 wt %, (f) DOC 0.25 wt %, (g) DOC 0.5 wt %, and (h) DOC 2 wt %.

did not change clearly with an increase in elution time at each concentration. Only one optical absorption spectrum is presented for each DOC concentration. The absorption spectra of the collected S-SWCNT with 0.1, 0.25, 0.5, and 2 wt % DOC solutions respectively correspond to spectra e, f, g, and h in Figure 2. With an increase in the concentration of DOC eluants, a great change in the optical absorption spectra was observed, indicating that an increase in the concentration of DOC eluants leads to the diameter-selective enrichment of S-SWCNTs. When we continued to employ DOC solutions with concentrations higher than 2 wt %, no more SWCNTs were collected, implying that most of the S-SWCNTs trapped in the gel had been eluted at the concentration of 2 wt %. To clarify the diameter-selective enrichment with increasing elution time at the DOC concentration of 0.05 wt % and with increasing the concentration of DOC eluant from 0.05 to 2 wt %, we carefully investigate the optical absorption spectra of the different S-SWCNT fractions. As shown in Figure 2, a corresponding assignment of chiralities is given based on the spectra of the single-chirality semiconducting species in HiPco SWCNTs.17 The wavelength of the S11 band in the absorption spectra generally increases as the diameter of the SWCNTs increases. In Figure 2, when we divide the S11 band into two sub-bands (indicated with different-color shadows), we can easily conclude that, in both sub-bands, smaller-diameter enriched S-SWCNTs are eluted first with the lower-concentration DOC eluant, and subsequently larger-diameter enriched S-SWCNTs are eluted with increasing elution time at 0.05 wt % and with increasing the DOC concentration from 0.05 to 2 wt %. Because the S-SWCNTs with similar diameters coexist in HiPco SWCNTs, it is difficult to precisely distinguish their absorption peaks in optical absorption spectra. For example, the absorption peaks of (8, 4), (9, 4), and (7, 6) are close to each other and usually form a single broad peak.17 Accordingly, we present the corresponding PL maps of the collected SWCNT portions in Figure 3, which gives a clear visualization of the chirality distribution of each collected SWCNT fraction. On the basis of the assignment of chirality in the PL maps, we can conclude that elution of semiconducting tubes roughly follows the order (9, 1), (8, 3), (6, 5), (9, 4), (7, 5), (8, 4), (8, 6), (7, 6),

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Figure 3. Normalized photoluminescence maps of the selectively enriched S-SWCNTs fractions by fractional collection with DOC eluants having different concentrations: (a) DOC 0.05 wt %, fraction 1, (b) DOC 0.05 wt %, fraction 3, (c) DOC 0.05 wt %, fraction 6, (d) DOC 0.05 wt %, fraction 9, (e) DOC 0.1 wt %, (f) DOC 0.25 wt %, (g) DOC 0.5 wt %, and (h) DOC 2 wt %.

Figure 4. Photographs of the S-SWCNT solutions corresponding to panel b (left) and panel g (right) in Figure 3.

(10, 2), and (9, 5). These results reveal the same consistent enrichment trend as that observed in the absorption spectra (Figure 2). We know that both isolated and bundled SWCNTs contribute to optical absorption spectra, whereas isolated SWCNTs mainly contribute to PL spectra. The consistent enrichment in the chiralities between PL spectra and optical absorption spectra indicates that most of the collected SSWCNTs are isolated. The absorbance of S11 bands is known to shift to longer wavelengths with increasing diameter, which gives rise to SWCNTs with different colors. As shown in Figure 4, the color difference between the S-SWCNTs in fraction 3 of 0.05 wt % DOC eluant and those in the fraction of 0.5 wt % DOC eluant also demonstrates that diameter-selective enrichment of S-SWCNTs occurs. In our previous paper,20 it was demonstrated that high-purity S-SWCNTs could be obtained at the expense of the purity of M-SWCNTs with use of agarose gel method. Similarly, in the present study, we selected the separation parameters for highpurity S-SWCNTs trapped in the gel, thus resulting in the presence of some large-diameter enriched S-SWCNTs in the first separated M-SWCNT solution (as shown in Figure 1d). Therefore, it is necessary to improve the purity of M-SWCNTs by the second separation. Since diameter-selective enrichment of the S-SWCNTs can be performed by gel column chromatography, it is expected that the diameter distribution of large-

diameter S-SWCNTs in M-SWCNTs can be refined by the second separation of M- and S-SWCNTs. To achieve this, we repeated the separation of M- and S-SWCNTs of the first separated M-SWCNTs followed by fractional collection of S-SWCNTs at each concentration of the DOC eluants ranging from 0.05 to 2 wt %. Figure 5 shows the optical absorption spectra and PL maps of the collected S-SWCNT fractions. The results clearly demonstrate diameter separation of the SSWCNTs in the first separated M-SWCNT solutions. The Raman spectra shown in Figure 5b indicate that the purity of the M-SWCNTs was greatly improved by the second separation of M- and S-SWCNTs. Successful diameter separation of S-SWCNTs by changing the DOC concentration suggests the possibility of diameter separation of M-SWCNTs by using SDS eluants with different concentrations. To demonstrate this, the first separated MSWCNTs were condensed by centrifugation and diluted in 0.1 wt % SDS solutions, and then employed as the starting material. SDS solutions with concentrations of 0.1, 0.25, and 2 wt % were used as eluants in sequence. The optical absorption spectra in Figure 6a show little change in the absorption peaks of M-SWCNTs with increasing SDS concentration. This is possibly due to the presence of S-SWCNTs in the separated M-SWCNTs and partially overlapping of the M11 band with the S22 band of HiPco SWCNTs. Raman spectroscopy is a powerful tool for characterizing the diameters of M-SWCNT. A careful study of the plots given in ref 31 reveals that the excitation wavelengths of 488 and 633 nm are suitable for characterizing smallerdiameter and larger-diameter M-SWCNTs, respectively. To confirm diameter separation of M-SWCNTs, the collected M-SWCNTs portions were further characterized by Raman spectra obtained by using excitation wavelengths of 488 and 633 nm. As shown in Figure 6b, in the case of 633 nm excitation wavelength, the peak intensity ratio of larger-diameter (196 cm-1) and smaller-diameter (223 cm-1) metallic nanotubes clearly increased with increasing SDS eluant concentration. Similarly, an increase in the peak intensity ratio of the largerdiameter (259 cm-1) and smaller-diameter (308 cm-1) MSWCNTs was also observed in the case of 488 nm excitation wavelength (Figure 6c). These results confirm that diameterselective enrichment of M-SWCNTs was achieved by employing

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Figure 5. (a) Optical absorption spectra and (c, d) normalized photoluminescence maps of S-SWCNT portions by fractional collection with DOC eluants having different concentrations. (b) Raman spectra of the pristine HiPco SWCNTs and the corresponding first and second separated M-SWCNTs obtained with an excitation wavelength of 633 nm. In panel a, spectra a and b are the optical absorption spectra of the S-SWCNTs of the fraction with DOC 0.05 wt % and the fraction with DOC 0.5 wt %, respectively. (c, d) The corresponding PL maps of spectra a and b in panel a.

Figure 6. Optical absorption spectra (a) and Raman spectra (b, c) of M-SWCNTs with 0.1%, 0.25%, and 2% SDS elutions, respectively. Raman spectra with excitation wavelengths of (b) 633 and (c) 488 nm.

a series of SDS eluants with different concentrations. Additionally, panels b and c of Figure 6 show that the higher relative abundance of S-SWCNTs was present in the M-SWCNTs fraction collected with the SDS concentration of 0.1 wt %. With an increase in the SDS eluant concentration, the relative abundance of S-SWCNTs to that of M-SWCNTs decreased greatly. The reason for the change of the relative abundance of S-SWCNTs with an increase in the SDS concentration is still unclear. An attractive feature of the column chromatography method is that the gel can be used to repeatedly separate nanotubes after re-equilibration with SDS solutions.21 However, as the number of separation cycles increases, the color of the gel become increasingly green and the effectiveness of nanotube separation decreases gradually. These results indicate that more and more species become permanently trapped in the gel. We directly characterized the species remaining in the gel by Raman and optical absorption spectroscopies. The results in panels a and b of Figure 7 reveal that the main component of the permanently

trapped species is S-SWCNTs and their diameter distribution is similar to that of the pristine HiPco SWCNTs, indicating that permanent trapping of the SWCNTs is not diameter selective. To determine the nanotube morphologies of the trapped SWCNTs in the gel, we desorbed the nanotubes from the gel by sonication of the gel in a 2 wt % DOC solution for 60 min followed by the centrifugation at 22 600 g for 2 h. The supernatant solution was characterized by Raman and absorption spectroscopies. The Raman and absorption spectra were the same as those obtained by direct measurement of agarose gel, indicating that SWCNTs had been successfully removed from the gel. AFM analysis (Figure 7c) reveals that the morphologies of the SWCNTs remaining in the gel differ little from those of the semiconducting nanotubes previously eluted with DOC elutions (Figure 8). In the present experiment, we believe that the separation of M- and S-SWCNTs still resulted from the selective adsorption of the SDS surfactants on SWCNTs based on electronic types. SDS surfactants are likely more strongly adsorbed on M-

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Figure 7. (a) Raman spectra (normalized, excitation wavelength 633 nm), (b) optical absorption spectra (normalized at 620 nm) of SWCNTs captured in gel and pristine HiPco SWCNTs, and (c) AFM image of the nanotubes captured in the agarose gel. The nanotubes permanently remaining in the gel have been desorbed from the gel by sonication in 2% DOC solutions for 60 min and centrifugation at 22 600 g for 2 h.

Figure 8. AFM images (a, b) of the S-SWCNTs corresponding to panels b and g in Figure 3.

SWCNTs than S-SWCNTs, resulting in higher-density SDS on the M-SWCNT surface. The high-density SDS suppressed the interactions between the M-SWCNTs and the gel, causing M-SWCNTs to pass directly through the gel by flow, while S-SWCNTs were trapped in the gel because of lower-density SDS on their surfaces and thus stronger interaction with the gel. When DOC surfactants were applied, the S-SWCNTs were fully coated by the DOC surfactants, and then desorbed from the gel. Interestingly, our experimental results reveal the diameter-selective enrichment of SWCNTs when we employed a series of eluants with different concentrations and fractionated SWCNTs. Diameter separation of SWCNTs by gel chromatography has been reported in ref 28. It was reported that length separation of SWCNTs caused this diameter separation. In this present study, to investigate the mechanism of the diameter-selective enrichment of SWCNTs, we used AFM to characterize the SWCNT portions enriched with different diameters. Figure 8 shows AFM images of the S-SWCNTs enriched with smallerdiameter nanotubes collected with 0.05 wt % DOC and those enriched with larger-diameter nanotubes collected with 0.5 wt % DOC. Clearly, the S-SWCNTs enriched with smallerdiameter and larger-diameter nanotubes have similar morphologies. Consequently, the diameter separation of SWCNTs in this present study cannot be attributed to differences in their lengths. The diameter-selective enrichment of S-SWCNTs with increasing elution time at the DOC concentration of 0.05 wt % and with increasing DOC eluant concentration implies that the DOC surfactant possibly has selectivity in the adsorption on different-diameter S-SWCNTs. On the basis of this, we propose the following mechanism for diameter separation. When we

employed single-concentration 2 wt % DOC as the eluant, diameter-selective enrichment of S-SWCNTs was not achieved. This may be because the concentration of the DOC surfactants is sufficiently high to cover the surfaces of most of the S-SWCNTs within a short time, resulting in that diameterselective adsorption of the surfactant was observed with difficulty. At lower DOC concentrations of 0.05 wt %, the DOC surfactant cannot coat most of the S-SWCNTs and it is preferably adsorbed on smaller-diameter SWCNTs possibly due to their higher surface energies. The DOC coatings on these smaller-diameter S-SWCNTs suppress their interaction with the gel. Thus, S-SWCNTs enriched with smaller-diameter SWCNTs were eluted with a low-concentration DOC eluant at an earlier elution time. With increasing elution time and DOC concentration, larger-diameter S-SWCNTs are coated with DOC surfactants and are then eluted. As a result, we collected differentdiameter enriched S-SWCNTs by fractional collection. To obtain a narrower diameter distribution, we also tried employing a DOC concentration lower than 0.05 wt %. Unfortunately, no SWCNTs were collected. Therefore, a DOC concentration of 0.05 wt % is probably the critical concentration for collecting S-SWCNTs. The diameter-selective enrichment of M-SWCNTs can also be explained by this mechanism. Here, selective adsorption of surfactants on the nanotubes means that the sidewalls of some nanotubes cannot be fully coated by surfactants, especially at low eluant concentrations. This causes their stronger adsorption onto the gel and longer retention time due to electrostatic interactions or van der Waals forces. However, this may also cause some nanotubes to become permanently trapped in the gel, such that they cannot be removed by flow pressure. With increasing number of nanotube separation cycles, more and more nanotubes were accumulated in the agarose gel, eventually causing deactivation of the gel. 4. Conclusions We successfully realized simultaneous M- and S-SWCNT separation and diameter-selective enrichment of SWCNTs by gel chromatography. During the separation, M- and S-SWCNTs were eluted with SDS and DOC eluants in sequence. Diameterdependent enrichment of M- and S-SWCNTs was demonstrated by employing a series of SDS/DOC solutions with different concentrations to fractionate M-/S-SWCNTs. Such diameter separation of SWCNTs must be due to selective adsorption of the SDS/DOC surfactants on nanotubes with different diameters. These findings open up a novel and simple way to sort SWCNTs based on electronic structure.

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