J. Phys. Chem. C 2008, 112, 18889–18894
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Separations of Metallic and Semiconducting Carbon Nanotubes by Using Sucrose as a Gradient Medium Kazuhiro Yanagi,*,† Toshie Iitsuka,† Shunjiro Fujii,† and Hiromichi Kataura†,‡ Nanotechnology Research Institute (NRI), National Institute of AdVanced Industrial Science and Technology (AIST), Tsukuba 305-8562, Japan, and JST-CREST ReceiVed: July 31, 2008; ReVised Manuscript ReceiVed: September 9, 2008
The separation of metallic and semiconducting single-wall carbon nanotubes (SWCNTs) was achieved using sucrose as a gradient medium in density-gradient ultracentrifugations (sucrose-DGU). By lowering the temperature during sucrose-DGU and tuning the concentrations of the surfactants, metallic and semiconducting SWCNT samples were obtained in high purity. The purity of the metallic and the semiconducting SWCNTs obtained by the sucrose-DGU was estimated to be 69% and 95%, respectively, from their optical absorption spectra. It is well-known that the amounts and types of surfactants significantly influence the separations. However, the authors found that the temperature during centrifugation was also an important parameter that improved the metal-semiconductor separation capability. Introduction Single-wall carbon nanotubes (SWCNTs) exhibit metallic and semiconducting characteristics depending on their chirality.1 Many production techniques for SWCNTs have been proposed. However, the reported samples were produced in a mixed state of metallic and semiconducting SWCNTs. Such mixture characteristics have impeded the development of device applications for SWCNTs as well as a detailed understanding of their physical properties. The development of a technique to separate the metallic and semiconducting types has been crucial for their applications, and thus many different kinds of separation techniques have been proposed. (Note the recent review2 by M.C. Hersam.)3-7 Density-gradient ultracentrifugation (DGU) is a well-known technique to purify macromolecules and biological materials. Recently,Arnoldetal.werethefirsttoachievemetal-semiconductor separation of SWCNTs using DGU.7 Upon the basis of their fundamental technique,7,8 various DGU techniques concerning metallic-SWCNT purifications, length, and chirality selections of SWCNTs have been reported.9-14 DGU is one of the most suitable techniques for obtaining high-purity SWCNTs of a single-electronic type. It is noteworthy that reported DGU applications have always used iodixanol as a gradient medium. (In this study, this technique is abbreviated as iodixanol-DGU.) Separation using other gradient media such as sucrose, which is the most popular gradient medium, has not been reported yet. It is well-known that surfactants play an essential role in metal-semiconductor separation using DGU.7,11,12 However, the role of the gradient media and their effects on the separation capability have not been understood well. If iodixanol is the only gradient medium that can separate metallic and semiconducting types of SWCNTs, the interaction between the SWCNTs and iodixanol must be taken into account to be able to correctly understand the mechanisms of the metal-semiconductor separation in DGU. Moreover, the use of iodixanol presents the following three * Corresponding author. E-mail:
[email protected]. † AIST. ‡ JST-CREST.
problems. (1) Iodixanol has iodine atoms (its chemical structure is shown in Figure S1 of the Supporting Information). If iodine remains in the semiconducting SWCNT sample obtained by iodixanol-DGU, the iodine will work as an electron acceptor for the SWCNT.15,16 The remaining iodine will increase the conductivity of the semiconducting SWCNTs and will degrade their semiconducting characteristics. (2) Iodixanol is a relatively large molecule (molecular weight: 1550), thus it takes considerable time to remove iodixanol from SWCNT samples. (3) Iodixanol is an expensive reagent. In light of the above problems, it is important to clarify whether it is possible to separate metallic and semiconducting SWCNTs effectively using a gradient medium other than iodixanol. In this study, we first report on the separation of metallic and semiconducting SWCNTs using sucrose as the gradient medium. (Hereafter, this technique is abbreviated as sucroseDGU.) Compared to iodixanol, sucrose has the following advantages: (1) Sucrose does not work as an electron donor or accepter for SWCNTs. (Its chemical structure is described in Figure S2 of the Supporting Information. It does not have any reactive atoms and aromatic groups.) Thus sucrose contamination in the semiconducting SWCNTs will not strongly influence their semiconducting properties. (2) Sucrose is a small molecule (molecular weight: 342) compared to iodixanol. Sucrose can be easily removed from SWCNT solutions using a centrifuge filter (e.g., Amicon 30k NMWL, Millipore Co.). (3) Sucrose is inexpensive. Metal-semiconductor separation through sucroseDGU is typically not possible as shown in Figure 1. However, the metal-semiconductor separation capability can be significantly improved by tuning the concentrations of the surfactants and the temperature during DGU. As a result, we obtained highly enriched solutions of metallic and semiconducting SWCNTs using sucrose-DGU. Experimental Sample Preparation. SWCNTs produced by arc discharge were used in this study because absorption bands due to metallic and semiconducting nanotubes (the first optical transition of the metallic, M11, and the second optical transition of the semiconducting, S22, nanotubes) can be easily identified as shown in
10.1021/jp806822x CCC: $40.75 2008 American Chemical Society Published on Web 11/11/2008
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Yanagi et al. dispersion (10 µL) in dimethylformamide (DMF) was dropped onto a SiO2 (100 nm)/Si substrate with the Au (100 nm)/Cr (10 nm) deposited in advance. Then the dispersion was blown from the substrate using nitrogen gas. The residual DMF solution on the substrate was dried by heating at 100 °C. The electrical measurements were conducted in air at room temperature using a semiconductor parameter analyzer (4200-SCS, Keithley). The Si substrate was used as a back gate electrode. The channel length and width were designed to be 2 and 200 µm, respectively. Results and Discussion
Figure 1. (a) Picture of a centrifuge tube after sucrose-DGU at 25 °C, SC 2%, and SDS 1%. (b) The absorption spectrum and (c) the ratio of peak intensities of absorption bands of the metallic (M11) and the semiconducting nanotubes (S22) at each fraction. Dotted red lines indicate the absorption spectrum and the M11/(M11 + S22) value of initial SWCNT () 0.25).
Figure 1. The SWCNTs were purchased from Meijo NanoCarbon Co. (ARC-APJ). The average diameter and the diameter distribution of the SWCNTs were estimated to be approximately 1.4 and 0.1 nm, respectively, from their optical absorption spectra.17 Before DGU, the SWCNTs were purified as follows. An amount of 100 mg of the SWCNTs was dispersed in 100 mL of a sodium cholate (SC, Sigma-Aldrich Inc.) 1% (weight per volume) solution by sonicating them for 20 h (Branson Co. Sonifier S-250D). In this study, the concentrations of surfactants were adjusted in weight per volume. The dispersed solution was centrifuged at 288 000 g for 1 h, and then supernatant was collected. The collected sample was centrifuged again at 288 000 g for 20 h, then the supernatant was removed and the condensed part (pellets) at the bottom of the centrifuge tube was collected. In this step, the sample was slightly purified because most of the supernatant was amorphous carbon. Then the pellets were dispersed again into a SC 1% solution. The dispersed solution was mixed with a controlled amount of sucrose solution, and the concentration of the sucrose in the solution was adjusted to 40 wt % (wt % means weight per weight). The concentrations of the SC and sodium dodecyl sulfate (SDS, Sigma-Aldrich Inc.) in the SWCNT solution were also adjusted in this step. The solution was sonicated for 2 h and centrifuged for 1 h at 288 000 g, and then the supernatant was collected. The supernatant SWCNT solution was used for DGU. DGU Process. A density gradient in a centrifuge tube before centrifugation was formed from 1.29 g/mL (60 wt % sucrose) to 1.20 g/mL (45 wt % sucrose) by layering with pipettes. The concentrations of the surfactants in the gradient solution were adjusted to be the same as those in the SWCNT solution prepared above. The SWCNT solution was layered on top of the gradient solution. Then the centrifuge tube was centrifuged at 402 000 g for 20 h (Rotor P65VT3, CP100WX, Hitachi Koki Co.) at a controlled temperature. Fractions of the solution in a centrifuge tube after centrifugations were collected in a manner similar to that in ref 11. Optical Characterization. The optical absorption spectra of the SWCNT solutions were measured using a commercial spectrophotometer (SolidSpec-3700DUV, Shimazu Co.). Fabrication and Electrical Measurement of an SWCNTField Effect Transistor (FET). A FET was fabricated by solution process as follows. Metal or semiconducting SWCNT
First we briefly discuss the theoretical background of metal-semiconductor separation using DGU. During ultracentrifugation, the nanotubes are moved in the density gradient solution by the centrifuge force. The velocity ν of the nanotubes can be simply written using Stokes’ equation as [ref 18]
V∝
|FCNT - Fm| 2 ω η
(1)
Here, FCNT and Fm are the densities of the SWCNT and solvent, respectively. η is the viscosity, and ω is the rotation speed of centrifugation. According to eq 1, it is clear that it is necessary to enlarge the difference between the densities of the metallic, Fmetal, and the semiconducting SWCNTs, Fsemi, to improve the separation capability. Moreover, the viscosity also influences the separation capability. The viscosity of sucrose is larger than that of iodixanol. For example, the viscosity of sucrose is several times larger than that of iodixanol at 20 °C (estimated from an application sheet of Optiprep and ref 18). Thus in our sucroseDGU process, we chose to use a rotor (Rotor P65VT3, Hitachi Koki Co.) that we could rotate at high speed to compensate for the increase in viscosity. We assumed the following simple model to determine a way to improve the separation capability in sucrose-DGU. It is known that the amount and the types of surfactants attached to the surfaces of metallic and semiconducting SWCNTs significantly influence Fmetal and Fsemi.7,11 Therefore, similar to a model proposed in the Supporting Information of ref 8, we assumed that Fmetal and Fsemi were written as a linear combination of the intrinsic densities of SWCNT, Fcnt, and the additional densities due to the surfactants attached to the surfaces of the metallic and semiconducting nanotubes, Fadd_metal and Fadd_semi, respectively.
Fmetal ) Fcnt + Fadd_metal Fsemi ) Fcnt + Fadd_semi
(2)
We assumed that the intrinsic densities of the metallic and semiconducting nanotubes were the same because the density of metallic SWCNTs is almost similar to that of semiconducting SWCNTs when they are dispersed in a single surfactant solution.7,12 The additional density depends on the concentrations of surfactants attached to the metallic SWCNT, Xmetal, and the semiconducting SWCNTs, Xsemi, which can be written as19
( (
∆µmetal kT ∆µsemi Xsemi ) X1 exp kT Xmetal ) X1 exp -
)
) (3)
Here ∆µmetal (∆µsemi) are the differences in the chemical potentials between a surfactant molecule adsorbed on the metallic (the semiconducting) SWCNT surfaces and a surfactant dispersed in the solution. X1 is the concentration of singly
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Figure 2. (a) Picture of centrifuge tube after sucrose-DGU at 12.5 °C, SC 2%, and SDS 1%. (b) The absorption spectrum and (c) the ratio of peak intensities of absorption bands of metallic (M11) and the semiconducting nanotubes (S22) at each fraction. Dotted red line indicates the M11/(M11 + S22) value of initial SWCNT () 0.25).
dispersed surfactants in the solution. (Thus X is related to density.) In eq 3, T and k are the temperature and Boltzmann constant. When Fadd_metal and Fadd_semi are assumed to be proportional to Xmetal and Xsemi, the difference of Fmetal and Fsemi can be simply written as
(
|Fmetal - Fsemi| ) |Fadd_metal - Fadd_semi| ∝ |exp -
(
exp -
)
)
∆µmetal kT
∆µsemi A B | ) | + 2 + · · · | (4) kT T T
Here A and B, which are derived from the Taylor expansion of the equation, are (∆µsemi - ∆µmetal)/k and (∆µmetal2 - ∆µsemi2)/ 2k2, respectively. Equation 4 suggests that the difference between Fmetal and Fsemi becomes smaller as T increases. Therefore, the model suggests that it is not good to perform DGU at a high temperature but better at a low temperature to improve the separation capability. Figure 1 shows the results of sucrose-DGU at 25 °C. The DGU was performed at SDS 2% and SC 1% concentrations. After centrifugation, the SWCNT band was observed at the middle part of the centrifuge tube. The band was divided into five fractions (in approximately 1 mL steps), and the absorption spectra of the fractions were recorded. Then the ratio of metallic and semiconducting SWCNTs was discussed from the optical absorption spectra.6 For comparison, the absorption spectrum of the initial SWCNT is also described in Figure 1. In all fractions, the ratio of the peak intensities of the absorption bands of the metallic (M11) and the semiconducting (S22) SWCNTs [M11/(M11 + S22) value] was almost the same as that of the initial SWCNTs [M11/(M11 + S22) of the initial sample was 0.25]. These results indicate that there was no enrichment of the metallic and the semiconducting SWCNTs in sucrose-DGU under these conditions. Figure 2 shows the results of sucrose-DGU at 12.5 °C and concentrations of SDS 2% and SC 1%. Remarkably, after centrifugation, the colors of the top and the bottom parts of the SWCNT band in a centrifuge tube were slightly green and red, respectively. The band was divided into five fractions, and the absorption spectra of the fractions were recorded. In the fraction obtained from the top part, the amount of metallic SWCNTs was more than that in the initial sample. In the fraction from the bottom part, the amount of the semiconducting SWCNTs was enriched. The densities of the metallic enriched and the
Figure 3. Dependence of the ratio of peak intensities of the absorption bands of metallic (M11) and semiconducting SWCNTs (S22) on temperature during centrifugation. Dotted red line indicates the value of M11/(M11 + S22) () 0.25) in an initial SWCNT. These experiments were performed at SC 2.0% and SDS 1.0% concentrations. The curved line is drawn for a guide to the eye.
semiconducting enriched fractions were 1.22 and 1.24 g/mL, respectively. These results indicate that metal-semiconductor separation became possible when the temperature was lowered. This tendency was consistent with the prediction from our simple model in eq 4. To clarify the relationship between the temperature and the separation capability in detail, we systematically changed the temperature and checked the M11/(M11 + S22) value in the fraction of the top part of the centrifuge tubes after sucroseDGU. As shown in Figure 3, the M11/(M11 + S22) value at 25 °C was the same as that of the sample before centrifugation, 0.25. This fact indicates that metal-semiconductor separation could not be achieved at room temperature. However, the value at 12.5 °C was approximately two times larger, indicating clear enrichment of the metallic SWCNTs. As the temperature approached 2 °C, the value tended to be closer to that of the initial sample. Therefore, the separation capability worsened at temperatures that were too low. As the temperature decreases, the difference between the densities of the metallic and the semiconducting nanotubes is assumed to be enlarged according to the model of eq 4. However, at too low of a temperature, (1) the viscosity of the solution became large and (2) the solubility of the surfactants worsened and small crystals formed during DGU. Such factors, we assume, degraded the separation capability. In addition, we checked the influence of the concentrations of surfactants on the separation capability as follows. First we performed sucrose-DGU at 12.5 °C [case (a)] with SC 1.5% and SDS 1.5% concentrations (the ratio of SC to SDS was 1:1) and [case (b)] with SC 2.4% and SDS 0.6% concentrations (the ratio of SC to SDS was 4:1), and we compared these to the results [case (c)] when the ratio of SC to SDS was 2:1 (SC 2%, SDS 1%). In all cases, the SWCNT band in the centrifuge tube after sucrose-DGU was divided into five fractions, and the absorption spectra of the fractions were recorded. Then the values of M11/(M11 + S22) in the fractions were evaluated. Figure 4 shows the M11/(M11 + S22) values of the fractions in cases (a), (b), and (c). The concentrations of the surfactants significantly influenced separation capability. The amount of metallic SWCNT in fraction 1 of case (a) was smaller than that in case
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Figure 4. M11/(M11 + S22) values of fractions after sucrose-DGU at 12.5 °C (a) with SC 1.5% and SDS 1.5% and (b) with SC 2.4% and SDS 0.6% and compared to the results (c) with SC 2.0% and SDS 1.0%.
Figure 5. Relationship between values of M11/(M11 + S22) and concentrations of SDS and SC. The value of a fraction at the top part of the SWCNT band in a centrifuge tube was evaluated. Red bars indicate conditions in which highly enriched metallic SWCNT solutions were obtained. Green line in the X-Y plane indicates concentrations at which the ratio of SC to SDS was 2:1. Cross-hatched region at the bottom indicates concentrations at which it was difficult to dissolve the amount of SDS into a sucrose 60 wt % solution at 12.5 °C. The bottom value of the z-axis was set to be the M11/(M11 + S22) value of the initial SWCNT, 0.25. Therefore, the positive bars indicate solutions in which the metallic SWCNTs were enriched.
(c). In case (b), the amounts of the metallic and the semiconducting SWCNTs were not enriched in any of the fractions. In Figure 5, we summarized the relationship between the concentrations of SC, SDS, and the separation capability. The separation capability was evaluated from the M11/(M11 + S22) value in the fraction at the top part of the SWCNT band (e.g., fraction 1 in Figures 2 and 4) after sucrose-DGU at 12.5 °C. We could not perform separation experiments when the concentration of SDS was more than 1.5% because it was quite
Yanagi et al. difficult to dissolve such a large amount of SDS into a sucrose 60 wt % solution at 12.5 °C. Separation experiments were performed when all of the surfactants could be dissolved into the sucrose solutions at 12.5 °C. As far as we checked, we can apply sucrose-DGU only to SC/SDS cosurfactant solutions. For example, when sodium dodecylbenzene sulfonate (SDBS) was used as a cosurfactant, it was not possible to achieve metal-semiconductor separation by sucrose-DGU (see Figure S3 of Supporting Information). Figure 5 clearly indicates that highly enriched metallic SWCNT solutions can be obtained when the ratio of SC to SDS was 2:1. The separation capability was still good when the concentrations of SC and SDS were 0.3% and 0.15%, respectively. In iodixanol-DGU, the total concentrations of the surfactants (summation of the concentrations of the surfactants) were from 1% to 3%.7-11 In sucrose-DGU metal-semiconductor separation can be achieved in total concentrations of 0.45%, less than 1%. When total concentration was 0.15%, however, no separation was observed. Therefore, critical micelle concentration of this system is supposed to be located from 0.15% to 0.45%. The relative concentrations of surfactants significantly affected the metal-semiconductor separation capability in sucrose-DGU. Such behavior is consistent with the results in iodixanol-DGU reported by Arnold et al.7 As proposed in ref 7, separation by DGU is expected to be caused by a difference in the organization of surfactants on SWCNTs of different electronic types. Energetic balance among nanotube-surfactant, water-surfactant, and surfactant-surfactant interactions is the critical parameter for the organization.7 In our model of eq 4, such an energy balance is simply described as ∆µ. The observed results suggest that the concentrations of surfactants would influence the ∆µ value. Modification of dielectronic properties of SWCNTs by adsorption of surfactants,20 difference in the nanotube-dispersion conditions of anionic alkyl surfactants and bile salts,21 and selective interactions of molecules with metallic and semiconducting nanotubes22 may describe the physical backgrounds of the observed concentration dependence. The energy balance of the above-mentioned interactions of surfactants in a sucrose solution is expected to be different from that in an iodixanol solution; for example, solubility of SDS in a sucrose solution was lower than that in an iodixanol solution. Thus the suitable concentration (SC/SDS ) 2:1) for separation in sucrose-DGU was slightly different from that (SC/SDS ) 3:2) in iodixanolDGU.7 Figure 6 shows a picture and the absorption spectra of the highly enriched metallic and semiconducting SWCNT solutions. The enriched solutions were obtained by sucrose-DGU at the following experimental conditions: SC 0.3%, SDS 0.15%, and 10 °C. These absorption spectra were recorded in SC 1% solutions. The sample solutions were prepared after the exchange/ removal process of surfactants/sucrose using a centrifuge filter (Amicon 30kNMWL, Millipore Co.). As shown in Figure 6a, the colors of the metallic enriched and semiconducting enriched solutions were slightly green and red, reflecting their absorption spectra. The purity of the metallic and the semiconducting nanotubes was roughly estimated to be 69% and 95%, respectively, from the ratio of the absorption area of the M11 and the S22 bands. The above results were obtained from sucrose-DGU for SWCNTs produced by an arc-discharge method. As shown in Figure S4 of Supporting Information, the sucrose-DGU can apply to other kinds of SWCNTs, such as SWCNT produced by a high-pressure CO method (HiPco, Carbon Nanotechnology Inc.).
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Figure 7. Source-drain current (Isd) vs gate voltage (Vg) characteristics (Vsd ) 1 V) of FET fabricated using metallic (blue curve) and semiconducting enriched SWCNTs (red curve) obtained by sucroseDGU. Figure 6. (a) Picture of metallic SWCNT enriched (left) and semiconducting SWCNT enriched solutions (right) obtained by sucroseDGU. (b) Absorption spectra of initial SWCNT, metallic enriched, and semiconducting SWCNT enriched solutions (SC 1%).
Finally, we characterized SWCNT FETs fabricated by using metallic and semiconducting enriched SWCNTs obtained by sucrose-DGU. AFM images of the channel (Figure S5 of the Supporting Information) showed the lengths of SWCNTs were approximately 0.3 to 1 µm, that is, smaller than the channel length (2 µm). Figure 7 shows typical source-drain current (Isd) vs gate voltage (Vg) characteristics. The Isd of the metallic SWCNTs was almost constant over the entire gate voltage range. In contrast, Isd of the semiconducting SWCNTs was modulated by 3 orders of magnitude without an electrical breakdown procedure. From the histogram of on/off current (Ion/Ioff) ratios (Figure S6 of the Supporting Information), 9 out of 16 devices made of semiconducting SWCNTs showed Ion/Ioff > 102, while all devices made of metallic SWCNTs showed Ion/Ioff < 102. The observed typical Ion/Ioff value (103-102) of the semiconducting enriched sample by sucrose-DGU is comparable to or smaller than that of semiconducting enriched samples reported previously,7,23,24 102 by DNA sorting,23 104 by iodixanol-DGU,7 and 105 by extraction of semiconducting nanotubes using polyfluorene.24 Thus we expect that the purity of the semiconducting sample used here was similar to that in the DNAsorting23 but lower than that in the iodixanol-DGU7 and the extraction with polyfluorene.24 We assume that the small amounts of the metallic SWCNTs were still contained in the sample of sucrose-DGU and that the presence of such metallic SWCNTs decreased the Ion/Ioff ratios. However, there were clear differences between the electronic characteristics (Ion/Ioff and its histogram) of the metallic and the semiconducting enriched samples obtained by sucrose-DGU. Therefore we concluded that sucrose-DGU could obtain the metallic and the semiconducting enriched SWCNTs.
Conclusions We clarified that it is possible to achieve metal-semiconductor separations using sucrose as a gradient medium. This study is the first report on the metal-semiconductor separation using a gradient medium other than iodixanol. Although separations by sucrose-DGU are not possible at 25 °C, the separation capability can be significantly improved by tuning the temperature and the concentrations of the surfactants. It is known that the concentrations and types of surfactants are the most important parameters for improving separation capability. This study proposes that temperature is also an important factor that significantly influences separation capability. As mentioned in the Introduction section, sucrose has various advantages over iodixanol as a gradient medium. Therefore, the separation by sucrose-DGU that we report herein will stimulate large-scale separation of metallic and semiconducting SWCNTs by DGU. Acknowledgment. This study was supported by the industrial technology research grant program from New Energy and Industrial Technology Development Organization (NEDO) of Japan. Supporting Information Available: Chemical structures of iodixanol and sucrose, sucrose-DGU using SDBS as a cosurfactant, sucrose-DGU for HiPco nanotubes, AFM images of SWCNTs on SiO2/Si substrate between source/drain electrodes of metallic and semiconducting devices, the histogram of distribution of Ion/Ioff ratios, and a table of relationship between concentrations of surfactants and separation capability. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) Saito, R.; Fujita, M.; Dresselhaus, G.; Dresselhaus, M. S. Appl. Phys. Lett. 1992, 60, 2204–2206. (2) Hersam, M. C. Nat. Nanotechnol. 2008, 3, 387–394. (3) Krupke, R.; Linden, S.; Rapp, M.; Hennrich, F. AdV. Mater. 2006, 18, 1468–1470.
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