ARTICLE pubs.acs.org/JPCC
Adsorbability of Single-Wall Carbon Nanotubes onto Agarose Gels Affects the Quality of the Metal/Semiconductor Separation Atsushi Hirano,†,‡ Takeshi Tanaka,† and Hiromichi Kataura*,†,‡ † ‡
Nanosystem Research Institute, National Institute of Advanced Industrial Science and Technology, Tsukuba, Ibaraki 305-8562, Japan JST, CREST, Kawaguchi, Saitama, 332-0012, Japan ABSTRACT: Although the metal/semiconductor separation of single-wall carbon nanotubes (SWCNTs) using an agarose gel column has been successfully developed, little is known about the mechanism of the separation. In this study, we examined the agarose concentration dependence of the separation using this column. The amount of SWCNTs obtained from the eluate decreased with increasing agarose concentration, indicating that a smaller amount of SWCNTs was adsorbed onto the agarose gels at higher concentrations. To clarify the relation between the adsorbability of SWCNTs and the agarose concentration, various concentrations of SWCNTs were adsorbed onto 2, 4, and 6% agarose gels in a quasi-equilibrium state. The results showed that the concentration of the unadsorbed SWCNTs increased with increasing agarose concentration. By assuming that the adsorption follows the Langmuir isotherm, it was found that the effective gel volume for the adsorption was negatively correlated with the agarose concentration; specifically, the adsorbability was associated with the effective gel volume. Finally, an effect of the adsorbability on the separation using the agarose gel columns was considered. With decreasing adsorbability of SWCNTs onto agarose, the amount of SWCNTs adsorbed onto the top of the column decreased and the distribution of the SWCNTs became broadened along the column. Thus, the agarose concentration dependence of the separation quality is attributable to the adsorbability of SWCNTs onto the agarose gels.
’ INTRODUCTION Single-wall carbon nanotubes (SWCNTs) are expected to have potential applications, such as conductive films,1,2 highperformance field-effect transistors,3 nanoscale sensors,4,5 cancer therapy,6 and imaging7,8 because of their prominent mechanical, thermal, optical, and electrical properties. A key challenge for the industrialization of SWCNTs is a high-quality separation of SWCNTs into metal and semiconductor species (metal/ semiconductor separation) because as-prepared SWCNTs are obtained as mixtures of both species and contain much chirality.9 Several methods for metal/semiconductor separation have been reported, such as ultracentrifugation,1013 dielectrophoresis,1416 gel electrophoresis,17 selective oxidation,18,19 amine extraction,20,21 aromatics extraction,22,23 polymer wrapping,24,25 DNA wrapping,2629 and protein wrapping.30 Recently, our group has developed a technique to separate SWCNTs into metal and semiconductor species using agarose gel columns.31,32 In this method, sodium dodecyl sulfate is used to disperse the SWCNTs in an aqueous solution and to allow unadsorbed (metal-rich) SWCNTs to flow through the column. In addition, sodium deoxycholate is used for the elution of adsorbed (semiconductor-rich) SWCNTs. This method will be more useful for the metal/semiconductor separation than will the other techniques mentioned above because it can be readily conducted in an aqueous solution without complex technical processes. Interestingly, diameter-selective metal/semiconductor separation was observed through the control of the concentration of sodium deoxycholate, although the selectivity was not very high.32 r 2011 American Chemical Society
More recently, our group has developed a new technique to separate the chiral species of SWCNTs using an allyl dextran-based gel.33 Despite the development of these separation techniques using the gel columns, little is known about their mechanisms. It was reported that interaction of surfactants with SWCNTs is a dominant factor in the separation.34 However, agarose concentration, pH, ionic strength, and temperature are also considered to be dominant parameters for the metal/semiconductor separation. Among them, the agarose concentration is a controllable parameter influencing the adsorption of SWCNTs onto the agarose gels; nevertheless, the influence of this parameter has not yet been discussed. Thus, in this study, the effect of the agarose concentration on the metal/semiconductor separation was examined. The results showed that the agarose concentration actually accounts for the quality of the metal/semiconductor separation. The control of the agarose concentration, based on the results in the present study, should play a key role in the industrialization of the separation method.
’ MATERIALS AND METHODS Dispersion of SWCNTs. SWCNTs produced by high-pressure catalytic CO (HiPco) decomposition were purchased from NanoIntegris and were used as the starting material for the Received: August 14, 2011 Revised: September 24, 2011 Published: September 27, 2011 21723
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Figure 1. Schematic diagram of the metal/semiconductor separation using an agarose column. The separation is achieved by three steps as shown above: (1) loading of the SWCNTs dispersed in SDS solution onto the column, (2) the addition of SDS solution to allow the unadsorbed (metal-rich) species to flow though the column, and (3) the addition of DOC solution to elute the adsorbed (semiconductor-rich) species. M and S depict the metalrich species and the semiconductor-rich species, respectively.
metal/semiconductor separation. Aliquots of 30 mg of HiPco SWCNTs were predispersed in 30 mL of purified water with 1 wt % sodium dodecyl sulfate (SDS, 99%, Sigma-Aldrich) at pH 8 using an ultrasonic processor (Nanoruptor NR-350, Cosmo Bio) for 1 min at a power of 350 W. Subsequently, the solutions were dispersed using an ultrasonic homogenizer (Sonifire 250D, Branson) equipped with a 0.5 in. flat tip for 1 h at a power density of 20 W cm2. To prevent heating during sonication, the bottle containing the sample solution was immersed in a bath of water at 18 °C. To remove the residue of the catalytic metal particles, the nanotube bundles and impurities, the dispersed sample solution was centrifuged at 210000 g for 1 h using an ultracentrifuge (S80AT3 rotor, Hitachi Koki). The upper about 70% of the supernatant was collected and used for the separation. Metal/Semiconductor Separation of SWCNTs Using Agarose Gel Columns. The metal/semiconductor separation of SWCNTs was performed using agarose gel columns. In this method, several 10 mL syringes, the tops of which were open, were filled with 7 mL of 2, 4, or 6% agarose gel beads (Sepharose 2B, 4B, and 6B, GE Healthcare), the diameters of which were 60200, 45165, and 45165 μm, respectively. A schematic of the SWCNT separation process is presented in Figure 1. After the gel columns were equilibrated with a 1 wt % SDS solution, 0.5 mL of the SWCNT dispersion was applied onto the top of the column. Subsequently, a 1 wt % SDS aqueous solution was added onto the top of the column. The unadsorbed SWCNTs flowed out of the column and were collected from the bottom of the column as a flow-through fraction. The species that were adsorbed onto the gels were desorbed and were flowed out of the column by the addition of 1 wt % DOC (sodium deoxycholate) at pH 8. Batch Separation of the SWCNTs. To estimate the amount of SWCNTs adsorbed onto the agarose gel, a batch separation was conducted as follows: 125 μL of the 2, 4, or 6% agarose gel beads was mixed with 625 μL of the SWCNTs dispersed in 1 wt % SDS solution; following gentle rotation for various times, the mixture was briefly centrifuged (500 g for 1 min); and the resulting supernatant was collected as the unadsorbed fraction. Measurement of Absorption Spectra. Absorption spectra of the SWCNTs were recorded from 200 to 1350 nm using an UVvisNIR spectrophotometer (SHIMADZU UV-3600) with a quartz cell having a path length of 10 mm. In this study, the absorption peaks at approximately 9401350 nm and 620 940 nm were assigned to the first and second optical transitions
Figure 2. Photographs of the open columns containing 2, 4, and 6% agarose gel beads after allowing the unadsorbed SWCNTs to flow through the column, corresponding to the right column on the second step in Figure 1. The distribution of the adsorbed SWCNTs, visible to the eye (black bars), narrowed as the agarose concentration increased.
of the semiconducting species, designated as the S11 and S22 bands, respectively, whereas the absorption peak at approximately 400620 nm was assigned to the first optical transition of metallic SWCNTs, designated as M11.35 Normalized spectra were obtained by the normalization of the absorbance at 620 nm.
’ RESULTS AND DISCUSSION The general metal/semiconductor separation of SWCNTs using agarose gel columns is illustrated in Figure 1.31 This separation method is composed of three steps, that is, the loading of dispersed SWCNTs onto the column, recovering the SWCNTs that were unadsorbed to the gel and eluting the adsorbed SWCNTs. As shown in the figure, SDS is used for the dispersion of the SWCNTs and the collection of the unadsorbed species, whereas DOC is used for the elution of the adsorbed species. Despite the utility of this method to obtain metal or semiconductor SWCNTs, little is known about the separation characteristics, such as the effects of the agarose concentration, the temperature and the cosolvent on the separation quality. In this study, we focus on the agarose concentration dependence of the separation quality. For the separation of the SWCNTs in the column, 2, 4, and 6% agarose was used. As is evident from Figure 2, the distribution of adsorbed SWCNTs on the agarose columns after allowing the unadsorbed species to flow through the column depended on the concentration of the agarose. Because the same amount of dispersed SWCNTs was loaded onto the respective columns, 21724
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Figure 3. (A) Absorption spectra of the unadsorbed SWCNTs in the flow-through. (B) Absorption spectra of the SWCNTs eluted by the DOC solution. (C) Normalized absorption spectra at 620 nm for the unadsorbed SWCNTs (black lines) and the adsorbed SWCNTs (gray lines). The solid lines, dotted lines and dashed lines represent the spectra of the SWCNTs obtained from the columns containing 2, 4, and 6% agarose gel, respectively.
the change of the distribution associated with the agarose concentration was assumed to be due to a decrease in the adsorption of SWCNTs. To confirm this assumption, absorption spectra of the unadsorbed species contained in the flow-through the column and the adsorbed species obtained from the eluate were recorded (Figure 3). The intensity of the adsorption spectra of the unadsorbed species increased with increasing concentration of agarose (Figure 3A), whereas that of the adsorbed species decreased with increasing concentration (Figure 3B). To compare the components of the collected solutions from the columns, the spectra normalized by the absorbance at 620 nm are shown in Figure 3C. The absorbance of the M11 band for the adsorbed species was lower than that for the unadsorbed ones, whereas the absorbance of the S11 and S22 bands for the adsorbed species was higher than that for the unadsorbed ones (see Materials and Methods section for the assignment of the bands). This result indicated that the unadsorbed species (i.e., the flowthrough) mainly contained metallic SWCNTs, whereas the adsorbed species (i.e., the eluate) mainly contained semiconducting SWCNTs, as shown previously.31,32 Interestingly, the M11 band for the unadsorbed species decreased with increasing agarose concentration, whereas the S22 band for these species increased with increasing agarose concentration, indicating that the amount of semiconducting SWCNTs contained in the unadsorbed species increased with increasing agarose concentration. In contrast, the spectra of adsorbed species obtained from the eluate were virtually identical; specifically, the components of the solution were comparable irrespective of the agarose concentration. These results indicate that a part of the adsorbed SWCNTs (i.e., the semiconducting species) flowed out of the column and was collected as flow-through. Thus, as assumed above, the difference in the distribution of the adsorbed species onto the columns was attributable to the adsorbability of the semiconductor SWCNTs. Importantly, the observed identical spectra of the adsorbed species irrespective of agarose concentration demonstrates that the adsorbability of the semiconducting SWCNTs onto agarose is independent of their chirality. The quantification of the adsorbability of SWCNTs onto the agarose gels is thought to be important in understanding the difference in the separation quality using the column. Here, the time course of adsorption of the SWCNTs onto the agarose gels was measured by a batch separation method where dispersed SWCNTs and gel beads were mixed and the adsorbed and
unadsorbed fractions were subsequently separated by brief centrifugation.36 Figure 4AC depicts the absorption spectra of the unadsorbed species obtained from the supernatants after centrifugation at each time point. As expected, the amplitude of each spectrum decreased with time and gradually converged to a particular value. Figure 4D shows the time course of the absorbance at 620 nm for the spectra. The absorbance for each agarose gel decreased with time. Importantly, the absorbance at each time point increased with increasing agarose concentration. While counterintuitive, this result indicates that the higher the concentration of agarose, the lower the adsorption of SWCNTs, implying that the effective adsorbability was decreased with increasing agarose concentration. Figures 4E and 4F show the ratio of the absorbance at 510 and 1120 nm to the absorbance at 620 nm for the spectra, respectively. The absorbances at 510 and 1120 nm correspond to the M11 and S11 bands, respectively. The ratio of the absorbance at 510 nm to the absorbance at 620 nm (A510nm/A620nm) increased with time, whereas the ratio of the absorbance at 1120 nm to the absorbance at 620 nm (A1120nm/ A620nm) decreased with time. This result indicates that the proportion of the metallic SWCNTs contained in the supernatants increased with time, whereas that of the semiconducting SWCNTs decreased with time. Because the metallic species did not substantially adsorb onto the agarose gels, as shown in Figure 3, the proportion changes in the supernatant can be accounted for by the gradual adsorption of semiconducting species over time. In addition, the difference in the absorbance ratios of the agarose gels at each time point can be ascribed to the adsorbability of the semiconducting species onto them. Next, the adsorption of SWCNTs onto the agarose gels at different initial concentrations of SWCNTs was estimated. The time for the adsorption was set to 180 min, where the adsorption should be in a quasi-equilibrium state (Figure 4D). As expected, the absorbance of the supernatants (i.e., the unadsorbed species) increased monotonously with the initial concentration of SWCNTs (Figure 5A). The final concentration of the unadsorbed species, plotted on the right axis, was estimated using a standard curve of the absorbance at 620 nm. Because the differential coefficient of each curve was less than one, the adsorption was not fully saturated in the initial concentration. If carbon nanotubes did not adsorb onto the gels at all, the coefficient would be one. To examine the component of the unadsorbed species in detail, A510nm/A620nm and A1120nm/A620nm are depicted in Figure 5B. Although A510nm/A620nm for each 21725
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Figure 4. Spectral changes of the unadsorbed SWCNTs on the 2 (A), 4 (B), and 6% (C) agarose gels over the course of time. The absorbance changes for the three samples at 620 nm are shown in panel (D). The ratios of the absorbance at 510 nm (A510nm/A620nm) and 1120 nm (A1120nm/A620nm) to the absorbance at 620 nm are shown in panels E and F, respectively. Closed circles, 2%; open circles, 4%; squares, 6%.
Figure 5. (A) Absorbance at 620 nm and final concentration of the unadsorbed SWCNTs on the agarose gels at various initial SWCNT concentrations. The final concentration was calculated from a standard curve at 620 nm. (B) The ratios of the absorbance of the unadsorbed SWCNTs at 510 (closed symbols) and 1120 nm (open symbols) to the absorbance at 620 nm. Circles, squares, and triangles represent the 2, 4, and 6% agarose gels, respectively.
agarose gel was almost constant in the initial concentration, A1120nm/A620nm increased monotonously with increasing initial concentration, supporting the idea that the major species adsorbed onto the agarose gels was semiconducting SWCNTs. Importantly, the curves seem to reach plateaus at high concentrations of SWCNTs, indicating the gradual saturation of the adsorption of SWCNTs onto the agarose gels. A510nm/A620nm decreased and A1120nm/A620nm increased with increasing agarose concentration. Thus, the adsorbability of SWCNTs in the quasi-equilibrium state was found to depend on the agarose concentration. As seen above, the adsorption of the SWCNTs onto the agarose increased and gradually saturated with increasing SWCNT
concentration. Such adsorption seems to be described by the Langmuir isotherm. By assuming that the adsorption follows the Langmuir isotherm, the relation between the initial concentration of the SWCNTs mixed with the agarose gels and the final concentration of the unadsorbed SWCNTs was calculated theoretically as follows: c ¼ c0 θF
Vgel Vgel Veff ¼ c0 θFϕ ¼ c0 θα Vsol Vsol Vsol
ð1Þ
where c and c0 are the final concentration and the initial concentration of SWCNTs, respectively; Veff is the effective gel volume of the agarose gel onto which the SWCNTs can be 21726
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Figure 6. Relations between the concentrations of the unadsorbed SWCNTs and the initial concentrations calculated from eq 3 with various values of α. (A) K = 10 mg1 mL. (B) K = 50 mg1 mL.
adsorbed; F is the saturation weight of the adsorbed SWCNTs per unit of the effective gel volume (possibly independent of the agarose concentration); Vgel and Vsol are the volumes of the agarose gel fraction and the aqueous solution fraction, respectively; ϕ is the effective gel volume per unit gel volume, that is, ϕ = Veff/Vgel; α is the saturation weight of the adsorbed SWCNTs per unit gel volume, that is, α = Fϕ; and θ is the fractional coverage of the agarose gels, which is given by the following equation based on the Langmuir isotherm: θ¼
Kc 1 þ Kc
ð2Þ
where K is the equilibrium constant between the unadsorbed and adsorbed forms. By combining eqs 1 and 2, the relation between the initial concentration of the SWCNTs and the final concentration of the SWCNTs can be described as follows: 8 9 ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi s Vgel Vgel 2 1< 1 1 4c0 = c0 α c¼ þ c0 α þ 2: K K Vsol Vsol K; ð3Þ Figure 6 shows two examples of the equation, where Vgel and Vsol are 125 and 625 μL, respectively, as described in Materials and Methods. When the equilibrium constant was 10 mg1 mL, the final concentration increased almost linearly with increasing initial concentration (Figure 6A). In contrast, when the equilibrium constant was 50 mg1 mL, the final concentration increased nonlinearly with increasing initial concentration (Figure 6B). Importantly, the slopes of the curves at each initial concentration decreased with an increasing value of α, which means more SWCNTs were adsorbed onto the agarose gel with higher values of α. Because a change in the value of α reflects a change in the value of ϕ, the results indicate that the adsorption of SWCNTs increases with increasing effective gel volume per unit gel volume. If the effective gel volume is zero (ϕ = 0, i.e., α = 0), and thus the SWCNTs are not adsorbed onto the agarose at all, the curve can be on the unit-slope line as shown in Figure 6. Thus, assuming that the equilibrium constant is identical for every agarose gel, the observed increase of the final concentration with increasing concentration of the agarose gels, as shown in Figure 5A, can be attributed to the decrease of the effective gel volume per unit gel volume (ϕ). Although a detailed mechanism is not clear at present, the reduction of the effective adsorption area due to steric hindrance may be responsible for the decrease of the value of ϕ. A network of agarose gel becomes closer as
Figure 7. Mechanistic insight into the metal/semiconductor separation using different concentrations of agarose. The distribution of the adsorbed SWCNTs along the column is shifted from the solid line to the broken line when the concentration of agarose increases; specifically, the amount of SWCNTs adsorbed to the top of the column decreases and the distribution of the adsorbed SWCNTs broadens when the agarose concentration increases. The dotted line indicates the distribution of the adsorbed SWCNTs when the flow velocity was decreased and exhibits a tendency contrary to the broken line.
concentration of the agarose gels is increased, allowing the steric hindrance to become more dominant. The difference in the size of the agarose gel beads may also be responsible for the variation of F, although this is unlikely because the size of the agarose gel tends to decrease or remain constant with agarose concentration (see Materials and Methods). Decreasing the size commonly results in an increase of the adsorption area, that is, the effective gel volume. In the above discussion, the equilibrium constant (K) was assumed to be independent of the concentration of the agarose gel. Here, the validity of this assumption will be discussed. As seen in Figure 6, an increase of the equilibrium constant shifts the curves downward. Therefore, the upward-shift of the curves with increasing agarose concentration, as shown in Figure 5A, would be ascribed to a decrease of the equilibrium constant, if the adsorption of the SWCNTs onto the agarose gels depends on the equilibrium constant. However, this parameter is unlikely to contribute to the curve shifts because the saturation concentration of the SWCNTs seems to decrease or remain constant with increasing agarose concentration, as seen in Figure 5B, which indicates that the equilibrium constant actually increases or remains constant with increasing agarose concentration. Consequently, the decrease of the adsorption with increasing agarose concentration shown in Figure 5A is not due to the equilibrium constant. Based on the results shown above, mechanistic insight into the quality of the metal/semiconductor separation using different concentrations of agarose will be presented. For simplicity, assuming that the time for the adsorption of the SWCNTs onto each position along the column is sufficiently shorter than that for equilibration, the distribution of adsorbed SWCNTs along the agarose columns is theoretically described as follows: ( dyðtÞ ¼ kϕydt ð3Þ dxðtÞ ¼ vdt 21727
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where y(t) is the SWCNT concentration in the agarose column at time t, x(t) is the position of the SWCNTs along the column at time t, k is the adsorption rate of SWCNTs onto the agarose gels and v is the flow velocity. By combining the above two equations, the distribution of the adsorbed SWCNTs expressed as C(x) can be described as follows: CðxÞ ¼
dy kϕ ¼ C0 eðkϕ=vÞx dx v
ð4Þ
where C0 is the total concentration of the SWCNTs applied onto the top of the column. This equation is illustrated in Figure 7. Importantly, as the effective gel volume per unit gel volume (ϕ) decreases, the concentration of the SWCNTs adsorbed onto the top of the column decreases and the distribution of them along the column broadens (see the shift from solid line to broken line in Figure 7). As mentioned above, the effective gel volume decreases with increasing agarose concentration, so it can be concluded that the broadening of the distribution is reflected by the increase in the agarose concentration, which is consistent with the observed result shown in Figure 2. Thus, the broadening of the distribution is possibly responsible for the decline in the quality of the metal/semiconductor separation that is seen in Figure 3; specifically, the broader the distribution, the more the flow-through. Incidentally, the effect of the flow velocity on the separation is also shown in Figure 7. A decreased flow velocity will result in an improved separation. Finally, it is worthwhile to compare the metal/semiconductor separation with chromatographic size separations. As shown in this study, metal/semiconductor separation occurs because metal-rich species flow out of the agarose columns, whereas semiconducting species adsorb onto the agarose gels. In contrast, size exclusion chromatographies separate SWCNTs according to their sizes (lengths or diameters) with no regard to metal and semiconductor species.3742 It is interesting to note the difference in mechanism between the metal/semiconductor separation and the chromatographic size separations. The columns that are used in the chromatographic separations contain some other gel than agarose gel, which is the reason that the size exclusion chromatographies displayed no metal/semiconductor separation of the SWCNTs. In other words, the metal/semiconductor separation seen in this study is a selective interaction between the semiconductor species and the agarose gel, as also reported previously.34 Thus, the metal/semiconductor separation of SWCNTs is subject to their specific adsorbability onto agarose gels rather than to their sizes, as shown in Figure 7.
’ CONCLUSIONS In this study, the effects of the agarose gel concentration on the quality of the metal/semiconductor separation of SWCNTs were assessed using columns. Unexpectedly, the quality of the separation deteriorated with increasing agarose concentration. By assuming that the adsorption of SWCNTs onto the agarose gel follows the Langmuir isotherm, it was found that the effective gel volume for the adsorption decreased with increasing agarose concentration. In addition, the effective gel volume can account for the distribution of the SWCNTs along the column. Therefore, the increased agarose concentration results in the broadening of the distribution, which is the reason that the quality of the separation depends on the agarose concentration. No doubt other factors, such as the flow velocity and the translational diffusion of SWCNTs in the column, also affect the quality of the
separation; nevertheless, the effective gel volume can be a primary factor. The present study does not conclude that the agarose concentration should be reduced as much as possible because an extreme reduction of the agarose concentration decreases the stability of the gel beads and hinders the formation of the gels. There should be an optimum concentration of agarose for the separation, probably depending on pH, ionic strength, and temperature of the solution. Thus, the optimization of the agarose concentration should be just one of the key challenges for the industrialization of the separation method.
’ AUTHOR INFORMATION Corresponding Author
*Tel.: +81-29-861-2551. Fax: +81-29-861-2786. E-mail: h-kataura@ aist.go.jp.
’ ACKNOWLEDGMENT This work was supported in part by KAKENHI (23651122) of MEXT of Japan to T.T. We thank M. Taga for technical assistance. ’ REFERENCES (1) Wu, Z.; Chen, Z.; Du, X.; Logan, J. M.; Sippel, J.; Nikolou, M.; Kamaras, K.; Reynolds, J. R.; Tanner, D. B.; Hebard, A. F.; Rinzler, A. G. Science 2004, 305, 1273–1276. (2) Green, A. A.; Hersam, M. C. Nano Lett. 2008, 8, 1417–1422. (3) Javey, A.; Guo, J.; Wang, Q.; Lundstrom, M.; Dai, H. Nature 2003, 424, 654–657. (4) Barone, P. W.; Baik, S.; Heller, D. A.; Strano, M. S. Nat. Mater. 2005, 4, 86–92. (5) Star, A.; Gabriel, J. C. P.; Bradley, K.; Gruner, G. Nano Lett. 2003, 3, 459–463. (6) Kam, N. W.; O’Connell, M.; Wisdom, J. A.; Dai, H. Proc. Natl. Acad. Sci. U.S.A. 2005, 102, 11600–11605. (7) De la Zerda, A.; Zavaleta, C.; Keren, S.; Vaithilingam, S.; Bodapati, S.; Liu, Z.; Levi, J.; Smith, B. R.; Ma, T. J.; Oralkan, O.; Cheng, Z.; Chen, X.; Dai, H.; Khuri-Yakub, B. T.; Gambhir, S. S. Nat. Nanotechnol. 2008, 3, 557–562. (8) Welsher, K.; Liu, Z.; Sherlock, S. P.; Robinson, J. T.; Chen, Z.; Daranciang, D.; Dai, H. Nat. Nanotechnol. 2009, 4, 773–780. (9) Saito, R.; Fujita, M.; Dresselhaus, G.; Dresselhaus, M. S. Appl. Phys. Lett. 1992, 60, 2204–2206. (10) Arnold, M. S.; Green, A. A.; Hulvat, J. F.; Stupp, S. I.; Hersam, M. C. Nat. Nanotechnol. 2006, 1, 60–65. (11) Ghosh, S.; Bachilo, S. M.; Weisman, R. B. Nat. Nanotechnol. 2010, 5, 443–450. (12) Yanagi, K.; Miyata, Y.; Kataura, H. Appl. Phys. Express 2008, 1, 034003. (13) Moshammer, K.; Hennrich, F.; Kappes, M. M. Nano Res. 2009, 2, 599–606. (14) Krupke, R.; Linden, S.; Rapp, M.; Hennrich, F. Adv. Mater. 2006, 18, 1468–1470. (15) Krupke, R.; Hennrich, F.; Weber, H. B.; Kappes, M. M.; von Lohneysen, H. Nano Lett. 2003, 3, 1019–1023. (16) Krupke, R.; Hennrich, F.; Lohneysen, H.; Kappes, M. M. Science 2003, 301, 344–347. (17) Tanaka, T.; Jin, H. H.; Miyata, Y.; Kataura, H. Appl. Phys. Express 2008, 1, 114001. (18) Miyata, Y.; Maniwa, Y.; Kataura, H. J. Phys. Chem. B 2006, 110, 25–29. (19) Miyata, Y.; Kawai, T.; Miyamoto, Y.; Yanagi, K.; Maniwa, Y.; Kataura, H. J. Phys. Chem. C 2007, 111, 9671–9677. 21728
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